Nghiên cứu các màng mỏng tử giảo đơn lớp, đa lớp chứa đất hiếm và các khả năng ứng dụng của chúng trong các kỹ nghệ kích thước bé

Đại học Quốc gia Hà nội
5jc ĩỊc ĩỊí ĩỊf ĩỊí *Ị^ V ĩ|*
B á o c á o k ế t q u ả th ự c h iệ n
Đ ề t à i k h o a h ọ c Đ H Q G đ ặ c b iệ t
Q G . 99 . 0 8
Chủ trì đề tài:
TS. Nguyễn Hữu Đức
Bộ môn vạt lý Nhiệt độ thấp,
Khoa Vật lý,
Trường Đại học Khoa học Tự nhiên
4-2001
Báo cáo kết quả thực hiện đề tài khoa học ĐHQG đặc biệt
QG. 99. 08
Tên đề tài: Nghiên cứu cá c m àng mỏng từ giởo đơn IỚỊ1, da lớp chứa ổ ất hiếm và các
khả nâng ứng dụng của chúng trong các kỹ nghệ kích thước bé
Chủ trì đề tài: TS. Nguyễn Hữu Đức
Bộ mồn Vật lý Nhiệt độ thấp, Khoa v ạ t lý, Trưòng Đại học Khoa học Tự nhiên
Đ H Q G H à nội.
Các cán bộ thực hiện chính:
Trần Mâu Danh, NCS, trung tâm IT ĨM S và Bộ môn v ạ t lý Nhiệt độ thấp
Nguyễn Pliíí Thuỳ CBG D, Bộ môn Vật lý Nhiệt độ (hấp
Hoàng Ngọc Thành CBG D, Bộ môn V ật ]ý Nhiệt độ thấp
Trẩu Đình Thọ K ỹ thuật viên, Bộ môn Vật lý Nhiệt độ thấp
Nguyễn Anh Tuấn CB hợp đồng, Bộ môn V ậí lý Nhiệt độ thấp
Nguyễn Thành Nam Học viên cao học, Bộ môn Vật lý Nhiệt độ thốp
Nguyễn Thị Ngọc Anh Học viên cao học, Trung trim IT IM S
Đỗ thị Hương Giang Sinh viên khoa Vật lý
Vũ Nguyên Thức Sinh viên khoa V ật lý
M ột số cán bộ khác của Bộ môn Vật lý Nhiệt độ thấp dã tham gia thảo luận xây
dựng Hệ do từ giảo, Bộ môn Vật lý chất rắn và Trung tâm khoa học vật liệu dã giúp đỡ
đo nhiễu xạ Rơnghen, Trung tâm IT IM S đã cho phép sử dụng máy sputtering để chế

tạo mẫu.
Các hợp tác khoa học với Viện Từ học Louis Néel, Grenoble và PTN từ hoc,
Đ H TH Rouen, Pháp đã giiìp các phép đo độ dày màng mỏng, và phổ Mossbauer.
9
Thời gian thực hiện: 2 năm (từ tháng 4/1999)
1. Tóm tỉít các kết quả chính đà đạt được
i.a. Các màng mỏng tờ giảo đon lóp
Đ ã chế tạo thành công ờ V iệt nam và nghiên cứu có kết quả các màng mỏng từ giảo
đơn lớp Tb(F e,C o)2 và Tb(Fe,C o), V Đã tìm hiểu các cơ chế vật lý của hiện tượng từ
giảo trong các màng mỏng đất hiếm và tìm được các thông số tối ưu cho vật liệu. Từ
giảo đã đạt được X ~ 10'? ở H = 0.7 T. Đặc biệt, ở tìr trường thấp 10 m T, độ cảm tír giảo
«
Xx/' - 1 -8x I0 '2 T 1. Tính chfTt vi mô của các màng mỏng cíĩng đã được nghiên cứu bằng
phổ Mossabuer. ĐAy là các vậl liệu có các lliông số so sánh được với các vật liệu cíing
loại đang được nghiên cứu ở các phòng thí nghiệm trên thế giới. Các kết quà nghiên
cứu thực hiện tron g năm 1999 vh đà có 3 bài háo đãng ở lên các tạp chí J. Appl. Pliys.
và J. of Phys.: Contiens. Matter, (xem các bài bán kèm tlieo trong phần phụ lọc)
l.b. Các màng mỏng từ giảo đa lốp
Chế tạo các màng mỏng từ giảo đa lớp dã được thực hiện với kỹ thuật phức lạp
hơn. Hai loại màng mỏng đa lớp đã được chế tạo và nghiên cứu (i) Tb(Fe,Co), ,/Fe và
(ii) Tb(Fe,Co), 5/(Y Fe). Ưu điểm cluing của cả hai loại vật liệu này là tính từ mềm tốt
nhưng vẫn giữ được độ cảm từ giảo cao nhờ cấu trúc composit của chííug. Loại đa lớp
thứ nhất mà đề tài đã chế tạo được có tính chất lương lự như các vạt liệu cùng loại mà
các phòng thí nghiệm ở trên thế giới dang đạt dược. Thế mạnh của chúng tôi là khai
thác một số ý tưởng vật lý liên quan đến cấu trúc interface của vật liệu. Bằng việc xử lý
nhiệt, ngoài việc cải thiện tính chất từ giảo của mẫu, vùng interface đã đựơc mờ rộng l a
vh dị hướng từ vuông góc đã hình thành. Với tính chất này, vật liệu CÒ
11
có khả năng
ứng dụng trong kỹ thuật ghi và lưu trữ thông tin. Các kếl quả đã dược xử lý, báo cáo và

đăng trong tuyển tập Hội nghị vạt lý Châu Á- Thái bình dương, 8/2000. Đặc biệt, các
kết quà Iihy cĩiiig đã được xuất bả» ở tạp chí Appl. Physics Letter., (20 01), xem phụ
lục. Trong bài báo này, ngoài việc (hông báo giá trị 1ỚI1 của dộ cám từ giáo, chúng tỏi
CÒI1 đề xuất một giải pháp chọn điểm làm việc cho các vật liệu từ giảo.
Loại vật liệu đa lớp thứ hai Tb(Fe,Co), ,/(Y Fe) là một (hành công hất ngờ.
Thông (hường, các lớp FeCo trong các màng mỏng đa lớp đều có cấu trúc tinh thể. Đó
cũng vừa là tru điểm (cho độ từ hoá cao), nhưng cũng là một hạn chế vì nó khống chế
luôn cả giới hạn dưới cùn lực kháng từ. Trong công trình của cluing tôi, trạng thái vô
định hình trong lớp Y Fe đã hình thành. Xuất phái từ (rạng thái này, nhờ quá trình II
nhiệt, cấu trúc vi liạt đã đạt được và kéo tlieo là những lính chất từ và từ giảo lý tưởng:
H t = 0.3 inT và độ cảm từ giảo kỷ lục Xx = 13 .102 T 1. Các kết quả đang được gửi
dăng ở lạp chí J. Appl. Phys. (USA). Các kếl quả nhân clirợc của đề thi đang mờ ra một
ImỚMg nghiên cứu lý thú và có nhiều ý nghĩa thực liễn ờ cả (rong nước và tiên lliế giới.
Với thành tựu này, cluing tôi đã đirợc ban tổ chức hội Iiglv I '-'-'-' w quốc tế IEM S '01
(28/8-1/9/200Í, Grenoble, Pháp) mời tham dự để đọc báo cáo mời tại hội thảo. Các kết
quả nghiên cứu tìm hiểu các cơ chế vật lý của loại vật liệu mới này trong từ trưòng cao
và ở nhiệt độ thấp cũng đang được xử lý để xuất bản.
2. Danh mục các công trình khoa học đã hoàn thành
2.a. Các bài báo đã đãng trên các tạp chí quốc tế:
T?) N.H. Due, K. Mackay, J. Betz and D. Givorđ,
Magnetic and magnetostrictive properties in amorphous (Tb, Dy) (Fe i-xCo^ỉ films
J. Appl. Phys., 87 (2000) 834 (USA).
ĩ) T M Danh, N.H Due, H.N. Thanh and ì. Teillet,
Magnetic, M ỏssbauer and magneíostrícíive studies o f amorphous
(Tb,Dy) (Fei.xCoJi.i films
J. Appl. Phys., 87 (2000) 7208 (USA)
/3 n) N H Due, T.M Danli, H.N. Thanh, J. Teillet and A Lienard,
M ag nelic, M o s sb a uer a n d m ag neto,sfrictive s tu d ie s o f am o rph o us TbfF eo .isC oo js) I.s
films
I of Phys : Condens. Matter., 12 (2000) 8283 (U K)

0 N H. Due, N.A. Tuan, D.T.N. Anh, T M Danh, N p. Thuy
Giant mơgtỉeíostriction o f TbFeCo single-layer and TbFeCo/Fe multilayer pirns
Proceedings of the 811' Asia Pacific Physics Conference, Taipei, August 7-10, 2000
Large magnetostrictive susceptibility in Tb-FeCo/FeCo multilayers
Appl. Phys. Letter (USA), 78,11 (2001)
2.b. Các bài báo đã gửi đăng ử các tạp chí quốc tế:
( T ) N.H. Duc,
Development of giant low-field magnetostriction in rare-earth based single, multilayer
and sandwich films
Invited talk to be presented at JEMS’01, Grenoble, 8-2001
2. N.H. Duc, T.M. Danh, N A Tuan and J. Teillet
E xce lle nt m ag n eto strictio n softn ess in am o rp h ous -T b -F eC o/p o ly m otp hotts-Y F e
multilayers
Submitted to J. Appl. Phys.
2.C. Sách chuyên khảo bànjî tiếng Anh
(T ) N.H. Duc,
Giant magnetostriction in lanthanide - transition m etal thin films
Handbook on the Physics and Chemistry of Rare Earths, Vol. 32,
eds K.A. Gschneider, Jr and L. Eyring, Elsevier Science, (Amterdam), 2001 in press.
N.H Duc, T.M. Danh, N.A. Tuan and J. Teillet
2.d. Các bàí báo đã đăng ở các tuyển tập hội nghị khoa học quốc tê tổ chức ỏ Việt
nam:
1. N H. Duc, T.M. Danh, N.T. Nam, H.N. Thanh, N P. Thuy, J. Teillet and D Givord
M a g n etic a n d m ag nefo stric tiv e p ro p er ties in a m orp h ous Tb(F et)ỊiC oo 4í) ĩ film s w ith
p e rpetidìcttla r a n isotrop y
Proceedings of the International Workshop on Materials Science, Hanoi, 11/99, p 230
2 N.H. Duc, T.M. Danh, H N. Thanh, N p. Thuy
G ian t m ag ne to stric tiv e T b-F eC o thin film s a n d app lic a tio ns
Proceedings of the Third Vietnamese-German Workshop on Physics and Engineering,
Ho Chi Minh City, 3-8 April 2000.

3. N.H. Duc, F. Richomme, N.A Tuan, D T líuong Giang and J Teillet
M a gne tic s oftn e ss in m a gnetostrictive T bF eC o/YF e m illlila ye rs
To be presented at JEMS 01, Grenoble, 8-2001
2.C. Các bài báo đã báo cáo tại các hội nghị khoa học trong nước
1. N guyễn Anli Tuấn, Trần MẠu Danh, Nguyỗn Hữu Đức, Npuyỏn Thị N gọ c Anh,
Đỗ Thị Hương Giang, Vũ NguyCn Thírc
Độ cảm tứ giào của các màng mỏng TbFeCo và TbFeCo/Fe
Hội nghị Khoa học Trường ĐHK H Tự nhiên, 11/2000
2. Nguyễn Hữu Đức, Trần Müll Danh và Nguyễn Anh Tuấn
Tính từ mềm dặc biệt ỉrong các' inàiĩg mòng từ giảo dơ lớp vô địnlt hình và vi hạt
Hội nghị vật lý loàn quốc, 3/2001.
3. Báo cáo tài chính:
Năm 1999: Đ ã được cấp 45 triệu đổng. Đã quyết toán xong.
Nãm 2000: Kinh phí đựơc cấp: 15 triệu đồng. Đã quyết toán xong.
4. Kiến nghị:
Đây là một đề tài thực hiện có hiệu quả và chất lượng nghiên cứu khoa học đạt trình độ
quốc tế. Cấc chỉ tiêu về vật liệu, về số lượng và chất lượng các cồng trình khoa học đều
đã đạt và vượt chỉ tiêu đăng ký. Đ ề thi cũng có tác đụng nflng cao chất lượng đho tạo ( I
NCS đang hoàn thành luận án, í cao học đã bảo vệ, 2 luận văn cử nhân cao học và 2
báo cáo khoa học sinh viên, (rong đó có một giải nhất của Đ H Q G Hà nội). Hướng
nghiên cứu của đề tài đang mở ra rất có ý nghĩa về khoa học cơ bản và cả khả năng ứng
dụng. Nhóm nghiên cứu của đề tài đã trở nên có uy tín (rên thế giới. Cán bộ của đề thi
đã được mời tiếp tục viết sách chuyên khảo về chủ đề. M a g n e to e ia stic ity in A rtific ia lly
Structured M ateria ls cho Handbook of Magnetic Materials do K.H.J. Buschow biên
tập và sẽ xuất bản ờ Amsterdam.
Đé nghị được nghiệm thu đúng thòi Itọn (4/2001) và tạo điểu kiện dể dề tài
được tiếp tục đăng ký trong hai năm tiếp then (2001-2003).
Ngày 10/04/2001
Chủ trì đề thi
/ [ Ị b u d h ^ —-

TS. Nguyễn Hữu Đức
©Ạt H ỌCQ UỔC GIA H Ì N ố i -
TRUNGTÁM ĩỉlÔHGTIN.Ti IƯ VI,¿ 'I
N o D r/2 0 1 ’ "
Phụ lục
M ột sô bài báo tiêu biểu đã xuất bản
trê n các tạp chí quốc tế
của đề tài QG. 99. 08
\
v/i * likij rn i ait.)
VOI.UMi; S7. Nt'MHIR 2
15 JANUARY
Magnetic and magnetostrictive properties in amorphous
(Tb0.27Dy()73)(F e1_xCox)2 films
N. H. Duca)
Cryogenic Laboratory. Faculty o f Physics, Notional University o f Hanoi. .I.U-Nyiiycn In n . Thanh Xuan.

Hanoi. Vietnam
K. Mackay, J. Betz, and D. Givord
Laboratoire rie Magnetisme Louis Neel, C NRS, 3X042 Grenoble. Cerle.x
9.
Francr

(Received 5 May 1999: accepted for publication 16 September 1999)
Magnetic and mngnetostrictive properties have been investigated for amorphous (Tb0 37Dyo 71)
(Fe|_,Co,)j thin films. An increase in the 3rf magnetic moment due to the enhancement of T -T
interactions in substituted (Fe. Co) alloys was found. This leads to stronger R-(Fe.Co) exchange
cnergifcs and then to enhancements of R—sublattice magnetization as well as magnetostriction in
these amorphous R(Fe.Co) thin films. In addition, a well-defined in-plane anisotropy is created by
magnetic-ficld annealing for the Co-rich films. A large magnetostriction of 4R0X 10'* developed in

low fields of 0.3 T was observed for films with v = 0.47 after magnetic-field annealing. The differing
roles of Fe and Co atoms on the magnetization process have also been discussed. ©
2000

Am e rican Institute o f Physics,
[S0021-8979(99)06624-4]
I. INTRODUCTION
Over the past few years there has been a growing interest
in magnetic thin films with large magnetostriction.1 ' This
interest is motivated by the potential such films show for use
in microsystems actuators.
R-Fe ( R = rare earth) based alloys offer the possibility to
develop very large magnetostriction al room temperature.
This is due to the highly aspherical 4 / orbitals remaining
oriented by the strong coupling between R and Fc moments.
In order to exploit (his property at reasonably low fields, it is
essential to have low macroscopic anisotropy. A first route to
low anisotropy is by using cubic compounds 111 which the
second-order anisotropy constants vanish. This is the case for
the RFei laves phase compounds of which TbFe2 (tcrlcnol),
a fcrrimagnet with 7'C = 710K. is probably the best known.1
having 1753X 10“f’. The anisotropy can be further de
creased by substitution of Th and Dy in these compounds.
This is due to Dy and Tb having opposite signs of the
Sloven's (3, coefficient and thus their contribution to the
fourth-order anisotropy being of opposite sign. This leads to
the magnetostriction, albeit less than in pure TbFe,. being
saturated in much lower fields. This is the case for the
terfenol-D material, the crystalline (Tbf|27Dy07,)Fci com
pound. which has found many applications as high-power

actuators.
An alternative route to low macroscopic anisotropy is by
using amorphous materials. In Fe-based amorphous alloys,
both positive and negative exchange interactions exisl' lead
ing to magnetic frustration in the Fe sublallicc. In amorphous
o-YFc alloys, this results in a concentrated spin-glass behav
ior below room temperature In a-RFc alloys, where R is a
magnetic rare earth, the additional contributions of R-Fe ex-
‘’Aultior to whnm cpmrspundence should be addressed; electronic mail:
ducfo'cryolnh cdu.vn
0021 ’8979/2OOO/87(2)/034/6/$ 17,00
change and local crystalline electric-field interactions lead to
the formation of sperimagnelic structures ’ The ordering
temperatures are above room temperature [ r r = 410K for
fl-Tbn }iFe0fi£ (Refs, 6 and 7)]. It is. however, still rather low
and is thus detrimental lo large magnetostrictions being ob
tained in such materials at room temperature.
Actually, with a view lo obtaining large magnetostric
tions in the amorphous stale, it is interesting to consider the
equivalent n-RCo-based alloys. Although crystalline RCo-
compounds order below 300 K as Co is merely
paramagnetic.* the amorphous state stabilizes a moment on
the Co sublatlice due to band narrowing. These Co moments
are strongly lerromagneticalh coupled. A sperimagnctic
structure occurs as in a-RFe alloys but the ordering tempera
ture is now raised up to 600 K (Ref, 7) for Tb„i,Cow. Re
cently, we have studied rt-Tb.Co, _ , and shown that large
magnetostrictions of />V’ = 300X 10 at 300 K are obtained
ror.T~0.33.9
In general, however. R- Fe exchange energies are larger

than the equivalent R-Co interaction energies 1,1 This arises
from the fact the Fe moment is significantly larger than the
Co one. while the R -T intersublattice exchange constant
(T=!ransition metal) is approximately the same for T=Fe
and Co. In addition, the T -T interactions tend to be stronger
in (FeCo)- than in either Fe- or Co-based alloys.1* This re
sults in an increase of
T r
for a given R:T ratio. The stronger
R-FeCo exchange energies should then lead to an enhance
ment of the R moment at room temperature and thus
the magnetostriction in these amorphous alloys. Rcccntly,
we have studied the magnetostriction in amorphous
(Tb| - vDy, )(Fe0.i5CO|i55)21 thin films. A magnetostriction of
1020X 10 was obtained for amorphous
Tb(Fe0jjCoo55)3 1 Indeed, this is much larger than that
seen in other amorphous films of either TbFe or TbCo.
© 2000 American Institute of Physics
B34
I
dI
Due el ai 835
n ihe present article, we have studied the influence of
e:Co ratio on the magnetization and magnetostriction of
:
7
Dyo
7
i)(F e,_ ,Co v) 2- We will show that the Fe:Co ra-
f 50:50 responds approximately to the optimum compo-

i for the giant magnetostriction.
<PERIMENT
The films were prepared by rf magnetron sputtering. The
al power during sputtering was 300 W and the Ar pres-
was 10
~ 2
mbnr. A composite target was used allowing a
range of alloys to be made in a controllable way with-
large cost of materials. The target consisted of 18 seg-
s of about 20°, of different elements (here, Tb, Dy, Fe,
These were made by spark cutting pure element disks,
were then assembled and stuck to a Cu sample holder
! silver paint. It was verified by Rutherford backscatter-
;pcctroscopy (RBS) and X-ray energy-dispersive spec-
opy (XEDS) measurements that no Cu and Ag contami-
n has occurred, The target-subslrnte distance was
8
cm.
substrates were glass microscope cover slips with a
nal thickness of 150 /uni, Roth target and sample holder
water cooled.
The ratio of the deposition rates of R=Tb. Dy lo T=Fe,
s 0.85. Thus, for the (Tb
0 27
Dyi)
7 1
)(Fe
1
_,CoT), films
: here, the Tb(Dy) and Fc(Co) concentrations could, in

iple, be varied in steps of about 14% and 9%. respec-
y. The resulting composition, contamination, and the
losition homogeneity were measured using XEDS and
analyses. The thicknesses were measured mechanically
> an rr-stcp and the sample mass was determined from
nass difference of the substrates before and after sput-
g. The typical film thickness was 1.2 /im. X-ray 0 - 1 0
action showed the as-deposited samples to be amor
s.
Samples were annealed al 150° and 250 °C for I h under
ignetic field of 2.2 T in order to relieve any stress in
d during the sputtering process and to induce a well
ed uniaxial in-plane anisotropy, Subsequent x-ray 0 - 2 0
iction showed no evidence of recrystallization after an-
ng.
The magnetization measurements were carried out using
•rating sample magnetometer in a field of tip
8
T from
a 800 K.
The magnetostriction was measured using an optical de-
imeter (resolution of 5 X I0 ~
8
rad), in which the bend-
>f the substrate due to the magnetostriction in the film
measured. This allows the magnetoelastic coupling co
gent of film (/j) to be directly determined
1' 14
using
_ a bj Es

’~ T T f 6(1 + 1/,)- (,)
e « is the deflection angle of the sample as a function of
ed field,
I .
is the sample length, and £ , and
v ,
are the
ig's modulus and Poission's ratio for the substrate
li are taken to be 72 GPa and 0.21, respectively. hs and
e the thicknesses of the substrate and film, respectively,
s typically of the order of 13 mm.
FIG I. Hyntcrcsis liKips m 4.2 K for scvcr;il ITtin -jDy,,71>t FC| ,Co,); lliin
films: (I) -,< =n. (3) *=0.31. and (.
1
) —.r =
1
.
0
. •
b is proportional to the magnetostriction via the Young's
modulus (Ef) and Poisson's ratio (v,) of the film. These
cannot be reliably measured for thin films. However, for
comparison, we also give values of \ calculated using
— 6 (1 +
v ,)
\ =

~

12)

kr
where Ef and i\ are taken to be 80 GPa and 0.31. respec
tively.
We measured two coefficients at saturation, hv and b ,
which correspond to the applied field, always in the film
plane, being, respectively, parallel anti perpendicular to the
sample length (i.e the measurement direction). In addition,
the perpendicular direction corresponds to the easy axis in
duced after field annealing. The intrinsic material-dependent
parameter h Y2 (or X’ -1) is just the difference bf — b1 (or
— X L . respectively).
III. EXPERIMENTAL RESULTS
A. M agnetization
Figure I presents the hysteresis loops for several as-
deposited (Tbn ;
7
Dy
0 7
,)(FC| - ,C o,): films at 4.2 K. The co
ercive fields arc very large for all samples and the magneti
zation does not completely saturate even at
8
T. Such large
coercive fields are typical of amorphous RT alloys at low
temperatures, where R is a non S' state rare earth. They are
related to the strong local anisotropy of the R atoms and their
random distribution of easy axes present in such ¿perilling-
netic systems. The high-íield susceptibility (,yhf) is also typi
cal of sperimagnetic systems and is associated with the clos
ing of the cone distribution of R moments as the field is

increased
.5
The coercive fields (¿tn/Vr ) reach their highest value of
3.4 T for.r = 0. With increasing Co concentration, coercivity
decreases rapidly down to about 0.5 T for 0.67'=.t« 1.0 [sec
Fig. 2(a)]. The * 1l( also decreases with increasing Co concen
tration, to a minimum at ,v = 0.47 and then slightly increases
with further increasing ,v.
In all cases, fxnHr also decreases with increasing tem
perature [sec the inset in Fig. 2(a)], while the * hf is strongly
enhanced. This is due to the rapid decrease local anisotropy
of the R atoms as the temperature is increased compared lo
Due el at.
2. (a) Coercive field
ixnH r
as a function or Co concentration at 4.2 K.
shows the temperature dependence of
finI Ir
for jr = 0 83. (b) Coer-
field as a function of Co concentration at 300 K: (I) the as-
ited films. (2) after annealing at liO"C, anti (3) after annealing
re.
ixchange field. In Fig. 2(b), we prescnl fi0Hc at 300 K
motion of x. All tbe films are magnetically rather soft at
n temperature and there is a maximum in at x
63, •
The spontaneous magnetization values at 4.2 and 300 K
the as-deposited (Tb0
27
Dyo71)(Fe| _,C o

t ) 2
films ex-
olatcd to zero field are shown in Fig. 3. At 4.2 K there is
nximum at r = 0.47 while at 300 K, within experimental
rs. the magnetization is independent of the Co conccn-
on. This is in contrast with (he behavior observed for the
esponding crystalline alloys where M s always shows a
inium in the middle of the composition range due to the
incemcnt of the 3d magnetic moment ( M w ). In the
irphous case, however, an increase in will close the
x
3. Variation of spontaneous magnetization as a function of
x
at 4.2 anil
K for (Tbo?1Dyn70(Fe,_tCo,)2 thin films.
(T)
FIG. 4. Hysteresis loops for the (Tbq:,Dy„,j)Co2 (1) as-deposited film and
(2) after annealing along induced easy axis and (3) hard axis.
R-sperimagnetic conc. The maximum in M , at .r = 0.47 re
flects that, at low temperature, the enhancement of M is
smaller than the associated increase in the magnetization of
the R sublattice ({A /R)).
Samples were annealed at temperatures between 150 and
250 0C in an applied magnetic field of 2.2 T. The field de
pendences of the magnetization before and after annealing
are shown in Fig. 4 for x ~ 1. For the as-dcposited samples,
the magnetization reversal process is progressive and isotro
pic with a rather large coercive field. This property is often
observed in sperimagnetic systems where domains of corre
lated moments are formed due to the competition between

exchange interactions and random local anisotropy. These
domains, termed Imry and Ma domains,|, ,r' are oriented
more or less at random in zero field but can be reoriented
relatively easily under applied field.
After annealing, there are a number of clear differences
in the magnetization proccss. First, (he coercive field is
strongly rcduccd. Figure 2(b) shows the coercive field as a
function of composition before and after annealing. After
annealing at 250 BC, Unlfc is less than 0.002 T for samples
with r = 0.0 and 1.0, A slight maximum of fxnHc around the
middle of the composition range is siiII observed, however,
with /xo/Yc~ 0.0 06 T only. Second, for this sample, there is
now a well-defined easy axis with an increased low-ficld
susceptibility. These properties are characteristic of systems
which show uniaxial anisotropy. This field-annealing in
duced anisotropy suggests that a process of single-ion direc
tional ordering
17
has occurred, in which there is a local re
orientation of the Tb easy axes along the field direction. The
composition dependence of this uniaxial anisotropy is, how
ever, more complex and will be discussed further in connec
tion with the magnetostriction data. The field annealing also
causes a reduction in ^(lf, indicating that the cone distribu
tion of the Tb moments is somewhat closed,
B. M agn etostriction
In general, the comparison of f>, and hL indicates clearly
the anisotropy state of the sample. If the zero-field state is
fully isotropic, then bi = —2b 1 , and if it is isotropic in the
plane, then b,= — hL . IS For a well-defined in-plane, uniaxial

system, magnelizaiion reversal under a field applied along
the easy axis, occurs by 180° domain-watl displacement. Ne-
t
Appl Phys., Vol. 87, No, 2, 15 January 2000
Due el al. 837
M, " O)
710. 5. (o) Magnotoslriction for t = 0.83: (I) as-deposited film, (2) anneal
ng al I50°C. and (3) 250'C. (b) Magnetostriction for jr = 0: (I) as-
lepositcd film, and (3) 250 °C.
¡lecting domain-wall contributions, no magnetostriction is
associated with this process. Thus, hi should be zero and
b, = b '-\ ' .
Figure 5 shows the effect of annealing on the magneto
striction for two alloys with * = 0.83 and * = 0. For jr = 0.83
[see Fig. 5(a)], we see that annealing increases the ratio of i>n
lo b± while b y7 rests roughly constant. This is due to the
creation of an in-plane uniaxial anisotropy as seen from mag
netization measurements. In addition, we see that this anisot
ropy is completely induced after annealing at 150°C and is
accompanied by a reduction in the saturation field. Subse
quent annealing at 250 °C simply further reduces the satura
tion field. For the * = 0 sample [see Fig. 5(b)], we see a
different behavior. Before annealing, the approach to satura
tion is rather slow and the ratio of b to b± indicates an
initial anisotropy. After annealing, the saturation field is re
duced and this initial anisotropy is destroyed, leaving the
sample almost isotropic. However, b
r 2
(measured at 1.8 T)
actually increases after annealing probably due lo the reduc

t io n in the saturation field.
These differences are reflected across the whole compo
sition range and the results obtained are summarized in Fig.
6
(a). As outlined above, it is clear that the annealing affects
very differently the Fe-rich alloys compared to the Co-rich
ones. For the Co-rich alloys, bt increases significantly after
annealing while b r2 rests virtually unchanged. For the Fe-
rich alloys, we see the opposite effect in that by 2 increases
significantly after annealing while b n rests virtually un
changed. The annealing seems to destroy the initial as-
deposited anisotropy and does not induce an in-plane
uniaxial anisotropy. These differences in anisotropy are also
X
X
FIG 6. (a) Magnetostriction Xy2(1.8T) and X,(0.06T) for the
(Tbo.27Dyo.73HFei ,0 ^)2 as-deposited thin films (I) and (I), films nn
nealed at I50°C (2 and
2 ')
and at 250*0 (3 and ,V). (b) Ratio
b%lb ±
as a
function of
x
before and after annealing
reflected in Fig
6
(b), which shows the ratio of b„ to />,
before and after annealing. This will be discussed later. The
largest magnetostriction of \ r'2= 480X 10~s and Xn = 250

X
1 0
is found in the middle of the composition range at
x = 0.47 and can be obtained in very low applied magnetic
fields of 0.06 T.
IV. DISCUSSION
The magnetic properties of these alloys are rather com
plex but it is important to attempt to understand them in
order to better optimize the magnetostrictive properties of
such alloys with respect to potential applications. One of the
main differences between the magnetic properties of amor
phous R T
2
alloys and their crystalline counterparts is the
sperimagnetic distribution of R and Fe moments in the amor
phous case
, ' 2
This sperimagnetic structure arises from the
competition between exchange interactions and random local
anisotropy and leads to the formation of domains of corre
lated moments. These domains are oriented more or less at
random in zero field and ihc macroscopic anisotropy energy,
which determines the coercive field, is an average of the
random local anisotropy over the volume of each domain
.10
At low temperature, these domains are small and this ex
plains the large coercivc fields found in these alloys. The
sperimagnetic cone, within which the Tb and Dy moments
lie, can be somewhat closed due to an increase in the mo
lecular field of the T sublaltice acting on them and this could

account for the maximum seen in M s and the minimum in
Afhf f° r * = 0.47. At room temperature, however, this en
hancement of the 1 sublattice moment is less clear. The mag-
w.
Due et si.
FIG. 7. Calculated variation of
(M u)
and
M
from magnetostriction data
as a function of jr.
netostriction is, on the other hand, much more sensitive to
changes in the R-sublattice magnetization and we will now
discuss this effect.
Assuming that the R moments have the same value as in
the crystalline laves phase, we can estimate the magnetostric
tion of a sperimagnelic system with respect to a collinear
femmagnctic one using
^■2= ^ ( < « Z) M ) .
where is the direction cosine for each rare-earth moment
with respect to the field direction and
/ ;£,2
is the intrinsic
magnetoelastic coupling coefficient (i.e., that of the collinear
ferrimagnct). Here, we take bjn7 = 127 MPa, the room-
temperaturc value of b y'7 in isotropic polycrystallinc crystal
line (Th()
27
Dy

0 7 1
)Pe2.70 Assuming a uniform probability dis
tribution of easy axes within a cone, we can deduce the
characteristic sperimagnelic cone angle ( 6). For the films un
der consideration, this gives values of between 48° and 53°,
which are typical of those reported in the literature
.5,21
This
variation in 0 implies that there is a variation in the average
(Tb. Dy) moment as a function of x. Using M (Tb.Dy)
= 7.27/ifl, the room-temperature value in
(Tb
0
,
7
Dy
0 7
,)Fej
. 4
we can deduce (M n ny) = W (TfcDy){n-,), as
a function of .r. and this is plotted in Fig. 7. From the mea
sured magnetization data, we can now deduce as a func
tion of x (Fig. 7), The values thus determined are in good
agreement with those found for AiJ<f in “ pure" o-TbCo
2
and
d-TbFe
2
alloys
6

at room temperature. This clearly indicates
that there is an enhancement in M for the substituted
rr-R(Fe,Co
)2
alloys and a maximum is reached Tor jt = 0.47
where there is sufficient Co to ensure good ferromagnetic
T -T coupling as well as sufficient Fe giving the larger mag
netic moment. We have, of course, neglected the variation in
ordering temperature, and hence, the intrinsic R-moment
value at room temperature associated with such an enhance
ment of the T -T interactions. However, this simple analysis
illustrates the importance of considering the influence of the
spcrimagnetic structure on the magnetostriction and the mag
netic properties of such alloys.
An intriguing aspect in this study is the variation of the
anisotropy state as a function of T composition, before and
after annealing. The comparison of bt to bL is a useful tool
for understanding the role of Co in these alloys [Fig.
6
(b)],
For the Fe-rich alloys before annealing. h ,/hL is large indi
cating a well-defined initial anisotropy. After annealing,
btlbL^ - 2 suggests that the zero-field magnetization State
is isotropic. The as-deposited material is not completely satu
rated at 1.8 T, while after annealing saturation is achieved at
around I T. This leads to the measured increase in (¿>„
— foj ) at 1.8 T after annealing. For the as-deposited Co-rich
alloys, bnlbL * * - I indicates that (he film is isotropic in the
plane. After annealing at 250 °C, this ratio is significantly
increased showing that a well-defined in-plane anisotropy

direction has been induced. Figure
6
(b) shows the variation
of bl>fb1 as a function of Co concentration. It clearly indi
cates that after annealing the easy axis becomes better de
fined with increasing Co content This may be accounted for
as follows. During the annealing process, it is the local in
ternal molecular field that is responsible for the reorientation
of the R moments. The external field merely saturates the
material in a given direction. For the Fe-rich alloys, the
sperimagnelic nature of the Fe-sublattice distribution is con
veyed to the R sublattice and gives no net anisotropy. How
ever. the strongly ferromagnetically coupled Co sublatticc is
well ordered and its molecular field acts to orient the R sub
lattice in one direction, giving rise to the observed uniaxial
anisotropy. The differing anisotropies seen in the as-
deposited stale are more difficult to account for precisely, but
it has often been noted that Fe-based RT compounds have a
different anisotropy state compared to their Co-based coun
terpart.
We can further illustrate this variation in anisotropy by
associating the field dependence of the magnetostriction with
different types of magnetization processes, For a system of
randomly oriented spin and random distribution of domain
walls, the magnetization process takes place in two steps.2’
First, the motion of 180° domain walls leads to a magnetiza
tion of M q without any contribution to magnetostriction. In
the second step, the spins rotate into the direction of the
applied magnetic licld leading to the change of both magne
tization and magnetostriction. For the case M0 — the

relation between magnetostriction and magnetization is
given asis
(3)
For the rotation of magnetization out of the easy axis, the
magnetostriction is related to magnetization as follows
:22
X (//)/X nu, = [W (ff)/W in„ ] 2. (4)
The results of this analysis are presented in Fig.
8
. The
experimental data for the (Tb. Dy)Fe, film are rather well
described by Fq. (3). With increasing Co concentration, the
vs curves shift towards the line described by
Eq. (4). This further confirms that Co substitution is advan
tageous to the creation of a well-defined easy axis in this
system.
Finally, the room-temperature magnetostriction is
strongly influenced by the Curie temperature of the investi
gated alloys. It is worth reporting here that one has found the
Tc value of 440 K for the n-(Tbn
27
Dy
0 7
j)(Fe, - rCo
, ) 2
film
with * = 0.63. Indeed, this Tc value is much higher than that
reported for ii-(Tbr)27Dyn73)Fe2(7'f:=3 70 K. see also, e.g
Ref. 23). The larger 7> is associated also to the stronger
Duc et al. 839

M/M
max
}. 8. Experimental and theoretical relations between normalized magne-
triction and magnetization for amorphous (Tbn jjDyn 71)(Fe, _ .Co,), thin
is.
-FeCo exchange energies. This is one of the reasons why
i room-temperature magnetostriction was enhanced in
lorphous (Tb, Dy)(Fe, Co) films.
CONCLUDING REMARKS
In conclusion, we would like to point out that
ger magnetostrictions are obtained in amorphous
b, Dy)(Fe,Co) films as compared to their parent amor
ous films of either (Tb, Dy)Fe or (Tb, Dy)Co. This has
en explained in terms of an increase in the ferromagnetic
upling strength wiihin the (Fe, Co) sublattice. In addition,
well-defined uniaxial anisotropy can be induced by
ignetic-ficld annealing for Co-rich films.
It is well known that the substitution of Dy for Tb gives
e to the increase of the magnetostriction at low magnetic
Ids, through the reduction of the saturation field However,
is also accompanied by a reduction in the saturation mag-
tostriction. In this study, we have shown that Co substitu-
in, coupled with the effects of annealing, results in an en-
ncement of both the low-field and saturation
agnctostriction. Thus, we can expect a further enhancement
the magnetostriction in these alloys by increasing the Tb
ncentration. Indeed, we have obtained a giant magneto-
■iction or \ y2= 1020X l (T
6
at 1.8 T with \„ = 585X 10

~ 6
0.1 T in amorphous Tb(Feg
5
;Cbo
45) 2 . 12
ACKNOWLEDGMENTS
The authors thank Dr. E. du Tremolet de Lncheisscrric
for helpful discussions. This work was carried out as part of
the E. C. funded "M A GN IF1T " project (Contract No.
BRE2-0536). The work of one of the authors (N. H. D.) is
partly supported by the National University of Hanoi within
Project No. QG.99.08.
1 E. Quandt. J. Alloys Compd. 258. 126 (1997).
3E. Tremolet de Lachcisserise, K. Mackey,
1.
Betz, and J. C. Peuzin, J
AppI Phys. 275-277, 685 (1998).
1N H. Due. in
Handbook on the Physics and Chemistry o f Rare F.anhs,

edited by K. A. Gschneidner, Jr. and L. Eyring (Nortb-Holland, Amster
dam), Vol. 28. (lo be published).
J A E. Clark, in
Ferromagnetic Materials,
edited by E. P. Wohlfarth, Vrl
I (North-Ilolland, Amsterdam, 1980), Vol. I. p 531.
5J. M. D. Coey, D. Givord, A. Lienard, and J. P. Rebouiltal, J. Phys. F II,
2707 (1981).
ftP. Hansen. G. Much, M. Rosenkranz, and K. Witter, J. Phys. 66, 756
(1989).

7K. Lee and N, Heimon, AIP Conf. Proc. 18. 108 (1973).
*R. Lemairc R., Cobalt (Engl. Ed.) 1968. 33.
?j. Betz, Thesis, University Joseph Fourier of Grenoble (1997).
,0J. P. Liu, F. R. de Doer, P. F. de ChStel. R. Coehoom, and K. H. J.
Buschow, J Magn. Magn Mater 134. 159 (1994).
" j. P. Gavigan, IX Givord, II. S, Li. and J. Voiron, Physic» B 149, 345
(1988).
,! N. H. Duc, K. Mackay. J. Betz, and D. Givord, J. AppI, Phys. 79, 973
(1996)
E. Tremolet de Lacheisserise and J. C. Peuzin, J. Magn. Magn. Mater.
136, 189 (1994).
1dJ. Betz, E. du Tremolet de Lachcisserise. and L. T, Baczewski, AppI.
Phys. Lett. 68. 132 (1996).
15 Y. Imry and S Ma. Phys. Rev. Lett. .15, 1399 (1975).
1f B. Boucber, A. Lienard. I, P. Rebouillat, and J. Schweizer, J. Phys. F 9,
1421 (1979)
,7L. Neel, Compte Rendu 273, 1468 (1953): 1. Phys. Radium 15, 225
(1954).
"F. Schatz, M Hirscher. M. Schnell, G Flik, and Ft. Kromuller. J. AppI
Phys. 76. 5380 (1994).
19R. Alben, J. I. Bundrik. and G. S. Cargill,
Metallic Glasses
(American
Society for Metals, Metals Park, OH, 1978), Chap. 12.
jnThe values given in Ref. 4 are for somewhat lex lured samples. Here, we
calculate
b
, ! for an isotropic polycryslalline sample of Tb027Dyn jjFe,.
using single-crystal data. frT,I=3GXs with l/(2G) = 2/5Jr+3/5j, and
= 0.6\ i,i , E. du Tremolet de Lacheisserise (private communication).

31 P. Hansen, in
Ferromagnetic Materials,
edited by K. H, J. Buschow
(North-Holland, Amsterdam, 1991). Vol. 6, p 289
22S. Chikazumi,
Physics o f Magnetism
(Willey, New York, 1964).
” K. Ried, M. Schnell, F. Schatz, M. Hirscher, B. Ludescher, W. Sigle, and
H, Kromuller, Phys. Status Solidi A 167, 195 (1998).
JOURNAL OF APPLIED PHYSICS VOLUME H7. NUMHF.R M)
15 MAY ;<XX)
M agnetic, Mossbauer and magnetostrictive studies of am orphous
Tb(Fe0 55COo.45)i.5 films
T. M. Danh, N. H. Duo,3’ and H. N. Thanh
Crvoyenic La boratory, Fa cu lty o f Physics, N ationa l University of Hano i. 334 N quven Trat. Thanh Xuan.

H a n o i, Vietnam
J. Teillet
Lab ora toire tie M aynetisme et Applications. G M P - U M R 6634 Université de Rouen,
768 21 Mfi’ il-S ainl-Aigna n. France
(Received 13 January 2000: accepted tor publication 19 February 2000)
The Tb(Fe,, jjCoo^sJi 5 films were fabricated by rf magnetron sputtering from a composite target.
Samples were investigated by means of x-ray diffraction, vibrating sample magnetometer,
conversion electron Mossbauer spectra, and magnetostriction measurements. The as-deposited film
is an amorphous alloy with a perpendicular magnetic anisotropy and an intrinsic magnetostriction
\ = I080X IO-,> in an applied field of 0.7 T. In this state, it was determined that the hvperfine field
/?t,r= 23.5 T and the cone-angle between the Fe moment direction and the film-normal direction
/3= 12°. Alter annealing in the temperature range of TA = 250-450 "C the amorphous structure still
remained, however the anisotropy was changed fo a parallel one. The soft magnetostrictive behavior
has also been improved by these heat treatments: the parallel magnetostriction \ B = 465X 10'* was

almost developed in low applied fields of less than 0.1 T and, especially, a huge magnetostrictive
susceptibility = c/\,|/î/(/(a0//) = I.8X 10~: T _I was obtained at /¿n//= ! 5 m T <D 2000
American Institute of Physics. [S0021 -8979(00)06210-1 ]
I. INTRODUCTION
It has been known for a few years that there has been a
growing interest in magnetic thin films with large
magnetostriction.I"* This interest is motivated bv the poten
tial such films show for use in microsystems actuators. For
these applications, large low-field magnetostrictive suscepti
bilities = d\/cl(finH)>2X 10"2T _i. and low coercive
fields 100 mT, are required. R-F e (R=rare earth)
based thin films offer the possibility of developing very large
magnetostriction at room temperature. Numerous investiga
tions on Tb-Fe and (Tb. Dy)-Fe based thin films have been
carried out. In order to get low macroscopic anisotropy, ma
terials have been used in the amorphous state. In Fe-based
amorphous alloys, however, both positive and negative
Fe-Fe exchange interactions exist.5 leading to magnetic frus
tration in the Fe sublattice. in amorphous R -Fe (o-RFe) al
loys, where R is a magnetic rare-earth, the additional contri
butions of R -Fe exchange and local crystalline electric field
interactions lead to the formation of sperimagnetic
structures.5 6 The ordering temperatures are above room tem
perature [7> =4I0K for fi-TbnjiFcoift, Ref. 6 and refer
ences therein). It is. however, still rallier low and is thus
detrimental to large magnetostrictions in such materials at
room temperature f\= s 300x IO- * in — I T). The opti
mization of magnetostriction and ordering temperature have
been reported for TbDyFe/Nb multilayers by combining the
advantages of a crystallized film (high T and giant X) with

' \ u I hi >r I o whom torrevpomletice 'iliould he .ultlrcssed: clccirnmc mail:
iliiCfcrvi'lah .etlu.vn
soft magnetic properties of an amorphous phase.' In these
materials, however, the coercive fields were raised up
< HciHc” 100 mT)
Regarding the large magnetostrictions in the amorphous
state, the equivalent <i-RCo based alloys can be considered.
Although crystalline RCoj compounds order below 300 K as
the Co is merely paramagnetic, the amorphous state stabi
lizes a moment on the Co sublattice due to band narrowing.
These Co moments are strongly ferromagneticallv coupled.
A sperimagnetic structure occurs as in d-RFe alloys but the
ordering temperature is now raised up to 600 K (Ref. 6) for
Tb„jjCo**. In practice, Betz7 has investigated rt-TbTC o ,_ r
films and shown that large magnetostriction of \ = 350
X l()-fi at 300 K was obtained for .t~0.33.
Recently, we have studied the magnetization and mag
netostriction in the amorphous (Tbl _,D yr HFe045Co0 55)2 i
(Ref. 8) and (Tbo 2?Dyo 7i)(F e| _ cCor)2 (Ref. 9) thin films. In
these alloys, the R-FeCo exchange energies are stronger
than those in the "p ure" a-RFe and <i-RCo alloys:*lt was
thought to be ihe reason for the enhancement of the R mo
ment at room temperature and thus the magnetostriction. In
deed. a magnetostriction of I020X 10"'’ was obtained in the
applied field of 2 T for amorphous Tb(Fe,)15Con jj) In
this article, we studied zero-lieid annealing effccts on the
magnetization. Mossbauer spectra, and magnetostriction of
ihe amorphous Tb(Fen,55^ 045)1 j films with a perpendicular
anisotropy The obtained magnetic and magnetostrictive
characters of these annealed films proved to be rather prom

ising for application requirements.
0021 - 0979/2OOO/87( 101/7208/5/Î17 00
7208
'0
2000 American Institute of Physics
J. Appl. Phys., Vol. 87, No. 10. 15 May 2000
Danh el al 7209
2
- theta (degrees)
FIG. I. X-ray diffraction patterns of the TbfFe035Coo4j)15 films.
II. EXPERIMENT
The films were prepared by rf magnetron sputtering. The
typical power during sputtering was 400 W and the Ar pres
sure was 10'* mbar. A composite target consisted of 18 seg
ments of about 20°, of different elements (here Tb, Fe. Co).

The substrates were glass microscope cover slips with a
nominal thickness of 150 ¿im. Both target and sample holder
were water cooled. The thickness was measured mechani
cally using an a step and the sample mass was determined
from the mass difference of the substrates before and after
sputtering. The typical film thickness was 1.2 ¿un without
any coating. The rilm structure was investigated by x-ray
{0-76) diffraction (XRD) with Cu Kn rays. The'results
showed the as-deposited samples to be amorphous (see
Fig. I).
Samples were annealed at temperatures from 250 to
450°C for 1 h in a vacuum of 5 X !0 _4 mbar in order to
relieve any stress induced during the sputtering process. Sub
sequent

0 -20 XRD showed no evidence of a global crystal
lization after annealing, but the peaks of Tb oxides and
a-(Fe, Co) (see also Fig. 1) appear to be due to the surface
oxidation.
The magnetization measurements were carried out using
a vibrating sample magnetometer in a field of up to 1.3 T at
300 K.
The conversion electron Mossbaucr specira (CEMS) at
room temperature was recorded using a conventional spec
trometer equipped with a homemade hclium-methane pro
portional counter. The source was a ,7Co in rhodium matrix.
The (ilms were set perpendicular to the incident y beam. The
spectra were titled with a leasl-squares technique using a
histogram method relative to discrete distributions, con
straining the linewidths of each elementary spectrum to he
the same. Isomer shifts are given relatively to a-Fe at 300 K.
The average "cone angle" (3 between the incident y-rnv di
rection (being, m the film-normal direction I ant) lhat of the
hypertine Held (or the Fe magnetic moment direction) is
estimated from the Iine-intensity ratios 3 :.v :l:l:r:3 o| the
six Mossbaucr lines, where r is related to 0 by sin /J
= lv/i4 + vl
m ait. <n
nJi.cn
ujtwin
in
FIG,
2.
Magnetic hvsieresis loops in the imemal maenelic fields al 300 K
for TWFen,,Coi>j5), s litms: ial the as-dcpowed films, (bl aller annealine .n

250°C, (cl 150'C. and Id) 450 "C.
The magnetostriction was measured using an optical de-
(lectometer Iresolution of 5 * 10 *’ rad), in which the bend
ing of the substrate due to the magnetostriction in the film
was measured
.'1" 11
III. EXPERIMENTAL RESULTS AND DISCUSSION
A. M agnetization
Figure 2 presents the magnetic hysteresis loops mea
sured with applied magnetic field in the film-plane and film
normal directions for the as-deposited and several annealed
Tb(Fen
33
Coo
45
)i
5
films. The magnetization curves have
been plotted versus the internal field = /xnW.„
-N M ) using a usual demagnetization factor N = N■, = 0 in
the film-plane direction but an experimentally determined
value of N = N l in the film-normal direction. The value ol
N l was chosen in a way that the steepest part of the magne
tization curve is transformed into a vertical line
. 12
The result
ing effective values of N, are equal to 0.5. 0,9, 0.95. and 1.0
for the as-deposited film, film annealed at r
1
= 250°C.

350 'C. and 4 5 0 ^ . respectively. Comparing to /V '*= l as
expected for an infinite plate, the obtained value for the as-
deposited film is too small. This is, however, characteristK
of the nucleation of tripe domains
.11
For all samples, the in-plane magnetization is almost iso
tropic. The as-deposited sample shows a perpendicular tnac-
netic anisotropy [Fig. 2(a)]. Its coercive Held is rather larse
(film-normal coercivity ^ oWC i= 0.1 32 T and film-plane co-
ercivity Hi\Ht — 0.08 T) and the magnetization docs not
completely saturate even at 1.3 T. While, intrinsically related
to the strong local anisotropy of the R atoms and iheir ran
dom distribution ot easy axes present in such sperimaunetk
systems, the coerciviiv is strongly affected by internal stress
microstructure, and homosteneitv
. 14
The high-tield susccptt-
'210 J. Appl. Phys Vol. 87. No. 10. 15 May 2000
Danh el ai
TABLE I, Room temperature ma|n<lic and mngnetostriclive chnncteriMiCfl
>f ihe amorphous Tb(Fe^ jjConjjJi j films:
T A
,
M
<.
n „ H a , (B u). 0.
and
*■ A, — K L) ore Ihe annealing temperature. saturation magnetization, film-
ilane coercive Held, avenge hypertine field. Fc-spin oriented angle, and
ntrinsic magnetostriction, respectively.

r^tKi
M
j (kA/ml
,u„«n (mT)
<oM)m
0
tdeg) \( n r ‘)
js-de posed
320 SO
22.5±l),3
I2±5 1080
250 268 25
— —
soo
350
250
10
— —
820
450
240
6
—
—
930
lility is also typical of sperimagnetic systems and is assor
ted with the clcsing of the cone distribution of R moments
s the field is increased.*
After annealing, there are a number of clear differences
n the magnetization process. First, the magnetic anisotropy

hanges from a perpendicular to a parallel one [see Figs. 2(c)
nd 2(d)], Second, the coercive field is strongly reduced
Table I) for instance, with the annealing at 450 °C. Mo^ch's
qual to
6
mT. Third, the saturation magnetization decreases
see also Table I) but can be easily reached at low magnetic
¡elds. In agreement with the XRD results, the reduction of
le magnetization may relate to the process of oxidation dur-
lg vacuum heat treatment (see Sec. Ill B). This effect was
reviously reported by Wada et ai.'! Finally, the annealing
Iso causes a reduction in the high-tield susceptibility, indi-
ating that ihe cone distribution of the Tb moments is easy to
lose.
From the magnetization curves measured with applied
lagnetic field in the film-plane and film-normal directions.
'C estimated the uniaxial anisotropy constant K„ to be'117
_I/m
3
for the as-deposited film. This value is comparable
'ith that reported in literature {e.g Refs. 12 and 14). Re-
arding the magnetoelastic anisotropy, any magnetostrictive
laterial always tries to compensate the external or internal
ress by appropriate rotation of spins. For a film with posi-
ve magnetostriction, tensile stress leads to a spin orientation
i the film plane, whereas for compressive stress the spins
rient along the film normal. At present, since the thermal
tpansion coefficients of the Tb-FeCo film and the glass
ibstrate would result in an in-plane anisotropy, it is possible
tat the observed perpendicular anisotropy must be of intrin-

c origin, associating with the structural anisotropy induced
jring Ihe sputtering process. This was already confirmed
rectly in the most careful studies of the local structures
. 1* 17
^ he elimination of the coercivity and anisotropy with anneal-
g reflects that an isotropic amorphous structure has lower
lergy than the as-deposited anisotropy state. The relaxation
the anisotropy without crystallization, thus, is a simple
laxation of the amorphous structure resulting in a more
able and homogenous film structure.
. M ossb auer sp ec tra
The CEMS is suitable to investigate hvperfine param-
srs of the iron nuclei within a depth range of about
2 0 0
nm
:>m the film surface. Within this space, however, ihe con-
button of the iron nuclci to the CEM spectrum is not the
Velocity ftnm/4)
m a U A
s
<-» . ■
/V m J\
I ' 0
1 ■
n
. , - r \ . . . i t .
to M SO
*0
,
Ml

ib) ■ 1 j
£ 20
<b) »
Á \ü J m
£. 10
. • \
'■fír ' , ■'
0
*
10 20 JO 40
- *
ttwr (T)
FIG 3 Mossbauer spectra and hyperfine-field distributions of
Tb(Fe^5!Con«)i j films: fa) Ihe as-depnsiled film, (bl after annealing at
450 “C
same, but is strongly reduced with increasing depth Figure 3
presents the CEM spectra for Ihe as-deposited and the
450 °C-annealed films. Although ihe statistics are not as
good, the information about the average hvperfine field
((B M)) and the Fe spin reorientation (/? angle) can be ex
tracted from these spectra. The perpendicular anisotropy of
the as-deposited film is characterized by the almost disap
pearing second and fifth Mossbauer lines [Fig. 3(a)). For ihis
sample, the spectrum has been fitted with a wide contribution
of hyperfine field P ( /?hr) to take into account all the environ
ments experienced by Fe
’ 7
nuclei. This provides an average
value of (B m}~ 23.5 T and (p ) = \2° It is worth mentioning
here that beside the peak at 22.5 T. which corresponds to the

magnetostrictive Tb(Fe.Co>|
5
alloy, the P {B hf) distribution
extends also to higher hvperfine fields (“"30 T). The spec
trum of the 450'’C-annealed lilm [Fig. 3(b)] is fitted with a
wide distribution of hyperfine field P(B h() too. For this
sample, the peak at 22.5 T still exists in the P (B hf) curve,
however, it is weakened and broadened. Moreover, the high
hyperfine-field contribution becomes dominant. A sharp
P (B i,r) peak is reached at 34.5 T. In accordance with the
XRD results, this major ferromagnetic component (82% of
the total spectrum area) is associated with the contribution of
the crystallized o'-(Fe.Co) phase formed at die film surface
due to the oxidation. The fraction of the magnetostrictive
alloy (18% of the total spectrum area) is small. As already
mentioned in the beginning of this subsection, this reflects
that the thickness of the oxidation layer is sufficiently thick
in annealed films.
The (Bi,,) values obtained for the ri-Tb(Fe
0 33
Co
0
45)|
5
phase are comparable with those reported for the Laves
phase RFe compounds. Such a value implies a strong
} d - }U exchange coupling. The 3</ magnetic moment ,W1(/
is determined by scaling with (B hf). taking (#*M) =
3 3
T and

W 1</ = 2.2,uH/atotns for »-Fe. It results in Myd- 1
atoms. This finding is in good agreement with that deduced
from magnetization data lor«-(Th. Dv)(Fe.Co): films ’ This
large room-tcmpernutrc 3r/ magnetic moment indicates that
in the composition under consideration there was sufficient
Co to ensure good lermmagnelic T - P coupling as well as
sufficient Fe giving the large magnetic moment
*ppl. Phys Vol. 87. No. 10, 15 May 2000
Danh el al. 7211
-0.8 -0.4 0 0.4 0.«
ti,H (T)
!. 4. Parallel magnetostrictive hysteresis loops in (he external fields for
Tb(Fe<)jjCo04j)| j films: (I) as-deposited film and (2) after annealing at
“C and (3) 450 °C.
IW M ,„
FIG. 5. Experimental and theoretical relations between normalized muene-
tostriction and magnetization for amorphous Tb<Fe«|jjCoo4j)| j films. (I)
and f2) theoretical curves described for Eqs. (I) and (2). respectively. (■) as
deposed. (• ) r, = 250°C. fAl 350 ”C. and fO) 450'C.
M agnetostriction
We measured two coefficients and , which coiTe-
)nd to the applied field, in the film plane being, respec-
ely, parallel and perpendicular to Ihe sample length. For
: films under investigation, magnetostriction is almost iso
pic in the plane V>i / X ^ — 1. The intrinsic magneiostric-
n data \ = . measured in the applied magnetic field
fjLnff = 0.7T. are listed in Table I. It is clearly seen that the
ignetostriclion of a magnitude of 10
“ 3
was achieved. The

rallel magnetostrictive hysteresis loops are shown in Fig.
For the as-deposited sample, the magnetostriction in-
:ases almost linearly in Ihe investigated magnetic fijld
iges. This implies that it is rather difficult to rotate spins
o the film plane. The largest magnetostriction obtained at
* T is \i| = 550x I0~ 6. The annealing at temperatures be-
een 7^ = 250 and 450°C reduces the high-field magneto-
iction but enhances the low-field magnetostriction. The
timum annealing is at 7’A=450°C . In this case, the mag-
tostriction of Xi = 465X 10~s is saturated at ¿¿
0
7/ = 0 .IT
d \ , = 340X 10
- 6
is already developed in very low applied
ignetic fields of 20 mT. In addition, its coercive field is
;s than 5 mT. It is worthwhile to mention that in the ap
ed field of 15 mT. ihe magnelostrictive susceptibility has
iched its maximum value
1-8
10
” 2
T -1. These magnetic and magnetostrictive character-
ics are very promising for microsystem applications.
^ We can further associate the field dependence of the
ignetostriction with different types of magnetization pro-
sses. For a system of randomly oriented spin and random
itribulion of the domain walls, the magnetization process
ces place in two steps
, 111

First, the motion of 180” domain
ills leads to a magnetization of M n without any comribu-
m to magnetostriction. In the second step, the spins rotate
ihe direction of the applied magnetic field, leading to the
ange of both magnetization and magnetostriction. For
lorphous alloys of randomly oriented spins and ol a ran-
m distribution of domain walls, the M „ is expected lo be
M mJ 2. In this case, ihe relation between macnetostric-
>n and magnetization is given as
= I |1,3 II)
For the motion of 90° domain walls, i.e the rotation of
magnetization oul of the easy axis, the magnetostriction is
related to magnetization as follows:IS
= (2)
Taking the values measured in /_in// = 0.7T as and
the relation between the normalized magnetostriction
and magnetization is presented in Fig. 5 For the as-deposited
film with the perpendicular anisotropy, almost no magneto
striction takes place at M /M mix< 0.3 and the experimental
data are close to the curve described by Eq.
1
2) II is possible
that, in this film, the magnetization process is governed
mainly by the rotation of spins. As the annealing temperature
increases, the \ /X m„ starts at a higher M IM m,t value, lor
instance /V//Aimlx=0.6 and 0.75 for 7"., = 250 and 350 “C.
respectively. This further confirms the randomly oriented
spin structure. At 7\t = 450°C, the vs M/M mtx curve
also starts at A//A fm„= 0.6 , and follows well that of 250°C.
IV. CONCLUDING REMARKS

It is well known that, in a traditional route, the substitu
tion of Dy for Tb has been used to compensate the anisot
ropy and Ihen to increase the magnetostriction at low mag
netic fields. However, it is also accompanied by a reduction
in the saturation magnetostriction At present, we have
shown that Co substitution, coupled with the effecty&f zero-
field annealing, results in an enhancement of both the low-
field and the saturation magnetostriction. Actually, we have
obtained a giant magnetostriction of \„ =3 4 0X 10~IS in field
fj.nH < 2 0 mT and '
8
x 10" ~ T _ in amorphous
Tb(Fe
0 5
,Co
0
jj), s. Such films are very promising for appli
cations.
In this study, the Co substitution led to the enhancement
ol the 3i/(Fe.Co)-3r/(Fe.Cot as well as 4 /(T b)-3 </(Fe.C oi
exchange energies and then the intrinsic magnelosiriution.
The soli magnetic and magnetostrictive behaviors, on one
hand, relate to the closing of the Tb-cone anale. but on ihe
oiher. may also be associated with the soft magnetic
ar-iFc. Co) layer, which was scurecaicd at the surface durinu
7212 J. Appl. Phys Vo). 87, No. 10, 15 May 2000
Oanh et al.
ihe heal treatments. The study of the influence of the soft
magnetic layer on the soft magnetostrictive character is in
progress.

ACKNOWLEDGMENTS
This work was granted by the National University of
Hanoi within Project No. QG.99.08. The authors express
their thanks to Dr. Le Van Vu for the x-ray diffraction mea
surements. The .stay of N. H. Due at the GMP, University
of Rouen is supported by the Ministère Française
de l’Education Nationale, de la Recherche et la Technologie.
' E. Quandt. J. Alloys CompA 258, I26 (I997).
!E. Trémolet rte Lacheisserise. K. Mackey, J. Betz. and J. C, Peu/in. I.
Alloys Compd 275-277, 685 (I998),
3S. F. Fischer. M. Kelsch. and H. Kronmüller. J. Maen. Mapn. Mater. 195.
545 (1999).
*N. H. Duc. in Handbook on ihe Phvsics and Chemixtrv nf Rare Earths.
edited by K. A. Gschneidner. Jr. and L. Eyring (North-Holland. Amster
dam. to be published). Vol. 28.
!J. M. D. Coey, D. Givord. A. Liénard. and J. P. Rebouillat. J. Phys. P:
Met. Phys. II. 2707 (1981).
6P. Hansen. G. Much. M. Rosenkranz, and K. Witter. J Phys 66. 756
(I9H9): P. Hansen, in Fermmaqnefic Materials, edited hy K. H. J. Bus-
chow (North-Holland. Amsterdam. 1991). Vol. 6. p. 289.
J. Bet7. thesis. Universrty Joseph Fourier nf Grenoble. 1997.
HN. H. Duc. K. Mackav. J. Betz. and D. Givord. J. Appl. Phvs. 79. 973
f 1996).
'N. H. Duc, K. Mackay. J. Betz. ;md D. Givord. J. Appl. Phys. 87. 834
(2000).
lf)E. Tremolet de Lacheissense and J. C. Pcuzm. J, Magn. Maen Mater.
136. 189 (1994).
NJ. Det7. E. du Tremolet de Lacheisserise. and L. T. Baczewski, Appl
Phys. Leit. 68, 132 fl 996).
L K. Ried. M. Schnell. F. Schatz, M. Hirscher. B Ludescher. W Sigle, and

H. Kronmüller. Phys. Status Solidi A 167. 195 (1998)
13 A. Forkl. M. Hirscher. T. Mizoguchi. H. Kronmüller. and II. U. Haber
meier. J. Magn. Maen. Mater. 93, 261 (1991).
"F. Hellman. M. Messer, and E. N. Aborra. J. Appl. Phys. 86. 1047 (1999).
I5M Wada. II Uchida. and II Kaneko. J Alloys Compd 258. 169 (1997)
lftA. Hemando, C. Prado*. and C. Prieto. J. Maßn. Maen. Mater. 157/158.
501 (19961
17 V. G. Harris. K. D. Avlesworth. B. N. Das, W. T. Ela m. and N. C Koon.
Phys. Rev Leu 68 1939 f 1992)
HS. Chikaztimi. Phvsics nf Maqneiixm (Wi)ey. New York. 1964).
19 F. Schatr. M. Hirscher. M. Schnell. G Flik, and H. Kronmüller. J. Appl.
Phys. 76. 5380 (1994)
*
J. Phys.: Condens. Matter 12 (2000) 8283-8293. Printed in the UK
PII: S0953-8984(00) 15123-9
Structural, magnetic, Mossbauer and magnetostrictive studies
of amorphous Tb(Feo.
5
sCoo.
4 5
)i
.5
films
N H DuctU, T M Danhf, H N Thanhf, J Teilleti and A Liénard§
t Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai,
Thanh Xuan, Hanoi, Vietnam
t Laboratoire de Magnétisme et Applications, GMP-UMR 6634, Université de Rouen,
76821 Mont-Saint-Aignan, France
§ Laboratoire de Magnétisme Louis Néel, CNRS, 38042 Grenoble Cedex 9, France
E-mail: duc«cryolab.edu.vn

Received 3 July 2000
Abstract. Films with a nominal composition of TbfFeo.ssCoo
45
)
1.5
were fabricated by rf-
magnetron sputtering from a fixed target configuration at various Ar gas pressures. Samples
were investigated by means of x-ray diffraction (XRD), scanning electron microscopy (SEM),
vibrating sample magnetometer (VSM), conversion electron Mossbauer spectra (CEMS) and
magnetostriction measurements. As the Ar pressure increases, the Tb and Fe content increases
slightly, whereas the Co content decreases. In addition, a small amount of Ar is introduced into
the films. The as-deposited films are amorphous alloys, but their magnetic behaviour depends
strongly on the deposition conditions: a perpendicular magnetic anisotropy is obtained only in film
deposited at lowest Ar pressure and a parallel magnetic anisotropy exhibits in remaining films.
These samples show an intrinsic magnetostriction (A = I0 - ^) in an applied field of 0.7 T. In this
state, it was determined that the hyperfine field reaches the value fl*/ = 24.5 T. Effects of the heat
treatment on the magnetostrictive softness are also reported. The parallel magnetostriction with a
huge magnetostrictive susceptibility (xk = 1.8 x I0
~ 2
T~') obtained at ( / i o / / = 10 mT) makes
these materials useful for applications.
1. Introduction
Giant magnetostrictive materials in thin film form are very useful for microactuator
devices [1-4]. For these applications, large low-field magnetostrictive susceptibilities,
(Xx = dX||/d(//0tf ) > 2 x 10~2 T~i), and low coercive fields (/¿o#c 100 mT), are
required f5]. Research on these materials, thus, concentrates on reducing the necessary driving
magnetic fields. Different approaches have been taken based on amorphous (Tb, Dy)(Fe, Co)2
asperimagnets [6,7], In these alloys, the R-FeCo exchange energies are stronger than
those in the ‘pure’ a-RFe and a-RCo alloys. This was thought to be the reason for the
closing of the cone type distribution of (Tb, Dy) moments, then the enhancement of the R

moment at room temperature and thus the magnetostriction. Recently, we have studied the
magnetization and magnetostriction in the amorphous (Tb, Dy)(Fe, Co)2.i thin films [6,7].
Indeed, a magnetostriction of 1020 x 10-6 was obtained in the applied field of 2 T for amorphous
Tb(Fe04;iCoo.55)2.i [6]. The optimization of magnetostriction and ordering temperature has
been reported forTbDyFe/Nb multilayers by combining the advantages of a crystallized film
II Corresponding author.
095.1-R9R4/OO/388283+11S30.00 © 2000 IOP Publishing Ltd
8283
8284
N H Due el al
Dispersive X-ray energy (keV)
Figure I. Dispersive x-ray energy for the Tb(Fe()
55
Con.
45
)i
5
film. The peak of Ar is obviously
recognizcd.
(high Tc and giant A.) with soft magnetic properties of an amorphous phase [5]. In these
materials, however, the coercive fields were raised (noHe % 100 mT). In principle, it is well
known that the richer the rare-earth compound, the larger the spontaneous magnetostriction
is. Thus, another optimization of room-temperature magnetostriction can also be obtained
by combining the rare-earth composition and their ordering temperature. In view of this,
the crystallized R-Fe alloys with the greatest potential to achieve large room temperature
magnetostrictions are the cubic Laves phase RFe2 compounds [81, whereas the amorphous
ones are the RFei.s alloys [9, 10|. In addition, it is also expected that similar effects of the
substitution of Fe by Co on the enhancement of the magnetostriction found for 1:2 thin films
would also be observed for this 1: 1.5 amorphous phase [6, I 11.
In this paper, we studied the structure, magnetization, Mossbauer spectra and

magnetostriction of the amorphous lilms with a nominal composition of Tb(Fe<>.55Coo 45)i >;
fabricated in different Ar gas pressures. Annealing effects make these films magnetically soft
and. thus, useful for applications.
2. Experiment
The films with a nominal composition of Tb(Fco 55C00 4s)t.s were fabricated by rf-magnetron
sputtering from a fixed target configuration at various Ar gas pressures. The typical power
during sputtering was 400 W and the Ar gas pressure was around 10-2 mbar. A composite
target consisted of 18 segments of about 20 degrees, of different elements (here Tb, Fe, Co).
The substrates were glass microscope cover-slips with a nominal thickness of 150 /tm. Both
target and sample holder were water cooled. The thickness was measured mechanically using
an »-step. The film thickness was ranging from 0.5 to 1.2 /<m without any coating. The film
composition was determined by energy-dispersive x-ray (EDX) (figure I) and microstructure
investigation (figure 2) was performed using a scanning electron microscope. The film structure
was investigated by x-ray 9-29 diffraction (XRD) with Cu Key rays. The results showed the
as-deposited samples to be amorphous (figure 3).
Samples were annealed at temperatures from 250 to 450 C for I hour in a vacuum of
5 x I0 -4 mbar in order to relieve any stress induced during the sputtering process. Subsequent
Tbị Fe 0.55 Cf>0.45 h .5 films
8285
(h)
Figure 2. Micrograph for films A: (¡1) as-deposilcd and (h) annealed al 350 c.
8286 NHDucelal
2- theta (degrees)
Figure 3. X-ray diffraction patterns of film A.
x-ray $-20 diffraction showed no evidence for a global crystallization after annealing, but the
peaks of Tb oxides and «-(Fe, Co) appear due to the surface oxidation, see also figure 3.
The magnetization measurements were carried out using a vibrating sample magnetometer
(V SM ) in a field of up to 1.3 T at 300 K.
The conversion electron Mossbauer spectra (CEMSs) at room temperature were recorded
using a conventional spectrometer equipped with a home-made helium-methane proportional

counter. The source was 57Co in a rhodium matrix. The films were set perpendicular to the
incident y-beam. The spectra were fitted with a least-squares technique using a histogram
method relative to discrete distributions, constraining the line-widths of each elementary
spectrum to be the same. Isomer shifts are given relative to or-Fe at 300 K. The average
‘cone-angle’ ^ between the incident y-xay direction (being the direction normal to the film)
and that of the hyperfine field Bhf (or the Fe magnetic moment direction) is estimated from
the line-intensity ratios 3:jc: 1:1 :x:3 of the six Mossbauer lines, where x is related to fi by
sin2 p — 2jc/(4 + .*■)•
The magnetostriction was measured using an optical deflectometer (resolution of
5 x 10~6 rad), in which the bending of the substrate due to the magnetostriction in the film
was measured [12,13].
3. Experimental results and discussion
Four samples, named A, B, C and D, were deposited from a fixed target configuration, but with
different Ar gas pressures (ranging from 5 to 15 mbar, see table 1). The resulting thickness,
composition and Ar contamination are summered in table 1. It can be seen from this table
that as the Ar gas pressure increases, the deposition rate and the Co content decrease, whereas
only a weak increase is observed for the Tb and Fe content. These composition variations can
be understood by the difference in the scattering of sputtered Tb, Fe and Co particles [14].
Table 1. Sample characterization of Tb(Feo 55Con 45)1 s lilms.
Tb(FeotfCoo45)\5 films 8287
Samples
Thickness
{/im)
Ar gas
pressure
(mbar)
Deposition
rate
(nm min“ 1)
Tb

content
(%)
Fe
content
<%)
Co
content
(%)
Ar
content
( * )
A
1.15
5
3.5
40.4 32.0
27.0 0.6
B
0.7
8 3.1 40.6 32.2 26.4
0.8
C 0.5
10 2.7
40.8 32.6 25.6 1.0
D
0.6
15 2.4
41.2 32.6 25.0
1.2
Moreover, as indicated by the EDX results (figure 1), a small amount of Ar atoms is introduced

into the films. This Ar content increases with increasing Ai gas pressure.
The magnetic hysteresis loops measured with applied magnetic field in the film-plane and
film-normal directions are presented in figures 4 and 5 for the films A and D, respectively. It is
clearly seen from these figures that magnetic properties o f these films are rather different. The
as-deposited film A exhibits a perpendicular anisotropy, whereas the films B, C and D show
an in-plane anisotropy (see, e.g. figures 4(a) and 5(a)). Here, corrections with regard to the
demagnetizing fields are not made, so the loops shown in the figures are more inclined than
the ‘true’ ones. In [14], it was reported that the direction of (he anisotropy depends mainly
on the film com position, but it is nearly independent of the Ar gas pressure. In addition, the
anisotropy change (from perpendicular to in-plane) can also be connected to the effects o f
sputtering condition [2 ,9, 10]. At present, this argument seem s to be the case because the
Tb com position was observed to vary very little. As regards the magnetoelastic anisotropy,
any m agnetostrictive material always tries to compensate the external or internal stress by
appropriate rotation o f spins. For a film with positive magnetostriction, tensile stress leads to
a spin orientation in the film plane whereas for compressive stress the spins orient along the
film normal. The observed anisotropy change may imply that the sign and the magnitude o f
the film 's stress were changed by the fabrication conditions.
The demagnetization process and the coercive field are also different in the as-deposited
films under consideration. For the films C and D, the demagnetization shows a two-step-like
process and a rather large coercivity (noHc = 0.23 T for film C and 0.34 T for film D).
For film A, however, the two-step-like demagnetization disappears and iiqHc is reduced (e.g.
the film-normal coercivity, /x0H a. — 0.132 T and film-plane /xo^cn — 0.08 T). While,
intrinsically, related to the strong local anisotropy o f the R atoms and their random distribution
o f easy axes present in such sperimagnetic system s, the demagnetization process and coercivity
are strongly affected by internal stress, microstructure and hom ogeneity [ 14]. At present, the
origin of the tw o-step-like demagnetization is still not clarified.
The CEMS is suitable to investigate hyperfine parameters of the iron nuclei within a depth
range o f about 200 nm from the film surface. Figure 6 presents the CRM spectra for the
as-deposited films A, C and D. The spectra are typical o f a distribution of iron environments
and the slight asymmetry could be taken into account by a correlation between the isomer shift

and the hyperfine field. However, due to the poor statistics, the spectra were fitted only with
a distribution of hyperfine fields. The fine structure of the fitted spectra is probably due to
least-squares fitting problems related to the poor statistics. Despite this less good statistics,
the information about the average hyperfine field ((/?/,/)) and the Fe-spin reorientation {(ft)
angle) can be extracted from these spectra. The perpendicular anisotropy o f the film A is
characterized by the near disappearance o f the second and fifth Mossbauer lines (figure 6(a)).
For the films B and C. the in-plane anisotropy is evidenced by the enhancement of the second
and fifth M ossbauer lines with rpcn^rt to the remaining lines (figures 6(b) and (c)). The fit by
8288
N H Duc et al
H-HCD h „H(T)
H„H m |i-H (T )
Figure 4. Magnetic hysteresis loops in the internal magnetic fields al 300 K for film A: (a) the
as-deposited films, (b) after annealing at 250"C. (c) 350"C and (d) 450 C.
(he distribution of hyperfine field provides an average value of (#/,/) = 23.5 T and
(P) = 12 degrees for film A and of (Bi,f) = 24.5 T and (fi) = 76 degrees for films D and
C. The {But) values obtained for these amorphous Tb(Fen ssCon45)1 s phases are somewhat
larger than that of 21 T reported for the amorphous TbFe2 alloy [17] Such a result implies
stronger 3d—3d exchange interactions. The 3d magnetic moment (M^j) is determined by
scaling with (#/,/), taking (/?/,/) = 33.4 T and Afv — 2.2 /</?/al for a-Fe. This results in
^ 1 . 6 fig/at. This finding is iivgood agreement with that deduced from magnetization data
for a-(Tb, Dy)(Fe, Co)2 Rims [7]. This large room-temperature 3d magnetic moment indicates
that in the composition under consideration, although the Tb composition is rich, there was
sufficient Co to ensure good ferromagnetic T-T coupling as well as sufficient Fe giving the
large 3d magnetic moment.
The heat treatment causes a number of clear differences in the magnetization process.
Firstly, the magnetic anisotropy changes from perpendicular to parallel (see figures 4(a)—(d)).
Secondly, the coercive field is strongly reduced: for instance, with the annealing at
Ta = 450 °C, /x0 Wen is equal to 6 mT. Thirdly, the saturation magnetization decreases, but can
be easily reached at low magnetic fields. In agreement with (he XRD results, the reduction of

the magnetization may relate to the process of oxidation during vacuum heat treatment. This
effect was previously reported by Wada el al [181. The annealing effects on the improvement
of the magnetic softness are considered to be the best for the film D annealed at TA = 350 "C:
a typical hysteresis loop with Ho He of below 5 mT and saturating at 25 mT, see figure 5(c).
The elimination of the coercivity and anisotropy with annealing rcfiects that an isotropic-
amorphous structure has lower energy than the as-deposited anisotropy stale. The relaxation
of the anisotropy without crystallization, thus, is a simple relaxation of the amorphous structure
resulting in a more stable and homogenous film structure, see also the micrograph in figure 2 (b).
Tb(Fe0 5sCoo45)\.5 films 8289
-1.5 -I -0.5 0 0.5 1 1.5
H .H (T)
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
HoH (T)
Figure 5. Magnetic hysteresis loops in (he internal magnetic fields at 300 K Tor film D: (a) the
as-deposited films, (b) after annealing at 250 "C and (c) 350 X .
The CEM S spectra was also recorded and Titled with a wide distribution of hyperfine
fields for the film A annealed at T\ = 450 rC. For this sample, the peak at 23.5 T, which
corresponds to the magnetostrictive T b-FeC o phase still exists in the P{l1i,r) curve; however,
it was weakened and broadened. Moreover, the high-hyperfine-field contribution becom es
dominant. A sharp peak is reached at 34.5 T. In accordance with the XRD results,
this major ferromagnetic component (82% of the total spectrum area) is associated with the
contribution of the crystallized or-(Fe, Co) phase formed at the film surface due to the oxidation.