DSpace at VNU: Nanostructure and magnetization reversal process in TbFeCo Y-x(FeCo)(1-x) spring-magnet type multilayers

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Journal of Magnetism and Magnetic Materials 282 (2004) 44–48

Nanostructure and magnetization reversal process in
TbFeCo/Yx(FeCo)1Àx spring-magnet type multilayers
N.H. Duca,*, D.T. Huong Gianga, N. Chaub
b

a
Cryogenic Laboratory, Faculty of Physics, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
Center for Materials Science, Faculty of Physics, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

Available online 28 April 2004

Abstract
Studies of the naturally formed nanostructure and magnetization reversal process were performed for the sputtered
Tb(Fe0.55Co0.45)1.5/Yx(Fe0.7Co0.3)1Àx multilayers (0pxp0.2) with a TbFeCo layer thickness tTbFeCo=12 nm and
YFeCo layer thickness tYFeCo=10 nm. The structural investigations showed that nanocrystals are naturally formed and
coexist within the amorphous matrix in Y0.1(FeCo)0.9 layers. In this state, low magnetic coercivity and large parallel
magnetostrictive susceptibility are observed. The results are discussed in terms of the crystalline discontinuity of the soft
YFeCo layers.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.60.Jk; 75.70.Cn; 81.07.Bc
Keywords: Spring-exchange multilayers; Nanocrystalline structure; Magnetization reversal; Giant magnetostriction

1. Introduction
The exchange-spring concept [1] opened an
alternative route towards new high-performance
hard magnetic materials. By associating a coercive
hard magnetic phase with a large magnetization

soft phase, it was expected that new high-energy
product materials could be prepared. Exchangespring behavior was found in various systems.
However, as far as our knowledge, no material has
been found with properties clearly superior to
those of usual hard magnetic materials. Mean*Corresponding author. Tel.: +84-8-7680978; fax: +84-48340724.
E-mail address: (N.H. Duc).

while, this concept has successfully been applied to
the so-called giant magnetostrictive spring-exchange multilayers, where high magnetostrictive
layers and soft magnetic layers alternate [2].
Indeed, Quandt and Ludwig [3] obtained a
magnetostriction as large as 890 Â 10À6 and a
huge parallel magnetostrictive susceptibility
(wlJ=dlJ/dH) of 8 Â 10À2 TÀ1 in an applied field
of about a few 10 mT for magnetostrictive/soft
magnetic TbFeCo/FeCo multilayers. In this case,
magnetization reversal is thought to be nucleated
within the soft layer in a low applied field and
propagates from the soft layers into the magnetostrictive layers [4]. In the TbFeCo/FeCo multilayers, the soft FeCo-layer is continuous (Fig. 1a),
thus the nucleation of reversal occurs at some

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.04.010


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N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48

Fig. 1. Illustration of the magnetostrictive exchange-spring
multilayers with a structural continuous (a) and discontinuous

(b) soft magnetic layer.

defect points on the sample surface and interfaces.
In this context, one expects that the reversal can be
nucleated in an easier way in discontinuous soft
phase, i.e. in layers in which the FeCo nanograins
are embedded within a non-magnetic matrix (Fig.
1b). Practically, an excellent magnetic softness has
recently been reported for Tb(Fe0.55Co0.45)1.5/
Y0.2(Fe0.8Co0.2)0.8 (denotes as Terfecohan/
Y0.2(Fe0.8Co0.2)0.8 multilayers, in which the nanostructure of YFeCo layers was formed from an
amorphous phase by heat treatments [5–6].
As far as our knowledge, the transformation of
the amorphous state in the RCoFe (R = rare
earths and/or light transition metals) layer is
shown to be dependent on the R-concentration
[7]. At a critical R-concentration, the nanostructured RFeCo layer is expected to be naturally
formed in as-deposited multilayers. By this way,
an optimization of the magnetostriction and
magnetostrictive softness can be reached right
after depositing or/and annealing at low temperatures.
In this paper, a direct approach to the natural
nanostructure and large parallel magnetostrictive
susceptibility will be applied for the Terfecohan/
Yx(Fe0.7Co0.3)1Àx multilayers. The results are
discussed in terms of the structural discontinuity
caused by the formation of the nanostructure in
the soft magnetic layers.

2. Experimental

{Terfecohan/Yx(Fe0.7Co0.3)1Àx}n
multilayers
with x=0, 0.1, 0.2, n=50 and the individual layer

45

thicknesses are tTbFeCo=12 nm and tFeCo=10 nm
were fabricated by RF-magnetron sputtering. The
typical power during sputtering was 200 W and the
Ar pressure was 10À2 mbar. Composite targets
were used which consisted of segments of different
elements (here Tb, Y, Fe, Co). The substrates were
glass microscope cover slips with a nominal
thickness of 150 mm. Both target and sample
holders were water-cooled. Samples were annealed
at different temperatures TA=200 C, 300 C,
350 C, 400 C and 450 C for 1 h in a vacuum of
5 Â 10À5 mbar.
The crystal structure of the sample was investigated by X-ray diffraction using the D5005
Siemens with a copper anticathode. The magnetization was measured with a vibrating magnetization magnetometer (VSM) in a magnetic field upto
1.4 T at room temperature. The magnetostriction
was measured by using an optical deflectometer
(resolution of 5 Â 10À6 rad), in which the bending
of the substrate due to the magnetostriction in the
film was determined.

3. Experimental results and discussion
In the x=0 sample, the large X-ray diffraction
intensity at 2y=45 is characteristic of the (1 1 0)
reflection of BCC-Fe (Fig. 2). No other diffraction

peaks are observed indicating that the TbFeCo
layer is amorphous. The intensity of the BCC-Fe
reflection is strongly reduced in the x=0.1 sample.
This is attributed to the formation of BCC-Fe
nanocrystals in the YFeCo layers. The crosssectional HRTEM image shown in Fig. 3 reveals
the coexistence of nanograins (with an average
grain size of about 10 nm) and of an amorphous
phase in the Y0.1(Fe,Co)0.9 layers. This transformation to the nanostructure was associated with
the reduction of the thermodynamic driving force
for the crystallization caused by substitution [8].
Finally, the (1 1 0) reflection almost disappears at
x=0.2 reflecting the fact that the whole layer is
now amorphous. Similar phenomenon was observed for the Terfecohan/YxFe1Àx multilayers [4].
Low-temperature annealing (at TAp350 C) is
usually performed to relieve the stress induced
during the sputtering process. At present, as


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N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48

magnetization. By, increasing x from 0 to 0.1,
the (magnetic) coercivity (MHC) decreases from 4.8
to 3.1 mT then it increases again to 6.2 mT for
x=0.2. In these samples, the YFeCo composition
is ferromagnetic at room temperature [7]. It is that
the FeCo (and/or YFeCo)/TbFeCo coupling imposes in-plane magnetization. The smallest coercivity value found in the sample with x=0.1 may
be attributed to the specific nanostructure of this

sample as observed by X-ray diffraction results at
room temperature. In the TbFeCo/FeCo multilayer, the soft FeCo-layer is continuous. We thus
expect that the nucleation of reversal occurs at
some defect points on the sample surface. In

Fig. 2. X-ray diffraction patterns of as-deposited Terfecohan/
Yx(Fe,Co)1Àx multilayers.

Fig. 4. X-ray diffraction patterns of 350 -annealed Terfecohan/
Yx(Fe,Co)1Àx multilayers.

Fig. 3. Cross-sectional HR-TEM image of the as-deposited
Terfecohan/Y0.1(Fe,Co)0.9 multilayer.

illustrated in Fig. 4, the microstructure of the
350 C-annealed samples with x=0 and 0.1 is
almost the same as that of the corresponding asdeposited ones. However, the modification of the
amorphous state to form BCC-Fe nanostructured
phase is observed in the Y0.2(Fe,Co)0.8 layers.
The magnetic hysteresis loops measured as a
function of the magnetic fields applied in the film
plane are presented in Fig. 5. For all samples, the
observed curves are characteristic of in-plane

Fig. 5. Magnetic hysteresis loops of as-deposited Terfecohan/
Yx(Fe,Co)1Àx multilayers.


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N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48

TbFeCo/Y0.1(FeCo)0.9, the FeCo grains are nanocrystallized and embedded within an amorphous
matrix. Each FeCo nanocrystals are decoupled
from each other. Soft phase reversal can then be
nucleated at any of the nanocrystals, on a defect
position. Statistically, it is expected that nucleation
will be easier than in FeCo-pure system. This
explains qualitatively the observed difference in
coercive field values between the samples with
x=0 and 0.1. For x=0.2, the whole Y0.2(FeCo)0.8
layer becomes continuous in the amorphous state,
then the magnetic coercivity is enhanced again.
Magnetic softness improvement due to stress
releasing effects is clearly provided by the reduction of the magnetic coercivity with the same
factor of 2 in the 350 C-annealed samples with
x=0 and 0.1. This coercivity decreasing factor
increases up to 4 in the x=0.2 sample. Moreover,
it is worthwhile to note that the coercivity in the
350 C-annealed samples with x=0.1 and 0.2 is
almost comparable (i.e. MHC equals to 1.7 and
1.6 mT). In this context, it is possible to argue that,
besides the stress releasing effects, the nanostructure formed in Y0.2(FeCo)0.8 layers due to heat
treatment must be the reason for the low magnetic
coercivity mechanism.
Magnetostriction l (=lJ-l>) data are determined. For all samples, the magnetostriction
obtained is comparable to the value deduced from
the data of the single-layer Terfecohan samples,
e.g. lTbFeCoB10À3 [9–12] using the following
expression [2–3,6]:
/lS ¼

lYFeCo tYFeCo þ lTbFeCo tTbFeCo
:
tYFeCo þ tTbFeCo

Low-field parallel magnetostriction lJ data are
presented in Fig. 6(a) for the as-deposited and
350 C-annealed films with x=0.1. Like in magnetic hysteresis loops, there is a so-called (magnetostrictive) coercive field (lHC), where l=0 in the
magnetostrictive hysteresis loops. Experimentally,
the lHC is observed to be equal to the MHC value
obtained from the magnetization measurements.
In addition, it is in good agreement with that
already reported in Ref. [12] that the magnetostrictive response to applied fields is always
strongest in the magnetizing fields just above the
coercivity. Because the performance of microsys-

47

Fig. 6. Low-field magnetostriction (a) and parallel magnetostrictive susceptibility (b) data of the as-deposited and 350 Cannealed x=0.1 multilayer.

tems is determined by the parallel magnetostrictive
response to an applied field, the observed behavior
is an important factor to consider the working
point for the magnetostrictive films in microsystems. In this case, the value of the parallel
magnetostrictive susceptibility wlJ is usually discussed. The low-field dependence of the parallel
magnetostrictive susceptibility is shown in
Fig. 6(b) for the corresponding films. It can be
seen from this figure that the as-deposited x=0.1
sample with the natural nanostructure exhibits
already a wlJ value as large as 3.2 Â 10À2 TÀ1. This

value is almost 5 times higher than that of asdeposited x=0 sample and is comparable with
that of 350 C-annealed x=0 one. The magnetostrictive softness, in particular, is strongly improved
after annealing at 350 C: wlJ reaches a maximal
value of 14.5 Â 10À2 TÀ1 at the magnetic field


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N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48

conjunction with large wlJ. This is because
magnetostriction is intimately associated with
magnetocrystalline anisotropy and it is well known
that the coercivity tends to be high in large
anisotropy systems. The obtained spectacular
result illustrates the significance of the approach,
which we have developed in view of optimizing
both magnetostriction and magnetostrictive susceptibility in the spring-exchange magnet type
multilayers with structurally discontinuous soft
layers.

Acknowledgements
This work was granted by the State Program for
Natural Scientific Researches of Vietnam. The
authors acknowledge discussions with D. Givord.

References

Fig. 7. Low-field magnetostriction (a) and parallel magnetostrictive susceptibility (b) data of the 400 C- annealed x=0.2

multilayer.

1.9 mT. This wlJ value is almost 15 times higher
than that obtained in Terfenol-D [13] and 2 times
higher than that obtained in multilayers by
Quandt et. al. [2,3]. Low-field lJ and wlJ data are
presented in Fig. 7 for the x=0.2 films annealed at
400 C (i.e. for the thermally induced nanostructured film). It turns out that the maximal wlJ value
equals 15.6 Â 10À2 TÀ1. This is in good accordance
with the coercivity data that the excellent magnetic
as well as magnetostrictive softness can be
obtained either in naturally or in thermally formed
nanostructured multilayers.

4. Concluding remarks
For conclusion, it is worthwhile to mention that,
in general, a large l value is not obtained in

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