(Luận án tiến sĩ) ảnh hưởng của các nguyên tố ti, v lên từ tính và tương tác trao đổi trong một số hợp chất đất hiếm sắt

M O T SO KI IIIKU VA
Clio ki

11/

VIETTAT

hieu

R: Nguyen to dat hicm
T: NguyCn to kim loai chuyen tiep
M, A: ki hiCu cho nguyen lo dimg thay the cho kim loai T.
ARR, ART, All: tifong irng la he so luang lac trao doi giua cac spin R, giua spin R va
spin T, giua eac^spin T.
HRR, nRx, \\y\'. lirang irng la he so Irircrng phan it'r cua phan mang dat hiern, giua cac
phan mang va cua phan mang kim loai chuyen tiep.

^

Ml', MR, MJ, M^.,r: lirang ung la mOmen tir phan mang kim loai chuyen tiep, momcn lij
phan mang dctl hiem, momcn Itr 3d, momcn cam irng.
SR, S]-: momcn spin irng \6\ ion R va ion T.
Tc, TK;: tuong ling vcfi nhiet dp Curie, nhiet dp bti trCr.
( X)- : \\t so lijr hoa phan inang T.
at. : ki hieu cho 1 nguyen lir.
hyb. : lai hoa, ve sir lai hoa.
f.u.

: mot don vi cOng Ihire.

Tii: viet tat

KHTN : Khoa hoc Tu nhien
Bp KHCN & moi Irudng : Bp Khoa hoc Cong nghe va moi Iruofng.
TT KHTN & CN Quoc gia: Trung lam Khoa hoe Tir nhien va COng nghe Quoc gia.
HFFP: High Field Free Powder method: phuong phap do m&u bOt rdi du6i lu Irucfng cao
DOS: Density Of States: mM dp Irang thai.
NMR: Nuclear Magnetic Resonance method: phuang phap do c6ng huang lijr hat nhan.
LT: Light Transition Elements: kim loai chuyen liep nhe (kim loai chuyen tiep dau
day)
HT: Heavy Transition Elements: kim loai chuyen tiep nang (kim loai chuyen liep cuoi
day)
XPS: X-ray Photoelectron Spectroscopy metliod: phuang phap pho phal xa quang dien.


MUCLUC
Md din

Trang

Chuong I. Mo men tijf va tucmg tac trao doi trong cac hgp chat

5

lien kim loai dat hiem-kim loai chuyen tiep
1.1 Nhan xet chung
1.2 Mo hinh Campbell w6 cau hinh spin va tuang tac trao doi
4f-3d trong cac hop chat R-T.
1.3 Hien tuang lai hoa va cau hinh spin trong cac hap chat R-T
1.3.1 Cac trang thai lai
1.3.2 Miic dp lai hoa phu thuoc vao nong dp dat hiem
1.3.3 Mo men tu cam ling 5d va su phu thupc ciia no vao

n6ng dp dat hiem

5
6
8
8
10
|j

1.3.4 Ca che tuang tac trao ddi 4f-3d va sir phu thupc ciia
cucfng dp tuang tac 3d-4f vao nong dp dat hiem
1.4

Mot so khao sat thuc nghiem ve tuang tac trao doi cua cac
hap chat R-T dira tren cac phep do xac dinh nhiet do trat
tu va phep do dp tij" hoa bao hoa

1.5 Mo hinh hoa tri tir

12
14
17

1.6 Ket luan chuang
Chirang n . Cac phuang phap nghien cu*u

23

2.1 Cac phuang phap thuc nghiem

23

2.1.1. Phuang phap tao mSu

23

2.1.2 Dotijdp

24

2.1.3 He do ph6 Mossbauer

26

2.1.3.1 Hieu ung Mossbauer

26

2.1.3.2 Kha nang quan sat ph6 Mossbauer

26

2.1.3.3 Cac tuang tac sieu tinh te c6 ihi quan sat

27

2.1.3.4. Cac tham so do ca ban cua he do ph6 hap thii

28

Mossbauer
2.1.4 Cac phuang phap thirc nghiem khac:
2.1.4.1 Phan tich gian d6 nhieu xa Ranghen (X- Ray)

29
29


2.1.4.2 Do tir do a nhiet dp thap trong tir trudng cao

29

2.1.4.3 ChupanhSEM

29

2.2 Xac dinh he so tuang tac trao doi
2.2.1 Xac dinh he so tuong tac trao doi (A^y) trong cac hap chat
R-T
2.2.1.1 Mo ta tuang tac trao doi trong phep gan dung
trucrng phan tir
2.2.1.2 Xac dinh dp Ian ciia tuang tac trao d6i tiJ" nhiet dp
trattLr(Tc)
'
'
2.2.1.3 Xac dinh do Ian tuang tac trao doi tij nhiet do bii
tru(TJ
2.2.2 Xac dinh gia tri nhiet dp chuydn pha T^ bang duang cong tir
nhiet

29
29
29
30
32
32
33

2.2.3 Xac dinh gia tri momen spin thuan t\x hieu dung
2.3 Ket luan chuang

^^

Chuang i n Nghien ciJu tinh chat ti^ va tuang tac trao d6i

34

trong cac hop chat R(Fe|.^Ti^)2
3.1 Ma dau

34

3.2 Cau true tinh th^

38

3.3 Nghien ciJu pho Mossbauer

41

3.4 Tudp

46

3.4.1 Tir dp bao hoa a T = 4,2 K

46

3.4.2 Cac duang cong tu nhiet

49

3.5 Thao luan cac ket qua thuc nghiem

53

3.5.1 So sanh sir thay doi cua truing sieu tinh te va momen tiJ M^

53

3.5.2 Hoa tri tij*

54

3.5.3 He so tuang tac trao ddi 3d-4f (Ap.-p)

56

3.6 Ket luan chuang
Chuang IV. Nghien cu*u tinh chat tij va tuang tac trao doi

59
60

trong cac hop chat he R(Fe,.^Tij3
4.1 Ma dau

60

4.2 Cau true tinh the ciia cac hap chat R(Fe,.^Tij3

62


4.3 Tudp bao hoa a T = 4,2 K

64

4.4 Duang do tir nhiet va nhiet dp Curie

66

4.5 Phan tfch cac ket qua va thao luin

68

4.6 Ket luan chuang

71

Chuang V. Nghien cuu anh huang ciia nguyen to thay the len tinh chat

^2

ttrva tuang tac trao doi trong mot so hgp chat R(T,M),2
5.1 Nhirng nhan xet chung
5.2 Cac nghien ciru thuc nghiem tren he hgp chat RFe|2.xV^ (R = Gd,Y
va Lu)

72
80

5.2.1 Ket qua phan tich nhieu xa tia X

80

5.2.2 Nhiet dp Curie

82

5.2.3 Momen tiJ phan mang Fe trong cac hgp chat LuFei2.xVx

§4

5.2.4 Tuang tac trao doi giira cac phan mang (AcdPe)

85

5.2.5 Nhan xet cac ket qua thuc nghiem

gg

5.3 Cac nghien cixu thirc nghiem tren he hgp chat RCoi2.xV^

gg

5.4 Ket luan chuang

03

Chuang VI. Mot so nhan xet tdng quat v6 anh huang ciia hieu
ung lai hoa len tinh chat til ciia cac hgp chat R-Fe

94

6.1 Momen tij" cam ung trong cac hgp chat R-T

94

6.2 Moi lien he giOa tuang tac trao doi 3d-4f va momen tir cam ting

99

6.3 Cuang dp tuang tac trao doi 3d-4f (ART) trong cac he hgp
chat R-T va momen tij* cam ung.

102

6.4 Anh huang ciia Si t6i tuang tac trao doi 3d-4f tren cac he
hgp chat RFe,2.,Si,

103

6.5 Buc tranh lai hoa 3d(T)-3d(LT) va buc tranh lai hoa p-3d(T)

104

6.6 Ket luan chuang

107

Vn. Ket luan chung

109

Danh muc c6ng trinh nghito cuu

110

Tai liau tham khao

112

Phuluc


1

M6DAU
Nghien cuu cac vat lieu lien kim loai dat hiem - kim loai chuydn tiep R-T,

dac biet vdi T = Fe, Co va Ni la mot doi tupng c6 y nghla ca v6 khoa hpc ca ban
va ung dung. Cac nguyen to R c6 lap v6 dien tu 4/'kh6ng lap day voi dac tinh
dinh xu cao. Trong cac nguyen to nay, ca momen quy dao Ian momen spin deu
dong gop vao momen tir nen chiing c6 di huong tij va tu giao Ian. Tuy nhien, dac
tfnh dinh xu ciia cac ham song 4/lai c6 tuong tac trao doi yeu, do do nhiet dp trat
tu ti^r thap (7^ < 300K). Ngupc lai, cac nguyen to kim loai T c6 quy dao 3 J dupe
ma rpng ve mat khong gian, gay nen tuong tac tir manh va nhiet dp Curie cao.
Cac vat lieu lien kim loai R-T do vay ket hpp dupe cac uu diem v^ tir tinh ciia m6i
nhom nguyen to R va T de tao nen cac vat lieu ixx c6 di huang tu Ian, dp tiJ giao
kh6ng 16, tir dp va nhiet dp Curie cao. Cac vat lieu nhu vay da dupe ung dung de
che tao cac nam cham vTnh ciru cao cap (vi du nhu SmCo^, Nd2Fe,4B), cac vat lieu
til' giao TbFe2 (Terfenol va (Tb,Dy)Fe2 (Terfenol-D), cac vat lieu ghi quang tu
TbFeCo...
Tinh chat tir cua cac vat lieu kim loai R-T chii yeu dupe qui djnh bai tuang
tac trao doi va di huang trucmg tinh thi. Chung c6 the' dupe xem xet la vat lieu c6
hai phan mang R(4/) va T(3d}. Trong cac hpp chat nay, noi chung c6 3 loai tuong
tac trao doi: tuang tac R-R giua cac nguyen tii thupc phan mang dat hiem; tuo'ng
tac T-T giiJa cac nguyen tu ciia phan mang T va tuang tac giua hai phan mang RT. Tuong tac R-R la tuong tac trao d6i gian tiep; c6 ban chat va ca che kieu
RKKY (Ruderman-Kittel-Kasuya-Yoshida). Tuong tac R-R rat yeu nen trong rat
nhieu truong hpp c6 thi bo qua dupe. Tuang tac T-T la tuang tac s4t tij" quyet dinh
sir phu thupc nhiet dp ciia tu dp va gia tri nhiet dp Curie cao ciia cac hpp chat RT. Dong gop ciia tuang tac 4f-3d{l\xc la tuong tac R-T) vao 7^- cung khong Ion,
nhung tuong tac nay quy dinh cau true tir, xac dinh sir on dinh nhiet ciia momen
tu 4fvk su phu thupc nhiet dp ciia di huang tir ciia phan mang dat hiem. Do do,
trong cac vat lieu nay, di huang tix tinh the c6 nguon goc bai ca hai phan mang tii'
R va T. O nhiet dp thap, di huang tii tinh the ciia dat hiem la Ion. Di huang nay
dupe chuyen tai tai phan mang T nha tuang tac R-T. O nhiet dp cao, di huong ttr


ciia cac kim loai chuyen tiep tra nen manh hon dj huang tii ciia cac kim loai dat
hiem. Luc nay tuong tac R-T lai dong vai tro chuyen tai lien ket di hu6ng tir phan

mang T sang phan mang R. Vi nhung ly do vira neu, tuang tac trao doi 4f-3d da
dupe nghien cuu manh me trong nhung nam tam muai ciia the ky 20. Tuang tac
trao doi da dupe nghien cuu va xac dinh bang nhieu phuang phap thuc nghiem
khac nhau nhu:

phuong phap do tir dp trong tir truong cao tren cac m^u bpt rofi

(High Field Free Powder - HFFP) [72,103], cac phuang phap nhiiu xa natron
(Neutron Diffraction : ND), tan xa natron (Inelastic Neutron Scatterring: INS)
[70,72], tij* nhiet dp Cuire (Tc) va nhiet dp bii trij (Ti^) [57],... Cac ket qua thu dupe
thuong kha phij hpp nhau va do do mot so qui luat bien doi ciia h^ng so tuang tac
R-T (ART) d^ dugc tim thay. Do la: (i) qui luat ARJ giam khi di X\x dau day den
cuoi day dat hiem, (ii) qui luat A^j tang khi tang nong dp dat hiem va (iii) qui
luat bien dcii cua Ap.j theo ban chat kim loai T (Fe, Co hay Ni) |57|. Tren ca so
cac ket qua thuc nghiem do, ca che ciia tuang tac 4f-3d cung da dupe khing dinh.
Do la ca che tuong tac gian tiep 4f-5d-3d, trong do su phan cue ciia cac dien tir
din 5d chii yeu xay ra do hieu ung lai hoa giua cac dien tii 3d va 5d.
Co the nhan dinh rang hai muai nam viia qua la thai gian phat trien manh
me ciia cac nghien cuu tren cac hpp chat lien kim loai R-T. Mot trong nhung
nguai ma dau cho huang nghien ciiu nay la giao su ngucfi Phap da qua c6 - giao su
R. Lemere. Dudi su lanh dao ciia ong, phong thi nghiem tix hpc Louis Neel a
Grenoble (Cong hoa Phap) da phat hien dupe nhilu tinh chat qui bau cua cac hpp
chat R-Co [69]. Cac nghien cuu nay Ian dSn sang cac phong thi nghiem khac tren
the giai, trong do Vien nghien cuu Van der Waals-Zeeman, Dai hpc Tong hpp
Amsterdam (Ha Ian) cung la mot trong nhirng trung tam nghien cun manh me cac
vat lieu nay. Tuy nhien, cac nghien cuu c6 y nghla ung dung true tiep lai thanh
cong a My (nam cham Sm-Co, vat lieu tij giao Terfenol, vat lieu c6 hieu ling tir
nhiet kh6ng 16) va a Nhat ban (nam cham Nd-Fe-B). Cac sang kien nh^m cai thien
tinh chat tu ciia he hpp chat R-Fe lai dupe Coey (Ireland) de xuat: che tao cac vat
lieu ditn ke dira tren phuang phap nitarit. Trong khuon kho hpp tac quoc te, cac

nghien cuu v6 he vat lieu R-T a Bp mon Vat ly Nhiet dp thap, Khoa Vat ly,
Truong Dai hpc Tong hop Ha noi (nay la Truong Dai hpc Khoa hpc Tu nhien. Dai


hpc quoc gia Ha noi) cung da thu dupe nhi6u ket qua dang chii y tren cac nghien
cuu ve di huang tu 3d, tuang tac 3d-4f, cac chuyen pha tir va cong nghe che tao
cac nam cham dat hiem.
Noi chung, nghien cuu ca ban ve cac hop chat R-T trong hai chuc nam
qua, ngoai mot so tinh loan cau true vijng, doi vai cac nghien cuu thuc nghiem,
phuong phap

thucfng dupe su dung nhieu nhat vAn la phuang phap thay the:

nghien cuu su thay doi cua tinh chat tiJ" khi thay the cac ion khong ti^r tfnh (Y, La,
Lu) vao phan mang 4f hoac thay the cac ion khong tir (Cu, Al, Si, B) vao phan
mang 3d. Cac nghien cuu nhu vay, chi dupe lien hanh va dupe hieu mot each don
thuan theo hieu ling pha loang nong dp cac ion tix tinh trong mSu de lam giam
truang phan tir, hoac de kiem tra mo hinh dan ion. Mot so giai thich sau s^c hon
CO di cap den hieu ung tij'-the tfch (thay doi ap suat tac dung len he m^u hoac dua
vao su thay doi kfch thu6c cac ion thay the). Gdn day, vai cac c6 g^ng che tao va
nghien cuu cac hop chat tij ciing moi hp 1:12 kieu RFe,2,xM^ va RCoj^.^M,, cac
nguyen to M thay the cung chi dupe hieu va danh gia tren vai tro on dinh pha, con
anh huang ciia no den tfnh chat tir ciia cac hpp chat noi tren vSn con ft dupe de
cap.
Hien nay, tinh hinh nghien cuu cac hpp chat R-T c6 ve nhu dang chung
lai, cac nha tu hpc tren the giai duang nhu tam b^ng long vcfi khoi lupng kien
thuc khong 16 ma hp da kham pha ra trong hai chuc nam visa, qua mac dii vSn con
mot so cau hoi rat dan gian nhu: tai sao cuang dp tuang tac trao doi 4f-3d lai
tang trong cac hpp chat (R,Y)-T, tuc la khi da lam loang nong dp cac ion tix [57]?
Ngoai ra, mot cau hoi len han van c6 the dupe dat ra: neu xet ve vj trf trong bang

he thong tu^n hoan va cau hinh cua cac lap v6 dien tii d, s thi cac nguyen to kim
loai dat hiem c6 thi xem nhu cac nguyen to chuyin tiep ddu day nhu Sc, Ti, V
(Light Trasition Elements - LT). Vay cac nguyen to LT c6 the c6 vai tro nhu cac
dien tii" 5d cua nguyen to dat hiem trong cac hpp chat R-(Fe,LT) va R-(Co,LT)
hay khong? Them vao do, cac nguyen to a kim c6 lap v6 vai cac dien tir p khi
duac thay the vao trong cac hpp chat R-T se c6 vai tro nhu the nao? Theo quan
niem cua chung toi, mot khi ca che tuong tac da dupe hieu la do hieu ling lai hoa
thi cac hpp chat ki6u nhu R-(Fe,LT) va R-(Co,LT), neu dupe nghien cuu, se la


cac doi tuong rat qui di c6 co so kh^ng dinh them ca che cua tuang lac trao d6i
tren nhi^u hp hop chat hon va vai nhieu kieu dien tu linh dpng han. Do vay, luan
an nay da chpn doi tupng nghien cuu la cac hpp chat R(Fei.^Tij2. R(Fe,,^TiJ3,
RFe]2.^V^, RCoi2_xVx vai R = Gd, Lu va Y. Mot so ket qua cua nhom nghien cuu
tren mot so hpp chat khac nhu (Er,Y)Fe2, GdFei2-xSix cung se dupe neu lai di ban
luan ket qua mot each tong quat.
Luan an dupe bo cue nhu sau:
Sau phan ma dau la chuong 1. Trong chuang nay, cac diem chinh \'i bien
d6i momen tii va tuang tac trao d6i trong cac hop chat R-T da dupe nhin nhan
duai quan niem ciia mo hinh lai hoa tren ca sa xem xet cac dai lupng vat ly Hen
quan la he so tuong tac trao doi giua hai phan mang va momen tij trung binh cua
hpp chat (trong sir phu thupc tham so hoa tri tir). Chuang 2 trinh bay cac phuang
phap CO lien quan t6i viec thu thap cac tham so thuc nghiem ma luan an su dung.
Cac chuang 3, 4 va 5 trinh bay cac thuc nghiem da dupe thuc hien tren cac he
mau ma muc lieu nghien cuu cua luan an di ra. Chuang 6 trinh bay viec phan tfch
cac ket qua thuc nghiem va cac ket luan ma luan an da rut ra tren ca so tra lai mot
so cau hoi chua giai dap truac kia ve vai tro cua cac nguyen to kim loai chu}'en
tiep nhe (Ti,V...) khi tham gia nhu thanh phan thu ba trong cac hpp chat gia hai
nguyen R-Fe-LT (R = Gd,Y,Lu).

MOMEN TtrVA

TUDNG

Chuang L
TAC TRAO DOI TRONG CAC HO? CHAT LIEN

KIM LOAI DAT HIEM-KIM LOAI CHUYEN TIE?
1.1 Nhto xet chung
Cac nguyen to dat hiem (R) va cac kim loai chuyen tiep 3d(T) c6 thi ket hop vai
nhau de tao ra cac hop chat lien kim loai dat hiem-kim loai chuyen tiep (R-T) vai mot
so nong dp xac dinh, vi du cac he hpp chat RT2, RT3, RT<;, RjT^j, RT,,,... Cac hpp
chat nay c6 cau true tinh thd khac nhau, tfnh chat lix khac nhau,... Tuy nhien, tii' rat
nhieu nghien cuu thuc nghiem va If thuyet, c6 the tong ket dupe mot so dac diem
chung sau day vi tix tinh ciia cac he hop chat R-T.
2.5
2.0

3

1.5
1.0

0.5
0.0

tV{E)\ N(E)]^ N{E}\

N(t)l

N{E}\

N{E)\

Hinh 1.1 Mo ta sir lap day dan vung
3d dan tdi cac truang hpp: (a), dac tinh
sat tir yeu, (b). sat tCr manh, (c). thuan
tij", trong cac hpp chat R-T [66].

0.1
0.2
0.3
NOng (16 d a i hie'iii

0.4

Hinh 1.2 Su phu thupc ciia momen tiJ
trung binh 3d vao n6ng dp thanh phan R
cho mot so day hpp chat Y-Fe, Y-Co va
Y-Ni(l57],[70])'

(i) Trong cac he hpp chat R-T, tfnh dinh xu cao ciia cac dien tii 4f thuong van dupe
giu nguyen nhu trong cac kim loai dat hiem. Momen tii" cua cac ion dat hiem dupe
xac dinh bang bieu thiJe MR = gj J|ip, trong do gj la thira so Lande, J la momen loan
phan ciia nguyen tii R (J = L-S vai dat hiem nhe, J = L+S v6i dat hiem nang) va \x\^ la
magneton Bohr.
(ii) Tfnh chat tir ciia cac dien tir 3d(Fe,Co,Ni) dupe mo ta bai dac tfnh linh dpng cua
chiing, do do phu thupc rat nhieu vao cau true viing nang lupng. Trong mot so mo ta
cac tinh chat vT mo, ngudi ta thuong sir dung hinh thuc "ciing" (rigid band) cua cac

vLing nang lupng. Khi do, trong phep gan diing bac nhat, su bien doi ciia momen tu
3d CO the dupe doan nhan tii' mire dp lap day vijng (tiic la tix vi trf cua miic nang lupng


Fermi). Cu the v6i cau hinh 3d", momen tii" cue dai c6 the nhan gia tri (10-n))aji cho
truong hpp lap day qua nua viing hoac n|^p cho truong hpp lap chua day niia vung. Tij
tfnh cLia hop chat cung bien doi theo dp tach miic viing 3d va phu thupc vj trf tuang
doi CLia muc Fermi trong hpp chat. Cac truong hop dien hinh dupe mo ta tren hinh 1.1
(iii) Trong cac hpp chat R-T, momen tir phan mang 3d giam khi tang nong dp dat
hiem va bien mat a mot gia tri nong dp t6i han (x^). Vf du v6i cac hpp chat RxNit.^, x,
= 0,15 con doi v6i hop chat R^Co,.^, x^= 0,33 (xem hinh 1.2).
(iv) Trong cac hpp chat R-T, momen tix 3d sap xep song song v6i momen tir cac dat
hiem nhe nhung lai sap xep phan song song vai momen tir cac kim loai dat hiem
nang. Neu chii y tai lien ket giua momen spin (S) va momen qui dao (L) cua dat hiem
(J = L-S vai dat hiem nhe con J = L+S v6i dat hiem nang) ta c6 th6 nhan xet rang lien
ket spin 3d va spin 4f luon luon la phan song song v6i tat ca cac hop chat R-T.
Truac day, cac qui luat tren ve tii" tfnh cac hpp chat R-T dupe dua ra chi nhu cac
nhan xet rut ra tu* cac quan sat thuc nghiem (vi du cac nhan xet trong gia thuyet ciia
Campbell [32]). Khoang 10 nam lai day, cac dac diem do da dupe xem la he qua ciia
hieu ling lai hoa giua cac trang thai 5d-3d. Cac quan niem nhu vay dupe cung c6 bai
cac tfnh loan If thuyet (dien hinh la cac cong trinh cua Brook va dong nghiep [2325]). Tren ca sa ap dung cac khai niem do, tac gia N. H. Due [57] da he thong hoa
dupe mot so qui luat ciia tuong tac 4f-3d. Trong chuang nay, chiing toi se trinh bay
each mo ta mot so tfnh chat tir ciia cac hpp chat R-T du6i each nhin ciia hieu ung lai
hoa va theo each sii" dung mo hinh hoa tri tii.

1.2 M6 hinh Campbell v^ du hinh spm va tirong tdc trao ddi 4f-3d
trong cac hop ch^t R-T.
Mo hinh hien tupng luan ciia Campbell ve ca che ciia tuang tac trao doi 4f-5d3d da dugc di xuat dua tren cac nhan xet thuc nghiem sau day. Chung ta biet khi
diing mot kim loai sat tix lam nin pha tap vod mot kim loai khac a ciling phfa voi no
trong day kim loai chuyen tiep thi se nhan dupe momen lien ket duang (lien ket sat

til', VI du truang hpp NiCo). Con khi pha tap vai kim loai thupc dau phi'a ben kia ciia
day thi se nhan dupe momen lien ket am (lien ket phan sat tiJ, vf du NiCr2). Thupc
tfnh nay ban dau dupe Friedel chi ra va tiep tuc dupe thao luan kl trong cac mo hinh


khao sat sau nay cua nh\iu tac gia. Tir cac quan sat nay c6 the dua ra mot nguyen tac
chung CO tfnh chat kinh nghiem cho su ket cap cac momen khi mot hpp kim dupe
thuc hien v6i hai kim loai deu thupc day chuyen tiep: ket cap se la sat ti^r khi ca hai
kim loai a ciing niia vi mot phfa ciia day kim loai chuyen tiep va ngupc lai khi hai
kim loai thupc ve hai niia a hai phfa khac nhau thi ket cap se la phan sat tir.
Tren ca sa cac nhan xet do, Campbell da di xuat ca che tuang tac trao d6i gian
tiep 4f-5d-3d v6i mot so y tuong ban dau nhu sau:
(i). Cac dat hiem (v6i dac diem c6 cau hinh dien tu 5d') dupe xem la ciing thupc day
voi cac kim loai chuyen tiep 3d" va dupe xep vao dau day v6i nhom nguyen to kim
loai chuyen tiep nhe (LT: light transition metal). Vi vay c6 thi ap dung qui tac kinh
nghiem nhae den a tren cho sir ket hpp. Tuong tac giira cac momen tii" d(5d) cua cac
dat hiem vdi nhau la tuang tac sat tiJf (ciing phfa cua day). Ket cap hieu dung giija
chiing Cling se la sat tii:. Tuy nhien, cac dien tu 5d(R) ung v6i cac nguyen to dau day
lai CO lien ket phan sat tir vdi cac dien tir 3d cua Fe, Co, Ni la cac nguyen to cuoi day
(HT: heavy transition metals) nen tuang tac 3d(Fe, Co, Ni)-5d(R) la tuang tac phan
sat tij.
(ii). Cac dat hiem pha loang vao vdri kim loai chii thuan tii* thupc ve nira dau day kim
loai chuyen tiep (nhom LT) se c6 ket cap sat tir vori momen cam ling d tai cac vi trf
ion kim loai chii. Khi do, cac dat hiem dupe xem la ciing phfa vai cac nguyen to kim
loai chuyen tiep khong tir (Ti, V, Cr). Cac quan sat cho thay momen tii trung binh cua
nguyen tir Gd trong hpp kim (vi du LuGd, LaGd, YGd...) la Ion han gia tri momen tir
ciia ion Gd trong kim loai khong pha. Tuong tu, dat hiem khi dupe pha loang vao kim
loai ehu la nguyen to thupc ve nua cuoi day HT (vf du GdPd, Lai.^Gd.Ni^) thi tinh
trang ket cap giiJa cac momen dat hiem v6i momen cam ling d tai vi tri ion kim loai
chii se la phan sdt tii:. Momen ti^ trung binh cua nguyen tir dat hiem liic nay lai nho

han momen tii' ion tu do. Nhung nhan xet tren da dupe xac nhan tren ket qua thuc
nghiem do pho NMR ciia cac hpp kim Gd [72].
Nhung nhan xet c6 tfnh chat hien tupng luan nay la noi dung ciia mo hinh
Campbell [32]. Mo hinh nay da cho phep giai thfeh dupe qui luat lien ket spin phan
sat tix 3d-5d trong cac hpp chat R-T va di xuat y tuang ve ca che tuang tac trao doi
gian tiep 4f-3d thuc hien thong qua vai tro trung gian cua cac dien tu 5d. Chiing toi


se tiep tuc cac phan tfch chi tiet hon nham mo ta nguon goc ciia ca che nay trong cac
phan duai day.
1.3 Hidn tuong lai ho^ va c & hinh spin trong cdc hop chS^t R-T
1.3.1

Cdc trang thai lai
1

1

1

1

1

1

I

•

ti'angtli^i kirn loai

f — d — J * "
f — d — J l ^

t

1

1

1
1

fl(£}

•

mifenlaihoi3d4d[5d)

p—

'

1

1

1
4

1
5

sa luang lu n

1
6

1

^

Hinh 1.3 Lupc do trinh bay su phan bo
cac mure va cac phan muc ling vai cac
trang thai nang lupng kha dT cua cac dien
tu trong nguyen tir cac nguyen to [153].

Hinh 1.4
Lupc d6 mo ta v6 su
phan bo mat dp trang thai ciia kim
loai 3d(T) va kim loai dat hiem
(R: 4d, 5d) trong kim loai don
chat (a) va trons hop cha't (b).

Nhu da biet, cac nguyen to 4f va 3d khac nhau nhilu vi dp am dien va the xuyen
tam. Dieu nay da dan den kha nang cau thanh rat nhieu dang hop chat lien kim loai
R-T v6i eac thanh phan va dac tfnh rat khac nhau [63, 70, 73]. Tuy theo su phan bo
cac nguyen tir trong khong gian cau true nen hpp chat, cac dam may dien tii xung
quanh cac hat nhan thuong bien dang di so v6i liic ton tai trong kim loai khong pha.

Khi khao sat cac hpp chat R-T, c6 the thay rang do sir khac biet dp am dien giua cac
nguyen to tham gia hpp chat ma trong cau true vung nang lupng cua chat ran, phan
bo cac miic nang lupng ung voi trang thai spin 5d hoac 4d (khi xet cac hpp chat v6i
Y) thucng bi chuyen dich tai gan cac miic 3d chua dupe lap day (xem mo ta a cac
lupc d6 tren hinh 1.3 va hinh 1.4). Su hinh thanh man chan tinh dien ciia cac the hat
nhan (dupe tao bai su tham gia ciia ca cac dien tir dan) trong hpp chat cung bi thay
doi di so v6i trong kim loai nguyen ban dau. Hai viing 3d va 5d (hoac 4d) liic nay
thuang ton tai vai mot so miJc phan bo kha gan nhau. Di6u do da dan den kha nang


9

xuat hien su iron lin cac miie va xay ra hien tupng lai hoa. Cac trang thai nang lupng
3d se CO them mot so mii'c do lai hoa v6i cac trang thai 5d va tuang tu, cac trang thai
5d cung se c6 them mot so miic do lai hoa ciia cac trang thai nang lupng 3d. Qua
phan tfch so lieu tren cac day R-Fe(Co,Ni) [25, 66, 72, 157], c6 the cho rang ban chat
cac moi lien ket va bien doi tuong tac xay ra a day khong phai la su chuyen dich dien
tir thong thuong ma do la do qua trinh xuat hien cac miic lai khi c6 su iron lan ciia
cac trang thai 5d trong viing 3d va ngupc lai. Cac miic lai c6 mat ca trong phan viJng
lien ket cung nhu trong phan viing phan lien ket. Su choan lan vi mat khong gian
ciing nhu nang lupng vao nhau ciia cac trang thai 5d va 3d (dp phu viang) la phu
thupc vao su phan chia hinh hpc cac khoang each giiJa nguyen tu R va nguyen tu Fe
trong khong gian phoi trf ciia chiing.

n(E)

Mure Fermi

/\

(a)
Hinh 1.5 So do ca'u true vijng
nang lupng ciia cac dien tir 3d va 5d
minh hoa cho su lai hoa 3d-5d. Cac
phan dien tich trSng ti le vai so
trang thai 3d va cac phan dien tfch
den ti le vai so trang thai 5d. Cac
trucfng hpp: (a). Lai hoa 3d-5d yeu,
(b). Lai hoa 3d-5d manh [57].

3d
5d

n(E)

/\

4

Mure Fermi
(b)

3d

5d
^

E
Do dac diem phan bo cac trang thai dien tu dan trong hpp chat cua kim loai
T(Fe,Co), cac trang thai 5d tham gia vao sir hinh thanh viing 3d se lam thay doi dang

dieu cua phan bo mat dp trang thai n(E) cua viing 3d quanh miic Fermi. Cau triic dien
tu ciia vung din trong hpp chat R-T kha phirc tap nen de don gian cac phan tfch, thay
cho eac thoi quen trinh bay cu (nhu tren hinh 1.1), chiing toi se diing cac lupc do hinh
chij* nhat ma dien tfch cua no ti le (mot each tuang doi) vai so lupng dien tu dan
tuang ling ma no se mo ta (xem hinh 1.5). Tiep tuc cac phan tfch chiing ta thay rang
miic dp lai hoa se phu thupc dp phu viing (thuong do vi trf tuang doi giua cac miic
tham gia lai hoa) cQng nhu phu thupc khoang each giua nang lupng trung binh ciia
cac viing 5d va 3d. Tat nhien khe nang lupng giua cac viing se co hep lai (vi vi trf


10

tuang doi) khi mat dp cac trang thai Iron lan vao nhau tang len (lupng cac miic lai
tang len). Dieu do tao dieu kien thuan Ipi cho tuong tac giira cac spin 5d va spin 3d
(xem hinh 1.5). Trong khao sat nay, ta can chii y rang tuong quan giQa dp Ion ciia the
Coulomb va be rpng vijng dien tir dan cua cac kim loai thanh phan tham gia hpp chat
la cac yeu to quan trpng [70].
TIJ: cac phan tfch tren, xu huong tu nhien co the doan nhan dupe la miic dp lai
hoa (bi rang bupe true tiep bai dp phii viing 5d-3d) se dan tai anh huang khac nhau
ciia cac miic nang lupng 5d trong viing 3d. Didu nay lai dan toi cac xu huong khac
nhau cua su djch miic Fermi. Miic dp tham gia ciia cac trang thai dien tu spin 5d cang
tang, momen tij 3d se cang giam. Se xay ra tinh huong ton tai nhung m'lin n6ng dp
(ciia m6i nguyen to thanh phan R va T) trong hpp chat phi^i hpp ma tai do lieu chuan
sat tu Stoner co the bi vi pham dan tai su bien mat ciia tfnh sat tu, Nhan xet nay co
the cho phep la If giai dupe hien tuong thuc nghiem da quan sat thay ve su ton tai gia
tri nong dp tai han x^ (xem hinh 1.2). Moi lien he giira su bien d6i tir tfnh hpp chat
v6i miic dp lai hoa theo each mo ta dupe di cap tren day la co siic thuyet phuc vi no
giiip chi ra dupe sir phu thupc cua gia tri n6ng dp tai han vao miic dp lap day vung
3d. Miic dp lap day cang cao, nong dp dat hiem can thiet (xj de tang cuang cac trang
thai 5d tham gia vao viing 3d nham dua miic Fermi dich den vi tri tuang ung vcfi

trang thai thuan tii: cang thap. Vf du trucng hpp Co: x^ = 0,33 con vai Ni: x, = 0,15.
Do la he qua dau lien co the riit ra tic hieu ling lai hoa.
1.3.2 Sir phu thu6c cua miic d6 lai hoa vao n6ng d6 dit hidfm
Cac quan sat ve mat dp trang thai (density of sate: DOS) va mo ta cau triic viJng
da cho chiing ta thay rang trong eac hop chat R-T, viing 3d co cac miic nang lupng
thap va tuong doi hep, con vung 5d co cac miic nang lupng cao hon va dp rpng vung
\6n han [15, 47, 91, 95]. Do vay thucng xay ra su phu vung len nhau mot phan lam
cho mot so dien tir 5d co the ton tai ling v6i cac miic 3d chua dupe lap day (va ngupc
lai). Cac trang thai lai thong thuong xuat hien lai dinh vung 3d va day viing 5d. Miic
Fermi ciia hpp chat thuong nam trong viing lai hoa, a do mat dp trang thai (DOS)
bien doi rat manh [57, 70]. Muc dp lai hoa 5d-3d true liep bi phu thupc vao dp rpng
cua khe nang lupng tao thanh giira hai viing 3d va 5d va phu thupc vao so dien tu 3d
va 5d chiem day cac trang thai a viing quanh miic Fermi (Ep).


11

Bay giof chiing ta hay xet su phu thupc cua miic dp lai hoa vao nong dp dat hiem.
Co the nhan xet rang, khi tang nong dp dat hiem, n6ng dp dien tii 5d se tang len lam
tang kha nang cac dien tu cua phan vijng nang lupng 5d tham gia vao vijng 3d. Theo
suy luan tu nhien, Ep phai dich v^ phfa nang lupng cao. Nhu vay, so trang thai 3d co
thi se tang len. Trong trudng hop nay, de giii cho so dien tu 3d khong d6i va tang kha
nang cho cac dien tu 5d tham gia vao viing 3d, khoang each nang lupng giira cac
vung 3d va 5d phai giam di. Do dac diem dp am dien cua cac nguyen tu kim loai
3d(Fe, Co, Ni) thuong kha Ion so v6i dp am dien cua cac nguyen tii dat hiem nen xu
huong xay ra thuong la su ha thap phan viing 5d. Nhu vay, tinh huong nay se lam cho
su phu giiia cac viing co the cang tra nen manh hon. Mat dp trang thai (DOS) cua cac
trang thai 3d phai giam di con mat dp trang thai 5d thi cao han lai viing co cac trang
thai lai (xem hinh 1.5.b). Nhu the, tuy miic Fermi bi dich chuyen len phfa nang
lupng cao han nhung so dien tu 3d lai van co the dupe giu nguyen. Tii do co the

k6'l luan rang miic dp lai hoa 3d-5d se tang theo nong dp cac nguyen tii dat hiem
tham gia thanh phan hpp chat. Lai hoa manh nhat trong cac hop chat RT2 va yeu
nhat trong hop chat R2T[7 va RT,2. Ket luan nay giiip chiing la doan nhan dupe qui
luat thay doi ciia mot so dai lupng vat ly co lien ket chat che vai hien tupng lai
hoa ma chiing toi se khao sat qua cac phan tfch thuc nghiem cua luan an nay. Do
cQng la ca so ma lir do, lac gia N. H. Diic dua ra nhan xet la: khi tang nong dp dat
hiem, momen tii 3d giam nhung cuang dp tuang tac 3d-4f lai tang [57].
1.3.3 Mdmen tCr cam ling 5d va su phu thu6c ciia no vao n6ng d6 dSit hi^m

Hinh 1.6 Gian do minh
hoa su lai hoa giira cac
trang thai 3d-4d trong
cac hpp chat da't hiem
khong tii tinh Y-T hoac
Lu-T [59]

nfE)


12

Tfnh chat tii cua cac hpp chat R-T(Fe, Co, Ni) thucmg bi chi phoi bai dac diem
phan bo miic va su tach vijng 3d (hinh 1.1 va hinh L2). Tii dac diem su tach vijng 3d,
CO the suy luan rang: lai hoa xay ra giCra cac trang thai 5d(4d)-3d se khong giong
nhau theo cac hucng phan cue spin-st va spin-sj,. Quan sal hpp chat LuFe2 trong
trang thai tii hoa bao hoa cho tha'y ro su tach viJng 3d xay ra khac nhau theo cac
huong phan cue spin-st va spin-s| (hinh 1.6). Vi the, hien lupng lai hoa tat se xay ra
khac nhau of hai phan viing lien ket. Tii day co the thay rang, do su tach vijng 3d, khe
nang lupng giira cac phan viJng spin-st (5d va 3d) se Idfn hon khe nang lupng giua cac
phan viing spin-sj, giiia chiing. Kei qua lai hoa 3d-5d giiia cac trang thai spin-sj. se

manh hon lai hoa 3d-5d giua cac trang thai spin-st. Theo mo la tren hinh 1.6, so dien
tu cua mdi phan viing spin: n^^r^ HJ^I, n^,^^ va n^^i la ti le vdfi dien tich m6i phan viJng
tuang ling a du6i miic Fermi. Khi Ug^i 16n hon n^^t, momen tii Lu co tri am (m^^ =
"5dT - Hsdi) va ngupc huong vdi momen tii 3d. Tuong quan nay se lam phat sinh ra
momen 5d am tai cac vi trf nguyen tu dat hiem. Do uu the lai hoa ciia cac trang thai
nang lupng spin-sj,. Momen 5d long cong se phai ngupc huong so v6i momen 3d.
Do do hop chat LuFe2 bieu hien dac tfnh feri tii hon la tfnh sat tii. Tfnh loan da cho
gia tri momen tii dinh xu: lai vi trf Lu trong LuFe2 bang -0,41|j.B/at [14]. Cac thuc
nghiem cung xac nhan momen tii dinh xii lai cac vi trf nguyen tii Y va Fe tuang
ling la -0,67|aB/at va \,71\xjsit trong hop chat YFe2 [57, 72,157]. Tii cac so lieu
thuc nghiem, thupc tfnh feri tii da dupe khang dinh la co ngu6n goc do hien tupng lai
hoa 3d-5d(4d) xay ra v6i miic dp khac nhau giiia cac phan vijng spin-st va spin-sj.
trong hop chat. Do cung la nguyen nhan lam xuat hien momen tii 5d. Voi he Y-Fe,
momen tii 5d tang theo nong dp Y, tii gia tri 0,2

IIB/^I

trong Y2Fe|7 den 0,44 [i^at

trong YFe2 (xem hinh 1.2) [57]. Nhu da noi b tren, qui luat thay doi momen tii 5d
nay dupe qui dinh bai hieu ling lai hoa va la nguyen nhan lam giam momen tu
trong cac hpp chat Y(Lu)-T. Khi nong dp dat hiem tang, momen tii giam. Ket qua
quan sat thuc nghiem mo la a hinh 1.2 la phii hpp v6i cac phan tfch neu tren.
1.3.4 Ca ch^ tuong tdc trao d6i 4f-3d vk su phu thu6c cua cucfng d6 tuong tac 3d-4f
vao n6ng d6 dit \n6m


13

Tir cac quan niem lruy6n thong, ban chai tuong tac trao doi 4f-3d da dupe thiia

nhan la tuong tac trao doi gian tiep (thong qua cac dien tu dan 5d) va la tuong tac am.
Co che cua tuang tac nay cung da dupe giai thfeh theo mo hinh Campbell. Tren co so
quan niem nay, he so tuong tac trao d6i A^^_^^ co thi dupe xac dinh nhu tfch cua hai
he so A4f.5d va A5d.3d (I'^^f.id = A4f.5d . A^^.^d)- Tuy nhien cac ket qua nghien ciiu gan
day (Brook va Johanson[23-25], N. H. Diic va D. Givord [54, 55]) da cho thay lien
ket phan sat tir giira cac spin 3d va 5d khong phai chi dupe qui dinh bai tuong tac trao
doi am giiia cac spin 5d va spin 3d ma no con la he qua cua chfnh su lai hoa giCia cac
trang thai 5d va 3d. Lien ket 5d-3d va su xuai hien momen tii 5d am (so voi momen
3d) do do CO Cling ngu6n goc.
Co che tuong tac trao doi 4f-3d trong cac hpp chat R-T co the dupe mo la
tren hinh 1.7. Trong biic tranh nay, hien tupng lai hoa 5d-3d da dupe xem la co vai
tro quan trpng tham gia co che hinh thanh tuong tac 5d-3d ciia hpp chat.

Theo cachS'nay, mdtsddiim

c6 thSnh^ manh vk Imi f nhu sau:

(i). Trong cac hop chat R-T, cau true phan song song giixa cac momen tu: cua
nguyen tu R va T vin ton tai ca khi R la nguyen to khong tit. Dieu nay co nguon
'^/

Ssd
Hinh 1.7 Gian
do mo ta tuang
tac trao doi giiia
cac momen tir
trong cac hpp
chai R-T. F va
AF: kf hieu lien
ket sat tii va

phan sat tii;
Hyb: kf hieu ton
tai vai tro lai
hoa 3d-5d; T^^_^^:
tuang tac dinh
xLr4f-5d; S.O.:
tuang tac spinquT dao

4j

^3d

Hyb.

S.O.

HyhJ

t
\

Mj

Mj^

{a)
Dat hiem khing tir

Mj,

Mj^

(b)
Dat hiem Dh§

My

Mj.

(c)
Bat hiem n^ng


14

goc do hien tuang luon xuat hien mdmen cam ifng 5d(R) tai cac vi trf nguyen tu R
trong ca hai trudng hpp co va khong co su tach vijng 3d.
(ii). Hieu ung lai hoa khong nhirng qui dinh lien ket Sd-Sd la phan sdt tir ma cdn
quyet dinh ca dp Idn cua lien ket ay.
(iii). Chfnh hieu ung lai hoa 3d-5d (giua cac trang thai spinsj.), da gay nen sir phan
cue cua cac dien tu5dch(x khong phai tuong lac trao doi 4f-5d (r4f.5d)L4

M6t s6' khao sat thuc nghidm v6 tuong t^c trao d6i cua cac hap ch^t R-T
dua tr6n cac phep do xac dinh nhi6t d6 tr^t tu va d6 tii hoa bao hoa
Bang cac phuang phap thuc nghiem nham xac dinh he so tuang tac trao d6i A[^y

(tii kei qua do nhiet dp trat tu T^ hay kei qua do nhiet dp bii trir T,,) va so lieu do dp tii
hoa bao hoa ciia mSu, N. H. Diic va cac cong su da lien hanh hang loat cac khao sat
mot each he thong tren cac he hop chat R^T,^ va mot so he ba nguyen. Cac gia tri thu
dupe bang phep do nhiet dp trat tu T^ hoac nhiet dp T^ da dupe so sanh voi nhau va

so sanh vai kei qua do ciia cac tac gia khac. Cac so lieu nhan dupe theo phuang phap
nay kha phii hpp vai so lieu theo phuang phap HFFP (J. P. Liu, [103]) va voi so lieu
xac dinh he so truong phan tir thu dupe qua phep do dudi tii truong cao (H. S. Li
[98])... So sanh nay cho thay cac kei qua thuc nghiem theo phuang phap ma N. H.
Diic ap dung la dang tin cay. Trong cong trinh [57], cac kei qua nay da dupe sap xep
mot each he thong vao cac bang thong ke (vf du bang 2.3 ma chiing toi se trich vao
chuang 2). Tac gia N.H. Diic trong cong trinh nay da riit diic mot each cong phu cac
tham so dac trung cho cau triic ciia nhieu he hpp chat R-T (thuong diing vao phep tinh
loan he so ART thuc nghiem). Tir cong trinh [57], mot so qui luat he thong ve tii tfnh
va tuong tac trao doi trong he hpp chai R-T da dupe xac dinh:
(i). Tuang tac trao doi 4f-3dphu thuoc mot each co qui luat vao nong do dat hiem.
Tren day la da chi ra rang sir tang cua miic dp lai hoa 5d-3d da dan den su tang
ciia momen Iii 5d. Di^u do cung din den he qua tuong tu vai cuang dp tuong tac 4f3d. Cac quan sal thuc nghiem vi gia tri he so tuong tac trao doi AR^ trong cac hpp
chai R-Fe, R-Co va R-Ni da dupe dua ra tren hinh 1.8 va hinh 1.9. Xu the A^p^, A^^o
va Aiij^i tang theo nong do dat hiem, theo hinh 1.8 la tuyen tfnh Hinh 1.9 bieu didn


15

quan sat thuc nghiem true tiep vi mdi quan he phu thupc cua he so tuang tac vao gia
trj momen tii 3d. Theo day quan sat, vai cac hpp chai co n6ng dp dai hiem giam dan,
CO the thay AR^ giam con Mj lai tang len. Cac qui luat phu thupc ciia gia tri momen
tii My va gia tri he so tuong tac ARJ (theo nong dp dai hiem) da dupe khang dinh nhu
cac qui luat thuc nghiem. Theo quan diem mo hinh lai hoa, cac qui luat nay lien quan
chat che t6i sir thay doi co he thong ciia miic dp lai hoa 3d-5d trong hpp chai R-T. Tu
hinh 1.8 co the thay miic dp khac biet ro ret ciia ART theo m6i he hpp chai vai T khac
nhau (= Co, Fe hay Ni). Dieu nay thuong dupe ly giai la do miic dp tach viJng 3d
khac nhau doi v6i m6i he hpp chat. Mot so dac diem co the nhan xet nhu sau:
20 I

1

1

1

1

GaCo B ^^"-"^"^

20

16
15

12
Uare

lO

• Gd-Co system
A. O d ' N I System
O.l

0.2

0,3

0.J

i,o

1.5

2,0

R • concentration

Hinh 1.8 Su phu thupc ciia he so
tuang tac ART trong cac day hpp
chai lien kim Gd-Co, Gd-Fe, Gd-Ni
{$,6 lieu dap tiJ" [571.

Hinh 1.9
Su phu thuoc A^-p vao
momen tii tren day cac hpp chai R-T
(so lieu dan tir [57),

(a). Trong cac hpp chai hai nguyen R-T, qui luat pho bien la cuofng dp tuong tac
(ARJ) tang theo n6ng dp thanh phan R trong khi do momen lir 3d(Mj) lai giam. Quan
sat nay da thay tren hau hei cac day hpp chai nhu Er-Fe, Gd-Fe, Gd-Co, Gd-Ni,...
Theo nhan xet cua N. H. Diic, dieu nay khong the chi hieu mot each don gian la su
phu thupc vao khoang each cac nguyen lu giiia cac phan mang (nhu J. P. Liu giai
thich [104]) VI ve ban chat, tuong lac ngoai nguyen tii 3d-5d khong the la yeu to
chfnh ciia hien tupng ma van de la do su rang bupc mot each chat che vao miic dp lai
hoa 5d-3d. Bien doi ciia tuang tac trao doi 4f-5d la gan kei vai thupc tmh nay.
(b). Dac diem giam My khi nong dp thanh phan R tang len thuong co the hilu do
su tang miic dp lai hoa 3d-5d (hinh 1.2). Tuy nhien, hien tupng con lien quan tai cac
chi tiet ciia miic do lao thanh momen cam ung 5d tai vi tri R; vl vay su phu thupc A^y

16

vao M j la khong don dieu (do M^ = M3d-M5d) theo nhu thuc nghiem da thay tren hau
het cac day R-Fe, R-Co. (hinh 1.9). Nhu cac phan tfch da chi ra, cac quan sat nay bieu
hien moi lien he chat che giiia su phat sinh momen 5d v6i dp tach miic ciia vCing 3d.
(c). Trong cac hpp chai R-T ma momen tii 3d Ihiei lap toi (da so cac hpp chat RFe va R-Co), su tang momen Mj thuong la bieu hien miic dp lai hoa giam. Xu huong
thay dupe a cac hpp chai giau kim loai T la momen tii M^ co gia tri tien tai rai sat vai
mo men tii kim loai don chai. Nhu tren da neu, dieu nay cung co nghla momen Mj^ bi
giam di (ung v6i giam miic dp lai hoa 3d-5d) va ham y su giam nong dp dien tir 5d
tren 1 don vi cong thiic hop chai. Thupc tfnh nay co the bpc 16 qua su tang dp phii giiia
viing 3d va vCing 5d. Nhu vay, 16 gic tu nhien co the quan sat dupe la tuang tac se
manh v6i hop chat giau kim loai T va yeu hon a hpp chai ngheo kim loai T. Dieu nay
CO lien quan tai viec gia tri mat dd dien tu 5d (tfnh tren 1 vi trf ion R) se tang len (Hie
la miic dp lai hoa 3d-5d tang len) la hoan loan dong nghla v6i viec A^j tang ti le ngupc
voi the tfch 6 mang ciing tuang ling vai su thay doi tCr tfnh cua hpp chai (dieu nay da
dupe nghien ciiu cua J.P. Liu chi ra [103, 104]).
(ii). Cuang do tuang tac (Aj^j-Jphu thupc ban chat cua thanh phan dat hiem.
Qui luat tii quan sal thuc nghiem la AR^ giam tii hpp chat vai dai hiem nhe tai hpp
chai vofi dai hiem nang. Hien tupng thuc nghiem nay co lien quan l6i su co lantan theo
day dai hiem. J. P. Liu [103], qua vf du so sanh hai he d6ng cau triic pha Laves GdFe2
va ErFe2, da thay A^j tang 10% khi tham so mang giam 1,5%.
40

T — r — 1 — T

1—r- - I — 1 — 1 — I — 1 —

•
•

Hinh 1.10
Bieu di6n
phu thupc cLia he so
tuang tac nhu ham ciia
dai hiem. Quan
sat tren cac day R-Fe, RCo,R-Ni [57].

1

•

30 --

20

A R.'^S^B
• RNi^

t

-

•

.

-

A.

i

10

RCo^B

•

-

"\^
1

•

" * - -

.

RR

••••

(

J

1

1

1

1

1

T

Ce
N d S m Gd Dy
Er
La P r P m
Eu
Tb Ho T m


17

Thuc chat, AR^- giam khi di tii dau day tod cuoi day dai hiem la do su tang khoang each
giua hai Idfp v6 dien tu 4f va 5d. Neu gpi r.^ la ban kfnh ion dai hiem, hien tupng co
lantan dan den r^^ giam ca 4% con r4f lai giam l6i 40% theo day tu La tai Lu [25, 57].
(iii). Cudng dd tuang tac Af^jphu thupc ban chat thanhphin kim loai T
So sanh cac day hpp chat dien hinh vai Fe, Co, Ni co the thay ngay qui luat nay
(xem hinh 1.8). Nguyen nhan cua hien lupng co the hieu do ban chai cua su tach miic
khac nhau ciia vijng 3d co lien quan tai chfnh dac diem ciia su phan bo mat dp trang
thai 3d quanh miic Fermi gay nen. Nhu phan tfch a cac phan tren da da cap, hau qua
do cac dac diem tach miic viing 3d chi phoi se din den nhung hien lupng kha phiic tap
cua su thiei lap tii tfnh chai ran, doi hoi cac quan sat, phan tfch chi tiei cho m6i he hop

chat cu the.
1.5 M 6 hinh ho^ tri tii.
Qui luat bien doi tfnh sat tii xac dinh theo su phu thupc cua gia tri momen tii hop
chat vao so dien tu trung binh tfnh tren mot nguyen tir da dupe the hien qua bieu didn
Slater-Pauling (hinh 1.11). Dinh ciia dudng Slater-Pauling (ung voi 2,5 16 trong/l
nguyen tu) the hien tfnh sat tii cua Fe va mot vai hpp kim tren x\ix\ sat v6i momen tii
gan bao hoa. Noi chung, tfnh sat lir ciia cac hop kim giiia kim loai T(Fe, Co, Ni) v6i
cac kim loai chuyen tiep khac ve ban chai co the' dupe xac dinh theo each danh gia su
khac nhau vi dp am dien giiia nguyen to kim loai chii vdi nguyen to kim loai hoa tan
vao no [18, 36, 70, 98,153]. Theo Friedel, khi mot nguyen l u c o nguyen tu s o Z , dupe
pha loang vao kim loai nen co nguyen tii so X^ thi dien tfch hat nhan du ra vai tri so
Zs - Z^= AZIel se lam cho cac dien tu linh dpng se tro nen dinh xii cho den khi lupng
dien tfch du nay lam cho su che chan bai man chan tinh dien dii thoa man linh huong
mdi lai vCing xung quanh vi trf ion bi thay the cua mang kim loai (do vi trf nay co
dien tfch hat nhan bi thay ddi khi mot nguyen ttr khac loai da dupe thay vao). Cong
ihiic xac dinh momen lir trung binh cho hop kim v(s\ nong dp pha tap c la:
M,b = M , , , - A Z c ^ B

(1-1)

V6i c6ng thiic nay co the nhan dupe cac kei qua tfnh momen tii phii hpp vdi kei qua
do nhieu xa natron (vf du v6i trucmg hpp Ni pha tap Fe). Khi tfnh cho hpp kim NiCr
va NiV thay cac gia tri nhan dupe lech ra ngoai nhanh chfnh cua duang cong [36].

TRlJNGTAMTHGNGTlN-'^i-;ij\-r:jl

v^.\J.'UlM\


18

ã Ft-V
0 Fe-NiO)

ã rô-ce
ONi-Co
dNi-Cu
ã Ni-Zn
0 Ni-Ct
tJ Ni-Mn
ACo-Cr
OCô-Mn
I>MNiằCo-V
OjoNiSOCo-Cr
0 Fe-NằU)
V INirc TDtUli

rc
Co
Ni
26
27
28
So dien tu trung binh/ Inguycn tiir

Hinh LU Biau di6n Slater-Pauling (din tij [36])

3.0
o

Fe-V
Fe-Ni
& Fe-Co
+ Ni-Co
X
Ni-Cu
• Ni-Zn
a Ni-V
•
Ni-Cr
o Co-Mn
• Co-Cr
•
Co-Ni-V
» Co-Ni-Cr
Fe-Pd
& Ni-AI
A
Ni-Sn
• Fe-Pt
A

:?j

2.5

2.0
3

1.5

•••

1.0

0.5

0.0

-1.0

0.5

0.0

0.5

1.0

1.5

2.0

Hoa tri tir trung binh (Zm)

Hinh 1.12 Bieu diln hoa trj tij*cLia cac hop kim goc Fe, Co, Ni (din tir [154J)


19

So sanh v6i thuc nghiem co the thay sai lech momen tij a day deu xay ra tai cac
vi trf Ni. Nguyen nhan la do cac kim loai nhe (LT) pha lap vao da lam giam momen
tu ciia cac nguyen tu thupc mang kim loai ehu xung quanh no. Ve dieu nay, Friedel
da cho rang do co cac dien tfch hat nhan am Ion (AZ < 0) cac miic nang lupng cua Cr,
V bi nhiau loan manh den miic vupt qua miic Fermi, cac dien lir spin s | (voi spin am )
se tham gia chiem cac miic trong phfa tren ciia dai 3d. Do di chuyen nay, cac dien tu
3d(LT) da chuyen tix spin-st thanh spin-si. Momen tij trung binh phai tfnh lai la[36]:
M , , = M,,„-(AZ+10)c^B.

(1.2)

Theo each phan tfch tir quan sat ciia Friedel, cac dien lir 3d(LT) da gay nen su
bien doi phan bo mat dp spin lai viing 3d(HT) lam thay doi gia tri momen tii" 3d cua
mang kim loai chii. Tang nong dp dien lir 3d(LT) se lam giam hon niJa momen tiJ:
hpp kim.
Ta da biet, tfnh chat tii" ciia hpp chat Tj.yAy giiia kim koai T(Fe, Co, Ni) voi cac
nguyen to kim loai khac (A) cung chii yeu dupe xac dinh bai su khac nhau ve dp am
dien giiia nguyen tii cua pha nin (T) va nguyen lijr thay the (A) [73], Vi vay mot each
tu nhien, co the mo ta tfnh chat tii ciia chiing cung b^ng mo hinh vira mo ta a tren.
Theo each da de cap, khi mot nguyen tii thay the dupe dua them vao pha nen, dien
tfch hat nhan du AZ se lam chuyen dich cac dien tfch linh dpng a gan muc Fermi cho
den khi cac man chan iTnh dien sinh ra bai cac dien tfch bi dich chuyen can bang
dupe v6i dien tfch hat nhan. Hieu ling nay chi xay ra xung quanh nguyen tir thay the.
Trong hop kim ciia cac kim loai sat liJ manh (vf du Co) mat dp trang thai a miic
Fermi cua phan viing spin-st la rat nho, nen man chan hau nhu dupe tao nen bai cac
dien tu 3d co spin-s|. Do do momen lijr ciia hpp kim dupe xac dinh nhu sau:
M=

MT-ZAMB

(1-3)

V6i M-y la momen tii" nguyen tu ciia pha nen con Z^ la hoa tri hoa hpc ciia nguyen to
pha tap. Doi v6i kim loai Fe ihi ngupc lai, miic Fermi cua phan viing spin-s; nam a
cue lieu cua dubng cong mat dp trang thai va cac dien tir linh dpng a miic Fermi chii
yeu la cac dien lir spin-st. Liic nay, mot each gan diing momen ti:r cua hpp kim la :
M =

M T + Z A I^B

(i-4)

Chii y rang cac thao luan a day chi ap dung trong gidi han cua phep gan diing vung


20

cung tiic la cau true vung nang luang ciia hpp kim duac gia thiet la khong thay dSi va
chi CO cac miic nang luang ciia cac thanh phin trong hap kim bi dich chuyen di so
vai trong kim loai. Tfnh chat trung binh cua hpp kim co thi thao luan dupe ngay ca
khi mat dp trang thai tai miic Fermi co cau triic dac biet. Khi cac dien tu dupe cung
cap them tii phfa cac nguyen to thay the se lap day dan vao viing 3d ciia kim loai ehu,
lam tang mat dp trang thai ciia chiing 6 miic Fermi, hoac theo each tuang tu, chiing se
lam cho vijng 3d(T) bat bi lap day (khi Z < 0). Do do momen tu' trung binh tren mot
nguyen tu trong hop chat Ti.^A^ la:
M,b =Uj+ayZ^\ji^

(1.5)

Voi y la nong dp cac nguyen tu pha tap, a = -1 voi Co va Ni, a = 1 vai Fe.

Mo hinh nay mo ta dupe cac thupc tfnh co ban ciia duang cong Slater-Pauling. So
sanh vai each quan sat tren cac hpp kim cua Co va Ni voi cac nguyen to kim loai
chuyen tiep dau day, theo giai thfeh cua Friedel, co the hieu cac nguyen tu A da sinh
ra nhxiu loan lam tach 5 trang thai lien kei a du6i miic Fermi ciia niia vijng 3d da lap
day, tiic 5 trang thai spin-st, de them vao 5 trang thai spin-sj cua chat pha tap a tren
miic Fermi. Khi do 5 dien lii tuang ling se dupe dien vao phan viing spin-st va gia trj
momen tir trung binh se mo ta tot kei qua thuc nghiem theo bieu thiic sau day:
M„ = MT-(Z^+10)y^B

(1-6)

Voi cac hop chat dat hiem kim loai chuyen tiep R-T, qui luat thuc nghiem da thay
la momen tiJ" phan mang T giam khi nong dp dat hiem tang. Hieu ling nay co the dupe
giai thfeh theo each xem cac nguyen to R vai cau hinh dien tu lop ngoai ciing: 5d'6s'
( vdi Y la 4d'5s") nhu cac kim loai chuyen tiep dau day (theo mo hinh Campbell).
Nhu vay, su thay doi momen lir trong hpp chat R-T do su thay doi nong dp cac dien
tii d da CO the dupe nhat quan trong ciing mot each phan tfch tuong tu nhu nhiing mo
ta a tren cho cac hop kim Ti.^A^ ciia kim loai 3d dau day vai cac kim loai nang
Fe(Co,Ni). Tuy nhien trong mo hinh hoa tri tii, mot uu viet r5 rang la: cac khiic mac
khi ly giai cac hien tupng tii tuong ting vai nhanh phai ciia duang Slater-Pauling da
dupe giai quyei tren ca so xet sir viec theo each phan tfch cac trang thai lai 3d(LT)3d(HT) nhu da lam khi xem xet hien tupng lai hoa 3d-5d. Cach xem xet nay the' hien
mot quan niem thong nhat khi danh gia nguyen nhan bien doi tij tfnh trong cac hpp
kim tu R-T la do vai tro ciia cac dien lu d.


21

D6 am dien Z ciia cac nguyen to da't hiem so voi cac kim loai Fe, Co, Ni tuong
ung la -5, -6 va -7. Trong cong thirc hpp kim T|.^,Ay da ham chua hien tupng cac dien
tii 5d(4d) se chuyen dich vao vung 3d (xem hinh 1.3 va hinh 1.4). Khi nay momen tiif

cung se dupe xac dinh theo each nhu tren ma khong can thiet phai quan tam den cac
CO che chi tiet cua qua trinh.
Nhieu loan do nguyen tii R dat len phan mang T cung rat manh. He thuc (1.6)
van apdung dupe de xac dinh momen tii M^. Theo William va d6ng nghiep[153]:
M-r = 110-N, + 2 N , J ^ B

(1.7)

v6i Nj la so dien tir hoa tri cua mang T va N.^ la so trang thai spin dien tu dan ung voi
miic s va p. Do do momen tii trung binh cua hpp chat co thi dupe xac djnh theo:

hoac:
6'day

M „ = [ 1 0 ( l - y ) - N , - y Z , + 2N,p]^B

(1-8)

M„ = [ Z„ + 2 N , J ^i^

(1.9)

Z,,^ = lO(l-y) - N^j-yZ[^

dupe dinh nghTa la hoa tri tii"ciia hpp chat R^Ti.,

Chii y rang vai viec ap dung khai niem hoa tri tii', phan anh huong ciia cac dien tii
sp vao gia tri momen lu' da dupe tach ra nhu la he so ciia phuang trinh duang thang.
NghTa la a day cac dien tu sp da dong gop vao momen tu mot lupng khong doi (dieu
nay phii hop vai quan niem cho rang cac bien dpng gia tri momen tii ciia hop chat ehu

yeu do vai tro ciia cac dien tii dan lap d). Viec tach anh huong ciia cac dien tu s va p
CO the da lam ro net hon qua trinh thay d6i tfnh chat tii" ciia he khao sat. Gfa tri N can
lira chpn tuy theo tfnh chat ciia cac dien tu tham gia vimg dan cua he. Thong thuang
thi N,,, - 0,3 - 0.45 (xem |38, 154]).
Luu y rang trong hpp chat bj pha loang theo each RyT|.y, N,i = (I-y) Zp voi Zj la
hoii tri hoa hpc ciia kim loai T, do do
Z,,= 1 0 ( l - y ) - ( l - y ) Z , - y Z Y

(I.IO)

Truong hop khao sat cac hpp chat ba nguyen, cong ihiic hpp chat ba nguyen dupe
di6n dat theo mau R^ T[.y A^

(vai

y = y ' + y " ) , khi do

Z„=10(l-y)-(l-y)Z,-y'ZR-y"ZJ

(1.11)

Nhu vay vdi khai niem hoa tri tu; dudng Pauling-Slater co the' duac thay bing mot
dudng thing, mo ta mot each tong quat han su bien doi momen tu 3d cua nguyen to
kim loai chuyen tiep co tu tfnh (Fe, Co, Ni) trong cac hap kim cua chiing vdi cac
nguyen to thay thi'la cac kim loai chuyen tiep, kim loai dat hiem hay cac a kim (xem