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ORIGIN OF HYPERFINE FIELDS ACTING ON 155Gd IN (Gd1-xRx) (Co1-yMy)5 (R = RARE
EARTH, M = Cu, Ni, Pt)
M. Seh, S. Cohen, I. Nowik
To cite this version:
M. Seh, S. Cohen, I. Nowik. ORIGIN OF HYPERFINE FIELDS ACTING ON 155Gd IN (Gd1-xRx)
(Co1-yMy)5 (R = RARE EARTH, M = Cu, Ni, Pt). Journal de Physique Colloques, 1980, 41 (C1),
pp.C1-201-C1-202. �10.1051/jphyscol:1980162�. �jpa-00219731�
JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n
"
1, Tome 41, janvier 1980, page C1-201ORIGIN OF HYPERFINE FIELDS ACTING ON 155 Cul IN (~d~-#,) (co~-$$)~ (R = RARE EARTH, M = Cut Ni, ~ t )
M. Seh, S. Cohen and I . Nowik
Racah I n s t i i x t e of Physics, The Hebrew University, JerusaZem, I s r a e l .
The study of hyperfine f i e l d s acting on 'loss- ' bauer probes in magnetic materials may yield useful information on the magnetic s t r u c t u r e , local magne- tism, conduction electron polarization and exchange interactions. As a f i r s t s t e p one has t o under- stand the various origins of the hyperfine f i e l d acting on the "lossbauer probe. Here we present studies of the hard magnetic materials ?Cog where we have chosen f o r a probe the isotope Gd155. Gd3+
i s a S s t a t e ion (6S7/2), i t s hyperfine f i e l d i s small, contributed by the core polarization mechan- ism (-340kOe). This enables a detailed study of the origins of other contributions t o the hyperfine f i e l d , in a s i m i l a r manner as has previously been done with E U ~ ' i n europium intermetall i c s [I].
Since E U ~ + i s equivalent t o Gd3' a l l the discussion given i n [ I ] f o r E U ~ ' i s also relevent f o r Gd3'.
The hyperfine f i e l d acting on the Gd nuclei i n (Gdl -xR,) (Co1-yEly)5 should be composed of four con-
t r i buti ons Q Co
Heft = Hcore + Ho + Hn' + Hn Hcore i s the core polarization, Ho i s the contribu- t i o n of t h e conduction electrons polarized by the Gd ion i t s e l f , H~~ and H,'O are the contributions of the conduction electrons polarized by the r a r e earth and cobalt s u b l a t t i c e s respectively. l~le were able t o determine the various contributions t o Heff through !lossbauer studies of the 86.5 key t r a n s i t i o n of ~ d ' ~ ~ .
The samples were prepared by melting the ap- propiate amounts of the elements i n an induction furnace. They were studied by X rays and were found t o be of single phase, t h e i r l a t t i c e parame- t e r s are given in Table 1 . Some of the obtained experimental Vossbauer spectra a r e shown in f i g . 1 . The detailed analysis of the spectra, by diagnoli- zation of the f u l l Hyperfine Hamil tonian and a l e a s t squares f i t procedure leads t o the parame- t e r s given i n Table 1 . One observes t h a t i n addi-
t i o n t o t h e isomer s h i f t , quadrupole interact'ion and byperfine f i e l d , one i s able t o determine a l s o
e ,
the anole between Heff and the c-axis. Observingthe data i n Table 1 ?re notice; t h e e l e c t r i c f i e l d gradient acting on t h e Gd nuclei i s almost the same in a l l samples. This tends t o indicate t h a t I1 con-
MC0.C"
PdCO,C">
Pdco2cu.
YCOCU.
W" 5
a ,La,,Ca5 Sd .U ,C03 Ed .La gco5 'id ,P? qCOI '4 ,va $0, Y ,a .co5 Pd Tb..Co, ,d 0s ?C.?, G3 , L . l c ~ , Ld ,EV >CO$
'dY1.
bdPK
- . - -. .- -
-
0 '198 -
S W 6 -
J 394 J 992 -
o m -
Fig. 1 Spectra of ~d~~~ .in a t 4.1
K.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980162
JOURNAL DE PHYSIQUE
Fig. 2 F i e l d s a c t i n g on ~ d 1 5 5 i n
t r i b u t e s very l i t t l e t o t h e e l e c t r i c f i e l d g r a d i e n t a t t h e Gd s i t e . This. conclusion i s confirmed by a p o i n t charge c a l c u l a t i o n which agrees w i t h t h e ex- perimental r e s u l t s i f i t i s assumed t h a t t h e M i o n i s n e u t r a l (Table 1). The o r i g i n s o f t h e magnetic h y p e r f i n e f i e l d can be found by p l o t t i n g Heff as a f u n c t i o n o f x f o r Gdl-xLaxCo5 and as a f u n c t i o n of y f o r ,Gd(Col ,yCuy)5 ( f i g . 2). I f we assume t h a t t h e c o n t r i b u t i o n s o f magnetic neighbors a r e addi- t i v e , then t h e average h y p e r f i n e f i e l d should be l i n e a r i n x and y . The dependence o f Heff on x and y can be expressed as;
(1) Heff = (Hcore+Ho)
+
HnCO(l -y) + H ~ ~ ~ ( ~ - X ) + H , * X R The slopes o f t h e two l i n e s shown i n f i g . 2 y i e l d t h e values H: = 220+5 kOe and H:~ = 7 0 t 5 kOe.Since Hcore i s about -340 k0e we o b t a i n f o r Ho t h e small value o f s l O kOe. I f we assume t h a t a l l pa- rameters
in
eq (1) a r e independent o f magnetization o r i e n t a t i o n and t h a t Ho, H? and H~~~ i n(Gd.lR.g)Co5 are t h e same as i n
(Gdl .x~ax) (Col -$uy)5 then H: i s given by;
(2) H ~ R =
+[kff -
( H ~ ~ ~ ~ + H ~ + H : o + O.IH;~)II .9.
The p o s i t i v e s i g n i s f o r t h e heavy r a r e e a r t h s whose moments a r e p a r a l l e l t o t h a t o f Gd, t h e nega- t i v e s i g n i s f o r t h e l i g h t r a r e earths whose mo- ments coupled t o t h e Co s u b l a t t i c e ferromagneti- c a l l y a r e a n t i p a r a l l e l t o t h e Gd ioment. I n f i g . 3 we show t h e experimental values o f Hn a g a i n s t R
R
and t h e expected values I f we assume t h a t H: i s
F i g . 3 The v a r i a t i o n o f
HZ
i n Gd.lR+9C05.p e ~ i m e n t a l and simpit? t h e o r j l r k s u l t s ; This may i n - d i c a t e t h e presence o f s t r o n g o r b i t a l e f f e c t s [23.
B u t even i f these are considered, i t i s s t i l l d i f - f i c u l t t o r j b t a i n f u l l agreement between t h e o r y and experiment. T h i s may i n d i c a t e t h a t our assumptions about t h e s t a b i l i t y o f Ho, H$O and H:~ under change o f magnetization o r i e n t a t i o n and exchange o f Gd by R a r e n o t completely j u s t i f i e d . A l l parameters may depend on o r i e n t a t i o n o f magnetization as i n t h e case o f Gd metal i t s e l f 131.
Our experimental r e s u l t s l e a d us t o t h e f o l - l o w i n g conclusions; The c o n t r i b u t i o n s t o t h e hyper- f i n e f i e l d a c t i n g on Gd i n (Gdl-xRx)(Col-yMy)B a r e a d d i t i v e and f o l l o w eq. 1. The h y p e r f i n e f i e l d and e l e c t r i c f i e l d g r a d i e n t a r e l i t t l e a f f e c t e d by t h e n a t u r e of t h e Y component. The h y p e r f i n e f i e l d pa- rameters a r e s e n s i t i v e t o magnetization o r i e n t a t i o n L31 and p o s s i b l y t o o r b i t a l e f f e c t s [2],
References
1. Dunlap, B.D., Nowik, I. and Wernick,J.H., Phys.
Rev. @ (1973) 238.
2. Dunlap, B.D., Nowik, I. and Levy, P.!I., Phys.
Rev. (1973) 4232.
3. Baumi nger, E. R.
,
Diamant, A., Fel ner, I.,
Nowi k,I. and Ofer, S., Phys. Rev. L e t t e r s
34
(1974) 962.l i n e a r i n t h e expectations value o f t h e r a r e e a r t h s p i n [2]. We observe no agreement between t h e ex-
