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KONDO EFFECT AND RKKY COUPLING IN
PdHx(α’) Fe
R. Brand, H. Georges-Gibert, C. Kovacic
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-862
KONDO EFFECT AND RKKY COUPLING IN PdH («') F
eX
R.A. Brand, H. Georges-Gibert and C. Kovacic
Labovatoire de Physique du Solids, Vniversite de Nanay - I
Case Offiaielle n° 140, 54037 Nanoy Cedex, France .
Résumé.- Le comportement du fer dans Pd H (a1) a été étudié par effet Mossbauer sous champ magnétique. Pour une faible concentration en fer, l'effet Kondo et l'effet d'in-teraction entre impuretés ont pu être séparés. Le moment magnétique reste constant dans un grand domaine de compositions, tandis que la température Kondo croît avec la
concen-tration en hydrogène.
Abstract. The Mossbauer effect in magnetic field is used to investigate the behaviour of Fe in Pd Hx (a'). It is shown that the single moment Kondo and impurity interaction
effects can be separated. It is found that the magnetic moment remains constant for 0.53 < x < 0.7 but that the Kondo temperature increases with hydrogen content.
The existence of Kondo effects and spin glass properties in PdH M alloys (M = transition metal impurity) has been known for some time /1,2,3/. In PdH , the role of the hydrogen is to progressively fill the 4d band of Pd, raising the Fermi energy into the 5s band and yielding an alloy with many of the electronic properties of Ag /4/. At ambient temperature PdH in the range of H/M = x from about 0.01 to about 0.57 /4/ exists as a two phase mixture a + a'. (In the literature, a' is frequently denoted as 8 ) . It has been found that the properties of PdH (a) Fe are very similar to those of PdFe ; Kondo and spin glass effects have been detected in PdH (a') Fe 151.
The advantages of using hyperfine techni-ques to determine the Kondo temperature T are
two-Is.
fold. First we cati obtain T at experimental tempe-ratures T > T„ (T„ *1 K ) , Second we can separate
mes K K
the single impurity Kondo from impurity interaction effects.Since only Fe has a convenient Mossbauer isotope in the first transition metal series, we are here concerned with PdH (a') Feo#
Q03-The technique used for preparing the PdFe alloy (supplied by I.A. Campbell have been descri-bed elsewhere /6/. The electrolytic charging with hydrogen was done at room temperature for H/M < 0.7 and at dry ice temperatures for H/M>0.7 llI.
Spectra were taken in a Mossbauer cryos-tat fitted with a 60 kOe magnet. Typical results are shown in Figure 1 a where the solid lines are the calculated fits to variable-width Lorentzians. From the single line spectra at zero field we cal-culate the isomer shift. In magnetic field, the single peak is split into 4peaks,numbered 1,3,4,6
(peaks 2 and 5 are not observed : gamma ray direc-tion parallel to magnetic field). This splitting is proportional to the total magnetic field at the nucleus H . . A random component in H produces
mes mes the broadening observed in the outer peaks for the
spectra in magnetic field, the inner peaks remai-ning essentially unchanged. First we discuss the measured isomer shifts and compare these with pre-vious results. Then we discuss the analysis of the average measured field H in terms of a Kondo
mes
moment. Third we discuss calculations of impurity interactions which justify using the average H for the single moment.
- 5 VELOCITY 0 (mm/s j 5
^ J OkOe « • « V,
Fig. 1 : Mossbauer spectra for H/M = 0.53. For He x t = 0, velocity scale expanded by a factor of 3.
Extensive hyperfine measurements were made on the compositions H/M = 0.53 and 0.7. The res-pective isomer shifts (IS) were 0.38 and 0.41 mm/s at 4.2 K (with respect to Fe in metallic Fe at 300 K ) . IS(0.53) agrees with that for a 'm i n re-ported in /8/ for 2 % Fe; IS(0.7) indicates a' as IS increases with H/M due to lattice expansion.
The h i g h e s t HIM > 0.7 y i e l d e d two phase s p e c t r a a t 77 and 4.2 K w i t h IS(aVmax) = 0.62 and IS(f3) =
0.89 m / s a t 4.2 K. Some p r e l i m i n a r y h y p e r f i n e measurements were made on a composition (IS = 0.60) c l o s e t o t h a t of armax.
Hmes i s composed of s e v e r a l terms
HHF i s t h e h y p e r f i n e f i e l d produced by t h e l o c a l moment,
YIF
= Ho mZ (Hext), and i s t h e q u a n t i t y of i n t e r e s t . The h y p e r f i n e c o n s t a n t H i s n e g a t i v e (due t o c o r e ~ o l a r i z a t i o n / 9 / . The Van Vleck term%
i s t a k e n t o b e independent of temperature. It w i l l be shown t h a t $ = HW/Hext i s z e r o , s o t h a t -%F = H s a t + Hmes'We a n a l y s e
HHF
i n terms of a modified B r i l l o u i n f u n c t i o n := Hsat BS (XI w i t h
x =
usat
Hext/kB(T+@I
T h i s form h a s no t h e o r e t i c a l j u s t i f i c a t i o n b u t i s numerically a c c u r a t e f o r a Kondo system i n t h e range 4T < T < 100 TK i f 8 = 4 TK / l o / . The
K
s a t u r a t i o n h y p e r f i n e f i e l d H i s g i v e n by H
u
s a t o s a t
whereusat = psatpB i s t h e e f f e c t i v e moment. Figu- r e s 2 ( a ) and (b) shows
%Insat
a s a f u n c t i o n o fusat
Hext/kB(T
+8) f o r HIM = 0.53 and 0.7 respec- t i v e l y . The parameters H s a t , Psat.and 8 a r e o b t a i - ned 6y numerical i t e r a t i o n (assuming 13 = 0 ) s t a r - t i n g w i t h t h e h y p e r f i n e s u s c e p t i b i l i t yx
HF=a
HHF
/
a
Hext which i n t h e l i m i t of z e r onext
i s g i v e n by :'
B
s
+
I P s a t Hext XHF =-
kg-
3 s T + 8 The e x t r a p o l a t i o n l/xHF = 0 g i v e s a s shown i n t h e i n s e r t s t o f i g u r e s 2 ( a ) and (b). From f i t t i n g HHF/Hsat t o BS(x) we o b t a i n . S = 1 and psat. Any non zero f3 can be d e t e c t e d by e x t r a p o l a - t i n gxHF
t o I / (T+
8) = 0 , shown i n t h e second i n s e r t s . The i n t e r c e p txHF
= 0 shows t h a t t h e r e i s no temperature independent term : 6 = 0 . R e s u l t s a r e given i n t a b l e I. Table I Summary of h y p e r f i n e r e s u l t s i n Pd Hx (a')Fe0.003. & S=l r e p r e s e n t s experimental r e s u l t s b u t i s n o t t h e unique s o l u t i o n . These r e s u l t s i n d i c a t e t h a t t h e Fe g i a n t mo- ment i n Fe ( 1 2 . 6 . ~ ~ ) i s d r a m a t i c a l l y reduced i n Pd H x ( a t ) Fe. For H/M < 0.7, "at remains c o n s t a n t-
a t about 4.2
ug and Hsat
a t about-
295 kOe. The s i g n and magnitude of.Hsat i n d i c a t e a moment of s p i n o r i g i n . 8 i n c r e a s e s w i t h H/M, a s was s e e n f o r T from r e s i s t i v i t y measurements / I / , b u t n o t T K g of t h e s p i n g l a s s t r a n s i t i o n which d e c r e a s e s / 2 / . Thus we conclude t h a t 8 i s r e l a t e d t o TK: 6 = 4 T K' For H/M> 0.7, p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t l H s a t l d e c r e a s e s , s i m i l a r t o what i s observed a t h i g h e r c o n c e n t r a t i o n s of Fe i n t h e m a g n e t i c a l l y ordered phase 1 5 , 11/. T h i s could s i g n a l t h e e x i s - t e n c e of an o r b i t a l component191
t o t h e moment, b u t f u r t h e r work i s needed t o c l a r i f y t h i s p o i n t .Fig. 2a.
F i g . 2b.
F i g . 2 : The reduced %/Hsat a s a f u n c t i o n of lleffHext/kB(T + 8) :
4.2 K, A 7 K, 1 10 K , m15 K. BS (x) indica- t e d by s o l i d l i n e s : (a) H/M = 0.53, (b) H/M = 0.7 I n s e r t s , . s e e t e x t .
a v e r a g e d e f i n e s t h e b e h a v i o u r o f t h e l o c a l mo- ment i n Hext, b u t o n l y a r e l a x a t i o n time model c a n
R e f e r e n c e s
g i v e s u c h a d i f f e r e n c e i n enlargement between i n n e r / 1 / Mydosh, J.A., Phys. Rev. L e t t . ,
2
(1974) 1562. and o u t e r l i n e s . / 2 / Burger, J . P . , Mac-Lachlan, D.S., - . M a i l f e r t , R.,We i n t e r p r e t t h e l i n e b r o a d e n i n g a s b e i n g and S o u f f a c h e , B . . P r o c . o f t h e 1 4 t h I n t e r n a - t i o n a l Conference o n Low Temperature P h y s i c s due t o t h e
RKKY
c o u p l i n g between moments which l e a d s . - ( F i n l a n d ) , (North-Holland, Amsterdam) 1975, Vol. a t h i g h e r Fe c o n c e n t r a t i o n s t o t h e s p i n g l a s s t r a n s i - . - 3 , 278. t i o n . R e l a x a t i o n time e f f e c t s a r e t h u s s i m i l a r t o t h e r e l a x a t i o n e f f e c t s s e e n i n s p i n g l a s s e s . F u r t h e r c a l c u l a t i o n s w i l l b e p u b l i s h e d l a t e r . I n c o n c l u s i o n , i t s h o u l d b e n o t e d t h a t t h e g r e a t a d v a n t a g e o f h y p e r f i n e t e c h n i q u e s a p p l i e d t o d i l u t e m a g n e t i c and e s p e c i a l l y Kondo systems i s t h e p o s s i b i l i t y t o s e p a r a t e s i n g l e moment b e h a v i o u r from r e l a x a t i o n and i n t e r a c t i o n e f f e c t s .One of t h e a u t h o r s (R.A.B.) would l i k e t o
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54
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