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Submitted on 1 Jan 1978
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LOW TEMPERATURE BEHAVIOR OF PdCo
ALLOYS
L. Moberly, O. Symko, G. Williams
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-870
LOW TEMPERATURE BEHAVIOR OF PdCo ALLOYS+
L.A. Moberly, O.G. Symko and G. Williams
Dept. of Physios, University of Utah, Salt Lake City, Utah 84112 , U.S.A. ,mDept. of Physios, University of Manitoba, Winnepeg, Canada
Résumé.- Des mesures d'aimantation sur des alliages Pd-Co montrent que lorsque la concen-tration du cobalt est réduite, l'ordre dans ces systèmes change du type ferromagnétique vers le type d'impureté isolé. Un tel régime peut s'interpréter grâce à la structure fine.
Abstract.- Magnetization measurements of dilute PdCo alloys show that as the concentration of Co impurities is reduced, the ordering changes from ferromagnetic character to single impurity behavior. Such behavior can be attributed to fine structure splitting.
It is known that Co impurities in a Pd host produce giant moments as a result of polari-zation of Pd atoms arising from a ferromagnetic coupling between the impurity and the d-band. Such coupling causes the overlapping polarization clouds centered on Co atoms to exhibit ferromagnetic or-dering. Many of the properties of this system, in particular the resistivity and nuclear orientation show anomalous features. Resistivity measurements of Pd Co alloys IM containing 500 - 2000 ppm of Co revealed a rapid decrease in incremental resisti-vity with decreasing temperature between 1.5 K and
10 K. Over this limited temperature range, the in-cremental resistivity varied approximately loga-rithmically with temperature prompting an inter-pretation of the data in terms of localized spin fluctuations or Kondo scattering. Nuclear orienta-tion studies HI have also been interpreted in terms of a possible Kondo-like state at low tempe-ratures. To investigate these effects, we have measured the magnetization of such alloys over a wide range of Co concentrations down to a tempera-ture of 10 mK;
The samples, ranging in concentration from 10.4 p.p.m. to 500 p.p.m., were mounted inside the mixing chamber of a 3He-'*He dilution refrigerator
and their magnetization was measured in a constant field of 10 Oe with a SQUID magnetometer. Tempera-ture was measured with a CMN thermometer. For the most dilute alloy, the impurity contribution was obtained by subtracting the alloy signal from a pure Pd sample with an astatic coil arrangement.
tSupported by NSF Grant No.DMR 76-21702-AOl
Figure 1 shows the magnetization data per Co impurity as a function of inverse temperature. At high temperatures, the magnetization of each sample follows a Curie-Weiss behavior; the Curie Weiss temperature 8 varies from 9 = + 300 mK + 50 mK for the 500 p.p.m. sample to 6 = 0 + 30 mK for
the 10.4 p.p.m. sample. The main error in 6 arises from the small range of high temperature data. The high temperature data implies ferromagnetic orde-ring for all samples, except perhaps the most dilute. The low temperature data indeed show such behavior.
The effective moment from high temperature Curie-Weiss behavior is 6.9u based on the nominal
p
250 ppm concentration. (The 500 ppm data was not taken to high enough temperatures to infer a Curie constant). A Brillouin function for this moment would give an indication of free spin beha-vior. The most dilute sample magnetization lies slightly below the Brillouin curve while the more concentrated ones are above it. The results indi-cate that as the ferromagnetic ordering is reduced by decreasing the concentration some other interac-tion becomes apparent; possibly single-impurity behavior is approached and it does not have to be Kondo behavior HI.
Although our data for the most dilute sam-ple does not necessarily represent a single impu-rity effect, it is important to look at the implications of such behavior. It is quite possi-ble that the 10.4 ppm sample still has RKKY inte-ractions present and that the single impurity limit has not yet been reached. Our data are con-sistent with the resistivity data where the possi-bility of a Kondo state is discussed.
F i g . 1 : The m a g n e t i z a t i o n p e r i m p u r i t y of Co impu- r i t i e s i n Pd i n an a p p l i e d f i e l d of 10 Oe a s a f u n c t i o n of I / T . ~ o m i n a l sample c o n c e n t r a t i o n s were used f o r t h e two concentrated samples. The two most d i l u t e c o n c e n t r a t i o n s were determined r e l a t i v e t o 240 ppm by s c a l i n g high temperature C u r i e c o n s t a n t s . The s o l i d l i n e r e p r e s e n t s t h e m a g n e t i z a t i o n p e r im- p u r i t y of a " f r e e s p i n " B r i l l o u i n f u n c t i o n w i t h g = 2, S = 3 ( g i v i n g a n e f f e c t i v e moment of 6 . 9 ~ ) . T h i s l i n e f i t s t h e d a t a i n t h e h i g h t e m p e r a t u r e B l i m i t .
Kondo s c a t t e r i n g would r e q u i r e a p o s i t i v e exchange coupling J ( o r p o s s i b l y a n e g a t i v e J w i t h s i z e a b l e p o t e n t i a l s c a t t e r i n g ) . Localized s p i n f l u c t u a t i o n s s c a t t e r i n g would r e q u i r e t h e corresponding charac- t e r i s t i c temperature t o be w e l l below 1K which appears i n c o n s i s t e n t w i t h t h e magnitude of t h e u s u a l parameters used t o s p e c i f y t h e r e g i o n of a p p l i c a b i l i t y of such an approach. The p r e s e n t ma- g n e t i z a t i o n d a t a and t h e e x i s t i n g r e s i s t i v i t y d a t a n e i t h e r r u l e o u t ~ o s s i b l e c r y s t a l f i e l d e f f e c t s . Unlike t h e Kondo e f f e c t , t h e s e m a n i f e s t themselves / 3 / i n a f i r s t Born approximation f o r r e s i s t i v i t y v i a a r e d u c t i o n i n t h e spin-disorder f a c t o r J ~ S
(S+1) through t h e removal of degeneracy (enhanced s c a t t e r i n g channels) a s t h e c r y s t a l - f i e l d s p l i t e i g e n s t a t e s become t h e r m a l l y depopulated w i t h de-
s c a l e involved i n b o t h m a g n e t i z a t i o n and r e s i s t i - v i t y s u g g e s t a r a t h e r small s c a l e f o r such s p l i t - t i n g s . T h i s would appear t o r u l e o u t 1st o r d e r o r b i t a l e f f e c t s and s u g g e s t s some h i g h e r o r d e r s p l i t t i n g s of an e s s e n t i a l l y s p i n o n l y f i n e s t r u c - t u r e ground s t a t e . Assuming Co i o n s t o be i n a 3d7 c o n f i g u r a t i o n / 4 / and i f t h e s i g n of t h e 4 t h o r d e r c r y s t a l f i e l d term i s n e g a t i v e (C,<O), t h e n a
r2
o r b i t a l s i n g l e t r e s u l t s l e a v i n g a spin-only C.F.ground s t a t e t o 1 s t o r d e r (a p o s i t i v e C 4 term would y i e l d a
r,
o r b i t a l t r i p l e t a s t h e lowest s t a t e w i t h 1 s t o r d e r s p i n - o r b i t s p l i t t i n g which would b e too l a r g e f o r our r e s u l t s ) . The measured Curie-Weiss temperature of 0 a 0 K i s c o n s i s t e n t w i t h t h e expec- t e d behavior of f i n e s t r u c t u r e s p l i t t i n g f o r a c u b i c c r y s t a l f i e l d . Such an i o n i c approach i n me- t a l s i s gaining t h e o r e t i c a l and experimental sup- p o r t . Au-Co / 5 / and Zn-Cr / 6 / a l l o y s t u d i e s show t h a t an i o n i c model h a s some v a l i d i t y which may be a p p l i c a b l e t o our r e s u l t s .Although our d a t a does n o t p r e c l u d e t h e pos- s i b i l i t y of a Kondo s t a t e a t v e r y low temperatures, t h e approach t o a s i n g l e - i m p u r i t y behavior i s s u g g e s t i v e of f o u r t h o r d e r f i n e s t r u c t u r e s p l i t t i n g .
References
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