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Submitted on 1 Jan 1978
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MAGNETIC SUSCEPTIBILITY OF METASTABLE
AuCo SOLID SOLUTIONS
D. Korn, D. Schilling, G. Zibold
To cite this version:
D. Korn, D. Schilling, G. Zibold.
MAGNETIC SUSCEPTIBILITY OF METASTABLE
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-899
MAGNETIC SUSCEPTIBILITY OF METASTABLE AuCo SOLID SOLUTIONS
D. Korn, D. S c h i l l i n g and G. Zibold
Faahbeveioh Physik, Universitat Konstanz, Postfaeh 7733, D-7750 Konstcmz, Germany
Résumé.- La susceptibilité initiale des alliages artificiels de AuCo est mesurée dans les champs magnétiques alternatifs avec une fréquence variant de 12 à 5500 Hz. Le maximum de la susceptibilité X(T) se déplace linéairement avec la concentration de Co. De plus, un déplacement du maximum avec la fréquence du champ magnétique existe suivant la loi d'Arrhenius.
Abstract.- The initial susceptibility of artificially produced AuCo alloys (4 at% to 23 at%Co) is measured in ac magnetic fields with frequencies varying between 12 and 5500 Hz. As a function of
temperature the susceptibility shows a smooth maximum at a temperature T_. T, increases with the frequency of the magnetic field obeying Arrhenius law.
AuCo solid solutions are obtained from the vapour phase by quench condensation onto a sapphire
substrate held at 4 K. Fractionation and diffusion are avoided by the production procedure. The sta-tistical distribution of the Co atoms as present in the vapour phase is spatially frozen in the conden-sed matter. The magnetic susceptibility of the films is measured in situ with a magnetic ac field of 22 G and 1100 Hz if no otherwise stated. (Magnetic fields are parallel to the plane of the film.) Figure 1 shows a representative example. The initial
suscep-$
I'O-
5^] Au*12at%Co
;-\ . 22G-15 " ; '•• '22G-*1UG= * 1 0" '\ T0=295K \.T„ = 80K "••. . " " * • • • • . . 0" 1 1 1 0 10 20 30K TemperatureFig. 1 : Mass susceptibility of a AuCo film as a function of temperature after annealing at T . (The ac field of 22 G has a fre-quency of 1100 s-1)
tibility x/p as a function of temperature T is the same for quench condensed films and those annealed at T = 8 0 K. After annealing at 295 K the suscep-tibility has higher values and the maximum of the susceptibility lies at a higher temperature. This change in x(T) is due to a decrease of lattice di-sorder and, or together with, a rearrangement of the magnetic atoms. A deviation from statistic distri-bution of the Co atoms with respect to the lattice sites must be taken into account for T = 295 K.
a
Annealing to 600 K causes strong diffusion and hence precipitation of ferromagnetic Co together with a total change in x(T).
The susceptibility maximum of quench conden-sed films has a natural smoothness (dx/d't continuous). Figure 1 shows the depression of the maximum of the susceptibility when a dc field is applied. Films annealed at 80 K do not have this maximum decrease except Au + 23 at%Co. That means the decrease of the maximum of the susceptibility is due to special clusters existing either above the percolation limit or in annealed films or bulk alloys below the per-colation limit (compare also G. Zibold about AuFe films).
At temperatures above the maximum of the susceptibility the Curie-Weiss law can be applied. The magnetic moments of the 80 K annealed films obey the formulas given by Boucai et al. /I/ for a sta-tistical distribution of the Co atoms on the lattice sites.
In figure 2 is plotted the temperature T , of the susceptibility maximum as a function of Co
Fig. 2 : Temperature Tf of t h e maximum of t h e i n i t i a l 0.04 0.045 0.05
L[K-']
s u s c e p t i b i l i t y of &Co f i l m s taken f o r twoT
d i f f e r e n t t e m p e r a t u r e s T of a n n e a l i n g . (Magnetic a c f i e l d 22 G i n d frequency 1100 s - I ) F i g . 3 : l o g a r i t h m of t h e frequency v of t h e magne- t i c a c f i e l d a g a i n s t t h e i n v e r s e tempera- Inv 1000 Hz 100 10 c o n c e n t r a t i o n . T i n c r e a s e s l i n e a r l y w i t h Co concen- f --
- t r a t i o n c b u t Tf i s n o t p r o p o r t i o n a l t o c e s p e c i a l l y i n t h e c a s e when t h e f i l m s a r e annealed a t 80 K. T h i s behaviour i s d i f f e r e n t from t h a t of correspon- d i n g AgMn f i l m s1 2 1 .
The e f f e c t o f a n n e a l i n g w i t ht u r e Tf of the-maximum of t h e i n i t i a i s u s c e p t i b i l i t y
References
r e s p e c t t o Tf d e c r e a s e s w i t h r i s i n g Co concentration / I / Boucai, E . , Lecoanet, B . , ~ i l o n , J . , Tholence, J . L . and T o u r n i e r , R., Phys. Rev. and no e f f e c t i s observed f o r Au
+
23 at%Co (Fig. 2).(1971) 3834
/ 2 / Korn, D., Z . Phys.
214
(1968) 136 T of quench condensed AuCo f i l m s depends on ft h e frequency of t h e magnetic f i e l d . The r e l a t i v e s h i f t ATf/Tf i s n e a r l y independent of concentration. F i g u r e 3 shows t h a t Arrhenius law v = vo e x p ( - ~ ~ / ~ ~ ) h o l d s f o r f i l m s annealed a t 80 K. The a c t i v a t i o n
energy corresponds t o TA = 650 K f o r Au + 23 at%Co and f a l l s w i t h d e c r e a s i n g Co c o n c e n t r a t i o n .
With T depending on t h e time of measurement f a n analogy t o a fundamental p r o p e r t y of g l a s s i s found. However i t should be remarked t h a t & C o i s
