CONCENTRATED SPIN GLASS SYSTEMS: A M O R P H O U S MnSi AND M n G e J. J. H A U S E R Bell Laboratories, Murray Hill, NJ 07974, USA
Both amorphous MnSi and MnGe display a spin glass transition respectively at 22 and 48 K as shown by a sharp cusp in the ac susceptibility. While the magnitude of the susceptibility and of its peak increases with increasing deposition temperature as a result of clustering, the spin glass transition remains unchanged.
While crystalline MnSi is a ferromagnet with a 30 K Curie temperature, amorphous MnSi (aMnSi) exhibits a spin glass transition with a spin glass temperature (Tsc) of (22 ___2) K [1]. The magnitude of the susceptibility cusp for the fully amorphous films increases with deposition temperature T D. This effect can be understood in terms of amorphous clusters of Mn atoms. The presence of these amorphous clusters was clearly established [1] by the susceptibility anomaly around 30 K and by the temperature dependence of the magnetic susceptibility [2]. Although the most random distribution of Mn atoms is achieved by deposition at 77 K and clustering increases with T D, Tsc remains fixed at (22 ___2) K. The susceptibility peak as well as its dependence on T D is shown in fig. 1 for two a-MnGe films. Similar to a-MnSi one finds that the film deposited at 77 K displays the smallest susceptibility peak. The increasing clustering with increasing T D is best measured by the ratio R = X(Tsc)/X(4.2 K); R increased from 2.2 to 4.3 for a-MnSi and from 4.3 to 27 for a-MnGe as T o was increased from 77 K to respectively 625 and 370 K. As with a-MnSi, Tso remains fixed at 48 K when T D is raised to 170 K (fig. 1) but it increases to 56 K (as determined by both ac and dc measurements) when To is raised to 370 K. The susceptibility measurements on a-MnSi were found [2] to be mostly consistent with the cluster mean field theory (CMFT) [3] which concludes that
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(1)
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Fig. 1. Susceptibility versus temperature showing the spin glass cusp at Ts6 for various deposition conditions.
dominant effect of M c. This view is supported by the measurements of the spin glass order parameter q(T) shown in fig. 2. The value of q(T) was obtained in a manner similar to that used on both a-MnSi [1] and a-GdA12 [4]. While the values of q(T) for a-MnSi were essentially independent of To, it is clear from fig. 2 that there is a definite increased deviation from mean field theory [5] with increasing TD for a-MnGe films. Furthermore, the values of q(T) show greatest deviations from mean field theory for a-MnGe than for a-MnSi. This is consistent with the greater degree of clustering present in a-MnGe and with the C M F T which suggests the greatest deviations from mean field for ferromagnetic clusters. Another typical behavior of a spin glass is the smearing of the susceptibility cusp by an externally applied dc field. The effect is best analyzed by plotting the field susceptibility XH[XH = X(Tsc, 0) -- X(TsG, H)] as a function of magnetic field. The interest of such a plot stems from the fact that the critical exponent 8 can be extracted from XH since as pointed out by Chalupa [6]. q ( T s c ) - - x . ( r s o , H ) ~ H 2/~.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1387-1388 ©North Holland
(2) 1387
J. J. Hauser/ Concentratedspin glass systems
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Fig. 2. Dependence of the spin glass order parameter q(T) on the reduced spin glass temperature for various a-MnGe films. Also shown as the solid line the mean-field theoretical prediction of ref. [5] and as a dashed line the results obtained on a-MnSi.
In the case of a-MnSi there was a crossover from a low field value 8 = 2 (mean field value) to a high field value of 3.3 to 4.1 which is close to the value reported for GdA12 [4, 6]. As s h o w n in fig. 3 the behavior of a - M n G e is very similar to that of a-MnSi: there is a crossover f r o m a low field value of 6 = 2 to 8 = 4 at high field. It is also noteworthy that this crossover occurs (just as in aMnSi) at lower fields for the films with the greatest degree of clustering.
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I w o u l d like to thank S. Greenberg Kosinski and J. E. Bernardini for their technical assistance.
References [1] [2] [3] [4]
J. J. Hauser, Solid State Commun. 30 (1979) 201. J. J. Hauser, Phys. Rev. B., to be published. C. M. Soukoulis, Phys. Rev. B18 (1978) 3757. T. Mizoguchi, T. R. McGuire, S. Kirkpatrick and R. J. Gambino, Phys. Rev. Lett. 38 (1977) 89. [5] D. J. Thouless, P. W. Anderson and R. G. Palmer, Phil. Mag. 35 (1977) 593. [6] J. Chalupa, Solid State Commun. 24 (1977) 429.