Through a direct comparison of experimental results in amorphous and crystalline forms of Fe-Cr alloys we examine the effect of topological and compositional disorder on magnetic properties. Both types of disorder result in a decrease in the Curie temperature, the magnetic moment at T = 0 and the exchange stiffness constant. These results are discussed in the framework of several models.
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Ferromagnetic (FM) - spin glass (SG) transitions --which are predicted by various theoretical models I-2 --have now been found in several magnetic systems. 3-8 In recent 8 elastic neutron scattering studies of crystalline FexCrl_ x alloys, spin wave excitations served to probe FM-SG transitions. We present here magnetic measurements on amorphous (FexCrI_x)75PI6B6AI 3 alloys which also exhibit FM-SG transitions. The magnetic species in this system are Fe and Cr --P, B and A1 serve to stabilize the amorphous phase. Thus this system can be considered as the amorphous analog of FexCrl_ x. The present work presents the phase diagram and an interpretation of the low temperature magnetization in this amorphous system in terms of a spin-wave model. This is the first direct comparison between the crystalline and amorphous magnetic analogs where both systems exhibit the FM-SG transition, Amorphous Fe-Cr alloys were prepa~:ed by centrifugal spin quenching. 9 Small ribbons (5 mm x 1 mm x 25 ~m) were used in a low field ac susceptibility bridge and in a ¢ibrating sample magnetometer. In Fig. 1 we present the temperature dependence of the ac susceptibility (measured at ac fields of =30e rms) for x=.70, This Figure exhibits the typical behaviour of a reentrant ferromagnet with a low temperature transition to the spin-glass phase. 3,7 The sharp rise at T c ~ 85 K marks the Curie temperature while tLe sharp drop at Tfg = 20 K, the freezing temperature, in Fig. 2 we present the temperature dependence of the magnetization in applied field (50 G - i0 kG) for the x = .70 alloy. The mag- *Supported in part by the National Science Foundation under the MRL Grant DMR-77-23999. +Present address: Department of Solid State Physics, Royal Institute of Technology, Stockholm, Sweden.