Theoretical Investigations of Structure vs Magnetism in Amorphous Alloys Showing Anomalous Magnetic Behaviors

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We have developed a finite-temperature theory of amorphous magnetic alloys on the basis of the functional integral method, combining the distribution function method describing random distribution of local magnetic moments with the geometrical-mean model for electronic structure calculations in amorphous alloys. applying our theory to amorphous Fe-Zr alloys whose ferromagnetism is destroyed by structural disorder, and amorphous Co-Y alloys with ferromagnetism enhanced by the structural disorder, we clarified the effects of structural disorder on their electronic structures and various magnetic properties. Next, we applied the theory to amorphous TM-Y (TM=Mn, Fe, Co, Ni) alloys showing peculiar magnetism, and showed that the most random atomic configuration explains the over-all features of their magnetic phase diagrams such as the appearance of spin glass (SG) in Mn-Y,the SG-F (ferromagnetism)-P (paramagnetism) transition in melt-spun Fe-Y,and the F-P transition in Co-Y and Ni-Y alloys … More . The theory also explained the concentration and temperature dependences of magnetizations and susceptibilities, semi-quantitatively. These magnetism is caused by the antiferromagnetic couplings between Mn local moments (Mn-Y), the local environment effects of magnetic moments and the nonlinear magnetic coupling between Fe local moments (Fe-Y), the atomic-size effects (Fe-Y and Co-Y), and 3d-4d hybridization (Co-Y and Ni-Y). We also examined the effects of atomic short range order in Fe-Y,and found that the F-SG transition occurs with increasing Fe-Fe pairs because of the increasing coordination number of Fe via atomic-size effects and the nonlinear magnetic coupling. This gives a mechanism for the single-phase SG in sputtered Fe-Y amorphous alloys. Finally, we proposed a theory which interpolates between crystal and amorphous structure. The application to Fe led to the conclusion that the ferromagnetism of bcc Fe becomes unstable when the fluctuation of interatomic distance (DELTA) becomes more than 0.05 along the bcc-amorphous line. In the case of Ni, we showed that the Curie temperature is rapidly decreased by increasing DELTA along the fcc-amorphous line. Less