Theoretical investigations of Anomalous Magnetism in Amorphous Transition Metals and Alloys.

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Recently, anomalous magnetic behaviors such as the spin glass (SG) state and high Curie temperatures (>1800 K) have been found in Fe-rich and Co-rich amorphous transition metal alloys. In order to clarify the anomalous behaviors, we have investigated the magnetism of amorphous transition metals on the basis of the finite temperature theory of local environment effect in amorphous metallic magnetism. Our conclusions are summarized as follows.1. The simple ferromagnetism in Fe, Co, and Ni is drastically changed by the structural disorder ; (1) Amorphous Fe shows the itinerant electron SG because of the nonlinear magnetic couplings between Fe local moments and the local environment effects on the amplitudes of local moments. (2) The Curie temperature in amorphous Co is enhanced by about 400 K because of the magnetic energy gain due to the shift of the non-interacting density of states. (3) The strong ferromagnetism in amorphous Ni is weakened because of the same reason.2. We examined the SG around amorphous Fe, so that we found the following new results. (1) There exists the cluster SG region with the ferromagnetic clusters between the SG and the ferromagnetism when the d electron number is varied. (2) Re-entrant SG was found near the ferromagnetic boundary. Theoretical results semi-quantitatively explained the magnetic-field and pressure effects on the re-entrant SG in Fe-rich Fe-Zr and Fe-La amorphous alloys. (3) The strong anomaly of the forced volume magnetostriction at the re-entrant SG temperature is characteristic of the itinerant SG which is never seen in the insulator systems.