Local moment disorder in ferromagnetic alloys

TLDR

Local moment disorder, a random arrangement of two distinct magnetic states of the same atomic species in a metallic system, is examined using the Korringa‑Kohn‑Rostoker coherent‑potential approximation with local‑density‑functional theory for Fe‑Cr, Ni‑Fe, and Ni‑Mn alloys. The study aims to analyze how local moment disorder and the resulting volume contraction from magnetic‑moment reversal affect the magnetic behavior of these alloys, particularly in relation to Invar anomalies. Calculations show that in Fe‑Cr alloys the disordered‑moment state is energetically unfavorable compared to ferromagnetism, failing to account for the observed spin‑glass state at Fe₀.₁₄Cr₀.₈₆; in Ni‑Fe alloys the model predicts a first‑order transition from ferromagnetism to paramagnetism near the Invar region, contrary to experiments; and in Ni‑Mn alloys it predicts coexistence of parallel and antiparallel Mn moments above 15 at % Mn, consistent with NMR data.

Abstract

Local moment disorder, defined as a random arrangement of two distinct magnetic states of the same atomic species in a metallic system, is discussed in the framework of the Korringa-Kohn-Rostoker coherent-potential approximation combined with the local-density-functional method and applied to Fe-Cr, Ni-Fe, and Ni-Mn alloys. For Fe-Cr alloys it is found that the disordered-moment state has a higher energy than the ferromagnetic state in the entire region of Fe concentrations. Thus the theory fails to explain the spin-glass state observed around ${\mathrm{Fe}}_{0.14}$${\mathrm{Cr}}_{0.86}$. The theory, on the other hand, can explain the transition of Ni-Fe alloys from ferromagnetism to paramagnetism around the Invar region; the transition, however, is of first order, in contrast to experimental indications. The volume contraction due to the reversal of the magnetic-moment alignment from parallel to antiparallel with respect to the bulk magnetization is also discussed in connection with the Invar anomalies. For Ni-Mn alloys the calculation shows that, when the Mn concentration is larger than 15 at. %, magnetic states with local moments parallel and antiparallel to the average magnetization coexist even in the ferromagnetic region. The results are quite consistent with NMR experiments, which clearly show the existence of the antiparallel Mn local moments in addition to the parallel ones.