Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy
M. Ekholm, H. Zapolsky, A. V. Ruban, I. Vernyhora, D. Ledue and I. A. Abrikosov
Phys. Rev. Lett. 105, 167208 (2010)
In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.
Magnetization m for Ni0.75Fe0.25 as a function of temperature obtained from Heisenberg MC calculations (solid line) shown together with the chemical transition temperature calculated using the PDLM reference state corresponding to each value of m (dashed line with circles). For comparison, we include the experimental transition temperature (vertical dashed line).
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