First-principles investigation of the impact of chemical order on the magneto-crystalline anisotropy in L10 FeNi (tetrataenite)

Abstract

The volatility in price and uncertainty in supply of the rare earth elements makes it highly desirable to find alternatives to rare-earth based magnets, in order to meet the increasing global demand for permanent magnets. One interesting candidate in this quest is the chemically-ordered L10 phase of Fe50Ni50 (tetrataenite) found in iron meteorites. While the laboratory synthesis of the ordered phase is very difficult due to the slow diffusion of atoms at the rather low order-disorder transition temperature, several attempts have been made to achieve samples with a high degree of chemical order. Nevertheless, synthesis of a fully ordered system remains challenging. Using first-principles-based density-functional theory calculations in combination with Monte Carlo (MC) simulations, we investigate the interplay between the degree of chemical order and the magnetic properties of L10 FeNi [1]. Our calculations demonstrate a strong effect of the magnetic order on the chemical order-disorder transition temperature, and a strong effect of the chemical order on the magnetic transition temperature, in agreement with previous studies. Furthermore, we investigate the dependence of the magneto-crystalline anisotropy on the chemical long range order. Our results indicate that the anisotropy does not decrease significantly as long as the deviations from perfect order are not too large. Moreover, we also find that a slight disorder in the alloy can in principle result in an even higher anisotropy than for the fully ordered structure. We further analyze the correlation between the magneto-crystalline anisotropy and the orbital magnetic moment anisotropy. This allows us to study the effect of the local chemical environment on both quantities, potentially enabling further optimization of the magneto-crystalline anisotropy with respect to both long-range order and stoichiometric composition.