Enhancing the magnetism and giant anomalous Hall effect in thin Fe-Al films via B2 nanophase growth

Abstract

The properties of alloys that undergo to chemical order-disorder transformations depend heavily on the degree of ordering in the crystal lattice. In the literature, it is well established that the ordering in a magnetic alloy such as Fe-rich Fe_xAl_1-x (x>0.5) leads to reducing its magnetization and even to a transition from the ferromagnetic (FM) to paramagnetic (PM) state at x<0.7. Studying the ordering kinetics in thin (50 nm) Fe_xAl_1-x films with a non-stoichiometric composition (0.5<x<0.7), we demonstrate the opposite behavior: When the alloy is aged at a high temperature Ta>600 {\deg}C, the ordering process is accompanied by an increase in magnetization and related properties. For example, we find the further enhancement of the giant anomalous Hall (AH) effect found recently in Fe_xAl_1-x alloys. Based on both experimental data and theoretical modeling, we argue that these properties are enhanced due to the nucleation and growth of the B2-Fe_0.5Al_0.5 phase. Growing B2 nanocrystals enable segregation and clustering of excess Fe in the alloy. It has been revealed that the PM phase, which is formed in the aged samples and contains Fe-enriched superparamagnetic clusters, contributes to the AH resistivity even more than the FM phase in the as-grown sample. Our findings open a route for improving the properties of functional alloys.