Magnetism of ordered and disorderedNi2MnAlfull Heusler compounds

Abstract

Based on ab initio calculations and Monte Carlo simulations, we present a systematic study of the magnetic ground state and finite temperature magnetism of ordered and disordered ${\mathrm{Ni}}_2\mathrm{MnAl}$ full Heusler compounds. By increasing the degree of the long-range chemical disorder between the Mn and Al sublattices, the magnetic order progressively changes from the ferromagnetic state in the ordered $L{2}_1$ phase toward a fully compensated antiferromagnetic state in the disordered $B2$ phase and we also conclude that the Ni atoms exhibit induced moments. We determine the Mn-Mn interactions by using the magnetic force theorem and find dominating, but rather weak ferromagnetic couplings in the ordered $L{2}_1$ phase. We used a recently proposed renormalization technique to include the weak Ni moments into the spin model, which indeed remarkably increased the nearest-neighbor Mn-Mn interaction. In accordance with the total energy calculations, in the disordered compounds, strong antiferromagnetic site-antisite Mn-Mn interactions appear. Determining the spin-spin correlation functions from Monte Carlo simulations, we conclude that above the transition temperature, short-range antiferromagnetic correlations prevail between the Mn atoms. In view of the potential application of disordered ${\mathrm{Ni}}_2\mathrm{MnAl}$ as a room temperature antiferromagnet, we calculate the magnetic anisotropy energies of tetragonally distorted samples in the $B2$ phase and find that they are smaller by two orders in magnitude than in the frustrated antiferromagnet ${\mathrm{IrMn}}_3$.