Magnetic and structural properties of all-$d$ metal Mn-Ni-Ti Heusler alloys

The paper presents a theoretical study of the effect of different atomic and magnetic orderings on the structural and magnetic properties of Mn$_2$Ni$_{1+x}$Ti$_{1-x}$ alloys, which are composed entirely of transition metals. Using the density functional theory, we predict the structural ground states and magnetic reference states of compounds with $x = 0, 0,25$, $0,5$ and $0,75$ in both cubic austenite and tetragonal martensite phases. Partial substitution of Ti atoms with Ni leads to an increase in the energy barrier between structural phases, to a change from a layered atomic ordering to an alternating staggered order, and to a change from antiferromagnetic to ferromagnetic spin alignment in the cubic phase. All compounds with tetragonally distorted structures reveal the out-of-plane spin configuration and easy axis magnetocrystalline anisotropy except tetragonal L1$_0$ phase of Mn$_2$NiTi. For the latter structure, easy-plane magnetic anisotropy is observed. The calculated values of anisotropy are comparable with those of tetragonal L1$_0$-FeNi.