Large anisotropy of spin polarization in Heusler alloy Ni ₂ MnGa induced by martensitic transformation

Spin polarization both in the cubic austenitic and tetragonal martensitic phases of the Ni2 MnGa alloy has been investigated using first-principles calculations combined with classical Bloch-Boltzmann transport theory. It is shown that the degree of spin polarization, while decreasing from 42% in the 〈001〉 directions of the austenitic phase to 30% in the [100] direction of the martensitic phase, rises to 75% in the [001] direction of the martensitic phase, resulting from a preferential reconstruction of the spin-down Fermi surfaces upon martensitic transformation. With this finding, various recent intriguing electrical measurements upon Ni2 MnGa across the martensitic transformation can find an explanation. This also opens a way of searching for giant magnetoresistance materials.