Magnetic properties of ultrathin tetragonal Heusler D 0₂₂-Mn₃Ge perpendicular-magnetized films

We investigated the crystal structure and magnetic properties of Manganese-germanium (Mn₃Ge) films having the tetragonal D0₂₂ structure, with varied thicknesses (5-130nm) prepared on chromium (Cr)-buffered single crystal MgO(001) substrates. A crystal lattice elongation in the in-plane direction, induced by the lattice mismatch between the D0₂₂-Mn₃Ge and the Cr buffer layer, increased with decreasing thickness of the D0₂₂-Mn₃Ge layer. The films exhibited clear magnetic hysteresis loops with a squareness ratio close to unity, and a step-like magnetization reversal even at a 5-nm thickness under an external field perpendicular to the film's plane. The uniaxial magnetic anisotropy constant of the films showed a reduction to less than 10 Merg/cm³ in the small thickness range (≤20nm), likely due to the crystal lattice elongation in the in-plane direction.