Interface alloying of ultra-thin sputter-deposited Co₂MnSi films as a source of perpendicular magnetic anisotropy

Novel spin-electronic devices require electrodes that inject electrons with both high spin-polarization and perpendicular magnetic anisotropy (PMA). Several full-Heusler compounds are expected to be half-metallic ferromagnets, e.g. chemically-ordered L2₁ or B2 Co₂MnSi. However, most cubic full-Heusler alloys have small magneto-crystalline anisotropy meaning that PMA is difficult to achieve in thin film geometries of devices. Addressing this limitation, Butler et al. (2014) calculate PMA and full spin-polarization in ultra-thin (2–3 nm) chemically-ordered Co₂MnSi with an epitaxial coherent interface to rock-salt MgO (0 0 1). Experimentally, PMA in sputter-deposited ultra-thin films with full-Heusler compositions was reported though with adjacent layers of Pd or Pt. We investigate structural origins of such PMA using a test case of ultra-thin Co₂MnSi films prepared by magnetron sputter-deposition, adjacent to a MgO layer to represent a tunneling barrier, and to a Pd buffer layer. We measure PMA, with an energy density of 7.8 ± 1.8 Merg/cc at 5 K, when a Pd layer is adjacent to Co₂MnSi, following annealing in a narrow temperature range, around 350 °C. This ferromagnetism originates from nanometer scale regions, below 5 nm in size, having a relatively low saturation magnetization, 550 ± 50 emu/cc at low temperatures. Following thermal annealing, significant compositional intermixing between Co₂MnSi films and adjacent layers, Co with Pd and Mn with MgO, was measured by electron energy-loss and angle resolved X-ray photoelectron spectroscopies. Aberration-corrected transmission electron microscopy shows that Co₂MnSi does not crystallize while at the interface with Pd, nanometer-scale crystallites of FCC solid solution CoPd with {1 1 1} texture are identified. We conclude that these Pd rich CoPd crystallites, characterized by large magnetostriction, are a source for PMA.