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

•Structural source for PMA in ultra-thin (2–3 nm) sputter-deposited Co2MnSi was studied.•PMA observed with adjacent Pd layer and following thermal annealing.•PMA related to CoPd nanocrystallites due to intermixing of Co with adjacent Pd layer.•Intermixing of Mn with an adjacent MgO layer affects magnetic properties.•Necessity of direct structural characterization of ultra-thin Co2MnSi films is shown. 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.