Negative spin polarization and effect of composition on the atomic order and electronic structure of Mn2VAl Heusler alloy thin films

Magnetic materials with high negative spin polarization can increase design freedom of spintronic devices for high-performance operations. We studied Mn2VAl (MVA) Heusler alloy thin films to investigate the potential for negative spin polarization materials from the viewpoints of electronic structure, composition tuning, and spin injection in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices. Density-of-states calculations showed that MVA has a gap in the majority-spin band at the Fermi energy, leading to negative spin polarization, which is higher in the L21-ordered state than in the B2-ordered state. VMn antisites (V atoms occupying Mn sites) introduce an in-gap state detrimental to the negative spin polarization, whereas MnV and AlV antisites exhibit a smaller impact, preserving the gap. High-quality MVA films were fabricated by sputter-deposition at elevated temperatures, achieving the B2 and L21 order parameters of 0.88 and 0.5 at 600 °C, respectively. The comparison of stoichiometric and various off-stoichiometric samples revealed that Mn-rich and Al-rich compositions exhibited improved ordering and reduced detrimental VMn antisites. The advantage of these off-stoichiometric compositions was demonstrated by the negative magnetoresistance measured in the epitaxial CPP-GMR devices consisting of MVA/Ag spacer/CoFe. The devices with Mn2.2V0.6Al1.2 exhibited a very large negative magnetoresistance of −4.4%, indicating high negative spin polarization of MVA. In addition, highly efficient spin-transfer torque generation via the spin injection from MVA was demonstrated with the opposite torque direction to that from the conventional positive spin polarization materials, paving the way for a new class of spintronic devices.