Large Spin Coherence Length and High Photovoltaic Efficiency of the Room Temperature Ferrimagnet Ca 2 FeOsO 6 by Strain Engineering.

The influence of epitaxial strain on the electronic, magnetic, and optical properties of the distorted double perovskite Ca 2 FeOsO 6 is studied. These calculations show that the compound realizes a monoclinic structure with P2 1 /n space group from -6% to +6% strain. While it retains ferrimagnetic ordering with a net magnetic moment of 2 μ B per formula unit at low strain, it undergoes transitions into E-antiferromagnetic and C-antiferromagnetic phases at -5% and +5% strain, respectively. It is shown that spin frustration reduces the critical temperature of the ferrimagnetic ordering from the mean field value of 600-350 K, in excellent agreement with the experimental value of 320 K. It is also shown that the critical temperature can be tuned efficiently through strain and that the spin coherence length surpasses that of Sr 2 FeMoO 6 under tensile strain. An indirect-to-direct bandgap transition is observed at +5% strain. Localization of the valence and conduction states on different transition metal sublattices enables efficient electron-hole separation upon photoexcitation. The calculated spectroscopic limited maximum efficiency of up to 33% points to excellent potential of Ca 2 FeOsO 6 in solar cell applications.