Ferrimagnetism in the double perovskite Ca2FeOsO6: A density functional study

Using density functional calculations, we find that the newly synthesized Ca2FeOsO6 has the high-spin Fe3+ (3d(5))-Os5+ (5d(3)) state. The octahedral Os5+ ion has a large intrinsic exchange splitting, and its t(2g up arrow)(3) configuration makes the spin-orbit coupling ineffective. Moreover, there is a strong antiferromagnetic (AF) coupling between the neighboring Fe3+ (S = 5/2) and Os5+ ( S = -3/2), but the AF couplings within both the fcc Fe3+ and Os5+ sublattices are one order of magnitude weaker. Therefore a magnetic frustration is suppressed and a stable ferrimagnetic ground state appears. This ferrimagnetic order is due to the virtual hopping of the t(2g) electrons from Os5+ (t(2g up arrow)(3)) to Fe3+ (t(2g up arrow)(3)e(g up arrow)(2)). However, if the experimental bended Fe3+ -O2- -Os5+ exchange path gets straight, the e(g) hopping from Fe3+ (t(2g up arrow)(3)e(g up arrow)(2)) to Os5+ (t(2g up arrow)(3)) would be facilitated and then a ferromagnetic (FM) coupling would occur.