Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6

Abstract

Abstract The multi-sublattice magnetism and electronic structure in double-double perovskite compound CaMnCrSbO6 is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order (TC∼49 K) between Mn2+ and Cr3+ spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2), which have tetrahedral and planar oxygen coordinations respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than expected from a two sublattice magnetic system. The separations between the onsite energies of the d-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting.
While the DOS obtained from non spin polarized calculations show a metallic character, starting from Hubbard U=0, the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases with Ueff(U-J) thus showing a Mott-Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap due to states associated with the interchanged atoms and also reduction in total magnetic moment.
Due to the ∼90o superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those obtained for various magnetic perovskite and double-perovskite compounds. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is ferromagnetic. The ferrimagnetism arises from the differences between the exchange interaction strengths between Mn(2)-O-Cr which is smaller than Mn(1)-O-Cr superexchange strength.
From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strength JAFM and also for the weaker ferromagnetic JFM. The relative strengths of JAFM for the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength ( ˜ JSS), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT.
Our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+ spins and a single magnetic sublattice for Cr3+spins yields a much improved TC, while a simple two magnetic sublattice model yields a much higher TC.”