Magnetoelastic octahedral breathing mode in the ferrimagnetic L a2CoIr O6 double perovskite

Abstract

Distortion of transition-metal ion-oxygen octahedra in perovskite structure has been a long standing subject for exploring the physics of charge-orbital-spin-lattice couplings. Recent findings in nickelates or BaBiO3 have suggested that a charge disproportionation in the transition-metal ions can evoke certain cooperative breathing distortions (CBD), constituted by the symmetric expansion and shrinkage of the octahedra in contrast to the Jahn-Teller-type asymmetric distortion. A double perovskite ferrimagnet, La2CoIrO6 bears inherently the charge disproportionation in the cationic sites with Co2+ and Ir4+ sublattices. In light of seeking the clues for the CBD and its coupling to the magnetic order, the crystal structure and magnetic structure of La2CoIrO6 were examined at various temperatures by using neutron diffraction. Structure refinements and mode decomposition analyses revealed that, indeed, a CBD mode exists substantially in La2CoIrO6 while preserving the symmetry of the lattices (P21/n) and the magnetism [Γ1(Ag)]. And more importantly, the magnitude of the symmetric distortion is correlated strongly with the size of the magnetic moment at temperatures below the Curie temperature (∼95 K). This strongly suggests a magnetoelastic coupling of the CBD mode. The order parameters for CBD, e.g., the movement of O1 toward Ir ions at low temperatures, are found to be in a linear-quadratic relationship with the value of the magnetic moment (∼0.002Å/μB2MCo2), manifesting the magnetoelastic CBD in double perovskite. © 2018 American Physical Society