Electronic structure and phonons in La<SUB>2</SUB>CoMnO<SUB>6</SUB>: a ferromagnetic insulator driven by Coulomb-assisted spin-orbit coupling

Using first-principles density functional calculations, we study the electronic structure of double perovskite compound, La<SUB>2</SUB>CoMnO<SUB>6</SUB>, which is reported to be ferromagnetic insulator with a Curie temperature of about 240 K. Our calculations show that the insulating state in this compound is driven by Coulomb-assisted spin-orbit coupling operative within the Co-d manifold, with a rather large orbital moment of ≈0.17 μ<SUB>B</SUB> at Co site. Motivated by the report of magnetodielectric behavior in this material, we also investigate the response of infrared active phonons to the underlying spin ordering. Our calculations exhibit existence of very soft phonon modes which show a substantially large response to the change in magnetic ordering, proving the existence of strong spin-phonon coupling in this material, as found in case of another related compound, La<SUB>2</SUB>NiMnO<SUB>6