Using first-principles density functional calculations, we study the electronic and magnetic properties of the ferromagnetic insulating double perovskite compound ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$, which has been reported to exhibit an interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals the existence of very soft infrared active phonons that couple strongly with spins at the Ni and Mn sites through modification of the superexchange interaction. We suggest that these modes are the origin for the observed dielectric anomaly in ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$.

• Received 9 July 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.186402