Magnetic, dielectric, and magnetoelectric properties in a spin-state transition system are examined, motivated by the recent discovery of a multiferroic behavior in a cobalt oxide. We construct an effective model Hamiltonian based on the two-orbital Hubbard model, in which the spin-state degrees of freedom in magnetic ions couple with ferroelectric-type lattice distortions. A phase transition occurs from the high-temperature low-spin phase to the low-temperature high-spin ferroelectric phase with accompanying an increase of the spin entropy. The calculated results are consistent with the experimental pressure-temperature phase diagram. We predict the magnetic-field induced electric polarization in the low-spin paraelectric phase near the ferroelectric phase boundary.
@article{arxiv.1803.09881,
title = {Magneto-Electric Effect in a Spin-State Transition System},
author = {Makoto Naka and Eriko Mizoguchi and Joji Nasu and Sumio Ishihara},
journal= {arXiv preprint arXiv:1803.09881},
year = {2018}
}