Magneto-Electric Effect in a Spin-State Transition System

TL;DR

This paper investigates the magnetoelectric properties of a spin-state transition system, modeling the coupling between spin states and lattice distortions, and predicts magnetic-field induced polarization near phase boundaries.

Contribution

It introduces a new effective Hamiltonian model linking spin-state transitions with ferroelectric distortions, explaining experimental phase diagrams and predicting novel magnetoelectric effects.

Findings

Phase transition from low-spin to high-spin ferroelectric state

Model reproduces experimental pressure-temperature phase diagram

Prediction of magnetic-field induced electric polarization

Abstract

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.