Cross-caloric effect in multiferroic bismuth ferrite

Multiferroic materials consisting of multiple and mutually coupled order parameters including polarization can have an intriguing response to applied electric field, and give rise to the so-called cross-caloric effect, as the nondipolar degrees of freedom can contribute to the electrocaloric effect (ECE) via cross couplings to the electric dipoles. In this study, we use a first-principles-based effective Hamiltonian combined with Monte Carlo simulations to investigate such an effect systematically in the well-known room-temperature multiferroic system BiFeO₃ (BFO), in which there exists three coupled order parameters that can respond to electric field, i.e., polarization, octahedral tiltings, and antiferromagnetism. We adopt and compare three methods based on the cumulant formula, Maxwell relation, and a Landau model, respectively. The Landau approach allows the computation of decomposed contribution from each order parameter, and magnetism is found to contribute significantly to the ECE near the N\'eel temperature. The electrocaloric (EC) adiabatic temperature changes and the EC coefficients are computed with various field directions and magnitudes, with which sizable positive and negative ECE are found.