Microscopic model for magnetoelectric coupling through lattice distortions

Abstract

We propose a microscopic magneto-electric model in which the coupling between spins and electric dipoles is mediated by lattice distortions. The magnetic sector is described by a spin S=1/2 Heisenberg model coupled directly to the lattice via a standard spin-Peierls term and indirectly to the electric dipole variables via the distortion of the surrounding electronic clouds. Electric dipoles are described by Ising variables for simplicity. We show that the effective magneto-electric coupling which arises due the interconnecting lattice deformations is quite efficient in one-dimensional arrays. More precisely, we show using bosonization and extensive DMRG numerical simulations that increasing the magnetic field above the spin Peierls gap, a massive polarization switch-off occurs due to the proliferation of soliton pairs. We also analyze the effect of an external electric field E when the magnetic system is in a gapped (plateau) phase and show that the magnetization can be electrically switched between clearly distinct values. More general quasi-one-dimensional models and two-dimensional systems are also discussed.

Topological solitons and bulk polarization switch in collinear type-II multiferroics

“…It should be stressed that long range dipole-dipole interaction leads to a richer phase diagram, with respect to the first neighbors interaction case [19]. It includes a new exotic phase where dipoles order with a period of three sites, not found before.…”

“…Following our proposal in [19], the magnetic ions interact via nearest neighbors (NN) antiferromagnetic couplings J 1 . Frustration is introduced by next nearest neighbors (NNN) antiferromagnetic couplings J 2 .…”

“…Following this description, and in order to unravel the physical mechanism leading to multiferroicity by lattice distortions, we will assume that the undistorted lattice hosts electric dipoles amidst magnetic ions, with a natural magnitude p 0 and a preferred axisê oriented perpendicular to the chain (this choice of axes can be easily generalized to deal with more general situations, but the main novelty of the mechanism presented here is already contained in this simplified description). Under distortions δ i the local dipole magnitude is modified through a pantograph mechanism [19,53,54]. This is modeled in a linear approximation by p i = p i (σ i , δ i )ê with a component…”

“…We have performed an iterative numerical analysis based on DMRG to solve the magnetic and electric sectors in the adiabatic equations (6), along the lines stated in [58] and implemented in a similar context in [19]. The ground state for the spin system is obtained by the DMRG algorithm for each δ i and σ i configuration.…”

“…In a previous paper [19] we have introduced a simple microscopic multiferroic model describing a system with magnetic and electric dipolar degrees of freedom coupled via lattice distortions. This mediated coupling is ubiquitous in magnetoelectric phenomena and may be enhanced by the strong influence of the lattice in multilayer multiferroics, as in some cases the lattice mismatch of the layer and the substrate can generate enormous lattice distortions and trigger giant multiferroic responses [20,21].…”