Enhanced strain-induced magnetoelectric coupling in polarization-free Fe/BaTiO3 heterostructures

Abstract

Magnetoelectrics (ME), materials which possess a coupling between their magnetic and electric orders, are the most studied type of multiferroic (MF) materials due to their obvious potential in technological applications such as: ultralow power logic-memory devices, magnetic sensors, energy harvesting/conversion devices, spintronic devices, among others [1-4].Still, the ME effect presents an inherent obstacle which is related to the apparent necessity of empty d orbitals (d0) for Ferroelectrics against semi-filled d orbitals (dn) for magnetic order [4-5].Therefore, the search for magnetoelectric materials typically revolves around the struggle to simultaneously coexist magnetic and ferroelectric orders in the same material, either using an intrinsic or extrinsic/composite approach. In fact, due this paradoxical limitation when having an electric and magnetic order in a single phase, extrinsic MF have been showing the most promising results, managing to achieve substantial ME couplings even at temperatures close to room temperature [4,6-8].Via ab initio calculations of a prototypical Fe/BaTiO3 interface, we predict that it is possible to tune the magnitude of the individual magnetic moments, even for non-polar BaTiO3 (BTO), through a uniaxial strain in cubic and tetragonal Fe/BTO heterostructures.By comparing polar and non-polar Fe/BTO heterostructures, we show that the Fe, Ti and equatorial O atomic magnetic moments are induced and enhanced as a result of their local crystal field. The crystal field may be controlled solely by the manipulation of the inter-atomic distances of their neighboring atoms (which will affect their electrostatic fields and orbital hybridizations), or by the BTO electric dipole moments, working as a local polarization.Contrary to conventional assumptions, a Fe/BTO interface does not require electrical polarization to be able to tune its magnetic moment. In fact, by applying strain, the sensitivity of the non-polar region is higher than that of its polar equivalent. Additionally, comparing the tensile region of the cubic and tetragonal supercells, it is clear that the non-polar cubic supercell reaches higher total magnetizations than its polar tetragonal counterpart. When the polarization is present, it dominates the crystal field contributions, thus constraining the effects of other perturbations such as strain.The results of our work reiterate the extrinsic magnetoelectric potentialities and reduces the requirements for an electric control of magnetism. Therefore, we should rethink the conventional search and optimization of ME materials, without the limitations which come with magnetic ferroelectrics. **