Control of magnetism in strain-coupled multiferroic heterostructures with in-plane and perpendicular magnetization

Modern data storage devices use the magnetization in a material to store information. Current research to improve the density and stability of magnetic storage devices faces difficulties due to material limitations and increasing energy consumption. Mechanisms that couple magnetization to other ferroic properties can reduce power losses. These multiferroic materials combine two ferroic orders in a single phase, for example ferroelectric polarization and spontaneous magne-tization. Coupling between these two order parameters has attracted interest because it allows for electric field control of magnetism. Multiferroic materials are insulators, and therefore, the application of an electric field yields a small electrical current. This could limit the energy usage. However, materials with coexisting ferroelectric and ferromagnetic properties are rare. Moreover, the coupling between both order parameters tends to be weak or occurs at low temperatures. Composites with physically separated ferroelectric and ferromagnetic phases, so-called multiferroic heterostructures, can be used as an alternative. In this thesis, I experimentally study elastic coupling between a ferroelectric BaTiO3 substrate and a ferromagnetic thin film and demonstrate electric field control of magnetism. The experimental results are supported by micromagnetic simulations. BaTiO3 exhibits a tetragonal lattice structure at room temperature, and it is divided into regular stripe domains. For this thesis, ferroelastic domains with 90 degree rotation of the polarization are used. Strain transfer from the BaTiO3 substrate to the ferromagnetic thin film induces a magnetic anisotropy, i.e., a preferred orientation of magnetization. As a result, the domain structure of the ferroelectric substrate is imprinted into the ferromagnetic film. The application of an electric field changes the ferroelectric domain structures in a non-volatile manner, which is transferred to the ferromagnetic film. Two ferromagnetic materials are considered in this thesis: amorphous CoFeB films and epitaxial Cu/Ni multilayers. In the case of CoFeB, the periodic modulation of strain periodically rotates the uniaxial magnetic anisotropy by 90 degrees in the film plane between domains. Because of this anisotropy modulation, magnetic domain walls are strongly pinned onto ferroelectric domain boundaries and two types of walls can be initialized by an external magnetic field: charged (head-to-head/tail-to-tail) and uncharged (head-to-tail). These domain walls differ in width and energy. Based on these findings, a reconfigurable magnetic logic operator is proposed. In Cu/Ni multilayers, strain transfer from BaTiO3 imprints domains with alternating in-plane and perpendicular magnetic anisotropy. The walls that separate these domains can be reversibly driven by electric-field pulses.