Ferromagnetic-Ferroelectric Domain Coupling in Multiferroic Heterostructures

In this thesis, strain-mediated coupling between magnetic films and ferroelectric BaTiO3 substrates with regular ferroelastic domain structures is studied. Using optical polarization microscopy, it is shown that imprinting of ferroelectric domains into magnetic polycrystalline CoFe, amorphous CoFeB and crystalline Fe films can be achieved. The ferroelectric polarization and elongated c-axis of the BaTiO3 substrates rotate by 90o at ferroelectric domain boundaries. Transfer of this strain to the adjacent magnetic film induces local magnetoelastic anisotropy whose orientation and symmetry depends on the underlying ferroelectric domain. Furthermore, abrupt changes in the magnetoelastic anisotropy pin the magnetic domain walls onto the ferroelectric domain boundaries. As a result, the magnetic domain walls do not move in an applied magnetic field resulting in the formation of magnetically charged and uncharged domain walls at different field directions. The strong coupling between magnetic and ferroelectric domains is used to demonstrate local magnetic switching and magnetic domain wall motion by purely electrical means. It is shown that a regular magnetic stripe pattern can be reversibly written and erased by the application of an electric field across the BaTiO3 substrate. Moreover, the magnetic domain walls are dragged along by their ferroelectric counterpart in an external electric field. Both effects are explained by 90o rotations of the ferroelectric polarization and the resulting strain-induced modification of the local magnetoelastic anisotropy. Similar strain-mediated effects, including local in-plane magnetization rotation by 90o, are obtained when the multiferroic heterostructures are cooled or heated through the structural phase transitions of BaTiO3