STRAIN-MEDIATED MAGNETO ELECTRIC COUPLING AND BEYOND: CASE STUDIES BY IN-OPERANDO SPECTROSCOPY

The work presented in this thesis addresses the Electrical control of Magnetism. Its implementation in multiferroic heterostructures may provide an ideal solution to efficiently manipulate magnetism at the nanoscale. In-operando spectroscopic techniques, in which electric and magnetic field can be applied on the sample during the experiment, are a powerful tool to understand the different mechanisms at the origin of magneto-electric coupling in multiferroic heterostructures. In this thesis X-ray Magnetic Circular Dichroism (XMCD), Magneto Optical Kerr Effect (MOKE) and related techniques were employed in in-operando experiments to investigate two prototypical systems composed by ferromagnetic thin films deposited on ferroelectric crystals. In the first one ultrathin La0.65Sr0.35MnO3 (LSMO) films were deposited epitaxially on BaTiO3 (BTO) ferroelectric substrates. It was shown that magnetic phase transitions are induced in LSMO both by variations in temperature, corresponding to structural transitions of BTO, and also by applied electric field, which results in ferroelectric domain rotations in BTO. The results can be explained comprehensively in terms of strain, which tunes the competition between ferro- and antiferro-magnetic interactions in manganites. The second one concerns metallic Fe deposited on 60% PbMg1/3Nb2/3O3 - 40% PbTiO3 (PMN-PT) substrates. Local variations of the magnetic anisotropy, fully reversible, are observed when the ferroelectric polarization of PMN-PT is switched between opposite directions out-of-plane. Correspondingly, surface cracks appear and disappear reversibly on the surface of the sample. The relation between these morphological and magnetic modifications is discussed. These results elude interpretations based on the most common mechanisms for magneto-electric coupling, suggesting that morphology could be considered as a new ingredient for electrical control of magnetism.