Strain and electric field mediated manipulation of magnetism in La(1-x)SrxMnO3/BaTiO3 heterostructures

Heterostructures of ferromagnetic La(1−x)SrxMnO3 (LSMO) and ferroelectric BaTiO3 (BTO) were produced and investigated for their structural and magnetic properties. The combination of these ferroic properties can lead to an artificial multiferroic. A possible magneto-electric coupling at the interface was proposed by Burton et al. [1]. Thus, special emphasis was given to the manipulation of magnetic properties by applying electric fields. A magneto-electric coupling could be observed in the heterostructures under investigation. Epitaxial LSMO thin films were grown on BTO substrates using a stateof-the-art oxide molecular beam epitaxy (OMBE) and a high oxygen sputtering system (HOPSS). Stoichiometric La(1−x)SrxMnO3 films with doping levels of x = 0.5 and x = 0.3 were produced. The film quality in terms of roughness and crystalline structure was confirmed by X-ray scattering methods. The presence of structural domains in the BaTiO3 single crystal substrate, whose proportion could be altered due to the application of electric fields, was shown by X-ray diffraction. Tensile strain is induced into the epitaxial La(1−x)SrxMnO3 films in the whole temperature range under investigation. The magnetization of LSMO alteres by the variation of strain induced into the film, generated by the different structural phases of single crystal BaTiO3 substrates. The magnetization shows sharp steps at the structural phase transition temperatures of BTO. The evaluation of magnetic hysteresis loops reveals a change of the magnetic anisotropy of LSMO for each structural phase of BTO, but also within the orthorhombic phase. Special focus was given to the manipulation of magnetic properties by the application of electric fields. A newly established measurement option was used to determine the magnetic response to an applied electric field as a function of temperature and magnetic field. The electrically induced modification of the magnetization is profound near the structural phase transition temperatures. Electrical hysteresis loops give a detailed view on the influence of the electric field on the magnetization. The magnetic coercivity field shifts by the application of electric fields giving rise to a change of the magnetic anisotropy. Polarized neutron reflectivity measurements yield the magnetization profiles of the LSMO/BTO heterostructures to clarify a limitation of the effect to the interface. Samples grown by OMBE indicate a better epitaxial crystal structure due to a strain induced reduction in the magnetization at the interface. Samples produced by HOPSS show a reduced magnetization for higher layer thicknesses, which might be related to oxygen vacancies. Simulations of the polarized neutron reflectivity data for different electric field directions reveal that the observed differences in the reflectivity are mostly related to altered structural properties. Several mechanisms, which might be responsible for the observed effects in LSMO on BTO, are discussed. Strain effects via the elastic channel can lead to a rotation of the magnetic anisotropy, a change of the orbital ordering or the exchange interaction. Also carriermediated effects and oxygen diffusion under applied electric fields have to be considered. Furthermore, the ferroelectric properties of the BaTiO3 substrates and their response to the electric field is of crucial importance to explain the observed effects.