Ferroelectric control of magnetism in artificial multiferroic composites

In this thesis, we studied ferromagnet/ferroelectric heterostructures, so-called artificial multiferroic composites, which exhibit magnetoelectric coupling between different ferroic order parameters. For a range of material combinations, we found that electrical switching of the ferroelectric polarization induces non-volatile reversible magnetization changes in the magnetic constituent and we contributed to the understanding of the underlying interface coupling mechanisms.
The ferromagnet/ferroelectric system La_{0.7}Sr_{0.3}MnO_{3}/ [Pb(Mg_{1/3}Nb_{2/3})O_{3}]_{0.68}-[PbTiO_{3}]_{0.32} (011) (LSMO/PMN-PT) enables magnetoelectric control of the double exchange interaction via strain. Reversible electrical switching of the ferroelectric polarization induces a 10 K shift of the magnetic Curie temperature Tc. A similar magnitude in Tc change has been previously only observed under applied electric fields. Sweeping between oppositely out of plane (OOP) poled ferroelectric polarization directions, PMN-PT (011) may exhibit an in-plane (IP) poled state where the ferroelectric polarization lies in the surface plane. OOP and IP poled configurations are stable at remanence and reciprocal space maps highlight the accompanying lattice parameter changes which impose a biaxial strain on the manganite thin film. The magnetic response to the strain changes is probed by temperature dependent Mn L_{3,2} x-ray magnetic circular dichroism (XMCD) providing quantitative values of the Mn spin and orbital moment. X-ray natural linear dichroism spectra for both strain states probe changes in the valence charge anisotropy. Multiplet and density functional theory calculations support the picture that the existing population imbalance between out of plane and in plane oriented orbitals increases further with tensile strain, favoring orbital occupation in the surface plane. An increase in tensile in-plane strain leads to an increased energy difference between the two e{_g} orbitals and a larger Mn-O-bond length. Increasing the electron-lattice coupling and reducing the e{_g} electron itinerancy that leads to ferromagnetism due to the double exchange interaction, results ultimately in lower Tc values in agreement with the Millis model.
In Co/PMN-PT (011), we disentangle the strain and charge contributions to the magnetic response upon electrical switching, using XMCD at the Co L_{3,2} edges as the main probe.
Our results evidence the coexistence of two coupling mechanisms leading to three distinct magnetization states upon electrical switching. If the ferroelectric polarization is switched to the IP poled state, the corresponding lattice parameter changes in the PMN-PT exert a strain on the Co layer and induce an anisotropy change with higher remanent magnetization along the [011-] direction.
When comparing oppositely OOP poled ferroelectric polarization configurations, an additional Co anisotropy change is observed. Since the structure of PMN-PT in the two OOP poled states is equivalent, this dependence of the anisotropy must stem from the substrate polarity. The bound charge at the interface is expected to be screened by the cobalt metal within the Thomas Fermi screening length of a few Angstroms. We use a Co wedge geometry to study the magnetic response as a function of Co layer thickness employing XMCD with surface sensitive total electron yield detection. Consequently, the anisotropy change induced by the charged substrate is observed for the thinner part but absent in the thicker part of the Co wedge.
Lattice parameter values for cobalt and PMN-PT obtained by x-ray diffraction as well as domain distributions obtained from atomic force microscopy serve as an input for density functional theory calculations which reproduce the experimentally observed anisotropy behaviour for fcc (111) textured cobalt as a function of the lateral strain and charge.
Our investigation unravels how magnetoelasticity and interfacial charge density trigger changes in the magnetic anisotropy. The observed coexistence of multiple coupling mechanisms opens up the possibility to tune and enhance the cross-coupling between layers in heterostructures.
The possibility to induce ferromagnetism in a per se paramagnetic system via electrical switching is explored for a Pd/Pb(Zr_{0.2}Ti_{0.8})O_{3} heterostructure.
Pd has a large magnetic susceptibility and is close to fulfilling the Stoner criterion for magnetism. According to calculations the polarity of adjacent ferroelectric layers could trigger a paramagnetic/ferromagnetic transition in paramagnetic metals by introducing shifts in the density of states.
No XMCD difference signal upon ferroelectric switching was found within the noise ratio of 0.2% at the M_{3,2} edge and of 1% at the L_{3,2} edge.