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

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School of Science | Doctoral thesis (article-based) | Defence date: 2017-09-29
Degree programme
90 + app. 58
Aalto University publication series DOCTORAL DISSERTATIONS, 155/2017
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.
Supervising professor
van Dijken, Sebastiaan, Prof., Aalto University, Department of Applied Physics, Finland
nanomagnetism, multiferroics, spintronics, electric-field control of magnetism, perpendicular magnetic anisotropy
Other note
  • [Publication 1]: Kévin J. A. Franke, Diego López González, Sampo J. Hämäläinen and Sebastiaan van Dijken. Size dependence of domain pattern transfer in multiferroic heterostructures. Physical Review Letters, 112, 017201, January 2014.
    DOI: 10.1103/PhysRevLett.112.017201 View at publisher
  • [Publication 2]: Yasuhiro Shirahata, Ryota Shiina, Diego López González, Kévin J. A. Franke, Aiji Wada, Mitsuru Itoh, Nikolay A. Pertsev, Sebastiaan van Dijken and Tomoyasu Taniyama. Electric-field switching of per-pendicular magnetized multilayers. NPG Asia Materials, 7, e198, July 2015.
    DOI: 10.1038/am.2015.72 View at publisher
  • [Publication 3]: Arianna Casiraghi, Teresa Rincón Domínguez, Stefan Rössler, Kévin J. A. Franke, Diego López González, Sampo J. Hämäläinen, Robert Frömter, Hans Peter Oepen and Sebastiaan van Dijken. Influence of elastically pinned magnetic domain walls on magnetization reversal in multiferroic heterostructures. Physical Review B, 92, 054406, August 2015.
    DOI: 10.1103/physrevb.92.054406 View at publisher
  • [Publication 4]: Diego López González, Arianna Casiraghi, Ben Van de Wiele and Sebastiaan van Dijken. Reconfigurable magnetic logic based on the energetics of pinned domain walls. Applied Physics Letters, 108, 032402, January 2016.
    DOI: 10.1063/1.4940119 View at publisher
  • [Publication 5]: Diego López González, Yasuhiro Shirahata, Ben Van de Wiele, Kévin J. A. Franke, Tomoyasu Taniyama and Sebastiaan van Dijken. Electricfield-driven domain wall dynamics in perpendicularly magnetized multilayers. AIP Advances, 7,035119, March 2017.
    DOI: 10.1063/1.4979267 View at publisher
  • [Publication 6]: Diego López González, Arianna Casiraghi, Florian Kronast, Kévin J. A. Franke, and Sebastiaan van Dijken. Influence of magnetic field and ferromagnetic film thickness on domain pattern transfer in multiferroic heterostructures. Journal of Magnetism and Magnetic Materials, 441, 404-408, June 2017.
    DOI: 10.1016/j.jmmm.2017.06.004 View at publisher