Topological spin systems and novel multiferroic materials investigated by optical spectroscopy and neutron scattering

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Clemens Ulrich

Electronic correlations in transition metal oxides lead to novel properties such as high temperature superconductivity, multiferroic properties, or topologically protected spin structures. Our group used advanced techniques of optical spectroscopy (2 laboratories at the UNSW) in combination with neutron scattering (at ANSTO) and advanced X-ray synchrotron techniques (at the Australian Synchrotron in Melbourne) to shine light on the physical processes behind theses fascinating phenomena. Facilities for the growth of these novel materials are also available (growth of single crystals: group of C. Ulrich, thin film preparation: school of materials science and engineering).



Recent advances in the growth techniques, in particular pulsed laser deposition, allow for the growth of thin films of novel multifunctional materials based on transition metal oxides, with atomic precision. This enables to combine effects such as magnetism or superconductivity with quantum confinement and offer therefore the opportunity to exceed the capabilities of semiconducting technology by far. Multiferroic materials combine electric polarization and magnetic order and allow for a switching between them, i.e. an electric field creates magnetic order and vice versa [1]. The potential for future technological applications in information technology or as novel sensors is tremendous. For example, combining both polarizations, electric and magnetic, would allow to increase the data storage capacity of hard drives by 5-6 orders of magnitude [2].



The aim of the proposed project is to investigate novel multiferroic materials by Raman light scattering and neutron scattering techniques. In our previous project we have investigated thin film systems of BiFeO3. This material is of special interest since it is the rare case where both, electric and magnetic polarization co-exist at room temperature, making it the most promising candidate for technological applications. In a first project the interfaces between multiferroic and ferromagnetic layers in a heterojunction used for spin-polarized tunnelling was investigated with atomic precision in order to determine magnetic damping mechanisms right at the interface [3]. Furthermore, the stability of the magnetic structure, which is amazingly a spin cycloid, was investigated by neutron diffraction. This did allow us to systematically improve the stability through a careful choice of the epitaxial growth conditions [4]. The aim of this project would be to extend the current investigations on novel multiferroic materials such as BiFeO3 or SrCoO3 [5].

In an extreme case, spins can arrangement in topological spin vortices, the so called skyrmions [6]. A skyrmion is a topological stable particle-like object comparable to spin vortex at the nanometre scale. It consists of an about 50 nm large spin rotation which order in a 2 dimensional, typically hexagonal superstructure perpendicular to an applied external magnetic field. Its dynamics has links to flux line vortices in a high temperature superconductors and its theoretical description has similarities to the Higgs Boson, making it an ideal test candidate for fundamental theories. Thin film skyrmion samples have been grown in the group of A/Prof. Jan Seidel at the UNSW and are available for this project.

Spin manipulation and coherent transport of the phase of the spins is a new emerging field of research and potential applications are called magnonics. This could be detected by Raman light scattering (Laser spectroscopy), a technique available in our group in the School of Physics, and by neutron diffraction at ANSTO.

The project offers the opportunity to obtain a deeper knowledge about the coupling between magnetism and electric polarization. By performing Raman light scattering experiments, various experimental techniques like Laser spectroscopy or cryogenics, i.e. the handling of ultracold liquids (Helium and Nitrogen) will be learned. In addition, neutron diffraction experiments will be performed at the new OPAL research reactor at ANSTO.

Figure 1: Neutron diffraction data obtained on the multiferroic material BiFeO3. The double peak structure in 1.a) demonstrates that the spins possess a cycloidal arrangement (paper submitted to Nature Com. [4]).

Figure 2: Schematic of a topological stable spin rotation, a skyrmion.



[1] S.-W. Cheong and M. Mostovoy, Nature Materials 6, 13 (2007).
[2] J. F. Scott, Nature Materials 6, 256 (2007).
[3] J. Bertinshaw, C. Ulrich, et al., Phys. Rev. B 90, 041113(R) (2014).
[4] J. Bertinshaw, C. Ulrich, et al., submitted to Nature Comm. (2015).
[5] S. Callori, C. Ulrich, et al., Phys. Rev. B 91, 140405(R) (2015).
[6] S. Seki, X.Z. Yu, S. Ishiwata, and Y. Tokura, Science 336, 198 (2012).