Browsing by Author "Spasovski, M"

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    Increase of the stability range of the skyrmion phase in doped Cu2OSeO3
    (Australian Institute of Physics, 2020-02-04) Sauceda Flores, JA; Rov, R; Camacho, L; Spasovski, M; Vella, J; Yick, S; Gilbert, EP; Han, MG; Zhu, Y; Seidel, J; Kharkov, Y; Sushkov, OP; Söhnel, T; Ulrich, C
    A skyrmion is a topological stable particle-like object comparable to a spin vortex at the nanometre scale. It consists of an about 50 nm large spin rotation and its spin winding number is quantized. Once formed, the skyrmions order in a two dimensional, typically hexagonal superstructure perpendicular to an applied external magnetic field (see Fig. 1). Its dynamics has links to flux line vortices as in high temperature superconductors. Cu2OSeO3 is a unique case of a multiferroic materials where the skyrmion dynamics could be controlled through the application of an external electric field. The direct control of the skyrmion dynamics through a non-dissipative method would offer technological benefits and unique possibilities for testing fundamental theories also related to the Higgs Boson whose theoretical description has similarities to skyrmions. Important for technological applications is a stability range of the skyrmion phase up to room temperature. While room temperature skyrmion materials exist, Cu2OSeO3 orders magnetically below 58 K. Our combined small angle neutron scattering (see Fig. 2), SQUID magnetization measurements and electron microscopy investigations did provide direct evidence that the stability range of the skyrmion phase can be extended in Te-doped Cu2OSeO3. The understanding of this effect will help to obtain deeper insights in the magnetic correlations in charge of the skyrmion formation and will thus help to systematically search for skyrmion materials with phase transition temperatures towards room temperature.
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    Scaling behaviour of the skyrmion phases of Cu2OSeO3 single crystals from small angle neutron scattering
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Sauceda Flores, JA; Jorge, A; Rov, R; Pervez, MF; Spasovski, M; O’Brien, J; Vella, J; Seidel, J; Yick, S; Gilbert, EP; Tretiakov, OA; Soehnel, T; Ulrich, C
    A skyrmion is a topological stable particle-like object comparable to a spin vortex at the nanometre scale. It consists of an about 50 nm large spin rotation and its spin winding number is quantised. Skyrmions emerge in chiral crystals as the result of competing symmetric exchange and asymmetric Dzyaloshinskii-Moriya (DM) interactions and typically form two dimensional hexagonal lattices perpendicular to an applied magnetic field. Its dynamics has links to flux line vortices as in high-temperature superconductors [1-2]. Cu2OSeO3 is a unique case of a multiferroic material where the skyrmion dynamics could be controlled through the application of an external electric field. The direct control of the skyrmion dynamics through a non-dissipative method would offer technological benefits applicable in energy-efficient data storage and data processing devices or for testing fundamental theories also related to the Higgs Boson whose theoretical description has similarities to skyrmions [3]. The technological applications crucially depend on the stability conditions of the skyrmion phase up to room temperature. While some materials host skyrmion lattices above room temperature [3], Cu2OSeO3 is the only insulating skyrmion material discovered so far, which orders magnetically below 58 K. It is interesting to note that the appearance of two different skyrmion phases have been reported along the temperature and magnetic field phase diagram of Cu2OSeO3 when the sample is aligned with its main crystallographic axes parallel to the incoming neutron beam and performing Zero Field Cooling (ZFC) or Field Cooling (FC) across the high-temperature skyrmion phase. However, the stabilisation processes of these two phases and their thermodynamic connection are still under debate [4-6]. We have used small angle neutron scattering and Lorentz transmission electron microscopy [7] to study the scaling behaviour of helical phase and the magnetic skyrmion lattices, i.e. the systematic change of their distances in single crystals of Cu2OSeO3 in order to gain insight on the balance between the different competing magnetic exchange interactions. Therefore, we have examined the field, temperature and sample alignment dependence of the scaling behaviour of skyrmions as an order parameter for the emergence of the two aforementioned skyrmion phases. The obtained data provide valuable information on the formation mechanism of the skyrmions and their stability range. This is an important step towards the understanding of the manipulation of skyrmions, which is required for technological applications. © The Authors
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    Synthesis and structural determination of the disordered bixbyite Cu3-xSb1+xO5.5+3x/2 with spin-glass behaviour
    (Wiley, 2019-01-28) Spasovski, M; Avdeev, M; Söhnel, T
    The ternary copper antimony oxide Cu3-xSb1+xO5.5+3x/2 (x=0.23) has been synthesized under 0.8–1.3 MPa pO2 at 1022–1082 °C. Rietveld refinements of X-ray and neutron powder diffraction patterns concluded that the oxide adopts a bixbyite type structure, crystallising in the cubic space group Ia-3 with the unit cell parameter a=9.61164(4) Å at room temperature from powder neutron diffraction data. The cationic 8b and 24d sites were found to be occupationally disordered where both Cu and Sb could be found on both sites. This is supported by X-ray absorption spectroscopy experiments showing more than one possible Cu environment. There was a significant net deficiency of oxygen in the compound which was first inferred from observations of a thermochromic-like phenomena and also seen from in situ high temperature neutron diffraction experiments. Magnetic susceptibility and magnetization measurements show paramagnetic behaviour with spin-glass like transition below 6 K. © 1999-2021 John Wiley & Sons, Inc.
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    Synthesis of new cuprate’s through high pressure chemical vapour transport
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Spasovski, M; Avdeev, M; Soehnel, T
    Chemical vapour transport (CVT) reactions has allowed for the growth of many inorganic single crystals which would be difficult or completely impossible to grow using alternative methods like flux related methods or from congruent melt. Most CVT reactions are done in evacuated and sealed quartz tubes where the internal pressure is typically in the range from 1 to 1^10-3¬ bar where diffusion is the dominant contributor to transport kinetics.[1] Diffusion limited transport is preferred over convective transport because it minimises nucleation, favouring the growth of larger single crystals with fewer defects. Many metal oxides are simply not thermodynamically stable under these conditions making it difficult to transport and crystallise the desired phase or composition. We have found this to be the case for many cuprates with the braunite, parwelite and various Cu3TeO6 related structures. To circumvent this limitation we have explored the unconventional high pressure CVT (HPCVT) method. As a result of these experiments we have been able to successfully grow and solve the structures of single crystals of new polymorphs and structures, this includes Cu5Sb2SiO12, Cu4MnSb2SiO12, Cu2GaSbO6 and Cu3Ga3SbSiO12. Samples have been characterised by X-ray and neutron diffraction and magnetic susceptibility measurements. These structures exhibit exotic gallium and copper coordination environments making them suitable candidates for studying various magnetic phenomena. HPCVT is a useful method not only for the growth of new inorganic compounds but also as an alternative, environmentally friendly method for growing known structures. Under pressure, water seems to be a major contributor to the transport reaction making it possible to grow samples without a reliance on halogens or commonly used salts like HgBr or TeCl4. Since the transport rates are high as a result of greater convective currents, a significantly smaller temperature gradient is necessary to conduct the experiments making much simpler experimental designs possible and accessible without the need for multi-zone furnaces. © The authors.
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    Tuning magnetic frustration in bixbyites
    (Australian Institute of Physics, 2020-02-05) Spasovski, M; Avdeev, M; Söhnel, T
    The cubic bixbyite structure (α-Mn2O3) has a deep history in solid-state chemistry, the first structural solution was completed by Zachariasen and later corrected by Pauling.[1] Related to the fluorite structure with ¼ of the anions removed, these vacancies force the displacement of the remaining anions changing the coordination from cubic to strongly distorted octahedra. Today bixbyites are commonplace in every household found in everything from batteries, TV monitors and touch screens to industry catalysts. The ternary oxide Cu3TeO6 is an ordered bixbyite that has been the focus of intense study due to its interesting magnetic spin-web structure.[2-5] We have synthesized powders and single crystals from the solid solution Cu2.25+3x/4Sb1-xTexO4.75+5x/4, Ga and Mn doped variants. Through single crystal, powder X-ray diffraction (XRD) and neutron diffraction (ND) we have seen that the bixbyite lattice can accommodate for a large variation of dopant pressure through site disorder and defects.[6] K and L-edge X-ray absorption spectroscopy has shown the bixbyite structure will also accommodate a large variation of charged species which when coupled with defects and site disorder manifests itself into interesting frustrated magnetic structures varying from canonical spin-glasses to complex anti-ferromagnetic order.