Browsing by Author "Soehnel, T"

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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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    Striped magnetic ground state of the ideal kagomé lattice compound Fe4Si2Sn7O16
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetisation measurements [1]. The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K [2]. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions — i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, ½, ½) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry-breaking ground states for perfect kagomé lattices.
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    Striped magnetic ground state on an ideal S = 2 Kagomé lattice
    (International Union of Crystallography, 2017) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetization measurements. [1] The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). [2] Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range ordered down to at least T = 0.1 K. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions – i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, 1/2, 1/2) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry breaking ground states for perfect kagomé lattices. © International Union of Crystallography
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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.