Browsing by Author "Chang, FF"

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    Crystallographic and magnetic structure study in SrCoO3-x by high resolution x-ray and neutron powder diffraction
    (Australian Institute of Physics, 2016-02-04) Chang, FF; Reehuis, M; Hester, JR; Avdeev, M; Xiang, F; Wang, X; Seidel, J; Ulrich, C
    Transition metal oxides (TMOs) represent a wide set of materials with a broad range of functionalities, including superconductivity, magnetism, and ferroelectricity, which can be tuned by careful choice of parameters such as strain, oxygen content, and applied electric and magnetic fields. This tunability makes TMO’s ideal candidate materials for use in developing novel information and energy technologies and SrCoO3 provides a particularly interesting system for investigation due to its propensity to form oxygen-vacancy-ordered structures as the oxygen content is decreased. The ties between structural and functional properties of this material are obvious as it undergoes simultaneously structural and magnetic phase transitions between two topotactic phases: from a ferromagnetic perovskite phase at SrCoO3.0 to the antiferromagnetic brownmillerite SrCoO2.5. In this study we have determined their crystallographic and magnetic structures of SrCoO2.50, SrCoO2.875, and cubic SrCoO3.00 using high resolution X-ray and neutron powder diffraction from 4 K to 600 K. The correct structure of oxygen-deficient end-member SrCoO2.5 was determined in space group of Imma, instead of Pnma or Ima2 proposed previously, with G-type antiferromagnetic order up to TN = 570 K. In SrCoO2.875, clear peak splitting was observed from (200) in cubic phase to (004) and (440) in tetragonal phase, indicating that the precise structure is I4/mmm with a = b = 10.829(9) Å and c = 7.684(2) Å at 95 K, and the corresponding magnetic structure is ferromagnetic with 1.86(4) μB per formula, in accordance to a spin configuration of cobalt ions with an intermediate spin state of both on Co3+ and on Co4+. The end member SrCoO3.00 possesses a simple cubic crystal structure with a = 3.817(2) Å at 95 K, and ferromagnetic order up to 280 K. The magnetic moment of 1.96(8) μB /Co4+ corresponds to an intermediate spin state of Co4+.
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    Evolution of crystallization and magnetic phase transition in Cu1-xZnxFe2O4 studied by neutron powder diffraction
    (American Physical Society, 2017-03-13) Chang, FF; Avdeev, M; Deng, G; Hester, JR; Wang, X; Ulrich, C
    High resolution and high intensity neutron powder diffraction were applied to study the crystallographic and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Structural phase transition from cubic to tetragonal phase was observed in CuFe2O4. Ferrimagnetic order was observed in CuFe2O4 and short-range antiferromagnetic scattering was observed below 10 K in cubic ZnFe2O4 which is strongly restrained by addition of slightly amount of Cu2+ ions. Upon doping, ferromagnetic order temperature was gradually reduced from 789 K. Collinear spin setting was observed and no indication of frustration was found even up to doping rate of x = 0.6. Highly frustrated Cu0.04Zn0.96Fe2O4 and ZnFe2O4 behave short-range antiferromagnetic order, induced by the competing between ferromagnetic interaction from first-nearest neighbor and antiferromagnetic interaction from the third-nearest neighbor in tetrahedron formed by Fe ions on B sites. © 2021 American Physical Society
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    Evolution of magnetic phase and cation distribution in Cu1-xZnxFe2O4 studied by neutron powder diffraction
    (Australian Institute of Physics, 2015-02-06) Chang, FF; Deng, DC; Avdeev, M; Hester, JR; Bertinshaw, J; Ulrich, C
    CuFe2O4 is a highly interesting material as it is a ferrimagnet with an unusual high magnetic ordering temperature of 780 K. ZnFe2O4, on the other hand, is a frustrated spin system with antiferromagnetic order below 10 K. By doping nonmagnetic Zn ions in CuFe2O4, frustration can be introduced and interesting properties might emerge. Given that, high resolution and high intensity neuron powder diffraction techniques have been applied to study the structural and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Coexistence of cubic and tetragonal structure in CuFe2O4 was observed in a wide temperature range, which indicates a second order phase transition nature. This transition is caused by Jahn-Teller distortion of the CuO6 octahedra. Although CuFe2O4 and ZnFe2O4 are inverse and normal spinels, respectively, mixed cation distribution was found in doped samples, with Cu and Zn ions sitting both either on the tetrahedral or the octahedral sites. All the doped Cu1-xZnxFe2O4 (x = 0.2 - 1) samples crystallise in the cubic structure and order in the ferrimagnetic spin configuration. Upon doping, the value of oxygen position parameter μ increases, indicating the compression of the octahedra with increasing Zn-composition. Short-range antiferromagnetic order was observed below 10 K in cubic ZnFe2O4. The spin frustration, which leads to the antiferromagnetic order in Cu0.04Zn0.96Fe2O4 and ZnFe2O4 is induced by the competing interaction between the first nearest neighbor and the third nearest neighbour tetrahedra formed by Fe ions on B sites.
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    Investigation on the nature of the Verwey Transition in Cu-doped Fe3O4
    (Australian Institute of Physics, 2018-01-31) Kareri, Y; Chang, FF; Hester, JR; Ulrich, C
    Magnetite (Fe3O4), the oldest known magnet, is still a hotly debated material in scientific research, due to its complex magnetic, electronic and transport properties. One of the most interesting physical phenomena associated with Fe3O4 is the occurrence of a metal-insulator transition at ~120 K (TV), the so-called Verwey transition, which was associated with charge ordering below TV, accompanied by a structural transition from the cubic phase to the monoclinic phase. However, due to the twinning of crystal domains, the detailed crystallographic structure is not fully solved yet and different charge ordered and bond-dimerized ground states have been proposed. In order to overcome this problem, we have investigated Cu-doped Fe3O4 to approach the problem through the determination of the phase diagram of Fe1-xCuxFe2O4. Using neutron diffraction and high resolution X-ray synchrotron diffraction we have investigated both the crystallographic and magnetic structure of Cu-doped Fe3O4 in order to elucidate the effect of doping on the Verwey transition. Data obtained from both complementary diffraction techniques indicate that the Verwey transition temperature and the magnetic structure, in particular the magnetic moment, remain unchanged up to high doping levels of 85% Cu-substitution. This is a surprising result at first glance and required a systematic investigation. The analysis of our high resolution X-ray synchrotron diffraction data allowed us to extract detailed information on the precise doping mechanism, including the distribution of Cu-ions between tetrahedral and octahedral sites in the spinel structure. The diffraction data therefore provide valuable information on the detailed mechanism behind the Verwey transition.
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    Investigation on the nature of the Verwey transition in Cu-doped Fe3O4
    (Australian Institute of Physics, 2017-01-31) Kareri, Y; Chang, FF; Hester, JR; Ulrich, C
    Magnetite (Fe3O4), the oldest known magnet, is still a hotly debated material in scientific research, due to its complex magnetic, electronic and transport properties. One of the most interesting physical phenomena associated with Fe3O4 is the occurrence of a metal-insulator transition at ~120 K (TV), the so-called Verwey transition, which was associated with charge ordering below TV, accompanied by a structural transition from the cubic phase to the monoclinic phase. However, due to the twinning of crystal domains, the detailed crystallographic structure is not fully solved yet and different charge ordered and bond-dimerized ground states have been proposed. In order to overcome this problem, we have investigated Cu-doped Fe3O4 to approach the problem through the determination of the phase diagram of Fe1-xCuxFe2O4. Using neutron diffraction and high resolution X-ray synchrotron diffraction we have investigated both the crystallographic and magnetic structure of Cu-doped Fe3O4 in order to elucidate the effect of doping on the Verwey transition. Data obtained from both complementary diffraction techniques indicate that the Verwey transition temperature and the magnetic structure, in particular the magnetic moment, remain unchanged up to high doping levels of 85% Cu-substitution. This is a surprising result at first glance and required a systematic investigation. The analysis of our high resolution X-ray synchrotron diffraction data allowed us to extract detailed information on the precise doping mechanism, including the distribution of Cu-ions between tetrahedral and octahedral sites in the spinel structure. The diffraction data therefore provide valuable information on the detailed mechanism behind the Verwey transition.
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    Low pressure synchrotron x-ray powder diffraction of Cu5-xMxSbO6 (M=Cr, Mn, W)
    (Australian Institute of Physics, 2016-02-04) Wilson, DJ; Söhnel, T; Smith, KL; Brand, HEA; Ulrich, C; Graham, PJ; Chang, FF; Allison, MC; Vyborna, NH
    The large crystallographic and chemical diversity of copper-based metal oxides is one of their highlighting features and cause for pursuit into copper based material research. An interesting feature seen in copper based metal oxides is the coexistence of different copper oxidation states, in different crystallographic positions, within the same compound. This can lead to a mixture of magnetically active Cu2+ and magnetically inactive Cu1+ within the same compound, with different structural motifs. One interesting compound that demonstrates this coexistence of mixed copper oxidation states is Cu5SbO6, which crystallises in a modified delafossite structure type (CuFeO2). Here, the magnetically active brucite-like CuO2 layer was diluted in an ordered fashion with non-magnetic Sb5+. These layers were separated by linearly coordinated, magnetically inactive Cu1+. Rietveld refinements on a range of preparation temperatures revealed a low-temperature (LT) and high-temperature modification (HT) phase transition. This is related to an ordering (HT)/disordering (LT) effect of the Sb5+/Cu2+ brucite-like layers between the Cu1+ ions. Substituting the Cu2+ or Sb5+ in the layers with other transition metals (Cr, Mn, W) could present interesting changes to the properties of the material, and potentially influence the ordered/disordered stacking of the layers. By using solid-state Raman spectroscopy, we could show that this structure displayed a pressure-induced phase transition at room temperature for the ordered modification, which was not observed for the disordered modification. Lowering the pressure from ambient down to 20 mbar showed phonon modes at about 700 cm-1 and 550 cm-1 disappeared almost completely. Neutron powder diffraction experiments were conducted at atmospheric and low pressure on both ordered and disordered modifications. On analysis of the neutron diffraction patterns, we could show a very small shift in the reflections, and thus changes in the unit cell parameters, for the ordered modification, while these shifts were not observed for the disordered modification. These shifts should also be observed in synchrotron powder diffraction patterns. Therefore, we investigated the nature of this phase transition with variable pressure synchrotron X-ray powder diffraction.