Browsing by Author "Kareri, Y"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • No Thumbnail Available
    Item
    Large easy-plane anisotropy induced spin reorientation in magnetoelectric materials (Co4−xMnx)Nb2O9
    (IOP Science, 2019-03-29) Deng, GC; Yu, YS; Cao, YM; Feng, ZJ; Ren, W; Cao, SX; Studer, AJ; Hester, JR; Kareri, Y; Ulrich, C; McIntyre, GJ
    Neutron powder diffraction experiments were carried out on the magnetoelectric compound series (Co4−xMnx)Nb2O9 (x  =  0, 1, 2, 3, 3.5, 3.9, 3.95 and 4) from base temperature to above their Neel temperatures. Their magnetic structures were analysed by using the irreducible representation analysis and Rietveld refinement method. Similar to Co4Nb2O9, the compounds with x  ⩽  3.9 have noncollinear in-plane magnetic structures (Γ6) with magnetic moments lying purely in the ab plane with certain canting angles. Mn4Nb2O9 has a collinear antiferromagnetic structure (Γ2) with magnetic moments aligning along the c axis. The compound of x  =  3.95 shows two magnetic phases in the magnetization, which was confirmed to have the Γ2 magnetic structure above 60 K and develop a second Γ6 local phase in addition to the main Γ2 phase due to doping. This study indicates 2.5 at% Co2+ doping is sufficient to alter the collinear easy-axis magnetic structure of Mn4Nb2O9 into the noncollinear easy-plane magnetic structure, which is attributed to the large easy-plane anisotropy of Co2+ and relative small Ising-like anisotropy of Mn2+. The doping effects on the Néel temperature and occupancy are also discussed. © 2019 IOP Publishing Ltd