Browsing by Author "Edge, AVJ"

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    Magnetic and crystal field properties of thulium calcium manganite
    (Australian Institute of Physics, 2004-02-04) Stewart, GA; Edge, AVJ; Studer, AJ; Elcombe, MM; Hovat, J; Lewis, RA
    For sufficiently large average Ln/A radii, the hole-doped manganites Ln2/3A1/3MnO3 (Ln = lanthanide, A = divalent metal) exhibit “colossal” magneto-resistance (CMR), which is associated with a transition from paramagnetic insulator to a low temperature ferromagnetic metal phase. However, with decreasing radius, the transition temperature is lowered and the ordered phase eventually reverts to that of a magnetic insulator. In this work, 169Tm Mössbauer spectroscopy has been used to investigate Tm2/3Ca1/3MnO3, which falls into the latter category. From the temperature of the onset of line broadening, the magnetic ordering temperature is determined as Torder ≈ 40 K (compared with ≈ 32 K from a.c. and d.c. susceptibility). The local Tm3+ magnetisation is almost certainly that of an isolated pseudodoublet ground state driven by a weak (compared with the crystal field) Mn-Tm exchange interaction. To a good approximation, the low temperature 169Tm spectrum is the superposition of a relaxation-broadened sextet and a paramagnetic doublet. This is consistent with regions of large slowly fluctuating magnetic clusters and regions of smaller rapidly fluctuating clusters, as has been observed for the generic CMR manganite, La2/3Ca1/3MnO3. However, recent neutron diffraction measurements performed on the Bragg Institute's HRPD facility are insensitive to this weak magnetism, suggesting a complex (perhaps spin-glass in nature) Mn sub-lattice magnetisation. The refinement of the position parameters for the nearneighbour oxygen atoms has assisted with the interpretation of the temperature-dependent 169Tm quadrupole interaction data in terms of a crystal field scheme for the Tm3+ ion. The appropriateness of this scheme will be considered in terms of the above observations. This work was supported by separate grants for source irradiation and neutron diffraction from the Australian Institute of Nuclear Science and Engineering.
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    Magnetic structures and valence states of YbMn2SixGe2-x
    (Australian Institute of Physics, 2004-02-04) Grimm, D; Hofmann, M; Campbell, SJ; Edge, AVJ; Studer, AJ
    Rare-earth intermetallic compounds containing ytterbium exhibit a wide range of interesting and unusual physical and magnetic properties. This occurs mainly as a result of their mixed valence states (II/III) or changes from one valence state to the other. Here we present a set of results for the YbMn2SixGe2-x series (8 samples from x = 0 to 2), focusing on the magnetic structure and valence state of these compounds as investigated by neutron powder diffraction (2-530 K). We have found that the valence state of the Yb ion changes from the trivalent state of YbMn2Si2 to a divalent-like behaviour for YbMn2Ge2 with increasing Ge concentration x, with significant changes in the magnetic structure taking place around 1.5