Browsing by Author "Stusser, N"

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    Magnetism and magnetic structures of PrMn2Ge2-xSix
    (IOP Publishing, 2013-09-25) Wang, JL; Campbell, SJ; Hofmann, M; Kennedy, SJ; Zeng, R; Din, MFM; Dou, SX; Arulraj, A; Stusser, N
    The structural and magnetic properties of seven PrMn2Ge2-xSix compounds with Si concentrations in the range x = 0.0-2.0 have been investigated by x-ray diffraction, magnetic (5-350 K), differential scanning calorimetry (300-500 K) and neutron diffraction (3-480 K) measurements. Replacement of Ge by Si leads to a contraction of the unit cell and significant modifications to the magnetic states-a crossover from ferromagnetism at room temperature for Ge-rich compounds to antiferromagnetism for Si-rich compounds. The compositional dependence of the room temperature lattice parameters exhibits non-linear behaviour around x = 1.2, reflecting the presence of magnetovolume effects. Re-entrant ferromagnetism has been observed in both PrMn2Ge1.0Si1.0 and PrMn2Ge0.8Si1.2 compounds with co-existence of canted ferromagnetism and canted antiferromagnetism detected, with both compounds exhibiting a larger unit cell volume in the canted Fmc state than in the canted AFmc. Combined with earlier studies of this system, the magnetic phase diagram has been constructed over the full range of PrMn2Ge2-xSix compositions (x = 0.0-2.0) and over the temperature range of interest (T = 3-480 K). In common with other systems in the RMn2X2 series, the overall magnetic behaviour of PrMn2Ge2-xSix compounds is governed by the strong dependence of the magnetic couplings on the Mn-Mn spacing within the ab-plane. Both total manganese moment mu(Mn)(tot) and in-plane manganese moment mu(Mn)(ab) at 5 K are found to decrease with increasing Si content, which can be ascribed to the reduction of Mn-Mn separation distance and stronger Si-Mn hybridization compared with Ge-Mn hybridization. Pr site ferromagnetic ordering occurs for x < 1.6 below T-C(Pr). © 2013, IOP Publishing Ltd.
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    Structural and magnetic phase separation in PrMn2Ge2-xSix compounds
    (Australian Institute of Physics, 2011-02-03) Wang, JL; Kennedy, SJ; Campbell, SJ; Hofmann, M; Zeng, R; Dou, SX; Arulraj, A; Stusser, N
    Ternary intermetallic compounds of RMn2X2 (where R = rare earth or Yttrium and X = Si or Ge) display a rich variety of magnetic structures due to sharp changes in magnetic exchange interactions between neighbouring manganese atoms, resulting from changes in chemical pressure. Observed variants in the magnetic structure include ferromagnetic (f), collinear & non-collinear antiferromagnetic (a/f), mixed axial f + planar a/f, and even incommensurate a/f structures. This remarkable behaviour is symptomatic of subtle changes in interatomic bond lengths, differentiated at sub-picometre length scales. Transformation between magnetic variants is often accompanied by structural distortions due to magnetoelastic coupling. Further to this, we find that some pseudoternaries, in which one or more sites has mixed occupancy (e.g. La & Y mixed on the R site or Si & Ge mixed on the X site), simultaneously display two structural variants with different axial magnetic order (f or a/f). Such behaviour is most clearly seen in PrMn2Ge2-xSix compounds where x ≈ 1. We report a neutron diffraction study on the PrMn2Ge2-xSix system, through which we gain new insights into the magnetic and structural origins of the curious behaviour of these compounds. In certain regions of the phase diagram we clearly see phase separation (both structural and magnetic), which leads us to propose a two-phase structural model driven by changes in the Mn-Mn magnetic exchange energy, and related to variations in local strain propagated by the shared crystallographic sites. This interpretation brings into question whether a random substitution could produce such remarkable magnetoelastic phenomena or whether local site-specific atomic order is prevalent in the family of mixed 122 compounds.