Nuttall, Christopher John; (1998) Bimetallic tris-oxalate magnets : Synthesis, structure and properties. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis the series of compounds (cation)MᴵᴵFeᴵᴵᴵ(ox)₃ {Mᴵᴵ = Mn and Fe, ox = C₂O₄²⁻} are studied for cation = NPr₄, NBU₄, Npent₄, PPh₄, AsPh₄, PBU₄ and PPN. X-ray diffraction reveals all 14 compounds crystallise with layer honeycomb structures. A linear correlation is found for both the inter-layer and the intra-plane unit-cell parameters between analogous MnFe and FeFe compounds. The profiles have been compared to single crystal structure models of (cation)MM'(ox)₃ in the form of partial refinement. For cation = Npent₄ the powders can be described by the C222(l) Npent₄MnᴵᴵFeᴵᴵᴵ(ox)₃ single crystal structure (solved at 120K). In the case of cation = NPr₄, NBu₄ and PBU₄, the compounds have found to be bi-phasic containing R3c and P6(3) phases, similar to the R3c NBu₄MnCr(ox)₃ and P6(3) NBu₄MnFe(ox)₃ single crystal structures. A stacking faulting model, allowing the layer sequence to vary between R3c and P6(3) structures, was found to explain the broadening apparent in the X-ray profiles. (cation)MᴵᴵFeᴵᴵᴵ(ox)₃ {Mᴵᴵ = Mn, Fe} compounds present a variety of magnetic ordering phenomena. (cation)MnᴵᴵFeᴵᴵᴵ(ox)₃ compounds are nominally antiferromagnet with Tc's in the range 24-27K. Neutron diffraction experiments on PPh₄(-d₂₀)MnᴵᴵFeᴵᴵᴵ(ox)₃ reveal the magnetic structure to be well explained by antiparallel spin alignment along the c-axis with R3c magnetic Shubnikov symmetry. The low temperature uncompensated moments exhibited in the MnFe compounds were reasoned to be the result of sublattice disorder. (cation)FeᴵᴵFeᴵᴵᴵ(ox)₃ compounds are ferrimagnetic and have an unusual cation dependence. For cation = NPr₄, NBU₄, Npent₄, PBU₄ and PPN Tc = 43.5-45K and the temperature dependent magnetisation approximates a variant of Néel type N ferromagnetic order, with a temperature of compensation (Tcomp) and a low temperature negative magnetisation. For cation = PPh₄ and AsPh₄ compounds Tc = 34-37K and the magnetization temperature dependence is similar to Néel type Q ferrimagnetic order, with low temperature positive magnetisation. The magnetic order in the FeFe series has been investigated by AC and DC SQUID magnetometry, MuSR spectroscopy, Mössbauer spectroscopy (experiments not performed by the author) and, in the case of PPh₄(-d₂₀)FeFe(ox)₃, neutron diffraction. The compounds with low temperature negative magnetisation exhibit evidence of a magneto-strictive transition (Tcomp < T < Tc) which stabilises the negative remanance. The compounds with a low temperature positive magnetisation exhibit considerable evidence for only a glassy-type magnetic order developing at low temperature. This has been correlated to Fe and consequent Feᴵᴵ deficiencies in the compounds.

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