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Title: An investigation of relaxor ferromagnets
Author: Chin, Chun Mann
ISNI:       0000 0004 7971 7206
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Q-dependent diffuse scattering was observed at low angles in a neutron diffraction experiment performed on the relaxor ferromagnet La3Ni2SbO9 using a spin-polarised beam. The nuclear component could be modelled using direct Monte-Carlo methods if it was assumed that short (~5.5 Å) Sb-Sb contacts were energetically disfavoured. The magnetic component of the diffuse scattering at 110 K (TC = 108 K) was analysed using both direct and reverse Monte-Carlo techniques. When antiferromagnetic nearest-neighbour coupling was assumed to be dominant in this perovskite, second neighbour coupling was found to be weak but non-zero with third-neighbour coupling being negligible. The short-range structural and magnetic orderings were found to have correlation lengths of 15 and 9 Å, respectively. The consequences of replacing d10 Sb5+ by d0 pentavalent cations were investigated; all the compounds reported are isostructural with La3Ni2SbO9. La3Ni2TaO9 was found to be a relaxor ferromagnet but La3Ni2NbO9 behaved as a spin glass below 35 K. Solid solutions La3Ni2SbxTayNb1-x-yO9 were synthesised and characterised. It was again shown that the relaxor behaviour is incompatible with the presence of Nb5+ in the system. In contrast, doping La3Ni2B'O9 (B' = Sb, Ta or Nb) with Cu2+ cations was found to increase TC and the bulk magnetisation. For example, La3Ni1.75Cu0.25NbO9 exhibits ferrimagnetic behaviour below ~90 K with the development of long-range G-type magnetic ordering. It is proposed that relaxor regions and long-range ordered regions coexist in this system. The sensitivity of cation ordering, and hence magnetic behaviour, to the charge difference between Ni2+ and the diamagnetic six-coordinate cation was studied by replacing Sb5+ by Te6+ or W6+. Electron microscopy revealed a significant level of cation disorder in a severely-twinned sample of SrLa2Ni2TeO9; spin-glass and antiferromagnetic phases coexist below 35 K. The Ni2+ cations in CaLn2Ni2WO9 (Ln = La, Pr or Nd) form a spin glass, although the behaviour of each composition reflects the magnetic properties of Ln3+.
Supervisor: Battle, Peter Sponsor: University of Oxford
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemistry, Inorganic