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Magnetic properties of the jarosites : model kagome antiferromagnets

The long-range ordered ground state of KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> has been determined unambiguously through Spherical Neutron Polarimetry on a natural single crystal. The magnetic arrangement has been shown to be the q = 0 structure with triangles of only positive chirality. A new transition to a state of higher magnetisation has been identified in a synthetic sample at T ~ 7K. The driving force for this has been attributed to anisotropy in the system and theory provides a possible ordering mechanism with weak ferromagnetic order. Magnetic neutron scattering confirms an intermediate phase at 52K < T < 65K with the coexistence of Bragg peaks and significant diffuse scattering. D<sub>3</sub>OFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OD)<sub>6</sub> has been shown to exhibit spin-glass type behaviour with a freezing temperature, T<sub>f</sub>, that is dependent on the timescale of the measurement. Intentional dilution of the magnetic lattice with Al<sup>3+</sup> induces a transition to long-range order at a temperature dependent on the level of doping. Mössbauer spectroscopy shows two equally-weighed magnetic sites with local field value and probability in excellent agreement with theoretical predictions. A new mixed-cation compound, H<sub>3</sub>OFe<sub>2.4</sub>Cr<sub>0.3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>, has been successfully prepared and represents the first experimental example of mixed magnetic ions on a kagome lattice. Field-cooled (FC) and zero-field-cooled (ZFC) dc susceptibility data cross at a temperature dependent on the magnitude of the applied field and hysteresis measurements suggest the presence of ferrimagnetism. High resolution neutron diffraction on D<sub>3</sub>OCr<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OD)<sub>6</sub> has shown that the crystal structure is slightly distorted from the <i>R ?m </i>to the <i>R 3 m </i>space group. A magnetic transition is observed at 2K through a sharp peak in ac susceptibility accompanied by a small FC/ZFC splitting. Polarised neutron diffraction shows no change in the magnetic correlation length, ξ, on cooling from 15K to 0.25K where only short-range order exists with ξ ~ 6.6Å, about half the width of a kagome star.
Date January 2002
CreatorsPettigrew, Keith G.
PublisherUniversity of Edinburgh
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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