Shirai, M; (2007) Experimental investigations of frustrated antiferromagnets. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes the investigation of frustrated magnetic systems, focusing on the pyro-chlore antiferromagnets (AFM). The rare-earth pyrochlore oxides R2B2Oj {lcub}R+ = rare earth, = transition metal), crystallizes in the Fd3m space group, with the magnetic ions (R) and the non-magnetic transition metal (B) ions situated on the 16c and on the 6d sites, respectively. As a result, the magnetic ions form lattices of corner-sharing tetrahedra-a stage in which frustration is manifested. Erbium titanate Er2Ti2C>7 and stannate ESO are both frustrated pyrochlore antiferromagnets which are expected to have very similar magnetic properties. However, Er2Ti207 magnetically orders at 7n 1.1 K but Er2Sn207 remains disordered down to F 150 mK. The main aim of this thesis is therefore to investigate the origin of this difference. The magnetic properties of the series Er2Ti2.jrSn.xO7 were studied as a function of jc (jc = 0 2). Firstly, from the powder neutron diffraction patterns obtained, it was shown that the detected order diminishes as jc * 2. However, in contrast to the previously reported result, a long-ranged ordering transition was detected for Er2Ti207 (at 80 mK) as well as for Er2Ti2<37 (at 1.2 K). The ordered magnetic structure was determined while Er2Ti207 orders into the Champion state (one of the ground state of the pyrochlore (lll)-XY AFM), Er2Sn207 orders into a ground state of a Heisenberg pyrochlore AFM with dipolar interaction (Palmer-Chalker state). Secondly, the existence of the quantum critical point (QCP) in the composition range 1.5 < x < 2 was investigated by means of low temperature magnetization measurements. The critical composition jccrjticai 1-55 was determined. Thirdly, the single ion anisotropy of the series ETiSnxO (x = 0 2) was studied by analyzing the crystal electric field (CEF) measured by the inelastic neutron scattering. As a result, the single-ion moments were shown to retain a constant size ( 3 Ub) throughout the series. In contrast, the nature of the single ion anisotropy changed from being XY-like for jc = 0 into being Ising-like for jc = 2 (with respect to the <111)- axes). Finally, the temperature-field phase diagram of the experimental realization of the Heisenberg pyrochlore AFM -Gd2Ti207, was investigated by means of powder neutron diffraction.

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