Magnetic frustration in insulators and conductors

• Main Author: Fenner, L. A.
• Published: University College London (University of London) 2011
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625701

This thesis presents experimental investigations of magnetically frustrated materials. The first part describes an investigation of the zero-point entropy of spin glasses. Heat capacity measurements on the insulating spin glass series ZnxCu1−xAl2O4 were conducted, and their zero-point entropies were found to agree well with the predicted value for the Tanaka and Edwards model of a Sherrington–Kirkpatrick XY spin glass, confirming that this model is applicable to real systems. The majority of the thesis focuses on structural, magnetic and electron transport studies on members of the metallic Fe–Sn family. Powder neutron diffraction showed that FeSn and Fe3Sn, with crystal structures based on single kagome layers, are simple collinear antiferroand ferromagnets respectively. In contrast, the kagome bilayer based Fe3Sn2 was found to be a rare example of a non-collinear, frustrated ferromagnet (TC ≈ 640 K). The complexity of the magnetic interactions in this material was further demonstrated by the emergence of a re-entrant spin glass phase at low temperature (Tf ≈ 80 K), as evidenced by magnetic susceptibility and thermoremanent magnetisation measurements. The magnetic moments in frustrated Fe3Sn2 were found to rotate towards the basal plane upon cooling. This rotation may be the cause of the temperature dependent muon spin depolarisation rate seen in μSR measurements, in addition to the evolution of the complex magnetic surface domain pattern, as illustrated by magnetic force microscopy. Furthermore, the non-collinearity of the moments could be the origin of the unusual anomalous Hall effect (AHE) observed in this material. Measurements show that Fe3Sn2 has a large saturated Hall resistivity (3.18 ± 0.02 μΩcm) at room temperature, and indicate that the AHE cannot be ascribed to any of the conventional intrinsic mechanisms. These findings highlight the potential of Fe3Sn2 for AHE based applications, and its importance for future investigation of the relationship between non-collinearity and the AHE.