Magnetic and transport properties of electronically spin polarised double perovskites and Heusler intermetallics

Abstract

Half-metallic ferromagnets with 100 % electronic spin polarisation are an interesting class of materials for new spin transport electronics applications. Some of the double perovskites and Heusler alloys are predicted to be half-metallic with Curie temperatures above room temperature. This thesis presents the results from an experimental study of polycrystalline double perovskites Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆, and ordered and disordered epitaxial thin films of Co₂MnSi Heusler alloys. A magnetothermopower was observed in Sr₂₋ₓLaₓFeMoO₆ and Ba₂₋ₓLaₓFeMoO₆. This magnetothermopower can be explained in terms of a spin-tunnelling contribution to the thermopower between grains that changes in an applied magnetic field. The results from the high temperature (above 400 K) magnetisation studies on Ba₂₋ₓLaₓFeMoO₆ in the paramagnetic region reveal that a localised electron model with antiferromagnetic coupling to itinerant electrons can account for the carrier concentration dependent effective moments. The correlation between the bare itinerant electron susceptibility and the Curie-Weiss temperature supports the kinetic energy driven model that has been used to account for the electronic spin polarisation and high Curie temperatures.

Antisite disorder is evident in the Co₂MnSi thin films that leads to a reduction in the saturation magnetisation. The resistivity of the ordered Co₂MnSi thin film is linear in temperature whereas the resistivity of the disordered film increases at low temperature due to weak localisation. A magnetoresistance is observed in ordered and disordered films. At low fields (below 0.1 T) the magnetoresistance is likely to be due to domain wall scattering. For magnetic fields greater than 0.1 T there is likely to be a contribution from a magnetic-field-induced suppression of the weak localisation resistivity. Similar magnetoresistance behaviour was observed for ordered and disordered films. There is a large anomalous Hall resistivity observed in the ordered and disordered Co₂MnSi thin films. In the case of the ordered film it is found that the anomalous Hall effect is dominated by skew scattering.