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

<p>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.</p>

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

<p>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.</p>

Magnetic Properties and Phase Diagram of Quasi-two-dimensional Na2Co2TeO6 Single Crystal Under High Magnetic Field

We investigated the magnetic characteristics of Na2Co2TeO6 at different temperatures and magnetic field. The experimental results indicated that the magnetic field can disturb the antiferromagnetic interaction and lead to the disorder. Magnetization curves measured with different angles θ (θ is between the magnetic field direction and c axis) express the magnetocrystalline anisotropy in this system. when the angle θ=0 (magnetic field parallel to c axis), two continuous magnetic phase transitions at critical temperature TN1 and TN3 were observed. As θ changes, TN1 is almost independent on θ, indicating the magnetic ordering at TN1 was a spontaneous behavior with a robust AFM characteristic. On the other hand, as θ increases from 0 to 180, TN3 presents extreme value at θ=90 (magnetic field perpendicular to c axis). It indicates that TN3 were sensitive to temperature and magnetic fields. At some angles closing to ab plane, an additional phase transition was observed at TN2. This phase transition at TN2 may mainly result from the long range antiferromagnetic ordering within ab-plane. Furthermore, the magnetization measurement up to 50 T revealed the strong antiferromagnetic coupling in the system, and in which the magnetic coupling within the honeycomb layers is strong and the magnetic coupling interaction between honeycomb layers is weaker. Based on the experimental results, we have obtained the complete magnetic phase diagram.

Novel eco-friendly low cost and energy efficient synthesis of (Nd–Pr–Dy)2Fe14B magnetic powder from monazite concentrate

Syntheses of Nd2Fe14B magnetic powder by conventional method is a complicated multi-step process, which produces harmful pollutants and consumes a huge amount of energy and resources. Herein we report a simple chemical route for the preparation of (Nd–Pr)2Fe14B magnetic powder using monazite concentrate as a precursor. Th, U, Sm, and La impurities were removed from monazite leachate by roasting, solvent extraction and leaching the concentrate. Purified leachate consisting of Nd and Pr Chlorides was added to the FeCl3 solution, and the solution produced was co-precipitated with NaOH. RE and Fe hydroxide precipitates were converted to the oxides by annealing at 700 °C. Boric acid and CaH2 were added in the RE and Fe oxides produced, and this mixture was reduced and diffused to (Nd–Pr)2Fe14B. Magnetic properties of the (Nd–Pr)2Fe14B produced were enhanced by introducing antiferromagnetic coupling, induced by Dy addition and efficient removal of CaO byproduct through ball milling in ethanol which increased the BHmax from 3.9 to 11.45 MGOe. Process reported is energy efficient, environment-friendly, time saving and low-cost.

Effect of Mn Substitution Fe on the Formability and Magnetic Properties of Amorphous Fe88Zr8B4 Alloy

Elemental substitution is commonly used to improve the formability of metallic glasses and the properties of amorphous alloys over a wide compositional range. Therefore, it is essential to investigate the influence of element content change on the formability as well as magnetic and other properties. The purpose is to achieve tailorable properties in these alloys with enhanced glass forming ability. In this work, the glass-forming ability (GFA) and magnetic properties of the minor Mn-substituted Fe88Zr8B4 amorphous alloy were investigated. The addition of Mn improving the amorphous forming ability of the alloy. With the addition of Mn, the magnetic transition temperature, saturation magnetization and the magnetic entropy changes (−ΔSm) peaks decreased simultaneously, which is possibly caused by the antiferromagnetic coupling between Fe and Mn atoms. The dependence of −ΔSmpeak on Tc displays a positive correlation compared to the −ΔSmpeak- Tc−2/3 relationship proposed by Belo et al.