Xiao-Yu Zhang

Enhanced room-temperature magnetoresistance in half-metallic CrO2/polymer composites

Abstract Granular CrO 2 /polystyrene composites were prepared by evaporating polystyrene solution. The composites consisting of the acicular half-metallic CrO 2 particles encapsulated properly with a thin layer of insulating polymer can exhibit a large enhancement in room temperature magnetoresistance (MR), up to −8.2% at a low field of 6.5 kOe. The microstructures and thermal behaviors of the composite are analyzed by XRD, SEM, TGA and DSC. The temperature dependence of the electrical resistance is studied at the temperature range (77–298 K). We also argue that the influence of polymer on room temperature MR is more significant than that at low temperature, which may be mainly attributed to spin-dependent tunneling between particles.

Study of the conductance and magnetotransport of CrO2–TiO2 composites

Abstract This letter presents an experimental study of the correlation between low field magnetoresistance (MR) and percolation process, and the dependence of conductance on temperature between 77 and 300 K in the CrO 2 –TiO 2 composites. We also discuss the conduction mechanisms of the composites and conclude that the conductance is mainly attributed to higher-order hopping at higher temperature.

Large magnetocaloric effect in chromium dioxide with second-order phase transition

A magnetic entropy change of 25.4 mJ cm−3 K−1 at H = 1.5 T in CrO2 nano-rods is observed, which is one of the largest magnetocaloric effects ever reported among those oxides. In this work, a molecular field model, Landau theory and Debye approximation are used to unveil the origins of the magnetic entropy change associated with a second phase transition in the system. The theoretical estimates indicate that the entropy change results mainly from a field-induced spin ordering, whereas the lattice entropy contribution is approximately 0.112–0.335 mJ cm−3 K−1. Finally, the electronic entropy change is considered to be negligible. The work is of significance not only in the understanding of the magnetic oxide refrigerant with large magnetic entropy change but also in the exploitation of a new class of potential magnetic refrigerants with large magnetostriction.

Studies of magnetic entropy change and phase transitions in SrRu1−xMnxO3 perovskite

This paper reports a systematical investigation on the role of magnetic phase transitions on magnetocaloric effect in polycrystalline SrRu1−xMnxO3 (0⩽x⩽0.72), combining with the temperature dependent magnetization measurement by a superconducting quantum interference device. Mn substitution drives the magnetic phase transition from ferromagnetic state to antiferromagnetic state via a short-range magnetic order. The magnetic entropy change (ΔSM) presents negative values and a maximum at Curie temperature TC and/or spin freezing temperature Tf for 0⩽x⩽0.27, which is due to the contribution of the spin ordering induced by magnetic field. In addition, we also observe positive magnetic entropy changes (ΔSP) at Neel temperature TN for high Mn concentration (x⩾0.41) as the charge ordering is negligible. Since the magnetization is insensitive to external field in the vicinity of TN, the positive magnetic entropy change is considered to stem from the dramatic temperature dependence of the magnetization.

AC magnetotransport property enhancement of Fe3O4 particles by modifying tunnelling barrier

We report an enhanced room temperature magnetoimpedance (−12.3%) effect in ball milled Fe3O4 particles. The crystalline structure, magnetic and transport properties indicate that an insulating α-Fe2O3 layer exists on the surface of the Fe3O4 particles during the ball milling process. Based on Simmons model, a series of non-linear current–voltage (I–V) curves display the change of barrier thickness and barrier height with respect to ball milling time. More importantly, Nyquist plots show that the grain boundary resistivity is sensitive to the applied magnetic field and dominates the magnetotransport properties. We assume that an appropriate barrier thickness plays a crucial role in the enhancement of the magnetoimpedance effect based on spin-dependent tunnelling.

Complex magnetoimpedance analysis of interparticle boundaries in Fe3O4 powder compact

Recent experimental results of Fe3O4 in polycrystalline film, powder compact, and single crystal form have indicated that boundaries play an important role in magnetoresistance effect. In this paper, the complex impedance analysis is used to evaluate the contribution of particle boundary to conduction process in Fe3O4 powders. Considering particle boundaries with different thicknesses, we developed a random resistor-capacitor network model to investigate the influence of magnetic field on impedance behaviors. The fitting results to experimental data indicated that two kinds of conduction mechanisms, spin-dependent tunneling and spin-independent hopping, dominate the transport process in particle boundaries. The capacitive transport through particles with a size of 10–20nm may yield negative magnetoreactance. It is also found that magnetic field has little influence not only on the competition between resistive and capacitive path, but also phase angle Φ(f). Finally, it can be deducted that the magnetoreac...