Magnetic and magnetotransport properties of half-metallic CrO_2-SnO_2 composites

摘要

Half-metallic (CrO_2)_(1-x)-(SnO_2)_x composites were prepared under high temperature and high pressure conditions. The composites are composed of large rod-like CrO_2 grains and small SnO_2 nanoparticles. The CrO_2 in the composites is very pure and its saturation magnetization is very close to the theoretical value. The composition dependence of magnetic and magnetotransport properties of the composites was studied. The coercive force (H_c) and remanence ratio (M_r/M_s) of the composites increase dramatically with increasing SnO_2 content x for x ＞ 0.6. This should be due to that the CrO_2 grains have been well separated by SnO_2 nanoparticles and the magnetic interactions among CrO_2 grains become weak when x ＞ 0.6. The resistivity and magnetoresistance at 5 K of the composites increase with increasing x, and the increase quickens up at x - 0.5. When x≥0.5, the (CrO_2)_(1-x)-(SnO_2)_x composites show insulator behavior, and the temperature dependence of the resistivity can be well described by fluctuation-induced tunneling model. But when x≤0.4, the (CrO_2)_(1-x)-(SnO_2)_x composites show insulator-metal transitions, and the transition temperature increases with increasing SnO_2 concentration. Below the transition temperature, their resistivity can also be explained by fluctuation-induced tunneling model. The (CrO_2)_(1-x)-(SnO_2)_x composites show greater magnetoresistance than pure CrO_2 at low temperature, which is attributed to enhancement of tunneling magnetoresistance by adding of SnO_2.