The Investigation Of Electrical And Magnetic Transport Properties Of Field Aligned CrO₂ Composites

Half metals are unusual ferromagnets that have electrons at the Fermi level in a single spin state, either spin up or spin down. Theoretical and experimental works have suggested that CrO2 is the simplest half-metallic ferromagnets with near 100% spin-polarization and as high as 392~396 K. Consequently, CrO2 is considered to be an ideal,potential material in Magneto-electronics devices . At the same time, magnetic composites have been reported to exhibit different, and often improving properties of electrical and magnetotransport of compared to its bulk counterpart. In addition,half metals are completely spin polarized, tunneling junctions made from such materials would have an extremely large magnetoresistance (MR) and significant switching effect at low field . Dai etal. have reported a large negative MR of about 41% in field-aligned CrO2 powders. But these MR significantly decreased with increasing temperature and the spin independent hopping conduction is suggested to be responsible for the suppression of MR at high temperature.To obtain the greater room temperature magnetoresistance,an insulating polymer as a barrier between CrO2 particles were introduced, and then aligned in a strong magnetic field in this paper. For such aligned CrO2/polymer composites, their structure,magnetization and magnetotransport properties were studied. From X-ray diffraction (XRD) data, it was found that the our aligned composite is strongly anisotropic and crystal structure of polycrystalline aligned CrO2 is not changed, with the PS phase lying in the grianboundaries probably. The magnetic and magnetotransport properties of field-aligned half-metallic CrO2/PS composite have been studied. One can see that when the applied field is parallel and perpendicular to the easy axis, the hysteresis loop is close to a square with a large remanence. When the magnetic field is perpendicular to the easy axis, the hysteresis loop becomes more gradual and rounded with a much-reduced remanence.The magnetoresistance variation with the angle is periodic at the positive and negative field and does not change at zero magnetic field. The results of enhanced magnetoresistance measurements at low temperature showed that, the PS insulator strengthened their insulated barrier among granulas, which favored the tunneling at the grain boundaries for spin-polarized electron. This behavior is related to the existence of the PS insulating phase, which improves the spin-polarized electron inter-granular tunneling.The measurements of resistivity at zero field showed that, with the increase of PS proportion, the resistivity was enhanced evidently. Futher more, the conductance showed a strong temperature dependence. At low temperature , the spin-dependent intergranular tunneling across the grain boundaries induced by the Coulomb block is the dominant transport mechanism. While at high temperature, the inelastic hopping of spin-independent electrons through the localized states induced by vices and impurities in the grain-boundary barrier occupies the dominant status.