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Resistive, magnetoresistive and magnetic properties of four kinds of pressedCrO2 powders, synthesized by hydrothermal method of chromic anhydride have beeninvestigated. The particles in powders constituted of rounded particles(diameter 120 nm) or needle-shaped crystals with an average diameter of 22.9 nmand average length of 302 nm. All of the particles had a surface dielectricshell of varying thickness and different types (such as oxyhydroxide -CrOOH orchromium oxide Cr2O3). For all the samples at low temperatures we foundnon-metallic temperature dependence of resistivity and giant negativemagnetoresistance (MR). The maximum value of MR at low temperatures (T \approx5 K) is \approx 37% in relatively small fields (0.5 T). At higher temperaturesthere was a rapid decrease of MR (up to \approx 1% / T at T \approx 200 K). Themain objective of this work was studying the influence of properties andthickness of the intergranular dielectric layers, as well as CrO2 particleshape, on the magnitude of the tunneling resistance and MR of the pressedpowder. The new results obtained in this study include: (1) detection at lowtemperatures in powders with needle-like particles a new type of MR hysteresis,and nonmonotonic MR behaviour with increasing magnetic field (absolute value ofthe MR at first grows rather rapidly with the field, and then beginsdiminishing markedly, forming a maximum), and (2) detection of non-monotonictemperature dependence, where - a field in which the resistance in a magneticfield has a maximum, as well as finding discrepancies in values of andcoercivity fields, (3) detection of the anisotropy of MR, depending on therelative orientation of the transport current and the magnetic field, (4) a newmethod of synthesis, to regulate the thickness of dielectric coating.

Resistive and magnetoresistive properties of CrO2 pressed powders with different types of inter-granular dielectric layers