Resistive and magnetoresistive properties of CrO2 pressed powders with   different types of inter-granular dielectric layers
  Resistive, magnetoresistive and magnetic properties of four kinds of pressed CrO2 powders, synthesized by hydrothermal method of chromic anhydride have been investigated. The particles in powders constituted of rounded particles (diameter 120 nm) or needle-shaped crystals with an average diameter of 22.9 nm and average length of 302 nm. All of the particles had a surface dielectric shell of varying thickness and different types (such as oxyhydroxide -CrOOH or chromium oxide Cr2O3). For all the samples at low temperatures we found non-metallic temperature dependence of resistivity and giant negative magnetoresistance (MR). The maximum value of MR at low temperatures (T \approx 5 K) is \approx 37% in relatively small fields (0.5 T). At higher temperatures there was a rapid decrease of MR (up to \approx 1% / T at T \approx 200 K). The main objective of this work was studying the influence of properties and thickness of the intergranular dielectric layers, as well as CrO2 particle shape, on the magnitude of the tunneling resistance and MR of the pressed powder. The new results obtained in this study include: (1) detection at low temperatures in powders with needle-like particles a new type of MR hysteresis, and nonmonotonic MR behaviour with increasing magnetic field (absolute value of the MR at first grows rather rapidly with the field, and then begins diminishing markedly, forming a maximum), and (2) detection of non-monotonic temperature dependence, where - a field in which the resistance in a magnetic field has a maximum, as well as finding discrepancies in values of and coercivity fields, (3) detection of the anisotropy of MR, depending on the relative orientation of the transport current and the magnetic field, (4) a new method of synthesis, to regulate the thickness of dielectric coating. 
N. V. Dalakova, B. I. Belevtsev, E. Yu. Beliayev, O. M. Bludov, V. A.   Pashchenko, M. G. Osmolovsky, O. M. Osmolovskaya
cond-mat.str-el cond-mat.mtrl-sci