Resistive and magnetoresistive properties of compacted CrO2 powders with different types of intergranular dielectric layers

Resistive, magnetoresistive and magnetic properties of four types of pressed CrO2 powders synthesized from chromic anhydride by the hydrothermal method were investigated. The new synthesis method allowed controlling the thickness of dielectric shells. The powders consisted of rounded particles (approximate to 120 nm in dia.) or acicular crystals (approximate to 22.9 nm in dia. and 302 nm in length). The particles were covered with a surface dielectric shell of different thickness and type (e.g. beta-CrOOH oxyhydroxide or chromium oxide Cr2O3). The influence of the properties and the thickness of intergranular dielectric layers as well as the shape of CrO2 particles on the tunneling resistance and magnetoresistance (MR) of the pressed powders was studied. It was found that at low temperatures all the investigated samples displayed a nonmetallic temperature dependence of resistance and a giant negative MR. The maximal values of MR at T approximate to 5 K were found to be approx. 37% in relatively low magnetic fields (0.5 T). The MR decreased rapidly with increasing temperature (down to approx. 1% in 1 T at T approximate to 200 K). At low temperatures the powders with acicular particles exhibited a new type of MR hysteresis and nonmonotonous dependence of MR with increasing magnetic field. A nonmonotonous temperature dependence H-p(T), where H-p is the field in which the resistance is maximal, mismatch between the values of H-p and coercive force H-c, and the anisotropy of MR as a function of mutual orientation of transport current and magnetic field were observed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770508]