Magnetoresistance of Chromium Dioxide Powder Compacts

Cold-pressed powders of the half-metallic ferromagnet ${\mathrm{CrO}}_2$ are dielectric granular metals. Hysteretic magnetoresistance with maxima at the coercive field arises from interparticle contacts. Dilution with insulating antiferromagnetic ${\mathrm{Cr}}_2{\mathrm{O}}_3$ powder reduces the conductivity by 3 orders of magnitude, but enhances the magnetoresistance ratio which reaches 50% at 5K. The negative magnetoresistance is due to tunneling between contiguous ferromagnetic particles along a critical path with a spin-dependent Coulomb gap.