Powder magnetoresistance (invited)

Magnetoresistance observed in pressed powder compacts of half-metallic ferromagnetic oxides is reviewed. The main, isotropic negative magnetoresistance, which exceeds 50% in CrO2 at low temperature, is due to alignment of the the ferromagnetic moments of contiguous ferromagnetic grains. The effect is related to interparticle tunelling and shows hysteresis similar to that of the bulk magnetization. Spin-dependent Coulomb blocade observed in the smallest particles. There is also, an anisotropic magnetoresistance of up to 1% and a high-field increase in conductivity of order 1%/T in the powder compacts. (C) 1999 American Institute of Physics. [S0021-8979(99)74208-8].