Phase transition and magnetotransport properties of ball-milled half-metallic CrO2

Small CrO2 particles with mean diameters ranging from 11 to 25 nm have been prepared by ball milling. X-ray diffraction studies show a continuous lattice expansion with increasing milling time. A phase transition to Cr2O3 also occurs with a sudden increase in the amount of Cr2O3 found between 5 and 8 h of milling. The decreases of low-field magnetoresistance with increasing milling time is correlated to the expansion of the lattice parameters of CrO2, which probably leads to the reduction in its spin polarization. High-field magnetoresistance increases with the milling possibly due to the enhanced mixed valence of the chromium, which supports the double exchange model. (C) 2002 American Institute of Physics.