Horielyi V. Magnetoresistive properties of granular oxide systems with spin-dependent electron tunneling

Magnetoresistive phenomena in compacted nanopowders, composed of half-metal CrO2 nanoparticles, coated with dielectric shells of different thickness and composition, were studied. The experiments revealed anisotropic properties of the tunneling magnetoresistance for the powder samples, which were related to the anisotropy of the shape and magnetic properties of the nanoparticles they were composed of. Percolation effects in the studied samples led to the changes in current carrying channels with decreasing temperature and increasing magnetic field, and allowed for the first time to observe unusual shapes of magnetoresistance hysteresis loops. The reasons for discrepancy between the fields of magnetoresistance coercity and the coercity of the samples’ magnetization, which appeared with decreasing temperature were identified for the first time and were explained by the multidomain magnetic state in some of the needle-shaped nanoparticles with maximal length. The scientific results obtained in this work are not of theoretical only, but also of practical interest. The high magnitudes of tunnel magnetoresistance allow to use CrO2 nanocomposites in spintronics devices and the anisotropy of the coercive force in pressed samples suggests using the nanostructured media for high density information recording.