Effective medium theory of magnetoresistance in half-metallic granular systems

Abstract

We present a conductance network model to study magnetotransport properties of half-metallic granular systems. The transport process here is supposed to be controlled by both the charging energies and the spin-dependent tunneling between the grains. Using the effective medium theory, we obtain the conductivity of the system as a function of the temperature T and the applied magnetic field H. The effect of the charging energy and the spin polarization on the magnetoresistance is studied. It is also shown that the rapid decrease of the magnetoresistance with temperature may result from an unusual exponential decay of spin polarization as a consequence of the strong temperature dependence of grain surface magnetization. Our results are compared with the experiments on pressed powders of half-metallic CrO 2 and a good agreement is found.