Thermal dependence of magnetotransport in nanogranular magnetic media

Magnetotransport simulation is performed on granular nanomagnets (Co) in insulator, of average radius of 2.5 nm, over a temperature range (5<T<1000 K) which straddles the single-domain and superparamagnetic regimes. The M-H hysteresis is calculated based on a two-state model, which is solved analytically using the Master Equation. The two-state model is then refined to account for fluctuations into states in the vicinity of the two minima. The occupation probability of these states is determined by a birth-death chain analysis. The resulting M-H hystereses show decreasing coercivity with T. At higher T>200 K, the M-H curve approaches the Langevin function, but with a small discrepancy, due to the intrinsic anisotropy of Co. The magnetization results are then combined with a stochastic Monte Carlo transport model which combines the effects of stochastic spin-polarized tunneling, Coulomb blockade, and the magneto- and electrostatic influence of the contacts. The tunneling magnetoresistance shows a complex thermal dependence, with distinct behavior for different types of contact electrodes used. (C) 2003 American Institute of Physics.