Model of the magnetic properties of FePt granular media

We have used a Monte Carlo model of an interacting fine particle system to study the magnetic properties of FePt nanoparticle arrays. The model is capable of calculating the temperature dependence of the hysteresis loop and dc demagnetization remanence curves. We have made a detailed fit of the theory to experimental data over the temperature range 10-350 K. The value of the best-fit anisotropy is K=2.35x10(7) erg/cc. This value is around one third the value expected for bulk FePt, possibly due to chemical disorder. The determination of K by application of Sharrock's law to the temperature dependence of H-c gives a value of 7x10(7) erg/cc. It is shown that this arises from a breakdown of Sharrock's law for systems containing a significant fraction of superparamagnetic particles. (C) 2002 American Institute of Physics.