THE COERCIVITY OF PARTICLE ARRAYS

Thermal effects in an array of nonidentical, noninteracting single domain ferromagnetic particles are studied. Two sources of stochasticity are recognized within the array: the intrinsic stochastic nature of thermal relaxation and variations within the particles' volume and orientation with respect to applied field, both are discussed in turn. We use the log-normal volume distribution {approximately (sigmaV)-1 exp[-ln2(V/V0)/(2sigma2)]} and a cos(nu) psi angular distribution to calculate the mean coercivity B(c)BAR of a particle array. We find that B(c)BAR increases rapidly with increasing average volume [V] = V0 exp (sigma2/2) while for misaligned particles (nu < infinity) we observe a sudden decrease in B(c)BAR accompanied by a gradual loss of frequency dependence.