Numerical calculations of the entropy and magnetization of magnetic fluidswith chain aggregates

The dimensionless entropy change ((S) over bar) and the equilibrium reduced magnetization ((M) over bar) for magnetic fluids with Linear chain aggrega tes were calculated numerically as a function of volumic packing fraction ( rho), magnetic field (H), relative strength of the magnetic interactions (l ambda), and radius of colloidal particles (a). For each S curve the results were compared with the corresponding results obtained using the classical theory of Superparamagnetism. For each (S) over bar versus alpha (= mu(0)mH /kT) curve a minimum was observed which varies in magnitude and position de pending on the parameters lambda and rho, and was found to be associated wi th an optimum chain number and their chain length. The reduced magnetizatio n ((M) over bar) was found to increase with increasing lambda and rho, and its values are higher than the values for noninteracting superparamagnetism using classical theory. (C) 1999 Elsevier Science B.V. All rights reserved .