Modified mean-field theory of the magnetic properties of concentrated, high-susceptibility, polydisperse ferrofluids

The effects of particle-size polydispersity on the magnetostatic properties of concentrated ferrofluids are studied using theory and computer simulation. The second-order modied mean-eld (MMF2) theory of Ivanov and Kuznetsova [Phys. Rev. E 64, 041405 (2001)] has been extended by calculating additional terms of higher order in the dipolar coupling constant in the expansion of the
initial magnetic susceptibility and the magnetization curve. The theoretical predictions have been tested rigorously against results from Monte Carlo simulations of model monodisperse, bidisperse, and highly polydisperse ferro
uids. Comparisons have been made between systems with the sameLangevin susceptibility and the same saturation magnetization. In all cases, the new theoretical magnetization curve shows better agreement with simulation data than does the MMF2 theory. As for the initial susceptibility, MMF2 theory is most accurate for the monodisperse model, while the new theory works best for polydisperse systems with a signicant proportion of large particles. These results are important for the analysis and characterization of recently synthesized
polydisperse ferrofluids with record-breaking values of the initial magnetic susceptibility.