The quasi-magnetic-hysteresis behavior of polydisperse ferrofluids with small coupling constant

Abstract

The magnetization behaviors of ferrofluids based on γ-Fe 2 O 3 /Ni 2 O 3 composite nanoparticles of size about 11 nm have been investigated. The dipole coupling constant λ of these particles is so small (0.43) that they cannot form aggregates through magnetic interaction alone. Experimental results have shown that for a polydisperse ferrofluid with a particle volume fraction of ϕ V =2.4%, the magnetization curve exhibits quasi-magnetic-hysteresis behavior, i.e., the demagnetization curve lies above the magnetization curve in a high field. However, for a more dilute γ-Fe 2 O 3 /Ni 2 O 3 ferrofluid with ϕ V =0.94%, the magnetization curve does not show such behavior. According to the bidisperse model for polydisperse ferrofluids, these magnetization behaviors may be attributed to field-induced effects of self-assembled pre-existing chain-like aggregates. For such pre-existing chain-like aggregates, the orientation of the moments inside the particles is not co-linear, so that during the magnetization and demagnetization processes, their apparent magnetizations at the high-field limit are different. As a consequence, the magnetization curve of the ferrofluid with ϕ V =2.4% displays quasi-magnetic-hysteresis.