Magnetization behavior of ferrofluids with cryogenically imaged dipolar chains

Abstract

Theories and simulations have demonstrated that field-induced dipolar chains affect thestatic magnetic properties of ferrofluids. Experimental verification, however, hasbeen complicated by the high polydispersity of the available ferrofluids, and themorphology of the dipolar chains was left to the imagination. We now present theconcentration- and field-dependent magnetization of particularly well-defined ferrofluids,with a low polydispersity, three different average particle sizes, and with dipolar chainsthat were imaged with and without magnetic field using cryogenic transmissionelectron microscopy. At low concentrations, the magnetization curves obey theLangevin equation for noninteracting dipoles. Magnetization curves for the largestparticles strongly deviate from the Langevin equation but quantitatively agreewith a recently developed mean-field model that incorporates the field-dependentformation and alignment of flexible dipolar chains. The combination of magneticresults and in situ electron microscopy images provides original new evidence forthe effect of dipolar chains on the field-dependent magnetization of ferrofluids.