The effect of particle interactions on Curie–Weiss behavior in ferrofluids

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Abstract
The initial susceptibility of a number of Fe3 O4 particle ferrofluids with volume concentrations, ε=0.08, 0.07, 0.05, 0.03, 0.02, and 0.01 (prepared by diluting a stock fluid of Ms =40 kA m−1) has been measured in the temperature range 220–450 K. The mean particle sizes Dm (magnetic) and Dp (physical) are 74 and 81 Å, respectively. The ferrofluids exhibit Curie–Weiss‐type behavior with negative (Néel) temperature temperature intercepts To . The form of the Curie–Weiss behavior is interpreted as arising from particle interactions which are strong enough to reduce the initial susceptibility below the noninteracting value in the temperature range of measurements. It is suggested that interactions lead to the formation of closed particle structures of zero moment in zero field and, as would be the case for an antiferromagnetic material, leads to Curie–Weiss behavior with a negative ‘‘Néel’’ temperature. Negative values of To are thus characteristic of an interacting particle system.

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