The differential magnetic susceptibility of [(${\mathrm{CH}}_3$${)}_3$NH]${\mathrm{FeCl}}_3$⋅${2\mathrm{H}}_2$O has been measured as a function of temperature in the range 4.2 K<T<20 K in fields of 0, 500, 1000, and 1500 Oe. To interpret the data, a simple model for a fermion gas of noninteracting clusters of various sizes is presented. The analysis suggests that single-spin reversals make only a small contribution to the susceptibility and that to obtain even qualitative agreement with data the effects of large spin clusters and interaction between spin clusters must be included.

• Received 19 August 1991

DOI:https://doi.org/10.1103/PhysRevB.45.3122