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Using the optical techniques described previously, the mean dipole moments and anisotropy fields, as well as the switching field distributions were measured on gamma ferric oxide, cobalt-doped gamma ferric oxide and chromium dioxide particles suspended in a liquid. A new method for the determination of switching field distributions is described; its results agree with those of the older one. The particles were about 100 × 20 × 20 nm. The anisotropy, fields (for small reversible changes of the magnetization) ranged from 60 to 100 kA/m, i.e. much lower than expected for shape anisotropy. The observed mean switching fields (for large irreversible changes) were all between 63 and 75 kA/m, and appeared to be independent of particle size, shape or material.