Magnetic properties of small acicular particles

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.