Theoretical single-domain size of NiZn ferrite

Abstract
The critical radical, R_c, below which a spherical particle of NiZn ferrite becomes a single domain in zero applied field, is computed by minimizing the energy under the only constraint that the magnetization configurations have a cylindrical symmetry. The rigorous result for an isolated particle is R_c = 22.3 nm. Several theories are used to correct for the soft environment of closely-packed particles, thus increasing R_c up to 0.727 µm. This value, however, is still too small to account for the experimental value of 1.4 ± 0.05 µm in NiZn ferrite, which means that the problem of the effect of the soft environment needs a more careful theoretical study.