Stoner-Wohlfarth model applied to ferromagnetic particle aggregates rotated in fixed magnetic fields

The classic Stoner-Wohlfarth model for ferromagnetic particle aggregates is extended to the magnetization-vector (M) behavior of samples rotated in fi xed magnetic fields (H). The predicted behavior depends critically on the s ize of H relative to H-K, the effective anisotropy field of each constituen t particle. For H less than or equal to H-K, M has a rotational component M -R, which turns synchronously with the sample, plus a component parallel to H, which grows with increasing H. For H-K < H < 2H(K), M turns initially u ntil it reaches and remains (as M-F) at a fixed frictional angle relative t o H, while the sample continues to rotate; the frictional angle diminishes to zero at H = 2H(K). This frictional behavior derives directly from the ab rupt orientational changes of the particle magnetizations that occur over t his range of H. Corresponding rotational M vector experiments were carried out on a sample disk cut from a commercial magnetic memory material. The re sults reveal a coexistence of rotational M-R and frictional M-F components, but M-R is predominant at low H and M-F becomes increasingly predominant a t higher H. The gradual changes with H seen experimentally indicate a fairl y broad distribution in the values of H-K for the different particles, The average value of H-K deduced from the rotational data agrees closely with t he average value indicated by a measured hysteresis loop that also suggests that H-K has a broad range of values, (C) 2001 Elsevier Science B.V. All r ights reserved.