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 fixed magnetic fields (H). The predicted behavior depends critically on the size of H relative to H_K, the effective anisotropy field of each constituent particle. For H≤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 until it reaches and remains (as M_F) at a fixed frictional angle relative to H, while the sample continues to rotate; the frictional angle diminishes to zero at H = 2H_K. This frictional behavior derives directly from the abrupt orientational changes of the particle magnetizations that occur over this range of H. Corresponding rotational M vector experiments were carried out on a sample disk cut from a commercial magnetic memory material. The results 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 at higher H. The gradual changes with H seen experimentally indicate a fairly 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 the average value indicated by a measured hysteresis loop that also suggests that H_K has a broad range of values.