Relaxation spectrum and dynamic magnetic response of magnetically hard single-domain particles suspended in a fluid

A theory of the magnetic response of single-domain particles suspended in a fluid to a linearly polarized harmonic field of arbitrary amplitude is developed for the case when a stationary magnetizing field is present. The theory regards magnetically hard particles without internal diffusion of the magnetic moment. The dependence of the magnetic relaxation spectrum of such particles on the magnitude of the applied field is analyzed. It is shown that their dynamic magnetic susceptibility comprises a countable set of Debye-type terms. However, the longest-lived relaxation mode predominates both in the longitudinal and transverse cases, and the dynamic magnetic susceptibility can be found up to good accuracy by means of a relatively simple formula for any direction of the bias field. The dynamic magnetic hysteresis of the particles in strong alternating fields is studied. It is demonstrated that the characteristics of the cyclic remagnetization curve strongly depend on the amplitude and frequency of the field. It is shown that the bias field can qualitatively change the magnetic response of the particles. In particular, it definitely leads to shrinkage of the hysteresis loop. Additionally, both vertical and horizontal shifts of the latter are expected if the angle between alternating and stationary fields is not right. The case of transverse biasing is special: the magnetization curve should always remain symmetric with respect to the origin.