On the Theory of Dynamic Susceptibility of Soft Magnetic Colloids

The magnetization reversal kinetics has been theoretically considered for a nanosized ferromagnetic particle in a soft viscoelastic medium. In contrast to well-known works, we have considered the simultaneous action of the Néel mechanism of magnetization reversal of a particle (overcoming the potential barrier of magnetic anisotropy by its magnetic moment), and rotation (turn) of the particle body as a result of a change in the external magnetic field. The case of a high magnetic anisotropy of the particle is considered, i.e., it has been assumed that its energy significantly exceeds the thermal energy of the system and the energy of the interaction between the particle and the magnetic field. No other limitations have been imposed on field strength. The case of low fields has been considered in greater detail in the linear approximation of the dependence of magnetization on the field. The components of the complex susceptibility of the composite are calculated within the framework of this approximation. It has been shown that the real component of the susceptibility decreases monotonically with the field frequency. If the stiffness of the composite is high or low, the imaginary component has one maximum, which corresponds to the Néel mechanism of magnetization reversal or particle rotation in a viscoelastic medium, respectively. At intermediate values of composite stiffness, the frequency dependence of the imaginary susceptibility has two maxima.