Multipeak dynamic magnetic susceptibility of a superparamagnetic nanoparticle suspended in a fluid

Abstract

Linear response theory for the case of a uniaxially anisotropic superparamagnetic nanoparticle suspended in a fluid is developed for the situations where, along with the probing field, a stationary bias field is present. The built up description allows for both mechanisms of magnetic relaxation available to the particle: the internal (relaxation of the magnetic moment inside the particle) and external (relaxation together with the particle body due to its Brownian orientation diffusion in a fluid). In this framework, longitudinal dynamic magnetic susceptibility of such a particle is considered. It is confirmed that at zero bias field, frequency dependence of the out-of-phase component of the dynamic susceptibility (absorption spectrum) has two maxima if anisotropy energy is only several times greater than thermal energy. The presence of these peaks is a direct consequence of the bistability of uniaxial magnetic nanoparticles. The magnetizing field changes position and height of these maxima. Moreover, it is shown that in a presence of the bias the spectrum can acquire a third maximum if the Brownian rotation of the suspended particle is retarded with respect to establishment of the intrinsic magnetic equilibrium in it. Necessary conditions for such a situation are analyzed, and criteria for the possible appearance of the additional third absorption peak are indicated.