Third harmonic of the dynamic magnetic susceptibility of a concentrated ferrofluid: Numerical calculation and simple approximation formula.

Information about the nonlinear magnetic response of dispersions of magnetic particles is the basis for biomedical applications. In this paper, using analytical and numerical methods, the third harmonic of the dynamic susceptibility of an ensemble of moving magnetic particles in an ac magnetic field with an arbitrary amplitude is studied, taking into account interparticle interactions. A simple approximation formula is proposed to predict the third harmonic as a function of two parameters: the Langevin susceptibility χ_{L}, which is used to estimate the particle dipole-dipole interactions, and the Langevin parameter ξ, which represents the ratio of the energy of the magnetic moment interacting with the magnetic field to the thermal energy.

Influence of a bias dc field and an ac field amplitude on the dynamic susceptibility of a moderately concentrated ferrofluid.

In this paper, we study the effect of a bias dc field on the dynamic response of a moderately concentrated ferrofluid to an ac magnetic field of arbitrary amplitude. The ferrofluid is modeled by an ensemble of interacting moving magnetic particles; the reaction of particle magnetic moments to ac and dc magnetic fields occurs according to the Brownian mechanism; and the ac and dc magnetic fields are parallel. Based on a numerical solution of the Fokker-Planck equation for the probability density of the orientation of the magnetic moment of a random magnetic particle, dynamic magnetization and susceptibility are determined and analyzed for various values of the ac field amplitude, the dc field strength, and the intensity of dipole-dipole interactions.

Influence of field amplitude and dipolar interactions on the dynamic response of immobilized magnetic nanoparticles: Perpendicular mutual alignment of an alternating magnetic field and the easy axes.

In this paper, the dynamic magnetic properties of an ensemble of interacting immobilized magnetic nanoparticles with aligned easy axes in an applied ac magnetic field directed perpendicular to the easy axes are considered. The system models soft, magnetically sensitive composites synthesized from liquid dispersions of the magnetic nanoparticles in a strong static magnetic field, followed by the carrier liquid's polymerization. After polymerization, the nanoparticles lose translational degrees of freedom; they react to an ac magnetic field via Néel rotation, when the particle's magnetic moment deviates from the easy axis inside the particle body.

Dynamic magnetic susceptibility of a ferrofluid: The influence of interparticle interactions and ac field amplitude.

Based on numerical results of dynamic susceptibility, a simple theory of the dynamic response of a ferrofluid to an ac magnetic field is obtained that includes both the effects of interparticle dipole-dipole interactions and the dependence on field amplitude. Interparticle interactions are incorporated in the theory using the so-called modified mean-field approach. The new theory has the following important characteristics: in the noninteracting regime at a weak ac field, it gives the correct single-particle Debye theory results; it expands the applicability of known theories valid for high concentrations [Ivanov, Zverev, and Kantorovich, Soft Matter 12, 3507 (2016)10.

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: a combined experimental and theoretical study.

The interweave of competing individual relaxations influenced by the presence of temperature and concentration dependent correlations is an intrinsic feature of superparamagnetic nanoparticle suspensions. This unique combination gives rise to multiple applications of such suspensions in medicine, nanotechnology and microfluidics. Here, using theory and experiment, we investigate dynamic magnetic susceptibility in a broad range of temperatures and frequencies.

Revealing the signature of dipolar interactions in dynamic spectra of polydisperse magnetic nanoparticles.

We investigate, via a modified mean field approach, the dynamic magnetic response of a polydisperse dipolar suspension to a weak, linearly polarised, AC field. We introduce an additional term into the Fokker-Planck equation, which takes into account dipole-dipole interaction in the form of the first order perturbation, and allows for particle polydispersity. The analytical expressions, obtained for the real and imaginary dynamic susceptibilities, predict three measurable effects: the increase of the real part low-frequency plateaux; the enhanced growth of the imaginary part in the low-frequency range; and the shift of the imaginary part maximum.