Influence of dipolar interactions on the magnetic susceptibility spectra of ferrofluids

Abstract

The frequency-dependent magnetic susceptibility of a ferrofluid is calculated under the assumption that the constituent particles undergo Brownian relaxation only. Brownian-dynamics simulations are carried out in order to test the predictions of a recent theory [A. O. Ivanov, V. S. Zverev, and S. S. Kantorovich, Soft Matter 12, 3507 (2016)1744-683X10.1039/C5SM02679B] that includes the effects of interparticle dipole-dipole interactions. The theory is based on the so-called modified mean-field approach and possesses the following important characteristics: in the low-concentration, noninteracting regime, it gives the correct single-particle Debye-theory results; it yields the exact leading-order results in the zero-frequency limit; it includes particle polydispersity correctly from the outset; and it is based on firm theoretical foundations allowing, in principle, systematic extensions to treat stronger interactions and/or higher concentrations. The theory and simulations are compared in the case of a model monodisperse ferrofluid, where the effects of interactions are predicted to be more pronounced than in a polydisperse ferrofluid. The susceptibility spectra are analyzed in detail in terms of the low-frequency behavior, the position of the peak in the imaginary (out-of-phase) part, and the characteristic decay time of the magnetization autocorrelation function. It is demonstrated that the theory correctly predicts the trends in all of these properties with increasing concentration and dipolar coupling constant, the product of which is proportional to the Langevin susceptibility χ_{L}. The theory is in quantitative agreement with the simulation results as long as χ_{L}≲1.

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

“…To this extent, earlier this year, we put forward a new theoretical approach 73 based on the analytical solution of the Fokker-Plank equation with an additional term allowing for the interparticle interactions and system polydispersity. We tested our theory against Brownian dynamics computer simulations but only for a monodisperse case 74 .…”

Effect of agglomeration of magnetic nanoparticles on the Casimir pressure through a ferrofluid

“…The magnetic permeability of a ferrofluid with account of the effect of agglomeration is found from Eqs. (6) and (7). Thus, if magnetite nanoparticles have d = 10 nm diameter, one obtains µ Here and below all computations are performed at room temperature T = 300 K for a water-based ferrofluid containing Φ = 0.05 volume fraction of magnetite nanoparticles.…”

“…During the last few years much attention was given to ferrofluids which are colloidal liquids consisting of magnetic nanoparticles suspended in some carrier liquid (see, e.g., the monograph [1] and Refs. [2][3][4][5][6][7][8][9]). Ferrofluids are used in optical switches, optoelectronic communications, mechanical and medical applications [4,6,[10][11][12], and also in microdevices playing a broad spectrum of roles [13][14][15][16][17][18].…”

“…if half of all nanoparticles are merged into clusters by two and three particles, respectively.In a similar way, if the nanoparticle diameter is d = 20 nm, one obtains from Eqs. (6) and(7) µ…”

Influence of Electric Field on AC Magnetic Susceptibility of a Mineral Oil Based Ferrofluid

“…On the other hand, for concentrated ferrofluids, the dipole-dipole interactions are predicted to increase the Brownian relaxation times, while the Néel mechanism is not affected significantly. Such effects have been detected experimentally [7] and recently novel numerical and theoretical approaches were published [8,9]. By means of temperature and concentration dependent AC magnetic susceptibility investigation and theoretical * corresponding author; e-mail: rajnak@saske.sk analysis it was shown [10] how to separate the effects of superparamagnetic polydispersity and interparticle magnetic interactions on the dynamic spectra, which deepens the understanding of the dynamic nanoparticle selfassembly.…”