Dynamics of magnetic nanoparticle in a viscous liquid: Application to magnetic nanoparticle hyperthermia

Abstract

It is shown that the magnetic dynamics of an assembly of nanoparticles dispersed in a viscous liquid differs significantly from the behavior of the same assembly of nanoparticles immobilized in a solid matrix. For an assembly of magnetic nanoparticles in a liquid two characteristic mode for stationary magnetization oscillations are found that can be called the viscous and magnetic modes, respectively. In the viscous mode, which occurs for small amplitude of the alternating magnetic field H0 as compared to the particle anisotropy field Hk, the particle rotates in the liquid as a whole. In a stationary motion the unit magnetization vector and the director, describing the spatial orientation of the particle, move in unison, but the phase of oscillations of these vectors is shifted relative to that of the alternating magnetic field. Therefore, for the viscous mode the energy absorption is mainly due to viscous losses associated with the particle rotation in the liquid. In the opposite regime, H0 ≥ Hk, the director oscillates only slightly near the external magnetic field direction, whereas the unit magnetization vector sharply jumps between magnetic potential wells. Thus, a complete orientation of the assembly of nanoparticles in the liquid occurs in the alternating magnetic field of sufficient amplitude. As a result, large specific absorption rates, of the order of 1 kW/g, can be obtained for an assembly of magnetic nanoparticles in viscous liquid in the transient, H0 ∼ 0.5Hk, and magnetic modes at moderate frequency and alternating magnetic field amplitude.

Long‐Term Stability and Reproducibility of Magnetic Colloids Are Key Issues for Steady Values of Specific Power Absorption over Time

“…This linear trend demonstrates that the thermal insulation of the sample space is good enough to consider it as quasi-adiabatic even for long times (minutes). [38] Regarding the effect on the heating efficiency, the basic mechanisms by which the transformation from magnetic energy to heat takes place has been recently modelled under some simplifying assumptions [39] with good agreement to several experimental systems [40] [41] [3] Irrespective of the theoretical approximation to the SPA heating mechanisms, all models contains a sharp dependence with magnetic volume and dispersity of the ensemble, mainly due to the exponential dependence of the thermally assisted magnetic relaxation (Néel relaxation) with particle size.…”

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Magnetic nanoparticle motion in external magnetic field

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