Properties of assembly of superparamagnetic nanoparticles in viscous liquid

N A Usov,R A Rytov,V A Bautin

doi:10.1038/s41598-021-86323-x

Abstract

Detailed calculations of the specific absorption rate (SAR) of a dilute assembly of iron oxide nanoparticles with effective uniaxial anisotropy dispersed in a liquid are performed depending on the particle diameters, the alternating (ac) magnetic field amplitude H0 and the liquid viscosity. For small and moderate H0 values with respect to particle anisotropy field Hk the SAR of the assembly as a function of the particle diameter passes through a characteristic maximum and then reaches a plateau, whereas for sufficiently large amplitudes, H0 ~ Hk, the SAR increases monotonically as a function of diameter. The realization of viscous and magnetic oscillation modes for particle unit magnetization vector and director for moderate and sufficiently large H0 values, respectively, explains this behavior. It is found that the SAR of the assembly changes inversely with the viscosity only in a viscous mode, for nanoparticles of sufficiently large diameters. In the magnetic mode the SAR of the assembly is practically independent of the viscosity, since in this case the nanoparticle director only weakly oscillates around the ac magnetic field direction. The conditions for the validity of the linear response theory have been clarified by comparison with the numerical simulation data.

Highlights

Detailed calculations of the specific absorption rate (SAR) of a dilute assembly of iron oxide nanoparticles with effective uniaxial anisotropy dispersed in a liquid are performed depending on the particle diameters, the alternating magnetic field amplitude H0 and the liquid viscosity
It should be noted that theoretical calculations of SAR of dilute assemblies of magnetic nanoparticles in a viscous liquid are complicated by the fact that a magnetic nanoparticle in a liquid can rotate as a whole under the action of fluctuating thermal torques and external ac magnetic field
A brief description of this approach is given in “Methods” section. Using this technique in this work we investigate in detail the dependence of the SAR of a dilute assembly of quasi-spherical single-domain iron oxide nanoparticles with a saturation magnetization Ms = 350 emu/cm[3] and an effective uniaxial anisotropy constant K1 = 105 erg/cm[3] on the particle size in the range of diameters D = 15–55 nm, on the amplitude of the ac magnetic field, H0 = 50–500 Oe, and the kinematic viscosity of the liquid, η = 0.01–0.2 g/(cm s)