Low frequency hysteresis loops of superparamagnetic nanoparticles with uniaxial anisotropy

Abstract

The low frequency hysteresis loops of superparamagnetic nanoparticles with uniaxial anisotropy are calculated as a function of the particle diameter, alternating magnetic field amplitude H0, frequency, and particle magnetic parameters both for oriented and nonoriented assemblies. The magnetic field frequency is assumed so high, f>50–100 kHz, that the mechanical rotation of a particle in surrounding medium is restricted. Thus, only the Neel–Brown magnetization relaxation process is taken into account. Pronounced dependence of the specific loss power on the particle diameter is found in the linear regime pertaining to small magnetic field amplitudes. For an oriented assembly of Co nanoparticles with optimal diameter D≈6 nm the specific power loss can be as high as 1600 W/g for typical values H0=200 Oe and f=500 kHz. It is three times less for the corresponding nonoriented assembly. Nevertheless, using of particles with low anisotropy field, i.e., particles of soft magnetic type, seems to be preferable for the purpose of magnetic nanoparticle hyperthermia. For such an assembly the nonlinear regime can be easily accessed. Hence the hysteresis loop area increases and is comparable with its maximum possible value.