Determination of the magnetic anisotropy constant of a gas of superparamagnetic nanoparticles using thermodynamic method

Author: Simon Chkhaidze
Co-authors: A.Ugulava, G.Mchedlishvili, M.Verulashvili
Keywords: superparamagnetism, magnetic nanoparticles, nanomaterials.
Annotation:

Magnetic anisotropy plays an important role in the process of magnetization of a super-paramagnetic gas. At low temperatures, the anisotropy energy can significantly affect its thermal capacity. At sufficiently low temperatures, when anisotropy energy of the uniaxial magnetic nanoparticles exceeds the energy of thermal fluctuations, the anisotropy energy can be expressed as a sum of the energies of two thermodynamic subsystems (the two potential wells). One of these subsystems is composed of magnetic nanoparticles oriented predominantly along the axis of anisotropy, and the other - the particles of opposite orientations. There is an analogy between the considered anisotropy energy and quantum two-level system. Therefore, the temperature dependence of the magnetic part of the specific heat (similar to Schottky anomaly) will have a sharp peak. At low temperatures, on the curve of the temperature dependence of the specific heat, besides a usual background, wavy curve with a pronounced maximum is monitored. The relation between the maximum heat capacity with a constant of magnetic anisotropy is derived. By using this relation and measuring the maximum heat capacity at a given temperature the numerical value of the magnetic anisotropy constant can be obtained.