The ensemble of immobilized superparamagnetic nanoparticles: the role of the spatial distribution in the sample

ABSTRACT In this work, static, thermodynamic and magnetic properties of interacting superparamagnetic nanoparticles have been studied using theory and computer simulation. Two types of particles’ distributions in the sample have been considered: (a) at the nodes of the simple cubic lattice and (b) by the random way. It was assumed that the directions of the easy axes for all particles were parallel to each other and directed at an angle to the external magnetic field. The theoretical approach is based on the expanding of the Helmholtz free energy into the classical virial series up to the second virial coefficient. The analytical expressions of the Helmholtz free energy for both textures allow us to obtain theoretical predictions for the static magnetization and the isochoric heat capacity. These characteristics turned out in a good agreement with the Monte-Carlo simulation data in the broad range of considered system parameters. In a zero and moderate external magnetic fields, the new theory allows to describe the numerical calculations much more efficient than the ideal approximations, for which the interparticle dipole-dipole interactions were neglected.