A phenomenological approach to study the effect of uniaxial anisotropy on the magnetization of ferromagnetic nanoparticles

Abstract We study the effect of the uniaxial anisotropy in a system of ideal, noninteracting ferromagnetic nanoparticles by means of a thermodynamical model. We show that the effect of the anisotropy can be easily assimilated in a temperature shift T a ∗ , in analogy to what was proposed by Allia et al. (2001) in the case of interacting nanomagnets. The phenomenological anisotropic T a ∗ parameter can be negative, indicating an antiferromagnetic-like behavior, or positive, indicating a ferromagnetic-like character as seen in the inverse susceptibility behavior as a function of temperature. The study is done considering an easy axis distribution to take into account the anisotropy axis dispersion in real samples (texture). In the case of a volumetric uniform distribution of anisotropy axes, the net effect makes T a ∗ to vanish, and the magnetic susceptibility behaves like a conventional superparamagnetic system, whereas in the others a finite value is obtained for T a ∗ . When magnetic moment distribution is considered, the effect is to enhance the T a ∗ parameter, when the dispersion of the magnetic moments becomes wider.