Temperature dependence of magnetic anisotropy in nanoparticles of CoxFe(3−x)O4

In this paper we present a study of the magnetic anisotropy constant of nanocrystalline magnetic particles of Co_xFe_(3−x)O₄, with x ranging from 0.05 to 1.6, synthesized by a combustion reaction. The magnetic anisotropy constants were obtained by fitting the high-field part of the major hysteresis loops with the law of approach equation down to temperatures of 4 K and up to fields of 60 kOe. The anisotropy constant depends strongly on both temperature and cobalt content x, exhibiting a nonmonotic dome-shaped dependence on x with a maximum at x=1.0. We found that fits at lower temperatures, i.e., 4 and 72 K, give values of K₁ that are approximately one order of magnitude higher than those at higher temperatures, i.e., 272 and 340 K. For example, K₁ for specimens with x=0.8 and 1.0 are 4.21×10⁷ and 4.22×10⁷ ergs/cm³ at 4 K, and 7.64×10⁶ and 7.51×10⁶ ergs/cm³ at 340 K, respectively. Thus, our determination of temperature-dependence of the anisotropy constant represents an improvement over existing works.