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 CoxFe(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 4K 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 K-1 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 4.21x10(7) and 4.22x10(7) ergs/cm(3) at 4K, and 7.64 x 10(6) and 7.51x10(6) ergs/cm(3) at 340 K, respectively. Thus, our determination of temperature-dependence of the anisotropy constant represents an improvement over existing works. (C) 2007 Elsevier B.V. All rights reserved.