Journal of Magnetism and Magnetic Materials 118 (1993) 193-199 North-Holland /1 1" On the static and time-dependent magnetic properties of Fe304 fine particles: effect of oxidation S.H. Mahmood and I. Abu-Aljarayesh Physics Department, Yarmouk University, Irbid, Jordan Received 11 March 1992; in revised form 4 May 1992 In this work, the results of static and time-dependent magnetization measurements on three systems of Fe304 fine particles are reported. Measurements of the static magnetization were carded out in the temperature range 293 K < T < 423 K, and of the remanent magnetization in the temperature range 5 K < T < 300 K over the period of time 30 s < t < 300 s. The remanent magnetization was found to obey the relation M = C - S In t. Annealing the sample at 250"C was found to increase its magnetic anisotropy constant. The results were analyzed within the context of Ndel's theory of magnetic clusters. 1. Introduction Ferromagnetic or ferrimagnetic fine particles can be found in many magnetic materials of sci- entific and technological importance, such as rocks, magnetic fluids, magnetic recording media, particulate permanent magnets, and some bio- magnetic organisms [1-5]. Therefore, the behav- ior of magnetic fine particles is of considerable interest, both from a scientific and from a techno- logical point of view [1]. Since there is a distribution of particle sizes in real systems of fine particles, and due to the relatively large surface to volume ratio in fine particles, the physics of fine particles involve both volume and surface effects. Most workers have explained the magnetic properties of fine particles within the theory of superparamagnetism [6,7]. Superparamagnetic particles are ferromagnetic or ferrimagnetic sin- gle-domain particles so small that the thermal energy is of the order of magnitude of the anisotropy energy barrier. Correspondence to: Prof. S.H. Mahmood, Director, Center for Theoretical and Applied Physics, Yarmouk University, Irbid, Jordan. The behavior of a fine particle system is in many aspects similar to that of a spin-glass mag- netic alloy [8-11]. In fact, many workers have suggested that the behavior of a spin glass can be interpreted within the Ndel theory of magnetic clusters [12]. In this paper, we present the results of a study of static and time-dependent magnetic properties of three samples of Fe30 4 fine particles. The organization of this paper is as follows: In section 2 a theoretical background for the magnetic re- manence is introduced. In Section 3 the sample preparation and measurement techniques are briefly explained and in section 4 the experimen- tal results, analysis and conclusions are pre- sented. 2. Remanent magnetization The magnetic relaxation time for an isolated single-domain particle in the absence of an ap- plied magnetic field is governed by the Ndel- Arrhenius law [11] z = "ro exp(Ea/kaT), (1) where Ea(= KF) is the anisotropy barriers (as- sumed uniaxial), ~'0 is a relaxation time (= 10 -8- 0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved