The anisotropy of very small cobalt particles

Abstract

2014 Torque and ferromagnetic resonance measurements on a single crystal of a copper 2014 2 % cobalt alloy have been made at various temperatures to determine the magnetocrystalline anisotropy of small single domain particles of precipitated cobalt. The anisotropy at 4.2 °K is found to be independent of particle radius from 21 A to 77 A, lending support to a crystal field model of the origin of anisotropy. A slight variation of anisotropy with particle size was observed at higher temperatures, and was related to a slight variation in saturation magnetization with particle size at these temperatures. LE JOURNAL DE PHYSIQUE ET LE RADIUM TOME 20, FEVRIER-MARS 1959, The magnetocrystalline anisotropy of bulk materials is observed to vary with temperature as a high power of the saturation magnetization the tenth and twentieth power for iron and nickel respectively [1]. Thus the change in anisotropy of small ferromagnetic particles as a function of their size may reflect in a sensitive fashion the change in saturation magnetization with size. Although there has been some disagreement [2], [3], there is no evidence for an abnormal saturation magnetization in particles of iron [4], [5]jor cobalt [6] down to 15 A radius. Naive use of the results on thin films [7], [8] would imply an effect much larger than that observed on small particles. There remains, however, the problem that a transition must exist between atomic paramagnetism and the coupled behavior of a many atom particle. Measurement of the anisotropy of small particles may also yield useful information concerning the origin of anisotropy. There are two basically different approaches to this problem within the framework of a Heitler-London approximation to a solid. The first approach (crystal field) [9], [10], [11] assumes the anisotropy to arise because of the electrostatic environment of a magnetic atom without reference to the magnetism of the adjacent atoms. The second approach (anisotropic exchange) [10], [11], [12] emphasizes the fact that the ordinarily isotropic exchange interaction between magnetic atoms may be modified by the presence of spin-orbit coupling of the initially quenched orbital states to become slightly anisotropic. Consider the anisotropy at absolute zero of one atom of cobalt in an electrostatic environment identical to that of cobalt metal but in which its neighbors are non-magnetic. According to the crystal field picture the anisotropy of this atom is identical to that of the ferromagnetic metal, while in the framework of dominant anisotropic exchange in the ferromagnet, the anisotropy per atom of the single atom case would be much less than in the ferromagnet. We may approach this conceptual experiment by measuring the anisotropy of small particles of cobalt in copper. A spherical particle 20 A in radius will have approximately 25 % of its atoms on the surface. About one-third of the nearest neighbors of each surface atom are non-magnetic atoms of similar electronic structure and lattice spacing. The properties of cobalt-rich single domain particles formed by precipitation in copper have been studied in several laboratories [3], [6], [13], [14], [15]. It has recently been demonstrated that the particles in such alloys are initially equiaxed in shape, fcc in structure and coherent with the lattice of the copper matrix and therefore aligned with each other [13], [16], [17]. When these particles are so small that their direction of magnetization can fluctuate thermally [18] they show a magnetic behavior that has been termed paramagnetique apparente [19] or superparamagnetic [20]. Experimental techniques. A single crystal of 2 % cobalt, balance copper, was grown usingthe Bridgman technique. From this crystal a sample was prepared in the shape of an oblate spheroid with (110) as the major plane. The major axes were 4.3 mm and 1.2 mm. The cobalt was retained in solution by quenching into an ice-water mixture from’ 1 000 °C. Precipitate particles of an average radius of 21 A, 43 A, and 77 A were then produced by aging 15, 150 and Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3029800