IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. zyxwvuts 5, SEPTEMBER zyxwvuts 1984 zyxwvuts MIXED ANISOTROPIES IN SMALL FERROMAGNETIC PARTICLES. K. O'Grady ,R.W. Chantrell and E.P. Wohlfarth 1). Physics Department, U.C.N.W. , Bangor, Gwynedd, U.K. 2). Division of Physics and Astronomy, Preston Polytechnic, Preston , Lancs., PR1 2TQ, U.K. 3). Department of Mathematics, Imperial College, London, SW7 ZBZ, U.K. zyxwv 1) 2) 3) 1849 ABSTRACT - Low temperature magnetic measurements have been carried out on a system ,of finecobaltparticles in a solid matrix. Samples frozen in a large applied field were found to exhibit the shifted hysteresis loop indicative of exchange anisotropy. The magnetic properties of the system were found to be determined by the mixture of the unidirectional exchange aniSOtrOpY and the predominant uniaxial shape anisotropy of the particles. The exchange- ani sotropy requires the presence of an antiferromagnetic surface layer on the particles and this is consistent with the observed differences in particle size obtained from electron microscopy and room temperature magnetic measurements. Changes in the coercivity factor and the interaction field factor on freezing the samples in a large applied field have also been measured. The observed changes were found to be consistent with the expected changes in the switching field distribution of the particles and the local order within the system. The coercivity factor and interaction field factor are thus shown to be useful parameters in characterising the properties of fine particle systems. INTRODUCTION. When a system of small ferromagnetic particles with antiferromagnetic surface layers i s frozen in a large applied magnetic field the phenomenon of exchange anisotropy is observed as long as the system i s cooled through the Nee1 temperature of the antiferromagnetic component. Exchange anisotropy was f i r s t observed by Meiklejohn and Bean (1) and arises from the strong coupling between the ferromagnetic particle and the antiferromagnetic surface layer. This is a unidirectional anisotropy, resulting in the observed shifted hysteresis loops. _-___ We have investigated this phenomenon in systems consisting of partially oxidised cobalt particles. The samples were prepared as colloidal dispersions in a low vapour pressure diester, solid samples being prepared by freezing the carrier fluid. Samples exhibiting exchange anisotropy were prepared by freezing in- a large applied field. This field freezing, however, has two additional effects; firstly an alteration of the switching field distribution of the particles due t o the easy axis alignment and secondly a change in the spatial ordering of the particles, which might be expected to have a bearing on the magnetic interactions between particles. These additional effects of the field freezing were investigated using the coercivity factor (CF) and the interaction field factor (IFF) defined by Corradi and Wohlfarth (2). EXPERIMENTAL. Measurements were made on samples containing fine cobalt particles prepared by the method of Hess and Parker (3). This involves the thermal decomposition of cobalt carbonyl in toluene solution in which there i s also dissolved surfactant. The s u r f a c t a n t (in this case Sarkosyl ) has the effect of control1 ing the particle size produced by the reaction and also stabilises the resulting colloidal dispersion. After preparation the particles were transferred to a low vapour pressure diester carrier fluid to avoid problems due to evaporation. The particle sizes within the sample were measured directly using a GEC-AEI Corinth 275 electron microscope with a linear resolution of 0.8 nm. The particle size distribution was obtained from the electron micrographs using the techniques described by O'Grady and Bradbury (4). An approximately Gaussian distribution of particle sizes was found, having a mean diameter of 12nm and a reduced standard deviation (relative to the mean diameter) of 0.2. Room temperature magnetic measurements were made on the colloidal dispersions using a vibrating sample magnetometer. The dispersions exhibited superparamagnetic behaviour and from the initial susceptibility and approach to saturation the particle size distribution parameters were calculated using the method of Chantrell e t a1 (5). This technique, which effectivelyfitstheaatato a lognormal distribution, gave, for the parameters of the distribution, values of median diameter= 8.5 nm and standarddeviation = 0.42. The difference between the 'magnetic' and 'physical' diameters cannot be accounted for entirely by the differences between the distribution functions used in the fitting procedures given in (4) and (5). In fact it is more 1 ikely that the particles have a non-magnetic surface layer. This is indicative of a partial oxidation of the particles, although a chemical reaction between the surfactant and the particle, producing a non-magnetic compound, is also 1ikely. Measurements in the solid state were made after cooling to 77K, which is below the freezing point of the carrier fluid.Field frozen samples were prepared by cooling gradually in a field of 8kOe. A variety of magnetic measurements were then made at 77K using a vibrating sample magnetometer. On all samples the d.c. magnetisation curve was measured in order to investigate the exchange anisotropy. To study the effects of field-freezing on the switching field distribution and the interparticle interactions, the isothermal remanent magnetisation curves and d.c. demagnetisation remanence curves were measured. From these measurements the CF and IFF were determined. ___- RESULTS. The static magnetisation curve measured a t 77K for the system of cobaltparticlesfrozeninzerofieldis shown in Fig (1). The sample has a 1 arge reduced remanence ($-/Is where 1,- is the remanence and Is is the saturation magnetisation of the sample) and a coercivity of 1.9kOe. The f a c t t h a t I, /Is is close to the Stoner-Wohlfarth (6) value of 0.5 indicates that the fraction of superparamagnetic particles within the system a t 77K i s negligible. In this case the bulk of the particles exhibit irreversible magnetic behaviour,and it can - be shown (6) that the reduced magnetisation (I) in small fields after previous saturation is - I = 1/2 + 2H / 3HR (1) where Hk =2K / Is zyxw is the anisotropy fie1 d of the particles with K tke anisotropy constant and ISb the saturation magnetisation of the bulk material. From equation (1) the slope of themagnetisation curve in small fields after previous saturation in a 0018-9464/84/0900-1849$01.0001984 IEEE