Particle size effects on magnetic garnets prepared by a properties of yttrium iron sol-gel method

We present detailed measurements of field-and temperature-dependence of magnetization in nanocrystalline YIG (Y3Fe5O12) particles. The fine powders were prepared using sol-gel method. Samples with particle sizes ranging from 45 to 450 run were obtained. We observe that the saturation magnetization decreases as the particle size is reduced due to enhancement of the surface spin effects. Below a critical diameter D-s congruent to 190 nm, the particles become single domains and the coercive forces reaches a maximum at diameters close to the critical value. As the particle size decreases the coercivity diminishes and at D-p similar or equal to 35 nm diameters the upper limit of superparamagnetic behavior is reached. A quantitative comparison of temperature and particle size dependence of coercivity shows a satisfactory agreement that is expected for an assembly of randomly oriented particles. (C) 2002 Elsevier Science B.V. All rights reserved.