Magnetic behaviour of iron oxide nanoparticles dispersed in a silica matrix

At room temperature the magnetic behaviour of non-interacting gamma-Fe2O3 nanoparticles dispersed in a silica matrix is superparamagnetic. At a lower temperature (77 K) hysteresis was observed. The appearance of the hysteresis loop is attributed to the Neel magnetic relaxation processes; the easy magnetisation axes of the nanoparticles being randomly oriented, the magnetic moments will rotate in an external magnetic field with a relaxation time c, which depends on the effective anisotropy constant K-eff. A supplementary anisotropy, e.g. stress anisotropy and the surface anisotropy leads to a total value (K-eff), approximately 10 times higher than the bulk magnetocrystalline anisotropy K. At the surface of embedded nanoparticles a modification of the superexchange interaction takes place-due to specific particle-matrix interactions-on decreasing the temperature from 300 to 77 K; the magnetic moment of the particles increases 1.6 times. This behaviour can be attributed to the increase of the nanoparticles magnetic core volume of aligned spins and is not due to the variation of the spontaneous magnetisation with temperature. (C) 2002 Elsevier Science B.V. All rights reserved.