ZnFe2O4 Nanoparticles: Different Magnetic Behavior When They Are Hosted in Porous Structures

ZnFe2O4 nanoparticles have been prepared encased into sepiolite and faujasite matrixes. Transmission electron microscopy (TEM) images show hosted particles from 1 to 3 nm. From the zero field cooling (ZFC) and field cooling (FC) magnetic susceptibility data, the blocking temperatures distribution has been obtained, which reflects the particle size distribution. In both samples, the particles seem to present two well-defined sizes. Taking into account the adsorption results, the smallest particles must be located in the micropores of the sepiolite and faujasite structures, while the largest ones are probably hosted in structural defects. On the other hand, when the thermal energy overcomes the anisotropy barrier, at temperatures higher than TB, the abrupt decreasing of magnetization observed in the ZFC and FC curves would indicate that the hosted nanoparticles are free to rotate. This leads one to believe that they do not present any mechanical impediment that could avoid their free rotation. This fact can be attributed to the good dispersibility of the nanoparticles in the matrixes, which implies noninteracting particles, and to the negligible interaction with the surface of the cavities.