Superspinglass behavior of maghemite nanoparticles dispersed in mesoporous silica

γ-Fe₂O₃ nanoparticles were dispersed in a mesoporous silica (SBA15)matrix using the co-precipitation method. While structural properties were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, micro-Raman and transmission electron microscopy, magnetic studies were performed with DC and AC magnetometry. The γ-Fe₂O₃ nanoparticles have the regular cubic spinel structure. A strong reduction in the saturation magnetization (∼30 emu/gFe, 300 K)was measured and can be mainly attributed to the large fraction (60%)of surface spin disorder. This phenomenon was studied using the modified Kneller's law and a modified Bloch spin wave model with a surface disorder term. Values of spin wave stiffness constant D of 391 meVŲ and exchange integral J_ex equal to 1.12 meV were obtained, suggesting the presence of a block spinel ferrimagnetic state. The Vogel-Fulcher relation was applied to study the nanoparticles magnetic interactions. The values of zv = 5.3 (5), T_f = 184 K and τ₀=10^-12s suggest a superspinglass behavior. Thermoremanent experiments have also shown that magnetic relaxation occurs even below T_f (180 K)with an exponential decay with scale parameters in the range from 0.56 to 0.6, in agreement with the slowing down law. These facts indicate that the superspinglass effect is mainly caused by the strong surface spin disorder and frustrated collective state related to an interacting system with a broad particle size distribution.