Spin-glass-like ordering of the magnetic moments of interacting nanosized maghemite particles

Samples of interacting nanosized maghemite particles have been studied by M\"ossbauer spectroscopy and magnetization measurements. The apparent blocking temperatures obtained from M\"ossbauer spectroscopy and zero-field-cooled magnetization curves are nearly identical, but the values obtained from measurements of the decay of remanence are much lower. These results are not in accordance with the N\'eel model for superparamagnetic relaxation, but can be explained by the formation of an ordered spin-glass-like state at low temperatures. At a critical temperature a transition to a superparamagnetic state takes place. By varying the average interparticle distance it is possible to change the ordering temperature.