Thermally reversible nanoparticle aggregation explains magnetic moment increase with temperature

Gd-substituted Mn-Zn ferrite (Mn_0.5Zn_0.5Gd_0.02Fe_1.98O₄) nanoparticles were synthesized using chemical co-precipitation. Measurements of magnetization as a function of the applied field were performed at several temperatures. As expected, the saturation magnetization was found to decrease with temperature. However, fitting the magnetization data to a single Langevin function yielded an anomalous increase in the average magnetic moment per particle. The increased magnetic moment is attributed to thermally increased nanoparticle volume which is caused by thermally induced aggregation of the nanoparticles. Inter-particles dipolar interactions, responsible for clustering, are believed to increase with thermally induced surface layer spin. This agglomeration is thermally reversible up to a critical temperature after which thermal energy overcomes dipolar interactions. Thermally induced agglomeration can be used as temperature sensor.