Slow magnetic relaxation in well crystallized, monodispersed, octahedral and spherical magnetite nanoparticles

Thermally activated relaxation over energy barriers concurrently related to local properties and interparticle interactions constitutes a major contribution to both the coercivity and the applied field frequency dependence of that quantity. We have measured the slow magnetic relaxation of magnetite nanoparticles (NPs), synthetized by using the oxidative precipitation technique, having spherical and octahedral shapes, monodispersed size distributions and similar transverse dimensions. From our relaxation data we evaluated the temperature dependencies of a) the irreversible demagnetization susceptibility, b) the fluctuation field (associated to the thermally induced demagnetization occurring during the measuring time range) and c) the activation volume (corresponding to the demagnetization produced by the fluctuation field). We conclude that i) the irreversible susceptibility peaks in both samples at ca. 135 K (Verwey transition temperature) and ii) the monotonically increasing temperature variation of the activation volume shows the same values in both samples for temperatures below ca. 135 K and at 290 K reaches values corresponding to 10 and 30 times the average particle volume of the spherical and octahedral NPs, respectively. Those large increases of the activation volume are compatible with a transition from local to collective of the thermally activated processes.