Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter 〈ϕ〉 = 15 nm have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ′) and out of phase (χ″) ac-susceptibility. The studies were performed in non-interacting and interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 and 17.6%. The ac-susceptibility of the non-interacting system, measured with HDC = 0 Oe, presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)–ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system shows two anomalies. One frequency independent peak associated to TC and a second low temperature maximum was observed in χ″, located at TP. The position of the TP maximum shifts to higher temperature when the frequency increases. The relation between the relaxation time and TP was well described by the Vogel–Fulcher law. When the susceptibility was measured with HDC = 20 kOe, the PM–FiM transition was observed in both systems. Remarkably, in the non-interacting system, the low temperature anomaly is evidenced by the magnetic field. This anomaly is present as a well-defined maximum, which shifts to higher temperature when the frequency increases.