Evidence of low-temperature superparamagnetism in Mn3O4 nanoparticle ensembles.

Using ac-susceptibility, dc-magnetization, and transmission electron microscopy, we have investigated the magnetic behavior of Mn3O4 nanoparticle ensembles at temperatures below the paramagnetic-to-ferrimagnetic transition of the title material ( K). Our data show no evidence of the complex magnetic ordering exhibited by bulk Mn3O4, or of a magnetic behavior around TN that has a dynamic (relaxation) origin. Instead, we find a low-temperature (at ~ 11 K) magnetic anomaly that manifests itself as a peak in the out-of-phase component of the ac-susceptibility. Analysis of the frequency and average-particle-size dependence of the peak temperature demonstrates that this behavior is due to the onset of superparamagnetic relaxation, and not to a previously hinted at spin-glass-like transition. Indeed, the relative peak temperature variation per frequency decade ΔT/TΔlog(f) is 0.11, an order of magnitude larger than the value expected for collective spin freezing, but within the range of values observed for superparamagnetic blocking. Furthermore, attempts to fit the frequency f/observation time τ = 1/2πf dependence of the peak temperature by a power law led to parameter values unexpected for a spin-glass transition. On the other hand, a Vogel–Fulcher law τ = τ0exp[EB/kB(T − T0)]—where EB is the energy barrier to magnetization reversal, kB is the Boltzmann constant, τ0 and T0 are constants related to the attempt frequency and the interparticle interaction strength—correctly describes the peak shift and yields values consistent with the superparamagnetic behavior of a slightly interacting system of nanoparticles. In addition, the peak temperature T is sensitive to minute changes in the average particle size D, and scales as , another signature of superparamagnetic relaxation.