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 Mn(3)O(4) nanoparticle ensembles at temperatures below the paramagnetic-to-ferrimagnetic transition of the title material (T(N) approximately equal 41 K). Our data show no evidence of the complex magnetic ordering exhibited by bulk Mn(3)O(4), or of a magnetic behavior around T(N) that has a dynamic (relaxation) origin. Instead, we find a low-temperature (at approximately 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 DeltaT/TDeltalog(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 tau = 1/2pif 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 tau = tau(0)exp[E(B)/k(B)(T - T(0))]-where E(B) is the energy barrier to magnetization reversal, k(B) is the Boltzmann constant, tau(0) and T(0) 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 (T - T(0) is proportional to(D)3, another signature of superparamagnetic relaxation.