Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe0.67Mn0.33)OOH particles

The magnetic properties of ultra-small (similar to 2 nm) delta-(Fe0.67Mn0.33)OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at similar to 150 K (T-mh), and a narrow one at similar to 30 K (T-ml). The two temperatures exhibit a different frequency dependence: T-mh follows a Vogel-Fulcher law tau = tau(0) exp[(E-a/k(B))/(T - T-0)], indicating a blocking of weakly interacting nanoparticle moments, whereas T-ml follows a power law tau = tau(0)(T-g/T-m nu - T-g)(alpha), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T-ml and T-mh and with the temperature dependence of the coercivity, strongly increasing below 30 K.