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

Abstract

The magnetic properties of ultra-small (∼2 nm) δ-(Fe_0.67Mn_0.33)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 ∼150 K (T_mh), and a narrow one at ∼30 K (T_ml). The two temperatures exhibit a different frequency dependence: T_mh follows a Vogel–Fulcher law τ = τ₀ exp[(E_a/k_B)/(T − T₀)], indicating a blocking of weakly interacting nanoparticle moments, whereas T_ml follows a power law τ = τ₀(T_g/T_mν − T_g)^α, 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.