Field dependence of blocking temperature in magnetite nanoparticles

Spherical magnetite nanoparticles having average particle size <d> = 5 nm have been synthesized by coprecipitation of Fe(II) and Fe(III) salts in KOH with Polyvinylalcohol (PVA). The resulting dry powder displayed superparamagnetic (SPM) behaviour at room temperature, with a transition to a blocked state at T-B similar to 45 K for applied field H-app = 500 Oe. The effect of dipolar interactions was investigated by measuring the dependence of T-B on the applied field H-ap and driven ac field in susceptibility data. A thermally activated model has been used to fit the dynamic data to obtain the single-particle energy barriers E-a = KeffV, allowing us to estimate the contributions of dipolar interactions to the single-particle effective magnetic anisotropy K-eff. We have measured the dependence of T-B with H-ap in order to draw the transition contours of a H-T diagram. Two different regimes are found for the (T-B-T-0) similar toH(lambda) dependence at low and hi-h fields, that can be understood within a pure SPM relaxation-time (Neel-Brown) landscape. The T-B(H) data shows a crossover from lambda = 2/3 to lambdasimilar to2 for applied magnetic fields of approximate to 550 Oe.