Field Dependence of Blocking Temperature in Magnetite Nanoparticles

Details Field Dependence of Blocking Temperature in Magnetite Nanoparticles Field Dependence of Blocking Temperature in Magnetite Nanoparticles

2011-03-19

arxiv.org/abs/1103.3784v2

Journal of Metastable and Nanocrystalline Materials (2004), 20-21, 673

Physics

Condensed Matter

Materials Science

6 pages, 4 figures

Scientific paper

10.4028/www.scientific.net/JMNM.

Spherical magnetite nanoparticles having average particle size = 5 nm have been synthesized by coprecipitation of Fe(II) and Fe(III) salts in KOH with Polyvinylalcohol (PVA). The resulting dry powder displayed superparamgnetic (SPM) behaviour at room temperature, with a transition to a blocked state at TB ~ 45 K for applied field Happ = 500 Oe. The effect of dipolar interactions was investigated by measuring the dependence of TB on the applied field Hap 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 Ea = KeffV, allowing us to estimate the contributions of dipolar interactions to the single-particle effective magnetic anisotropy Keff. We have measured the dependence of TB with Hap in order to draw the transition contours of a H-T diagram. Two different regimes are found for the (TB-T0) ~H{\lambda} dependence at low and high fields, that can be understood within a pure SPM relaxation-time (N\'eel-Brown) landscape. The TB(H) data shows a crossover from {\lambda} = 2/3 to {\lambda} ~2 for applied magnetic fields of \approx 550 Oe.

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