Regensburg 2004 – scientific programme
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MA: Magnetismus
MA 13: Poster:Schichten(1-23),Spinabh.Trsp(24-41),Exch.Bias(42-56),Spindyn.(57-67),Mikromag.(68-76),Partikel(77-90),Spinelektr.(91-97),Elektr.Theo.(98-99),Mikromag+PhasÜ+Aniso.(100-105),Magn.Mat.(106-118),Messmethod.(119-121),Obflm.+Abbverf.(122-123)
MA 13.83: Poster
Tuesday, March 9, 2004, 15:00–19:00, Bereich A
Magnetic ordering transition in a dense frozen ferrofluid — •S. Bedanta1, S. Sahoo1, Xi Chen1, W. Kleemann1, D. Sudfeld2, K. Wojczykowski2, and A. Hütten2 — 1Angewandte Physik, Universität Duisburg-Essen, D-47048 Duisburg — 2Fachbereich Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld
Magnetic properties of a dense frozen ferrofluid [Fe55Co45/hexane] are measured with SQUID magnetometry and susceptometry. Above the blocking temperature, Tb≈ 25 K, of the particles with size 4.6 nm, Langevin-type M(H) curves reveal small moments per particle, µ ≈ 50 µ B, and biased minor hysteresis loops (non-saturating for µ 0H ≤ 5 T) after field-cooling to below Tb. Sharp peaks of χ ´(T) and a sizeable decrease of the coercive field at T c ≈ 9 K seem to indicate a transition with critical behavior of both susceptibility and spontaneous magnetization. Two heterogeneous models are discussed to explain these features. Either the nanoparticles contain a ferromagnetic core and a disordered shell [1], possibly stabilized by an oxidic surface layer, or they are non-collinearly antiferromagnetic with uncompensated net moments [2]. Within the frame of the above models the observed transition might then be either exchange-driven into an intra-shell frozen cluster arrangement [1] or dipolar-driven into an inter-particle ferromagnetic ground state [3].
[1] R. D. Zysler et al., J. Magn. Magn. Mater. 266, 233 (2003).
[2] R. H. Kodama et al., Phys. Rev. Lett. 79, 1393 (1997).
[3] J. P. Bouchaud and P. G. Zérah, Phys. Rev. B 47, 9095 (1993).