Dresden 2006 – wissenschaftliches Programm
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MA: Magnetismus
MA 20: Poster: Films(1-36) Transp(37-56) Ex.Bias(57-67) Spindyn(68-80) Micromag(81-95) Particle(96-109) Imag.+Surface(110-113) Spinelectr(114-122) Theory+Micromag(123-131) Spinstr+Aniso(132-142) MagMat(143-156) Meas(157,158) MolMag+Kondo(159-162) Postdead(163-)
MA 20.97: Poster
Dienstag, 28. März 2006, 15:15–19:15, P1
Interacting ferromagnetic nanoparticles in the superspin limit: from modified superparamagnetism to collective magnetic states — •S. Bedanta1, X. Chen1, W. Kleemann1, O. Petracic1, E. kentzinger2, P. Fischer3, S. Cardoso4, and P. P. Freitas4 — 1Universität Duisburg-Essen, 47048 Duisburg, Germany — 2Forschungszentrum Jülich, 52425 Jülich, Germany — 3Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA — 4INESC, 1000 Lisbon, Portugal
Discontinuous multilayers [CoFe(tn nm)/Al2O3(3 nm)]10 of soft ferromagnetic Co80Fe20 nanoparticles embedded in an Al2O3 matrix are considered as homogeneously magnetized superspin systems exhibiting randomness of size (viz. moment), position and anisotropy. With increasing particle concentration (viz. nominal CoFe layer thickness tn), but prior to physical percolation, one observes superparamagnetic (SPM), superspin glass (SSG) and finally superferromagnetic (SFM) domain state behavior. Dipolar interaction weakly modifies the SPM relaxation properties at tn≤ 0.5 nm, but dominates the SSG state at 0.7≤ tn≤ 1.1 nm, which is unambiguously characterized by memory and aging effects. Tunneling exchange between the nanoparticles due to atomically small magnetic clusters around the nanoparticles is suspected to mediate the SFM interaction at 1.2≤ tn≤ 1.6 nm. SFM domains have been imaged by transmission X-ray microscopy. They show temporal relaxation as evidenced by SQUID magnetometry and polarized neutron reflectometry. Owing to random pinning their walls reveal different dynamic modes as identified by the complex ac susceptibility in Cole-Cole presentation.