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    Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy

    S. Disch1,2,*, R. P. Hermann1,3, E. Wetterskog4, A. A. Podlesnyak5, K. An6,7, T. Hyeon6,7, G. Salazar-Alvarez4, L. Bergström4, and Th. Brückel1,†

    • 1Jülich Centre for Neutron Science JCNS and Peter Grünberg Institut PGI, JARA-FIT, Forschungszentrum Jülich, D-52425 Jülich, Germany
    • 2Institut Laue-Langevin, F-38042 Grenoble, France
    • 3Faculty of Science, University of Liège, B-4000 Liège, Belgium
    • 4Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden
    • 5Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
    • 6Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Korea
    • 7School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea

    • *Present address: Department Chemie, Universität zu Köln, Luxemburger Strasse 116, 50939 Köln, Germany.
    • †T.Brueckel@fz-juelich.de
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    Phys. Rev. B 89, 064402 – Published 5 February, 2014

    DOI: https://doi.org/10.1103/PhysRevB.89.064402

    Abstract

    We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Landé factor g=2.05(2) as expected for maghemite (γ-Fe2O3) nanoparticles. A large effective anisotropy field of BA,eff=0.45(16) T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles.

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    Phys. Rev. B 89, 064402– Published 5 February, 2014
    • Received 2 December 2013
    • Revised 18 January 2014

    ©2014 American Physical Society

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