Tunneling and charging effects in discontinuous superparamagnetic Ni81Fe19/Al2O3 multilayers

R. Bručas, M. Hanson, P. Apell, P. Nordblad, R. Gunnarsson, and B. Hjörvarsson
Phys. Rev. B 81, 224437 – Published 29 June 2010

Abstract

The magnetic and transport properties of films based on discontinuous layers of Ni81Fe19 (Py) embedded in Al2O3 were investigated. In films with nominal Py thicknesses 6 and 8Å superparamagnetic particles with median diameters Dmed=2.8 and 3.1 nm and distribution widths σD=1.2 and 1.3 nm were formed. Current voltage (IU) curves were measured with the current perpendicular to the film plane. The analyses show that the charge transport occurs via tunneling; with the charging energy supplied by thermal fluctuations at high temperature, T100K, and by the electric field at low temperature, T<10K. The separation of the two regimes allows independent estimates of the mean charging energy EC40meV for both samples; from the resistance R versus T analyzed in an effective-medium model at high temperature and from I versus U at 4 K. In order to obtain a consistent description of the transport properties, the size distributions must be included to account for the deviation from the single size behavior Rexp(EC/kBT) at high T. The scaling parameter in the relation I(UUth1)γ, where Uth is the threshold for conduction, is estimated to γ2 at 4 K. The superparamagnetic relaxation of the particles becomes blocked below a temperature T20K respective 30 K for 6 and 8Å. The magnetic field (B) dependence of the resistance R(B) displays a single maximum of the ratio MR=[R(B)R(2T)]/R(2T) in zero field at room temperature and a characteristic splitting of the peak at 4 K, attributed to the blocking. The maxima, 0.9% for 6Å and 1.1% for 8Å, are positioned at fields about a factor of two to three higher than the coercive fields of the samples.

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  • Received 10 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.224437

©2010 American Physical Society

Authors & Affiliations

R. Bručas, M. Hanson, and P. Apell

  • Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

P. Nordblad

  • Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden

R. Gunnarsson

  • Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Göteborg, Sweden and HLK, Jönköping University, SE-551 11 Jönköping, Sweden

B. Hjörvarsson

  • Department of Physics and Materials Science, Uppsala University, P.O. Box 530, SE-751 21 Uppsala, Sweden

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Issue

Vol. 81, Iss. 22 — 1 June 2010

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