Tunneling and charging effects in discontinuous superparamagnetic Ni81Fe19/Al2O3 multilayers
Artikel i vetenskaplig tidskrift, 2010

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 8 angstrom superparamagnetic particles with median diameters D-med = 2.8 and 3.1 nm and distribution widths sigma(D)= 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, T >= 100 K, and by the electric field at low temperature, T < 10 K. The separation of the two regimes allows independent estimates of the mean charging energy approximate to 40 meV 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 R similar to exp(E-C/k(B)T) at high T. The scaling parameter in the relation I proportional to (U/U-th-1)(gamma), where U-th is the threshold for conduction, is estimated to gamma approximate to 2 at 4 K. The superparamagnetic relaxation of the particles becomes blocked below a temperature T approximate to 20 K respective 30 K for 6 and 8 angstrom. The magnetic field (B) dependence of the resistance R(B) displays a single maximum of the ratio MR = [R(B)-R(2 T)]/R(2 T) in zero field at room temperature and a characteristic splitting of the peak at 4 K, attributed to the blocking. The maxima, approximate to 0.9% for 6 angstrom and 1.1% for 8 angstrom, are positioned at fields about a factor of two to three higher than the coercive fields of the samples.



room temperature


transport properties

electrical conduction

granular films



giant magnetoresistance


Rimantas Brucas

Chalmers, Teknisk fysik, Fasta tillståndets fysik

Maj Hanson

Chalmers, Teknisk fysik, Fasta tillståndets fysik

Peter Apell

Chalmers, Teknisk fysik, Kondenserade materiens teori

P. Nordblad

Robert Gunnarsson

Chalmers, Mikroteknologi och nanovetenskap, Kvantkomponentfysik

B. Hjorvarsson

Physical Review B - Condensed Matter and Materials Physics

24699950 (ISSN) 24699969 (eISSN)

Vol. 81 22 224437-


Nanovetenskap och nanoteknik






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