Abstract
The magnetic and transport properties of films based on discontinuous layers of (Py) embedded in were investigated. In films with nominal Py thicknesses 6 and superparamagnetic particles with median diameters and 3.1 nm and distribution widths and 1.3 nm were formed. Current voltage 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, , and by the electric field at low temperature, . The separation of the two regimes allows independent estimates of the mean charging energy for both samples; from the resistance versus analyzed in an effective-medium model at high temperature and from versus 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 at high . The scaling parameter in the relation , where is the threshold for conduction, is estimated to at 4 K. The superparamagnetic relaxation of the particles becomes blocked below a temperature respective 30 K for 6 and . The magnetic field dependence of the resistance displays a single maximum of the ratio in zero field at room temperature and a characteristic splitting of the peak at 4 K, attributed to the blocking. The maxima, for and 1.1% for , are positioned at fields about a factor of two to three higher than the coercive fields of the samples.
1 More- Received 10 November 2009
DOI:https://doi.org/10.1103/PhysRevB.81.224437
©2010 American Physical Society