Magnetic behavior of Fe : Al2O3 nanocomposite films produced by pulsed laser deposition

Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al2O3) have been grown by sequential pulsed lase r deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5x10(15) atoms/c m(2). Increasing the Fe content leads first to the formation of elongated n anocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this ca se, a reduced moment per Fe atom (1.4 mu (B)/atom) with respect to the valu e for alpha -Fe (2.2 mu (B)/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4x10(6) J/m(3)) a nd the low temperature coercivity in these samples are attributed to a stro ng surface contribution to anisotropy, whereas the temperature dependence o f coercivity is attributed to thermal activation. (C) 2001 American Institu te of Physics.