Structural investigations of Co/ZrO 2 discontinuous multilayers by x-ray absorption fine structure spectroscopy

X-ray absorption spectroscopy (XAS) is used to probe the evolution of the local order around the cobalt atoms in Co/ZrO 2 discontinuous multilayers as a function of the nominal Co layer thickness. A decrease of the mean number of Co nearest neighbours around Co atoms is observed as the Co layer nominal thickness decreases. XAS analysis is made according to a justified simple model based on two possible Co sites referred to as metallic (cobalt coordinated) and oxide (oxygen coordinated) sites. XAS results are in agreement with the results obtained by other structural characterizations using transmission electron microscopy and by magnetic and transport measurements showing the morphological evolution of the Co layers from the discontinuous state (multilayer of Co nanoparticles embedded in the oxide) to the continuous one by increasing the Co nominal thickness. 1. Introduction The spin dependent tunnel effect has attracted great interest in the last seven years. Besides its fundamental aspect, the tunnel magnetoresistance (MR) of magnetic tunnel junctions (MTJ) could be useful for applications, especially in magnetic random access memories [1-4]. As MTJ(two continuous ferromagnetic electrodes separated by an insulating layer),ferromagnetic nanoparticles embedded in an insulating matrix also exhibit tunnel MR but with a weaker amplitude [1, 5-7]. In order to obtain high MR sensitivity in metal/oxide systems, the ferromagnetic particles can be arranged to form discontinuous multilayers (DCM) [8, 9]. Compared to MTJ, DCM are easier to prepare by physical vapour deposition such as sputtering. Besides the intrinsic physical properties of the magnetic metal and of the oxide, the local atomic order at the metal-oxide interface has been shown to have a strong influence on the spin dependent tunnel mechanisms, in particular on the polarization rate of the tunnelling electrons [10, 11]. Dispersion of the metallic species into the insulating matrix may lead to a drastic change in the magnetic and electronic properties of the metal/oxide/metal device. Moreover, the relative orientation of the magnetic moments of neighbouring particles, which is modified by the applied magnetic field H, also determines the shape of the MR variation [12]. As a result, the variation of the magnetization and of the electrical resistance of the DCM with H is mostly governed by the morphology of the nanogranular metal-oxide layers. This paper presents the results of the characterization of th