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

International audienceX-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 the local structure around the Co atoms and of the morphology of the Co layers in Co/ZrO2 multilayers as a function of the nominal deposited metal thickness (λ) and deposition rate (v). We compare the results obtained by several techniques which are sensitive to different length scales, from 1 to a few tens of Å. The central technique of this study is x-ray absorption spectroscopy (XAS), which appears to be particularly suitable for such systems without long-range order and composed of nanometric particles [13]. X-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) investigations have been performed at the Co K-edge to probe the structural local environment of the Co atoms. A quantitative analysis of the EXAFS data allows the determination of the nature and the number of nearest atomic neighbours (NN) at distance R from the probed central Co atoms, and the so-called Debye-Waller (DW) factor, corresponding to the mean-square relative displacement of the R distribution. In this paper the sensitivity of the EXAFS signal to the morphology of this nanoscaled metallic layer is stressed. Moreover, the investigation of the pre-edge and XANES features provides information about the electronic structure of the Co atoms and the number that are located in metallic sites. Both structural and morphological results from XAS analyses are compared to those deduced from transmission electron microscopy (TEM) micrographs and transport and magnetic measurements