Relationship Between Thermally Activated Atomic Diffusion Processes in Co-Cu Nanoscale Granular Films and Electric/Magne
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characterised by structural defects such techniques are not always effective in providing an estimation of the dimensions or the distribution of the grains [5]. In this case magnetisation curves, susceptibility and resistivity measurements can give insights about the nanometric structure of superparamagnetic-diamagnetic granular media. In this paper, we induce the
diffusion of atomic species submitting Co-Cu granular systems to thermal annealings with different heating rate (HR). We then study the modifications the system undergoes to with
magnetic and resistive measurements and perform XPS analysis to detect possible changes in samples composition.
EXPERIMENTAL The granular materials have been obtained depositing alternately very thin cobalt and copper layers onto Si substrates by RF-sputtering in Ar atmosphere. The nominal thickness are 0.4 nm for Co and 1.4 nm for Cu; the whole film is 80 nm thick. Resistivity measurements at various temperatures have been performed with the Van der Pauw method. Magnetisation curves have been measured with an alternating gradient field magnetometer (AGFM) at room temperature and with a vibrating sample magnetometer (VSM) as a function of temperature. Thermal treatments have been performed by a differential scanning calorimeter (DSC) in
609 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
scanning mode with Ar flowing inside the sample holder in laminar regime. We used two different HR, 10 K/min and 20 K/min, and during each annealing the temperature of the sample was varied starting from 373 K (initial temperature) up to 773 K (final temperature). Every sample has been submitted to more than one thermal treatment with the same HR. XPS measurements have been performed with a synchrotron light source at the SuperEsca beamline with photon energies equal to 183 eV and 650 eV. The XPS data have been collected at various depths inside the sample after etching procedure. RESULTS The in-plane magnetisation curve of the as-prepared sample is reported in fig. 1(a): it doesn't show a coercive field and displays superparamagnetic features. The loop can be well approximated by a weighted sum of Langevin functions [5), EN I pjL(jH/kT), where pi is proportional to the volume Vi of the non interacting Co particles. The best fit is obtained when we use just two values of [p, p, and 12; if we suppose to have disk-like particles and that [la-Ms.gV we find that their mean diameters are 8 and 2 nm, respectively. The superparamagnetic nature of the samples is confirmed by the presence of a broad maximum in the susceptibility curve as a function of temperature ascribable to the average blocking temperature of the cobalt particles [6]. Resistivity of the as-prepared samples, Po, depends linearly on temperature between 40 K and 300 K and Opo/OT > 0 (fig. 1(b)). Its value at room temperature (RT) is 31 pQ-cm and the value of Po extrapolated at 0 K is 23 pQ.cm. The higher value of po( 300 K) respect to those of Co and Cu (Pco equals to 5.8 p/Qcm and Pcu to 1.7 pfbcm at RT) and th
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