Magnetic and Structural Properties of Iron Nitride thin Films Obtained by Argon-Nitrogen Reactive Radio-Frequency Sputte
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Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
These samples were structurally characterized by X-Ray Diffraction (XRD) with a Kpfiltered Co Ku, radiation (1.78892 A) on a 0/20 Philips goniometer operating with a Raytech Position Sensitive Detector. Crystallographic phases were deduced from comparison of experimental diffraction profiles with standard ones (JCPDS data). Some uncertainty is left for the determination of e-Fe2 -3N phase because of the relatively large variations of its crystalline parameters among the compositional domain where it exists. Structure was also checked by Transmission Electron Microscospy (TEM) with a Philips CM20 microscope operating at 200 kV. Selected Area Electron Diffraction (SAED) results were consistent with XRD. Electron Energy Loss Spectroscopy (EELS) experiments were performed on this TEM fitted with a Gatan (model 666) spectrometer. We could measure the atomic abundances of iron and nitrogen from the respective intensities of their characteristic absorption edges, estimate the oxidation state of iron (L 2/L 3 intensity ratio) and get some qualitative informations about the atomic structure of the samples. Local magnetic properties have been investigated by Conversion Electron Mdssbauer Spectroscopy (CEMS). M6ssbauer spectra were recorded at room temperature in the backscattering mode with a He (5% CH 4 ) gas flow proportional counter. This allows a non destructive study with a sampling depth of about 2500 A, encompassing therefore the whole thickness of iron nitride films. The source drive and data storage were of usual design. The 57Fe hyperfine pattern was fitted with standard routines where Lorentzian line shapes were assumed. Bulk magnetization measurements have been performed with either a Vibrating Sample Magnetometer or a Quantum Design SQUID down to 5K. Room temperature Kerr rotation cycles have also been performed in both longitudinal and polar geometries, they give similar results than the VSM ones but faster and with a better accuracy for determining coercive fields. (211)CFe 2N
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Figure 1 XRD experiments performed on the series of Fe-N films deposited at room temperature as a function of the nitrogen concentration in the plasma.
400
600 800 AE (eV)
1000
Figure2 EELS spectra recordedfor a series of samples deposited on unheated TEM grids and increasing nitrogen concentrationsin the plasma (0.00 < xN2 < 41%). The N/Fe atomic ratio evolves from 0 (Fe) to 0.5 (Fe2N).
104
RESULTS 1. Crystallographic Structure and Stoichiometry Structural properties of the films deposited on unheated substrates are illustrated in figure 1 which represents the evolution of XRD peaks as a function of the nitrogen partial pressure in the plasma. These results can be summarized as follows : - At very low nitrogen concentrations, films only sh
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