Structure and Magnetic Properties of Nd-Co-Fe-Si-C Intermetallics
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Structure and Magnetic Properties of Nd-Co-Fe-Si-C Intermetallics Monica Sorescu, A. Grabias and M. Valeanu* Duquesne University, Bayer School of Natural and Environmental Sciences, Physics Department, Pittsburgh, Pennsylvania 15282, U. S. A. *Institute of Atomic Physics, National Institute of Materials Physics, R 76900 Bucharest, Romania ABSTRACT Intermetallics of the type Nd2Co15-xFexSi2Cy (x=7; 9; 12 and y=0; 1) were prepared by arc melting in order to study the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in these systems. The investigations were performed by x-ray diffraction, Mössbauer spectroscopy and hysteresis loop measurements. Both site and average hyperfine magnetic field values were analyzed in order to discriminate between the Fe and Co contributions to the local atomic environments in these systems. While a higher Fe content as well as a higher Co content can both lead to an increase in the site magnetic fields, it was found that the model based on hyperfine field distributions provides a better description of the trends observed in hyperfine magnetic fields in these compounds. INTRODUCTION In the search of new materials for permanent magnet applications, much attention has been focused during the past few years on the synthesis of iron-rare earth compounds with noncubic structures and high values of the saturation magnetization and anisotropic field strength [1-3]. In particular, the iron-rich side of the phase diagram corresponding to the iron-rare earth alloys is expected to present such interesting magnetic properties. The R2Fe17 compounds (R=Y or rare earth) have the highest saturation magnetization among R-Fe systems [4]. The rather low Curie temperatures in these compounds are attributed to complex magnetic interactions (both positive and negative exchange interactions) between iron located at various lattice sites [5]. It has been proposed, however, that nitrogenation may increase substantially the Curie temperature of these compounds due to volume expansion, as a result of the increase in the distances between iron atoms and the corresponding diminution of negative exchange interactions [6-11]. In the present work we report on the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in Nd-based intermetallics. EXPERIMENTAL The intermetallics of the type Nd2Co15-xFexSi2Cy (x=7; 9; 12 and y=0; 1) were prepared by melting the components in an arc furnace under a protective argon atmosphere. The ingots were turned several times to avoid inhomogeneities and annealed at 950-1050o C for 10 days. XRD patterns were recorded on a powder diffractometer using the Kα1 radiation of a Cu cathode at λ=1.5404 Å. Room temperature transmission Mössbauer measurements were made with a constant acceleration spectrometer . The 25 mCi gamma ray source was 57C
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