Local Magnetic Structure and Magnetic Properties of Nd 50 Fe 40 Si 10-x Al x Bulk Glassy Hard Magnets
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Local Magnetic Structure and Magnetic Properties of Nd50Fe40Si10-xAlx Bulk Glassy Hard Magnets H. Chiriac, N. Lupu, R.E. Vandenberghe1, and K.V. Rao2 National Institute of Research and Development for Technical Physics, Iasi, RO-6600, Romania 1 Department of Subatomic and Radiation Physics, Gent University, Gent, B-9000, Belgium 2 Department of Materials Science, Royal Institute of Technology, Stockholm, S-10044, Sweden ABSTRACT The large values over 300 kA/m at room temperature were found for the coercive field of Nd50Fe40Si10-xAlx (x = 0; 10) amorphous melt-spun ribbons and cast rods. The dependence of the coercive field on the thickness and the preparation method, namely the cooling rate, is ascribed to the existence in the amorphous matrix of very small ferromagnetic clusters, which approach of a single magnetic domain. Coercivity increases substantially up to 600 kA/m by decreasing the temperature down to 200 K, although the measurements are carried out in low fields (up to 800 kA/m). The Mössbauer spectra can be fitted to two Fe sites with magnetic moments ranging from 1.25 to 2.20µB depending on the additional element.
INTRODUCTION Rare-earth (RE) and transition metal compounds possess many unique magnetic properties and they are widely used as permanent magnets, magneto-strictive devices or magneto-optical recording media. Many efforts are devoted to search new binary or ternary RE-transition metal alloys that are interesting for both research and applications. It was found by Croat [1,2] that RE-Fe melt-spun amorphous ribbons exhibit coercivities over 10 kOe at room temperature, and their strong dependence on the cooling rate and temperature was associated with the existence in the amorphous phase of one metastable hard magnetic phase. Recently, it has been reported that Nd-Fe alloys with additions of Si or Al exhibit both a large glass-forming ability and good hard magnetic properties at room temperature [3,4]. The mechanisms that govern the hard magnetic behaviour of these ternary amorphous alloys are not well understood. This paper focuses on structural and magnetic behaviour of Nd50Fe40Si10-xAlx (x = 0; 10) melt-spun ribbons and cast rods in the temperature range 5 – 300 K. Comparative studies as a function of composition, the method of preparation and the degree of crystallisation are presented.
EXPERIMENTAL DETAILS The master alloys were prepared by arc melting pure elements in an argon atmosphere, and re-melted several times for complete homogenisation. Ribbons with thicknesses between 25 and 145 µm prepared by single roller melt-spinning method and rods with diameters up to 2 mm obtained by suction casting in Iasi, Romania, were used for detailed structural and magnetic investigations. Ribbons with different thicknesses were obtained by maintaining constant the orifice size and the ejection pressure and changing the wheel velocity. In the suction casting method the master alloy was re-melted in a water cooled Cu crucible and L8.3.1
sucked up in a cooled Cu mould with different diameters. The str
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