Structural and magnetic properties of Nd-Fe-Mo-(N) melt-spun ribbons with ThMn12 structure

Abstract

The influence of quenching rate and nitrogenation in melt-spun Nd 1.2 Fe 10.6 Mo 1.4 has been investigated in terms of microstructure, phase formation and magnetic properties. Increasing the quenching rate leads to smaller grain size. However, it also implies a change in the crystallized phase structure. We obtained a pure ThMn 12 (1:12) structure at quenching rates up to 30 m/s, leading to an average grain size of 220 nm. Magnetic measurements of the as-spun ribbons revealed a reduction of the saturation magnetization for samples quenched above 30 m/s. This is attributed to the formation of a paramagnetic phase and/or magnetic phase with a Curie temperature (T C ) close to room temperature which is confirmed by 57 Fe Mössbauer spectroscopy. The analysis of the spectra rules out the presence of a ferromagnetic TbCu 7 (1:7) phase, which is usually reported in such system. The ribbons were nitrogenated in order to form the harder magnetic phase Nd 1.2 Fe 10.6 Mo 1.4 N x . The ribbon quenched at 30 m/s with the pure ThMn 12 nitride structure is the optimum sample for getting hard magnetic properties, with a coercivity of 0.6 T, saturation magnetization of 1.15 T and Curie temperature of 350 • C. Finally, we show the good stability of the later phase structure at elevated temperatures (≤ T C ), making this compound a good candidate for permanent magnet applications.

Denitrogenation process in ThMn12 nitride by in situ neutron powder diffraction

“…2. The saturation magnetization is reached for the parent compound, giving a value M s = 90 Am 2 /kg, which is consistent for Nd-Fe-Mo alloys [15]. For the nitrides, the saturation is not reached at 4 T. However, this field is high enough to observe a coercive field >0.5 T, suggesting a proper nitrogenation and proving its effect on the anisotropy of these compounds.…”

“…Both samples show a quasipure ThMn 12 structure with a small amount (<2 wt.%) of impurities, mostly α-(Fe,Mo) and a few traces of metallic Nd (dhcp). For the nitrides, we observe a shift of the peak positions to lower angles, owing to the nitrogen inclusion in the 2b site of the lattice, resulting in an increase of both the a and c cell parameters [15].…”

“…Flakes with a nominal composition (Nd 0.75 , Pr 0.25 ) 1.2 Fe 10.5 Mo 1.5 were prepared by the strip cast method using metals with purity >99.9%. An excess of (Nd,Pr) mixture (20 at.%) was added in order to enhance the hard magnetic properties [15,20]. The obtained alloys were then melt-spun by ejecting the molten alloy through an orifice (0.8 mm) from the bottom of a quartz tube onto a copper wheel under argon atmosphere.…”

“…The quenching rate was optimized to obtain a pure ThMn 12 structure with the finest microstructure possible, i.e., about 200 nm in grain size. The ribbons were crushed into powder (<40 μm) and then nitrogenated at 873 K for 3 hours under a N 2 atmosphere, as described previously [15]. To investigate the thermostability of the magnetic properties, the nitride powder was sealed in quartz ampoules under vacuum, and then heat treated at various temperatures between 873 and 973 K for 30 min followed by quenching in air.…”

“…Even though powder nitrides have recently shown success in obtaining hard magnetic properties [14,15], one of the remaining challenges is to transfer the good properties of the powder into a useful bulk magnet [16,17]. One weakness of the ThMn 12 nitrides is their tendency to disproportionate or eventually denitrogenate at temperatures which are required for sintering [18].…”