Structural and magnetic characterization of Nd-based Nd-Fe and Nd-Fe-Co-Al metastable alloys

The aim of the present work is to characterize a metastable hard magnetic phase referred to as "A1" in Nd-Fe alloys, which forms as a part of the fine eutectic depending on the composition and cooling rate. In order to define the range of composition for the formation of A1, Nd100-xFex (x = 20, 25, 40) alloys are cooled at about 150 K/s. The results indicate that for a cooling rate of 150 K/s, the hypereutectic Nd100-xFex (x = 20) alloys solidify into hard magnetic A1 whilst the hypoeutectic alloys (x = 40) show the formation of Nd2Fe17 crystallites. However, no sample cooled at 150 K/s shows the peaks of Nd5Fe17 as expected from the equilibrium Nd-Fe phase diagram. The effect of cooling rate on the formation of hard magnetic A1 is studied by investigating the Nd80Fe20 alloys cooled at different rates. The microstructure of hard magnetic Nd80Fe20 alloys displays a fine eutectic-like matrix consisting of Nd-richer and Fe-richer regions. The Nd-richer regions are identified as dhcp Nd and fcc Nd-Fe solid solution. However, the Fe-richer regions also referred to as A1, are diffuse and give an average composition of Nd56Fe44. These regions yield complex electron diffraction patterns, which do not match with any known Nd-Fe phase. HRTEM images of the Fe-richer regions reveal the presence of 5-10 nm crystallites embedded in an amorphous phase. Thus the Fe-richer regions of the hard magnetic Nd80Fe20 specimens are not a single homogeneous phase rather they are mixture of finely dispersed nanocrystallites in an amorphous phase. The demagnetization curves the hard magnetic Nd80Fe20 measured at temperatures above 30 K are typical of a hard magnetic material. The coercivity increases from 0.48 to 4.4 T with the temperature decreasing from 300 to 55 K. The demagnetization curves change from single to two-phase type when the temperature approaches 29 K, ordering temperature of fcc Nd-Fe solid solution. The measurements of initial magnetization, field dependence of coercivity, and temperature dependence of coercivity suggest the Stoner-Wohlfarth type magnetization reversal process for the hard magnetic A1. The values of anisotropy constant are estimated by fitting the magnetization data to the law-of-approach to saturation. The temperature dependence of anisotropy constant and the coercivity indicate that the origin of coercivity is magnetic anisotropy. A cluster model with sperimagnetic arrangement of Nd and Fe spins is used to explain the hard magnetic behavior of the mold-cast Nd80Fe20. Structural and magnetic properties of multicomponent Nd60Co30-xFexAl10 (0 < x < 30) alloys are compared with the binary Nd-Fe alloys. Magnetic measurements of the multicomponent alloys show that the magnetic properties are controlled by the fraction of the Fe content. The coercivity of the Nd60Co30-xFexAl10 mold-cast rods does not vary much with the Fe-content for more than 10 at.% Fe but the remanence and the maximum magnetization increase linearly with the Fe content. The temperature dependence of coercivity, effective anisotropy constant, and anisotropy field are identical to those for the binary Nd80Fe20 mold-cast rod. These results clearly suggest that the binary Nd80Fe20 and the multicomponent Nd60Co30-xFexAl10 (x > 5) mold-cast rods are magnetically identical