Magnetic properties of B-rich nanocomposite REy(TM)90-y-xMxB10 (RE=Nd+Pr, TM=Fe, Fe+Co, M=Nb, Ta, y=8,10, x=0-4) alloys

In this work, magnetic properties of melt-spun B-rich (10 at%) and Nb/Ta-containning nanocomposite alloys based on the formula (Nd_0.75Pr_0.25)_y(TM)_90-y-xM_xB₁₀ (TM=Fe, Fe_0.90Co_0.1, M=Nb, Ta; y=8, 10; x=0,2,4) were determined by means of VSM measurements. Nanocomposite alloys were obtained from devitrification of initially fully amorphous precursors. Considerable enhancement of the intrinsic coercivity (iHc) together with reasonable remanence (Jr) and energy density (BHmax) values were observed for Co-free alloys. Most favorable properties were observed at y=10 and 2 at%Nb concentration (iHc=912 kA/m, Jr=0.96 T, (BH)max=140 kJ/m³). On the other hand, for Co-containning alloys, optimum combinations of properties were attained for y =10 and 2 at% Nb (iHc=1042 kA/m, Jr= 0.88 T and (BH)max=123 kJ/m³). Alloy microstructure characterization by means of TEM observations, exhibited a clear grain size refining effect for Nb-containing alloys alongside with a homogenous grain size distribution, presumably comprising soft alpha-(Fe/Fe,Co) grains interspersed with RE2Fe14B hard grains, according to XRD analysis. Intrinsic coercivity and remanence enhancements were explained in terms of alloy microstructure features as well as exchange interaction among the very fine grains observed.