| The magnetic and structural properties of melt-spun nanocomposite {dollar}rm Rsb6Fesb{lcub}87{rcub}Nbsb1Bsb6{dollar} and {dollar}rm Rsb{lcub}3.5{rcub}Fesb{lcub}91{rcub}Nbsb2Bsb{lcub}3.5{rcub}{dollar} magnets (R=Nd, Pr, Dy, and Tb) consisting of a mixture of soft magnetic {dollar}alpha{dollar}-Fe and hard magnetic {dollar}rm Rsb2Fesb{lcub}14{rcub}B{dollar} phases have been investigated using calorimetry, x-ray diffractometry, magnetometry, and electron microscopy.; The as-spun samples of the boron-rich composition were amorphous or non-crystalline in structure while the ribbons of low-boron alloys contained an {dollar}alpha{dollar}-Fe component. Crystallization studies on these ribbons revealed at least two exothermic peaks. In {dollar}rm Rsb6Fesb{lcub}87{rcub}Nbsb1Bsb6{dollar} samples with R=Nd, Pr, and Dy the first exotherm corresponds to the formation of metastable {dollar}rm Ysb3Fesb{lcub}62{rcub}Bsb{lcub}14{rcub}{dollar}-type structure along with {dollar}alpha{dollar}-Fe. The corresponding metastable phase in {dollar}rm Tbsb6Fesb{lcub}87{rcub}Nbsb1Bsb6{dollar} samples was identified as TbCu{dollar}sb7{dollar}-type. Upon crystallization, the low-boron ribbon samples consisted of predominantly {dollar}alpha{dollar}-Fe, and no other metastable phase was detected by the techniques used in this study. All the samples transformed into the expected mixture of {dollar}rm Rsb2Fesb{lcub}14{rcub}B + alpha{dollar}-Fe phases when annealed at higher temperatures, and the transformation temperature was found to strongly depend on the rare-earth element.; Smooth loops, concealing the individual hysteresis behavior of the constituent phases of the composite magnet, were observed. High values of remanent magnetization (110 to 145 emu/g), reduced remanences, {dollar}rm Msb{lcub}r{rcub}/Msb{lcub}s{rcub},{dollar} in the range 0.6-0.8, and modest room-temperature coercivities of 2.7-4.5 kOe with energy products in the range of 10-14 MGOe were obtained. The microstructure of coercivity optimized samples reveals a uniform mixture of {dollar}rm Rsb2Fesb{lcub}14{rcub}B{dollar} and {dollar}alpha{dollar}-Fe phases with grain sizes in the 30-50 nm range.; The presence of high reduced remanences (greater than 0.5) in the isotropic samples and the greater reversibility observed in the demagnetization curves are the result of exchange coupling among the 2:14:1 and {dollar}alpha{dollar}-Fe grains. |
