Characterization of Nd(Tb)–Fe–B-based exchange-spring nanocomposite magnets

The authors attempt to optimize for performance the composition of R ₂Fe₁₄B/Fe₃B-based hard nanocomposite alloys with R = neodymium (Nd) and terbium (Tb) by optimizing phases in order to develop hysteresis parameters such as maximum magnetization, coercivity and maximum energy product through variation in composition and heat treatment. The samples were prepared from arc-melted ingots of different compositions. The crystallization temperatures were obtained from the differential scanning calorimeter traces of the samples with the composition Nd_4−x Tb _x Fe₇₁Co₅Cu_0·5Nb₁B_18·5 (x = 0·0, 0·2, 0·4, 0·6, 0·8 and 1·0). Depending on their crystallization temperatures, the samples were annealed at 600, 625, 650, 675 and 700°C for 10 min. Amorphosity and crystallization behavior were studied by X-ray diffraction using copper (Cu) K_α radiation (1·5418 Å). The Mössbauer spectra of all the samples were recorded at room temperature by using iron-57 (⁵⁷Fe) Mössbauer spectrometry in transmission geometry with a constant-acceleration spectrometer. Magnetic properties were determined by a vibrating-sample magnetometer in the as-cast and annealed conditions. Although the highest values of coercivity and remanent ratio were found to be 4·18 kOe and 0·72, respectively, for the sample of composition Nd₃Tb₁Fe₇₁Co₅Cu_0·5Nb₁B_18·5 annealed at 650°C with a higher concentration of terbium, the highest maximum energy product was determined to be 12·67 MGOe for the sample of Nd_3·8Tb_0·2Fe₇₁Co₅Cu_0·5Nb₁B_18·5.