Abstract

Ce-Fe-Co-B is a promising system for permanent magnets. A high-throughput screening method combining diffusion couples, key alloys, Scanning Electron Microscope/ Wavelength Dispersive X-ray Spectroscope (SEM/WDS), and Magnetic Force Microscope (MFM) is used in this research to understand the phase equilibria and to explore promising magnetic phases in this system. Three magnetic phases are detected and their homogeneity ranges are determined at 900°C, which are presented by the formulae: Ce2Fe14-xCoxB (0≤x≤4.76), CeCo4-xFexB (0≤x≤3.18) and Ce3Co11-xFexB4 (0≤x≤6.66). The phase relations among the magnetic phases in this system have been studied. Ce2(Fe, Co)14B appears to have stronger magnetization than Ce(Co, Fe)4B and Ce3(Co, Fe)11B4 based on MFM analysis when comparing the magnetic interactions of selected key alloys. Two non-magnetic compounds have been detected in this system, a Co-rich with CeCo12-xFexB6 (0≤x≤8.74) formula; and a B-rich with Ce13FexCoyB45 (32≤x≤39, 3≤y≤10) formula. The crystal structure of this B-rich phase could not be found in the literature and its XRD pattern is extracted in the current study. Moreover, ternary solid solutions ε1 (Ce2Fe17-xCox (0≤x≤12.35)) and ε2 (Ce2Co17-xFex (0≤x≤3.57)) are found to form between Ce2Fe17 and Ce2Co17 in the Ce-Fe-Co ternary system at 900°C.
The intrinsic magnetic properties of Ce2Fe14-xCoxB (0≤x≤4.76) are studied at 25°C using key alloys annealed at 900°C for 25 days. The saturation magnetization (Ms) and the Curie temperature (Tc) of Ce2Fe14-xCoxB increase with Co content. However, the anisotropy field (Ha) of Ce2Fe14-xCoxB diminishes precipitously with Co content. The process of crystal structure refinement indicates that the saturation magnetization of Ce2Fe14-xCoxB is related to the site occupancy of Co atoms at different Fe atomic sites. Co atoms prefer to occupy 8j2 site, followed by 16k2, 4e and 16k1 sites sequentially. Moreover, Co atoms occupying 8j2 site are more effective leading to an increase in the Ms. The individual effects of Ni or Cu on the intrinsic magnetic properties of Ce2Fe12.98-xCo1.02NixB and Ce2Fe12.98-yCo1.02CuyB are evaluated. The maximum solid solubilities of Ni and Cu in Ce2Fe12.98Co1.02B at 900ºC are found to be 8 at.% and 0.8 at.%, respectively. Ni or Cu enhances Tc, but decreases both Ms and Ha of Ce2Fe12.98Co1.02B. This work also discussed the combined effects of Ni and Cu on the intrinsic magnetic properties of Ce2Fe12.98Co1.02B. The Ms of Ce2Fe12.98-x-yCo1.02NixCuyB (0≤x≤0.41, y≈0.119) increases after doping with both Ni and Cu, reaching around 155 emu/g. Meanwhile, the Ha and the Tc are measured to be near 24 kOe and 280°C, respectively.
The domain width and domain wall energy of the Ce2Fe14-xCoxB solid solution are studied for the first time in this work. The influence of Co content on these properties has been analyzed with the aid of magnetic force microscopy using diffusion couple and key alloys. The domain widths of Ce2Fe14-xCoxB decrease with increasing Co content at about 0.02 μm per 1 at.% Co. In Ce2Fe14-xCoxB, phase shift, domain width and saturation magnetization are related in a way that lower average domain width is associated with higher phase shift and higher saturation magnetization. The highest domain wall energy of Ce2Fe14-xCoxB is measured as 31.7 erg/cm2 after dissolving 14 at.% Co (x=2.38). The effects of Ni and Cu on the domain width and domain wall energy of Ce2Fe14-xCoxB (x=1.02) are also studied and reported using response surfaces. The domain width and domain wall energy of this solid solution increase after doping with 1 at.% Ni at constant Co content of 6 at.%, measuring 1.39 μm for domain width and 33.4 erg/cm2 for domain wall energy. Both properties are determined as 0.71 μm and 18.6 erg/cm2, respectively, after doping with 0.8 at.% Cu, while keeping Co content constant at 6 at.%. When Ce2Fe14-xCoxB (x=1.02) is doped with both Ni (1 at.%) and Cu (0.8 at.%), the domain width and domain wall energy measured 0.99 μm and 33.8 erg/cm2, respectively.