A new magnet material with ThMn12 structure: (Nd1−xZrx)(Fe1−yCoy)11+zTi1−zNα (α=0.6-1.3)

We previously reported a new compound for permanent magnets, (Nd0.7Zr0.3)(Fe0.75Co0.25)11.5Ti0.5N0.56, which has a high saturation polarization (Js) of 1.68 T and a high magnetocrystalline anisotropy field (Ha) of 2.88-4.0 MA/m. Here, we examined the effects of substituting Co and Ti at the Fe sites and Zr at the Nd sites. Js increased with Co substitution at the Fe sites, and higher Fe and Co content could be achieved by decreasing the Ti content to −Ti0.5. The ThMn12 structure with high Fe and Co content (i.e. low Ti content) was stabilized mainly by −Zr0.3 substitution at the Nd sites. The Zr substitution resolves the local mismatch in atomic size in the structure. Specifically, the atomic radius of Zr is about 88% that of Nd, so the local structure surrounding Nd (2a site) shrinks as a result of the substitution and resolves the size mismatch in the three types of local Fe six-fold symmetric hexagons in the structure. The effect of the α-(Fe, Co) phase on Js was evaluated from the phase's volume fraction measurement of 7.7% (8.1 wt%) by electron backscatter diffraction. Nitrogenation of the starting alloy also resulted in augmentation of the c-axis magnetocrystalline anisotropy. Finally, we confirmed a high Js(−N1.3) of 1.71T (=1.67 T, −0.04 T for the α-(Fe, Co) phase) and Ha of 2.9-5.25 MA/m (Ha(−N1.3)=5.25 MA/m) in the (Nd0.7Zr0.3)(Fe0.75Co0.25)11.5Ti0.5Nα (α=0.60-1.30) compounds.