Magnetic domain structure correlated with the microstructure of nanocrystalline Fe-M-B (M=Zr,Nb) alloys

Nanociystalline Fe-M-B (M=Zr or Nb) alloys prepared by crystallization of rapidly quenched amorphous ribbons are known as a new class of soft magnets with high saturation magnetization. In order to systematically understand the annealing temperature dependence of their soft magnetic properties, the investigation of magnetic domain structure correlating with their microstructure and magnetic properties was made.The alloys annealed at the optimum condition (at 923K for 1hr) consist of large and uniform domains in which no magnetization ripple can be seen. However, magnetization ripple was observed in the alloys annealed at lower temperature (773K for 1hr), in spite of nearly the same grain size and rather small magnetostriction than those annealed at the optimum condition. This is due to the reduced intergranular exchange coupling strength between bcc nanograins caused by the lower Curie temperature of residual amorphous phase surrounding the bcc phase. For over-annealed (≧ 973K) alloys with poor soft magnetic properties, much smaller domains than in the optimum state were observed. The subdivision of domains in the Fe-Zr-B alloys is attributed to the increase of effective magnetocrystalline anisotropy due to the coarsening of bcc grains. On the other hand, domains of the Fe-Nb-B alloys are refined by the wall pinning by the abnormally grown iron-borides, whereas the bcc grains are still nanoscaled. These results suggest that not only refinement of mean grain size but also the magnetic properties of intergranular residual amorphous phase and the homogeneous distribution of grains are important to achieve good soft magnetic properties in nanocrystalline materials.