Microstructure of nanocrystalline b.c.c. FeMB(MNb,Hf) soft magnetic alloys

The magnetic and structural properties of as-deposited and annealed Fe_87.9Hf_9.8B_2.3 films produced by a sputtering technique were examined. The nanocrystalline b.c.c. film annealed for 3.6 ks at 873 K exhibits a B_s value of 1.4 T and, at 1 MHz, it has a μ_e value of 1400. The structure of the annealed film was found by high resolution transmission electron microscopy (HRTEM) to consist of a mixture of a large portion of nearly spherical b.c.c. grains with sizes of 10-15 nm, and a small portion of amorphous layers less than 5 nm wide surrounding the b.c.c. grains. HRTEM and electron diffraction analysis using an electron beam with a diameter of 0.6 nm revealed that the amount of Hf in the amorphous layer was several times higher than that in the b.c.c. grain with a small amount of lattice defects in the film. Furthermore, the structure of a melt-spun Fe₈₄Nb₇B₉ ribbon annealed for 3.6 ks at 873 K (previously known to exhibit high B_s values and excellent soft magnetic properties with a mostly singl b.c.c. phase) was clarified to be almost the same as that of the annealed Fe_87.9Hf_9.8B_2.3 film; the amounts of Nb and B in the amorphous region were found to be considerably higher than those in the b.c.c. grain for the annealed ribbon. The amorphous region containing a large amount of solute elements is considered to have a high thermal stability and high Curie temperature, to repress the b.c.c. grain growth, and to play an important role in achieving good soft magnetic properties.