Chapter 5: E ffect of m agnetic field annealing on the properties of am orphous ribbons
5.2. Experimental Details
5.3.3. Effect of field annealing on domain structure
To improve our understanding on the evolution of soft magnetic properties with TA and to investigate the correlation between the microstructure, magnetic domain structure, and resulting magnetic properties, the magnetic domain structure was observed at room temperature in the Fresnel mode using Lorentz microscope. Figs.5.06 – 5.08 depict the corresponding micrographs for the x = 0, 5, and 10 samples annealed at different TA. For the as-spun Fe89Zr11 alloy, no
Chapter 5: E ffect of m agnetic field annealing on the properties of am orphous ribbons
domains were observed due to the PM nature of the sample in the demagnetized state. On the FIG. 5.06: Magnetic domain images of Fe89Zr11 alloy ribbons annealed at (a) TA = 673 K, (b) TA = 823 K and (c) TA = 923 K for 1 hour.
FIG. 5.07: Magnetic domain images of Fe84Zr11B5 alloy ribbons annealed at (a) TA = 673 K, (b) TA = 823 K and (c) TA = 923 K for 1 hour.
FIG. 5.08: Magnetic domain images of Fe79Zr11B10 alloy ribbons annealed at (a) TA = 673 K, (b) TA = 823 K and (c) TA = 923 K for 1 hour.
Chapter 5: E ffect of m agnetic field annealing on the properties of am orphous ribbons
other hand, the sample annealed at 673 K showed domains with smooth domain walls. The average size of the domains varied between 200 and 400 nm. Increasing TA to 823 K resulted in smaller domains with the average size between 90 – 150 nm and irregular domain walls. On further increasing TA to 923 K, the irregular domain walls are believed to be pinned by Fe3Zr compounds. The smaller domains in the Fe89Zr11 samples indicate a weak IEC. On the other hand, Fe-Zr-B samples annealed up to 823 K show very large-sized (2 and 5 µm) domains with smooth and relatively straight domain walls, indicating a strong IEC. These observations can be interpreted in the framework of the Random Anisotropy Model (RAM) [ALBE1978, HERZ1989, HERZ1990] that has been developed to account for the behavior of the HC with the grain size in nanocrystalline materials. According to this model, the average effective anisotropy resulting from strong (weak) IEC is (not) averaged out. Hence, domain wall pinning is expected by the fluctuation of anisotropy in weakly coupled system. These results are consistent with TA
dependent magnetic parameters depicted in Fig.5.05. For the alloys annealed at 923 K, the average domain size is very much smaller and pinned by the Fe3Zr compound. This suggests that the deterioration of soft magnetic property is caused by the structural inhomogenities, where the effective anisotropy is not averaged out and can no longer be neglected.
5.4. Effect of field annealing on the properties of Fe-Co-Zr-B ribbons 5.4.1. Effect of field annealing on microstructure
To understand the effect of Co substitution on the control of microstructure and soft magnetic FIG. 5.09: Bright-field TEM images and SAED patterns of Fe84-yZr11B5Coy (y = 0 (a), 5 (b), 10 (c)) alloys annealed at TA = 823 K for 1 hour.
Chapter 5: E ffect of m agnetic field annealing on the properties of am orphous ribbons
properties of Fe89-x-yCoyZr11Bx (x, y = 5, 10) alloys annealed at different TA. Figs.5.09 – 5.10 FIG. 5.10: Bright-field TEM images and SAED patterns of Fe79-yZr11B10Coy (y = 0 (a), 5 (b), 10 (c)) alloy ribbons annealed at TA = 823 K for 1 hour.
FIG. 5.11: Bright-field TEM images and SAED patterns of Fe79-yZr11B10Coy (y = 0 (a), 5 (b), 10 (c)) alloy ribbons annealed at TA = 823 K for 1 hour.
FIG. 5.12: Bright-field TEM images and SAED patterns of Fe79-yZr11B10Coy (y = 0 (a), 5 (b), 10 (c)) alloy ribbons annealed at TA = 923 K for 1 hour.
Chapter 5: E ffect of m agnetic field annealing on the properties of am orphous ribbons
show the bright-field TEM micrographs and SAED patterns of the alloys with x = 5 (y = 0, 5, 10) and x = 10 (y = 0, 5, 10) annealed at 823 K. It is to be noted that the alloys annealed below 773 K show only amorphous phase. For the Fe84Zr11B5 alloys annealed at 823 K, the formation of fine bcc nanocrystals with average size between 3 and 5 nm was observed. With the increase in Co content to 10 at.% in Fe84-yCoyZr11B5 alloy annealed at 823 K, the volume fraction of the nanocrystals increase (Fig.5.09c). The ring pattern in SAED supports the existence of nanocrystals. By comparing the SAED patterns of the Fe84-yCoyZr11B5 alloys, the halo effect in the alloy with y = 10 (Fig.5.09c) is weaker than that in the alloy with y = 5 (Fig.5.09b), suggesting that the volume fraction of amorphous phase has reduced. In addition, there were extra rings next to the first ring, which are indexed to be (200) and (211) reflections of the bcc Fe(Co) phase [SHUL1999]. On the other hand, for the Fe79-yCoyZr11B10 alloys with y = 0 and 5 annealed at 823 K, the bright-field TEM images revealed an even contrast and the SAED patterns contained concentric diffusive rings, a typical characteristic of an amorphous structure.
However, for the Fe69Co10Zr11B10 alloy, there was an extra ring pattern next to the first ring in SAED supports the existence of fine nanocrystals. The increased volume fraction of nanocrystals in the Co substituted Fe84-yCoyZr11B5 alloys annealed at 823 K is mainly due to the reduction in the crystallization temperature with increasing Co content. In other words, addition of Co seems to destabilize the amorphous phase [SHUL1999]. These results are in good agreement with the DSC results reported in chapter 4 (sec. 4.3.2). Figs.5.11 – 5.12 depict the bright-field TEM micrographs and SAED patterns of the Fe89-x-yCoyZr11Bx alloys with x = 5 (y = 0, 5, 10) (Fig.5.11) and x = 10 (y = 0, 5, 10) (Fig.5.12) annealed at 923 K. It can be seen that the average grain size increased to 20 – 40 nm and the uniform distribution of the crystalline grains gets changed with Co substitution. The SAED patterns show additional diffraction rings that are attributable to phase(s) other than the bcc Fe(Co) phase. It was found that the additional rings could be indexed to (Fe(Co))3Zr compound.