Domain imaging in nanometallic glasses

SILVEYRA JOSEFINA M (ORAL PRESENTATION); CREMASCHI VICTORIA J; VLASAK GABRIEL; SVEC PETER; JANICKOVIC DUSAN

Valparaíso, Chile

Congreso; Sólidos 09; 2009

There has been a great scientific and technological interest on the development of magnetic materials for more than a century. In 1988, Yoshizawa et al. [1] opened a whole new area for research and application purposes by developing the nanometallic glass called FINEMET (Fe73.5Si13.5 B9Nb3Cu1), which due to its special two phase structure, exhibits extremely soft magnetic properties together with high satu-ration magnetization. First, glassy ribbons are casted by a rapid quenching technique: planar flow cast-ing. Then, the material is subjected to a con-trolled annealing to induce the precipitation of nanometric FeSi grains (which are smaller than the magnetic correlation length). The magnetization behavior of a fer-romagnetic material depends on its domain structure which in turn is largely determined by magnetic anisotropies. Therefore, it is of great practical interest to characterize and understand the domain structure, which is essentially a fingerprint of the anisotropy distribution, not only in the as-quenched but also in the annealed state. For the observation of domain pat-terns, the magneto-optical Kerr technique is neither suitable for studies on rough surfaces nor to sharply disclose maze structures pre-sent in these ribbons. Therefore, Bitter tech-nique was used for observing magnetic do-mains in as-quenched (Fig. 1) and annealed FINEMET type samples. New advances in imaging techniques made this old and al-most abandoned technique much more prac-tical and powerful. The evolution on domain patterns with heat treatments at different temperatures was observed and compared with magnetostric-tion and coercivity variation. A heterogeneous magnetization –including out of the plane direction– was found on as quenched samples due to stresses induced during the casting. They were relieved with the annealing, turning magnetization towards the plane of the rib-bon, ordering domain structure, decreasing magnetostriction and coercivity. After an-nealing at 650 ºC during 1 h, magnetic prop-erties hardened. This could also be reflected on domain patterns, which looked disor-dered and moved only when a strong mag-netic field was applied.