Formation and evolution of the transverse anisotropy with nanocrystallization in amorphous Fe73.5CuNb3Si13.5B9 ribbons

The evolution of the magnetic domain patterns has been studied in amorphous Fe73.5CuNb3Si13.5B9 ribbons by suitable "long duration" thermal treatments. It is shown that by annealing at 550 degrees C for annealing time t(a) from 1 to 150 h. very fine nanocrystalline bcc-FeSi rains are homogeneously formed in the amorphous matrix. Although with increasing t(a) the grain size remains very fine (less than or equal to 11 nm), the coercivity H-c increases rapidly from 0.14 A/m for t(a) = 3 h to 133 A/m for t(a) = 150 h. For the nanocrystalline ribbons with t(a) = 3 h, the domain structure is characterized by a few broad longitudinal together with some broad transverse domain patterns, connected to the minimum coercive field. With increasing annealing time t(a) greater than or equal to 10 h, only transverse domain patterns are observed and the transverse domain width gradually becomes narrow. By applying an external magnetic field. the magnetization processes indicate that the easy magnetization may be between the longitudinal and transverse directions for the samples annealed for t(a) = 1, 3, and 10 h, while in the sample of t(a) = 30 h. the easy magnetization in domains is transverse to the ribbon direction. The induced transverse anisotropy of 380 J/m(3) in the sample of t(a) = 30 h is determined from the domain width. The estimated H-c for the coherent rotation process in the sample of t(a) = 30 h is compatible with the experimentally observed value.