Magnetic properties of nanocrystalline FeCuNbSiB with huge creep induced anisotropy

Ribbons of originally amorphous Fe73.5Cu1Nb3Si15.5B7 have been annealed under tensile stress for about 4 seconds at temperatures between 450 degrees C and 750 degrees C. Under such short time conditions the optimum nanocrystalline state is achieved for annealing temperatures between 600 degrees C and 720 degrees C. This is about 100 degrees C higher than in more conventional heat treatments with annealing times in the order of an hour. The stress annealed ribbons reveal an almost perfectly linear hysteresis loop and a transverse domain pattern with zigzag domain walls characteristic for a magnetic easy plane perpendicular to the stress axis. The induced magnetic anisotropy increases proportional to the annealing stress and is comparable to that obtained after more prolonged annealing. We succeeded to induce anisotropy constants as high as K-u approximate to 12 kJ/m(3) which even exceed the local magneto-crystalline anisotropy (K-1 approximate to 8 kJ/m(3)) of the crystalline Fe-Si phase. Nonetheless, the coercivity is still as small as H-c approximate to 8 A/m. This is to be compared to H-c approximate to 0.5 A/m for field annealed samples where K-u (approximate to 20 J/m(3)) is smaller by more than two orders of magnitude. The behaviour of Hc can be understood within the framework of the random anisotropy model.