Magnetic properties of nanocrystalline FeCuNbSiB with huge creep induced anisotropy

Ribbons of originally amorphous Fe_73.5Cu₁Nb₃Si_15.5B₇ have been annealed under tensile stress for about 4 seconds at temperatures between 450°C and 750°C. Under such short time conditions the optimum nanocrystalline state is achieved for annealing temperatures between 600°C and 720°C. This is about 100°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 ≈ 12 kJ/m³ which even exceed the local magneto-crystalline anisotropy (K₁ ≈ 8 kJ/m³) of the crystalline Fe-Si phase. Nonetheless, the coercivity is still as small as H_c ≈ 8 A/m. This is to be compared to H_c ≈ 0.5 A/m for field annealed samples where K_u (≈ 20 J/m³) is smaller by more than two orders of magnitude. The behaviour of H_c can be understood within the framework of the random anisotropy model.