Christian Polak

Effect of Stress Annealing on the Saturation Magnetostriction of Nanocrystalline Fe

Ribbons of amorphous Fe73.5Cu1Nb3Si15.5B7 have been crystallized by annealing under tensile stress. As typical for this composition, the annealed ribbons reveal a nanocrystalline structure with low coercivity, near-zero magnetostriction, and a strong creep-induced anisotropy proportional to the annealing stress. The present work shows that the tensile stress, ?a, applied during annealing also affects the saturation magnetostriction ?s. Accordingly, ?sdecreases linearly with increasing magnitude of ?a. The samples have an initially small positive magnetostriction and, hence, ?s even changes its sign from positive to negative with increasing annealing stress. This change of sign provides qualitative evidence that the effect is clearly beyond experimental error. Our finding is compared to the elastic stress dependence of the saturation magnetostriction constant. The latter is a well-established phenomena for amorphous Co-base alloys and, as we will demonstrate, can also be observed in nanocrystalline alloys. The results will be discussed theoretically in terms of the strain dependence of the magnetic anisotropy energy which ultimately provides the physical origin of magnetostrictive phenomena.

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Ribbons of originally amorphous Fe73.5Cu1Nb3Si15.5B7 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 Ku ≈ 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 Hc ≈ 8 A/m. This is to be compared to Hc ≈ 0.5 A/m for field annealed samples where Ku (≈ 20 J/m3) is smaller by more than two orders of magnitude. The behaviour of Hc can be understood within the framework of the random anisotropy model.