Development of magnetic cores with extremely loss and controllable permeability using creep-induced ansisotropy

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Magnetic anisotropy was induced to Fe-Nb-Cu-Si-B nanocrystalline ribbons by ciystallizing amorphous ribbons from the amorphous state under tensile stress, and extremely low loss and controllable permeability were achieved in them. Investigations on the induction and relaxation processes of the anisotropy revealed that the anisotropy is induced during the crystallization. Thus, the anisotropy was improved in magnitude by controlling the annealing proces s during crystallization.Subsequently, the methods of preparing toroidal cores from developed ribbons was studied. Among several methods investigated, the formation of a toroidal core by winding the ribbons after crystallization achieved the most superior magnetic properties. Therefore, we investigated this method in detail. The magnetic loss decreased with increasing the core diameter and it was found that deterioration of magnetic properties due to formation of a toroidal core was suppressed by making the core diameter larger than the critical diameter determined from the values of magnetostriction and induced anisotropy. Resultantly, toroidal cores with extremely low loss were achieved. The permeability of the developed cores was kept constant up to 1 MHz and their magnetic loss was much smaller than the loss values reported previously for nanostructured and amorphous cores with low permeability. The measured magnetic loss agreed with the calculated classical eddy current loss. In addition, the permeability and magnetic loss were kept constant up to the DC-bias field being three times higher than that applicable to a ferrite cut-core with the same permeability value.