Critical length and giant magnetoimpedance in Co69Fe4.5Ni1.5Si10B15 amorphous ribbons

We report a systematic study of the influence of sample length on the giant magnetoimpedance (GMI) effect and its field sensitivity (η) in Co69Fe4.5Ni1.5Si10B15 amorphous ribbons in the frequency range of 0.1-10 MHz. We show that there exists a critical length (L0 ∼ 8 mm) below which the maximum GMI and η decrease with decreasing sample length (L) and above which they increase with decreasing L. The critical frequency, at which the maximum GMI and η are achieved, reaches the largest value for samples with L = L0 and shifts to lower values for samples with L < L0 and L > L0. Our observations provide important practical guidance for optimization of the geometrical dimensions of magnetic sensing elements and design of GMI-based magnetic sensors. The demagnetization contribution for L < L0 and the origin of the existence of L0 and f0 are discussed in the context of the permeability and resistance dependences of GMI.