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.