Depth profile of transverse permeability spectrum in an annealed Co-based amorphous ribbon

Amorphous Co₆₆Fe₄B₁₅Si₁₅ ribbons were annealed at a temperature of 380°C for 8 h in open air under a field of 3 Oe applied along the sample axis. The annealed samples were etched in hydrofluoric acid solution during various times from 10 s to 30 min in order to the different thickness of material, t_etch, from ribbon surface. The static permeabilities due to domain wall motion μ_s,dw and magnetization rotation μ_s,rot are separated from the permeability extracted from the magnetoimpedance spectra. For the samples with thickness t_etch less than 2 μm, both μ_s,dw and μ_s,rot are nearly zero at H=-3 Oe, while they are relatively large at H=+3 Oe. As the etched thickness increases over 2 μm, μ_s,dw and μ_s,rot at H=-3 Oe start to increases and eventually reach similar values to those at H=+3 Oe. The variations of μ_s,rot and μ_s,rot with t_etch reflect that the sample annealed in open air is nearly saturated in the direction opposite to that of annealing field. This is due to antiferromagnetic coupling between the inner amorphous phase and the surface crystalline phase with thickness about 2 μm.