Dynamic circular magnetisation loops in Co-based amorphous wires

Summary form only given. Co-based slightly negative magnetostrictive amorphous wires possess a circumferencial anisotropy and a corresponding circular domain structure. This magnetic configuration has been proven to be very important for giant magneto-impedance effect (GMI), since the circular magnetisation processes are very sensitive to the application of a dc axial magnetic field H/sub ex/. At moderate frequencies up to few megahertz the change in the impedance with H/sub ex/ (and measured voltage) is mainly determined by the circular domain wall dynamics. If the amplitude of ac current in a wire is sufficiently large the irreversible domain wall processes are essential changing the waveform of the voltage signal. On the other hand, the circular hysteresis loops were measured only under quasi-dc conditions. This paper presents the investigation into dynamical circular hysteresis in CoFeNiSiB amorphous wires covered by glass under the effect of H/sub ex/. The experimental results obtained are used to find the relaxation parameters describing the frequency behaviour of the circular differential permeability and to model the voltage waveforms. The circular hysteresis loops (MH-loops) were measured for frequencies up to 200 kHz by filtering out and integrating an inductive voltage signal across the wire. For frequencies higher than a few kilohertz, the effect of distributed inductance and capacitance becomes significant introducing distortions into the output voltage signal. The MH-loop tracer was calibrated using a special integro-differential correction function. The driving current amplitude was 20-50 mA allowing the wire to be remagnetised (or partially remagnetised) at higher frequencies. The dc bias field H/sub ex/ varied in the range of 0-800 A/m. The measured hysteresis loops are plotted with frequency as a parameter. At H/sub ex/=0, with increasing frequency the coercivity also increases and differential permeability drops. However, if the magnetising curre- t amplitude is not large enough, there is no complete magnetisation cycle and narrower loops are seen in this case. The effect of H/sub ex/ is very strong at low frequencies (1-10 kHz) resulting in a transition from almost a square loop to a linear one. At higher frequencies (>50 kHz), the field effect is much weaker. Using these results, a differential permeability as a function of frequency, current amplitude and dc bias field was deduced to model GMI waveforms at MHz. frequencies.