Influence of magnetic relaxation on magnetoelastic resonance-based detection

Abstract

The phenomenon of magnetic relaxation in amorphous ferromagnetic alloys can result in an undesired time evolution of the magnetization that produces serious drawbacks in the use of these materials in sensor applications. The present work studies, at room temperature, the influence of magnetic relaxation on the performance of an amorphous ferromagnetic ribbon as the main element of a magnetoelastic resonance (MER)-based sensor. The time evolution was observed through the evolution of the MER signal, in particular through the variation experienced by the resonance frequency fr, which is the main parameter used for sensing. It is found that, after the bias field is changed to a given value, and under constant excitation conditions, fr increases with time in a typical relaxation behavior with a relaxation amplitude ∆fr and a relaxation time τ that depend on the excitation conditions. The amplitude of the excitation h turned out to be a key factor on the relaxation, since larger excitation field amplitudes (h ⩾ 100 mOe) result in a considerable decrease of relaxation times (τ < 460 s) and a reduction of the variation of the resonance frequency (∆fr < 77 Hz). The influence of this relaxation on the sensor performance and the possible approaches to overcome this problem are evaluated and applied to the case of a magnetoelastic sensor, operating as mass sensor, for monitoring a chemical precipitation reaction.