Magnetic Relaxation effects on Magnetoelastic Resonance Sensors

The phenomenon of magnetic relaxation in amorphous ferromagnetic alloys produces a time-dependence of the magnetization, and other magnetic related properties, that can be a source of error when these materials are used in sensor applications. The present work studies the influence of the magnetic relaxation on the performance of an amorphous ferromagnetic ribbon of composition Fe
73
Cr
5
Si
10
B
12
as the main element of a magnetoelastic resonance (MER) sensor. The relaxation behavior was observed through the evolution of the MER signal, in particular through the variation experienced by the resonance frequency f
r
, which is the most commonly used parameter in sensing applications. It was found that, after the application of the bias field, and under constant measurement conditions, f
r
increases with time in a typical relaxation behavior. The amplitude of the excitation magnetic field (h) turned out to be a key factor on this phenomenon, since larger excitation field amplitudes (h ≥ 100 mOe) reduce considerably its effect, decreasing the relaxation time (τ) and reducing the temporal variation of f
r
(Δf
r
) experience by the sensor. The influence of this relaxation on the sensor performance and the approaches to overcome this problem are evaluated and applied to the case of a real-time monitoring of a precipitation reaction.