Measuring the mass of thin films and adsorbates using magnetoelastic techniques

Magnetoelastic sensor techniques have the unique characteristics of being able to wirelessly detect resonant frequency shifts of a magnetoelastic foil in response to differences in the foil mass. However, the mathematical expression that links the resonant frequency shift with the change in the mass of the magnetoelastic foil is rarely reported. Furthermore, this relationship is not easy to ascertain due to potential changes in the Young's modulus of the sensor upon a change in mass loading. In this paper, we have shown that adsorption of water vapor from the gas phase by magnetoelastic ribbons coated with a two layer porous thin film (SiO2/Pt-TiO2) induces large changes in the effective Young's modulus of the sensor. We also demonstrated that the change in Young's modulus upon mass loading can be eliminated from the relationship between mass loading and shifts in resonant frequency by using a technique that we refer to as the two different length sensor method (TDLS). Ibis methodology permits the conversion of the magnetoelastic sensor into a microbalance. From data presented in this paper, we illustrate that the sensitivity for the same sensor can range between 214 Hz/mg for mass loadings of Au to 438 kHz/mg for acetone. In the case of water adsorption, frequency shifts varies from 20.0 kHz/mg when Delta m <= 0.01 mg to 2.00 kHz/mg for Delta m values between 0.05 and 0.10 mg.