Magneto-piezoresistance in Magnetorheological elastomers for magnetic induction gradient or position sensors

Abstract

Composite material constituted by Fe micro-particles homogeneously dispersed in a silicone matrix, at a volume concentration slightly above the percolation threshold but separated by a thin silicone layer, was produced. The particle magnetic softness and their average size, have been properly improved with respect to previous investigations in order to maximize the piezo-resistive and the piezo-magnetic effects. The optimal combination of magneto-elasticity and piezo-resistivity enables to achieve a record value of magneto-piezo-resistivity sensitivity. An analytical model is proposed to simulate the theoretically expected behavior of electric resistance vs. the applied induction field gradient, so to predict the magneto-piezoresistive response and explain the obtained material tailoring. The experimental results have been in good agreement with the theoretically predicted behaviors, so validating the employed model and the interpretation of the phenomenon. A simple basic application in position sensing is also reported. The analytical model presented in this paper has demonstrated its potentiality to project further improvements, while the experimental results allow for different innovative applications

Magneto-piezoresistivity in iron particle-filled silicone: An alternative outlook for reading magnetic field intensity and direction

“…The sample length can be easily reduced by a laboratory knife. All the process has been described in some detail in previous papers [9,20,21]. No methodology has been applied to induce a preferential orientation of the Fe microparticles; therefore they resulted homogeneously distributed and randomly oriented.…”

“…The peculiar morphology of magnetic particles used in this work is different from the one employed in previous investigations (pseudospherical morphology with moderate roughness). The reason of this choice is to improve composite material performances, delineated on the basis of previous studies [9,20,21], in order to achieve three specific characteristics: i) bigger size particles; ii) better silicone entrapping among particle cavities during the mechanical dispersion process to isolate each particle from nearest ones; iii) lower transversal contraction able to give particle contact as consequence of the irregular surface protrusions; iv) preferential in plane magnetization of a single particle due to shape anisotropy. The positive effects of these listed material characteristics were verified by means of the sample performance as will be shown and discussed in Section 3.…”

“…On the basis of this experimental expedient, a different MPR core elongation is obtained if the M i magnetic moment is directed in concordant or opposite direction with respect to M magnetization. According to the MPR model [20,21], a different sample elongation determines a different decrease of sample initial resistance R 0 . We will use the symbol R up for the sample resistance when M and M i magnetization directions are identical, while we will use R down in case they are opposite.…”

“…As a result of this strain, a decrease of average distance among the particles and a consequent decrease in resistivity occur (piezoresistivity). These are the fundamental mechanisms in the coupling of elastomagnetism and piezoresistivity which produce the magneto-piezoresistive (MPR) effect extensively treated in [20] and [21]. A magneto-piezoresistive sensitivity (change of resistivity on the inducing magnetic field gradient) higher than 10 11 !m/T has been both predicted by theoretical models and experimentally detected [20,21].…”

“…These are the fundamental mechanisms in the coupling of elastomagnetism and piezoresistivity which produce the magneto-piezoresistive (MPR) effect extensively treated in [20] and [21]. A magneto-piezoresistive sensitivity (change of resistivity on the inducing magnetic field gradient) higher than 10 11 !m/T has been both predicted by theoretical models and experimentally detected [20,21]. The main objective of this paper is to establish, by means of some basic experiments in the millimeter scale range, that the MPR effect can be used to detect the change of magnetic polarization direction at a magnetization intensity similar to that produced by the semi-permanent nanomagnets of a magnetic memory.…”