Giant Magnetoelectric Effect in Thin‐Film Composites

Abstract

Highly sensitive AC magnetic field sensors are presented using magnetoelectric composites consisting of magnetostrictive and piezoelectric phases. They are offering passive nature, high sensitivity, large effect enhancement at mechanical resonance, and large linear dynamic range. Thin-film magnetoelectric 2-2 composites benefit from perfect coupling between the piezoelectric and magnetostrictive phases and from the reduction in size which is essential for high spatial resolution. Their design uses AlN and a plate capacitor or PZT with interdigital electrodes and magnetostrictive amorphous FeCoSiB single layers or exchanged biased multilayers. At mechanical resonance and depending on the geometry, extremely high ME coefficients of up to 9.7 kV/cm Oe in air and up to 19 kV/cm Oe under vacuum were obtained. To avoid external DC magnetic bias fields, composites consisting of exchanged biased multilayers serving as the magnetostrictive component with a maximum magnetoelectric coefficient at zero magnetic bias field are employed. Furthermore, the anisotropic response of these exchanged biased composites can be utilized for three-dimensional vector field sensing. Sensitivity and noise of the sensors revealed limits of detection as good as to 2.3 pT/Hz 1/2 at mechanical resonance. Sensitivity between 0.1 and 1000 Hz outside resonance can be enhanced through frequency conversion using AC magnetic bias fields.

Measuring the magnetoelectric effect across scales

“…Its value must be maximized. Sensitivity of magnetoelectric sensors has meanwhile reached values comparable to the best known technique for magnetic field detection of today (which are SQUID devices, for an explanation and more details see Section 5) [8]. The highest coupling coefficient are reached at mechanical resonance of an oscillating crystalline bilayer cantilever [9][10][11].…”

“…The highest coupling coefficient are reached at mechanical resonance of an oscillating crystalline bilayer cantilever [9][10][11]. The maximum reached so far is a value of α ME = 19.000 V/(cm Oe) in vacuum [8]. Brain currents can be localized within an entire human head without the need for cooling the measurement device.…”

“…Conceptually, connectivity is a major factor affecting the modeling grounds for these composites [40]. Bilayer systems (2-2-connectivity, see Figures 2 and 21) of single crystal piezoelectric substrates and strongly magnetostrictive top layers furnish the best known sensors [8,[41][42][43][44][45][46]. If a metal is chosen for the magnetostrictive material, it can concurrently serve as electrode.…”

“…Modeling is rather straight forward in these systems due to the clear boundary conditions and the simple constitutive laws (at least those assumed in the model) [47]. A wealth of experimental www.gamm-mitteilungen.org examples exists due to the many possible material combinations [8,[48][49][50]. Modeling 1-3-composites is also well feasible, if the microstructure is periodic [40,[51][52][53][54][55].…”

Sequential piezoresponse force microscopy and the ‘small-data’ problem

“…Such composites are a realistic alternative for achieving the foreseen device applications at roomtemperature. [29][30][31] The ME effect in such composites takes place via stress mediation, where the field induced strain in one phase leads to a stress on the adjacent phase, which ultimately varies the order parameter (polarization/magnetization) of the other phase. [32][33][34][35][36] Interestingly, the performance of such composites, gauged by the ME coupling coefficient α, is found to be highly sensitive to microstructure.…”