Junqi Gao

An Extremely Low Equivalent Magnetic Noise Magnetoelectric Sensor

As a result of the coupling between their dual order parameters, multiferroic materials exhibit unusual physical properties and, in turn, promise new device applications. [ 1 , 2 ] Of particular interest is the existence of a cross-coupling between the magnetic and electric orders, termed the magnetoelectric (ME) effect. [ 3–5 ] Because no single-phase material has been put forward demonstrating a practical capacity for such coupling at room temperature, [ 8 ] many of the most promising applications offered by the ME effect, including magnetic fi eld sensors and electric write-magnetic read memory devices, have not been forthcoming. [ 6 , 7 ] Furthermore, the exploitation of high magnetic fi eld sensitivity in two-phase ferromagnetic/ferroelectric composites requires development and identifi cation of end users. [ 7 ]

Enhancement in the field sensitivity of magnetoelectric laminate heterostructures

The effect of magnetostrictive layer thickness on the magnetoelectric (ME) response and field sensitivity of Pb(Zr,Ti)O3-metglas based sandwiched ME heterostructures has been studied. Such structures hold promise for sensor applications. The increase in metglas thickness results in a significant increase in the ME response and magnetic field sensitivity. The ME coefficient and field sensitivity increase by about 1.5–1.75 and 2.7 times, respectively, for a structure with 150 μm thick six metglas layers on both sides of the Pb(Zr,Ti)O3, in comparison to a 50 μm thick two layered structure.

Enhanced sensitivity to direct current magnetic field changes in Metglas/Pb(Mg1/3Nb2/3)O3–PbTiO3 laminates

We have developed Metglas/Pb(Mg1/3Nb2/3)O3–PbTiO3 magnetoelectric (ME) laminates that have notably larger ME coefficients, with maximum values of up to 45 V/cm Oe. Based on this giant ME effect, the dc magnetic field sensitivity for Metglas/PMN–PT laminate sensors was improved by a factor of > 3, relative to that for Metglas/Pb(Zr,Ti)O3 (PZT)-based ones of the same geometry. Our new ME sensor can detect dc magnetic field changes as small as (i) 5 nT at 1 kHz and (ii) 1 nT near the resonant frequency in a shield chamber.

Comparison of noise floor and sensitivity for different magnetoelectric laminates

We present a comparison of the magnetoelectric (ME) response and magnetic-field sensitivities of engineered laminate sensors comprised of magnetostrictive and piezoelectric phases. The ME voltage coefficients for Metglas and single crystal fibers of Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) or Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) are about 2.8 times larger than those with Metglas-Pb(Zr,Ti)O3 (PZT) ceramic ones. This results in a 1.7 times enhancement in the magnetic-field sensitivity for the structures with single crystals. Accordingly, the noise floors are about three to four times lower for composites with PMN-PT or PZN-PT fibers than those with PZT.

A review on equivalent magnetic noise of magnetoelectric laminate sensors

Since the turn of the millennium, multi-phase magnetoelectric (ME) composites have been subject to attention and development, and giant ME effects have been found in laminate composites of piezoelectric and magnetostrictive layers. From an application perspective, the practical usefulness of a magnetic sensor is determined not only by the output signal of the sensor in response to an incident magnetic field, but also by the equivalent magnetic noise generated in the absence of such an incident field. Here, a short review of developments in equivalent magnetic noise reduction for ME sensors is presented. This review focuses on internal noise, the analysis of the noise contributions and a summary of noise reduction strategies. Furthermore, external vibration noise is also discussed. The review concludes with an outlook on future possibilities and scientific challenges in the field of ME magnetic sensors.

Theoretical and experimental investigation of magnetoelectric effect for bending-tension coupled modes in magnetostrictive-piezoelectric layered composites

In this paper, we discuss a theoretical model with experimental verification for the resonance enhancement of magnetoelectric (ME) interactions at frequencies corresponding to bending-tension oscillations. A dynamic theory of arbitrary laminated magneto-elasto-electric bars was constructed. The model included bending and longitudinal vibration effects for predicting ME coefficients in laminate bar composite structures consisting of magnetostrictive, piezoelectric, and pure elastic layers. The thickness dependence of stress, strain, and magnetic and electric fields within a sample are taken into account, as such the bending deformations should be considered in an applied magnetic or electric field. The frequency dependence of the ME voltage coefficients has obtained by solving electrostatic, magnetostatic, and elastodynamic equations. We consider boundary conditions corresponding to free vibrations at both ends. As a demonstration, our theory for multilayer ME composites was then applied to ferromagnetic-f...

Analysis of Noise in Magnetoelectric Thin-Layer Composites Used as Magnetic Sensors

Taking advantage of magneto-elasto-electric interactions, new laminated composites of magnetostrictive and piezoelectric layers have been developed for highly sensitivity magnetoelectric (ME) sensors. The ME sensor design chosen in this study was based on long-type Metglas composites laminated together with piezoelectric fibers. In this paper, we analyze the expected limit of the magnetic noise of ME sensor versus its intrinsic characteristics and electronic setup. Theory calculations and experimental results are compared, and are well supported by noise measurements. Moreover, the presented results show very impressive performances in terms of the equivalent magnetic sensor noise. A 65 pT/√Hz equivalent magnetic noise was achieved at 1 Hz, while reaching values as low as 70 fT/√Hz near the ME resonant frequency.

Magnetoelectric nonlinearity in magnetoelectric laminate sensors

A nonlinearity in the magnetoelectric coefficient, αMENonlin, of Metglas/Pb(Zr,Ti)O3 (PZT) and Metglas/Pb(Mg1/3,Nb2/3)O3-PbTiO3 (PMN-PT) laminate sensors has been observed. This nonlinearity was found to be dependent on the dc magnetic bias (Hdc) and frequency of the ac drive field (Hac). The maximum value of αMENonlin for both types of composites was found near the electromechanical resonance. For Metglas/PZT laminates, the maximum occurred under a finite bias of Hdc ≈5 Oe; whereas, for Metglas/PMN-PT, the maximum was found near zero dc bias. One application for αMENonlin is a cross-modulation scheme that can shift low frequency signals to higher frequency to achieve lower noise floor. For Metglas/PMN-PT, αMENonlin has another application: removal of the necessity of a dc bias, which helps to design high-sensitivity sensor arrays and gradiometers.

Thermal stability of magnetoelectric sensors

The effect of temperature on the equivalent magnetic noise floor of Metglas-Pb(Zr,Ti)O3 (PZT) laminate magnetoelectric (ME) sensors has been investigated in the temperature range of −50 °C to 50 °C. In detail, the parameters that control the noise floor of ME sensors, such as capacitance, tan δ, and ME charge coefficient, were characterized. The results show the noise floor was thermally stable around 30 pT/√Hz (f = 1 Hz) over the studied temperature range. To demonstrate the relative invariance of ME sensor at different temperatures over the range studied, a simulation based on a noise model was conducted, where the predicted and measured equivalent magnetic noise floors were found to well agree.

Enhanced dc magnetic field sensitivity by improved flux concentration in magnetoelectric laminates

In this letter, we present magnetostatic modeling results that show significant magnetic field concentration tunability through geometric modification of high-mu magnetostatic Metglas layers of laminate magnetoelectric (ME) sensors. Based on the modeling results, composite ME sensors were fabricated with longer Metglas foils and found to exhibit notably higher ME voltage coefficients at smaller DC magnetic biases in response to a 1 kHz driving signal. Such ME sensors have been used to detect DC magnetic field changes as small as 6 nT at 1 kHz, while maintaining a signal-to-noise ratio greater than 10. This represents an enhancement of ∼250% relative to values previously reported for Metglas/Pb(Zr,Ti)O3 laminates.