Marc Lam Chok Sing

Evaluation of Applied Axial Field Modulation Technique on ME Sensor Input Equivalent Magnetic Noise Rejection

By using nonlinearity effects in magnetostrictive -piezoelectric laminated sensors, modulation techniques can transfer low-frequency signals to higher frequencies. Theory predicts that the transfer ability depends mainly on the amplitude of the carrier signal and the sensor nonlinearity. This was confirmed by our experiments. We present the first analysis on a ME noise model associated to a modulation technique. Furthermore, the overall equivalent magnetic noise was analyzed, and shown to be dominated only by the signal transfer ability and the output electrical noise level appearing around the carrier frequency.

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.

Sensitivity and Noise Evaluation of a Bonded Magneto(elasto) Electric Laminated Sensor Based on In-Plane Magnetocapacitance Effect for Quasi-Static Magnetic Field Sensing

The quasi-static magnetic field detection of a layer-bonded magneto(elasto) electric (ME) laminate has been investigated by measuring the in-plane electric capacitance via its interdigital electrodes close to the piezoelectric resonant frequency. The ME-layered composite is considered as a stress-induced dielectric effect because there is practically no direct response of the electric capacitance to an external magnetic field. The sensitivity is dominated by the magnetoelastic coupling in the magnetic layer and on the stress induced by the permittivity change in the piezoelectric layer. The low-frequency magnetocapacitance effect is sensitive to an external magnetic bias which can modulate the electric permittivity by producing a stress. The magnetoelastic coupling is another important parameter for this magnetic field detection mode. For a given magnetic field, the amplitude of the magnetostriction is directly related to this parameter as well. Therefore, an optimal magnetic bias can maximize the induced strain or stress which is coupled into the piezoelectric layer through the change of the electric permittivity in this layer. To evaluate the sensitivity and the noise performance by the magnetocapacitance effect, we have used the piezoelectric and magnetic constitutive equations to predict the permittivity dependence. Experimentally, this sensor achieved an equivalent magnetic noise spectral density, presently still limited, by the noise of the detection electronics, ~100 pT/√Hz at 1 Hz and offered a dc detection capability. With the model and experimental nonlinear factors, an equivalent sensor noise spectral density close to the pT/√Hz can be ultimately predicted considering the mechanical loss limitation of the sensor.

Investigation of the Near-Carrier Noise for Strain-Driven ME Laminates by Using Cross-Correlation Techniques

The near-carrier noise around the longitudinal mechanical resonance of a magnetoelectric laminated composite has been investigated. By simultaneously applying a high-frequency electric field across the piezoelectric phase, the sensor response to low-frequency magnetic signals can be shifted around the “carrier” frequency as side band modulation signals. This magnetoelectric response can appear either as an electric charge via piezoelectric-to-piezoelectric (PP) modulation effects or as a magnetic signal via piezoelectric-to-magnetic (PM) modulation effects. These two signals are detected either with a charge preamplifier or with a coil surrounding the sample and the low-frequency sensor response to the applied magnetic field can be recovered by using two independent synchronous detectors. We have designed an experimental setup to observe the direct (passive) low-frequency noise and the noise corresponding to the two above modulations. Noise cross-correlating measurements were also carried out to investigate the origin of the near-carrier noise. No noise coherence was found between the direct low-frequency noise and the noise resulting from either the PP or the PM modulations. However, a noise coherence factor of more than 50% has been found between the signals recovered from the two modulation techniques. A simple model has been used to explain this effect. The magnetoelectric sensor is considered as a nonlinear forced vibration system. Noise sources passing through such a system can be amplified and distributed around the carriers as side band noise where it hampers the equivalent magnetic noise performance. Electronic-thermal noise caused by dielectric dissipations in the piezoelectric phase can be considered as a noise source with a negligible contribution to the total noise floor. Mechanical-thermal low-frequency excess noise is found to be the only intrinsic noise source which is filtered by the nonlinear ME system and is present as an output near-carrier noise which dominates the noise level after the demodulation processes.

Theoretical Intrinsic Equivalent Magnetic Noise Evaluation for Magneto (Elasto) Electric Sensors Using Modulation Techniques

The equivalent magnetic noise of magnetostrictive-piezoelectric composite sensors, in the passive mode or when magnetic modulation techniques are used, has been investigated theoretically and compared with measurements. Several main noise sources and their contributions to the equivalent magnetic noise spectral density have been analyzed using the fluctuation-dissipation theorem and modeled via Nyquists noise-expression in the linear and non-linear regime. These theoretical analyzes show that the mechanical loss, related to the interfriction of composites, appears as the dominant noise source for such magnetoelectric modulation techniques.

Hall sensor response to an inhomogeneous magnetic field

The response of a Hall-effect sensor to a spatially dependent magnetic field is of importance for many applications such as magnetic microscopy and nondestructive testing. Using the analytical expression of the response of a Greek cross Hall sensor response to an ideal field dot published a few years ago, we have calculated its sensitivity and its full width at half maximum for the field produced by a magnetic dipole and by two coplanar lines. The experimental results are in good agreement with theory. They show that the spatial resolution is roughly equal to the dimension of the central part of the Greek cross and that a flux-meter approximation is not appropriate for modeling such Hall-effect sensors for very close field sources.

Dynamic Sensitivity and Noise Floor of a Bonded Magneto(Elasto)Electric Laminate for Low Frequency Magnetic Field Sensing under Strain Modulations

The intermediated strain can convert a magnetic field to an electric output signal in a magnetostrictive-piezoelectric layered composite via three parameters: the magnetoelastic coupling, the piezoelastic coupling and the mechanical impedance. These three parameters are dominated respectively by the magnetostrictive coefficient, the piezoelectric coefficient and the mean flexibility of material in the composite. Focusing on these three parameters, many investigations on the ME enhancement have been carried out by choosing the correct material or by adjusting the ratio between the two phases in the composite [4]. Thereafter, the noise performance of ME laminates has been studied for applications as a magnetic sensor. In the last several years, the intrinsic noise sources for both the composite and the amplifier circuit have been mathematically modeled and experimentally characterized. The passively sensed signal can be amplified by either a voltage or a charge method. Furthermore, the noise contributions from the detection electronics were also integrated in the noise performance analysis [5]. According to these studies, dielectric dissipation in the piezoelectric phase is the main contribution to the noise floor for low-frequency magnetic field sensing even though the equivalent current noise source from the electronics induce fluctuations in the output signal of the low-frequency charge detection as well [6].

Design of a magnetically shielded helium cryostat insert with a variable temperature regulated stage

This note describes the design and realization of a liquid helium cryostat insert comprising a variable temperature regulated stage. The latter is magnetically shielded by a superconducting lead pot bathing in liquid helium. This apparatus is intended for the study of high Tc superconducting films and superconducting quantum interference devices submitted to different applied magnetic fields for temperatures ranging from 50 to 100 K.

Magnetic Sensors Based on AMR Effect in LSMO Thin Films

In this paper, the potentialities of the manganese oxide compound La0.7Sr0.3MnO3 (LSMO) for the realization of sensitive room temperature magnetoresistive sensors are discussed. LSMO films deposited on various types of substrates having different magnetic anisotropies were patterned to form rectangular stripes of width 100 µm and length 300 µm. It is shown that, apart from the well-known colossal magnetoresistance contribution, the anisotropic magnetoresistance effects can be used to exhibit competitive performance at room temperature benefiting from the very low noise of LSMO thin films.

Investigations on the Equivalent Magnetic Noise of Magneto(Elasto)Electric Sensors by Using Modulation Techniques

The equivalent magnetic noise of the magnetoelectric (ME) layered composite sensors has been investigated for various modulation techniques. The ME thin film response to an electric modulation or a magnetic modulation can be sensed by using either a charge amplifier or a coil wound around the sample and then demodulated by a synchronous detector. The equivalent magnetic noise for these excitations methods has been compared. As expected, the low-frequency fluctuations can be lowered when the magnetoelectric sensor is operated in a modulation mode. Results show that these methods can give the same level of equivalent magnetic noise for a certain strain-excitation. In theory, mechanical noise appears as the only dominant noise source after the demodulation process in the case of a certain strong amplitude excitation carrier signal. By using these modulation techniques, an equivalent magnetic noise level of 10-100 pT/Hz at 1 Hz was achieved with DC capability.