Asaf Grosz

A Three-Axial Search Coil Magnetometer Optimized for Small Size, Low Power, and Low Frequencies

A compact and sensitive three-axial search coil magnetometer has been designed, built, and tested. The magnetometer sensitivity threshold equals 12 pT/Hz0.5 at 1 Hz, and the magnetometer dimensions are 72 mm × 69 mm × 69 mm. All the magnetometer coils, the electronics, and batteries are accommodated within a single electrostatic shield and a single housing. A close to 2 aspect ratio (30 mm diameter, 58 mm total length) of the search coils provides a very high (~70%) volume utilization factor. Such a small aspect ratio is obtained due to employing 30-mm diameter, 4-mm thick flux concentrators. The magnetometer is optimized for 20 mHz to 7 Hz frequencies and for ultra-low (252 μW) power consumption. The ultra-low-power consumption enables a seven-year continuous operation from the four 1/2AA lithium batteries. The effect of the integration of three orthogonal search coils on the magnetometer sensitivity and accuracy has been investigated.

Analytical Optimization of Low-Frequency Search Coil Magnetometers

An analytical optimization is proposed for the sensitivity threshold of low-frequency search coil magnetometers employing disk-shape flux concentrators. The optimal diameters of the core and the wire are found for a given set of the optimization parameters: frequency, search coil volume and aspect ratio, relative permeability of the core and the flux concentrators, and the noise of the preamplifier. The proposed analytical optimization allows an immediate analysis of the theoretical limits of the magnetometer sensitivity threshold. An approximation is obtained to simplify and clarify the relationship between the minimum possible sensitivity threshold and the optimization parameters. Measurements performed with an experimental magnetometer model confirm the optimization.

A miniature and ultralow power search coil optimized for a 20 mHz to 2 kHz frequency range

We describe the design of a miniature search-coil magnetometer that is optimized in terms of resolution and power consumption for frequencies down to 20 mHz. Our aim is to come close to the size and resolution of fluxgate magnetometers, while reducing the power consumption by at least an order of magnitude. To reach this goal, we attach flux concentrators in the shape of thin disks to a ferrite core, employ an ultralow power, zero 1/f noise preamplifier, and finally optimize the diameters of the coil core and wire. The optimized search coil is of 54 mm length, 30 mm outer diameter, and includes 160 000 turns of a 50 μm copper wire. The coil resistance is 86 kΩ, the self-resonance frequency is 250 Hz, and the total weight is 210 g. Our experimental results are in close agreement with the theoretical calculations. For a power consumption of 5 mW, the coil resolution is 14 pT/√Hz at 1 Hz and reaches 350 fT/√Hz in the frequency range from 100 Hz to 2 kHz. For a power consumption of 0.17 mW, the coil resolutio...

Demagnetizing factors for two parallel ferromagnetic plates and their applications to magnetoelectric laminated sensors

An analytical expression is derived to approximate the magnetometric demagnetizing factors for two parallel ferromagnetic plates having the shape of rectangular prisms. The magnetometric demagnetizing factors relate the average magnetic fields within the plates’ volumes to an external magnetic field. Knowing this relationship is essential for describing the response of magnetoelectric sensors comprising two parallel magnetostrictive plates. It is shown that two separate ferromagnetic layers provide better field sensitivity than a single layer with a doubled thickness. The obtained results are in a good agreement with numerical calculations and experimental data.

Integration of the electronics and batteries inside the hollow core of a search coil

A novel design of a miniature search coil magnetometer is proposed based on the integration of the electronics and batteries within the hollow core of the search coil. In contrast with conventional designs, where the search coil and its electronics have individual housings and electrostatic shields, this paper presents a design in which the core of the search coil serves both as the housing and the magnetic shield for the electronics and batteries. Moreover, the electrostatic shield of the search coil also shields the electronics and batteries. We found that a thin-wall tube core with sufficient permeability is able to replace a solid rod core without decreasing the average flux sensed by the coil winding. In addition, the outer diameter of the tube core can be increased beyond its optimum size to provide more space for the electronics and batteries without considerably affecting the search coil resolution. To validate the effectiveness of this new design, a miniature search coil magnetometer was built an...

Planar Hall effect sensors with shape-induced effective single domain behavior

We show that shape anisotropy induces effective single domain behavior in elliptical structures of thin permalloy films with long axis ranging between several microns to several millimeters, provided that the ratio of the film long and short axes is large enough. We also show that the thin film elliptical structures exhibit a wide range of effective anisotropy fields, from less than 10 Oe up to more than 100 Oe. We discuss the advantage of shape anisotropy in the fabrication of planar Hall effect sensors with high field resolution.

Orthogonal Fluxgate Employing Digital Selective Bandpass Sampling

Orthogonal fluxgate employing digital selective band pass sampling is developed and tested. The fluxgate output is sampled only once, at a single time instance during a number N of excitation cycles. This provides reconstruction of a measured magnetic field with a finite bandwidth from dc to 1/2N Hz. The sampling time instances correspond to the minimum magnitude of the fluxgate equivalent magnetic noise within the excitation cycles. As a result, the fluxgate resolution is improved by 40% compared to that obtained by conventional methods, where the fluxgate output is sampled a number of times and then averaged over each excitation cycle. The proposed approach not only improves the fluxgate equivalent magnetic noise, but also simplifies the fluxgate output processing by eliminating the need for analog synchronous detection.

Compensation of Crosstalk in Three-Axial Induction Magnetometers

A method for the compensation of crosstalk in three-axial induction magnetometers is developed theoretically and verified experimentally. The compensation is based on deriving crosstalk-free magnetometer outputs from a system of equations describing the magnetometer total outputs as a function of the applied field, the parameters of the magnetometer coils, and the crosstalk factors for the applied and secondary magnetic fluxes. Processing the total outputs of an experimental magnetometer has demonstrated an effective reduction of the crosstalk: it has been reduced below 0.5% in the whole magnetometer bandwidth, including the frequencies near resonance, where the crosstalk is especially strong (20%). In comparison, the reduction of the crosstalk by applying magnetic feedback is much less effective: the crosstalk has been reduced down to 6% at resonance, remained unchanged at low and high frequencies, and even increased just below resonance. Moreover, magnetic feedback flattens the frequency response and significantly reduces the magnetometer selectivity, which can be advantageous in many applications. Employing magnetic feedback also increases the magnetometer complexity and its power consumption.

Planar Hall Effect Sensors With Subnanotesla Resolution

We report the fabrication of elliptical planar Hall effect sensors made of Permalloy with response determined by shape-induced uniaxial anisotropy. By using ac excitation and by optimizing the sensor thickness and the amplitude of the excitation current, we have obtained a magnetic field resolution which is better than 600 pT/ √Hz at 1 Hz and close to 1 nT/ √Hz at 0.1 Hz. We discuss possible routes for further improvement of the obtained resolution.

Inverse effect of magnetostriction in magnetoelectric laminates

We introduce the notion of inverse effect of magnetostriction for magnetostrictive-piezoelectric heterostructures and study this effect theoretically and experimentally. It is shown that the inverse effect of magnetostriction may crucially contribute to the mechanism of magnetoelectric coupling. It is shown that the studied effect essentially modifies the saturation magnetostriction of the whole structure as compared to its magnetic phase bulk and also induces an additional magnetic anisotropy. Our consideration provides useful insight into the fundamental issue of strain-mediated magnetoelectric coupling. Understanding this effect may lead to its utilization in original experimental concepts and the improvement of the ME coupling.