Presents a review of fluxgate magnetometry based on a survey of the literature and on the work of the author and colleagues. The theory is outlined and an evaluation of the merit of the different types of sensors is made based on the sensor geometry. The design of a second-harmonic feedback magnetometer is discussed with respect to sensor configuration, frequency response, noise, offset, temperature dependence and long-term stability. Examples of the parameters are compiled from the literature and some new results on the long-term stability are presented.

https://iopscience.iop.org/article/10.1088/0022-3735/12/4/001/pdf

The fluxgate magnetometer

Since the 1930s, fluxgate sensors have been used for measuring d.c. magnetic fields up to 1 mT with a maximum resolution of 10 pT. In the sensor core the flux is gated by the excitation field. The preferable sensor geometry is a ring-core; both crystalline and amorphous ferromagnetic materials can be used for the core. Although a lot of fluxgate magnetometer types have appeared, the classical type with detection of the second harmonics by a phase-sensitive detector is the most popular. Fluxgate sensors are reliable and rugged and their applications range from space research to submarine detection.

Review of fluxgate sensors

The application of lunar magnetic field measurements to the study of properties of the lunar crust and deep interior is reviewed. Following a brief description of lunar magnetometers and the lunar magnetic environment, measurements of lunar remanent fields and their interaction with the solar plasma are discussed. The magnetization induction mode is considered with reference to lunar magnetic permeability and iron abundance calculations. Finally, electrical conductivity and temperature calculations from analyses of poloidal induction, for data taken in both the solar wind and in the geomagnetic tail, are reviewed.

/pdf/magnetism-and-the-interior-of-the-moon-2pw5pcpdyz.pdf

Magnetism and the interior of the Moon

A generalized form of the matrix inversion lemma is shown which allows particular forms of this lemma to be derived simply. The relationships between this direct method for solving linear matrix equations, lower-diagonal-upper decomposition, and iterative methods such as point-Jacobi and Hotelling's method are established. The generalized form is used to derive a new factorization scheme and a new matrix inversion algorithm with a high degree of parallelism.

Generalization of the matrix inversion lemma

Fluxgate magnetometers, introduced in the 1930's, continue to be used in a variety of applications, such as magnetic air-borne detection, search and surveillance operations, nondestructive testing of materials, palaeomagnetism, and exploration of magnetic fields in space. This paper discusses recent analyses of fluxgate operation, the development of highly stable sensors for absolute measurement of weak fields, and the development of compact inexpensive low-power devices for various search and surveillance operations and for aspect sensing.

Recent advances in fluxgate magnetometry

A theoretical and experimental analysis has been made of the geometrical and magnetic factors affecting the sensitivity of the ring-core magnetometer first introduced by Geyger in 1961. Using a drive-to-pickup transfer function derived from an assumed dynamic hysteresis loop and a given dc input signal, the second-harmonic output voltage was derived for both current-source and voltage-source driving conditions. Results of the analysis, which were confirmed by experiment, show the following. 1) When a single, diametrically wound (solenoid-type) pickup winding is used with a ring-core sensor, simultaneous minimization of both fundamental and second-harmonic feedthrough can be achieved by a simple rotational adjustment. This optimization is possible only with a circular core. 2) Sensitivity can be increased by increasing frequency, number of pickup turns, dynamic differential permeability, cross-sectional area, and effective sensor length, and by decreasing the dc initial permeability (μ 0 ). 3) The dominant factor affecting the sensitivity is the demagnetizing term \mu_{0}K = \mu_{0}l^{-1.72} (4A/\pi)^{0.86} , which determines the entrance attenuation of the dc signal to be measured. Highest sensitivity is obtained when \mu_{0}K is small, which requires a low initial permeability, since a large dimensional ratio l/A^{1/2} is usually not attainable. When the demagnetizing term is not small, the sensitivity advantages of the other factors are negated by the input signal attenuation. Power considerations show that increasing sensitivity by raising area, length, or frequency results in increased power consumption. However, increasing sensitivity by using a pickup coil with a larger number of turns or by a sensor core with lower initial permeability can be achieved without additional power.

Factors affecting the sensitivity of gamma-level ring-core magnetometers

A new fluxgate magnetometer sensor, highly stable with time and temperature, was developed for space magnetic exploration. The sensor, of ring-core construction, uses 6-81 Mo-Permalloy tape. When coupled with the NASA Ames Research Center driving and detection electronics, the resulting magnetometer has a zero offset level which is adjustable to less than 0.1 γ fluctuates less than \pm 0.1\gamma in 24 hours at constant temperature, and shifts less than 0.4 γ when the temperature of the sensor is shifted from -30 to + 60°C. Room temperature noise is less than 0.2 γ peak to peak in the frequency band of 0-10 Hz. This performance, which was confirmed by Ames in its fluxtank, was measured at the Naval Ordnance Laboratory in its superconducting shield, in which static magnetic fields are less than 0.1γ, static gradients are less than 0.1 γ/in, and the attenuation to time-varying fields is 106.

R. Chiarodo

Papers

Factors affecting the sensitivity of gamma-level ring-core magnetometers

A fluxgate sensor of high stability for low field magnetometry

Demagnetization effect in closed-core magnetometers