Magnetic - Field - Measurement

Steven A. Macintyre. \

Copyright 2000 CRC Press LLC. .

Magnetic Field Measurement

48.1 Magnetic Field Fundamentals 48.2 Low-Field Vector Magnetometers

The Induction Coil Magnetometer ? The Fluxgate Magnetometer ? The SQUID Magnetometer

48.3 High-Field Vector Gaussmeters

The Hall Effect Gaussmeter ? The Magnetoresistive Gaussmeter

Steven A. Macintyre

48.4 Scalar Magnetometers

The Proton Precession Magnetometer ? The Optically Pumped Magnetometer

Macintyre Electronic Design

Magnetic field strength is measured using a variety of different technologies. Each technique has unique properties that make it more suitable for particular applications. These applications can range from simply sensing the presence or change in the field to the precise measurements of a magnetic field’s scalar and vector properties. A very good and exhaustive fundamental description of both mechanical and electrical means for sensing magnetic fields can be found in Lion [1]. Less detailed but more up-to-date surveys of magnetic sensor technologies can be found in [2, 3]. It is not possible to adequately describe all of these technologies in the space available in a Handbook. This chapter concentrates on sensors that are commonly used in magnetic field measuring instruments.

As shown in Figure 48.1, magnetic field sensors can be divided into vector component and scalar magnitude types. The vector types can be further divided into sensors that are used to measure low fields (<1 mT) and high fields (>1 mT). Instruments that measure low fields are commonly called magnetom-eters. High-field instruments are usually called gaussmeters.

The induction coil and fluxgate magnetometers are the most widely used vector measuring instruments. They are rugged, reliable, and relatively less expensive than the other low-field vector measuring instruments. The fiber optic magnetometer is the most recently developed low-field instrument. Although it currently has about the same sensitivity as a fluxgate magnetometer, its potential for better performance is large. The optical fiber magnetometer has not yet left the laboratory, but work on making it more rugged and field worthy is under way. The superconducting quantum interference device (SQUID) magnetometers are the most sensitive of all magnetic field measuring instruments. These sensors operate at temperatures near absolute zero and require special thermal control systems. This makes the SQUID-based magnetometer more expensive, less rugged, and less reliable.

The Hall effect device is the oldest and most common high-field vector sensor used in gaussmeters. It is especially useful for measuring extremely high fields (>1 T). The magnetoresistive sensors cover the middle ground between the low- and high-field sensors. Anisotropic magnetoresistors (AMR) are cur-rently being used in many applications, including magnetometers. The recent discovery of the giant

? 1999 by CRC Press LLC

FIGURE 48.1 Magnetic field sensors are divided into two categories based on their field strengths and measurement range: magnetometers measure low fields and gaussmeters measure high fields.

TABLE 48.1 Field Strength Instrument Characteristics

Comment

Cannot measure static fields

General-purpose vector magnetometer Highest sensitivity magnetometer Best for fields above 1T

Good for mid-range applications

General-purpose scalar magnetometer Highest resolution scalar magnetometer

Instrument Induction coil Fluxgate SQUID Hall effect

Magnetoresistance Proton precession Optically pumped

Range (mT) 10–10 to 106 10–4 to 0.5 10–9 to 0.1 0.1 to 3 ??104 10–3 to 5 0.02 to 0.1 0.01 to 0.1

Resolution (nT) Variable 0.1 10–4 100 10 0.05 0.005

Bandwidth (Hz) 10–1 to 106 dc to 2 ??103 dc to 5 dc to 108 dc to 107 dc to 2 dc to 5

magnetoresistive (GMR) effect, with its tenfold improvement in sensitivity, promises to be a good competitor for the traditional fluxgate magnetometer in medium-sensitivity applications.

The proton (nuclear) precession magnetometer is the most popular instrument for measuring the scalar magnetic field strength. Its major applications are in geological exploration and aerial mapping of the geomagnetic field. Since its operating principle is based on fundamental atomic constants, it is also used as the primary standard for calibrating magnetometers. The proton precession magnetometer has a very low sampling rate, on the order of 1 to 3 samples per second, so it cannot measure fast changes in the magnetic field. The optically pumped magnetometer operates at a higher sampling rate and is capable of higher sensitivities than the proton precession magnetometer, but it is more expensive and not as rugged and reliable.

Table 48.1 lists various magnetic field strength instruments and their characteristics.

48.1 Magnetic Field Fundamentals

An understanding of the nature of magnetic fields is necessary in order to understand the techniques used for measuring magnetic field strength. The most familiar source of a magnetic field is the bar

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FIGURE 48.2 Magnets produce magnetic fields. A magnetic field is a vector quantity with both magnitude and direction properties.

magnet. The field it produces is shown in Figure 48.2. Magnetic field is a vector quantity; that is, it has both a magnitude and a direction. The field of a bar magnet or any other magnetized object, when measured at a distance much greater than its longest dimension, is described by Equation 48.1:

r

H ??

r

3r ?????????m a?r a m ?r

???

r

3

(48.1)

where ar is a unit vector along r, r is the distance between the magnetic field source and the measurement point, and m is called the magnetic dipole moment. The derivation of this equation can be found in

many textbooks on electromagnetics. This is a very convenient equation for estimating the field produced by many magnetized objects.

The strength or intensity of a magnetized object depends on the density of its volume-distributed moments. This intensity is called its magnetization M, which is defined as the moments per unit volume:

r

M?????????????????????????????????????????????

volume

r m

(48.2)

Like magnetic field, magnetization is a vector quantity. Magnetization is a material property that can

arise from internal magnetic sources as well as be induced by an external magnetic field.

???

There is a third magnetic vector B called magnetic induction or flux density. In free space, magnetic

field and magnetic induction are proportional to one another by a constant factor ?0.

r

r

B????0H (48.3)

Things are different in matter. Equation 48.4 describes the relationship among the magnetic field, mag-netic induction, and magnetization vectors in matter:

r r r

B???0??H?M?

(48.4)

? 1999 by CRC Press LLC