GB2271644A

GB2271644A - Measurement of magnetic field by exposure of a magnetic crystal to the field and an applied rotating field

Info

Publication number: GB2271644A
Application number: GB9219497A
Authority: GB
Inventors: Alexander I Veronko; Peter M Vetoshko
Original assignee: VOLTAGEUNIT Ltd
Current assignee: VOLTAGEUNIT Ltd
Priority date: 1992-09-15
Filing date: 1992-09-15
Publication date: 1994-04-20
Also published as: GB9219497D0

Abstract

In the measurement of a magnetic field by placing a body of magnetic crystal material in the field to be measured, applying a further magnetic field of cyclically varying direction to the body and detecting cyclical fluctuations in the magnetic field in the body, the magnitude of which are proportional to the strength of the magnetic field to be measured, the crystal material (eg. LuxY3-xFe5-yScyO12 where y is 0.5 - 0.7, x is 0 - 1.2) defines a single magnetic domain (ie all the magnetic moments of the body are aligned) which remains constant. Body 3 is mounted on substrate 4 adjacent coils 1 and 2 so as to be exposed to a rotating field of fixed frequency established by passing current through the coils and to the field to be measured. The detecting means detects the cyclical fluctuations at twice the fixed frequency. The problem of noise swamping such fluctuations is alleviated. <IMAGE>

Description

MAGNETIC FIELD MEASUREMENT The present invention relates to an apparatus and method for measuring the strength of magnetic fields.

Various devices are available for measuring magnetic fields. For example, solid state Hall effect transducers are widely used in a variety of applications. Such devices have only limited sensitivity to weak magnetic fields.

Superconducting quantum interference devices (Squids) have been developed for the measurement of very weak fields. Such devices are capable of measuring a single quantum of magnetic flux and thus can provide whatever degree of resolution is required. Unfortunately Squid devices require superconducting junctions that with current technology require associated cryogenic systems and accordingly are too expensive for use in many applications.

If a magnetic crystal material is exposed to the combined effects of a rotating magnetic field and a constant magnetic field, the result is variations in magnetic flux including components at the fundamental frequency of rotation of the rotating magnetic field and harmonic components. Thus by monitoring the harmonic components a measure of the strength of the constant magnetic field component can be determined. If the constant magnetic field component field is weak however it will be impossible to derive useful information from the harmonic components because the desired harmonic components are swamped by noise.

It is an object of the present invention to provide an apparatus and method for measuring magnetic fields, including weak fields, which obviates or mitigates the problems outlined above.

According to the present invention there is provided an apparatus for measuring the strength of magnetic fields, comprising a body of magnetic crystal material in which all the magnetic moments are aligned in a predetermined manner, the body of magnetic crystal material being exposed to the magnetic field to be measured, means for applying a magnetic field to the body of magnetic crystal material such that at least the direction of the applied magnetic field varies cyclically at a predetermined frequency, the strength of the applied magnetic field being such that the predetermined magnetic moment alignment is maintained, and means for detecting cyclical fluctuations in the magnetic field in the body the magnitude of which fluctuations is proportional to the strength of the magnetic field to be measured.

It is believed that in practical devices all the magnetic moments of the magnetic crystal material will be aligned so that the material defines a single magnetic domain. It is theoretically possible, however, to have more than one magnetic domain and still obtain results, but the detailed design of a device having more than one domain would be difficult.

The applied magnetic field may rotate at a fixed frequency, harmonic components at twice the fixed frequency being detected to determine the strength of the field to be measured. Alternatively, the applied magnetic field may be the sum of a constant applied magnetic field and an alternating applied magnetic field of fixed frequency.

It is believed that the problem of noise referred to above is solved by ensuring that no magnetic domains are re-aligned during use of the device. Although the well known hysteresis loops that can be plotted for magnetic crystal materials are generally represented as smooth curves, on a sufficiently large scale these curves become staircase-like, with each step in the staircase representing the realignment of one domain. Using a material defining a single magnetic domain avoids stepped changes in flux which would be a source of noise.

The present invention also provides a method for measuring the strength of magnetic fields, comprising applying a magnetic field to a body of magnetic crystal material which is exposed to the magnetic field to be measured, at least the direction of the applied magnetic field varying cyclically at a predetermined frequency, and all the magnetic moments of the body of magnetic crystal material being aligned in a predetermined manner which remains constant, and detecting cyclical fluctuations in the magnetic field in the body the magnitude of which are proportional to the strength of the magnetic field to be measured.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings.

Referring to the drawing, this is a schematic illustration of an embodiment of the present invention. The embodiment comprises a first coil 1 extending parallel to the X-axis and a second coil 2 extending parallel to the Y-axis. A body 3 of magnetic crystal material is mounted on a substrate adjacent the coils 1 and 2.

The substrate 4 may be a gallium gadolinium garnet. The magnetic crystal material 3 may be a thin film single crystal of lutezium, yttrium, iron, scandium and oxygen. The chemical formula is preferably as follows: Lux Y3~X Fey Scy 12 where y is from 0.5 to 0.7 and x is from 0 to 1.2.

Typically the magnetic crystal material 3 will have a thickness of 10 microns and have a lmm square perimeter. Alternative configurations are possible however, for example circular rather than square. The magnetic crystal material 3 may be formed using liquid phase epitaxy techniques, for example as follows: a) prepare a substrate by mechanically and chemically polishing and then washing the polished surface b) place yttrium oxide (Y203) scandium oxide (so203), ferrous oxide (we2 02), lead oxide (PbO) and boron oxide (B203) in a platinum crucible c) heat the crucible to 15000C for approximately 12 hours d) cool the crucible e) dip the substrate in the molten material in the crucible f) etch the surface to define a lmm diameter surface using lithographic etching techniques g) test the magnetic crystal material 3 to ensure that it defines a single magnetic domain.

The magnetic domain testing step involves well known techniques. For example these techniques are described in the paper by P.M. Vetoshko, V.B. Volkovoy, V.N. Zalogin and A.Y. Toporov, J. Appl Phys. 70, N10, P6298, 1991.

The illustrated magnetic crystal material 3 is then placed adjacent the coils 1 and 2 and exposed to fields generated by currents passing through the two coils and to the magnetic field the strength of which is to be determined.

In a first embodiment of the invention, the current supplied to the coils 1 and 2 establishes a rotating magnetic field. Each coil generates a field parallel to its own length and those two fields combine to produce a rotating resultant. In the absence of an external field to be measured, the magnetic flux varies simply in accordance with the frequency of the applied currents. In the presence of an external magnetic field, however, the magnetic flux variation is the combination of two components, that is a first component at the frequency of the rotation of the rotating field and a second component at twice that frequency. The second component is proportional in magnitude to the strength of the field to be measured.

By way of further explanation, a simple mathematical analysis of the measurement technique is set out below using the following notation: Hx = field generated parallel to x axis when no external field present Hy = field generated parallel to y axis when no external field present H xm = component of field to be measured parallel to x axis Hum = component of field to be measured parallel to y axis Ix = current through coil 1 Iy = current through coil 2 10 = amplitude of the alternating current through coils 1 and 2 in the absence of the field to be measured w = frequency of the alternating current through the coils 1 and 2 Using the above, it will be apparent that: : Hx = Kilo sin wt Hy = K110 cos wt where K1 is a constant In the presence of the field to be measured: Ix = lo sin wt + K2 (Hxm/Hx) sin 2wt ly = 10 cos wt + K2 (Hym/Hy) cos 2wt where K2 is a constant.

Thus, by detecting the harmonic components represented above, which have a frequency of twice the fundamental frequency of the currents applies to the coils 1 and 2, it is possible to derive K2 HXm/Hx and K2 Hym/Hye The device can be calibrated using known external fields to determine K2 and thus the magnitude of Hxm and H ym can be determined.

In the example described above, the applied magnetic field rotates. Alternative arrangements for generating the required applied field that varies in direction cyclically are possible, however. For example, rather than applying an alternating current to both coils 1 and 2, the coil 1 may carry a DC current of amplitude 1o to generate a constant field and the coil 2 may carry an AC current of amplitude I,. Thus, if the current through coil 2 is IyX and using the same notation as above: ly = 10 sin wt x = 10 cos 2wt + K3 (Hym/Hy) sin 2wt where K3 is a constant Thus, H ym can be readily determined from the sin :2wt component.

To determined HXm, the inductance L of the coil 1 may be measured and the required information determined from L = K4/(HX + Hxm) where K4 is a constant.

It will be appreciated that alternative circuits for generating a cyclical magnetic field may be used. It will also be appreciated that null methods may be used to determined the external magnetic field strength, for example by applying a controllable magnetic field to the magnetic crystal material 3 and adjusting the controllable field until the harmonic components are substantially zero. Knowledge of the applied controllable field strength can then be used to derive the external field strength.

Claims

1. An apparatus for measuring the strength of magnetic fields, comprising a body of magnetic crystal material in which all the magnetic moments are aligned in a predetermined manner, the body of magnetic crystal material being exposed to the magnetic field to be measured, means for applying a magnetic field to the body of magnetic crystal material such that at least the direction of the applied magnetic field varies cyclically at a predetermined frequency, the strength of the applied magnetic field being such that the predetermined magnetic moment alignment is maintained, and means for detecting cyclical fluctuations in the magnetic field in the body the magnitude of which fluctuations is proportional to the strength of the magnetic field to be measured.

2. An apparatus according to claim 1, wherein all the magnetic moments are aligned such that the body defines a single magnetic domain.

3. An apparatus according to claim 1 or 2, wherein the applied magnetic field rotates at a fixed frequency, and the detecting means detects cyclical fluctuations at twice the fixed frequency.

4. An apparatus according to claim 1 or 2, wherein the applied magnetic field is the sum of a constant applied magnetic field and an alternating applied magnetic field of fixed frequency, and the detecting means detects cyclical fluctuations at twice the fixed frequency.

5. A method for measuring the strength of magnetic fields, comprising applying a magnetic field to a body of magnetic crystal material which is exposed to the magnetic field to be measured, at least the direction of the applied magnetic field varying cyclically at a predetermined frequency, and all the magnetic moments of the body of magnetic crystal material being aligned in a predetermined manner which remains constant, and detecting cyclical fluctuations in the magnetic field in the body the magnitude of which are proportional to the strength of the magnetic field to be measured.