IES990194A2 - Variable flux source - Google Patents

Abstract

A coaxial arrangement of three or more permanent magnet rings or cylinders generates a uniform magnetic field in a central volume. The field is variable in magnitude and direction. Access to the field is possible from different noncoplanar directions. The variable flux source may be combined with a tube furnace or electrochemcial cell to provide a versatile tool for processing and growth of thin-film devices and other materials in a magnetic field. The structure of permanent magnets will also permit the measurment of physical properties of materials in a variable magnetic field.

Description

Variable static magnetic fields are traditionally produced by passing an electric current through the coils of an electromagnet which may be wound from normal or superconductingwire. The electromagnet may incorporatean iron yoke to concentrate the magnetic flux in a region of interest. The magnitude of the field is readily altered by changing the current, but to change the direction of the field it is necessary to rotate the whole electromagnetassembly. An alternative way of generating a magneticfield is with an arrangementof permanent magnets. Rings or cylinders incorporating multiple magnet segments which are magnetized in different directions can produce a uniform magnetic field within some volume at the centre of the cylinder [H. A. Leupold, Static Applications in Rare Earth Iron Permanent Magnets, J.M.D.Coey, editor, Clarendon Press, Oxford 1996, Chapter8, pp 381 - 429], When manufactured from a rare earth or hard ferrite permanent manget possessing a large anisotropy field, the fields produced by the different magnet segments superpose. It is therefore possible to construct an arrangementof permanent magnet cylinders which produces a variable magnetic field. An arrangement described by Leupold on page 404 of the reference cited and in US patent 4682128. is composed of two concentric nested cylinders. Each cylinder produces a magnetic field, also known as magnetic flux density, of magnitude BI and B2, respectively. Each cylinder can be independently rotated about their common axis. The fields produced by the two cylinders add vectorially, so that by rotating the cylinders a field B of variable magnitude is produced. If Bl and B2 are equal, the resultant may have any magnitude between 0 and 2B1. The direction can be chosen anywhere in the plane perpendicularto the axis of the nested pair.

The present invention is a novel way of generating a variable magneticfield using permanent magnets. There is no need to nest the magnets one inside the other. Advantagesof the invention are that the field is uniform over a large volume and it can be generated using less magnetic material. Access is possible from many directions, including directions approximately perpendicular to the axis of rotation of the magnet rings or cylinders. Furthermore the magnet configurations of the invention are readily combined with a tube furnace for thermal treatment of devices and materials under a variable magnetic field or with an electrochemical cell for producing thin films by electroplating. Such treatments are needed, for example, for the processing of thin film heads used in magnetic recording.

Description The permanent magnet variable flux source of the invention is composed of three or more rings or cylinders in which permanent magnets are incorporated in order to generate INT CL i-K/F 7-bL IE 990194 a substantially uniform magnetic field in a central volume. The figures show a cross section of the permanent magnet structures containing the axis of the structure, which is drawn horizontal. These magnet assemblies may be rotated around a common axis. The magnet assemblies are divided into two sets 'X' and Ύ', each of which consists of one or more rings or cylinders. The two sets of magnets are not nested one within another. They are interleaved.

One set of magnets 'X' is typically a pair of identical magnet rings or short cylinders. The spacing of the rings and the orientation of the magnets in the rings is chosen so as to produce a distribution of magnetic flux B j which is substantially uniform in magnitude and direction in a region centered on the axis midway between the two I rings. If the internal radius of the rings is r, and their separation is 2a i the region where a uniform magnetic field is produced transverse to the axis is indicated by shading in figure I a. The field may be rotated by rotating this set of magnets about its axis.

In one embodiment of the invention, the second set of magnets Ύ' form a short cylinderof length 2a2 where a2 < a,. This cylinder is placed concentrically and coaxially between the first set of rings as shown in Figure lb. The internal radius of the cylinder Γ2 is similar but not necessarily identical to rj. The second set of magnets is designed to produce a uniform magnetic flux density B2 which is of the same magnitude as Bj throughout a similar central volume. The second set of magnets can be independently rotated about the common axis. To vary the magnitude of the magnetic field the two sets of magnets are rotated in opposite senses so that the vector flux density Btoial is the vector sum of the two components Β i and Bg. To vary the direction of the field, the two sets of magnets are rotated in the same sense. The rotation of the magnets may be effected by using motors such as stepper of servomotors, or it can be effected manually using a crank. In either case optionally gears may be used to reduce the torque necessary to rotate the magnets. A locking device may be incorporated to fix the relative orientation of the two magnet sets, and facilitate their rotation in the same sense.

In another embodiment of the invention, the second set of magnets may be composed of two or more rings which are symmetrically arranged about the centre. Some possible arrangements are illustrated in Figure 2. In Figure 2a and Figure 2b the second set of magnets Ύ' is composed of a pair of rings set inside or outside the first set. In Figure 2c, two pairs are used, with one on each side of each of the rings of the first set. In Figure 2d the second set is in three parts including a central cylinder and an outer pair. The different arrangements permit the field uniformity to be optimized in central zones of different lengths and radii using the smallest quantity of permanent magnets. Another possible arrangement of rings for the first and second sets of magnets is shown in Figure 3. Here two or more rings are used for both the first and second sets. Again the two sets may be independently rotated to achieve a field which is variable in magnitude IE 990194 and direction. These examples are illustrative, rather than exhaustive of the possible configurations for the two sets of magnets.

An important feature of the invention in,relation to previous designs based on nested Halbach cylinders is the possibility of placing the magnets close to the central region where it is desired to create the uniform field. In this case, access to the magnetic field in the central region is possible in directions transvers to the axis as well as along the axis. Different magnet rings may have different inner and outer radii without the constraint that one set fits inside the other. In this way different thicknesses of magnetic material may be accomodated in different parts of the structure. Additionally, the permanent magnet segments in each ring or cylinder may be composed of different grades of permanent magnethavingdifferentremanen'ceand coercivity. The magnets are chosen to best resist the demagnetizing fields created by the magnets themselves and the demagnetizing field created by the other magnets in the variable flux source. They are also chosen to compensate forfinite length effects which induce inhomogeneity in the field in the central plane normal to the axis. These inhomogeneities are also reduced by a configuration of separated rings instead of a continuous cylinder.

The magnetic flux density which can be produced by the magnets of the invention can typically range from 1 millitesla to more than 2 tesla. Smaller fields are more conveniently produced by arrangements of Helmholz coils. Larger fields, up to about 4 teslas may be generated using permanent magnets but the mass of magnet required is large in relation to the useful volume. It is impracticable to use magnetic fields to generate fields in excess of 2.5 tesla unless the field is needed in a very small volume.

The magnet configurations of the invention all have a common clear circular bore. This permits access to the field along directions close to the axis. Furthermore configurations such as those shown in Figures 2a - c and 3 additionally allow access in directions perpendicular to the axis or close to perpendicular to the axis. This is often required for measurements which require a beam of light or other radiation to be brought to a sample which is exposed to the magnetic field. Typical measurements are those such as optical absorption, transmission and scattering, magnetooptic Kerr or Faraday effect, neutron scattering, inelastic scattering of electromagnetic or other radiation. Access from directions perpendicular to the axis may facilitate the introduction of material such as samples or substrates in to the central region.

Anotheradvantage of the clear bore is the possibility of incorporating a tube furnace in the variable flux source of the invention, as shown in Figure 4. This allows thermal magnetic treatments to be carried out of materials and magetic thin-film devices such as read and write heads for magnetic recording, magnetic media and magnetic memory arrays. These thermal treatments may be carried out in vacuum or controlled atmosphere at high temperatures in magnetic fields which may be varied in magnitude and direction as required at different stages in the processing cycle. It is necessary to ensure that the outer IE 990194 wall of the furnace is kept cool so not as to heat the magnets of the variable flux source. An advantage of the variable flux source of the invention over the nested arrangement is that the balance between the fields BI and B2 is relatively insensitive to radial thermal gradients An electrochemical cell may be located in the bore of the permanent magnet structure. Electrodepositiopn of nonmagnetic metals such as copper or magnetic metals and alloys such as permalloy or Fe-Co-Ni can take place in a stready, variable and optionally rotating magnetic field in the permanent magnewt structure of the invention. Advantagesofelectoplating in a magnetic field include enhanced deposition rate, modified film morphology, improved deposition in restricted volumes such as submicron structures and induced anisotropy in feromagnetic films. These benefits are found in applied fields in the range 0.05 - 2 tesla or more. Fields of these magnitudes are produced by the permanent amgnet structure of the invention, typically using rare-earth permanent magnet material such as Nd-Fe-B or Sm-Co for fields in excess of 0.5 tesla, and hard ferrite for lower fields.

Claims (5)

1. I An arrangement of coaxial rings or cylinders incorporating permanent magnets which is composed of three or more rings or cylinders which form tw o sets. The two sets can be rotated independently about their common axis so producing a substantially uniform magnetic field in a central volume which is variable in magnitude and in direction in a plane perpendicular to the axis.

2. An arrangement of coaxial rings or cylinders incorporating permanent magnets which is composed of three or more rings or cylinders which form two sets. The (wo sets can be rotated independently about their common axis so producing a substantially uniform magnetic field in a central volume which is variable in magnitude and in direction in a plane perpendicular to the axis. The magnetic rings or cylinders of the device are arranged face to face and have substantially the same inner radii.

3. An arrangement of an even number of pairs of coaxial rings or cylinders incorporating permanent magnets which form two sets. The two sets can be rotated independently about their common axis so producing a substantially uniform magnetic field in a central volume which is variable in magnitude and in direction in a plane perpendicular to the axis. Preferred numbers of rings or cylinders are four or six. Mechanical access or access for beams of light or radiation to the to the magnetic field volume may optionally be provided in directions along or close to the common axis or in directions perpendicular or close to perpendicular to the common axis.

4. A magnetic processing chamber such as a tube furnace optionally with control of ambientatmosphereoran electrochmical deposition cell incorporated into an arrangement of coaxial rings or cylinders incorporating permanent magnets w hich is composed of three or more rings or cylinders which form two sets. The two sets can be rotated independently about their common axis so producing a substantially uniform magnetic field in a central volume which is variable in magnitude and in direction in a plane perpendicular to the axis.

5. A chamber for processing or measurement of materials or devices in a magnetic field constituting eithera tube furnace optionally with control of ambient atmosphere oran electrochemical deposition cell incorporated into an arrangement of an even number of pairs of coaxial rings or cylinders incorporating permanent magnets which form two sets. The two sets can be rotated independently about their common axis so producing a substantially uniform magnetic field in a central volume which is variable in magnitude and in direction in a plane perpendicular to the axis. Preferred numbers of rings or cylinders are four or six. Mechanical access or access for beams of light or radiation to the to the magnetic field volume may optionally be provided in directions along or close to