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Title:
MAGNET ASSEMBLY AND METHOD OF ASSEMBLING A MAGNET ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2021/048385
Kind Code:
A1
Abstract:
There is provided a method of modifying a magnetic field of a single magnet (110) and a corresponding magnet assembly (100) made by such a method. The magnetic field of a single magnet (110) is modified by affixing a plurality of modifying magnets (120, 130) to the single magnet (110) in order to increase an identified gradient of the magnetic field of the single magnet (110). The plurality of modifying magnets (120, 130) are affixed to the single magnet (110) such that the modifying magnets (120, 130) are oriented with their respective magnetic polarisations directed at least partially in the direction of the identified gradient of the magnetic field of the single magnet (110).

Inventors:
HØYER HENRIK (NO)
Application Number:
PCT/EP2020/075517
Publication Date:
March 18, 2021
Filing Date:
September 11, 2020
Export Citation:
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Assignee:
GIAMAG TECH AS (NO)
International Classes:
B03C1/033; B03C1/28; H01F7/02
Domestic Patent References:
WO2019175066A12019-09-19
WO2020148424A12020-07-23
Foreign References:
US8358190B12013-01-22
US20120262019A12012-10-18
US8405479B12013-03-26
Attorney, Agent or Firm:
RING, James (GB)
Download PDF:
Claims:
Claims

1. A method of modifying a magnetic field of a single magnet for separation of substances, the method comprising: selecting a single magnet having a magnetic field and a first magnetic polarisation; identifying a gradient of the magnetic field of the single magnet to be increased; selecting a plurality of modifying magnets for modifying the identified gradient of the magnetic field, each modifying magnet having a magnetic polarisation; determining an arrangement for the plurality of modifying magnets to increase the identified gradient, the arrangement including an orientation of the magnetic polarisations for each of the modifying magnets in which the magnetic polarisations of the modifying magnets are oriented at least partially in the direction of the identified gradient; and affixing the plurality of modifying magnets to the single magnet in the determined arrangement to thereby increase the identified gradient. 2. A method as claimed in claim 1, wherein the identified gradient is a first gradient and the plurality of modifying magnets are a first plurality of modifying magnets, the method comprising: identifying a second gradient of the magnetic field to increase, the second gradient being perpendicular to the first gradient; selecting a second plurality of modifying magnets for modifying the second gradient of the magnetic field, each of the second plurality of modifying magnets having a magnetic polarisation; determining an arrangement for the second plurality of modifying magnets to increase the second gradient, the arrangement including an orientation of the magnetic polarisations for each of the second plurality of modifying magnets in which the magnetic polarisations of the second plurality of modifying magnets are oriented at least partially in the direction of the second gradient; and affixing the second plurality of modifying magnets to the single magnet in the determined arrangement to thereby increase the second gradient.

3. A method as claimed in claim 2, wherein the arrangement of the magnetic polarisations of the second plurality of modifying magnets is such that they are oriented in a plane perpendicular to a plane within which are oriented the magnetic polarisations of the first plurality of modifying magnets.

4. A method as claimed in claim 2 or 3, wherein the single magnet has a first surface substantially perpendicular to the first magnetic polarisation, and wherein the first gradient is in the direction of the first magnetic polarisation and the second gradient is in a plane parallel to the first surface.

5. A method as claimed in claim 4, wherein affixing the first plurality of modifying magnets to the single magnet comprises affixing the first plurality of modifying magnets to the first surface of the single magnet.

6. A method as claimed in claim 4 or 5, wherein affixing the second plurality of modifying magnets to the single magnet comprises affixing the second plurality of modifying magnets to the surface of the single magnet.

7. A method as claimed in claim 4 or 5, wherein affixing the second plurality of modifying magnets to the single magnet comprises affixing the second plurality of modifying magnets to opposing sides of the single magnet.

8. A method as claimed in claim 1, wherein the identified gradient is a first gradient, the method comprising: identifying a second gradient of the magnetic field to increase, the second gradient being perpendicular to the first gradient; and wherein determining the arrangement for the plurality of modifying magnets includes determining the arrangement to increase the second gradient, the arrangement including an orientation of the magnetic polarisations for each of the modifying magnets in which the magnetic polarisations of the modifying magnets are oriented at least partially in the direction of the second gradient; and wherein affixing the plurality of modifying magnets to the single magnet comprises increasing the second gradient.

9. A method as claimed in claim 8, wherein the single magnet has a first surface substantially perpendicular to the first magnetic polarisation, and wherein the first gradient is in the direction of the first magnetic polarisation and the second gradient is in a plane parallel to the first surface.

10. A method as claimed in claim 9, wherein the arrangement of the magnetic polarisations of the plurality of modifying magnets is such that they are oriented in three dimensions.

11. A method as claimed in claim 9 or 10, wherein affixing the modifying magnets to the single magnet comprises affixing the modifying magnets to at least the first surface of the single magnet.

12. A method as claimed in any preceding claim, comprising increasing the identified gradient of the magnetic field by at least 5 times, preferably by at least 10 time, and more preferably by at least 20 times.

13. A method as claimed in any preceding claim, comprising affixing magnetically susceptible elements to the single magnet and/or to the plurality of modifying magnets.

14. A method as claimed in any preceding claim, wherein the arrangement of the plurality of modifying magnets is such that the magnetic polarisation of each magnet is angled with respect to that of its nearest neighbour of the plurality of modifying magnets.

15. A method as claimed in any preceding claim, wherein affixing the plurality of modifying magnets to the single magnet comprises affixing the plurality of modifying magnets so that the magnetic polarisations of the modifying magnets are oriented symmetrically with respect to the first magnetic polarisation of the single magnet.

16. A method as claimed in any preceding claim, wherein determining the arrangement for the plurality of modifying magnets comprises simulating the arrangement and calculating the gradient of the magnetic field.

17. A method as claimed in any preceding claim, wherein selecting a plurality of modifying magnets comprises selecting only magnets that are smaller than the single magnet.

18. A magnet assembly for separation of substances, comprising: a single magnet having a magnetic field and a first magnetic polarisation; and a plurality of modifying magnets each having a magnetic polarisation, wherein the modifying magnets are affixed to the single magnet and oriented with respective magnetic polarisations directed at least partially in a first direction to thereby increase the gradient of the magnetic field of the single magnet in the first direction.

19. A magnet assembly as claimed in claim 18, wherein the plurality of modifying magnets are a first plurality of modifying magnets, the magnet assembly comprising a second plurality of modifying magnets each having a magnetic polarisation, wherein the second plurality of modifying magnets are affixed to the single magnet and oriented with respective magnetic polarisations directed at least partially in a second direction perpendicular to the first direction to thereby increase the gradient of the magnetic field of the single magnet in the second direction.

20. A magnet assembly as claimed in claim 19, wherein the second plurality of modifying magnets are arranged with their magnetic polarisations oriented in a plane which is perpendicular to a plane within which are oriented the magnetic polarisations of the first plurality of modifying magnets.

21. A magnet assembly as claimed in claim 19 or 20, wherein the single magnet comprises a first surface substantially perpendicular to the direction of the first magnetic polarisation, and wherein the first plurality of modifying magnets are affixed to the first surface.

22. A magnet assembly as claimed in claim 21, wherein the second plurality of modifying magnets are affixed to the first surface of the single magnet.

23. A magnet assembly as claimed in claim 21, wherein the second plurality of modifying magnets are affixed to opposing side surfaces of the single magnet.

24. A magnet assembly as claimed in claim 18, wherein the modifying magnets are oriented with respective magnetic polarisations directed at least partially in a second direction perpendicular to the first direction to thereby increase the gradient of the magnetic field of the single magnet in the second direction.

25. A magnet assembly as claimed in claim 24, wherein the magnetic polarisations of the plurality of modifying magnets are oriented in three dimensions.

26. A method of separation of substances, the method comprising using the magnet assembly of any of claims 18 to 25 to gather magnetically susceptible material.

27. A method as claimed in claim 26, comprising disposing the magnet assembly in a sheath and collecting the magnetically susceptible material on the sheath.

28. A method as claimed in claim 26 or 27, wherein the magnetically susceptible material collected are sub-micron particles and/or nanoparticles.

29. A method as claimed in claim 26, 27 or 28, wherein the method comprises gathering magnetically susceptible material from viscous fluid, and/or from digestate.

30. A method as claimed in claim 26, 27 or 28, wherein the method comprises separating magnetically susceptible material from dry samples, from gas, and/or from solids.

31. A method of manipulating magnetically susceptible material using the magnet assembly of claims of claims 18 to 25, comprising using the magnetic field of the magnet assembly to move the magnetically susceptible material from a first location to a second location, preferably wherein the magnetically susceptible material comprises particles such as nanoparticles.

Description:
MAGNET ASSEMBLY AND METHOD OF ASSEMBLING A MAGNET ASSEMBLY

Field The invention relates to a method of modifying a magnetic field of a single magnet, particularly to modifying it for use in separation techniques, as well as to a magnet assembly made by such a method, and a separation method using the magnet assembly. Background

Magnets are used for various applications. One such application is the separation of magnetically susceptible material from other material. For example, a magnet may be used to gather particles that are affected by magnetic fields from a fluid, other particles, solids, gasses, or the like. In those methods, a single magnet will be positioned proximate the particles so that the magnetically susceptible particles are attracted to the single magnet and thereby separated from the other material.

In one such separation method, a magnet is placed inside a rack or case or the like, typically made of plastic or another material that is not susceptible to magnetic fields. A sample container or tube will then be placed on or near the rack and particles (or other magnetic material) inside the container will be gathered by the magnetic force of the magnet on the walls of the sample container. The sample container may then be removed from the rack to recover the particles. However, because magnetic forces are strongest close to a surface of a magnet, the use of a case will diminish the effectiveness of the magnet inside by some degree, because particles will be prevented from interacting with the strongest magnetic fields by the case and walls of the sample container. Relatively thick cases (e.g. 1 , 2 or 3 millimetres or more) may be used in some systems and hence may limit the effectiveness of the magnets for separation. To mitigate against this, single, lone magnets are used in the racks or cases because although single magnets generally exhibit weaker magnetic forces than e.g. magnetic arrays, single magnets exhibit stronger forces at a greater distance, whereas the strong forces of magnetic arrays are concentrated near their surfaces (e.g. within 1 or 2 millimetres). However, improvements are desirable to the methods and systems presently using single magnets for particle separation. For example, improvements in the efficiency and strength of particle capture are desirable.

Summary

According to a first aspect of the invention there is provided a method of modifying a magnetic field of a single magnet for separation of substances, the method comprising: selecting a single magnet having a magnetic field and a first magnetic polarisation; identifying a gradient of the magnetic field to be increased; selecting a plurality of modifying magnets for modifying the identified gradient of the magnetic field, each modifying magnet having a magnetic polarisation; determining an arrangement for the plurality of modifying magnets to increase the identified gradient, the arrangement including an orientation of the magnetic polarisations for each of the modifying magnets in which the magnetic polarisations of the modifying magnets are oriented at least partially in the direction of the identified gradient; and affixing the plurality of modifying magnets to the single magnet in the determined arrangement to thereby increase the identified gradient.

The force experienced by an element of a substance, such as a particle is a product of the magnetic field and the magnetic gradient. By increasing the identified gradient, the method may adapt the field of the single magnet to increase its capturing force for particles e.g. by increasing the gradient in a direction towards the magnet. The gradient is the change in the magnetic field in a predetermined direction, therefore the gradient may be different in different directions. In separation methods, it is desirable to draw particles or the like towards the magnet, and therefore the gradient may be in a direction normal to a surface of the magnet. Typically, the direction normal to the surface of the magnet is known as the z- direction. By increasing the gradient in one direction, the method adapts the single magnet so that particles experience a greater force in that direction. For example, by increasing the gradient in the z-direction, particles will experience a greater force drawings them towards the magnet.

The inventors of the present invention have understood that the gradient of a single magnet may be increased by attaching to the single magnet a plurality of modifying magnets with magnetic polarisations oriented at least partially in the direction of the gradient. However, since it is preferable to maintain the longer range of the magnetic field of a single magnet (compared to e.g. a magnetic array of equally sized magnets) for particle separation applications and the like, the modifying magnets attached to the single magnet should preferably be used to adapt and configure the magnetic field of the single magnet (e.g. by superposition principles) without significantly reducing it strength at long ranges. Therefore, the modifying magnets may be thought of as being used to refine the magnetic field of the single magnet e.g. for separation methods.

The single magnet selected by the method may be suitable for use in a method of separating substances such as separating particles even before it has been modified by the method of the first aspect. The method may therefore comprise selecting a single magnet which is suitable for use on its own in a separation method. However, the modification of the single magnet by the above method may adapt and improve the single magnet for use in such methods by configuring and/or modifying its magnetic field using the modifying magnets to increase the identified gradient. The method of the first aspect of the invention may therefore be a method of augmenting a magnetic field of a single magnet e.g. to improve its efficacy for capturing material for separation. The augmentation of the magnetic field is an increase of the gradient because the gradient of the magnetic field affects the force that the magnet exhibits upon particles or other magnetically susceptible material. As such, the method may be a method of increasing a gradient of a single magnet.

Therefore, the method provides a means of configuring the magnetic field of a single magnet by attaching thereto modifying magnets with predetermined orientations. Particularly, the modifying magnets are oriented so that the directions of their respective magnetic polarisations have a component in the direction of in the identified gradient. That is, the magnetic polarisations of the modifying magnets extend at least partially in the direction of the gradient and therefore have a component in that direction. The magnetic fields of the modifying magnets will superpose with the magnet field of the single magnet to thereby modify the magnetic field of the single magnet.

The method may provide a magnet assembly in which the single magnet carries the plurality of modifying magnets. The single magnet may carry all components of the magnet assembly e.g. by their being attached directly thereto. That is, the single magnet may not be part of another magnet assembly or magnet array, and may be attached only to the plurality of modifying magnets. The single magnet may therefore serve as a type of substrate for the magnet assembly. Systems for separation of substances may have existing cases, racks, sheaths or the like for holding a single magnet, and therefore the magnet assembly made by the method may be sized to fit those e.g. the same size as another single magnet previously used for separation in those systems. The plurality of modifying magnets may be recessed into the single magnet so that the magnet assembly has the same size as the single magnet prior to the method. The single magnet may span (i.e. extend the length of) one direction of a cross-section through the centre of the magnet assembly made by the method. The single magnet may span two orthogonal cross-sections through the centre of the magnet assembly. The single magnet may span two orthogonal directions of the cross section(s). The single magnet may extend more than 50% of the length of the cross-section(s), more than 75%, more than 90%, and/or 100% of the cross-sections. Thus, the magnet assembly may be a single magnet having modifying magnets attached thereto, and the majority of the magnet assembly may be provided by the single magnet. The single magnet may provide more than 50% of the volume of the magnet assembly. The single magnet may be isolated from any but the modifying magnets.

The single magnet may have a first surface, e.g. a top, substantially perpendicular to the direction of the first magnetic polarisation. Therefore, during use, the first surface may be used as an area towards which particles are gathered for separation. The single magnet may be substantially cylindrical or cuboidal, for example with a rectangular cross section. The single magnet may have any suitable shape and any suitable cross section. The single magnet may have at least one side substantially perpendicular to the first surface, and may have a second surface, e.g. a bottom, on an opposite side of the single magnet to the first surface. The second surface may be substantially parallel to the first surface and may therefore be substantially perpendicular to the first magnetic polarisation. The single magnet may have a quadrilateral cross-section or a regular quadrilateral cross-section. The cross-section may be rectangular or square. The single magnet may have a circular cross-section (e.g. instead of the quadrilateral cross-section, or in a plane perpendicular to that). The plurality of modifying magnets may be fixed to any surface of the single magnet. The plurality of modifying magnets may be fixed to the first surface of the single magnet, and may be arranged in a layer on the first surface of the single magnet to partially or completely cover the first surface of the single magnet. The plurality of modifying magnets may be affixed to the first surface of the single magnet in a line or strip spanning the first surface (i.e. from one side to the other). The plurality of modifying magnets may be fixed to other surfaces of the single magnet, for example to sides substantially perpendicular to the first surface. The modifying magnets may be adjacent one another and each modifying magnet may be fixed to at least one other modifying magnet in addition to the single magnet. Alternatively, the majority of modifying magnets may be fixed to at least one other modifying magnets and a minority may not be fixed to another modifying magnet. For example, each modifying magnet fixed at the edge of the single magnet may not be adjacent and fixed to another modifying magnet, while all modifying magnets not fixed at the edge of the single magnet may be adjacent another modifying magnet.

The plurality of modifying magnets may each be recessed into the single magnet and may be flush with the first surface of the single magnet. The method may therefore comprise defining a recess or a plurality of recesses in the single magnet e.g. in the first surface of the single magnet, for receiving at least one of the plurality of modifying magnets. The method may comprise affixing at least one of the modifying magnets into a recess in the single magnet.

The plurality of modifying magnets may be arranged on opposing surfaces of the single magnet, for example on the first surface and on the second surface. The plurality of modifying magnets may be arranged on side surfaces of the single magnet, and the single magnet may therefore be surrounded by modifying magnets, for example on all sides. Alternatively, sides (e.g. opposing ends) of the single magnet may not have magnets attached thereto.

The arrangement of modifying magnets may be symmetrical about a plane through the centre of the single magnet parallel to the first magnetic polarisation. Since the single magnet has only a single magnetic polarisation (the first magnetic polarisation), the plurality of modifying magnets may be arranged symmetrically with respect to that. Therefore, the magnetic field of the magnet assembly may be symmetrical. The magnet assembly may therefore be symmetrical in a plane through its centre parallel to the first magnetic polarisations (a symmetrical plane). The modifying magnets may be arranged in pairs symmetrically with respect to the symmetrical plane.

As noted above, the identified gradient may be in a direction parallel to the first magnetic polarisation. The identified gradient may therefore be in a direction normal to the first surface of the single magnet. By increasing the gradient normal to the first surface of the single magnet the capturing force in a direction towards the single magnet may be increased. However, the inventors have also recognised that the efficiency of capture of magnetically susceptible material may be increased by increasing not only the gradient of the magnetic field perpendicular to the first surface of the single magnet, but also by increasing a gradient of the magnetic field parallel to the first surface of the single magnet in order to improve transportation of particles (or transportation of any captured material) over the surface the single magnet. This may help concentrate captured material in a predetermined location, and/or may help move captured material to locations where the gradient in the normal direction is largest, thereby increasing efficiency of capture.

Therefore the identified gradient may be a first gradient and the plurality of modifying magnets may be a first plurality of modifying magnets. The method may comprise: identifying a second gradient of the magnetic field to increase, the second gradient being perpendicular to the first gradient; selecting a second plurality of modifying magnets for modifying the second gradient of the magnetic field, each of the second plurality of modifying magnets having a magnetic polarisation; determining an arrangement for the second plurality of modifying magnets to increase the second gradient, the arrangement including an orientation of the magnetic polarisations for each of the second plurality of modifying magnets in which the magnetic polarisations of the second plurality of modifying magnets are oriented least partially in the direction of the second gradient; and affixing the second plurality of modifying magnets to the single magnet in the determined arrangement to thereby increase the second gradient. The second plurality of modifying magnets may therefore be arranged with respective magnetic polarisations oriented at least partially in the direction of the second gradient to increase the gradient in that direction. For example the magnetic polarisations of the second plurality of modifying magnets may be directed at least partially within a plane substantially perpendicular to the first magnetic polarisations e.g. substantially parallel to the surface of the single magnet, with the first gradient in a direction normal to the surface. The magnetic polarisations of the second plurality of modifying magnets may be confined to a plane substantially perpendicular to the first magnetic polarisation (e.g. parallel to the first surface), and the magnetic polarisations of the first plurality of modifying magnets may be confined to a plane substantially parallel to the first magnetic polarisation (e.g. perpendicular to the first surface). Therefore, the first and second pluralities between them may have magnetic polarisations oriented in three dimensions (i.e. three orthogonal directions).

The method may therefore provide a means of increasing two gradients of the magnetic field of the single magnet using two pluralities of modifying magnets, the first plurality of modifying magnets having magnetic polarisations oriented at least partially in the direction of the first gradient, and the second plurality of modifying magnets having magnetic polarisations oriented at least partially in the direction of the second gradient, so the first gradient of the magnetic field of the single magnet is increased by the first plurality of modifying magnets, and the second gradient of the magnetic field of the single magnet is increased by the second plurality of modifying magnets. The first and/or second pluralities of modifying magnet may also be arranged to increase a third gradient of the magnetic field in a third direction perpendicular to the first and second gradients.

The magnetic polarisations of the first plurality of modifying magnets may be oriented orthogonally to the direction of the second gradient and/or the magnetic polarisations of the second plurality of modifying magnets may be oriented orthogonally to the direction of the first gradient. The arrangement of the magnetic polarisations of the second plurality of modifying magnets may be such that they are oriented in a plane perpendicular to a plane within which are oriented the magnetic polarisations of the first plurality of modifying magnets. The magnetic polarisations of the first plurality of modifying magnets may therefore be confined to a plane orthogonal to the first surface, and the magnetic polarisations of the second plurality of modifying magnets may be confined to a plane parallel to the first surface.

As noted above, the single magnet may have a first surface substantially perpendicular to the first magnetic polarisation. The first gradient may be in the direction of the first magnetic polarisation and the second gradient may be in the plane parallel to the first surface. The first gradient may therefore be normal to the first surface of the single magnet and the second gradient may be in a plane parallel to the first surface of the single magnet. The second gradient may be in a direction orthogonal to the orientation of any of the first plurality of modifying magnets. The second plurality of modifying magnets may have magnetic polarisations confined to the plane perpendicular to the direction of the first magnetic polarisation. The first plurality of modifying magnets may have polarisations confined to a vertical plane (e.g. a plane containing the first magnetic polarisation).

The magnetic assembly formed by the method may therefore be more efficient at capture than a single isolated magnet, because it is able to more efficiently draw material in the direction of the second gradient towards locations where the first gradient is highest, whereupon the captured material will be drawn efficiently to the surface of the magnet assembly.

The magnetic polarisations of the first plurality of modifying magnets may therefore be directed into and/or out of the first surface of the single magnet. The magnetic polarisations of the first plurality of modifying magnets may be confined to a vertical plane (i.e. a plane parallel to the first magnetic polarisation). The magnetic polarisations of the second plurality of modifying magnets may be confined within a plane substantially parallel to the first surface of the single magnet. The magnetic polarisations of the second plurality of modifying magnets may be orthogonal to the first magnetic polarisation of the single magnet.

The step of affixing the first plurality of modifying magnets to the single magnet may comprise affixing the first plurality of modifying magnets to the first surface of the single magnet. The step of affixing the second plurality of modifying magnets to the single magnet may comprise affixing the second plurality of modifying magnets to the first surface of the single magnet. The first plurality of modifying magnets and second plurality of modify magnets may therefore be disposed on the same surface of the single magnet and may cooperate to cover (e.g. partially or fully) the first surface of the single magnet.

The step of affixing the second plurality of modifying magnets to the single magnet may comprise affixing the second plurality of modifying magnets to sides of the single magnet e.g. to opposing sides. The step may comprise affixing the second plurality of modifying magnets at least partially to sides of the magnets of the first plurality of modifying magnets.

The first plurality of modifying magnets may be arranged centrally on the single magnet e.g. in the middle of the first surface. They may cover the centre of the first surface and may extend from one side of the first surface of the single magnet to an opposing side of the first surface. The first plurality of modifying magnets may be arranged in a row on the first surface of the single magnet. Each magnet of the first plurality of modifying magnets may have a magnetic polarisation rotated with respect to that of the nearest other magnet of the first plurality of modifying magnets. The magnetic polarisations of the first plurality may alternate between two different orientations along the line, and every other one of the first plurality of modifying magnets may have the same magnetic polarisation orientation. Each of the first plurality of modifying magnets may have magnetic polarisations oriented by between about 0° and 75° to a line parallel with the first gradient. The magnetic polarisations may be oriented by between about 30° and 60° to the line parallel with the first gradient, and may be oriented by about 45° thereto. Since the positive direction of a magnet’s polarity is a convention, the orientations of the modifying magnets may be towards, or away from, the single magnet. Alternating magnets of the first plurality may have magnetic polarisations oriented alternating sides of the first gradient, for example a first magnet oriented 45° clockwise to the first gradient, and the next magnet oriented 45° anticlockwise to the first gradients, and so on.

The alternating orientations may be other than 45°, and may be between 30° to 60°, or between 0° to 75°. Thus, the magnetic polarisations of the first plurality of modifying magnets may be oriented between about 15° to 90° to the first surface of the single magnet, and/or about 30° to 60° thereto, and/or about 45° thereto. Each of the first plurality of modifying magnets may be an elongate magnet and may be the same length as the single magnet so that each magnet of the first plurality covers a strip of the first surface of the single magnet from one side to the other.

The first plurality of modifying magnets may be arranged adjacent each other or may be spaced apart. Some of the first plurality of modifying magnets may be adjacent another of the first plurality of modifying magnets, and others of the first plurality of modifying magnets may be spaced from any others of the first plurality of modifying magnets. The first plurality of modifying magnets may be arranged in pairs symmetrically with respect to a plane through the centre of the single magnet parallel to the first magnetic polarisation. The first plurality of modifying magnets may be distributed over the first and second surfaces of the single magnet, and therefore may be distributed on opposing faces. The magnets of the first plurality on the second face may have any arrangement, and may be symmetrical to the arrangement on the first surface but with polarities reversed (e.g. reflected in a plane through the centre of the magnet device, then polarities reversed).

The second plurality of modifying magnets may be affixed to the first surface of the single magnet and therefore may be adjacent at least some of the first plurality of modifying magnets. The second plurality of modifying magnets may be affixed to sides of the single magnet (those sides being e.g. substantially perpendicular to the first surface), and may be affixed to opposing sides of the single magnet. The second plurality of modifying magnets may have magnetic polarisations oriented by between about 0° and 75° to a line parallel to the second gradient. The magnetic polarisations may be oriented by between about 0° and 45° to the line parallel to second gradient, and may be oriented by about 0° to the second gradient (i.e. parallel thereto).

The second plurality of modifying magnets may be distributed on opposing faces of the single magnet. For example, they may be disposed on the first and second surfaces. They may be disposed on opposing side faces of the single magnet, and/or may be disposed on the first and second faces as well as on the sides.

The method may comprise affixing the second plurality of modifying magnets to the single magnet to narrow the region of the magnetic field with an increased gradient in the first direction. That is, the second plurality of modifying magnets may reduce the size or width of the region above the magnet assembly in which the first gradient is increased. The second plurality of modifying magnets may be arranged either side of the first plurality of modifying magnets, and all magnets of the first plurality of modifying magnets may be between magnets of the second plurality of modifying magnets.

The method may comprise identifying a third gradient of the magnetic field of the single magnet to increase, wherein the third gradient is perpendicular to the first and second gradients; selecting a third plurality of modifying magnets for modifying the third gradient; determining an arrangement for the third plurality of modifying magnets to increase the third gradient, wherein the arrangement includes an orientation of the magnetic polarisations for each of the third plurality of modifying magnets in which the magnetic polarisations of the third plurality of modifying magnets are oriented at least partially in the direction of the third gradient; and affixing the third plurality of modifying magnets to the single magnet to thereby increase the third gradient. Instead of using a third plurality of modifying magnets, the method may comprise arranging the first or second plurality of modifying magnets to increase the third gradient.

Each magnet of the first and/or second plurality of modifying magnets may also be arranged such that each individual modifying magnet is oriented to increase a gradient of the magnetic field in more than one direction, e.g. in two perpendicular direction directions or three perpendicular directions.

Therefore, the identified gradient may be a first gradient, and the method may comprise: identifying a second gradient of the magnetic field to increase, the second gradient being perpendicular to the first gradient. Then, determining an arrangement for the plurality of modifying magnets may include determining the arrangement to increase the second gradient, the arrangement including an orientation of the magnetic polarisations for each of the modifying magnets in which the magnetic polarisations of the modifying magnets are oriented at least partially in the direction of the second gradient. Affixing the plurality of modifying magnets to the single magnet may comprise increasing the second gradient.

For example, the arrangement may be such that the modifying magnets have magnetic polarisations angled in two planes. Particularly, the magnetic polarisations may be angled partially in the direction normal to the first surface of the single magnet and partially in a direction parallel to the first surface of the single magnet. The first gradient may be in the direction of the first magnetic polarisation and the second gradient may be in a plane parallel to the surface. The modifying magnets may therefore have magnetic polarisation directions with a component in the direction of the first gradient and a component in the direction of the second gradient. The modifying magnets may therefore increase the gradients in both directions.

The arrangement of the magnetic polarisations of the plurality of modifying magnets may be such that they are oriented in three dimensions. The magnetic polarisations may be oriented at least partially in two dimensions in a plane perpendicular to the first magnetic polarisation and may further be oriented at least partially in a plane parallel to the first magnetic polarisation. Therefore, the modifying magnets may have magnetic polarisations oriented in three orthogonal directions, one of which direction may be parallel to the first magnetic polarisation. The method may comprise: identifying a third gradient of the magnetic field to increase, the third gradient being perpendicular to the second gradient and perpendicular to the first gradient. Then, determining an arrangement for the plurality of modifying magnets may include determining the arrangement to increase the third gradient, the arrangement including an orientation of the magnetic polarisations for each of the modifying magnets in which the magnetic polarisations of the modifying magnets are oriented at least partially in the direction of the third gradient. Affixing the plurality of modifying magnets to the single magnet may comprise increasing the third gradient. The first gradient may be normal to the first surface of the single magnet e.g. parallel to the first magnetic polarisation. For example, the modifying magnets may be disposed on the first surface of the single magnet with magnetic polarisations oriented inwardly and downwardly towards the centre of the single magnet. The modifying magnets may be disposed on the first surface of the single magnet with magnetic polarisations oriented inwardly and upwardly away from the surface of the single magnet. The modifying magnets may be arranged on opposing surfaces of the single magnet, and magnets on the opposing surfaces may be arranged symmetrically but with polarisations directions reversed.

The step of affixing the modifying magnets to the single magnet may comprise affixing the modifying magnets to the first surface of the single magnet. It may comprise affixing the modifying magnets to the first and second surfaces, and may include affixing the modifying magnets to sides of the single magnet.

The method may comprise increasing the gradient (e.g. the first gradient) of the magnetic field by at least 5 times compared to that of the single magnet. The method may comprise increasing the gradient (e.g. the first gradient) by at least 10 times, and more preferably by at least 20 times. The method may comprise increasing the first gradient by at least 5 times, preferably by at least 10 times, and more preferably by at least 20 times. The method may comprise increasing the second gradient by at least five times, preferably by at least 10 times, and more preferably by at least 20 times. The increase of the gradient may be within a predetermined region of the magnetic field.

The method may comprise increasing the gradient by at least 5 times at a distance of more than 1 millimetre from the surface of the magnet assembly formed by the method. The method may comprise increasing the gradient by at least 10 times at a distance of more than 1 millimetre from the surface of the magnet assembly formed by the method. The method may comprise increasing the gradient at a distance of more than 10 millimetres from the surface of the magnet assembly formed by the method. The method may comprise increasing the gradient at a distance of more than 20 millimetres from the surface of the magnet assembly formed by the method. The increase in the gradient may be confined to a region of the magnetic field. Therefore, the modifying magnets may have a surprisingly large influence on the gradient of the magnetic field of the single magnet, even where the modifying magnets are relatively small compared to the single magnet. It may therefore be possible using the method to configure the magnetic field of the single magnet so that regions of the field have a substantially increased gradient and therefore may be significantly more effective at capturing magnetically susceptible particles or the like than is the field of the single magnet alone. The use of a single magnet may allow the gradient to be increased at greater distance from the single magnet e.g. compared to using an array of equally sized magnets. As such, the method may provide a magnet assembly that allows capture of materials that could not be captured by the single magnet alone.

For example, the magnet assembly formed by the method may be suitable for capture of sub-micron sized particles, and/or nanoparticles. The modification of the magnetic field of the single magnet by the plurality of modifying magnets may be sufficient for use in separating sub-micron particles or nanoparticles. The magnet assembly may be suitable for separating particles that have low susceptibility to magnetic fields. The single magnet alone (i.e. not modified according to the present method) may not be suitable for capturing e.g. sub-micron particles, nanoparticles, or particles with low magnetic susceptibility.

The method may comprise affixing magnetically susceptible elements to the single magnet and/or to the plurality of modifying magnets. The magnetically susceptible elements may be any elements responsive to magnetic fields. The magnetically susceptible elements may be permanent magnets and/or may be magnetic elements susceptible to magnetic fields (e.g. magnetic metals and the like). Where magnetically susceptible elements comprise at least part of the magnet assembly, those magnetically susceptible elements may preferably have relatively high saturation magnetisations and/or relatively high magnetic permeabilities. The use of magnetically susceptible elements having higher saturation magnetisations and/or higher magnetic permeabilities may be preferred. The method may comprise affixing magnetically susceptible elements to the first surface of the single magnet and/or affixing magnetically susceptible elements adjacent (e.g. between) magnets of the plurality of modifying magnets (e.g. between magnets of the first and/or second plurality of modifying magnets). The magnetically susceptible elements may be formed of any material affected by magnetic fields, and may or may not themselves be permanent magnets.

The method may comprise attaching additional magnets to the plurality of modifying magnets. The additional magnets may further modify the magnetic field of the single magnet and may have magnetic polarisations oriented at least partially in the direction of the first and/or second gradient. For example, the method may comprise fixing magnets to at least one of the plurality of modifying magnets on an opposite side to the single magnet.

The arrangement of the plurality of modifying magnets (e.g. the first plurality and/or the second plurality of modifying magnets) may be such that the magnetic polarisation of each magnet is angled with respect to that of its nearest neighbour in the plurality of modifying magnets. The magnetic polarisation of each magnet of the plurality of modifying magnets may be oriented between about 0° and 75° to a line parallel to the gradient to be increased. The magnetic polarisation of each magnet of the plurality of modifying magnets may be oriented between about 0° and 45° to the line parallel to the gradient to be increased. The magnetic polarities may be oriented in either direction relative to the positive direction of the identified gradient of the magnetic field, so that the angle described herein is relative to a direction parallel to the gradient, rather than necessarily relative to the positive direction of the gradient.

The step of affixing the plurality of modifying magnets to the single magnet may comprise affixing the plurality of modifying magnets so that the magnetic polarities of the modifying magnets are oriented symmetrically with respect to the magnetic polarity of the single magnet. Since the plurality of modifying magnets are provided to modify the magnetic field of only the single magnet, the modifying magnets may be arranged symmetrically (e.g. oriented at 45° and -45° to the direction of the gradient). The plurality of modifying magnets may be arranged symmetrically about a plane through the centre of the single magnet parallel to the first magnetic polarisation direction. The magnet assembly formed by the method may be symmetric about a plane through its centre parallel to the first magnetic polarisation. It may be symmetric about two such planes orthogonal to each other. The step of determining the arrangement for the plurality of modifying magnets may comprise simulating an initial arrangement and calculating the gradient. The method may comprise calculating a value of the first gradient and/or calculating a value of the second gradient. The method may comprise comparing the calculated gradient to that of the single magnet and confirming that the increased gradient is more than a predetermined amount larger.

The step of selecting a plurality of modifying magnets may comprise selecting only magnets that are smaller than the single magnet. Therefore all of the modifying magnets may be smaller than the single magnet. The magnet assembly formed by the method may not comprise any magnets larger than the single magnet. Alternatively, the magnets of the second plurality of modifying magnets may be larger than the single magnet and may each occupy a greater volume that the single magnet. The single magnet may be a different size to any other magnet of the magnet assembly. The plurality of modifying magnets may all be the same size. The first plurality of modifying magnets may all be the same size. The second plurality of modifying magnets may all be the same size, and may be a different size to magnets of the first plurality. The magnets of the first plurality may have a height less than about 20% that of the single magnet, or less then about 15%. The magnets of the second plurality may have a height less than about 20% of the single magnet, or less then about 15%. The single magnet may comprise more than 50% of the magnet assembly by weight, or may comprise more than about 60% of the magnet assembly by weight.

The method may comprise using an even number of modifying magnets. The method may comprise using two or more magnets of the first plurality and/or two or more magnets of the second plurality. The method may comprise using 10 or more magnets of the first plurality, and/or may comprise using 4 or more magnets of the second plurality.

The method may comprise arranging the single magnet so that it spans (i.e. extend across) the entire width of a cross-section of the magnet assembly. The method may comprise arranging the single magnet so that it spans the entire width of two orthogonal cross-sections of the magnet assembly. The single magnet may span the entire width of three cross-sections of the magnet assembly. The method may comprise positioning the single magnet centrally in the magnet assembly. The method may comprise leaving end faces of the single magnet exposed and may therefore comprise not covering or otherwise fixing modifying magnets or the like to the end faces of the single magnet.

The method may comprise arranging the magnet assembly and/or orientations of the magnetic polarisations of the magnets in the magnet assembly symmetrically about a plane through the centre of the magnet assembly parallel to the first magnetic polarisation. The method may comprise arranging the magnet assembly and/or orientations of the magnetic polarisations of the magnets in the magnet assembly symmetrically about a two orthogonal planes through the centre of the magnet assembly parallel to the first magnetic polarisation. According to a second aspect of the invention there is provided a magnet assembly for separation of substances, comprising: a single magnet having a magnetic field and a first magnetic polarisation; and a plurality of modifying magnets each having a magnetic polarisation, wherein the modifying magnets are affixed to the single magnet and oriented with respective magnetic polarisations directed at least partially in a first direction to thereby increase the gradient of the magnetic field of the single magnet in the first direction.

The magnet assembly is therefore a single magnet carrying modifying magnets arranged to modify its magnetic field in a predetermined manner, particularly to increase its gradient in the first direction. The magnet assembly may be formed using the method of the first aspect of the invention and therefore may comprise any and all of the features of a magnet assembly described with reference to that aspect. The magnetic polarisations of the plurality of modifying magnets may be oriented between about 0° and about 70° to a line parallel to the gradient, and may be oriented by between about 0° and about 75° thereto. The magnetic polarisations of the plurality of modifying magnets may be oriented between about 0° and about 45° to the direction of the gradient. Since the magnet assembly is constructed around a single magnet, the single magnet may span (i.e. extend across) the entire width of a cross-section of the magnet assembly (e.g. as described herein with reference to the magnet assembly formed by the first aspect). The single magnet may span the entire width of two orthogonal cross-sections of the magnet assembly. The single magnet may span the entire width of three cross-sections of the magnet assembly.

The single magnet may be centrally located within the magnet assembly. The single magnet may comprise end faces that are not covered by other magnets or such as the modifying magnets and may be exposed and visible.

The magnet assembly and/or orientations of the magnetic polarisations of the magnets in the magnet assembly may be symmetric about a plane through the centre of the magnet assembly parallel to the first magnetic polarisation. The magnet assembly and/or orientations of the magnetic polarisations of the magnets in the magnet assembly may be symmetric about a two orthogonal planes through the centre of the magnet assembly parallel to the first magnetic polarisation.

The plurality of modifying magnets may be a first plurality of modifying magnets, and the magnet assembly may comprise a second plurality of modifying magnets each having a magnetic polarisation, wherein the second plurality of modifying magnets are affixed to the single magnet and oriented with respective magnetic polarisations oriented at least partially in a second direction perpendicular to the first direction to thereby increase the gradient of the magnetic field of the single magnet in the second direction.

The second plurality of modifying magnets may be arranged with their magnetic polarisations oriented in a plane which is perpendicular to a plane within which are oriented the magnetic polarisations of the first plurality of modifying magnets. The second plurality of modifying magnets may have magnetic polarisations confined to a plane orthogonal to the first magnetic polarisation and the first plurality of modifying magnets may have magnetic polarisations confined to a plane parallel to the first magnetic polarisation. The single magnet may comprise a first surface substantially perpendicular to the direction of the first magnetic polarisation, and the first plurality of modifying magnets may be affixed to the first surface. The second plurality of modifying magnets may be affixed to the first surface of the single magnet. The first and second pluralities of modifying magnets may therefore cover or partially cover the first surface of the single magnet and may form a layer disposed thereon.

The second plurality of modifying magnets may be affixed to a side of the single magnet, for example a side substantially perpendicular to the first surface of the single magnet. The second plurality of modifying magnets may be affixed to opposing sides of the single magnet and may be arranged symmetrically either side of the single magnet.

The modifying magnets may be oriented with respective magnetic polarisations directed at least partially in a second direction perpendicular to the first direction to thereby increase the gradient of the magnetic field of the single magnet in the second direction. Therefore the modifying magnets may be oriented with respective magnetic polarisations directed partially in the first direction and partially in the second direction, thereby increasing the magnetic gradient of the single magnet in the first direction and simultaneously increasing the magnetic gradient of single magnet in the second direction.

The first direction may be substantially perpendicular (i.e. normal) to the first surface of the single magnet. The second direction may be substantially parallel to the first surface the single magnet. The magnet assembly may therefore have an increased gradient in the direction directly towards its surface compared to that of the single magnet. It may also have an increased gradient in a direction parallel to its surface compared to that of the single magnet.

The magnetic polarisations of the plurality of modifying magnets may be oriented in three dimensions. The magnetic polarisations may be oriented at least partially in two dimensions in a plane perpendicular to the first magnetic polarisation and may further be oriented at least partially in a plane parallel to the first magnetic polarisation. According to a third aspect of the invention there is provided a method of material separation, for example a method of collecting magnetically susceptible material (such as particles), using the magnet assembly of the second aspect of the invention. The method may comprise disposing the magnet assembly in a rack or sheath and collecting the magnetically susceptible particles or other material on the sheath or rack. The method may comprise placing a sample container containing the material proximate the magnet assembly and collecting the captured material within the sample container. The method may comprise exposing the magnetically susceptible material to a magnetic field of the magnet assembly. The method may comprise collecting sub-micron particles and/or may comprise collecting nanoparticles. The method may comprise collecting particles having a low magnetic susceptibility that could not be collected using the single magnet unmodified. The method may comprise gathering magnetically susceptible particles from viscous fluid, for example a highly viscous fluid. The method may comprise gathering the particles from digestate, for example digestate from biogas production. The method may comprise DNA extraction. The method may comprise gathering material from a sample in which the concentration of the material to be gathered is low. The method may comprise separating magnetically susceptible material from dry samples, from gas, and/or from solids.

According to a fourth aspect of the invention there is provided a method of manipulating magnetically susceptible material (such as particles) using the magnet assembly as recited herein with reference to the second aspect of the invention, comprising using the magnetic field of the magnet assembly to move the magnetically susceptible material from a first location to a second location. The magnetically susceptible material may be particles, such as nanoparticles.

According to another aspect of the invention there is provided a method of assembling a symmetric magnet device, comprising: selecting a single magnet to be a central magnet and orienting the magnetic polarity of the central magnet in a first direction; selecting a pair of magnets to be modifying magnets; arranging the pair of magnets so that their magnetic polarities are oriented symmetrically about a plane through the centre of the symmetric magnet device in the first direction; and affixing the pair of magnets to the central magnet to thereby increase a gradient of the magnetic field of the central magnet. The method may include increasing the gradient by at least five times or ten times compared to that of the central magnet. The method may comprises any and all of the features of the invention described herein with reference to the other aspects.

According to another aspect of the invention there is provided a symmetric magnet device comprising: a central magnet having a magnetic polarity oriented in a first direction; and a pair of modifying magnets fixed to the central magnet, wherein the modifying magnets are arranged with their magnetic polarities oriented symmetrically about a plane through the centre of the symmetric magnet device in the first direction; wherein the modifying magnets increase a gradient of the magnetic field of the central magnet. The modifying magnets may increase the gradient by at least ten times compared to that of the central magnet. The symmetric magnet device may comprise any and all of the features of the invention described herein with reference to the other aspects.

According to another aspect of the invention there is provided a method of modifying a magnetic field of a single magnet for separation of substances, comprising: selecting a single magnet having a magnetic field and a first magnetic polarisation direction; identifying a first gradient of the magnetic field to increase; identifying a second gradient of the magnetic field to increase, the second gradient being perpendicular to the first gradient; selecting a plurality of modifying magnets for modifying the magnetic field of the single magnet, each modifying magnet having a magnetic polarisation; determining an arrangement for the modifying magnets to increase the first and second gradients, the arrangement including an orientation for the magnetic polarisations of each of the modifying magnets in which the polarisations of each of the modifying magnets are oriented in the direction of the first gradient and in the direction of the second gradient; and affixing the plurality of modifying magnets to the single magnet in the determined arrangement to thereby increase the first gradient and the second gradient magnetic field. The method may comprises any and all of the features of the invention described herein with reference to the first and/or third aspects, or any of the other aspects.

According to another aspect of the invention there is provided a method of separation of substances (such as particles), comprising: modifying the magnetic field of a single magnet by attaching thereto a plurality of modifying magnets having magnetic polarisations oriented to increase the gradient of the magnetic field of the single magnet in a direction perpendicular to it surface, and to increase the gradient of the magnetic field of the single magnet in a direction parallel to it surface. The method may comprises any and all of the features of the invention described herein with reference to the first and/or third aspects, or any of the other aspects.

According to another aspect of the invention, there is provided a method of adapting or modifying the magnetic field of a single magnet by attaching thereto a plurality of modifying magnets.

Figures

Certain preferred embodiments of the invention will be described herein by way of example only and with reference to the Figures in which:

Figure 1A shows a side-and-top-view of a magnet assembly;

Figure 1 B shows an end view of the magnet assembly of Fig. 1 A;

Figure 1C shows a vertical cross-section through the centre of the magnet assembly of Fig. 1A;

Figure 1D shows a top view of the magnet assembly of Fig. 1A;

Figure 1E shown a bottom view of the magnet assembly of Fig. 1A;

Figure 2A shows a side-and-top-view of a magnet assembly;

Figure 2B shows an end view of the magnet assembly of Fig. 2A;

Figure 2C shows a vertical cross-section through the centre of the magnet assembly of Fig. 2A;

Figure 2D shows a top view of the magnet assembly of Fig. 2A;

Figure 2E shown a bottom view of the magnet assembly of Fig. 2A;

Figure 3A shows a side-and-top-view of a magnet assembly;

Figure 3B shows an end view of the magnet assembly of Fig. 3A;

Figure 3C shows a vertical cross-section through the centre of the magnet assembly of Fig. 3A;

Figure 3D shows a top view of the magnet assembly of Fig. 3A; and Figure 3E shown a bottom view of the magnet assembly of Fig. 3A.

Description Throughout the Figures, the orientation of magnetic polarisations of magnets is indicated by an angle in degrees. That angle is measured anticlockwise from the horizontal per normal convention for angle measurement. The angle of magnetic polarisations are shown only for magnets where the magnetic polarisation is in the plane of the paper on which the figure is drawn. The angular orientations for magnets at least partially in to or out of the plane of the paper are not shown.

Figure 1A shows a side-and-top view of a magnet assembly 100. The magnet assembly comprises a single magnet 110 arranged centrally within the magnet assembly 110 and providing the majority of the magnet assembly 100. That is, the single magnet 110 is the largest (e.g. by volume, mass, and/or surface area) magnet in the magnet assembly 100.

A first plurality of modifying magnets 120 are fixed to an upper surface of the single magnet 110. The first plurality of modifying magnets 120 are disposed in a row running along the entire length of the single magnet 110 in the middle of the surface. A second plurality of modifying magnets 130 are also fixed to the upper surface of the single magnet 110 adjacent the first plurality of modifying magnets 120.

Figure 1B shows an end view of the magnet assembly 100 of Fig. 1A. The single magnet 110 has a first magnetic polarisation oriented at 90° anticlockwise from the horizontal and hence substantially perpendicular to its upper surface. The two magnets 120 of the first plurality are visible at the ends of their respective rows.

The ends of the magnets 130 of the second plurality can also be seen, with their magnetic polarisations oriented at 0° (directly right in the plane of the Fig. 1B) and 180° (directly left in the plane of Fig. 1B). The magnetic polarisation of the magnets of the second plurality 130 are confined to a plane perpendicular to the first magnetic polarisation. The magnet assembly is symmetric (by reflection) about a plane into the paper and parallel with the first magnetic polarisation through the centre of the magnet assembly. The single magnet 110 spans the width of the magnet assembly.

Figure 1C shows a cross-section through the centre of the magnet assembly 100. The single magnet 110 includes a series of protrusions 112 disposed between the magnets 120 of the first plurality, those protrusions separating the magnets of the first plurality into groups of one or two magnets 120. Thus, the single magnet 110 comprises a plurality of recesses housing the first plurality of modifying magnet 120. The protrusions 112, together with the magnets 120 of the first plurality, also separate the magnets 130 of the second plurality.

The first plurality of modifying magnets 120 have magnetic polarisations oriented at least partially in a direction normal to the upper surface of the single magnet 110. The orientations alternate back and fore along the row. All of the magnets 120 of the first plurality have magnetic polarisations oriented at 45° to the first magnetic polarisation of the single magnet 110. However, the direction of their orientation with respect to the first magnetic polarisation alternates along the row. Therefore, the left-most magnet 120 of the first plurality has a magnetic polarisation oriented at 45° to the horizontal (and therefore also at 45° to the vertical), while the next left most magnet 120 of the first plurality has a magnetic polarisation oriented at 135° to the horizontal (and therefore also at 45° to the vertical in the other direction to the leftmost magnet 120). The magnetic polarisations of the first plurality of modifying magnets 120 are confined to a vertical plane, i.e. a plane parallel to the first magnetic polarisation direction. The magnet assembly is symmetric (by reflection) in a plane parallel to the first magnetic polarisation direction through the centre of the magnet assembly and into the paper.

Half of the first plurality of modifying magnets 120 are disposed on a second surface of the single magnet 110, the second surface being opposite the first surface of the single magnet 110 and substantially orthogonal to the first magnetic polarisation. Their orientations mirror those of the magnets on the first surface but with polarities reversed.

Figures 1 D and 1 E show a top view and a bottom view of the magnet assembly.

The second plurality of modifying magnets 130 have the magnetic polarisation directions shown in Fig. 1B, 0° and 180° to the horizontal. The magnetic polarisations of the second plurality of modifying magnets are confined to a plane perpendicular to the first magnetic polarisation direction. ln the embodiment of Fig. 1, the first plurality of modifying magnets 120 are oriented partially in an upward direction (i.e. the direction of the first magnetic polarisation). The magnetic fields of those magnets 120 interact with the magnetic field of the single magnet 110 to modify its gradient in the upward direction (e.g. the z- direction) and increase it. The magnetic polarisations of the first plurality of magnets 120 are also oriented partially in a direction parallel to the surface of the single magnet (e.g. the x-direction). The magnetic polarisations of the second plurality of magnet 130 are oriented orthogonally to the magnetic polarisations of the first plurality of modifying magnets (e.g. in the y-direction). Their magnetic fields interact with the magnetic field of the single magnet 100 to increase the gradient in a direction perpendicular to the plane in which are confined the magnetic polarisations of the first plurality of modifying magnets.

Figure 2A shows another embodiment of a magnet assembly 100 comprising a single magnet 110 in the centre of the magnet assembly 100. A pair of modifying magnets 120 are disposed on each of a first (e.g. top) and an opposed second (e.g. bottom) surface of the single magnet 110. Magnetically susceptible elements 140 are provided in the form of strips and disposed between the pairs of modifying magnets 120 on each of the first and second surfaces of the single magnet 110.

Figure 2B shows an end view of the magnet assembly 100 of Fig. 2A. As with the magnet assembly of Fig. 1, the ends of the single magnet 110 are exposed and no modifying magnets are attached thereto. The first magnetic polarisation direction of the single magnet 110 is at 90°, and the pairs of modifying magnets 120 on the first and second surfaces have magnetic polarisations at 0° and 180°. The single magnet 110 spans the width of the magnet assembly, and the magnet assembly is symmetrical in a vertical plane through its centre (per the orientation of Fig. 2B).

Figure 2C shows a cross-section through the middle of the magnet assembly 100, wherein the single magnet 100 and magnetically susceptible elements 140 are visible. The magnetically susceptible elements 140 span the entire first and second surfaces of the single magnet to which they are attached. The single magnet spans the length of the magnet assembly, and the magnet assembly is symmetrical about a vertical plane through its centre (in the orientation of Fig. 2C). Figures 2D and 2E show top and bottom views of the magnet assembly 100 respectively. The magnetically susceptible elements 140 are shown, and the pairs of modifying magnets 120 are shown having magnetic polarisation directions of 0° and 180°.

Unlike the magnet assembly 100 of Fig. 1, the modifying magnets 120 of the magnet assembly 100 of Fig. 2 are confined to the plane perpendicular to the first magnetic polarisation. The pairs of magnets 120 therefore increase the gradient in the plane.

Figure 3A shows another magnet assembly 100 comprising a single magnet 110 arranged in the centre of the magnet assembly 100. The magnet assembly is symmetric in two planes like the magnet assembly of Figs. 1 and 2. A first plurality of modifying magnets 120 are provided on each of a first (e.g. top) and second (e.g. bottom) surface of the single magnet 110. In the magnet assembly shown in Fig. 3A, the first plurality of modifying magnets 120 are a pair of magnetic strips spanning the surface to which they are attached (i.e. extending the whole length from one side of the surface to the other). A second plurality of modifying magnets 130 is also provided, also in strips spanning the length of the surface to which they are attached.

Figure 3B shows and end view of the magnet assembly. As with the magnet assemblies 100 in Figs. 1 and 2, the ends of the single magnet 110 are exposed. The first magnetic polarisation of the single magnet is oriented at 90°. The orientations of the magnetic polarisations of the first and second pluralities of modifying magnets 120, 130 are also shown. The first plurality of magnets 120 have orientations of 45° and 135° on the upper (first) surface, and orientations of 135° and 45° on the lower (second) surface. As such, the magnetic fields of those magnets 120 interact with the magnetic field of the single magnet 110 to increase its gradient in the direction of the first magnetic polarisation.

The second plurality of modifying magnets 130 in this case do not have magnetic polarisations oriented orthogonally to those of the first plurality of modifying magnets 120. The magnets 130 increase the gradient in a plane orthogonal to the direction of the first magnetic polarisation of the single magnet 110.