GB2452474A

GB2452474A - Magnetic rotation sensor

Info

Publication number: GB2452474A
Application number: GB0712842A
Authority: GB
Inventors: Daniel A Allwood; Michael Richard John Gibbs
Original assignee: University of Sheffield
Current assignee: University of Sheffield
Priority date: 2007-07-03
Filing date: 2007-07-03
Publication date: 2009-03-11
Also published as: WO2009004293A2; GB0712842D0; WO2009004293A3

Abstract

A magnetic rotation sensor (20) comprises a first sensing layer (22) (which may comprise a magnetic nanowire (10)) within which a domain wall (12) separates regions of different magnetic orientation (ML, MR), the location of the domain wall being arranged to change when a magnetic field (H) rotates relative to the layer (22), and electronic measuring means arranged to measure the location of the domain wall (12, 42) electronically, and wherein the amount of relative rotation between the sensor (20) and the magnetic field (H) can be determined from the location of the domain wall (12, 42). The domain wall (12, 42) may be created at a nucleation zone in the form of an injection pad (36). The electronic measuring may comprise measuring resistance between electrical contacts (40, 42); the anisotropic magnetoresistance (AMR) will increase with the presence of more domain walls (12, 42) in the first, second, third and fourth portions (24, 26, 28, 30) of the first sensing layer (22). Movement of a domain wall (12, 42) over four portions (24, 26, 28, 30) corresponds to one complete turn of relative rotation (360{) of the magnetic field (H); so 4n portions are required to measure n turns. The invention has application in measuring the rotation of a shaft, particularly a vehicle steering wheel shaft.

Description

MAGNETIC ROTATION SENSOR, SYSTEM AND METHOD This invention relates to magnetic rotation sensors, systems and methods, particularly but not exclusively, for sensing shaft rotation.

Magnetic nanowires are wires having a very small thickness and width and which can be magnetised. For most applications where they are used, the shape and material chosen for magnetic nanowires is important. An example of a material suitable for a magnetic nanowire is NiKIFeI9 -this is a material from the generic Permalloy family. This material has very low magnetostriction and magnetocrystalline anisotropy. As a result the magnetisation within a NiFe19 nanowire is dominated by the wire geometry and is not influenced by intrinsic material properties. Other materials can also be used for the construction of nanowires.

These wires are generally planar, usually between 3 nm and 40 nm thick; between 50 nm and 1000 nm wide; and preferably at least 500 nm long.

These dimensions are small enough to force magnetisation to lie along the wire length in one dimension only (the wire effectively comprises one magnetisation layer). if the wire is significantly thicker, wider or shorter than this, then regions of the wire having differing magnetisations (i.e. stable magnetic regions pointing in different directions known as domains) may form adjacent to each other across the thickness or width of the wire. For the purpose of this invention, it will be appreciated that any configuration (whether in a nanowire or not) which comprises a layer capable of being magnetised in one dimension only may be utilised.

When the layer is capable of being magnetised in one dimension only, there are two possible (opposite) orientations of magnetisation in a simple, straight wire. The NLFe19 wire described above is considered generally planar due to its small thickness. Such a wire has a generally rectangular cross section across its length. Other geometries are possible to support the requirements of one dimensional magnetisation. For example, there is some research into substantially circular, trapezoidal or parallelogram cross-section nanowires which support this property.

Referring to figures Ia to Ic, a wire 10 supports magnetisation in One dimension only, i.e. along its length. Figure Ia shows the wire in a first condition in which, on its left hand side there is a region of magnetisation (also known as a domain) in which the magnelisation direction is indicated by the arrow ML and on its right hand side the magnetisation direction is represented by the arrow, MR. The two magrieisaEior1 directions M, and MR are opposed to each other. Between them, where the opposite magnetisation directions (domains) meet there is a transition region, called a domain wall. A domain wall separates regions of differing magnetisation. In a one-dimensional magnetisation system as discussed above, if a component of magnetic field is applied in the single magnetisation dimension of the wire then the position of a domain wall within the wire can be caused to change. In figure Ia, the domain wall 12 which separates the different magnetisation regions (or domains) is represented as a vertical wall at 90° to the different magnetisation directions. In practice, individual magnetic moments in the proximity of the domain wall 12 gradually re-orientate on both the left hand side and the right hand side of the wall 12. These types of domain walls are known as transverse domain walls. The invention is also applicable, in other embodiments, using vortex domain walls, in which magnetic moments rotate 360° about a central core position.

As discussed above, magnetic nanowires can be arranged to support one dimensional magnetisation by sufficiently restricting the space across the thickness, or the width of the wire in which further domain walls could be created. In other systems, a larger wire may be provided which supports magnetisation in one dimension only for use with the present invention.

Referring to figure 1 b, if a magnetic field (represented by the arrow H) is applied in a particular direction having a component parallel to the length of the wire, the domain wall 12 moves such that the domain with magnetisation M, which is parallel with the applied magnetic field, H, is enlarged and the domain, MR, which is anti-parallel with the applied field, H, is made smaller.

Figure Ic schematically shows the effect of applying a magnetic field, H, in an opposite direction to that of figure lb. The domain wall 12 moves in the opposite direction so that domain, MR, grows bigger and domain, ML, grows smaller.

A simple, straight magnetic wire with no domain walls and continuous unidirectional magnetisation throughout can undergo magnetisation reversal so that unidirectional magnetisation in an opposed direction results when the wire is subject to an applied magnetic field. With sufficient field strength, a domain wall can be nucleated at one or other of the ends of the wire before propagating rapidly through the wire.

For wires that are substantially identical, it is difficult to predict which end will have the lower nucleation field (i.e. at which end the domain wall will be created) since this is dependent upon random, nanoscale defects generated during fabrication of the wire. In order to provide a region in which domain walls can be reliably nucleated, a relatively large area of magnetic material can be fabricated on the end of a magnetic wire. This large area (e.g. in the form of a square, rectangle, ellipse, circle or any other suitable shape) will undergo magnetisation reversal at a much lower field than an isolated wire. Such a large area of material provides a nucleation zone and may be known as an injection pad since domain walls are injected from this pad into a magnetic wire.

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Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which; Figures la to Ic schematically illustrate movement of a domain wall along a wire layer in a one dimensional magnetisation system; Figure 2 is a schematic representation of part of a magnetic rotation sensor according to this invention; Figure 3 illustrates a cusp in the sensing layer; Figure 4 illustrates the passage of a domain wall through the cusp of

figure 3 in a rotating magnetic field:

Figure 5 illustrates alternative sensing layer cusp geometry; Figures 6a and 6b show sensors according to different embodiments of the invention; Figures 7 to 9 illustrate parts of sensors according to different embodiments of this invention; Figure 10 illustrates a voltage divider circuit formed by sensors of this invention; Figures ha to lic illustrate different sensing layer geometries according to this invention; Figure 12 is a schematic representation of part of a magnetic rotation sensor according to another embodiment;

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Figure 1 3 is a schematic representation of part of a niagnetic rotation sensor according to another embodiment: and Figure 14 is a schematic representation of the region around a stub of a magnetic rotation sensor according to another embodiment.

According to an embodiment of this invention there is provided a magnetic rotation sensor 20 for sensing the amount of rotation between itself and a magnetic field which rotates in a particular direction relative to it. Referring to figure 2, in this embodiment the sensor 20 comprises a magnetic nanowire of the type previously described. The nanowire has a first sensing layer 22 within which a domain wall can separate regions of different magnetic orientation in one dimension. As previously discussed the sensing layer 22 is arranged to allow stable magnetisation directions only parallel or anti-parallel to the direction of the wire. Domain walls cannot form across the width or thickness of the wire. In this embodiment the one dimension is the dimension parallel to the wire direction, i.e. it follows the curvature of the wire -the one dimension is not necessarily straight. In this embodiment this is achieved by having a NiHFe) nanowire, which is 5 nm thick and 200 nm wide. In this embodiment the first sensing layer has a substantially rectangular cross section across its length.

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In other embodiments the first sensing layer may comprise a different profile -for example it may be circular, or have an irregular shape, or any other regular shape. As discussed above, a domain wall present within the sensing layer 22 will move along the sensing layer if a magnetic field of sufficient magnitude rotates relative to it in the region of the wire and with a varying component in the plane of the wire. In this embodiment the magnetic field is between 800 Am' and 4OkAm. In

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other embodiments magnetic field magnitude may be between 8OAm' and 400kAm'. For example the magnetic field is about 4kAm' or may be

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about 8kAm.

In this embodiment the first sensing layer 22 comprises a regular series of first, second, third and forth portions 24, 26, 28, 30 respectively.

Between adjacent portions there are wire cusps 25, 27, 29. At a first end 34 of the sensing layer 22, a nucleation zone in the form of an injection pad 36 is attached such that it can communicate with the first portion 24 of the sensing layer 22. The wire cusps 25, 27, 29 are formed integrally at the sensing layer 22 and figure 3 shows the geometry of one of the cusps 25 of this embodiment in more detail. The integrally formed junctions provide regions at which a rotating magnetic field is required for a domain wall to pass through due to the physical change in direction of the wire. In this specification the term junction' is intended to mean any geometrical feature, such as a discontinuity, in the sensing layer. It does not indicate a physical break in the material of the sensing layer (although in some embodiments, there may be such a physical break). As described above the one-dimensional magnetisation follows the curvature of the wire -therefore the magnetisation direction follows the curvature of the wire cusps at the wire junctions.

The cusp 25 comprises converging branches leading from the first portion 24 and the second portion 26 of the first sensing layer 22. In this embodiment the radius of curvature of the converging branches is about lllm. In other embodiments the radius of curvature may be between lOOnm and 100.tm. Beyond a point at which the converging branches of the portions 24, 26 meet, the sensing layer 22 extends into an elongated stub 40. The width of the stub is about 200 nm. At a free end of the stub, the layer 22 narrows to a point, having an angle of about 60°.

Referring to figures 4a to 4d, the passage of a domain wall 42 through the first cusp 28 under the influence of a rotating magnetic field, H, is schematically illustrated. The applied magnetic field direction (assuming it is large enough in magnitude) determines the location of the domain wall within the wire. It will be appreciated that since regions to either side of a domain wall are aligned antiparallel to each other, the lowest energy state towards which the system moves is provided when the antiparallel magnetisation orientations (of the two distinct domains) are both orientated substantially perpendicularly to the applied magnetic field direction. In the Figure 4 example, the domain wall is substantially parallel to the applied field direction. As the applied field is rotated, the lowest energy configuration of the system changes and so the domain wall moves through the wire.

Referring to Figure 4a, as the magnetic field, H, is rotated anticlockwise the domain wall moves from the first portion 24 of the layer 22 towards the stub 40 until it reaches the junction point as shown in figure 4b. As the magnetic field is rotated further anticlockwise, the domain wall 42 is forced through the junction into the position shown in figure 4c, i.e. into the branch of the second portion 26 leading from the junction. As the magnetic field, H, is rotated further, the domain wall 42 moves away from the junction through the second portion 26 of the layer 22. :3

The amount of rotation of magnetic field, H, required to move the domain wall 42 through the junction from the first portion 24 to the second portion 26 is dependent upon the geometry of the first cusp 25. In this example, in order to move the domain wall 42 from a position X represented by a dotted line in figure 2, to a position Y represented by a dotted line in figure 2, where X and Y are corresponding positions within the first portion 24 and the second portion 26 of the layer 22 respectively, a magnetic field rotation of 1800 in the plane of the sensing layer 22 is required.

In more detail, when the domain wall is moving between the position shown in figure 4b and the position shown in figure 4c, in order to move the domain wall from the branch of the first portion 24 to the branch of the second portion 26, the magnetic field direction vector H actually needs to rotate beyond the vertical by an angle, 0. In fact, the cusp geometry and magnetic field magnitude determines how large the angle 0 is in practice. Therefore switching from the first portion to the second portion will actually take place when the magnetic field vector has rotated past the vertical direction by an angle of 0°. In general 0 will be larger in applied fields of smaller magnitudes, with all other factors being constant. For continuous field rotation in a single direction (e.g. clockwise or anticlockwise) the magnetic field rotation required for a domain wall to pass through the wire junction will be 180° as described above. When the field rotation direction changes however, the sense of 0 reverses such that the magnetic field vector has to reach the other side of the vertical direction relative to previously. In other words, without any previous knowledge of field rotation, a rotation of 180° � 20 may be required in order to be sure that a domain wall has passed back from the second portion 26 to the first portion 24 taking into account the extra amount of rotation which is necessary in order to force the domain wall through the junction. In some embodiments counting integer numbers of field rotations can be accomplished by combining the information from a high resolution, single turn angle sensor with the sensor of this invention.

Figure 5 shows an alternative geometry for a cusp which can be used in an alternative embodiment. If a sensing layer were to be formed from a series of cusps as shown in figure 5, a magnetic field rotation of 180° would be required in order to move a domain wall from a corresponding position between cusps.

Referring to figure 2, in one embodiment of this invention the sensing layer 22 is initially devoid of domain walls. A magnetic field is applied and rotated clockwise until a domain wall is positioned in the first portion 24 of the layer 22. The domain wall is created at the injection pad 36 and moves through the first portion 24. As the magnetic field is rotated a further 1800 clockwise, this domain wall will move through the first cusp 25 and into the second portion 26 of the layer 22 as previously described.

For simplicity it is assumed that an idealised situation exists in which 0 = 0. At the same time, a further domain wall will be created at the injection pad 36 and will move into the first portion 24 of the layer 22.

Therefore there will be a domain wall present within both the first portion 24 and second portion 26 of the sensing layer 22. The third portion 28 and the fourth portion 30 will not have a domain wall within them. If the applied magnetic field is now rotated in an anticlockwise direction, the domain walls will move back in the directions from which they came.

The domain wall in the first portion 24 will move back towards the injection pad 36 and eventually be absorbed by it. The domain wall in ihe second portion 26 will move back towards the first cusp 25 and if there is enough field rotation, it will pass beyond the cusp 25 and back into the first portion 24 of the layer 22.

If we consider once again the configuration in which a domain wall is

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present in the first portion 24 and the second portion 26, but not in the third portion 28 or the fourth portion 30 of the layer 22, it will be appreciated that from this position, if the magnetic field is rotated clockwise by 180°, the first three portions 24, 26, 28 will each include a domain wall. If the magnetic field is rotated clockwise by a further 180° then all four portions will include a domain wall. If there is a rotation of

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a further 1800 clockwise then the layer 22 will still only include four domain walls in total -the lowermost domain wall will annihilate at the end of the layer 22 and no longer exist. Similarly, if, from the starting position, there is a rotation of 360° anticlockwise and both domain walls are absorbed by the injection pad 36, any further anticlockwise magnetic field rotation will not change the total number of the domain walls within the layer 22 (since there will be no domain walls within the layer 22 to be further absorbed by the injection pad 36).

This invention provides an electronic measuring means which is arranged to electronically measure the location of a domain wall within the sensing layer 22 and to obtain information regarding the relative rotation between the layer 22 and the applied magnetic field by measuring the location of the domain wall within the layer 22. It will therefore be appreciated that using the sensing layer 22 shown in figure 2 initialised with two domain walls in the first 24 and second 26 portions of the layer 22, it is possible to measure a maximum relative rotation of � 360° between the layer 22 and the applied magnetic field by determining the location of the lowermost domain wall in the system. The lowermost domain wall is the one that is furthest from the injection pad 36. In other embodiments if it is desired to measure a greater degree of relative rotation then, assuming similar 180° junctions are provided between portions, 4n portions should be provided in series in order to measure �n turns, i.e. 4 portions are required to measure � 1 turn (i.e. �360°); 8 portions are required to measure �2 turns; 40 portions are required to measure � 10 turns.

Referring to figure 6a, in this embodiment the electrical properties of a domain wall are used to determine its location. The magnetic rotation sensor 20 comprises electronic measuring means in the form of a first electronic contact 40 which contacts the layer 22 between its first portion 24 and the injection pad 36, and a second electrical contact 42 which

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contacts the layer 22 towards a free end beyond the fourth portion 30.

The electronic measuring means is arranged to measure the resistance between the first 40 and second 42 electrical contacts using standard circuitry. The sensing layer 22 will have an intrinsic resistance. The presence of a domain wall within the layer 22 increases the electrical resistance which is measured between the first 40 and second 42 electrical contacts. Accordingly the presence of more domain walls in any of the first, second, third or fourth portions 24, 26, 28, 30 increases the electrical resistance between the electrical contacts 40, 42. The electrical contacts are used to measure anisotropic magnetoresistance (AMR) across a portion of the first sensing layer 22. In this embodiment the portion across which the AMR is measured is substantially the whole of the first sensing layer. In order to achieve this, the electrical contacts are placed at different points along the same layer 22.

If there is only one domain wall in the system (in the first portion 24) then the measured AMR will be less than if there are two domain walls in the system (i.e. in first 24 and second 26 portions) and this, in turn, will be less than the measured AMR if there are three domain walls present in the first 24, second 26 and third 28 portions, which will be less than the measured resistance if there are four domain walls present in the first 24, second 26, third 28 and fourth 30 portions. Accordingly, there will be four distinct resistance measurement states and from these it is possible to

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distinguish the number of domain walls within the layer 22 at any one time. Assuming there has only been a rotation of � n turns from the starting position (with n = I in this embodiment), then it is possible to determine the location of the lowermost domain wall from the measured AMR. As a result it is possible to determine the amount of relative rotation between the sensing layer 22 and the applied magnetic field relative to the starting position since the amount of movement of the domain wall is indicative of the amount of relative rotation.

Referring to figure 6b, in a further embodiment, electrical contacts 50, 52, 54, 56 and 58 are provided at each cusp, i.e. between each portion of the sensing layer 22. In this embodiment it is possible to measure the AMR for each distinct portion 24, 26, 28, 30 and determine whether or not a domain wall is present within that portion. The skilled person will appreciate that in this embodiment the passage of one particular domain wall can be tracked separately from the passage of other domain walls -it is not necessary to determine the total number of domain walls within the layer 22 in order to determine the location of the lowermost (or any other) domain wall. Therefore in this embodiment it. may be that the sensing layer 22 is initialised by having a domain wall in the second portion 26 only (with the first portion 24, the third portion 28 and the fourth portion 30 being void of domain walls). Then a field rotation of 180° anticlockwise can be detected by detecting the presence of the domain wall in the first portion 24: a rotation of 360° anticlockwisc can be detected by detecting the absence of a domain wall anywhere in the layer 22; a 180° clockwise field rotation can be detected by detecting the presence of the domain wall in the third portion 28; and a 360° clockwise field rotation can be detected by detecting the presence of the domain wall in the fourth portion 30. When the field is rotated 180° or 360° clockwise, the injection pad 36 will inject further domain walls into the layer 22. However the position of these further layers will not be required to be determined in order to measure the relative rotation, since this can be determined solely from the location of the lower most domain wall in this embodiment.

It is possible to obtain an initialised configuration in which there is only one domain wall present in the second portion 26 by injecting a domain wall at one of the intermediate cusps 25, 27, 29 during manufacture.

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In some embodiments in which there is no domain wall injection pad, this initialised configuration can be obtained by saturating and relaxing the wire chain followed by sufficient field rotation in one direction to remove all but one domain wall and then reversing the field rotation sufficiently to position the solitary domain wall in the second portion of the wire chain. This is achieved by applying a large magnetic field in a horizontal direction (i.e. parallel to the stub direction in this embodiment) to saturate the magnetisation at all places in the layer 22 (i.e. the field is large enough to force magnetisation everywhere to point in the direction of the field, regardless of the local wire geometry). Next this saturating field is removed. Upon the saturating field being removed, the magnetisation structure will relax to leave a domain wall at every curve apex (i.e. at the centres of the portions 24, 26, 28, 30). The magnetic field is then rotated in one direction only so that all but one of the douldin walls is removed from the layer. The magnetic field rotation direction is then rotated in the opposite direction a certain number of turns in order to achieve the desired initial configuration, i.e. until the solitary domain wall is present in the second portion in this example.

The electrical contacts 52, 54, 56, 58 at the cusps 25, 27, 29 of the layer 22 are attached to the stubs 40 of the cusps. Advantageously this provides efficient and reliable measurement of AMR since domain walls do not settle immediately adjacent the electrical contacts. For example if electrical contacts were to be provided within branches of the portions of the layer 22 and a domain wall were to settle directly above or below or alongside the electrical contact then an inconsistent or erroneous AMR measurement may be obtained since the domain wall may not be entirely on one or other side of the electrical contact relative to the other electrical contact between which resistance is being measured.

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In other embodiments each electrical contact may be replaced by a pair of electrical contacts -one for applying current and the other for measuring potential difference.

In one embodiment the sensor 20 is attached to a first object which is a frame within which a second object, in the form of a shaft, is arranged to -rotate and a permanent magnet is attached to the shaft. The permanent magnet provides a magnetic field of sufficient strength to drive a domain wall through the sensor 20. The shaft rotating within the frame is, in this embodiment, a vehicle steering rotation shaft in communication with a vehicle steering wheel and the frame is part of the frame of the vehicle.

The rotation sensor 20 therefore provides information regarding the amount of rotation of the steering wheel relative to the vehicle frame. In this embodiment the shaft rotation sensor is required to provide information on the incremental movement of the steering wheel relative to the frame, i.e. in 1800 increments. The sensor measures half turns of the steering wheel relative to the frame. It will be appreciated that, if necessary, more sensitive measurements may be provided by measuring the location of the domain wall more precisely. For example, this may be achieved by having electrical contacts at various positions throughout each portion of the sensing layer.

Referring to figure 7, the magnetic rotation sensor 20 comprises the sensing layer 22 and injection pad 36 as previously described in conjunction with a temperature compensator in the form of a first compensation layer 60. The first compensation layer 60 corresponds in shape, profile and its material properties (including composition) to the first sensing layer 22. The first compensation layer 60 does not have an injection pad attached to it and it also does not have any domain walls within it. Any rotating magnetic field within the plane of the compensation layer 60 does not cause the nucleation of any domain walls and so no domain walls ever move through the compensation layer 60.

Resistance, AMR, is measured across corresponding portions of the compensation layer 60 as across the portions of the first sensing layer 22.

Therefore if the AMR is measured across the entire sensing layer 22 then it is correspondingly measured across the entire compensation layer 60.

If the AMR is measured across each portion of the sensing layer 22 then it is measured across each portion of the compensation layer 60. The measured AMR values for the compensation layer 60 may vary with temperature and magnetic field. The magnetic field may be the applied magnetic field and any stray or background magnetic field which might be present. Differences in measured AMR values in the compensation layer can be used as a guide to how any differences in measured AMR values within the sensing layer 22 might be attributed to temperature variation or magnetic field (instead of the presence or absence or movement of a domain wall). Advantageously, a sensor is provided which will work across a wider range of temperatures.

Referring to figure 8, a sensor according to a further embodiment is shown. The sensor includes the sensing layer 22 and injection pad 36 of the previously described embodiment along with a sensitivity enhancer in the form of a second sensing layer 22' (which is substantially identical in form to the first sensing layer 22) and an injection pad 36' which is provided at an end of the sensing layer 22'. The sensing layers 22 and 22' are aligned with each other and in proximity to each other such that a rotating magnetic field in the plane of the layers 22, 22' will move any domain wall within the layers substantially equally. The injection pad 36 is provided at a first, top end of the sensing layer 22. The injection pad 36' is provided at a second, bottom end (opposed to the top end) of the sensing layer 22'. As can be clearly seen from figure 8, a clockwise rotation of a magnetic field in the plane of the layers 22, 22' will cause an additional domain wall to be injected into the sensing layer 22 (and r accordingly the resistance across the layer will increase), whilst the same rotation will cause the removal of a domain wall from the sensing layer 22' by means of absorption into the injection pad 36' and a decrease in the resistance across the entire layer 22'. By comparing measured AMR values from corresponding portions of the first sensing layer 22 and the second sensing layer 22' the relative rotation between the sensing layers 22, 22' and any applied rotating magnetic field can be measured more sensitively. In one embodiment, the sensor shown in figure 8 is required to be initialised by having domain walls present in the two portions 24, 26, 28', 30' closest to each injection pad 36, 36'. In order to achieve this H a large magnetic field is applied in a horizontal direction (i.e. parallel to the stub direction in this embodiment) to saturate the magnetisation at all places in the layers 22. 22' (i.e. the field is large enough to force magnetisation everywhere to point in the direction of the field, regardless of the local wire geometry). Next this saturating field is removed. Upon the saturating field being removed, the magnetisation structure will relax to leave a domain wall at every curve apex (i.e. in this embodiment at the centres of the portions 24, 26, 28, 30, 24', 26', 28', 30' in Figure 8) in both layers. The magnetic field is then rotated in one direction only so that all domain walls are removed from one of the layers (22, say).

During this rotation, the number of domain walls in layer 22' will remain constant since any domain walls lost from the end of the layer at position 31' are replenished by input from the injection pad 36'. The magnetic

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field rotation direction is then rotated in the opposite direction a certain number of turns in order to half-fill layer 22 and half empty layer 22' (in this embodiment it is rotated one whole turn to achieve the desired initial domain wall configuration).

In another embodiment, this initial configuration can be achieved by providing a magnetic field rotating in a first direction until one of the layers 22, 22' has a domain wall in every portion and the other layer 22', 22 has no domain walls in any of its portions and then following the subsequent procedure as described above, i.e. rotating the magnetic filed to remove two domain walls from the said one of the layers and introduce two domain walls to the said other layer.

Referring to figure 9 in a further embodiment a sensor 90 includes the first sensing layer 22, the second sensing layer 22' and the temperature compensator includes the first compensation layer 60 and a second compensation layer 62 for providing independent temperature compensation for measured AMR values from the second sensing layer 22'. In this embodiment the temperature compensator also provides compensation for changes due to applied field magnitude. These applied field changes are in the form of a sinusoidal modulation due to the rotation of the magnetic field. The applied field compensator allows for this modulation and therefore allows more convenient results processing.

The initialisation sequence for the sensor 90 is similar to that for the Figure 8 embodiment. The layers are rotated so that only layer 22 (or 22') is filled with domain walls while all other layers remain empty.

Alternatively, all layers are filled with domain walls by saturating with magnetic field and then the compensation layers and either layer 22 or layer 22' are emptied of domain walls in the first set of field rotations. ] In either case, the field rotation is reversed to half-fill the sensing layers and the compensation layers will remain empty due to the absence of an injection pad. The resistance can then be measured by placing the four layers in a simple electronic bridge arrangement.

In another embodiment, a compensating arrangement using two sensors 22 and two sensors 22' may be provided. This will have the advantage of retaining the temperature/field compensation feature whilst also maintaining a greater signal level. The system incorporating the first and second compensation layers 60, 62 will provide a relatively weaker

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signal. Therefore in a further embodiment, where two sensors 22 and two sensors 22' are provided, referring to Figure 10, a voltage divider circuit is formed, in which V and V are supply voltages. V is the measured potential difference. Each combination of layers 22 and 22' creates a voltage divider. Because the layers of the same type are used on opposite parts of each voltage divider, when the potential of one voltage divider increases, the other will have decreased. This maintains the signal that would ordinarily be measured from a single layer 22 but with

temperature and field compensation.

In yet further embodiments further pairs of first and second sensing layers may be provided to provide more sensitivity. Further temperature compensating layers may be provided.

The present electronic measuring system provides a compact magnetic rotation sensor and in some embodiments the rotation sensor may be provided as an integrated device. The rotation sensor may be provided integrally with an object or surface with which it is to be fixed in use.

Figures ha to lie show sensing layer geometries which may be used in alternative embodiments. In the sensing layer of figure 1 la, a relative field rotation of 90° is required to move a domain wall between junctions.

In the geometry shown in figure 1 lb a field rotation of 270° is required to move a domain wall between junctions. In the sensing layer geometry of figure lie a 3600 relative field rotation is required to move a domain wall between junctions. It will be appreciated that other geometries may also be provided as required. The rotation sensor of this invention is particularly useful in providing an indication of specific increments of relative rotation -for example, it may be desired to keep count of the number of whole or half rotations of a vehicle steering column relative to a vehicle frame. Other geometries will be apparent to the skilled person dependent upon the desired field rotation per junction. For example, the desired field rotation per junction maybe between 100 and 360°.

In other embodiments the magnetic field applier comprises a magnetic excitation coil or coils, an electromagnet or any other suitable field source.

In many of the embodiments described above the layer portions are arranged substantially linearly. It will be appreciated that this is not an essential feature of this invention -see for example Figure lic, which shows a non-linear arrangement of portions and junctions. Having a sensor according to this invention with the junctions arranged substantially linearly may provide some advantage since the electronic measurement points can be arranged substantially linearly and thus be provided in a simpler, more efficient form. For example if junctions are not linearly arranged, there may be a measurement point with which electrical contact needs to be made at an inner coil of a spiral (e.g. in the Figure 1 Ic embodiment) -the electrical contact may extend from an outer part of the spiral (e.g. if it is elongated or generally rectangular) and thus might be required to avoid contact with other (e.g. outer) parts of the spiral before eventually contacting the inner spiral part. This may increase the difficulty of manufacture of a non-linear' sensor relative to a linear' sensor. Also, if contact is made with an unnecessary' coil, it may need to be accounted for subsequently -this may lead to increased complexity in calculations and in processing of measured results.

In some embodiments the resistance measuring means is arranged to measure giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) instead of or in addition to anisotropic magnetoresistance (AMR).

Anisotropic magnetoresistance is measured by taking a resistance measurement between two points within a single sensing layer.

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GMR or TMR measurements require GMR/TMR detectors having three layers. In a GMR detector, the first layer is a layer in which the magnetisation direction can change (i.e. with the rotating magnetic field).

A second layer is a nonmagnetic conducting layer and is used to separate the first layer from a third layer which is a magnetic layer. The third layer has a magnetic field which undergoes magnetic reversal at higher fields than the first layer. Often, the third layer is effectively fixed in its orientation. In the case of TMR detectors, there are also three layers with the middle, second layer being an insulating layer.

A GMR/TMR resistance measurement is made by measuring resistance within a GMR/TMR detector placed spaced from the region of the sensing layer at which a measurement is to be made (in this embodiment this = location is the stub of the sensing layer). In some embodiments a GMR -measurement is obtained by measuring GMR between two points on the same detector layer, for example a few micrometres apart. If the magnetic orientation of the first detector layer is aligned with the orientation of the third detector layer then the resistance measurement will be different to the case where the magnetic orientation of the first detector layer is not aligned with the fixed magnetisation direction on the third detector layer. Methods of obtaining GMR and TMR measurements are known.

Referring to figures 12 to 14, in other embodiments, the magnetic rotation sensor comprises the sensing layer shown in figure 2, having first 25, second 27, third 29 cusps. In the region of each cusp beyond its free end, there will be a stray magnetic field which can be used to indicate the presence or otherwise of a domain wall between the cusp and an adjacent cusp (e.g. between cusps 25 and 27 in figure 11). Referring to Figure 12, stray field detectors 112, 114, 116. 118 are located close (ideally < nm but could be < 1 tm) to each cusp in the sensor shown in figure 12.

In some embodiments the stray field sensors 112, 114, 116, 118 are GIVIR or TMR elements, which are used to measure the stray field. in some embodiments the Orientation of the stray field is measured in order to determine whether or not it is the same as the orientation of the stray field of an adjacent cusp. In the embodiment shown in figure 12, the cusps require 180° field rotation in order for a domain wall to pass through them. If adjacent stray field detectors (e.g. 112, 114) detect stray fields having the same magnetisation orientation, this is indicative of a domain wall being present in the second portion between the cusps 25, 27.

If the stray field detectors 112, 114 detect stray fields at the cusps 25, 27 which have opposite magnetisation orientations then this is indicative of there being no domain wall between the cusps 25 and 27 (in the second portion). Therefore it may not be necessary to measure the magnitude of the stray field but simply its orientation at one cusp relative to its orientation at an adjacent cusp in order to determine whether or nor the portion between the cusps contains a domain wall or not. a

Referring to Figure 13, in another embodiment a single stray field detector 122 is provided spaced from, but sufficiently close to be in communication with each stub to measure its stray field. A single value (GMR or TMR) would be obtained from this detector 122 from which

individual stray field readings could be inferred.

In another embodiment the stray field sensors operate based upon the ordinary Hall effect. The ordinary Hall effect is due to the force experienced by a moving charged particle due to a magnetic field. In this case, the charged particles are electrons moving to create an electrical current in a nonmagnetic wire. The stray field sensor comprises a Hall sensor placed close to the end of each stub in order to sense the stray magnetic field emanating from the stub. The electrons (and hence, current) will flow in an in-plane direction perpendicular to the stubs (although configurations using other in-plane or out-of-plane current directions could be used). The sense (orientation) of the magnetic field from the stub will depend upon the direction of magnetisation in the stub.

For magnetic field of one sense, the electrons will experience a force moving them towards the upper surface of the conducting wire. This creates a charge imbalance between the upper and lower wire surfaces, which can be measured as a potential difference (in Volts). When the sense of magnetic field is reversed, the electrons experience a force directed towards the lower conductor surface, reversing the potential difference measured previously. The sense of the Hall-induced potential difference determines the orientation of magnetisation in the nearby stub.

Materials that have a high Hall coefficient include semiconductors, gold and bismuth. The Hall resistance can be defined as the ratio of the measured potential difference to the electrical current through the conducting wire.

In other embodiments the stray field sensors used arc planar (or extraordinary) Hall effect sensors. This effect is similar to the ordinary Hall effect except that the magnetic field, electrical current direction and Hall voltage are in-plane and a ferromagnetic material is required (Permalloy is an appropriate material). For currents and fields in certain orientations (both in-plane) electrons experience an in-plane force towards one side of the conductor. The resulting charge imbalance creates a potential difference across a wire. One possible arrangement (shown in Figure 14) is to use a cross shape that is at 45° to the stub (can use angles greater than 0° and less than 90°, but most sensitive when at 45°). With electrical current passing through two parallel arms of the cross, the field-induced potential difference can be measured using the other perpendicular arms. The Hall resistance is again defined as the ratio of the measured potential difference to the electrical current through the cross shape.

The stray field sensors of Figures 12 and 13 may be used as an alternative to or in addition to the previously described contact sensors.

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Claims

1. A magnetic rotation sensor comprising a first sensing layer within which a domain wall separates regions of different magnetic orientation, the location of the domain wall being arranged to change when a magnetic field rotates relative to the layer and electronic measuring means arranged to measure the location of the domain wall electronically and wherein the amount of relative rotation between the sensor and the magnetic field can be determined from the location of the domain wall.

2. A magnetic rotation sensor according to claim 1, wherein the electronic measuring means comprises resistance-measuring means arranged to measure resistance across a portion of the first sensing layer and the measured resistance is indicative of the location of the domain wall within the first sensing layer portion.

3. A magnetic rotation sensor according to claim 2, wherein the measured resistance is indicative of the total number of domain walls within the first sensing layer portion and the total number is indicative of the location of the domain wall.

4. A magnetic rotation sensor according to claim 2 or claim 3, wherein the first sensing layer portion comprises substantially the whole first sensing layer.

5. A magnetic rotation sensor according to claim 2 or claim 3, wherein the first sensing layer portion comprises part of the first sensing layer. a

6. A magnetic rotation sensor according to any of claims 2 to 5, wherein the resistance-measuring means is arranged to measure the resistance across further portions of the first sensing layer and the measured resistances are indicative of the location of the domain wall within the further portions.

7. A magnetic rotalion sensor according to any preceding claim, wherein the layer comprises a junction through which the domain wall can not pass unless there is a predetermined amount of rotation in a predetermined direction between the sensor and the magnetic field.

8. A magnetic rotation sensor according to claim 7 wherein the predetermined amount of rotation is about 180°.

9. A magnetic rotation sensor according to claim 7 or claim 8, wherein the junction comprises a cusp profile.

10. A magnetic rotation sensor according to any of claims 7 to 9, wherein the layer comprises a plurality of junctions.

11. A magnetic rotation sensor according to claim 10, wherein each junction requires substantially the same predetermined amount of rotation in a predetermined direction between the sensor and the magnetic field to

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allow the domain wall to pass through it.

12. A magnetic rotation sensor according to any of claims 7 to 11, wherein the resistance-measuring means is arranged to measure the resistance between junctions. a

13. A magnetic rotation sensor according to claim 12, wherein the resistance-measuring means is arranged to measure the resistance between adjacent junctions.

14. A magnetic rotation sensor according to any of claims 2 to 13, wherein the resistance-measuring means comprises a plurality of electrical contacts attached o the layer at points between which resistance is required to be measured.

15. A magnetic rotation sensor according to claim 14, wherein the plurality of electrical contacts are arranged substantially linearly.

16. A magnetic rotation sensor according to any preceding claim comprising a domain wall nucleation zone in communication with the first sensing layer and arranged to introduce a new domain wall to the layer, absorb a domain wall from the layer or both.

17. A magnetic rotation sensor according to claim 16, wherein the domain wall nucleation zone is provided in communication with a first end of the first sensing layer.

18. A magnetic rotation sensor according to claim 16 or claim 17, wherein the nucleation zone comprises a domain wall injection pad.

19. A magnetic rotation sensor according to any of claims 2 to 18, wherein the sensor further comprises a temperature compensator comprising a first compensation layer substantially corresponding in form to the first sensing layer, but not having a domain wall within it, wherein resistance-measuring means is arranged to measure the resistance across the or each compensation layer portion, the compensator being arranged to compare measured resistances from the or each corresponding layer portion of the first sensor layer and the first compensation layer and compensate for variation in measured resistance caused by temperature variation.

20. A magnetic rotation sensor according to claim 19, wherein the temperature compensator comprises a magnetic field compensator arranged to compensate for variation in measured resistance caused by

magnetic field variation.

21. A magnetic rotation sensor according to any of claims 2 to 20, wherein the sensor further comprises a sensitivity enhancer comprising a second sensing layer substantially identical in form to the first sensing layer, but arranged in an opposed configuration to the first sensing layer, wherein resistance-measuring means is arranged to measure the resistance across the or each second layer portion such that when a magnetic field rotates relative to the sensor in a predetermined direction, a measured resistance in the first sensing layer is arranged to increase or decrease and the corresponding measured resistance in the second sensing layer is arranged to correspondingly decrease or increase.

22. A magnetic rotation sensor according to claim 21 when dependent on any of claims 17 to 20 comprising a second domain wall nucleation zone in communication with the second sensing layer and arranged to introduce a new domain wall to the second sensing layer, absorb a domain wall from the second sensing layer or both, wherein the second domain wall nucleation zone is provided in communication with an end of the second sensing layer, which end is opposed to the first end of the first sensing layer.

23. A magnetic rotation sensor according to any of claims 20 to 22, wherein the temperature compensator further comprises a second compensation layer substantially corresponding in form to the second sensing layer.

24. A magnetic rotation sensor according to any of claims 19 to 23, wherein the sensor comprises one or more further sensing layers or compensation layers or both.

25. A magnetic rotation sensor according to any preceding claim, wherein the first sensing layer is formed in a wire, such as a nanowire.

and optionally, or each further sensing layers are formed in wires, such as nanowires.

26. A magnetic rotation sensor according to claim 25, wherein the or each wire comprises only one sensing layer.

27. A magnetic rotation sensor according to any of claims 2 to 26, wherein the resistance measuring means is arranged to measure anisotropic magnetoresistance (AMR).

28. A magnetic rotation sensor according to any preceding claim wherein the electronic measuring means comprises a stray field sensor

arranged to sense stray magnetic field.

29. A magnetic rotation sensor according to claim 28, wherein the stray field sensor is arranged to sense stray magnetic field at a sensing layer junction.

30. A magnetic rotation sensor according to claim 28 or claim 29, wherein the stray field sensor is arranged to measure giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR).

31. A magnetic rotation sensor according to any of claims 28 to 30, wherein the stray field sensor comprises an extraordinary Hall effect sensor.

32. A magnetic rotation sensor according to any of claims 28 to 30, wherein the stray field sensor comprises an ordinary Hall effect sensor.

33. A magnetic rotation sensor according to any preceding claim, wherein the wire comprises a Permalloy material.

34. A magnetic rotation sensor according to any preceding claim, wherein the thickness of the layer is between 3nm and 4Onm.

35. A magnetic rotation sensor according to any preceding claim, wherein the thickness of the layer is about 5nm.

36. A magnetic rotation sensor according to any preceding claim wherein, the width of the layer is between 5Onm and l000nm.

37. Rotation sensing system for measuring rotation of a first object relative to a second object comprising the rotation sensor of any preceding claim fixed relative to the first object and a magnetic field applier fixed relative to the second object.

38. A rotation sensing system according to claim 37, wherein the magnetic field applier comprises a permanent magnet.

39. A rotation sensing system according to claim 37 or claim 38, wherein the rotation sensor comprises an integrated device attached to the first object.

40. A rotation sensing system according to any of claims 37 to 39, wherein the rotation sensor is formed integrally with the first object.

41. A rotation sensing system according to any of claims 37 to 40, comprising a shaft rotation sensing system wherein one of the first object or the second object comprises a shaft and the other second object or first object comprises a frame relative to which the shaft is arranged to rotate.

42. Rotation sensing method comprising providing a rotation sensor according to any of claims 1 to 36, applying a magnetic field rotating relative to the sensor and electronically measuring the location of the domain wall in the sensor to determine the amount of rotation of the

sensor relative to the magnetic field.

43. A sensor, system or method substantially as herein described with reference to any one or more of the accompanying drawings.

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