GB2372574A

GB2372574A - Polarity sensitive magnetic sensor

Info

Publication number: GB2372574A
Application number: GB0104546A
Authority: GB
Inventors: Nicholas Rouse
Original assignee: TELCON Ltd
Current assignee: TELCON Ltd
Priority date: 2001-02-23
Filing date: 2001-02-23
Publication date: 2002-08-28
Anticipated expiration: 2021-02-23
Also published as: WO2002068979A1; GB2372574B; GB0104546D0

Abstract

A polarity sensitive magnetic field sensor device comprises at least first and second magnetic sensors 1a, 1c connected in a bridge arrangement. The device includes an arrangement for applying a magnetic field bias 5 equally to the said sensors 1a, 1c in a direction perpendicular to their sensing direction. A structure 6 of soft magnetic material channels the magnetic flux of an external magnetic field 8 along paths 6a, 6b in which equal parts of the magnetic flux are directed with 6a and against 6b the magnetic field bias 5 in first and second regions of the said structure 6, respectively. The first and second sensors 1a, 1c are located in respective gaps 7a, 7b in the said first and second regions of the structure 6. The magnetic sensors 1a, 1c may be Hall effect or magneto-resistive elements. The device may include a nulling coil 9 or conductor. The device may have a magnetic structure with four gaps each with a respective sensor. The four senors being arranged in a balanced bridge arrangement at zero applied magnetic field. The device provides accurate magnetic measurements which are unlikely to drift with temperature or time.

Description

IMPROVED POLARITY SENSITIVE MAGNETIC SENSORS The present invention relates to improved polarity sensitive magnetic sensors.

There are a number of applications of sensors of magnetic fields in which it is important that the sensor is polarity sensitive. That is to say, electrical output of the sensor is positive when the magnetic field along a particular axis has one polarity, is negative when the polarity of the magnetic field along that axis is reversed, and preferably this electrical output passes smoothly, accurately, and stably through zero as the magnetic field along that axis passes through zero strength during polarity reversal.

Fig 1 shows examples of the response curves of a polarity insensitive sensor (a) and the response curves of a sensor with the desired polarity sensitivity (b). In Fig 1 (a) the output of the sensor, shown on the vertical axis, is the same for given positive magnetic field strength, +B, shown on the horizontal axis, as it for an equal strength negative magnetic field-B. It is therefore impossible to determine the polarity of the magnetic field from the output of such a sensor. In the example shown the output is a minimum at zero magnetic field strength. Some polarity insensitive magnetic sensors will produce a maximum at zero magnetic field strength but this does not alleviate the problems of polarity insensitivity. In Fig 1 (b), when the polarity of the magnetic field reverses so does the polarity of the output. With sensors having this desirable feature the polarity of the field can be determined immediately.

Some basic magnetic field sensing phenomena such as the Hall effect naturally come close to producing the desired polarity sensitivity. However some other phenomena such as the giant magneto-resistive effect, which possess advantages which would otherwise make them attractive for such applications, produce the same polarity electrical output when the polarity of the magnetic field being sensed is reversed.

It is possible to determine the polarity of a magnetic field with a polarity insensitive magnetic field sensing element by applying a fixed bias field to a polarity insensitive magnetic sensing element and offsetting the electrical output caused by this bias field to shift the operating point along the response curve of the element. By this means, at least in the near vicinity of zero external field, the output of the sensor changes polarity when the external magnetic field changes polarity. However, the zero output with zero external magnetic field is obtained by balancing three factors: the element response curve, the magnetic bias and the electrical offset. Since these three factors will in general be affected differently by temperature, it is very difficult to obtain an accurate and stable zero output at zero magnetic field.

It IS also possible to determine the polarity of a magnetic field with a polarity insensitive magnetic field sensing effect by applying a small additional alternating field to the field to be measured and noting which polarity of the additional alternating magnetic field causes the output of the detector to increase. Magnetic field sensors can be built in this way, but they tend to require a substantial amount of electronic circuitry to implement them and the additional alternating magnetic field causes problems. Either the bandwidth of the sensor has to be limited to a frequency below that of the additional alternating magnetic field or there is a risk that frequency components in the field to be measured at or near the frequency of the additional alternating field will beat with the additional alternating field and cause spurious responses.

Another objective in the design of magnetic sensors is the ability to measure small magnetic fields with accuracy and stability by simply placing the sensor in the magnetic field to be measured and taking the output of the element as a measure of the magnetic field strength. When the sensors rely only on some basic magnetic field sensitive effect they tend to have a non-linear response to magnetic fields and have a sensitivity that changes with temperature and time. When such elements are mass produced there is usually a broad distribution of characteristics between individual elements from the production run requiring expensive individual calibration if the elements are used in a direct measuring mode.

A technique that has been widely used to overcome these problems is to use a 'nulling'or'compensating'approach. With care it is possible to make a structure of electrically conducting material such as a coil of wire such that when an electrical current is passed through the conductors a magnetic field is generated locally whose strength is accurately proportional to the current and whose strength for a given current is accurately calculable from the dimensions of the structure and which therefore needs no calibration. If the photolithographic techniques developed for the semiconductor industry to produce such structures can be used it is generally possible to produce them with great accuracy at low cost. To use the 'nulling'approach, a magnetic field sensing element is placed in such a conductive structure and both are placed in the magnetic field to be measured. If a current of the correct polarity and value is passed through the conductive structure it will normally be possible to produce a magnetic field that, at the position of the sensing element, is the same strength but opposite polarity to the field to be measured. This will reduce the total magnetic field at that point to zero. If the magnetic field sensing element gives zero output when the magnetic field at that point is zero, then this zero output can be used to indicate when the two magnetic fields are equal and opposite. From the current in the conductive structure and the calculable ratio of current to magnetic field strength for the structure it is possible to calculate the strength of the field to be measured.

There is no need to adjust the current in the conductive structure manually for each measurement. By amplifying the output of the sensing element by a large factor and feeding the output of the amplifier to the conductive structure in the correct polarity it is possible to form a closed feedback loop so that the current in the conductive structure will automatically be proportional to the magnetic field strength to be measured and will continuously track changes in that magnetic field and provide a measure of that strength.

By this means the accuracy and stability of the measurement is no longer dependant directly on the sensitivity and the stability of the sensitivity of the

sensing element, and the linearity of the measurement is now no longer dependent directly on the linearity of the sensing element. Hall elements have been widely used for such applications. However, Hall elements that have sufficient sensitivity at zero magnetic field tend to have a restricted range of operating temperatures, and the stability of the output at zero magnetic field is less than ideal, although sufficient for some applications. Furthermore, Hall elements having the desired sensitivity tend to be based on III-V semiconductors such as GaAs and InSb that are difficult to fabricate and involve the use of highly toxic starting materials.

The standard magnetoresistive effect is a bulk material effect. That is, it does not rely on any surface effects or internal structure. In its basic form it is polarity insensitive. Ways have been developed for embedding thin parallel strips of more conductive material In the less conductive magnetoresitive material. If a resistor is cut from such a composite such that the thin conductive strips run diagonally to the direction of current through the resistor, and an external magnetic field is applied along the axis defined by the direction of current flow, the point of minimum resistance will be shifted from zero external magnetic field (as would be the case for a homogeneous magnetoresistor) to a finite value of magnetic field of one polarity. If the resistor is rotated by 1800 so that the thin conductive strips run along the opposite diagonal direction, but connected with the current flowing in the same direction as before. the point of minimum resistance will be with a magnetic field of the same magnitude as the first configuration but of opposite polarity. If four identical resistors of this type are connected together electrically in a Wheatstone bridge arrangement and placed so that the physical direction of current flow is the same for all of them, and with the two resistors that form one pair of electrically opposite arms of the bridge having the thin conductive strips running along one diagonal direction and the other two resistors having thin conductive stripes running along the opposite diagonal direction, a sensor can be formed having the desired polarity sensitivity. If the four magnetoresistors are well matched, then the output of the bridge will be close to zero at zero external magnetic field. Because the resistors are well matched the changes in their

characteristics with temperature will track closely and therefore there will be little change with temperature in the output at zero external magnetic field. Applications have been found for these sensors which are commonly called'barberpole bridges'. However their sensitivity in not much greater than the best of Hall effect sensors and their greater complexity has limited their use.

The Giant Magnetoresitive effect can have a sensitivity many times greater than the best standard magnetoresistive effect devices and because this effect can be produced in devices formed only from metals and inorganic insulators sensors can be made that operate over a very wide temperature range. Its internal mechanism is radically different to the standard magnetoresistive effect and relies on a stack of a number of thin layers of different conductive materials one or more of which is magnetic. An external magnetic field has a very strong effect on the resistance to a current flowing through the stack of layers in a direction parallel to the interfaces between the layers. The basic effect is polarity insensitive. Because the effect is fundamentally bound up with the layer structure and of charge carriers either crossing or bouncing off the interfaces between the layers, the effect does not lend itself to the'barberpole bridge'technique.

The purpose of the present invention is to provide improved polarity sensitive magnetic sensors that substantially avoid the problems of prior art solutions.

It is a further object of the present invention to provide improved polarity sensitive magnetic sensors utilising basically polarity insensitive phenomena.

It is a further object of the present invention to provide improved polarity sensitive magnetic sensors wherein the electrical output of the sensors passes smoothly, accurately, and stably through zero as the magnetic field along a particular axis passes through zero strength during polarity reversal.

The present invention provides a polarity sensitive magnetic sensor comprising: at least one pair of magnetic sensing elements, wherein the first and second magnetic sensing elements of a first pair are connected in a bridge

arrangement ; a source of a magnetic field bias that acts substantially equally on the first and second magnetic sensing elements, said magnetic field bias being in a direction substantially perpendicular to a sensing direction of the sensor; a structure of magnetically soft material configured to channel a magnetic flux to be sensed and to rotate at least a portion of the said magnetic flux, whereby the magnetic flux in the soft magnetic structure has a component in the direction of the magnetic field bias in a first region of the structure, and an equal component in the direction opposite to the magnetic field bias in a second region of the structure; and wherein the first and second magnetic sensing elements are located in different gaps in the soft magnetic structure, the gaps being located in the first and second regions of the structure, respectively.

Preferably, the first and second magnetic sensing elements are polarity insensitive sensing elements. More preferably they are magnetoresistive sensing elements, and most preferably they are giant magnetoresistive effect (GMR) sensing elements.

In certain preferred embodiments the magnetic field sensing elements are formed from a multilayer film structure exhibiting the giant magnetoresistive (GMR) effect.

In such cases, the magnetic field bias to the giant magnetoresistive effect based sensing elements is preferably provided by a pinning layer that can provide a substantial degree of permanent magnetism with a direction of magnetisation parallel to the plane of the film.

The magnetic sensing elements are preferably substantially similar and are connected in a bridge arrangement, whereby temperature variations of the individual elements are cancelled out. In the case of a sensor having two sensing elements, preferably the bridge arrangement is such that the first and second magnetic sensing elements are connected in series between two electrical input connections and an electrical output connection is provided intermediate the first and second magnetic sensing elements.

In a simple two-element bridge, the output is measured against a reference potential that is independent of the applied magnetic field. For example, the bridge may further comprise two electrical elements, such as resistors, that are not magnetically sensitive, the two electrical elements being connected in series between the same two electrical input connections and an electrical output connection being provided intermediate the two electrical elements.

However, preferably, the output is measured against a reference potential that is itself dependent on the applied magnetic field in equal and opposite fashion to the output potential. This can be regarded as a magnetic Wheatstone bridge.

Therefore, preferably the bridge comprises two matched pairs of magnetic sensing elements. Preferably, the first and second magnetic sensing elements of the second pair are located in gaps in the soft magnetic structure such that they experience equal and opposite components of the magnetic flux in the direction of the magnetic bias as do the first and second magnetic sensing elements of the first pair, respectively. Preferably, the magnetic sensing elements are all substantially identical. Preferably, the magnetic bias applied to the sensing elements is substantially identical for all of the sensing elements.

Preferably, the bridge is substantially balanced in the absence of an external magnetic field in the sensing direction.

In certain preferred embodiments, the first magnetic sensing element of the second pair is located in the same gap in the soft magnetic structure as the second magnetic sensing element of the first pair, and the second magnetic sensing element of the second pair is located in the same gap in the soft magnetic structure as the first magnetic sensing element of the first pair. This ensures that the magnetic sensing elements of the second pair experience substantially the same magnetic bias and substantially the opposite applied flux in the direction of the bias as the respective magnetic sensing elements of the first pair.

Such four-element bridge arrangements are preferred for a number of reasons. In terms of photolithography, the four-element bridge is considerably simpler than the two element sensor, as well as having twice the sensitivity. This is because four elements, such as GMR elements, may be deposited at the same time, by choice of a suitable mask pattern. On the other hand, introducing stable resistors for the two-element sensor bridge would require an extra deposition step in the process as well as an extra mask and etching step Preferably, the bridge is substantially balanced in the absence of an external magnetic field. That is to say, the electrical potential at the output connection is substantially equal to the reference potential however generated.

Preferably, the elements are located close together in the sensor In order that they sense the same external magnetic field and are at the same temperature.

Preferably the elements are no more than 1 mm apart, more preferably they are no more than 0.1 mm apart at their closest point. Preferably, the magnetic sensing elements are formed on a single substrate, and more preferably source of magnetic bias is also formed on the same single substrate, for example by photolithography.

The soft magnetic structure may for example be U-shaped or zig-zag or serpentine so as to achieve the desired rotation and reversal of the magnetic flux direction relative to the magnetic bias direction. Preferably, the soft magnetic structure is branched to split the magnetic flux into two substantially equal parts. Preferably, the branches converge again at a position on the other side of the sensing elements from the position of branching. The branches diverge or converge relative to the sensing direction. Preferably, the resulting structure is has at least two fold rotational symmetry about the magnetic sensing direction. In certain embodiments the structure also has two fold rotational symmetry about the magnetic bias direction. The gaps may be at different locations in the same branch, or may be in different branches.

Preferably, the soft magnetic structure comprises a layer of magnetically soft material. More preferably, the soft magnetic layer is laid down in a pattern that divides a total magnetic flux therein in two components and the soft magnetic layer further guides these components to turn either clockwise or anti clockwise, whereby the portions of magnetic flux applied to the first and second magnetic sensing elements, respectively, have substantially equal and opposite components in the magnetic sensing direction. The magnetic flux components derived from the external flux to be measured, having been turned through 900 (in the preferred configuration) clockwise or anti-clockwise at the points at which these components of flux are applied to the sensors, will be perpendicular, not parallel to the measurement direction. One of the components will be parallel to the direction of the bias field, and will add to this bias field. The other component will be of equal strength and anti-parallel to the bias field, and will subtract from the bias field. One of the components will be applied to elements 1 (and 4), and the other will be applied to element 2 (and 3).

Optionally the sensor may incorporate conductor patterns to allow the generation of a local magnetic field that can cancel out the field due to the external magnetic field to be sensed and thus allow the'nulling'technique to be used. Such conductor patterns will typically be coils either adjacent to or surrounding the magnetically soft material at points where it carries all the magnetic flux, (that is before it splits the magnetic flux). The coils may be formed from thin film conductors laid down by photolithography above and/or below the soft magnetic layer.

The invention provides a class of sensors of magnetic fields characterised in that it provides an electrical output that is sensitive to the polarity of the magnetic fields, and which electrical output changes polarity when the polarity of the magnetic field in a particular direction changes and with this electrical output passing smoothly accurately and stably though zero when the magnetic field in the particular direction passes though zero as the polarity of that magnetic field changes. The invention allows these characteristics to be achieved using magnetic field sensing phenomena that are not inherently polarity sensitive and thus allows the other

advantageous properties of these phenomena to be exploited in sensors requiring polarity sensitivity.

Preferably, the poling field or other magnetic bias that has the same effect is applied uniformly to all sensing elements in a direction at right angles to the magnetic field to be sensed, so that in the absence of external magnetic field the bridge or bridge-like structure remains balanced and gives no output.

Preferably, a layer of magnetically soft material that is laid down in a pattern that channels the magnetic flux in the direction to be measured along the layer of magnetically soft material before splitting the flux in two or more parts and forcing these parts of the total magnetic flux to turn by 900 (or some other angle) either clockwise or anti clockwise leading the part of the total flux up to the sensing elements with gaps being provided in the magnetically soft material at these sensing elements so that the flux in the magnetically soft material is substantially perpendicular to the gap on either side of the gap and the sensing elements in the gap are exposed to a magnetic field that is accurately perpendicular to the gap.

Preferably, the magnetically soft material has a magnetic permeability of at least 100, more preferably at least 300, and most preferably at least 500 or 1000.

Preferably, the configuration of the turns and gaps in the soft magnetic layer is such that for a Wheatstone bridge the sensing elements in two diagonally opposite arms of the bridge are subject to part of the magnetic flux turned clockwise and the other two arms subject to an equal part of the flux turned an equal angle anticlockwise The invention thus permits the attainment of an accurate and stable zero output for zero external field by allowing the sensing elements to be constructed identically so that any change will affect all sensing elements equally and not cause an output drift The invention naturally extends to configurations other than the simple Wheatstone bridge provided these configurations preserve the following features.

Firstly, the matching of corresponding and oppositely acting elements in the

configuration, so that changes in the characteristics of these elements due to a common cause will not give rise to an output drift. Secondly, matching of the magnetic fluxes channelled to the sensing elements so that corresponding and oppositely acting elements are subject to equal magnitude but oppositely turned fluxes as a result of the application of an external magnetic field so that changes in the magnetic properties of the magnetically soft material will not cause on output drift Specific embodiments of the invention will now be illustrated by examples without the generality of the invention being limited by these examples. The examples will be explained with reference to the accompanying drawings, in which: Fig. 1 shows schematic response curves for (a) polarity insensitive and (b) polarity sensitive magnetic field sensors; Fig. 2 shows a schematic plan view of the sensor of Example 1; Fig. 3 shows a an expanded view of the gap regions of Figure 1; Fig. 4 shows the equivalent electrical bridge circuit of Example 1 and Example 2; and Fig. 5 shows a schematic plan view of the sensor of Example 2.

Example 1 Example 1 is shown in Fig. 2 with the gap region shown expanded in Fig. 3 and is constructed on a silicon wafer as a convenient non-magnetic substrate. The sensor is constructed from a series of thin layers of different materials built up on the substrate. A number of sensors can be constructed simultaneously on the same substrate and thereafter separated. The layers are formed and shaped using known processes commonly used in the manufacture of semiconductors. A layer is deposited of the entire substrate using sputtering and this layer is then shaped by photolithography using a suitable etchant. An oxide layer is formed on the silicon to insulate it. Next the multilayer giant magnetoresistive elements are formed. These take the form of a previously known pinned spin valve structure All the layers of the magnetoresistors are deposited first and then the stack of layers

is shaped to form the individual magnetoresistors 1a, 1b, 1c and 1d in Fig. 3.

These magnetoresistors are approximately 3pm wide and 25pm long The layers of the magnetoresistors are as follows :5nm of Tantalum (Buffer Layer) 6nm of Nickel-Iron 2nm of Cobalt-Iron 2.8nm of Copper 4nm of Cobalt-Iron 9nm of Iridium-Manganese (Pinning Layer) 5nm of Tantalum (Capping layer).

Copper conductors 2a, 2b, 2c and 2d connect the magnetoresistors 1 a, 1 b, 1 c and 1d together in a Wheatstone bridge configuration and connect the terminals of the Wheatstone bridge to the bonding pads 3a, 3b, 3c and 3d The copper conductors 2a, 2b, 2c and 2d are show as dotted lines where they pass under the soft magnetic layer 6. The bonding pads 3a, 3b, 3c and 3d allow the sensor to be mounted in a standard semiconductor package using standard wire bonding techniques. The electrical equivalent of these connections is shown in Fig. 4. To operate as a Wheatstone bridge two opposite terminals are selected as input terminals and a voltage applied between these terminals. For illustration we may select 3a as the positive input terminal and 3d as the negative input. The other two terminals then become outputs and the output voltage of the bridge measured across these outputs. We may nominate 3b to be the positive output terminal and 3c to be the negative output terminal. With this convention a positive output is achieved if the resistance of the magnetoresistors 1c and 1d increase while the resistance of the magnetoresistors 1 a and 1 b fall. Conversely a negative output is achieved if the resistance of the magnetoresistors 1a and 1b increase while the resistance of the magnetoresistors 1 c and 1 d fall.

The bottom half of each turn of the nulling coils 9 are formed from the same layer of copper as the conductors 2a, 2b, 2c and 2d. A silicon dioxide insulating layer is deposited over the magnetoresistors and bottom copper conductors. Holes are

created in the silicon dioxide layer at the points where connections are required to be made to the top copper layer.

The soft magnetic layer 6 is formed from nickel-iron and is 1um thick. The long axis of the soft magnetic layer 6 is aligned with the direction 8 of the external magnetic field to be measured. The magnetic flux due to this external magnetic field is channelled along the soft magnetic layer 6 to the point where it splits into two arms 6a and 6b which curve symmetrically down and up respectively before curving back towards the centre and joining back together to reform a single arm aligned with the external magnetic flux. The shape of the curve of the two arms is such that at one point arm 6a is leading back up towards the centre at right angles to the long axis of the soft magnetic layer 6 and the direction 8 of the external magnetic field. At this point a narrow gap 7a is made across the width of the arm

6a. This gap is 5pm wide and the two magnetoresistive elements 1c and 1d are situated in centre of this gap. At a point symmetrically opposite the gap in arm 6a with respect to the long axis of the soft magnetic layer 6 the arm 6b leads back down towards the centre at right angles with respect to the long axis of the soft magnetic layer 6. There is a gap 7b in arm 6b at this point that matches the gap 7a in arm 6a. The two magnetoresistive elements 1 a and 1 b are situated in centre of this gap 7b. Because the soft magnetic layer 6 has a high magnetic permeability the magnetic flux caused by the external magnetic field in the direction 8 will follow the path of the soft magnetic layer 6 and split equally in two to follow the two arms 6a and 6b. In particular at the points at which the magnetic flux crosses the two gaps 7a and 7b the magnetic flux will have split into two equal, parallel but oppositely directed parts both accurately at right angles to the direction of the external magnetic field to be measured.

A second silicon dioxide insulating layer is deposited over the soft magnetic layer 6 with holes formed in it where connection to the lower copper layer is required A top layer of copper is formed on top of the second silicon dioxide insulating layer. This is shaped to form the top half of every turn of the compensating coil 9.

The ends of these half turns connect through the holes in the silicon dioxide layers

to the ends of the half turns on the bottom copper layer to form the complete the compensating coil 9. The ends of the compensating coil are connected to the bonding pads 10a and 10b. A current flowing from pad 10a to pad 10b through the compensating coil 9 will induce a magnetic flux in the soft magnetic layer 6 that is opposed to that caused by the external field to be measured in the direction 8. If the correct value of current is flowing in compensating winding the magnetic field flowing across the gaps will be reduced to zero and the Wheatstone bridge will be balanced and give zero output.

The top copper layer can also be used to form the cross-over 4 that allows the magnetoresistive elements 1a, 1b, 1c and 1d to be correctly connected together.

When the fabrication of the sensor is complete the magnetoresistors 1a, 1b, 1c and 1d are poled. To do this the sensor is placed in a magnetic field in the direction of the arrow 5 which is at right angles to the direction 8 of the external field to be measured, in the same direction that the magnetic flux caused by the external magnetic field to be measured crosses the gap 7a and in the opposite direction to that which this magnetic flux crosses gap 7b. The sensor is heated up to above the Curie temperature of the poling layer. The sensor is then allowed to cool down through the Curie temperature and when the external magnetic is removed the sensor is found to have acquired permanent magnetism. The effect of the poling is to shift the response of the magnetoresistors 1a, 1b, 1c and 1d from one that has a is symmetrical peak in resistance at zero applied magnetic field to one that has a peak at a finite applied magnetic field of one polarity and in which the resistance falls as the applied magnetic field of that polarity falls to zero and furthermore continues to drop as the strength of the applied magnetic field passes through zero, reverses and builds up in the opposite polarity. Provided the magnetoresistors are well matched, poling will not destroy the balance of the Wheatstone bridge and with zero external magnetic field there will be a near zero voltage out. There will still be a balance if the characteristics of the magnetoresistors change with temperature or time provided that equal changes take place in all of them.

Example 2 Fig. 5 shows Example 2. This is similar to Example 1 except there are four gaps in the soft magnetic layer each containing one magnetoresistor in contrast to the configuration in Example 1 in which there are two gaps each containing two magnetoresistors. In Example 2 in Fig. 5 the gaps are labelled 7a, 7b, 7c & 7d. which contain respectively the four magnetoresistors 1 a, 1 b, 1 c & 1 d. These are connected together in a Wheatstone bridge configuration having the same electrical equivalent circuit shown in Fig. 4. The gaps in arm 6a of the soft magnetic layer 6 are 7c and 7b. An external magnetic flux to be measured, flowing in the direction shown by arrow 8, will cause a flux across gap 7c that is opposite in direction to the poling direction shown by arrow 5 and cause a flux in gap 7b that is in the same direction as the poling direction. The gaps in arm 6b of the soft magnetic layer 6 are 7a and 7d and the fluxes in these gaps will be respectively the same as, and opposite to, the poling direction 5 as a result of the external flux in the direction 8. Because of the double rotation of the flux, the Wheatstone bridge connection can be made correctly without the need there was in Example 1 for the crossover 4 Example 2 has the advantage over Example 1 of greater symmetry making if easier to obtain a good match between similar the elements of the sensor but the disadvantage that the two gaps in each arm are magnetically in series, raising the magnetic reluctance of the path through the arm and thus lowering the magnetic flux in the gap produced by a given external magnetic field.

The above examples have been described by way of example only. Many other examples falling within the scope of the accompanying claims will be apparent to the skilled reader.

Claims

1. A polarity sensitive magnetic sensor comprising : at least one pair of magnetic sensing elements, wherein the first and second magnetic sensing elements of a first pair are connected in a bridge arrangement; a source of a magnetic field bias that acts substantially equally on the first and second magnetic sensing elements, said magnetic field bias being in a direction substantially perpendicular to a sensing direction of the sensor a structure of magnetically soft material configured to channel a magnetic flux to be sensed and to rotate at least a portion of the said magnetic flux, whereby the magnetic flux in the soft magnetic structure has a component in the direction of the magnetic field bias in a first region of the structure, and an equal component in

the direction opposite to the magnetic field bias in a second region of the structure ; and wherein the first and second magnetic sensing elements are located in different gaps in the soft magnetic structure, the gaps being located in the first and second regions of the structure, respectively.

2. A sensor according to claim 1, wherein the first and second magnetic sensing elements are polarity insensitive sensing elements.

3. A sensor according to claim 2, wherein the first and second magnetic sensing elements are magnetoresistive sensing elements.

4. A sensor according to claim 3, wherein the first and second magnetic sensing elements are giant magnetoresistive effect sensing elements.

5. A sensor according to any preceding claim, wherein the bridge arrangement is such that the first and second magnetic sensing elements are connected in series between two electrical input connections, and an electrical output connection is provided intermediate the first and second magnetic sensing elements.

6. A sensor according to claim 5, wherein the bridge further comprises two electrical elements that are not magnetically sensitive, the said two electrical elements being connected in series between the said two electrical input connections and an electrical output connection being provided intermediate the said two electrical elements.

7. A sensor according to any preceding claim, wherein the bridge comprises two matched pairs of magnetic sensing elements.

8. A sensor according to claim 7, wherein the first and second magnetic sensing elements of the second pair are located in gaps in the soft magnetic structure such that they experience equal and opposite components of the magnetic flux in the direction of the magnetic bias as do the first and second magnetic sensing elements of the first pair, respectively.

9. A sensor according to any preceding claim, wherein the magnetic sensing elements are all substantially identical.

10. A sensor according to any preceding claim, wherein the magnetic bias applied to the sensing elements is substantially identical for all of the sensing elements.

11. A sensor according to any preceding claim wherein the bridge is substantially balanced in the absence of an external magnetic field in the sensing direction.

12. A sensor according to any preceding claim wherein the soft magnetic structure is branched to split the magnetic flux into two substantially equal parts.

13. A sensor according to claim 12, wherein the different gaps for the first and second magnetic sensing elements are present in one branch of the soft magnetic structure.

14. A sensor according to claim 12, wherein the different gaps for the first and second magnetic sensing elements are present in different branches of the soft magnetic structure.

15. A sensor according to any preceding claim wherein the magnetic sensing elements are formed on a single substrate.

16. A sensor according to claim 15, wherein the soft magnetic structure comprises a soft magnetic layer formed on the said single substrate.

17. A sensor according to claim 15 or 16, wherein the source of the magnetic field bias is also formed on the single substrate.

18. A sensor according to claim 17, wherein to the magnetic sensing elements are giant magnetoresistive effect based sensing elements and the bias thereto is provided by a pinning layer that can provide a substantial degree of permanent magnetism with a direction of magnetisation parallel to the plane of the substrate.

19. A sensor according to any preceding claim further comprising a structure of conductive material such as a coil incorporated into the sensor in such a way that it can be used to oppose the effect of the magnetic field being measured to return the output of the sensor to zero

20. A sensor according to claim 19, wherein the soft magnetic structure is a soft magnetic layer on a substrate and the structure of conductive material is a thin film structure defining a coil for magnetising said soft magnetic layer.

21. A polarity sensitive magnetic sensor substantially as hereinbefore described with reference to the examples.