GB2169434A - Magnetoresistive sensors - Google Patents

Abstract

A thin-film magnetoresistive sensor for detecting and/or measuring magnetic fields, e.g. from a track (12), has enhanced shape anisotopy in a direction (y) transversely to its longitudinal axis (x) by the use of a finned configuration (24) for the film (16). The fins (24) act as an integral flux guide from the track (12) to the main stripe (20) of the film positioned some distance from the track. The fins (24) may have alternative shapes (Figs. 4-7). Particularly preferred are fins broader in the x-direction adjacent to the track than they are adjacent to the main stripe, thereby to funnel flux away from the track (Fig. 7). The sensors have, for example, increased high frequency end bandwidth, increased transverse sensitivity, and burn-out current is higher. Tape, disc and credit card applications are specified. <IMAGE>

Description

SPECIFICATION Magnetoresistive sensors This invention relates generally to magnetoresistive sensors, and particularly to thin-film magnetoresistive replay sensors. Although the magnetoresistive sensors of the present invention are particularly useful for reading magnetically stored information, the sensors of the present invention have general application to the detection and/or measurement of magnetic fields.

Thin-film magnetoresistive sensors are used for detecting and/or measuring magnetic fields by detecting the effect which a magnetic field has on the magnetisation in the thin film. Such changes in the magnetisation in the thin film result in a change in the electrical resistance of the film. As shown in the simplified diagram of Fig. 1, a magnetic tape 10 having a central track 12 moves beneath a thin-film magnetoresistive element 14. The element 14 is arranged with its longitudinal X-axis at rightangles to the direction of movement of the tape. It has a width w in a direction perpendicular to the surface of the tape, and a thickness t in a direction parallel to the direction of movement of the tape. Information is stored on the tape in the form of magnetisation which varies in its direction and magnitude as a function of position on the tape.This produces an external magnetic field whose strength varies with position. When the tape is moved relative to the sensor element the variation of the magnetic field component Hy perpendicular to the plane of the tape causes the direction of the magnetisation in the sensor element to vary. The magnetic field which is produced by the tape causes a rotation of the magnetisation vector M5, which is normally directed along the longitudinal axis of the element, and rotates it away from this longitudinal axis.

A common application for magnetoresistive sensors is in the replaying of information from a magnetic storage medium, such as a magnetic tape or disc where the plane of the film can be perpendicular to the surface of the storage medium. The magnetic field arising from the storage medium is sufficient to rotate the magnetisation vector M5 or cause magnetic domain wall movement, or both, resulting in a change in the electrical resistance of the sensor. This change in resistance can then be used as an indication of the state of the magnetically stored information in or on the storage medium. The use of a sensor in this way can serve as a replay or read sensor for replaying information, for example from credit cards, magnetic tape and flexible or rigid computer memory discs.

One of the problems with the use of such sensors is that if the magnetically stored information has a short wavelength, then the thinfilm sensor has to have very small dimensions and be placed very close to the storage medium in order to provide an acceptable output signal of good resolution. For example, the thickness t in Fig. 1 may be of the order of Iqum and the spacing of the element from the storage medium may be 5,us. As information packing densities have increased, manufacturers have endeavoured to place the sensors nearer and nearer to the surface of the storage medium or else have flux-guiding magnetic circuits to channel the flux from the medium to the sensor which is electrically insulated from the magnetic circuit by a thin layer of electrically insulating material.However, fabrication of such flux-guiding magnetic circuits increases the cost. Additionally, the careful machining of the sensor substrate whichis necessary so that the sensor edge is very close to the magnetic storage medium also increases the manufacturing costs.

It is one object of the present invention to provide a thin-film magnetoresistive sensor which can be placed close to the storage medium but which can be produced at substantially the same cost as conventional sensors which are spaced by a relatively large distance from the storage medium and which use fluxguiding magnetic circuits.

It is a further object of the present invention to provide a thin-film magnetoresistive sensor which not only can be fabricated economically but which also has improved performance as compared with conventional sensors of this general type.

In accordance with the present invention there is provided a thin-film magnetoresistive sensor for detecting and/or measuring magnetic fields, in which the shape anisotropy of the sensor is enhanced in a direction transversely to the longitudinal axis of the sensor.

By enhancing the shape anisotropy of the magnetoresistive element in a direction transversely to its longitudinal axis, the sensitivity of the sensor film to magnetic fields, particularly magnetic fields transverse to the longitudinal axis of the element,is increased.

The shape anisotropy of the sensor can be increased in a direction transversely to the longitudinal axis of the sensor by selective extension of the film in a direction transversely to the longitudinal axis of the film. This can be achieved for example by forming the film with transverse fins. Such selective extension of the film in the transverse direction allows the magnetisation of the storage medium to transmit its effect to the main part of the film which is positioned some distance above the storage medium. In other words, the fins act as a "flux guide" which is not spaced from and electrically insulated from the sensor but is an integral part of the sensor itself.

If transverse fins provide the selective extension of the sensor, then they may take any one of several forms. Various embodiments will be described hereinafter.

In accordance with a preferred embodiment of the invention, the ends of the fins adjacent to the storage medium are widened as compared with the rest of the fin length, in order thereby to "collect" more flux. By this means the edges of the fins next to the storage medium are exposed to a much greater amount of the flux available across the track width.

In order that the invention may be fully understood, a number of embodiments of magnetoresistive sensor in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1, as referred to above, illustrates a conventional magnetoresistive element positioned across a magnetic tape; Fig. 2 shows a first embodiment of thinfilm magnetoresistive sensor in accordance with the invention; Fig. 3 shows a modification of the arrangement of Fig. 2, where the film is spaced from the surface of the storage medium; Figs. 4 to 6 show three alternative configurations of film with different shapes of transverse fin; and Fig. 7 shows a further embodiment of sensor in accordance with the invention, having a modified fin configuration.

It is well-known that if an external magnetic field is applied to a bar of magnetisable magnetic material, such as iron, then it is easier to magnetise it along its longitudinal axis than if the same external magnetic field is applied along its short, transverse axis. This is because the internal "demagnetising" field is less for the "long axis" case, so that the resultant field in the "long axis" case is greater than for the "short axis" case. This effect is referred to as shape anisotropy, and the iron bar in the example referred to above thus has a greater shape anisotropy along its long axis than along its shorter axes.

Referring now to Fig. 2, this shows a magnetic storage medium in the form of a tape 10 which has a central track 12 and which ismovable in the direction shown by the arrow beneath a thin-film magnetoresistive sensor.

The thin film, indicated generally at 16, is mounted on a substrate 18. The thickness t of the film 16 is exaggerated in the drawing for the sake of greater clarity. The film 16 comprises a main stripe 20 with leadouts 22 at each end. A sensing current I is supplied to one leadout 22 and is taken from the other leadout 22 to associated electrical or electronic circuitry (not shown). As shown in Fig.

2, the main stripe 20 which extends across the tape track 12 at right-angles to the direction of tape movement is provided on each side with transverse fins 24. In this particular embodiment the fins 24 are shown as being generally rectangular in shape and of equal size on each side of the main stripe 20. Although the bottom edges of the downwardly extending fins 24 may be either in contact with or slightly spaced from the surface of the storage medium, the main stripe 20 of the film is spaced away from the surface of the storage medium. The provision of the transverse fins 24 increases the shape anisotropy of the film in the transverse direction y.

The film 16 can be produced by appropriate photolithography techniques for example. With this finned structure the field Hy from the magnetic storage medium 10 will then more readily rotate the film magnetisation.

As is shown in Fig. 3, the ends of the fins 24 do not necessarily have to be in contact with the magnetic storage medium 10,12 for the sensor to be effective. In Fig. 3 the ends of the lower fins 24 are shown spaced from the surface of the magnetic medium 12 by a distance a. In one practical embodiment of thin-film sensor with a film configuration of the general type shown in Fig. 3, the film has 96 double fins equispaced along the main stripe 20. Each fin 20 has a length I along the x-axis of 1film, and the spacing d between adjacent fins along the x-axis is also 10#m, i.e. a mark/space ratio of 1:1. The width b of each fin 24 along the y-axis is 20#m. It is emphasised however that these values are given by way of example only.In particular, the width b could in practice be substantially more than 20#m. In a digital device where a signal is required from two magnetic stages, these can be with the magnetisation along the x-axis as shown in Fig. 3 and along the y-axis as shown in Fig. 3. The binary-coded magnetisation transitions on the storage medium then tend to switch the sensor magnetisation between the two states, and this provides an output signal by means of a change in the current I through the sensor, or in the voltage across the sensor.

Figs. 4 to 6 show three alternative fin configurations which differ from the rectangular shape shown in Figs. 2 and 3. Fig. 4 shows fins with a generally triangular configuration, although with the lower fins truncated at their apices. Fig. 5 shows fins of generally oval or elliptical configuration, and again with the lower fins presenting flat surfaces towards the storage medium. Fig. 6 shows a single-sided finned structure where the film presents a continuous straight edge to the storage medium but has transverse fins along the upper edge of the main stripe. Other configurations of fin structure can also be used.

Fig. 7 shows a further modified fin configuration. In Fig. 7 a magnetic storage medium in the form of a tape 10 is movable into the paper, as shown. A track on the tape backing is provided by a magnetic coating 12. The main stripe 20 which extends across the tape track at right-angles to the direction of tape movement is provided on each side with transverse fins 24. The upper fins 24 are rectangular, and may be longer than shown in the drawing. This embodiment is concerned with the lower fins which extend from the main stripe 20 towards the tape track 12.

The end portion of each lower fin 24 adjacent to the track is widened so that each of these lower fins has an inverted T-shape configuration. It is desirable that the fins should be widened as much as possible over the region adjacent to the tape without actually touching each other. This will lead to a proportionally higher amount of flux being transmitted to the sensor, thus giving a higher sensitivity and a greater output signal.

It is important that the widening of the lower fins 24 should not extend upwards away from the track right up to the main stripe 20 of the transverse sensor. If the widening continued right up to the main stripe, then the widened fins would effectively begin to short-circuit the sensor and lead to what would effectively be a single, wide sensor.

Such a wide sensor would not be suitable because it would have a low resistance and be particularly sensitive to the head profiling distance. It would also have a lower transverse anisotropy.

Although as shown in Fig. 7 the height of the broadened fin portion is substantially equal to the height of the narrower fin portion which connects that broadened portion to the main stripe 20, the length of the narrower fin portion can be increased in proportion to the length of the broadened portion.

Various other fin configurations can be envisaged within the scope of the present invention, each of which is based upon the fact that the end portions of the fins adjacent to the track are broader than portions of the fins further removed from the track, so that the flux is funnelled away from the track towards the main stripe. This funnelling of the flux can be achieved for example by providing the fins with curved sides rather than rectilinear edges as shown in the drawing. This would result for example in a "bottle-shaped" fin having its neck attached to the main stripe 20.

The fact that the transverse fins attract the magnetic flux from the storage medium and direct it to the main portion of the stripe makes the manufacture of the finished product much easier, as the control of the dimension from the main body of the stripe to the storage medium can have a greater tolerance.

The finned structure also reduces the effective separation between the stripe 20 and the storage medium 12, thereby reducing separation losses. There is also a "domino" effect in that when the fins are magnetised this magnetises the rest of the film. Magnetisation of the sensor is therefore easier and faster.

Experiments have shown that a finned film structure has greater transverse sensitivity. A finned sensor will reach its maximum change in magnetoresistivity before an equivalent "straight" sensor, i.e. at a lower value of transverse field Hy.

It has also been established that the finned sensor does have a "flux guiding" capability.

By comparing a finned sensor with a straight sensor, and plotting the frequency of the replayed magnetic information against the voltage output of the sensor for the same (constant) sensor current, it is found that the finned sensor has a larger bandwidth at the high frequency end. This would be achieved with a straight sensor only if it was placed very close to the storage medium. Additionally, the burnout current for a finned sensor is higher than for a straight sensor, partly due to its increased surface area for heat dissipation.

It has also been found that the fins of the sensor of the present invention are still effective if the fins are spaced away from the surface of the magnetic storage medium. This means therefore that a sensor in accordance with the present invention has application to the general detection and/or measurement of magnetic fields where enhanced transverse sensitivity is advantageous.

There are special benefits in using the sensor of the present invention for replaying information from a magnetic medium such as on credit cards, magnetic recording tape, and flexible and rigid computer discs. The enhanced shape anisotropy allows these sensors to be more tolerant of separation from the magnetic medium, especially if it is storing magnetic information at high "bit" densities.

The present invention therefore enables the creation of a magnetic replay sensor spaced from the magnetic medium and which would otherwise either have to be close to the magnetic medium to read efficiently or else be set into a flux guide structure or some other means to channel flux from the medium towards the sensor.

Claims (19)

1. A thin-film magnetoresistive sensor for detecting and/or measuring magnetic fields, in which the shape anisotropy of the sensor is enhanced in a direction transversely to the longitudinal axis of the sensor.

2. A magnetoresistive sensor as claimed in claim 1, in which the sensor comprises a thin film on a substrate, and in which the enhanced shape anisotropy is achieved by the configuration of the film.

3. A magnetoresistive sensor as claimed in claim 2, in which the film is selectively extended in a direction transversely to the longitudinai axis of the film.

4. A magnetoresistive sensor as claimed in any preceding claim, which comprises a film mounted on a substrate, with the film being provided with transverse fins extending at right-angles to the longitudinal axis of the sensor.

5. A magnetoresistive sensor as claimed in claim 4, in which the film comprises a stripe extending parallel to the longitudinal axis of the sensor, with said transverse fins extending from at least one side of said stripe.

6. A magnetoresistive sensor as claimed in claim 5, in which the fins are each of substantially rectangular shape.

7. A magnetoresistive sensor as claimed in claim 6, in which the fins are equispaced along the stripe with a mark/space ratio of substantially 1:1.

8. A magnetoresistive sensor as claimed in claim 6 or 7, in which the width of each fin from the stripe to the effective fin edge to be placed next to the storage medium is at least twice the length of the fin in the direction parallel to the longitudinal axis of the sensor.

9. A magnetoresistive sensor as claimed in claim 8, in which said fin length is of the order of 10#m and the said fin width is at least 20#m.

10. A magnetoresistive sensor as claimed in any of claims 4 to 9, in which the fins are all of equal size.

11. A magnetoresistive sensor as claimed in claim 4 or 5, in which the fins are substantially triangular in shape.

12. A magnetoresistive sensor as claimed in claim 11, in which the ends of the fins to be positioned next to the storage medium are truncated at their apices.

13. A magnetoresistive sensor as claimed in claim 4 or 5, in which the fins are of generally oval configuration.

14. A magnetoresistive sensor as claimed in claim 13, in which the ends of the fins to be positioned next to the storage medium are truncated to present a straight edge to the storage medium.

15. A magnetoresistive sensor as claimed in any of claims 5 to 9, in which the fins are provided only on the side of the stripe remote from the edge of the substrate to be positioned next to the storage medium.

16. A magnetoresistive sensor as claimed in claim 4 or 5, in which fins provided adjacent to the edge of the substrate to be positioned next to the storage medium are broader, in said longitudinal axis direction, at their ends adjacent to said edge than they are remote from said edge, thereby to funnel flux away from the storage medium.

17. A magnetoresistive sensor as claimed in claim 16, in which said fins are of inverted T shape configuration.

18. A magnetoresistive sensor as claimed in claim 16, in which said fins are of bottle shaped configuration with curved sides.

19. A magnetoresistive sensor as hereinbefore described with reference to Fig. 2 of the drawings, or Fig. 2 as modified in any of Figs.

3 to 7 of the drawings.