MAGNETIC FIELD RESPONSIVE DEVICE

Abstract: A magnetic field responsive device which comprises: an electrically insulating substrate (2), a layer of an electrically conductive soft magnetic material (3), a layer of giant magneto-resistive (GMR) material (5) arranged in electrical contact with the layer of electrically conductive soft magnetic material, the thickness of the GMR material layer being not greater than twice the mean free path of an electron in the said GMR material, the layers being arranged in any order or position on the substrate and having opposed directions of magnetisation in zero applied magnetic field.

Description: MAGNETIC FIELD RESPONSIVE DEVICE This invention relates to magnetic field responsive devices and is more particularly concerned with a magnetic field responsive device having improved sensitivity and performance and including one or more giant magneto resistive (GMR) materials. A variety of magnetic field sensors have been proposed hitherto. The simplest of these are Hall Effect magnetometers in which the size of the magnetic field is derived from measurement of a transverse voltage in a current-carrying semiconductor placed in the magnetic field. Such devices are exceedingly temperature sensitive, and in fact the measured values are exponentially dependent upon temperature. Classical magneto resistive materials, such as nickel iron, have also been used for stray field pick up, but such devices are too insensitive for many applications. Flux-gate magnetometers are also used to measure magnetic fields. In these devices a soft magnetic material is excited with an AC excitation field, with the response, as modified by the ambient field, being observed. Layered sandwich-type magneto resistive devices comprising at least two deposited layers of magnetic thin films separated by a non-magnetic thin film layer have been proposed for use as magnetic memory cells and magnetoresistive read transducer assemblies. Examples of such devices are illustrated in European Patent Specifications Nos. 0276784 and 0314343, US Patent No. 4897288 and International Patent Application No. W091/18424, the entire disclosures of which are incorporated herein by reference. Giant magneto resistive (GMR) materials of the iron chromium or cobalt copper type have been proposed for use in magnetic field sensors. The sensitivity of such devices incorporating GMR materials may be improved by the use of layered sandwich-type structures, but again the change in the electrical resistance at low magnetic fields is not enough for many applications. In high fields at room temperature, however, a 60% 6R/R is achievable. Other magneto resistive materials which have been suggested include cobalt silver, but this is a very fine grain material and inherently magnetically hard. It is only useful in devices for measuring high magnetic field strengths. In Phys Rev Lett 21 (1993) 2331, the entire disclosure of which is incorporated herein by reference, there is described a GMR material comprising a mixed oxide of lanthanum, barium and manganese with which a δR/R of 97% was achieved at room temperature in high fields. None of the magnetic field sensors which have hitherto been proposed have achieved the combination of sensitivity, size and ease of manufacture necessary for many applications and any improvement in this respect would be highly desirable. According to the present invention there is provided a magnetic field responsive device which comprises an electrically conductive soft magnetic material and a giant magneto resistive material proximate thereto and in electrical contact therewith. In one aspect the invention provides a magnetic field responsive device which comprises: an electrically insulating substrate; a layer of an electrically conductive soft magnetic material, a layer of a giant magneto resistive (GMR) material arranged in electrical contact with the layer of electrically conductive soft magnetic material, the thickness of the GMR material layer being not greater than twice the mean free path of an electron in the said GMR material, the layers being arranged in any order or position on the substrate and having opposed directions of magnetisation in zero applied magnetic field. In another aspect the invention provides a method of manufacturing a magnetic field responsive device which comprises: depositing onto an electrically insulating substrate, in any order, (i) an electrically conductive layer of a soft magnetic material, (ii) a layer of a giant magneto resistive (GMR) material, of a thickness not greater than twice the mean free path of an electron in the GMR material, the layers being deposited so as to be in electrical contact and, if necessary, treating the layers such that they have opposed directions of magnetisation in zero applied magnetic field. The magnetic field responsive device can be, for example, a magnetic field sensor, a proximity sensor, a security device, a magneto resistive read transducer, a flux gate magnetometer or a direction finding device. The electrically insulating substrate preferably has a smooth, and more preferably, a flat surface and can comprise, for example, a glass, a silicon wafer, a plastics material, or any other suitable electrically insulating layer, film or surface. In this specification, a soft magnetic material is one which has its direction of magnetisation changed in low magnetic fields, of the order of, for example, less than about lOOOAmps per metre. The electrically conducting soft magnetic material can be any suitable ferro-magnetic material or alloy, and for example very good results have been obtained with alloys of iron with for example, nickel and/or cobalt. The preferred soft magnetic material is Ni 3 Fe. In principle any suitable GMR material, preferably with low electrical conductivity, can be used in the devices of the invention. Preferably the GMR material is a semi-conductor or an electrical insulator, for example a magnetic oxide, such as, for example iron oxide; or a mixed oxide of lanthanum, a Group Ila metal, and manganese, such as, for example, lanthanum, barium, manganese oxide; lanthanum, calcium, manganese oxide; or lanthanum, strontium, manganese oxide. Particularly good results have been obtained using a lanthanum, barium, manganese mixed oxide of the type mentioned heretofore. A plurality of layers of soft magnetic material and GMR material may be used if desired. The layers of soft magnetic material and GMR material are arranged on the substrate such that an electric current passing through the layers must pass through alternately a layer of soft magnetic material and a layer of GMR material. In a first, preferred embodiment, the substrate, and the one or more layers of soft magnetic material layer and GMR material can, for example, be arranged in a sandwich type layered construction on the substrate. The preferred GMR oxide materials are semi¬ conductors or insulators and thus it is important that the thickness of the layer, or each of the layers, of GMR material is sufficiently small to enable electrons to pass transversely through the GMR material. Preferably the thickness of the, or each, layer is not greater than one and a half times the mean free path of an electron in the GMR material, and most preferably the thickness is equal to or less than the mean free path of an electron in the GMR material. In general, this requires the thickness of the layer or layers to be around 10 to 100 lattice spacings (0.2μ), and preferably from 500-1500 angstroms, at at least one point. The thickness of the layer, or each of the layers, of soft magnetic material is preferably also small, but in this case the thickness is not so critical, and for example thicknesses of from 1 to lOOμ have been found to be suitable. In a second preferred embodiment, the magnetic field responsive device comprises: an electrically insulating substrate, an electrically conductive layer on the substrate, the layer comprising a plurality of elongate strips of an electrically conductive soft magnetic material and a giant magneto resistive (GMR) material arranged alternately and side by side with their adjacent surfaces in electrical contact, the width of each of the strips of GMR material in the plane of the electrically conductive layer being not greater than twice the mean free path of an electron in the said GMR material, and the adjacent strips of soft magnetic material and GMR material having opposed directions of magnetisation in zero applied magnetic field. In this aspect of the invention such a device may be manufactured by a method which comprises: depositing an electrically conductive layer of a soft magnetic material onto a substrate, removing thin elongate strips of the soft magnetic material at intervals so as to form vias in the layer thereby producing in the layer elongate strips or mesas of soft magnetic material electrically insulated from each other by the vias, depositing an electrically insulating giant magneto resistive (GMR) material onto the layer so as to fill, or at least bridge, the vias in the layer, the width of each of the vias at at least one point along its length being not greater than twice the mean free path of an electron in the GMR material, and if necessary, treating the layer such that adjacent strips or mesas of soft magnetic material and strips of GMR material filling or bridging the vias have opposed directions of magnetisation in zero applied magnetic field. The strips or mesas of soft magnetic material and GMR material strips or filled vias are arranged alternately on the substrate such that an electric current passing across the layer must pass through alternate mesas of soft magnetic material and vias of GMR material. The width of each of the vias is sufficiently small to enable electrons to pass through the deposited layer of GMR material filling or bridging the via. Preferably the width of each via is not greater than one and a half times the mean free path of an electron in the GMR material, and most preferably the width is equal to or less than the mean free path of an electron in the GMR material. In general, this requires the width of the vias to be around ten to one hundred lattice spacings (0.2 / Ϊ) at at least one point. The thickness of the deposited soft magnetic layer is preferably small, consistent with the layer having a suitable electrical resistance. Thicknesses of about 0.1 to about 5 microns can be used, but preferably the thickness is around 1 micron. The width of each of the strips or mesas of soft magnetic material is preferably also small, and for example widths of from 1 to lOOμ have been found to be suitable. In some preferred devices according to the invention the soft magnetic material may be in intimate physical contact with the GMR material although this is not essential. Adjacent surfaces of soft magnetic material and GMR material can, for example, be separated by a thin layer of an electrically conductive metallic material, for example, a noble metal such as copper, silver or gold. Where such a separation layer is used, this is preferably a "flash" layer of, for example, gold, of a few nanometres thickness. Electrodes can be attached to the device by any suitable means. An electrode may, for example be soldered onto the layer of soft magnetic material, or attached by means of a conductive adhesive. An electrode may be attached to the layer of GMR material by, for example, applying a flash layer of a conductive material, for example, gold, and then soldering or applying a conductive adhesive to make the electrical connection. Although the invention is not intended to be bound by any particular theory, it is believed that by arranging for the soft magnetic material and GMR material to have opposed directions of magnetisation in zero applied magnetic field the conduction of electrons through the GMR material is assisted when the magnetism of the soft magnetic material is reversed, thereby increasing the efficiency of the device. If necessary the layers may be treated to ensure that the directions of magnetisation are opposed, by, for example, applying a magnetic field of sufficient intensity to change the direction of magnetisation of the soft magnetic material in a zero applied magnetic field. In the method of the invention the layer of soft magnetic material is preferably deposited onto the substrate, or the GMR material, by any of the known deposition techniques, for example, sputtering, molecular beam epitaxy, electrodeposition, or laser ablation, until a layer of up to about lOOμ thickness has been obtained. The mesas and vias are preferably obtained by an etching process, and lithographic techniques such as electron beam lithography or possibly X-ray lithography can be used. In electron beam lithography a mask is first used to make a pattern on the surface and the mesas and vias formed by plasma etching. The vias need not be parallel sided, and for example it may be advantageous for the vias to become progressively narrower in width towards their bottom, that is, approaching the substrate. It is important to arrange, however, that none of the soft magnetic material bridges the vias to avoid non- optimal exchange coupling between the magnetic materials. The GMR material may be deposited onto the surface of the substrate, or the soft magnetic material layer, by any suitable technique, but preferably deposition is by laser ablation. Sputtering may be used in some cases, but is not usually preferred. The GMR material may also be deposited onto the surface of the etched layer of soft magnetic material by any suitable technique, but preferably deposition is by laser ablation. This process is highly directional and can therefore more easily fill the vias with the GMR material. It has been found in practice that it is highly desirable to deposit the GMR material onto a layer of a suitable lattice matched oxide. Suitable oxides include, for example, yttrium/barium/copper oxide (preferably in its metallic rather than its superconducting phase). The method of the invention therefore preferably includes the step of depositing a layer of a lattice matched oxide before the deposition of the GMR material. After the deposition of the layers, the entire device may be annealed if desired, for example at a temperature of from 100 to 600°C. Preferred embodiments of magnetic field responsive devices according to the invention will now be described by way of example with reference to the accompanying Drawings in which: Figure 1 shows a diagrammatic representation of a first device in sectional side elevation; Figure 2 shows the device of Figure 1 in plan view; Figure 3 shows a diagrammatic representation of a second device in sectional side elevation; and Figure 4 shows the device of Figure 3 in plan view. Referring to the Drawings, the first device, illustrated generally at 1, comprises an electrically insulating silicon wafer substrate 2 having deposited thereon by a sputtering technique a Iμ thick layer 3 of nickel iron Ni 3 Fe. A flash coating 4 of a noble metal such as gold, titanium or tantalum of 1 to 100 nanometres thickness may then be applied to the deposited Ni 3 Fe layer by vapour deposition. The flash-coated Ni 3 Fe layer is next coated with a thin layer of yttrium, barium, copper oxide 4a, followed by deposition of a GMR material layer 5 by laser ablation. The laser-ablated GMR material is a mixed oxide of lanthanum, barium and manganese as described in Phys. Rev. Lett. 21 (1993) 2331, referred to previously. A second layer of Ni 3 Fe (not shown) may be deposited onto the GMR material layer if desired. Finally, a further flash coating 6 of gold, titanium or tantalum is applied to the surface of the deposited GMR material, and the device connectorised by making electrical connections 7,8 to the nickel iron layer 3 and the gold coating 6 respectively. If necessary, the device is "poled" by applying a magnetic field of sufficient intensity to change the direction of magnetisation of the Ni 3 Fe layer in a zero applied magnetic field, to ensure that the magnetic layers have opposed directions of magnetisation. To operate the device it is connected to an EMF source 9 and a voltmeter 10. In operation, the device is placed in the magnetic field whose intensity is to be measured and the EMF source switched on. Generally an applied current of about 1 to 10 milliamps will be suitable. The effect of the magnetic field intensity alters the number of electrons which can pass through the GMR material and thus alters the apparent resistivity. The change in potential is measured by the voltmeter and is directly related to field intensity. Referring now to Figures 3 and 4, the second device, illustrated generally at 11, comprises a plastics film substrate 12 having deposited thereon by a sputtering technique a lμ thick layer 13 of nickel iron Ni 3 Fe. The Ni 3 Fe layer has etched in its surface by electron beam lithography a plurality of parallel vias 14 which extend to the surface of the substrate 12 and insulate adjacent mesas of nickel iron 15 one from another. In the device illustrated the vias are parallel-sided, but other configurations are possible, for example the vias could become narrower in the direction of the substrate 12. On top of the nickel iron layer 13 there is deposited by laser ablation a GMR material layer 16 which comprises a mixed oxide of lanthanum, barium and manganese. As shown in Figure 3 the vias can be completely filled with GMR material as in 17, but this is not essential, and the GMR material could only partially fill the via or simply form a bridge between adjacent mesas as illustrated in 18. Electrical connections 19, 20 are made to the nickel iron layer, which extend to a current source 21 and a voltmeter 22. In operation the device is placed in the magnetic field whose intensity is to be measured and subjected to an applied current of from 1 to 10 milliamps. The change in potential is measured by the voltmeter and is directly related to the magnetic field intensity. As illustrated, the sensitivity of the device is greatest in a direction lying along the mesas. The dimensions of the device can, however, be chosen to give extreme directionality, or relative non-directionality, as desired. The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps or any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Claims: CLAIMS 1. A magnetic field responsive device which comprises: a electrically insulating substrate, a layer of an electrically conductive soft magnetic material, a layer of a giant magneto resistive (GMR) material arranged in electrical contact with the layer of electrically conductive soft magnetic material, the thickness of the GMR material layer being not greater than twice the mean free path of an electron in the said GMR material, the layers being arranged in any order or position on the substrate and having opposed directions of magnetisation in zero applied magnetic field. 2. A magnetic field responsive device according to Claim 1, which is a magnetic field sensor, a proximity sensor, a security device, a magneto resistive read transducer, a flux-gate magnetometer, or a direction finding device. 3. A magnetic field responsive device according to Claim 1 or 2, in which the substrate comprises a flat surface of a glass, a silicon wafer or a plastics material. 4. A magnetic field responsive device according to any of the preceding claims, in which the soft magnetic material comprises a ferro-magnetic material or alloy. 5. A magnetic field responsive device according to Claim 4, in which the soft magnetic material is nickel iron. 6. A magnetic field responsive device according to any of the preceding claims, in which the GMR material is a magnetic oxide. 7. A magnetic field responsive device according to Claim 6, in which the magnetic oxide is iron oxide or a lanthanum barium manganese oxide. 8. A magnetic field responsive device according to any of the preceding claims, in which the substrate and the layers of electrically conductive soft magnetic material and GMR material are arranged in a sandwich-type construction. 9. A magnetic field responsive device according to Claim 8, in which the thickness of the GMR material layer is equal to or less than the mean free path of an electron in the GMR material. 10. A magnetic field responsive device according to Claim 8 or 9, in which the thickness of the GMR material layer is from about 10 to about 100 lattice spacings at at least one point. 11. A magnetic field responsive device according to any of the preceding claims, in which the thickness of the soft magnetic material layer is from about 1 to about lOOμ. 12. A magnetic field responsive device according to any of the preceding claims, which comprises: a electrically insulating substrate, an electrically conductive layer on the substrate, the layer comprising a plurality of elongate strips of an electrically conductive soft magnetic material and a giant magneto resistive (GMR) material arranged alternately and side by side with their adjacent surfaces in electrical contact, the width of each of the strips of GMR material in the plane of the electrically conductive layer being not greater than twice the mean free path of an electron in the said GMR material, and the adjacent strips of soft magnetic material and GMR material having opposed directions of magnetisation in zero applied magnetic field. 13. A magnetic field responsive device according to Claim 12, in which the strips of electrically conductive soft magnetic material and GMR material are arranged respectively in a series of alternating mesas and vias. 14. A magnetic field responsive device according to Claim 13, in which the width of each via is equal to or less than the mean free path of an electron in the GMR material. 15. A magnetic field responsive device according to Claim 13 or 14, in which the width of each of the vias is from about 10 to about 100 lattice spacings at at least one point. 16. A magnetic field responsive device according to any of Claims 12 to 15, in which the thickness of the strips of electrically conductive soft magnetic material is from about 0.1 to about 5μ. 17. A magnetic field responsive device according to any of Claims 14 to 16, in which the width of each of the strips of soft magnetic material is from 1 to lOOμ. 18. A magnetic field responsive device according to any of the preceding claims, in which the soft magnetic material is in intimate physical contact with the GMR material. 19. A magnetic field responsive device according to any of Claims 1 to 17, in which adjacent surfaces of the soft magnetic material and the GMR material are separated by a thin layer of an electrically conductive metallic material. 20. A magnetic field responsive device according to Claim 19, in which the separation layer comprises a noble metal. 21. A magnetic field responsive device substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings. 22. A magnetic field responsive device substantially as hereinbefore described. 23. A method of manufacturing a magnetic field responsive device which comprises: depositing on to an electrically insulating substrate, in any order, (i) an electrically conductive layer of a soft magnetic material (ii) a layer of a giant magneto resistive (GMR) material of a thickness at at least one point of not greater than twice the mean free path of an electron in the GMR material, the layers being deposited so as to be in electrical contact, and, if necessary, treating the layers such that they have opposed directions of magnetisation in zero applied magnetic field. 24. A method according to Claim 23 which comprises: depositing an electrically conductive layer of a soft magnetic material onto a substrate, removing thin elongate strips of the soft magnetic material at intervals so as to form vias in the layer thereby producing in the layer elongate strips or mesas of soft magnetic material electrically insulated from each other by the vias, depositing an electrically insulating giant magneto resistive (GMR) material onto the layer so as to fill, or at least bridge the vias in the layer, the width of each of the vias at at least one point along its length being not greater than twice the mean free path of an electron in the GMR material, and if necessary, treating the layer such that adjacent strips or mesas of soft magnetic material and GMR material filling or bridging the vias have opposed directions of magnetisation in zero applied magnetic field. 25. A method according to Claim 23 or 24, in which the layer of soft magnetic material is deposited onto the substrate by sputtering, molecular beam epitaxy, electrodeposition, or laser ablation. 26. A method according to Claim 24 or 25, in which the mesas and vias are obtained by a lithographic etching process. 27. A method according to any of Claims 23 or 26, in which the GMR material is deposited by laser ablation. 28. A method according to any of Claims 23 to 27, in which, prior to the deposition of the GMR material, there is deposited a layer of a lattice matched oxide. 29. A method according to any of Claims 23 to 28, in which, after the deposition of the layers, the device is annealed by heating to a temperature of from 100 to 600°C. 30. A method according to any of Claims 23 to 29, in which a magnetic field is applied to the device of sufficient intensity to change the direction of magnetisation of the soft magnetic material in a zero applied magnetic field such that the directions of magnetisation of the soft magnetic material and the GMR material are opposed. 31. A method according to any of Claims 23 to 30 substantially as hereinbefore described.

Inventors: BLYTHE HARRY JARRATT, GEHRING GILLIAN ANNE, GIBBS MICHAEL RICHARD JOHN, BLYTHE, HARRY, JARRATT, GEHRING, GILLIAN, ANNE, GIBBS, MICHAEL, RICHARD, JOHN
Applicants: BLYTHE HARRY JARRATT, GEHRING GILLIAN ANNE, GIBBS MICHAEL RICHARD JOHN, UNIV SHEFFIELD, BLYTHE, HARRY, JARRATT, GEHRING, GILLIAN, ANNE, GIBBS, MICHAEL, RICHARD, JOHN, THE UNIVERSITY OF SHEFFIELD
Application year: 1995
Main classification: G01R33
IPC codes: G01R33, H01F1, H01F10, H01L43