WO2006003639A1

WO2006003639A1 - Magnetoresistance device

Info

Publication number: WO2006003639A1
Application number: PCT/IE2004/000093
Authority: WO
Inventors: Igor Shvets
Original assignee: The Provost Fellows And Scholars Of The College Of The Holy And Undivided Trinity Of Queen Elizabeth Near Dublin
Priority date: 2004-07-01
Filing date: 2004-07-01
Publication date: 2006-01-12

Abstract

A tunnel magnetoresistance (TMR) device (1) is provided. Illustrated only are the ancillary layers of such a device which comprises a film laminate having two electrode layers (2, 3) separated by a thin dielectric layer (4) for reception of an electric current directed substantially orthogonal to one of the major exposed surfaces (5, 6) of the device (1). At least one of the electrode layers (2, 3) is of a magnetic material and in contra distinction to the prior art, the dielectric layer is one of a magnetic material, a laminate of a ferromagnetic and an anti-ferromagnetic or may be a laminate of a non-magnetic dielectric material on a magnetic material. The device does not depend on change in magnetisation direction of one electrode (2) with respect to the other electrode (3). Indeed, these two electrodes (2, 3) can have substantially the same direction of magnetisation, but it is not essential that they do so.

Description

"Magnetoreaistance Device"

Introduction

The present invention relates to a tunnel magnetoresistance (TMR) device comprising a film laminate having two electrode layers separated by a thin dielectric layer for reception of electric current directed substantially orthogonal to one of the major exposed surfaces thereof. These devices are sometimes called simply "magnetoresistance devices" or "tunnel magnetoresistance devices".

The current flowing between the two electrodes of the magnetoresistive device is sensitive to the external magnetic field. Alternatively, one could drive through the device constant current and observe the voltage drop between the two electrodes, which should also be sensitive to the external magnetic field. One could, also, say that resistance of the device is sensitive to the external magnetic field. It will be appreciated, by those, skilled in the art, that all these manifestations of a magnetoresistance device: change in current, change in voltage drop and change in resistance are related to each other.

Ideally, a TMR should be so constructed whereby the resistance of the device can be switched by the current or voltage pulse.

Magnetoresistance devices of many different types are widely used in information and communication technologies e.g. in disk drive read heads, magnetic tape read heads, random access memory devices and in numerous other applications. For random access memory applications the typical role of the magnetoresistance device could be described as a sensor of the magnetic field created by the magnetic area storing information. Therefore, in this application the magnetoresistance device and the magnetic area storing information as well as, means for remagnetising, the magnetic area forms the foundations of the memory cell. Typically one random access memory chip contains many thousands of memory cells. Magnetoresistance devices are also commonly used as sensors for a magnetic field in applications that are not directly related to the domain of information and communication technologies, e.g. in automotive and aviation industries, security devices, position encoders, medical devices and numerous other applications.

Specific requirements to a magnetoresistance device depend on the application. Typically it is desirable to have the device providing a large current (voltage or resistance) change in response to the magnetic field. In practice for many applications the resistance change of about 5-20% in a field of approximately 0.01 T or 0.1 T is considered acceptable. For some applications the time response of the device is important, for example, for read heads of disk drives. The read head of the disk drive should be able to collect data at the speed of many Megabits/sec and even greater. Therefore, for this application the magnetoresistance device should be able to respond to a sudden change in the magnetic field within the time interval 10^'7 sec or shorter. For some applications it is important to have the resistance of the device within a certain range of values. For example the resistance of the device preferably should not be very small by comparison to the resistance of the wires that connect it to the electric circuit. On the other hand the resistance should not be very large to make signal coupling from the device into an amplifier more efficient. There are many other requirements similar to those applicable to any practical electronic device such as reliability, robustness, temperature stability, manufacturability and costs.

There are several classes of magnetoresistance devices. This invention is most closely related to the class of tunnel magnetoresistance devices that are based on spin-polarized electron tunnelling through a thin dielectric barrier known as tunnelling magnetoresistance (TMR). The theoretical foundations for these devices have been laid some time ago although their practical implementation commenced during the last decade. The phenomenon is based on the tunnel junction with two ferromagnetic electrodes. The tunnel current between the electrodes depends on their relative orientation of magnetisations with respect to each other [M. Julliere, Phys. Lett. 54A, 225 (1975); J. Slonczewski, Phys. Rev. B 39, 6995 (1989)]. In practice magnetisation of one of the electrodes is often pinned and the magnetisation of the second electrode is altered by the external magnetic field. In recent years numerous papers dedicated to this phenomenon have been published [J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. Rev. Lett. 74, 3273 (1995);

J. S. Moodera, J. Nowak, R. J. M. van de Veerdonk, Phys. Rev. Lett. 80, 2941 (1998); S. Gider, B. U. Runge, A. C. Marley, and S. S. P. Parkin, Science 281, 797 (1998); J. M. de Teresa, A. Barthelemy, A. Fert, J. P. Contour, F. Montaigne, and P. Seneor, Science 286, 507 (1999); S. Yuasa, T. Nagahama, and Y. Suzuki, Science 297, 234 (2002);

H. Yang and A. R. Smith, Phys. Rev. Lett. art. no. 226101 (2002); E. Yu. Tsymbal and D. G. Pettifor, J. Phys.: Condens. Matter 9, L411-417 (1997); D. Wortmann, S. Heinze, Ph. Kurz, G. Bihlmayer, and S. Blϋgel, Phys. Rev. Lett. 86, 4132 (2001); P. LeClair et al., Phys. Rev. Lett. 88, art. no. 107201 (2002)].

There are numerous inventions describing various aspects of magnetic tunnel junctions. US Patent Specification No 5,629,922 (Moodera et al) and subsequently US. Patent Specification No 5,835,314 (Moodera et al) describe devices from this class comprising of two ferromagnetic electrodes separated by a nonmagnetic dielectric layer thus forming a tri-layer tunnel junction. Magnetisations of one of the ferromagnetic electrodes can be reversed with respect to the other. As an electric current passes between the two magnetic electrodes, the current value is sensitive to the relative orientation of the magnetisation directions in them. Therefore, the direction of magnetisation of one of the layers with respect to the other one, which in turn is sensitive to the external magnetic field can be identified. US Patent Specification No 5,835,314 further suggests that the greatest magnetoresistance effect is obtained when the tunnelling resistance of the device is comparable to the electrode resistance. US Patent Specifications Nos 5,734,605 and 5,978,257 (Zhu et al) describe a tunnel junction element similar to the one described in US. Patent Specification No 5,629,922 and further teach how it could be utilised in a memory cell. US Patent Specification No 6,335,081 (ArakJ et al) describes an improved tunnel magnetoresistance effect element based on a tunnel multilayered film with a tunnel barrier having reduced roughness of the layers. In most magnetic tunnel junction devices magnetisation of one of the two ferromagnetic layers is pinned by exchange coupling to an antiferromagnetic layer. There are inventions that deal with improvements in the pinning characteristics. For example, US Patent Specification No 5,764,567 (Parkin) describes a magnetic tunnel junction device consisting of two ferromagnetic layers separated by a dielectric barrier layer. Magnetisation in one of the ferromagnetic layers is pinned to the antiferromagnetic layer. This invention teaches that an extra non-ferromagnetic layer should be added between the dielectric barrier layer and the second ferromagnetic layer in order to reduce the exchange coupling between the fixed and free ferromagnetic layers.

A related US Patent Specification No 6,069,820 (Inomata et al) also describes the spin dependent conduction device based on electron tunnelling in a multilayer system involving three or more metal electrodes. As in other inventions, nonmagnetic dielectric layers separate these magnetic conducting electrodes. The US Patent Specification No 6,069,820 further describes the embodiments where some of the intermediate layers comprise the conducting particles of magnetic material embedded in a matrix of nonmagnetic dielectric to achieve some type of resonance tunnelling. This approach was further developed in the US Patent Specification No 6,114,056 (Inomata et al). In this latter specification the inventors have replaced the stack of conducting ferromagnetic electrodes and nonmagnetic dielectric barrier layers with a granular film in which the conducting particles of ferromagnetic material are incorporated in a matrix of nonmagnetic dielectric material. US Patent Specification No 6,365,286 (Inomata et al) describes a magnetic element and magnetic memory device utilising spin dependent tunnelling between a ferromagnetic metal and a ferromagnetic-dielectric mixed layer. The tunnelling occurs through a nonmagnetic dielectric layer of AI₂O₃. The tunnel current depends on the orientation of magnetisations in the two layers: the ferromagnetic metal layer and the ferromagnetic-dielectric mixed layer. This patent also describes structures comprising of three ferromagnetic layers: ferromagnetic metal layer, ferromagnetic-dielectric layer and again ferromagnetic metal layer all separated from each other by nonmagnetic dielectric layers. It will be noted that these US Patents 6,069,820, 6,114,056 and 6,365,286 all have one inventor in common.

It is essential that all these inventions suggest using as dielectric layers of the tunnel barrier the same range of nonmagnetic dielectric materials, mainly AI₂O₃, MgO, AIN, SiO₂, MgF₂, B₂O₃, CaF₂, SrTiO₃. Furthermore, in virtually all of these inventions the sensitivity to magnetic field is based on tunnelling of spin-polarised electrons emitted from one of the magnetic electrodes to the spin-polarised empty electron levels of the other magnetic electrode (e.g. see US No 6,114,056 column 4 lines 50-60 and Fig. 3).

It is well known that in order to achieve magnetic sensitivity of the magnetoresistance device based on the tunnel junction, one should have both of the ferromagnetic electrodes with as high a degree of spin-polarisation as possible, as the magnetic sensitivity of the device is proportional to the polarisation of each of the two electrodes. Ideally one would like to have the electrodes with 100% spin polarisation. This would result in the strongest possible dependency of the tunnel current on the direction of magnetisation of the electrodes. There is a small group of known materials that are predicted to have nearly 100% spin polarisation at the Fermi level. These include Fe₃O₄, CrO₂, and Heusler alloys and some of the manganites. However, in most magnetic tunnel junctions the electrodes are made of common transition metals and their alloys, e.g. Fe, Ni, Co, permaloy. The reason is that although these materials have lower spin polarisation, they are more technology-friendly and the films of these materials have more consistent properties.

It is also known that spin-dependent current can be achieved in the case of tunnelling between an antiferromaghetic electrode and a ferromagnetic one across a nonmagnetic dielectric barrier. In this case the tunnel current is sensitive to the direction of magnetisation in the ferromagnetic electrode with respect to the antiferromagnetic direction of the other electrode [A.A. Minakov, IV. Shvets, Surf Sci. 236 (1990) L377-L-381]. This sensitivity was recently used to achieve spin dependent imaging with STM on the atomic scale whereby an antiferromagnetic MnNi tip was used instead of a conventional tungsten tip [N.Berdunov, S. Murphy, G. Mariotto, I.V. Shvets, Phys Rev. Lett, to be published; G. Mariotto, S.Murphy, I.V. Shvets Phys.Rev.B 66 245426 (2002)].

The present invention is directed towards providing an improved tunnel magnetoresistance -(TMR) device which will overcome certain of the problems with known TMR's and which will additionally provide a TMR whereby the resistance of the device can be switched by a current or voltage pulse. Further, the invention is directed towards providing an improved construction of such TMR.

Another objective of the present invention is directed towards providing a multi-stable switch, i.e. the resistive element whose resistance can be altered by a voltage or a current pulse.

Statements of Invention

According to the invention, there is provided a tunnel magneioresistance (TMR) device comprising:

a film laminate having two electrode layers separated by a thin dielectric layer for reception of electric current directed substantially orthogonal to one of the major exposed surfaces thereof characterised in that at least one of the electrode layers is of a magnetic material and in which the dielectric layer is one of:

a magnetic material;

a laminate of a ferromagnetic and anti-ferromagnetic material; and

a laminate of a non-magnetic dielectric material on a magnetic material.

The advantage of the present invention over a conventional TMR device is that the resistance change does not occur solely in response to the change in magnetisation direction of one electrode with respect to the other. The change in magnetoresistance occurs even when both electrode layers have the same direction of magnetisation. In contrast to conventional TMR devices, the resistance to change is not proportional to the spin polarisation of each of the electrode layers.

In particular, it will be noted that an advantage of the present invention is that the performance of the device is based on the alteration of the height of the tunnel barrier which alters the barrier transparency.

In one embodiment of the invention, both electrodes are of a magnetic material.

In another embodiment, both electrode layers have the same direction of magnetisation or in another embodiment, the electrode layers have different directions of magnetisation. The advantage of the latter is that the effect of spin polarisation can also be used with the present invention.

Ideally, in this latter embodiment, the electrode layers are of a material having a relatively high degree of spin polarisation.

In one embodiment of the invention, at least one of the electrode layers is of a ferromagnetic material, as is the dielectric layer.

In another embodiment, at least one of the electrode layers is of a ferromagnetic material and the other dielectric layer is of an anti-ferromagnetic material.

In a still further embodiment, at least one of the electrode layers is of an anti- ferromagnetic material and the other dielectric layer is of a ferromagnetic material.

In one embodiment of the invention, one of the electrode layers is of a non-magnetic material.

In another embodiment of the invention, the dielectric layer is a composite tunnel barrier layer comprising a laminate of a magnetic layer, sandwiched between two non-magnetic layers or alternatively it is a sandwich of a magnetic layer and a nonmagnetic layer.

In another embodiment of the invention, the dielectric layer is a composite tunnel barrier layer comprising a laminate of a non-magnetic layer, sandwiched between two magnetic layers.

In a still further embodiment, the dielectric layer is a laminate of non-magnetic dielectric material on a magnetic material in which the non-magnetic material is one of:

MgO MgAI₂O₄

AI₂O₃, and S_rT,O₃.

In another embodiment of the invention, the anti-ferromagnetic material is one of:

NiO

CoO

Ci-Fe₂O₃ a sulphate, a transition metal oxide, and a selenide.

In one embodiment of the invention, the material and thickness of the dielectric layer is chosen to have a resistance not greater than 10⁸Ω per μm². The electrode layers may be of a ferromagnetic spinel oxide.

Ideally, the thickness of the dielectric layer is between 0.2 and 20 nm.

In a still further embodiment of the invention, there is provided a multi-stable switch in which there are two magnetic electrode layers, the magnetisation of one of the electrode layers being pinned.

In one embodiment of a multi-stable switch, the electrode layers are of a ferromagnetic material.

In another embodiment of the invention, the direction of the magnetisation of the electrode layers is so chosen as to be different to provide a device, the operation of which mirrors that of a magnetic diode. With this latter embodiment, means are provided to alter one of the magnetisations of each magnetic electrode layer and the dielectric layer.

The material of the dielectric layer is so chosen that the sensitivity to the magnetic field of the device is based on the dependency of the height of the tunnel barrier resulting from exchange interaction of tunnelling electrons emitted by one of the electrode layers with electrons of the tunnel barrier.

Further, the direction of spins within the dielectric layer can be altered by the voltage or current pulse between the two electrode layers.

Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example with reference to accompanying drawings in which:

Figs. 1 to 6 are schematic cross-sectional views of various tunnel magnetoresistance (TMR) devices according to the invention.

None of the embodiments described below include connection wires and elements of the circuitry and circuit infrastructure that is required to form a functioning sensor or to integrate the sensor into the memory cell. These elements are common in the state-of-the-art and therefore are known to those skilled in magnetic tunnel junctions. For the same reason none of the embodiments illustrate substrate, seed layers, buffer layer, protection layer, exchange bias layer and other layers that are typically added in the stack of a magnetic tunnel junction. In a typical stack of magnetic tunnel junction, one could count up to 10 or 20 different layers. The use of these layers is known to specialists in the field and the specific arrangement depends on the specific application of the device, the materials and the fabrication processes used. In order to focus the attention of the fundamentals of the invention we have shown just the key functional layers.

In this specification the term "ferromagnetic" includes both notions as commonly known to those skilled in the art of magnetism, namely, ferromagnetic and ferrimagnetic. It should be appreciated that although no distinction between the two is often made, strictly speaking they relate to two different classes of materials. Ferrimagnetic is related to materials having more than one magnetic sub-lattice and having net magnetic moment. For example, the material Fe₃O₄ that is often referred to as being ferromagnetic, is in fact ferrimagnetic. Therefore, for the purposes of this invention we do not make any distinction between the two.

Further, the term "magnetic" is used to encompass any of the three: "ferromagnetic", "ferrimagnetic" and "antiferromagnetic". The term antiferromagnetic is used as known to specialists in the field and means magnetic material with more than one magnetic sub-lattice and effectively no net magnetic moment.

Referring to Fig. 1 , there is illustrated a tunnel magnetoresistance device, indicated generally by the reference numeral 1, comprising two ferromagnetic layers 2 and 3, sandwiching therebetween a further ferromagnetic dielectric layer 4. In this specification, the term "electrode layer" and "electrode" and similarly "dielectric layer" and "dielectric" are used somewhat interchangeably, since the electrode layer forms an electrode. Further, the direction of magnetisation in the electrodes and in the electrode layers 2 and 3, and in the dielectric layer or barrier 4, are identified by the arrow M and appropriate subscripts.

Like in the conventional spin-dependent tunnel junctions the electrode layers 2, 3 are characterised by a hicjh degree of spin polarisation at the Fermi level. However, contrary to the conventional approach for the spin-dependent tunnel junctions, we now assume that magnetisations M₂ and M₃ in both electrodes have the same direction. As it will be explained later it is not necessary for the operation of the device to always ensure that the two magnetisations M₂ and M₃ are parallel to each other. However, in order to explain more clearly the concept of the invention and the difference between the invention and the conventional TMR device, it is easier to assume that they are parallel. Crucially, contrary to the conventional approach, the tunnel barrier is composed of a ferromagnetic dielectric material. This could be one of the ferromagnetic oxides e.g. one of many ferromagnetic spinels like NiFe₂O₄, MnFe₂O₄, yttrium iron garnet, ferrites, and many other materials known to specialists in the field of magnetic materials. Like in the conventional TMR device the dielectric layer should preferably be uniform and free from pinholes, i.e. the two electrodes should not be in direct electric contact with each other. The thickness of the dielectric layer should be in the range of 0.2 nm to 20 nm (1 nm =10^'9 m) so that the tunnel barrier between the two electrodes should have a reasonable transparency, i.e. the resistance between the two electrodes is preferably not greater than some 10⁸ Ω even for the junction area as small as 1 μm². The direction of magnetisation M₄ in the barrier is different from the direction of magnetisations M₂ and M₃ in the electrodes . It appears that the tunnel current between the two ferromagnetic electrodes depends on the relative direction of the spin of electrons emitted by the electrodes with respect to the magnetisation in the tunnel barrier. The reason is that the tunnelling electrons see interaction with the spins of the dielectric layer through additional exchange energy /2 are

electron spins in the first electrode and barrier respectively. This additional energy either increases or decreases the effective tunnel barrier depending on the relative direction of spins in the electrodes and the ferromagnetic layer. Therefore, the tunnel current at low bias voltage V is

where signs - and + in (1) correspond to the cases of the spin directions in the tunnel barrier and in the electrodes being parallel and antiparallel to each other respectively, provided J is positive (ferromagnetic exchange) and vice versa for negative J. G is the barrier conductivity per unit area, m is the free electron mass, φ is the barrier height, d is the barrier width, % is the Plank constant. In writing (1) we assume 100% spin polarisation at the Fermi level. To simplify the formula one can assume that J«φ. Then, the barrier conductivity for the two cases of the spin direction in the barrier parallel and antiparallel to the spins of electrodes is:

G«G_oex_T| +^pA (2)

L NΦ J where G₀ is the barrier conductivity per unit area in the absence of the additional exchange. Hence the relative change of the conductivity is AG/ < G >= itssMk^d) ,

where

and <G> is the average conductivity. If we substitute in these formulae the typical values of the barrier width d=1nm, the barrier height φ=4eV and the exchange J=0.1eV, we obtain /c_e/?=0.29nm^"1 and the change of tunnel magnetoresistance (TMR) of 55%. Thus the TMR device according to the invention is based on the changes in tunnelling magnetoresistance in response to the change in the relative orientation of magnetisations in the electrodes and the magnetic dielectric barrier.

At this point one can summarise the key differences between the conventional tunnel magnetoresistance device and the one in accordance with the current invention.

i). In the conventional TMR device the resistance change occurs in response to the change in magnetisation direction of the first electrode with respect to the second one. In the embodiment of the invention described above, both electrodes have the same direction of magnetisation. It will be appreciated that the directions of magnetisation of the electrodes could also be different. This leads to a modification of the formula above. However, either way the response of the TMR device to the magnetic field is not solely dependent on the rotation of magnetisation M₁ of the first electrode with respect to the magnetisation M₃ of the second electrode.

ii). In the conventional TMR device the resistance change is proportional to the spin polarisation of each of the electrodes. This is well explained in the previously referenced paper [J. Slonczewski, Phys. Rev. B 39, 6995 (1989)]. Therefore, if the second electrode has zero spin polarisation, the conventional TMR device has no sensitivity to the magnetic field. In the present invention, the device will have sensitivity to a magnetic field even if the spin polarisation of the second electrode is zero.

iii). In the conventional TMR the dielectric barrier is nonmagnetic. In the present invention, it is magnetic. The spin polarisation of the dielectric barrier cannot be defined in the conventional sense of the definition of spin polarisation. Indeed a dielectric material has no electrons in both spin up and spin down bands at the Fermi level meaning that the spin polarisation would be calculated as the ratio of zero to zero, i.e. mathematical nonsense. iv). In the invention proposed the performance of the device is based on the alteration of the height of the tunnel barrier which alters the barrier transparency. In the conventional TMR devices the barrier is kept constant and the effect is based on the control of the electron density of the electrons capable of performing tunnelling to the empty states of the second electrode.

Referring now to Fig. 2, there is illustrated an alternative tunnel magnetoresistance device, again indicated generally by the reference numeral 2, in which parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the electrode layers 2 and 3 are of a ferromagnetic material, however, the dielectric layer 2, that is to say, the dielectric barrier, is of an anti-ferromagnetic dielectric material.

This could be e.g. an antiferromagnetic oxide (NiO, CoO, FeO, Ct-Fe₂O₃, etc) or some of the sulphates, tellurides or selenides. Again, like in the conventional TMR device the dielectric layer preferably should not contain pin-holes, which, as it will be appreciated by those skilled in the art, is required to ensure that the two electrodes are not in contact with each other. The thickness range of the antiferromagnetic dielectric barrier is comparable to the one indicated for the ferromagnetic barrier with reference to Fig, 1. In the same way as with the case of a ferromagnetic dielectric barrier, the exchange interaction should appear in the Hamiltonian for the barrier region and with it in the tunnelling. In this case the spin operator of barrier electrons S_b is replaced by the antiferromagnetic operator l__b and the angular dependency of the current on the direction of magnetisation in the ferromagnetic electrode should reflect the symmetry of the antiferromagnetic material. One can expect that the rotation of magnetisation M₂ by _% with respect to the antiferromagnetic direction A of the dielectric layer should not alter the current because of the symmetry considerations, whereas, rotation by π/2 should. This is similar to the angular dependency in the tunnel junction consisting of one ferromagnetic and one antiferromagnetic electrodes separated by a nonmagnetic dielectric layer, as previously referenced, [A.A. Minakov, I.V. Shvets, Surf. Sci. 236 (1990) L377-L-381]. Similarly, one could construct the TMR device with both electrodes of antiferromagnetic material and a ferromagnetic dielectric layer. As stated above, one could add further layers to the TMR device including protective layers, or each of the layers described could be a composite layer, i.e. it could consist of two more layers. This applies to any other embodiment described in the specification.

One could estimate the value of the exchange interaction J if the wave function of the electron state within the barrier is known. The conventional formula for the exchange energy is

J = jV«(«i Vl(«2)%(^rjH(^ri)*l*2 ^• (³)

Here ψ_a is the wave function of the tunnelling electron. This could be written as ^₀(¹O = V_o (O)exp(-fo) . where k = φ.mφ I % and ψ_b is the wave function of the electrons in the dielectric layer. H is the Hamiltonian. In the case of the weak electron interactions the exchange constant can be calculated using the simplified formula

and

Referring now to Fig. 3, there is illustrated a further TMR, again indicated generally by the reference numeral 1 and again, parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the dielectric layer 4 comprises a laminate, namely, a non¬ magnetic dielectric layer 4(a) and a magnetic dielectric layer 4(b), in this embodiment of a ferromagnetic material. Both of the electrodes 2 and 3 are of a ferromagnetic material. Essentially, the dielectric layer 4 is formed by depositing a layer of conventional nonmagnetic dielectric material on top of a layer of magnetic material. For example this could be a layer of MgO or MgAI₂O₄, AI₂O₃, SrTiO₃ or indeed another suitable nonmagnetic dielectric layer. The reason for this approach is that many dielectric materials do not readily form homogeneous layers in the thickness range of a few nanometers and thinner that is required to construct tunnel barrier of reasonable transparency and homogeneity. Therefore, in order to seal the pin-holes in the magnetic dielectric layer one may apply on top of the magnetic dielectric layer a nonmagnetic barrier with good dielectric properties in the small thickness range. This however, does not change the essence of the invention and the presence of the magnetic dielectric layer is the key to the functionality of the TMR device. Alternatively, one could first form the nonmagnetic dielectric layer free from pin-holes. Then on top of the nonmagnetic dielectric layer one could form the magnetic dielectric layer. This alteration does not affect the concept of the device and is merely related to the manufacturing procedures.

Referring now to Fig. 4, there is shown a still further alternative construction of TMR device, again indicated generally by the reference numeral 1 and again, parts similar to those described with reference to the previous drawings, are identified by the same reference numerals. In this embodiment, the electrode layer 2 is of a non-magnetic material, while the electrode layer 3 is of a magnetic material, sandwiching therebetween a ferromagnetic dielectric layer 4. The embodiment operates in much the same way as the embodiment described with reference to Fig. 1. In the same way the concept of the spin sensitivity is based on the dependency of the transparency of the tunnel barrier on the relative directions of the magnetisations in the magnetic electrode and the dielectric magnetic layer.

Referring now to Fig. 5, there is illustrated a further TMR device, again indicated generally by the reference numeral 1 , and is substantially similar to the device illustrated in Fig. 2, except now one of the electrode layers 2 is of a non-magnetic material. The dielectric layer 4 is of a anti-ferromagnetic material. One could also construct a TMR having a ferromagnetic dielectric layer sandwiched between an antiferromagnetic conducting electrode and a nonmagnetic electrode or between two ferromagnetic electrodes. In either of these two embodiments, the resistance of the TMR device changes when the magnetisation of the ferromagnetic layer changes with respect to the antiferromagnetic direction of the antiferromagnetic layer.

Referring now to Fig. 6, there is illustrated a further TMR, again indicated generally by the reference numeral 1 and again, parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the tunnel barrier or dielectric layer 4 is composed of at least two layers. One of them is a magnetic layer 4(a) and the other one is a dielectric layer 4(b) that is deposited to seal the pin-holes in the magnetic dielectric layer. The construction of this tunnel barrier 4 is similar to the one described with reference to Fig. 3. As explained in the embodiment relating to Fig. 3, the magnetic dielectric layer could be deposited first and a nonmagnetic layer on top or vice versa. One could also consider the case of a composite barrier consisting of more than two layers. For example the barrier could comprise of a magnetic dielectric layer sandwiched between two nonmagnetic dielectric layers or vice versa: a nonmagnetic dielectric layer interposed between two magnetic dielectric layers. Other cases with more than three layers can be constructed as well.

An interesting property of the TMR device 1 is shown in Figs. 4,5,6. The current in the device depends on the polarity of the bias voltage applied to it. This is very different from the conventional TMR device of two ferromagnetic electrodes separated by a nonmagnetic dielectric. In the conventional TMR device the tunnel current depends on the product of spin polarisations P₂ and P₃ of the two electrodes and also on the angle between the magnetisation directions of the electrodes. When the current direction is reversed the resistance of the junction is not affected. In the case of the TMR device 1 described in Figs. 4,5,6 the value of the voltage drop across the device is altered when the direction of the current through it is reversed. Therefore the device shown in Figs 4,5,6, effectively forms a magnetic diode, i.e. it is a diode as its I-V curve is asymmetric and furthermore the I-V curve can be controlled by altering the magnetisation of the magnetic electrode or the dielectric layer.

It should be pointed that magnetic moment of a ferromagnetic layer can be altered by current of spin-polarised electrons injected into the layer. This has been predicted theoretically in [Slonczewski J. C, Journal of Magn. Magn. Mater. 159 (1- 2) L1-L7 (1996)]. Yet, more experimental evidence needs to be collected to understand if and how this effect could be used to alter magnetisation in real systems. For example there are active efforts by many research groups to demonstrate a multistable switch based on magnetic tunnel junction. In this approach magnetisation of one of the ferromagnetic electrodes is pinned and the magnetisation of the second electrode is expected to be altered from one stable state to the other one when current pulse is applied between the two electrodes. It is envisaged that the same approach could be used in the case of the device described in the present invention.

The invention is not limited by the accuracy of the formulas (1-5). The formulas (1-5) are given here merely to explain the essence of the invention rather than be used as a quantitative guide. It is expected that the accuracy of the formulas depends on the detailed model used to describe different aspects on the tunnelling process and physics of exchange interaction. This will be apparent to those skilled in this technology.

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms "include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail within the scope of the claims.

Claims

1. A tunnel magnetoresistance (TMR) device (1 ) comprising:

a film laminate having two electrode layers (2, 3) separated by a thin dielectric layer (4) for reception of electric current directed substantially orthogonal to one of the major exposed surfaces (5, 6) thereof characterised in that at least one of the electrode layers (2, 3) is of a magnetic material and in which the dielectric layer (4) is one of:

a magnetic material;

a laminate of a ferromagnetic and anti-ferromagnetic material; and

a laminate (4(a), 4(b)) of a non-magnetic dielectric material on a magnetic material.

2. A TMR device (1) as claimed in claim 1, in which both electrode layers (2,3) are of magnetic material.

3. A TMR device (1) as claimed in claim 2, in which the electrode layers (2, 3) have substantially the same direction of magnetisation (M₂, M₃).

4. A TMR device (1) as claimed in claim 2, in which the electrode layers (2, 3) have different directions of magnetisation (M₂, M₃).

5. A TMR device (1) as claimed in claim 4, in which the electrode layers (2, 3) are of a material having a relatively high degree of spin polarisation.

6. A TMR device (1) as claimed in claim 1 , in which at least one of the electrode layers is of a ferromagnetic material, as is the dielectric layer (4).

7. A TMR device (1) as claimed in claim 1 , in which at least one of the electrode layers (2) is of a ferromagnetic material and the dielectric layer (4) is of an anti-ferromagnetic material.

8. A TMR device (1) as claimed in claim 1, in which at least one of the electrode layers (2) is of an anti-ferromagnetic material and the dielectric layer (4) is of a ferromagnetic material.

9. A TMR device (1) as claimed in claim 1 , in which one of the electrode layers (2, 3) is of a non-magnetic material.

10. A TMR device (1) as claimed in claim 1, in which the dielectric layer (4) is a composite tunnel barrier layer comprising a laminate of a magnetic layer (4(b)), sandwiched between two non-magnetic layers (4(a)).

11. A TMR device (1 ) as claimed in claim 1 , in which the dielectric layer (4) is a composite tunnel barrier layer comprising a laminate of a non-magnetic layer (4(b)), sandwiched between two magnetic layers (4(a)).

12. A TMR device (1 ) as claimed in claim 1 , in which the dielectric layer (4) is a laminate of non-magnetic dielectric material on a magnetic material in which the non-magnetic material is one of:

MgO MgAI₂O₄ AI₂O₃, and SrT₁O₃.

13. A TMR device (1) as claimed in any preceding claim, in which the anti- ferromagnetie material is one of:

NiO

CoO α-Fe₂O₃ . a sulphate, a transition metal oxide, and a selenide.

14. A TMR device (1) as claimed in any preceding claim, in which the material and thickness of the dielectric layer (4) is chosen to have a resistance not greater than 10⁸Ω per μm².

15. A TMR device (1) as claimed in any preceding claim, in which the electrode layers (2, 3) are of a ferromagnetic spinel oxide.

16. A TMR device (1) as claimed in any preceding claim, in which the thickness of the dielectric layer (4) is between 0.2 and 20 nm.

17. A TMR device (1) as claimed in any preceding claim, comprising a multi- stable switch in which there are two magnetic electrode layers (2, 3), the magnetisation of one of the electrode layers being pinned.

18. A TMR device (1) as claimed in claim 17, in which the electrode layers (2, 3) are of a ferromagnetic material.

19. A TMR device (1) as claimed in any preceding claim, in which the direction of the magnetisation of the electrode layers (2, 3) is so chosen as to be different to provide a device, the operation of which mirrors that of a magnetic diode.

20. A TMR device (1) as claimed in claim 19, in which means are provided to alter one of the magnetisation of the or each magnetic electrode layer (2, 3) and the dielectric layer (4).

21. A tunnel magnetoresistance (TMR) device (1 ) as claimed in any preceding claim in which the material of the dielectric layer (4) is so chosen that the sensitivity to the magnetic field of the device is based on the dependency of the height of the tunnel barrier resulting from exchange interaction of tunnelling electrons emitted by one of the electrode layers (2,3) with electrons of the tunnel barrier (4).

22. A tunnel magnetoresistance (TMR) device (1 ) as claimed in any preceding claim in which the direction of spins within the dielectric layer (4) can be altered by the voltage or current pulse between the two electrode layers (2,3).