WO2010131182A1

WO2010131182A1 - Magnetic cellular devices

Info

Publication number: WO2010131182A1
Application number: PCT/IB2010/052047
Authority: WO
Inventors: Rolf Allenspach; Leo Gross; Gerhard Meyer; Reto Schlittler
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2009-05-13
Filing date: 2010-05-10
Publication date: 2010-11-18
Anticipated expiration: 2011-11-13
Also published as: TW201106624A

Abstract

A magnetic cellular device (1) comprises a plurality of magnetically-coupled nano magnets (2). Each nano magnet (2) is bistable in states with opposite magnetization directions parallel to a first axis (A), and metastable in a state with a magnetization direction parallel to a second axis (B). The nano magnets (2) include at least one input nano magnet for receiving an input to the device via coupling of a local magnetic field with that nanomagnet. The arrangement is such that the nanomagnets (2) can be set to their metastable states by an applied magnetic field, and, on removal of the field, sequential switching of nanomagnets (2) from their metastable state to a bistable state can be initiated at the input nanomagnet(s), in response to the input, to produce an output state of the device (1). Devices (1) can be constructed for signal processing and logic gate functionality, and plural devices can be coupled together to form digital circuits in which an output state of at least one device (1) can provide an input for at least one other device (1).

Description

MAGNETIC CELLULAR DEVICES

This invention relates generally to magnetic cellular devices. The theory behind magnetic cellular devices, often called "magnetic quantum-dot cellular automata" (MQCA), is that arrangements of coupled nanomagnets can be employed for logic functionality and signal processing. The nanomagnets are sub-micrometer magnets of small enough size that each one contains only a single magnetic domain (in the ideal case), or sufficiently few magnetic domains that the domain behaviour is predictable. Currently, high-resolution electron beam lithography techniques can be used to form such nanomagnets as thin layers of magnetic material on a semiconductor substrate such as silicon. The individual magnets are formed as "dots", of circular or oval shape for example, and are magnetically coupled via local magnetostatic (magnetic dipole-dipole) interactions. The nanomagnets can be made to act as bistable elements having stable states in which the magnetization vector points in either of two, opposite directions parallel to a fixed axis. In particular, a shape anisotropy can be imposed by elongating a nanomagnet. The longitudinal axis then defines the axis of alignment for the bistable states. This is the axis of "easy" magnetization, whereby the nanomagnet can be magnetized such that its magnetization vector is aligned, in either direction, with this axis by temporary application of an external magnetic field. If an external magnetic field of sufficient strength is applied with a field direction perpendicular to the easy axis, the magnetization vector of the element can be temporarily rotated away from the easy axis to align with the applied field. When this field is removed, the magnetization vector will return to one of its two stable orientations along the easy axis. Which of the bistable states is adopted can be influenced by any stray magnetic field in the neighbourhood of the element. The nanomagnets are therefore arranged such that each is magnetically coupled with (i.e. within influence of the field emanating from) one or more of its neighbours. The principle is that, by arranging the nanomagnets appropriately, the nanomagnets can be caused to switch in a controlled manner to perform a desired function. It is for this reason that single-domain nanomagnets are highly preferred since the stray field is much stronger, providing better coupling between neighbouring elements. MQCA technology is currently in its infancy. Signal propagation along a line of circular nanomagnets is described in "Room Temperature Magnetic Quantum Cellular Automata", Cowburn and Welland, Science 287, 1466 (2000), and also in US Patent No. 6,774,391. A three input majority gate is described in "Majority Logic Gate for Magnetic Quantum-Dot Cellular Automata", Imre et al, Science 311, 205 (2006). This gate is also discussed in related papers: "Magnetic QCA Systems", Bernstein et al., Microelectronics Journal 36, (2005) 619-624; and "Magnetic Quantum-Dot Cellular Automata: Recent Developments and Prospects", Orlov et al., Journal of Nanoelectronics and Optoelectronics", Vol. 3, 1-14, 2008. The majority gate uses single-domain nanomagnets which are elongated to define the axis of easy magnetisation as described above. The nanomagnets are arranged in a cross formation with their easy axes parallel. Three additional nanomagnets are used as driver nanomagnets to apply inputs to the gate. These are positioned next to respective input nanomagnets which form three points of the cross. The driver nanomagnets are orientated with their easy axes perpendicular to that of the nanomagnets in the cross. In operation, an applied magnetic field (the "clock field ") aligned with the easy axes of the driver nanomagnets magnetizes these along their easy axes, and at the same time forces the gate nanomagnets into their null state. This state is magnetically unstable, however, persisting only for the duration of the applied field. When the clock field is removed, the driver nanomagnets remain in their bistable states, and magnetic coupling between these and the input nanomagnets influences the switching, or "decay", of the gate nanomagnets to one of their bistable states. The state of a nanomagnet at the fourth point of the cross provides the gate output. Being a majority gate, the bistable state of the output nanomagnet should have a magnetization aligned with that of the majority state of the three input nanomagnets. In practice, very tight constraints, e.g. on the form and arrangement of the nanomagnets and application of the clock field, are required to achieve operation of the prior systems. This is so even with relatively small numbers of nanomagnets, and even then operation is unreliable (see for example the results discussed in the Orlov reference above, on page 9, column 2 of that paper). For application in real devices, reliability must be greatly improved. Moreover, individual devices (or "cells") will need to be coupled, so that outputs of logic gates can form inputs to other logic gates, and provision must be made for signal amplification. No feasible concept for meeting these objectives has been proposed thus far.

One aspect of the present invention provides a device comprising a plurality of magnetically-coupled nanomagnets, wherein: each nanomagnet is bistable in states with opposite magnetization directions parallel to a first axis, and metastable in a state with a magnetization direction parallel to a second axis; the nanomagnets include at least one input nanomagnet for receiving an input to the device via coupling of a local magnetic field with that nanomagnet; and the arrangement is such that the nanomagnets can be set to their metastable states by an applied magnetic field and, on removal of that field, sequential switching of nanomagnets from their metastable state to a bistable state can be initiated at said at least one input nanomagnet, in response to said input, to produce an output state of the device. The present invention is predicated on the realization that problems with prior MQCA systems stem from the fact that the null state induced by the clock field is magnetically unstable. As soon as the clock field is removed, no force exists to prevent the nanomagnets decaying to a stable state. Hence all nanomagnets tend to switch substantially simultaneously. As a consequence, considering a sequence of nanomagnets between the input and output of a device, nanomagnets towards the output will tend to switch before the input signal has actually arrived. Effective propagation of an input signal along the sequence requires that each nanomagnet should only switch when the state of its predecessors in the sequence has been determined. The simultaneous switching prevents effective signal propagation and leads to unreliable outputs. This problem is exacerbated as the number of nanomagnets is increased. Moreover, individual cells cannot be sequentially coupled since their component nanomagnets will decay simultaneously, i.e. the second cell will have switched to a stable configuration before the signal from the first cell has arrived.

In contrast, devices embodying this invention use nanomagnets which have an additional, metastable magnetic state in which the magnetization direction is aligned along a different axis to the axis of magnetic bistability. This additional state is only metastable in that it constitutes a state of more delicate equilibrium than the lower-energy, bistable states where the magnetization is aligned with the first axis. However, being metastable, this state will persist unless the nanomagnet is sufficiently disturbed to switch to one of the more stable bistable states. The devices are arranged such that the nanomagnets can be set to their metastable states by an applied field such as a clock field, and sequential switching of nanomagnets to a bistable state can be initiated by applying a local magnetic field, to provide an input to the device, to one or more input nanomagnets of the device. The metastable state provides for sequential, rather than simultaneous, switching since each nanomagnet will hold the metastable state until the state of its predecessor(s) in the coupling sequence from the input nanomagnet(s) has been determined as required for operation of the device. In simple terms, a nanomagnet only switches when its predecessor(s) have switched (i.e. when the signal has arrived at that nanomagnet), this then providing a local coupling field sufficient to disturb the metastable state of that nanomagnet. In this way, the reliability problems of prior systems can be avoided and correct signal propagation can be assured. Moreover, correct switching can be achieved even with large numbers of nano magnets, and devices can be reliably coupled with the output of one device providing the input to another device. In addition, practical devices for signal amplification can be constructed as discussed further below. Embodiments of this invention thus provide all the ingredients for performing arbitrary logic operations and forming complex signal processing networks.

The bistable and metastable states of a nanomagnet can be defined by purely material (crystalline) anisotropies or by a combination of material and shape anisotropies. For example, materials can be produced whose crystalline properties provide inherent magnetic anisotropies of different character along two axes. The axis with lowest energy, i.e. in general with the higher coercive field, then provides the first axis and the other axis provides the second axis as defined above. Alternatively, for example, materials with biaxial anisotropy which is identical for each axis may be employed. A shape anisotropy can then be superimposed on this to increase the coercivity of magnetization states along one axis relative to another. This can be done simply by making the nanomagnet elongate along the direction of one axis, i.e. the first axis as defined above. In general, however, any combination of material and shape anisotropy which produces the required magnetic properties can be employed.

Nanomagnets might be envisaged which are metastable with a magnetization aligned in one direction, but not the other, along the second axis, and this is sufficient for the operation described. More usually, however, each nanomagnet is metastable in two states with opposite magnetization directions parallel to the second axis. Such a nanomagnet is effectively bistable along both axes, but more weakly so along the second axis. This allows the metastable state of nanomagnets to be induced by an applied field in either direction along the second axes. In particular, in some embodiments the device can be set to its metastable state by both polarities of an applied field.

Devices embodying the invention may of course include other elements in addition to the biaxially-anisotropic nanomagnets described above. For example, magnetic elements with uniaxial magnetic anisotropy might be included in devices, to boost signal levels for instance. In general, therefore, each of the nanomagnets described above may be directly or indirectly coupled to one or more of the other nanomagnets.

Various factors are relevant in determining a particular arrangement of the nanomagnets (and any other elements of a device) which can implement the required sequential switching from metastable states of the nanomagnets to produce an output state of the device. These include factors such as the size, shape, orientation and magnetic characteristics of the elements as well as of course on the relative positioning of elements and the intended function of the device itself. The key point is that the arrangement ensures that nanomagnets switch sequentially, i.e. that each switches in response to switching of its predecessor element(s) as described above. Of course, which particular nanomagnets switch, and in which direction, can depend on the particular input applied. Depending on device function, all, some or no nanomagnets may switch in any single operating cycle to produce the required output state. In the output state, the device output may be represented in general by the magnetization state of one or more elements of the device.

For simplicity in preferred embodiments, the nanomagnets are substantially identical elements. In particularly simple implementations, the first axis of each nanomagnet is substantially perpendicular to the second axis, and nanomagnets are arranged with their respective first and second axes substantially parallel to those of the others. Other arrangements can be readily envisaged however, for example where the axes of different nanomagnets are differently orientated. The characteristics of the external magnetic field required to induce the metastable state of nanomagnets will depend, inter alia, on such arrangement details.

The number of input nanomagnets will of course depend on the intended device function, and the overall device input signal may consist of individual inputs at more than one input nanomagnet. The local magnetic field which provides the input can be applied to an input nanomagnet in a variety of ways, e.g. by magnetizing an input element of the device which is locally coupled to one or more input nanomagnets. Preferred devices include an input element in the form of a conductor for carrying an electric current to apply the magnetic field to an input nanomagnet, or an input element which is set using the spin-momentum transfer torque effect (see J. Slonczewski, J. Magn. Magn. Mater. 159, Ll (1996); L. Berger, Phys. Rev. B 54, 9353 (1996)).

Devices embodying the invention can be constructed to operate as logic gates, an output nanomagnet of such devices adopting a state, after sequential switching in response to the applied input, which indicates an output of the logic gate. Embodiments can also be constructed to perform other signal processing functions, an important example being signal amplification. Particular examples of such embodiments will be described below. Moreover, as explained above, the metastability of nanomagnets enables devices to be coupled together to form more complex circuits. Thus, a second aspect of the invention provides a digital circuit comprising a plurality of devices according to the first aspect of the invention, the devices being magnetically coupled such that an output state of at least one device can provide a said input for at least one other device. Each individual device in such a circuit may include one or more of the preferred features discussed in relation to embodiments of the first aspect of the invention.

Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a first magnetic cellular device embodying the invention;

Figures 2a to 2f represent successive stages in a sequential switching process of the Figure 1 device; Figures 3 a and 3b demonstrate operation of the Figure 1 device with an alternative input; and

Figures 4a and 4b illustrate different output states of a second MQCA device embodying the invention.

Figure 1 illustrates the configuration of a first magnetic cellular device embodying the invention. The device 1 of this example is formed by an arrangement of seven substantially identical nano magnets 2 formed on a substrate 3. The nano magnets 2 are of generally oval shape, each nanomagnet being elongate in the direction of a first axis A and narrower in the direction of a second, perpendicular axis B. Each nanomagnet 2 is formed as a thin layer of a magnetic material. The crystalline structure of this material is such that the nanomagnet is magnetically anisotropic about both of the orthogonal axes A and B. This can be achieved using a material with fourfold crystalline anisotropy such as Co on stepped Cu(OOl) (see Weber et al, Phys. Rev. Lett. 76, 1940 (1996)), or thin films of Fe or FeCo on GaAs(OOl) (see Thomas et al., Phys. Rev. Lett. 90, 017205 (2003)). The orientation of the crystalline structure of the layer is such that the two axes of anisotropy are aligned with the axes A and B respectively.

The nanomagnets 2 are sufficiently small that each contains only a single magnetic domain. In general, the dimensional constraints for achieving this will depend on the material employed, though single-domain operation is typically obtained at element sizes in the region of lOOnm. In the present example, we assume that the nanomagnets are formed of FeCo on GaAs and have dimensions of 140nm and 60nm along axes A and B respectively. For the purposes of the simulations presented below, the material anisotropy (about both axes A and B) is taken as 2.65xlO⁵ J/m³. (This is a reasonable value, of the same order of magnitude as the crystalline anisotropies for Fe (about 5x10⁴ J/m³) and Co (about 5x10⁵ J/m³), the anisotropy in general being tunable by adjusting various factors such as material composition, film thickness, etc. as will be apparent to those skilled in the art). The device 1 can be produced using generally known techniques. For example, a thin film of FeCo can be deposited on a GaAs(OOl) substrate by molecular beam epitaxy (MBE). High-resolution electron beam lithography techniques can then be used to etch this film, leaving the pattern of nanomagnets 2 on the substrate 3.

The inherent biaxial material anisotropy of nanomagnets 2 means that each is effectively bistable in magnetic states with opposite magnetization directions along each of the axes A and B. However, the elongate shape of the element imposes an additional, shape- induced magnetic anisotropy about the "long" axis A. This makes the element more strongly bistable along this axis. That is, while the anisotropy value of 2.65xlO⁵ J/m³ is high enough to stabilize a magnetization vector aligned with the "short" axis B even in the absence of an applied field, these states are more weakly stable than those with magnetization directions along the long axis A. The nanomagnet is thus effectively metastable when magnetized in either direction along axis B since, if sufficiently disturbed, these states will decay to one of the lower-energy, bistable states in which the magnetization is aligned with axis A. When this occurs, the particular bistable state adopted can be influenced by any local magnetic field in the vicinity of the nanomagnet.

The nanomagnets of device 1 are arranged on substrate 3 such that the magnetic properties just described can be exploited to effect device operation. In particular, the nanomagnets are arranged such that each is magnetically-coupled, via local magnetostatic interactions, with at least one neighboring nanomagnet. In addition, the relative orientations are such that the nanomagnets can be set to a metastable state by application of an external magnetic field. In the simple embodiment shown, all nanomagnets 2 of device 1 are orientated with their short axes B parallel to the x-axis, and their long axes A parallel to the y- axis, of the Cartesian x, y space indicated in the figure. An external magnetic (clock) field can be applied to the device as a whole with a field direction parallel to the x-axis. Any convenient means, not shown in the figure, can be employed to apply this clock field as will be apparent to those skilled in the art. The field strength is sufficient to set all nanomagnets 2 to their metastable states, along short axes B, in line with the applied field direction. When the clock field is removed, all nanomagnets will hold this metastable state until sufficiently disturbed to decay to the ground state. To this end, device 1 includes an input element for applying a local magnetic field to the left-most nanomagnet 2 in the figure which serves as an input nanomagnet for the device. The input element is indicated schematically in the figure at 4. In this preferred embodiment, the input element is implemented by a conducting element such as a wire formed on substrate 3. Current flow in different directions along the wire produces a local magnetic field in the +x or -x direction, providing for two possible inputs to the device. (Alternative embodiments could use spin-transfer magnetic tunnel junction elements to set the input. In this case, a spin polarized current is injected into the input element setting its magnetization direction). The local field produced by input element 4 couples with the adjacent input nano magnet to trigger switching of this nano magnet from its metastable state to a bistable state. The particular bistable state adopted by the input nanomagnet depends on the direction of the local field applied by element 4. Thus two inputs, represented by the two possible bistable states of the input nanomagnet, can be applied to the device 1 by input element 4 on removal of the clock field. The ensuing operation of device 1 is described below with reference to Figures 2a to 2f.

Figures 2a to 2f illustrate the results of micromagnetic simulations for a device corresponding generally to device 1 described above. These simulations were performed using the Object Oriented Micromagnetic Framework (OOMMF) of the National Institute of Standards and Technology (see M. J. Donahue, D. G. Porter, OOMMF User's Guide Version 1.2 Interagency Report NISTIR 6376, http://math.nist.gov/oommf/). For the purposes of simulation, the input field is represented by the magnetization state of an input element in the form of a small magnetic element, of the same material as nanomagnets 2, whose long axis is parallel to the x-direction of Figure 1. Application of an external clocking field in the +x direction thus sets the magnetization of this input element to its bistable state aligned with the field direction. The clock field also sets all nanomagnets 2 to their metastable state with this alignment as already described. At time t=0, the clock field is switched off. The initial state at time t=0 is shown in Figure 2a, where the small arrows here and in subsequent figures represent schematically the magnetization state of individual elements. The nanomagnets 2 then decay sequentially from their metastable state to a bistable state as shown progressively in Figures 2b to 2f. In particular, the input nanomagnet switches first in response to the input field (Figure 2b). Magnetic coupling between this and the next nanomagnet then triggers switching of the next nanomagnet (Figure 2c), and so on. It is only under the influence of the local magnetic field emanating from neighbours that have already switched that the metastable state of a given nanomagnet becomes unstable. In the final, output state of the device (Figure 2f), the two right-most elements of the device are set to the same magnetization state. These two elements are the output nanomagnets of this device. Since setting of a single input nanomagnet results in setting of two output nanomagnets, the device 1 implements signal multiplication of the input signal, i.e. fan-out or amplification. It will be seen from the above that the configuration of nano magnets in Figure 1 provides a simple amplification device in which two output nanomagnets are magnetically coupled to a single input nanomagnet via a number of intervening nanomagnets. Sequential switching of the nanomagnets from their metastable states to a bistable state is initiated at the input nanomagnet to produce an output state in which the input signal is multiplied, in this case by a factor of two.

In order to prove correct functionality, simulations were performed using the same arrangement of nanomagnets but with an inverted input signal. In Figure 3a this is done by changing the position of input element 4. In Figure 3b the input element is unmoved but the polarity of the clock field is inverted. In both cases the final state of the two output elements is switched with respect to Figure 2f, indicating correct functionality.

Note that, in the example of Figures 2a to 2f, 3 a and 3b, the output nanomagnets switch from their metastable states to mutually- equivalent (here identical) bistable states which are opposite to that of the input nanomagnet. This device can therefore be viewed as an inverting amplifier. The device can easily be modified such that the output nanomagnets switch from their metastable states to equivalent bistable states to the input nanomagnet. In particular, by adding or removing elements in the anti- ferromagnetic ordering sequence between the input and output nanomagnets, the output can be brought into phase with the input. Considering Figure 2f for example, adding or removing a nanomagnet adjacent to the input nanomagnet would result in like states of the input and output nanomagnets in operation. Adding a nanomagnet before each output nanomagnet would have a similar effect, as would removing the two output nanomagnets thus making the preceding elements the output nanomagnets of the device.

While simple inverting and non-inverting amplifiers are described above, the actual gain can of course be tailored by changing the ratio of output to input nanomagnets.

Appropriate arrangements of nanomagnets to provide different gains will be readily apparent to those skilled in the art based on the operating principles described herein.

Devices embodying the invention can also be arranged to operate as logic gates. In this case, the magnetic state of an output nanomagnet (after the sequential switching process in response to the input field) provides an output of the logic gate. Figures 4a and 4b illustrate simulated output states for a two-input AND gate based on nanomagnets with the same basic characteristics as before. In this arrangement, however, there are two input nanomagnets magnetically coupled to a single output nanomagnet via four intervening nanomagnets as shown. The input elements are represented as before by smaller magnetic elements orientated at right angles to the input nanomagnets. In this example, however, an input of logic "1" is represented by a bistable state of an input nano magnet, induced by magnetization of a respective input element by the clock field as described above. Logic "0" is represented by a metastable state of an input nanomagnet, resulting from absence of an input field. Thus, the configuration of Figure 4a represents the AND gate in the "on" state (with inputs 1,1), and Figure 4b represents the equivalent gate in the "off state (with inputs 1,0). Note that the spacing of the elements in j-direction is increased compared to that in the earlier structure used for signal amplification. The central nanomagnet (and as a consequence the output nanomagnet) therefore switches only when both input nanomagnets have been set to like input states. This produces the output state of Figure 4a, representing an output of logic "1". Opposite inputs result in input nanomagnets in different magnetic states, one bistable and one metastable. In this case the central nanomagnet is not sufficiently perturbed to switch from its metastable state. Consequently, the output nanomagnet does not switch. This is the output state of Figure 4b, representing an output of logic "0". Thus, output "1" is obtained only for inputs (1,1), demonstrating correct AND-gate operation.

Other logic gates can of course be constructed using the basic principles described herein and possible structures will be apparent to those skilled in the art. By way of example, modified AND-gate arrangements having more than two inputs can be readily constructed using the principles described. Logic gates can also be constructed in which the two bistable magnetic states along the easy axis A of a nanomagnet are used to represent the "0" and "1" for binary logic, the input and output states being set accordingly.

It will be seen from the above that simple, effective and reliable magnetic cellular devices are provided for logic and signal processing functionality. The additional, metastable state of nanomagnets ensures that the nanomagnets switch only when required and in the correct sequence to give the output state appropriate to device functionality. Furthermore, simulations have been performed for all embodiments described above for the case of no input, i.e. with identical structures but without the input elements. In these cases no nanomagnet switched, i.e. all stayed in the metastable state. This proves that such automata can be used with delayed inputs, and can therefore be coupled sequentially and in parallel to form complex systems, since switching will not occur before the signals have actually arrived. Digital circuits for performing desired functions can thus be constructed using a plurality of individual devices of the type described herein, the devices being magnetically coupled such that an output state of at least one device can provide an input for at least one other device as required for circuit functionality. While simple embodiments have been described to illustrate the operating principles employed, numerous alternatives and modifications can be envisaged. For example, the embodiments described employ a material with biaxial material anisotropy which is identical about both easy axes, one of the easy axes being aligned with the easy axis imposed by the shape of the element to increase the stability of the bistable ground states relative to the metastable states. It is straightforward also to consider materials with more complex material anisotropies such as materials with inherent biaxial anisotropy with different characteristics about the two axes. Such materials might, for example, have a layer structure in which twofold and fourfold anisotropies are superimposed. Where material properties alone provide the difference in stability of magnetic states along two axes, shape anisotropy of elements may not be required. That is, nanomagnets need not necessarily be elongate. Of course, shape anisotropy may still be superposed if desired to tune the magnetic properties of elements.

Non-trivial angles between the axes of shape anisotropy and material anisotropies can also be envisaged, and in general the axes A and B of elements need not be orthogonal. Equally, nanomagnets need not necessarily be arranged with their axes mutually parallel, and clock fields could have different components (spatial and/or temporal) to allow particular nanomagnets to be set to their metastable states as required for device operation. Also, while use of identical nanomagnets is preferred for simplicity of design, this is not essential for operation. In general, the magnetic properties of devices can be tuned, by tuning anisotropies and/or adjusting form and arrangement of nanomagnets, to give the operation required. As one example, devices can be designed in which switching of any given nanomagnet to one of its bistable ground states requires a greater input field strength than switching to the other state. Moreover, additional elements may be included in devices, such as magnetic elements with only uniaxial anisotropy, if desired for device functionality, e.g. to boost signal levels at certain points.

Many other changes and modifications can be made to the exemplary embodiments described without departing from the scope of the invention.

Claims

1. A device (1) comprising a plurality of magnetically-coupled nanomagnets (2), wherein: each nanomagnet (2) is bistable in states with opposite magnetization directions parallel to a first axis (A), and metastable in a state with a magnetization direction parallel to a second axis (B); the nanomagnets (2) include at least one input nanomagnet for receiving an input to the device via coupling of a local magnetic field with that nanomagnet (2); and the arrangement is such that the nanomagnets (2) can be set to their metastable states by an applied magnetic field and, on removal of that field, sequential switching of nanomagnets (2) from their metastable state to a bistable state can be initiated at said at least one input nanomagnet, in response to said input, to produce an output state of the device (1).

2. A device as claimed in claim 1 wherein each nanomagnet (2) is a single-domain magnet.

3. A device as claimed in claim 1 or claim 2 wherein the nanomagnets (2) are substantially identical.

4. A device as claimed in any preceding claim wherein each nanomagnet (2) is metastable in two states with opposite magnetization directions parallel to the second axis (B).

5. A device as claimed in any preceding claim wherein the magnetic bistability of each nanomagnet (2) parallel to the first axis (A) is produced by a combination of material anisotropy and shape anisotropy.

6. A device as claimed in any preceding claim including an input element (4) for applying a local magnetic field, indicative of said input, to said input nanomagnet.

7. A device as claimed in any preceding claim wherein the first axes (A) of the nanomagnets (2) are substantially parallel.

8. A device as claimed in any preceding claim wherein the second axes (B) of the nanomagnets (2) are substantially parallel.

9. A device as claimed in any preceding claim wherein the first axis (A) of each nano magnet (2) is substantially perpendicular to the second axis (B).

10. A device as claimed in any preceding claim including at least one additional magnetic element magnetically coupled to a said nanomagnet (2).

11. A device as claimed in any preceding claim wherein the arrangement is such that the device is operative as a logic gate, the nanomagnets (2) including an output nanomagnet whose state in response to said input indicates an output of the logic gate.

12. A device as claimed in claim 11 including a plurality of said input nanomagnets, the arrangement being such that the output nanomagnet switches from its metastable state to a bistable state only in response to receipt of like inputs by said plurality of input nanomagnets.

13. A device as claimed in any one of claims 1 to 10 wherein the nanomagnets (2) include a plurality of output nanomagnets magnetically coupled to a said input nanomagnet, the arrangement being such that each output nanomagnet switches from its metastable state to an equivalent bistable state to the input nanomagnet in response to receipt of an input by the input nanomagnet.

14. A device as claimed in any one of claims 1 to 10 wherein the nanomagnets (2) include a plurality of output nanomagnets magnetically coupled to a said input nanomagnet, the arrangement being such that the output nanomagnets switch from their metastable states to mutually- equivalent bistable states opposite to that of the input nanomagnet in response to receipt of an input by the input nanomagnet.

15. A digital circuit comprising a plurality of devices (1) each as claimed in any preceding claim, the devices (1) being magnetically coupled such that an output state of at least one device (1) can provide a said input for at least one other device (1).