Abstract:
A device with adjustable resistance includes two magnetic elements separated by an insulating or semi-conductor element. The resistance of the device depends on the position of a magnetic wall in one of the magnetic elements, the magnetic wall separating two areas of said magnetic element each having a separate homogeneous direction of magnetization. The device comprises means for moving the magnetic wall in the magnetic element by applying a spin-polarized electric current, such that the resistance of the device is adjustable in a continuous range of values. The invention is useful in neuromimetic circuits, neural networks and bio-inspired computers.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a National Stage of International patent application PCT/EP2010/055902, filed on Apr. 30, 2010,which claims priority to foreign French patent application No. FR 0902122, filed on Apr. 30, 2009, the disclosures of which are incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an adjustable-resistance device, commonly called a memristor, that works through the movement of a magnetic wall by spin transfer. It is used for example in the field of chip-based neural network neuromimetic circuits and more generally in bio-inspired computer architecture. 
     BACKGROUND 
     In the last fifty years, purely digital Von Neumannian computing has progressed considerably. Nonetheless, even the most powerful computers using the most advanced algorithms are unable to quickly perform apparently simple processes, such as image interpretation, that are however performed in a fraction of a second by the human brain. The human brain indeed operates massively parallelly and analogically, unlike current computers. Chip-based analog neuromimetic circuits, which are intended to reproduce the operation of the human brain, make it possible to go beyond conventional architectures. Neural-network architectures work on the basis of learning methods: a circuit is caused to react in the desired manner to a given input. This is achieved by adjusting the values of the components of the circuit such as to converge on the desired output for a given input. Chip-based implementation therefore requires the use of nanometric, analog, reconfigurable and rapid components. 
     Until 2008, chip-based neuromimetic circuits were entirely built using transistors. Notably, several transistors were used to reproduce the plasticity of a single synapse connecting two neurons. 
     In 2008 however, the Hewlett-Packard team headed by Stanley Williams published several patents and articles proposing neural circuits built using one transistor per neuron and a “memristor” per synapse connecting two neurons (D. B. Strukov et al., Nature 453, 80 (2008) and J. J. Yang et al., Nature Nano. 3, 429 (2008)). A memristor is a nanometric resistor having a value configurable within a continuous range by the electric charge that previously crossed it. In the case of neural circuits, the main application thereof is to simulate the plasticity of synapses on a chip. A Hewlett-Packard memristor forms a structure comprising an insulating layer inserted between ordinary metal layers. By applying a current to the structure, oxygen vacancies are created, which migrate under the effect of the voltage and induce a resistance change. These memristors work on the basis of the effect of ion electromigration. Unfortunately, this ion electromigration effect involves high operating temperatures and therefore a potential fragility of the device. Above all, however, this ion electromigration effect results in low operating speeds, since it is related to the mobility of the ions: the resistance of Hewlett-Packard memristors changes very slowly. 
     This is why the applicant has turned to a technology radically different to the technologies used in known memristors, i.e. magnetoresistance. Indeed, the applicant has demonstrated in experiments that it is possible to move a magnetic wall in a magnetic element by spin transfer, in one direction or the other according to the sign of the current applied to the magnetic element. The applicant has notably measured this effect in spin valves, which are devices in which two magnetic layers are separated by a metal layer (J. Grollier et al., Appl. Phys. Lett. 83, 509 (2003)). Furthermore, it has recently been suggested (X. Wang et al., IEEE Electron Device Letters 30, 741 (2009)) that these spin valves could be used to make memristors. Surprisingly, this last publication gives no implementation details, since the realization of a memristor using a spin valve is difficult to imagine. This is because the memristive effect of the spin valves is negligible, because they provide limited magnetoresistance, about 10%. Consequently, a spin valve can only provide low-value resistance. This is why the realization of a memristor using a spin valve is difficult to envisage. 
     In 2005, the applicant filed international patent application WO 2006/064022 A1, which discloses a device having two magnetic elements separated by an insulating element. The device makes it possible to switch reproducibly between two stable magnetization states identifiable with logic states “0” or “1”, thereby enabling the storage of information. Switching between two magnetization states is effected by moving a magnetic wall in one of the magnetic elements between two stable positions. Unfortunately, this device does not make it possible to continuously vary the resistance within a range, since the magnetic wall only has two stable positions to which they gravitate irremediably. It therefore only provides two resistance values, each corresponding to one of the two magnetization states. Consequently, it is not a memristor. 
     SUMMARY OF THE INVENTION 
     The invention is notably intended to provide a magnetoresistive memristor, i.e. a resistor having a value that may be adjusted within a wide and continuous range, using the movement of a magnetic wall by spin transfer. For this purpose, the invention relates to a device with adjustable resistance comprising two magnetic elements separated by an insulating element or semi-conductor. The resistance of the device depends on the position of a magnetic wall in one of the magnetic elements, the magnetic wall separating two areas of said magnetic element each having a separate homogeneous direction of magnetization. The device comprises means for moving the magnetic wall in the magnetic element by applying a spin-polarized electric current, such that the resistance of the device is adjustable in a continuous range of values. 
     The invention also relates to an adjustable-resistance device having a stack of two magnetic layers separated by an insulating or semi-conductor layer, the stack forming a bar. A first of the two magnetic layers comprises a magnetic wall separating two zones having homogeneous directions of magnetization substantially parallel to the longitudinal axis of the bar but in opposing directions. The magnetic wall is substantially perpendicular to the longitudinal axis of the bar. The second of the two magnetic layers has a homogeneous direction of magnetization substantially parallel to the longitudinal axis of the bar. The device comprises means for moving the magnetic wall in the first magnetic layer along the bar by applying a spin-polarized electric current, such that the resistance of the device changes within a continuous range of values as the magnetic wall moves along the bar. 
     In a preferential embodiment, the means for moving the magnetic wall along the bar may include at least two electrodes arranged on the first magnetic layer and/or on the second magnetic layer, said electrodes making it possible to apply the spin-polarized electric current to the bar. 
     Advantageously, the magnetic wall can only move along the bar if the spin-polarized current applied between the electrodes has a current density above a given threshold. Above this threshold, the amplitude of movement of the magnetic wall along the bar may vary linearly with the duration of application of the spin-polarized current between the electrodes. The sign of the spin-polarized current applied between the electrodes can determine the direction of movement of the magnetic wall along the bar. 
     The device may include a nucleation reservoir at one extremity of the bar, to facilitate the creation of the magnetic wall. 
     In one embodiment, the magnetic wall in the first magnetic layer may be situated at one extremity of the bar, the first layer then having a single homogeneous direction of magnetization substantially parallel to the longitudinal axis of the bar, such that the resistance of the device is equal to one of the limits of the continuous range of values. The device may then be used as excitatory synapse between two neurons in a neural network, the current applied to the bar making it possible to reduce the value of the resistance from the upper limit of the continuous range of values. The device may also be used as inhibitory synapse between two neurons, the current applied to the bar making it possible to increase the value of the resistance from the lower limit of the continuous range of values. The device may also include artificial traps for the wall, such as to reproduce synaptic functions. 
     For example, the first magnetic layer may be a ferromaagnetic layer, such as a planar anisotropic magnetic material made of a NiFe alloy or a CoFeB alloy, or a perpendicular anisotropic magnetic material made of a CoPt alloy or of a FePt alloy, or a FePd alloy, or a Co/Ni stack, or a Co/Al 2 O 3  stack. 
     For example, the two magnetic layers may be separated by an insulating layer containing aluminum oxide Al 2 O 3  or magnesium oxide MgO. They may also be separated by a semi-conductor layer containing gallium arsenide GaAs or a GaAs-based compound. 
     For example, the second magnetic layer may also be a ferromagnetic layer. It may notably contain cobalt, or a NiFe alloy, or a CoPt alloy, or a FePt alloy, or a FePd alloy, or a Co/Ni stack or a Co/Al 2 O 3  stack. It may also contain a synthetic CoFeB/Ru/CoFeB antiferromagnetic. 
     The other main advantages of the invention are that, by using the effect of magnetoresistance, it provides a resistance having a value that can be adjusted quickly and up to very high values. Indeed, it has recently been demonstrated that a wall pushed by spin transfer can reach 175 m/s. This means that the resistance of a sub-micrometric memristor according to the invention based on spin transfer may be adjusted by current injection lasting several nanoseconds or less. 
     Finally, the invention limits the risk of deterioration of the component, because the current densities used to move the magnetic wall by spin transfer are less than 10 8  A.cm −2  (amps/cm 2 ), i.e. well below those used in electromigration techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention are disclosed in the description below in relation to the drawings attached, as follows: 
         FIG. 1 , which is a schematic drawing of an example magneto-resistive memristor according to the invention, 
         FIG. 2 , which contains two schematic drawings of an example magneto-resistive memristor according to the invention, used as a synapse in a neural network, 
         FIG. 3 , which is a perspective view of an example magneto-resistive memristor according to the invention in bar form, 
         FIG. 4 , which contains top views of several examples of magneto-resistive memristors according to the invention having complex forms, 
         FIG. 5 , which contains a side view and a top view of an example of command electrodes of a magneto-resistive memristor according to the invention, 
         FIG. 6 , which is a side view of an example magneto-resistive memristor according to the invention during writing of the initial state thereof, 
         FIG. 7 , which is a side view of an example magneto-resistive memristor according to the invention during reading of the resistance state thereof, 
         FIG. 8 , which is a side view of an example magneto-resistive memristor according to the invention during writing of a resistance state different to the initial state thereof. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of an example magneto-resistive memristor according to the invention, comprising two magnetic elements FM 1  and FM 2  forming elongate layers of length L, the elements FM 1  and FM 2  being separated by a non-magnetic element I also forming an elongate layer of length L. The layer I is an insulant or a semi-conductor. The direction of magnetization of the two magnetic layers FM 1  and FM 2  are shown by arrows. Thus, in the case of  FIG. 1 , a magnetic wall is present in FM 1  at a position x which may vary over time t, while the magnetization is homogeneous in FM 2 , which has no magnetic wall. 
     The resistance R of such a magneto-resistive structure is different depending on the arrangement of the magnetizations in the two layers. If the magnetizations are oriented in the same direction, the arrangement is referred to as “parallel”, i.e. both arrows in the same direction in  FIG. 1 , and the resistance is noted as R═R P . If the magnetizations are oriented in opposing directions, the arrangement is referred to as “antiparallel”, i.e. the two arrows in opposing directions in  FIG. 1 , and the resistance is noted as R═R AP . Thus, the resistance R of the structure is given by the following equation: 
             R   =         R   P     ⁢     x   L       +       R   AP     ⁡     (     1   -     x   L       )               
Advantageously, by injecting a spin-polarized current through the wall, it is possible to move the wall in one direction or in the other according to the sign of the current through the effect of spin transfer. Advantageously, for current densities above a critical value J c  defined notably by the initial trapping of the wall, the propagation speed u of the wall by spin transfer is proportional to the current i injected according to u=γi, where γ is a given coefficient. Again advantageously, the wall does not move below J c  (u=0) and the movement x at instant t is given by x(t)=γit=γq, where q is the total charge injected. The resistance of the device therefore depends on the charge and not just the current: this is the basis of the memory effect of the memristor. Thus, for a perfect sample with no trapping center, the “memristance” M as a function of the charge q is given by the following equation (1):
 
               M   ⁡     (   q   )       =       R   AP     ⁡     [     1   -           R   AP     -     R   P         R   AP       ⁢     γ   L     ⁢   q       ]             
Consequently, since the resistance of the device is a function of the charge, it is a multi-state device with resistance controllable by the charge injected, using wall movement by spin transfer. And if no current is applied, the device retains its last resistance value.
 
       FIG. 2  shows schematically how a magneto-resistive memristor according to the invention can advantageously be used as a synapse in a neural network. Indeed, for a single polarity of the current injected, i.e. for a single direction of propagation of the magnetic wall, a magneto-resistive memristor according to the invention makes it possible to implement either an excitatory synapse or an inhibitory synapse. An excitatory synapse becomes more conductive the more it is used, while an inhibitory synapse becomes more insulating each time it is stimulated. Thus, as shown in  FIG. 2  where R AP &gt;R P , by simply changing the direction of the magnetization of the layer FM 2 , the magneto-resistive memristor according to the invention can be used either as an artificial excitatory synapse, having an initial antiparallel state AP the resistance of which is reduced when a current is applied to it, or as an artificial inhibitory synapse, having an initial parallel state P the resistance of which increases when a current is applied to it. By changing the polarity of the current injected, it is also possible to switch from excitatory synapse behavior to inhibitory synapse behavior. 
       FIG. 3  is a perspective view of an example magneto-resistive memristor according to the invention in the form of a bar  5 . The bar may be of length L which may vary from several hundred nanometers to several microns. The width w of the bar  5  may be around a hundred nanometers or less. The thickness h of the magnetic stack formed by the memristor may be around several tens of nanometers. A layer 1 advantageously forms a protective layer of the layer FM 1 , typically Au, Ru, Ta or a more complex stack of these materials. The layer 1 may be omitted if the materials forming FM 1  so permit. The layer FM 1  may be a ferromagnetic layer where the magnetic wall is propagated. It may be a planar anisotropic material such as a NiFe or CoFeB alloy. To reduce the dimension of the wall to several tens of nanometers or less and therefore reduce L to around a hundred nanometers, perpendicular anisotropic materials may be used, such as CoPt, FePt, FePd alloys, or a stack of cobalt and nickel layers (stack hereinafter referred to as Co/Ni) or a stack of cobalt and aluminum dioxide layers (stack hereinafter referred to as Co/Al 2 O 3 ). Layer I may be an insulant or a semi-conductor material. Indeed, the magneto-resistive effect used by the invention is the tunnel magnetoresistance effect, which may reach 600%, in order to maximize the amplitude of variation of the resistance between R P  and R AP  and thereby enlarge the range of resistance. If I is an insulant, it may for example be a tunnel barrier such as Al 2 O 3  or MgO. If I is a semi-conductor, it may for example be GaAs. The layer FM 2  is a ferromagnetic layer having fixed magnetization. It may for example be a simple Co, NiFe, CoPt, FePt, FePd, CoNi or CoAl 2 O 3  ferromagnetic, or a synthetic CoFeB/Ru/CoFeB antiferromagnetic. Layer 2, which may be omitted, advantageously forms a layer enabling the FM 2  magnetization to be trapped, typically an anti-ferromagnetic material such as IrMn, PtMn or CoO. Layer 3 advantageously forms a buffer layer that may be useful for a correct growth of the materials, typically Ta, Ru or a more complex stack of the two. 
       FIG. 4  shows top views of several examples of magneto-resistive memristors according to the invention having more complex forms. Indeed, a nucleation reservoir may be used to facilitate the creation of the wall, as shown schematically in example (a) of  FIG. 4 . To obtain memristors that are more complex than the one in the equation (1) and therefore to reproduce synaptic functions, artificial traps for the magnetic wall may be included in a controlled manner, as shown schematically by examples (b) and (c) of  FIG. 4 . 
       FIG. 5  shows a side view (a) and a top view (b) of the same example magneto-resistive memristor according to the invention in the form of a bar  5  as shown in  FIG. 3 , but also showing command electrodes for writing then reading the resistance state. Electrodes  4  may advantageously be located at the top of the memristor, which may include other electrodes like the electrodes  4 . An electrode  6  may advantageously be located at the bottom of the memristor. 
       FIG. 6  is a side view of the same example magneto-resistive memristor according to the invention in the form of a bar as shown in  FIGS. 3 and 5 , during writing of the initial state. The initial state is either the parallel state P of the magnetizations for an inhibitory synapse, or the antiparallel state AP for an excitatory synapse. This initial state may be controlled by vertical spin transfer by applying a current i E  between the electrodes  4  and  6 . Depending on the polarity of the current i E  injected and for a sufficiently high current density i E , around 10 7  A.cm −2 , a spin transfer torque sets the state P or the state AP. 
       FIG. 7  is a side view of the same example magneto-resistive memristor according to the invention in the form of a bar as shown in  FIGS. 3 ,  5  and  6 , during reading of the resistance state. The resistance state is read by measuring the voltage V between the electrodes  4  and  6 . To do so, a current i L  that is sufficiently weak not to disturb the magnetic configuration (i L &lt;&lt;i E ) is also injected between the electrodes  4  and  6 . 
       FIG. 8  is a side view of the same example magneto-resistive memristor according to the invention in the form of a bar as shown in  FIGS. 3 ,  5 ,  6  and  7  during writing of a resistance state different to the initial state, also by moving the wall by spin transfer. Thus, after writing the initial state, subsequent writing may be effected in two different ways. A first technique involves applying the current i E  laterally between the electrodes  4 , as shown in example (a) of  FIG. 8 . A second technique involves injecting the current i E  vertically between the electrodes  4  and  6  as shown in example (b) of  FIG. 8 . 
     Another principle advantage of a memristor according to the invention described above is that, even when it is off, it conserves the resistance value it had at the time it was turned off.