Abstract:
A memory slot including a pad formed of a stack of regions made of thin layers, including a first region made of a nonmagnetic conducting material; a second region made of a magnetic material exhibiting a magnetization in a direction perpendicular to the principal plane of the pad; a third region made of a nonmagnetic conducting material of different characteristics to those of the first region; the pad resting on a conducting track adapted to cause the flow of a programming current of chosen sense, in which the pad has an asymmetric shape with respect to any plane perpendicular to the plane of the layers and parallel to the central axis of the track, and with respect to its barycenter.

Description:
[0001]    The present patent application claims the priority benefit of French patent application FR15/50273 which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    The present invention relates to a magnetic memory cell, and more particularly to a magnetic memory cell of current-induced reversal type. 
       STATE OF THE ART 
       [0003]    French patent application Nr. 2963152 describes a magnetic memory cell such as schematically shown in  FIGS. 1A, 1B, and 2 .  FIGS. 1A and 1B  respectively show a cross-section view and a perspective view of a magnetic memory cell such as described in relation with  FIGS. 1 c -1 f , 2 a   - 2   b,  and  3   a - 3   d  of French patent application Nr.  2963152 .  FIG. 2  is a simplified perspective view of this memory cell. 
         [0004]    As illustrated in  FIGS. 1A and 1B , the memory cell comprises, above a conductive track  1 , a pad  3 . Pad  3  comprises a stack of regions, each of which is formed of a portion of a thin layer or of a stack of a plurality of thin layers. Conductive track  1  is for example formed on a substrate  5  comprised of a silicon wafer coated with a silicon oxide layer and is connected between terminals A and B. The stack forming pad  3  successively comprises, from track  1 , a region  10  made of a non-magnetic conductive material, a region  11  made of a magnetic material, a region  12  made of a non-magnetic material, a region  13  made of a magnetic material, and an electrode  14 . The material of layer  12  may be conductive; it is preferably an insulating material, sufficiently thin to be crossed by electrons by tunnel effect. There is a structural difference between non-magnetic regions  10  and  12 , in order to have an asymmetrical system in a direction orthogonal to the plane of the layers. Such a difference may in particular result from a difference in material, thickness, or growth mode between these layers. 
         [0005]    Lists of materials capable of forming the various layers are given in the above-mentioned patent application. The magnetic materials of regions  11  and  13  are formed in conditions such that they have a magnetization directed orthogonally to the plane of the layers. The magnetic material of layer  13  is formed in conditions such that it keeps an intangible magnetization (trapped layer). Upper electrode layer  14  is connected to a terminal C. 
         [0006]    The programming of the memory cell is performed by circulating a current between terminals A and B, while a horizontally-directed field H (parallel to the plane of the layers and to the direction of the current between terminals A and B) is applied. According to the relative directions of the current between terminals A and B and of field vector H, layer  11  is programmed so that its magnetization is directed upwards or downwards. 
         [0007]    For the reading from the memory cell, a voltage is applied between terminal C and one or the other of terminals A and B. The resulting current between terminal C and one or the other of terminals A and B takes different values according to the relative direction of the magnetizations of layers  11  and  13 : high value if the two magnetizations are in the same direction and low value if the two magnetizations are of opposite directions. 
         [0008]    An important feature of the above-described memory cell is that its programming is only performed due to a current flowing between terminals A and B and to a magnetic field applied in the plane of the layers, parallel to the current. No current flows from terminal A or B to terminal C during the programming. This has the advantage of fully dissociating the memory cell read and write operations. 
         [0009]    Various alternative embodiments are possible. Particularly, each previously-described layer may be formed of a stack of layers as known in the art to acquire the desired characteristics. 
         [0010]    The portion of layer  10  of a non-magnetic conductive material may be omitted, provided for track  1  to be made of a non-magnetic material appropriate for the growth of magnetic layer  11 . Track  1  may then exhibit an overthickness under pad  3 . For the reversal of the magnetization in layer  11  to be possible, spin-orbit couples should also be present in the magnetic layer. To achieve this, the layer in contact with layer  11  (or separated therefrom by a thin separation layer) should for example be made of a material or made up of materials having a strong spin-orbit coupling. Another solution for example is for the contact between magnetic layer  11  and one or the other of layers  10  and  12  to create the spin-orbit coupling; this is what may for example occur by hybridization of magnetic layer  11  with layer  12  if the latter is made of an insulator (see “Spin-orbit coupling effect by minority interface resonance states in single-crystal magnetic tunnel junctions”, Y. Lu et al. Physical Review B, Vol. 86, p. 184420 (2012)). 
         [0011]    It should be noted that the memory cell of  FIGS. 1A and 1B  may be broken up into two elements: a storage element comprising track  1  provided with terminals A and B and layer portions  10 ,  11 , and  12 , and a readout element comprising, in the example given hereabove, layers  13  and  14  and electrode C. With the same storage element, various readout modes may be envisaged, for example, an optical readout. 
         [0012]      FIG. 2  shows a simplified perspective view of the memory cell of  FIG. 1B . Only track  1  and layer stack or pad  3  are shown, as well as electrodes A and B connected to contacts  20  and  21 . 
         [0013]    As previously indicated, the memory cell is programmable by application of a current between terminals A and B simultaneously to the application of a magnetic field having a non-zero component along the direction of the current. Examples of magnetic field generation means are given in the above-mentioned patent application. The application of an external field or the forming of specific magnetic layers capable of creating field H raises practical implementation issues. 
       SUMMARY 
       [0014]    An object of the present application is to provide a magnetic memory cell of the same type as that previously described and where the memory cell programming is simplified. 
         [0015]    More particularly, it is here provided for the memory cell to be programmable by the simple application of a current in the absence of a magnetic field. 
         [0016]    To achieve this object, it is provided to replace the symmetrical pad described in the above-mentioned application with an asymmetrical pad. It can then be observed that the memory cell may be programmed by simple application of an electric current in the track having the memory pad resting thereon, in the absence of a magnetic field. 
         [0017]    More particularly, a memory cell comprising a pad formed of a stack of regions in thin layers is here provided, which comprises: a first region made of a non-magnetic conductive material; a second region made of a magnetic material having a magnetization in a direction perpendicular to the main plane of the pad; a third region made of a non-magnetic conductive material having characteristics different from those of the first region; said pad resting on a conductive track capable of circulating a programming current of selected direction, wherein the pad has an asymmetrical shape, on the one hand with respect to any plane perpendicular to the plane of the layers and parallel to the central axis of the track, and on the other hand with respect to its barycenter, whereby the direction of the magnetization in the second layer is programmable by the circulation direction of said programming current, in the absence of an external magnetic field. 
         [0018]    According to an embodiment, the first region corresponds to a portion of the track underlying the pad or to an overthickness of this underlying portion. According to an embodiment, the pad has a triangular shape with a first side parallel to the direction of the programming current and two other sides for example forming an angle from ±20° to ±70° relative to the first side. 
         [0019]    According to an embodiment, the pad has a triangular shape with rounded corners. 
         [0020]    According to an embodiment, the pad only extends across a portion of the track width. 
         [0021]    According to an embodiment, the second region comprises an alloy having a specific perpendicular magnetic anisotropy, that is, in particular, FePt, FePd, CoPt, or also a rare earth/transition metal alloy, particularly GdCo, TbFeCo. 
         [0022]    According to an embodiment, the second region comprises a metal or an alloy having in the stack a perpendicular magnetic anisotropy induced by the interfaces, particularly Co, Fe, CoFe, Ni, CoNi. 
         [0023]    According to an embodiment, at least the first region or the third region is conductive and is made of a non-magnetic material, such as Pt, W, Ir, Ru, Pd, Cu, Au, Bi, Hf, or of an alloy of these metals, or is formed of a stack of a plurality of layers of each of these metals. 
         [0024]    According to an embodiment, the third region is made of a dielectric oxide such as SiOx, AlOx, MgOx, TiOx, TaOx, HfOx or of a dielectric nitride such as SiNx, BNx. 
         [0025]    According to an embodiment, the thickness of the first and third regions is in the range from 0.5 to 200 nm, more particularly from 0.5 to 100 nm, and preferably smaller than 3 nm. 
         [0026]    According to an embodiment, the third region is covered with a readout layer made of a magnetic material and with a readout electrode. 
         [0027]    According to an embodiment, the thickness of the second region is smaller than 3 nm. 
         [0028]    According to an embodiment, the first region is made of a conductive antiferromagnetic material. 
         [0029]    According to an embodiment, the memory cell comprises a conductive buffer layer between the first region and the track. 
         [0030]    According to an embodiment, the thickness of the first region made of an antiferromagnetic material is in the range from 1 to 200 nm, and preferably smaller than 10 nm. 
         [0031]    An embodiment provides a method of programming a memory cell such as hereabove, comprising the step of conducting in the track, and accordingly in the first region of antiferromagnetic material, a current having an intensity capable of heating the material to disorganize its magnetization, the direction of the current being selected to program the magnetization of the material in the storage layer according to a desired direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0032]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of dedicated embodiments in connection with the accompanying drawings, among which: 
           [0033]      FIGS. 1A, 1B, and 2  are simplified representations of a memory cell such as described in French patent application Nr. 2963152; 
           [0034]      FIG. 3  is a simplified perspective view of an embodiment of a memory cell; 
           [0035]      FIG. 4  is a simplified top view of the pad of  FIG. 3 ; 
           [0036]      FIGS. 5A to 5J  are simplified top views of contours of various embodiments of a memory cell; 
           [0037]      FIG. 6  is a simplified perspective view of another embodiment of a memory cell; and 
           [0038]      FIG. 7  shows various curves illustrating the operation of the memory cell of  FIG. 6 . 
       
    
    
       [0039]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, as usual in the representation of integrated components, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0040]    As shown in  FIG. 3 , an embodiment of a memory cell comprises a pad  30  integrally arranged on a conductive track  1  comprising, at its ends, connection layers  20  and  21  connected to terminals A and B. Pad  30  comprises the same stack of layers as previously-described pad  3 . Thus, pad  30  comprises a region  40  made of a non-magnetic conductive material, a region  41  made of a magnetic material, a region  42  made of a non-magnetic material, region  42  exhibiting a difference with respect to region  40 , a region  43  made of a magnetic material, and an electrode  44 . The same variations as those described hereabove and in the above-mentioned French patent application apply. 
         [0041]    Unlike pad  3  of  FIG. 2 , pad  30  has a doubly asymmetrical configuration. On the one hand, it is asymmetrical with respect to any plane perpendicular to the plane of the layers and parallel to the central axis of track  1 . On the other hand, it is asymmetrical with respect to its barycenter, which corresponds to point C in  FIG. 3 . As can be better seen in the top view of  FIG. 4 , this pad for example has a triangular shape. A first side is parallel to the direction of the programming current (from A to B). The two other sides for example form an angle in the range from ±20° to ±70° and preferably from ±30° to ±60° relative to the first side. Due to the photolithography defects, the corners of the triangle will generally be rounded. It may be intentionally desired to obtain such a rounded shape. 
         [0042]    It has been observed that with a pad having such doubly asymmetrical shape (with respect to a plane and with respect to its barycenter) and integrally arranged on a track, the application of a programming current between terminals A and B flowing in the track under the pad and around the pad is sufficient to program layer  41  so that its magnetization is directed upwards or downwards according to the direction of the programming current. This is done with no application of an external magnetic field. Of course, if a conductive region  40  is provided, its resistivity and that of track  1  are selected so that a significant portion of the current between terminals A and B flows through region  40 . It should be reminded that if track  1  is made of a non-magnetic material, region  40  may correspond to the track portion underlying the pad or to an overthickness of this region. 
         [0043]      FIGS. 5A to 5J  show as an example various shapes of asymmetrical pads which may be used on a track  1 . These drawings will be considered as incorporated herein by reference. 
         [0044]    Pads  50  and  51  of  FIGS. 5A and 5B  have triangle shapes like the pad of  FIG. 4 , but with different proportions and centers. 
         [0045]    Pad  52  of  FIG. 5C  has the shape of a crescent elongated in the track direction. In other words, pad  52  has a trapezoidal shape provided with two tapered protrusions  52 A and  52 B at the ends of the base of the trapeze. It is now considered that the presence of the tapered protrusions eases the triggering of the reversal of the magnetization at the passing of the programming current. 
         [0046]    Pads  53  to  55  of  FIGS. 5D to 5F  have herringbone shapes, the tip of the herringbone being directed towards a side of track  1 , the herringbones having various dimensions and positions relative to the track width. 
         [0047]    Pads  56  to  59  of  FIGS. 5G to 5J  are pads in the shape of triangles and of herringbones with rounded corners. 
         [0048]    The various shapes described hereabove may be combined and modified from the moment that the double asymmetry rule mentioned herein is respected. In particular, all the envisaged shapes may be “rounded” or only extend across a portion of the width of track  1 . 
         [0049]    As concerns the materials and the thicknesses of the various regions of the pad, reference may be made to the above-mentioned French patent application. 
         [0050]    As an example:
       the magnetic region ( 41 ) may comprise an alloy having a specific perpendicular magnetic anisotropy, that is, in particular, FePt, FePd, CoPt, or also a rare earth/transition metal alloy, particularly GdCo, TbFeCo.   the magnetic region ( 41 ) may comprise a metal or an alloy having in the stack a perpendicular magnetic anisotropy induced by the interfaces, particularly Co, Fe, CoFe, Ni, CoNi,   at least region ( 40 ) or region ( 42 ) may be conductive, made of a non-magnetic material, such as Pt, W, Jr, Ru, Pd, Cu, Au, Bi, Hf, or of an alloy of these metals, or in the form of a stack of a plurality of layers of each of these metals,   region ( 42 ) may be made of a dielectric oxide such as SiOx, AlOx, MgOx, TiOx, TaOx, HfOx or of a dielectric nitride such as SiNx, BNx, having a thickness capable of allowing a tunnel effect,   the thickness of one of the regions ( 40 ) and ( 42 ) may be in the range from 0.5 nm to 200 nm, more particularly from 0.5 nm to 100 nm, and preferably smaller than 3 nm,   the upper region ( 42 ) may be covered with a readout layer ( 43 ) made of a magnetic material, or of a compound of magnetic materials, or of a plurality of layers of magnetic and non-magnetic materials, and with a read electrode ( 44 ), and   the thickness of the magnetic region may be smaller than 3 nm.       
 
         [0058]    In top view, the lateral dimensions of the pad may be in the range from 10 to 100 nm. 
         [0059]    The above-described memory cells may be assembled in a memory array, as described in the above-mentioned French patent application. 
         [0000]    Embodiment with a High Thermal Stability 
         [0060]    The previously-described memory cells have satisfactory operations. However, to avoid having an excessive intensity of the current necessary for the programming, storage layers which are as thin as possible should be provided. Storage layers having a thickness smaller than 3 nanometers, and even having, for example, a thickness smaller than one nanometer, can thus be formed. Such a thickness decrease favors the decrease in the intensity of the programming current but results in a lack of stability of the memory cells. Indeed, memory cells comprising such a thin storage layer, risk incidentally switching state, particularly due to disturbances such as thermal agitation or parasitic magnetic fields. Thus, such memory cells have, in average, a limited thermal stability, which leads to providing a periodic reprogramming thereof. 
         [0061]    To overcome this disadvantage, it is provided to couple the magnetic, generally ferromagnetic, storage region with a region of an antiferromagnetic material. 
         [0062]      FIG. 6  shows an embodiment of a memory cell having an increased thermal stability, corresponding to the embodiment of  FIG. 3 . In  FIG. 6 , non-magnetic conductive layer  40  of  FIG. 3  is replaced with a conductive layer  60  made of an antiferromagnetic material. The antiferromagnetic material couples by exchange interaction with the magnetic, for example, ferromagnetic, material of storage layer  41 . This enables to trap the magnetization of this magnetic layer and to provide a thermal stability. 
         [0063]    This structure is likely to have a number of variations. Particularly, a conductive buffer sub-layer (not shown), particularly intended to enable to satisfactorily deposit antiferromagnetic material layer  60 , may be provided under antiferromagnetic material layer  60 . Further, a thin layer of a non-magnetic conductive material, for example, copper, may be arranged between the ferromagnetic and antiferromagnetic regions to decrease, if need be, the coupling between these regions. 
         [0064]    As illustrated in the curves of  FIG. 7 , the memory cell programming is carried out as follows. From a time tO, a current pulse I is sent between terminals A and B, the current having a direction capable of causing the reversal of the magnetization of magnetic layer  41 . First, nothing occurs due to the coupling between magnetic and antiferromagnetic layers  41  and  60 . However, temperature T af  of the antiferromagnetic material increases by Joule effect and the magnetization organization of the material, whereby the coupling between magnetic materials and antiferromagnetic layers  41  and  60  decreases. From a time t 1 , the coupling becomes sufficiently low for magnetization “pol” of layer  41  to reverse under the effect of the portion of current I flowing through the ferromagnetic and antiferromagnetic layers of the pad. Once the current pulse between terminals A and B stops, at a time t 2 , temperature T af  of the antiferromagnetic material decreases and the latter recovers, at a time t 3 , an organized state which couples back with the modified polarization of the magnetization in storage layer  41  and ensures its stability. To avoid possible write errors due to thermal agitation during the cooling of the memory cell, current I may be gradually decreased and controlled to control the temperature of the pad and the amplitude of the couples responsible for the reversal of the magnetization more independently. Current I will for example be gradually decreased over a time interval shorter than 1 μs, more particularly shorter than 100 ns, and preferably shorter than or equal to 10 ns. 
         [0065]    A layer of antiferromagnetic material of a nature such that temperature TR for which the decrease of the exchange coupling with the magnetic layer, which enables the magnetization therein to be freer to reverse, is much higher than the temperatures of use provided for the operation of the memory cell or of the memory where the memory cell is incorporated, will be selected. Temperature TR is for example in the order of 140° C. for slow writing operations and in the range from 220 to 300° C. for fast writing operations. 
         [0066]    As an example of antiferromagnetic materials which may be used herein, alloys such as for example those based on Mn such as IrMn, FeMn, PtMn, or alloys of these compounds such as PtFeMn, or also materials obtained by lamination of these compounds, or also oxides such as CoOx or NiOx may be mentioned, the magnetic material, preferably ferromagnetic, used for the storage layer then being a material such as previously described for layer  41 . Generally, the electric conductivity of the antiferromagnetic material should be sufficient for current to flow therethrough during the write phase and any ferromagnetic material complying with this condition, coupling by exchange with the ferromagnetic material and having a blocking temperature in the range from 120 to 450° C., may be used. 
         [0067]    Thus, the previously-described memory cell may be stabilized, even by providing a very thin magnetic layer, of a thickness smaller than 3 nm and preferably smaller than 1 nm. Antiferromagnetic material layer  60  will have a thickness in the range from 1 to 200 nm, more particularly from 1 to 50 nm, and preferably smaller than 10 nm. Such a thickness will depend on the material used, for example, in the order of 10 nm for FeMn, and in the range from 4 to 5 nm for IrMn. The time of reversal of such a magnetic cell may be very short. The current pulses I shown in  FIG. 7  may for example have a duration shorter than 15 nanoseconds.