Patent ID: 12239028

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIGS.1A-1D, there are shown perspective views of three embodiments of the hybrid superconductive-magnetic memory cell in accordance with the present invention. The memory cell is composed of a Josephson junction100formed with two segments1and3of superconductive layers and a barrier2between the superconductive layers, and a magnetic junction101formed with two segments4and6of ferromagnetic layers and a barrier5between the ferromagnetic segments. Further, the Josephson junction and the magnetic junction are situated in close proximity to each other and form a vertically integrated structure, wherein at least one edge of the Josephson junction protrudes beyond the edge of the magnetic junction. The proposed memory cell is a multi-terminal device wherein each superconducting layer and each magnetic layer, separately, may have at least one contact to feed electric current and to read-out voltage. InFIGS.1-2, only the contacts to superconducting layers are schematically shown for the sake of clarity of the drawings.

In a particular embodiments shown inFIGS.1A-1C, two opposite edges of the Josephson junction100protrude beyond the edges of the magnetic junction101in a symmetric way. Possible embodiments of the proposed cell are not limited to the symmetric configuration, but can also be asymmetric, as shown inFIG.1D. Such asymmetric configuration may be advantageous for obtaining definite suppression at least 20% of the critical current, Ic, in the Josephson junction for ‘1’ logical state, as was mentioned above. The lateral shape of the magnetic and Josephson junctions can be not only rectangular, but can also have other forms, for example, circular or elliptic. An elliptic or diamond-like form of the magnetic junction101allows one to beneficially exploit the shape anisotropy in the magnetization switching process. The magnetic junction101can be a true spin valve or a pseudospin valve. In the first case, an antiferromagnetic layer (not shown) is used to fix the magnetization vector in one of the magnetic layers4and6.

In the second case, there is no antiferromagnetic layer, but the magnetic layers4and6have different coercive fields, so that their magnetization vectors can be switched at different values of the magnetic field. The magnetization vectors of the magnetic layers can be oriented either parallel or perpendicular to the planes of the films comprising the cell structure. The barrier material5can be either an insulating or a conductive material that provides indirect exchange coupling between the magnetic layers4and6.

An example of such indirect exchange coupling is the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the case of a metallic layer5(see P. Bruno, Phys. Rev. B 52, 411 (1995), and references therein), or, in a magnetic tunnel junction (MTJ), an exchange coupling between the ferromagnetic electrodes due to a torque generated by spin currents flowing through the tunnel junction (see J. C. Slonczewski, Phys. Rev. B 39, 6995 (1989)). The barrier5can be made from a combination of these insulating and conductive materials.

Furthermore, two superconducting wires8and9, which are electric control lines, are situated in close proximity to the magnetic junction101, electrically insulated from each other and from the magnetic junction101. These wires, by feeding currents Ixand Iy, serve to provide an external magnetic field needed to change the magnetization vector direction of the soft magnetic layer (being one of the layers4and6) or both magnetic layers4and6. Electrical contacts are established on the exposed edges of the superconducting layer3and the superconducting layer1, and another electrical contact may be established to the superconducting layer7(not shown inFIGS.1A-1D) adjacent to the external magnetic layer6of the magnetic junction. The superconducting layer7may not be present in some preferred embodiments, as depicted inFIGS.1A-1Dand in the cross-sectional views inFIGS.2A and2B, but may be present in other preferred embodiments as shown in the cross-sectional views inFIGS.2C,2D,2E, and2F.

The operational principle of the memory cell in accordance with the present invention is explained by reference toFIGS.3A,3B,3C,5A, and5B. The memory cell function is based on the fact that the maximum Josephson current of the Josephson junction100can be controlled by a magnetic junction101in such a way that switching the magnetization direction in one of the magnetic layers4,6by 180° (π/2 radians) or 90° (π/4 radians) with respect to the fixed magnetization direction in the second of the magnetic layers4,6results in two distinct values of the maximum Josephson current in the Josephson junction100. These two Josephson current states represent the binary “0” and “1” logic states for a memory cell.

FIGS.3A and3Bshow the magnetic field distribution generated by the magnetic junction101in the case of parallel mutual orientation of the magnetization vectors in the magnetic layers4and6, whereasFIG.3Cshows the magnetic field distribution generated by the magnetic junction101in the case of anti-parallel mutual orientation of the magnetization vectors in the magnetic layers4and6. In the anti-parallel magnetization vectors orientation (shown inFIG.3C), the magnetic lines of force of the two magnetic layers4and6are configured into a closed circulating pattern in such a way that the magnetic flux is completely enclosed within the magnetic junction101and no magnetic flux penetrates the Josephson junction100. In this case, the Josephson junction100has maximum Josephson current value Ic0as depicted inFIG.5A. On the other hand, in the parallel mutual magnetization vectors orientation (shown inFIGS.3A and3B), there is a strong component of the magnetic field perpendicular to the superconducting layers comprising the Josephson junction100, which leads to a partial suppression of the Josephson current in the Josephson junction to the value of Ic1(seeFIG.5A).

In the case ofFIG.3A, the perpendicular component partially suppresses the superconductivity in both superconducting layers1and3composing the Josephson junction100(seeFIGS.1A,1B,1C,1D,2A,2B,2C,2D,2E, and2F). Physically, this takes place in the junctions where the thicknesses of the superconducting electrodes1and3, dS1and dS3, respectively, are smaller or comparable to the respective London penetration depths, λS1and λS3(typically, λ=80−90 nm for niobium thin films at 4.2 K). The partial suppression of the superconductivity in the superconducting electrodes leads to a reduction of the maximum Josephson current in the Josephson junction100from the level Ic0characteristic of the anti-parallel orientation to the level Ic1, as is illustrated inFIG.5A, where the dependences of the Josephson critical current vs. externally applied magnetic field, Ic(H), are shown by solid and dashed curves for the respective anti-parallel and parallel magnetization vectors orientations in the magnetic layers4and6. The two maximum Josephson current levels Ic0and Ic1, characteristic of the anti-parallel and parallel magnetization orientations, respectively, of the magnetic layers4and6, serve as logical “0” and “1” states according to the preferred embodiment of the present invention. A single cell has a scale of order 200 nm.

Referring toFIG.3B, if only the superconducting layer3has the thickness dS3≤λS3, whereas dS1>λS1, then, for the state with the parallel mutual magnetization vectors orientation in the magnetic layers4and6, the magnetic field from the magnetic junction101penetrates only the superconducting layer3of the Josephson junction100, and there is a magnetic field component inside of the Josephson junction100which is parallel to the planes of the superconducting layers1and3. As a result of such magnetic field penetration into the Josephson junction100, its maximum Josephson current value is not only partially suppressed, but also the entire dependence of the Josephson critical current vs. externally applied magnetic field, Ic(H), is shifted along the H axis, as is illustrated by the dashed curve inFIG.5B. In this case, the logical states “0” and “1” are determined by the maximum Josephson currents Ic0and I′c1(seeFIG.5B).

Referring toFIGS.1A-1C, the necessary switching of the mutual magnetization between the parallel and anti-parallel orientations can be performed by supplying appropriate electrical currents to the wires8and9. The two wires8and9enable half-selection of a specific memory cell within the memory array. Half-selection corresponds to an applied magnetic field that is approximately half of the value required to switch the magnetic film, so that two half-selection currents can switch the film. Furthermore, the two wires8and9may be configured in such a way (not shown) as to allow not only for parallel and anti-parallel mutual magnetization vectors orientation in the magnetic layers4and6, but also for rotation of the magnetization vector of the soft magnetic layer by 90 degrees with respect to the magnetization vector of the hard magnetic layer. Such 90 degree orientation of the magnetization vectors in the layers4and6will also result in two distinct maximum Josephson current levels (for zero applied external magnetic field H) in the Josephson junction100.

In a preferred embodiment, the Josephson junction may comprise superconducting thin films of niobium, with a critical temperature of 9.2 K. The barrier of the Josephson junction may comprise a thin layer of insulating aluminum oxide of order 1 nm thick. Alternatively, other superconducting materials can be used for the electrodes of the Josephson junction, for example, films of niobium nitride (NbN) with a critical temperature of about 16 K, or magnesium diboride (MgB2) with a critical temperature as high as 39 K.

Preferred magnetic materials may comprise both soft and hard magnetic materials. For example, one may use permalloy, a nickel-iron alloy with about 80% nickel. Other soft magnetic materials for cryogenic temperatures might include dilute alloys of Pd in Fe or copper-nickel alloys. The hard magnetic material may comprise pure nickel, or other materials known in the art.

The memory cell may be very compact, limited only by the size of the Josephson junction. Current Josephson junction fabrication technology enables junctions that are down to 200 nm in transverse dimensions. This is much smaller than prior art cryogenic memory technologies that comprise superconducting loops and multiple junctions, and have scales of several micrometers or more. Switching times for the write operation are limited by the switching time for the magnetic domain in the soft magnetic material, about 1 ns. Read times do not require switching magnetic domains, and can be of order 10 ps using low-power single-flux-quantum electronics.

FIG.4is a conceptual drawing of one embodiment of a memory cell, showing a small magnetic junction (MJ) on top of a larger Josephson junction (JJ). This also shows the magnetic field lines emanating from the ends of the MJ for the case of parallel alignment of the magnetic films, corresponding to a logical ‘1’. This will also include perpendicular components of the field that penetrate at least part of the superconducting film of the JJ. The penetration of the field into the film, typically in the form of vortices, is responsible for a local suppression of critical current density in the JJ. For this reason, penetration of the fringe field should ideally cover a major fraction of the area of the JJ, so that the total effect of Icsuppression will be significant. It is not necessarily required that the fringe fields are present on all sides of the MJ, but this configuration may be expected to provide the largest possible effect.

FIG.6shows a conceptual picture of a two-dimensional array of hybrid magnetic-superconducting memory cells described above, which may be used to construct an addressible random-access memory (RAM) array for use with a superconducting computing or digital signal processing system. Such a memory array may have rows and columns, whereby a particular element in a given row and column may be selected by simultaneous inputs on a row and a column, where both inputs are required to trigger the write or read operation.

The memory density of an array can be greater than 1 Mb/mm2.

The array is organized in such a way that a plurality of the Josephson junctions100are connected in series, using the wires200, in the rows along the x axis. The corresponding plurality of the magnetic junctions101do not need to have electric contacts in this embodiment; however, such electric contact may be realized in other embodiments in accordance with the disclosed invention. Magnetization reversal of the “soft” magnetic layer (the bottom layer4in the magnetic junctions101inFIGS.1and6) is accomplished by combined action of the magnetic field induced by the currents Ixand Iysupplied to the superconducting control lines running in perpendicular directions along the x and y axes. These control lines are insulated from the MJs and from each other by thin layers of dielectric material (e.g., SiO2). The level of currents Ix, Iyis such that all cells are half-selected by the fields created by each of these currents Ix, Iyduring the WRITE operation. However, for one selected cell (encircled with the thick dotted line inFIG.6), the fields from the two lines add up (for the shown directions of the currents), thus enabling the magnetization reversal of the “soft” magnetic layer, so that, in this example, the magnetic moments of the magnetic layers in the magnetic junctions101become oriented in parallel.

The READ operation for the memory array is illustrated inFIG.7, where the main lobes of the Ic(H) dependences are shown for the anti-parallel (AP) and parallel (P) orientation of the magnetization vectors in the magnetic junction101(thin and thick solid curves, respectively). We assume that the cell is configured according to the embodiment shown inFIG.3B, i.e., for the P magnetization orientations in the magnetic layers of the magnetic junction101, the Josephson critical current of the Josephson junction100in the same cell is not only suppressed to Ic1as compared with its magnitude Ic0(which Josephson junction100has for the AP magnetizations orientations in the magnetic layers of the magnetic junction101), but has actual value I′c1at H=0, because the respective Ic(H) dependence is shifted along the H axis due to the influence of the magnetic field component parallel to the superconducting layers; the resultant modified Ic(H) dependence as is shown by the thick solid line inFIG.7.

During the READ operation, a current pulse with the amplitude of Iris supplied to the desired row of Josephson junctions; simultaneously, a current pulse with the amplitude of I′yis supplied to the respective column in order to select a Josephson junction whose state needs to be read out. The magnitude of I′yis such that it creates a magnetic field that shifts Ic(H) along the H axis but cannot reverse the magnetization in any of the magnetic layers in the magnetic junction101. The shifted Ic(H) curves are shown as thin dashed line for the AP magnetization orientation in the half-selected cells and thick dashed line for the P magnetization orientation in the selected cell (encircled with the thick dotted line inFIG.6). This shift is needed in order to move the steep slope of the main lobe of the Ic(H) dependence close to H=0. The amplitude of the read current Iris chosen as shown by the horizontal line inFIG.7. Without any current pulse I′y, Iris lower than both I′c0and I′c1(values of the Josephson critical current in the Josephson junction100for the AP and P states in the magnetic junction101at H=0, respectively). However, if I′yis applied, it further shifts the Ic(H) dependence along the H axis, so that the new I″c1(H=0) value becomes lower than Ir(but in all other cells which are half-selected, the I′c0(H=0) value for the AP state is higher than Ir).

As a result of this READ procedure, the Josephson junction100in the selected cell undergoes transition into the resistive state if the magnetic junction101is in P state, but remains superconductive if the magnetic junction101is in AP state. The occurrence or absence of the switching event into the resistive state is registered as the presence or absence of a voltage pulse across the selected row, which determines the state of the selected cell.

Note that without the current I′ythat selects the cell for readout operation along the y axis, the read current Ircannot switch the Josephson junction100into the resistive state even if its critical current is depressed by the P state of the magnetic junction101. Therefore, all the cells with the depressed critical current (if any in the selected column), except for the selected cell, remain superconductive. Thus, the energy dissipation during the READ operation is minimal and is estimated to be ˜1 fJ for an estimated read latency of 0.1 ns. The WRITE operation causes negligible energy dissipation within the array, because the control current flows through the superconducting lines.

In another embodiment in accordance with the present invention, the memory cells and the control lines can be arranged in an array as shown inFIG.8. The difference between the memory array presented inFIG.6and that presented inFIG.8is that, in the latter case, the control lines8and9produce magnetic fields in mutually perpendicular directions within the planes of the layers in the memory cell. In this case, according to theoretical calculations disclosed by Mironov et al., Appl. Phys. Lett. 113, 022601 (2018), and Devizorova et al., Phys. Rev. B 99, 104519 (2019), a larger shift of the Ic(H) dependence along the H axis is expected for the Josephson junction100if the magnetization orientation of the magnetic layers4and6in the magnetic junction101(cf.FIG.1) is changed from the AP to the mutually perpendicular direction. This is more favorable to realize the READ operation according toFIG.7as described above in accordance to the present invention, especially if the lateral dimensions of the said magnetic cell are reduced. The WRITE operation for the embodiment shown inFIG.8is accomplished in the same way as that for the embodiment shown inFIG.6.

In yet another embodiment according to the present invention, the memory cells disclosed in this invention are arranged in an array shown inFIG.9, wherein the control lines8and9run in parallel with the diagonals of the rectangles representing the in-plane area of the magnetic junction101, and within the area of the magnetic junction101. This allows for making use of the configurational anisotropy of the magnetic layers4and6in the magnetic junction101. It is known for those skilled in the art that hysteretic properties, specifically, the value of the coercive force, of the magnetic elements depends on their shapes and the direction of the applied magnetic field. The embodiment shown inFIG.9allows for using lower switching fields of the “soft” magnetic layer in the magnetic junction101, and therefore, allows for a broader choice of the materials used for magnetic layers4and6in the magnetic junction101. In the particular embodiment shown inFIG.9, the Josephson junctions100are connected along one of the diagonals using the wires200; however, they can be connected in rows in a similar way shown for the array configurations inFIGS.6and8. Both the WRITE and READ operations for the embodiment shown inFIG.9is accomplished in a similar way as that for the embodiment shown inFIG.6, and takes place according to the description given above and is illustrated inFIG.7.

Note that, in addition to the embodiments presented inFIGS.6,8, and9, other configurations of arrays made of the memory cells disclosed in the present invention, are possible. Furthermore, the lateral shape of the Josephson junction100and that of the magnetic junction101, according to the present invention, is not limited to the rectangular shape. It can be oval, or diamond-like, or other shape or a combination of different shapes for the Josephson junction100and the magnetic junction101. Also, according to the present invention, the position of the magnetic junction101within the area of the Josephson junction100can be different. Moreover, according to the present invention, the magnetic junction101can be positioned not only on top, but also below the Josephson junction100. Yet furthermore, according to the present invention, the control lines8and9can be positioned, separately or jointly, both below and above of the memory cell. Yet furthermore, the lateral shape of different magnetic layers4and6in the magnetic junction101can also be different, in accordance with the present invention.