JPH0642314B2 - Information writing method in superconducting memory - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting memory device for storing information in the form of Abrikosov magnetic flux quantum in a superconductor, and in particular to an information book in the superconducting memory device. It is related to the inclusion method.

[Conventional technology]

A device that uses Abrikosov flux quanta as memory information is
Japanese Patent Publication No. 60-34194 and Applied Physics Letters, Vol.39 NO.12 Desember 1
981, pp, 992-993).

FIG. 7 is a schematic perspective view showing one of the conventional examples of the Abrikosov magnetic flux quantum memory device. A thin superconductor film 51 for holding Abrikosov magnetic flux quanta, and a Josephson junction 53 for magnetic flux quantum detection, in which a part of the superconductor film 51 is a lower electrode and the superconductor film 52 is an upper electrode. , The magnetic flux quantum write line 54 provided near the end 55 of the superconductor layer 51.
And a superconductor film 51 'that is thicker than the superconductor film 51 surrounding the periphery except the end 55 of the superconductor film 51. Note that B conceptually represents the Abrikosov magnetic flux quantum retained in the superconductor film 51.

This Abrikosov magnetic flux quantum memory device has a superconductor film 51.
The presence / absence of the Abrikosov magnetic flux quanta held therein is associated with "1" and "0" in the memory state. That is, when the Abrikosov magnetic flux quantum is retained in the superconductor film 51,
The fact that the magnetic flux quantum influences the Josephson junction 53 to lower the threshold current Ij in the Josephson junction 53 is utilized, and the change of the threshold current Ij is made to correspond to “1” and “0” of the memory state. Is there.

Information is read by applying a bias current Ib having an appropriate value to the Josephson junction 53. That is, if a bias current Ib having a value between the threshold current Ij when the Abrikosov flux quantum is held in the superconductor film 51 and the threshold current Ij when the Abrikosov flux quantum is not held in the superconductor film 51 is applied to the Josephson junction 53, the Abrikosov flux quantum is given. Is held, a voltage is generated in the Josephson junction 53 (voltage state)
However, if it is not held, the voltage becomes zero (zero voltage state).

[Problems to be solved by the invention]

However, this conventional Abrikosov magnetic flux quantum storage device has a very serious problem in writing.

To write the memory state “1”, a current is applied to the write line 54 in a predetermined direction to generate a magnetic field, and the superconductor film 51 has an end 5 at the end 5.
This is achieved by internally generating Abrikosov flux quanta from 5 in a predetermined direction (for example, upward). on the other hand,
Writing of the storage state “0” must be performed using a different procedure depending on the storage state at that time. That is, if the storage state before writing is “1”, a reverse current is applied to the write line 54 to cause downward flux quanta to be generated in the superconductor film 5.
Abrikosov magnetic flux quantum number is made zero by internally generating 1 and annihilating the upward magnetic flux quantum. Also,
If the memory state is "0", no current is passed through the write line 54 to maintain the state as it is, and "0" is written.

As described above, in the conventional Abrikosov magnetic flux quantum storage device, it is necessary to read the storage state each time before writing, and if the access time for writing is unnecessarily lengthened and a computer is configured using this storage cell, the machine cycle time There was a problem that it becomes longer.

In addition, when writing "0" from the memory state "1", it is very difficult to exactly eliminate the pair of magnetic flux quanta. If the pair annihilation is insufficient, a part of the upward Abrikosov magnetic flux quantum remains, and if the pair annihilation is excessively performed, the downward Abrikosov magnetic flux quantum is retained. Therefore, the write operation margin is extremely narrowed due to the necessity of annihilating the Abrikosov magnetic flux quanta without excess or deficiency, and stable operation is difficult. In addition, there is a problem in that residual Abrikosov magnetic flux quanta, which have not been completely pair-annihilated, gradually accumulate during repeated write operations, resulting in malfunction.

[Means for solving problems]

The superconducting memory device used in the information writing method of the present invention is made in view of the above-mentioned problems. When a magnetic field of a predetermined level or more is applied, Abrikosov magnetic flux quanta are internally generated and the Abrikosov magnetic flux quanta are generated in the magnetic field. Information writing having an information storage means having a superconductor layer which is self-holding even in the state where the above is removed, and a superconductor layer for generating a magnetic field applied to the superconductor layer of the information storage means when a current is applied. Means, and at least a Josephson junction element sensitive to the magnetic field created by the Abrikosov flux quantum self-held in the superconductor layer of the information storage means, and the magnetic field by the Abrikosov flux quantum in the Josephson junction element. Information reading means having a control line for generating a magnetic field that can be applied, the information of the present invention being provided. When a memory cell is selected from a memory cell array in which a plurality of memory cells including the information memory means, the information write means, and the information read means are arranged and writing is performed, the information storage of the selected memory cell is performed. A first write current corresponding to a write bit current is supplied to the superconductor layer forming the means and a write word current is supplied to the superconductor layer forming the information writing means of this memory cell. The second write current is supplied.

[Action]

By supplying the first write current and the second write current, the Abrikosov magnetic flux quantum is internally generated and held in the information storage means. At this time, "0" of the stored information is stored in the superconductor layer of the information storage means depending on the directions of the first and second write currents.
Alternatively, the upward or downward Abrikosov flux quanta corresponding to "1" are selectively retained. When the control current Ics is given to the control line and the bias current Is is given to the Josephson junction element, the Josephson junction quantum becomes either a voltage state or a zero voltage state according to the direction of the Abrikosov magnetic flux quantum.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 1 is a plan view showing an embodiment of the superconducting memory device according to the present invention, and FIG. 2 is a sectional view taken along the line AA '.
The information storage means 1 has a superconductor layer 2 as an information storage element that internally generates the Abrikosov magnetic flux quantum α when a magnetic field is applied and holds the Abrikosov magnetic flux quantum by itself even after the magnetic field is removed. . The superconductor layer 2 is formed, for example, in a rectangular shape or a rectangular shape, and when one pair of opposing sides are 3a and 3b and the other pair of opposing sides are 3c and 3d, respectively, the side 3
Slide-shaped superconductor layers 4a and 4b are extended outward from the sides 3a of a and 3b integrally with them. The storage state of the information “1” in the device of this embodiment means a state in which the Abrikosov magnetic flux quantum is held in the first direction (upward “↑”) in the superconductor layer 2 of the information storage means 1. The memory state of "0" is the same as that of the superconductor layer 2
Abrikosov magnetic flux quantum is in the second direction (downward "↓")
The state held in.

The information writing means 11 is supplied with a current,
A stripe-shaped superconductor layer 12 for generating a magnetic field applied to the superconductor layer 2 of the information storage means 1 is provided as an information writing element. In this case, since the superconductor layer 12 is juxtaposed with the superconductor layer 2 of the information storage means 1, the side 3c of the superconductor layer 2 is provided.
It extends along.

The information reading means 21 includes a Josephson junction element 22 sensitive to the Abrikosov magnetic flux quantum held by the superconductor layer 2 of the information storage means 1 and a control line 34 for applying a predetermined magnetic field to the Josephson junction element 22. It has as an element for reading information. The Josephson junction element 22 is configured by interposing a tunnel barrier layer 25 between a superconductor layer 32 as a lower electrode and a superconductor layer 26 as an upper electrode. That is, the superconductor layer 32 is formed as a lower electrode on the superconductor layer 2 of the information storage means 1 via the insulating layer 31, and the insulating layer 24 having the window 23 formed on the superconductor layer 32 is formed. Formed. Window 23 of superconductor layer 32
A tunnel barrier layer 25 made of, for example, an oxide of the material of the superconductor layer 32 is formed in a region facing the insulating layer 24.
A superconductor layer 26 is formed as an upper electrode so as to cover the window 23. The control line 34 is composed of a stripe-shaped superconductor layer, and is formed on the superconductor layer 26 via the insulating layer 33 so as to cross over the tunnel barrier layer 25. In addition, T1 to T8 indicate respective terminals when the present device is regarded as one memory cell.

Next, the operation of this embodiment will be described.

First, reading of information will be described. To detect the memory state, the control current Ics is passed through the superconductor layer 34 of the information reading means 21, and a predetermined bias current Is is applied to the Josephson junction element 22 of the information reading means 21.
And the tunnel barrier layer 25 and the superconductor layer 32. FIG. 3 is a diagram showing the threshold characteristics of the Josephson junction element 22, in which the horizontal axis represents the control current value flowing in the control line 34 and the vertical axis represents the bias current value flowing in the Josephson junction element 22. In the figure, the broken line 7 is the threshold characteristic curve when the Abrikosov magnetic flux quantum is not held in the information storage means 1, and the actual battle 8 is the threshold when the Abrikosov magnetic flux quantum (downward) corresponding to the “0” state is held. A characteristic curve, actual battle 9, is a threshold characteristic curve when the Abrikosov magnetic flux quantum (downward) corresponding to the “1” state is held. As described above, the threshold characteristic changes along the horizontal axis depending on the memory state because of the influence of the magnetic field created by the Abrikosov magnetic flux quantum retained in the superconductor layer 2 of the information storage unit 1.

Now, the bias current Is is passed through the Josephson junction element 22, and the control current Ics is applied to the control line 34 in the first direction.
It is possible to move the operating point to the position A by flowing. From the same figure, since the operating point A is inside the threshold characteristic curve 8 in the "0" memory state, the Josephson junction element 22 maintains the zero voltage state, and the operating point A is in the "1" memory state. Since it goes out of the threshold characteristic curve 9, it can be seen that the state changes to the voltage state. This phenomenon is "0"
In the state, the magnetic field created by the downward Abrikosov magnetic flux quantum and the magnetic field created by the control current Ics are the tunnel barrier layer 25.
And the control current I when the Abrikosov flux quanta are upward when they cancel each other out in the "1" state.
It is caused by mutual application of the magnetic field of cs. By such an action, the Josephson junction element 2 when a predetermined bias current Is and a control current Ics are simultaneously passed
It is possible to read out whether the storage state is "1" or "0" depending on the voltage state of 2.

Next, writing of information will be described. First, the operation principle of information writing will be described. In FIG. 1 and FIG. 2, when a current IA corresponding to the information “1” is passed through the superconductor layer 12 of the information writing means 11 in the first direction, the superconductor layer 12 responds accordingly. A magnetic field corresponding to the information “1” is generated, and the magnetic field is applied to the superconductor layer 2 of the information storage means 1. Therefore, if this magnetic field is strong enough, the Abrikosov magnetic flux quantum α corresponding to the information “1” is internally generated in the first direction (upward “1”) on the side 3c of the superconductor layer 2 of the information storage means 1. . Then, when the application of the magnetic field is continued, the Abrikosov magnetic flux quanta are generated one after another at the side 3c, and the Abrikosov magnetic flux quanta internally generated earlier are pushed into the information reading means 21 side by the repulsive force acting on each other.

The presence of the relatively thick superconductor layer 32, which is the lower electrode of the Josephson junction element 22, prevents further movement of the Abrikosov magnetic flux quantum that has moved to the information reading means 21 side. In this way, if the Abrikosov magnetic flux quanta enter the information storage means 1 and a certain amount or more of the Abrikosov magnetic flux quanta enter, this time it becomes difficult for them to enter due to the repulsive force of the Abrikosov magnetic flux quanta, and the saturated state occurs. Becomes The Abrikosov magnetic flux quantum that has entered in this way is self-held by the pinning force of the superconductor layer 2 (the force that traps the Abrikosov magnetic flux element).

Next, when a current IB corresponding to "0" of information is passed through the superconductor layer 12 of the information writing means 11 in the second direction opposite to the above-mentioned current IA, the superconductor layer 12 receives the above-mentioned current. A magnetic field in the opposite direction to the magnetic field corresponding to the information "1" is generated, and this magnetic field causes the Abrikosov magnetic flux quantum corresponding to the information "0" to correspond to the information "1" in the superconductor layer 2 of the information storage means 1. It is internally generated in the second direction (downward "↓"), which is the opposite of the Abrikosov magnetic flux quantum. The Abrikosov magnetic flux quantum corresponding to the internally generated information "0" enters like the above-mentioned upward Abrikosov magnetic flux quantum, and is coupled with the upward Abrikosov magnetic flux quantum corresponding to the information "1" that is already self-held. Then disappears. Furthermore, the Abrikosov magnetic flux quanta corresponding to the information "0" penetrates, so that finally only the Abrikosov magnetic flux quanta corresponding to the information "0" is included in the superconductor layer 2 of the information storage means 1.
Remain and are retained. This is the holding state of the information “0”.

Next, a more specific write operation based on the above principle will be described. FIG. 4 is a write characteristic diagram showing the relationship between the transition amount of the threshold characteristic of the Josephson junction device 22 shown in FIG. 3 and the write current. The horizontal axis represents the write current value and the vertical axis represents the transition amount. I am taking it. In FIG. 4, the write characteristic curve 41 is the case where a Lorentz current in the opposite direction to the write current is supplied to the superconductor layers 4a and 4b (I _L ≠ 0).
0 indicates the case where the Lorentz current is not supplied (I _L = 0). In any of the curves, the transition amount of the threshold characteristic of the Josephson junction element 22 shows a steep rising characteristic and a saturation characteristic symmetrical with respect to the write current with respect to the origin. The steep rise cannot generate Abrikosov magnetic flux quanta in the superconductor layer 2 of the information storage means 1 at a write current below a certain value, and when the current value is exceeded, a large number of Abrikosov magnetic flux quanta are generated. It is caused by the phenomenon of internal generation and invasion by a group. Also, the saturation characteristic is that the Abrikosov flux quanta intrude more than a certain amount as described above, and due to the repulsive force between the Abrikosov flux quanta, even if the write current is increased to a certain value or more, it becomes saturated and becomes difficult to enter. Occurs in

Further, from the comparison between the write characteristic curves 40 and 41, the flow in the opposite direction to the write current to the current terminals 4a and 4b of the Lorentz current I _L, the Josephson device 22 of the information writing means 11 The write current value at which the transition amount of the threshold characteristic becomes non-zero becomes smaller than that when the Lorentz current I _L is not passed. This is the superconductor layer 2 of the information storage means 1.
This is because a Lorentz current I _L flows in the side of the superconductor layer 2 to generate a self-magnetic field on the side 3c of the superconductor layer 2, which acts additively with the magnetic field generated by the write current. Also, Lorentz current I
_{When L} flows, the saturation value of the transition amount increases (D '→
D). This is because the Lorentz current I _L flowing in the superconductor layer 2 of the information storage means 1 causes a force to move the Abrikosov magnetic flux quantum that has entered the superconductor layer 2 to the information read means 21 side, and therefore the information read means 21. The Abrikosov flux quanta held near the
This is because the amount of magnetic flux interlinking with increases.

With such a writing characteristic, selective writing can be performed in a memory cell array in which a plurality of the present device are arranged in a plane as one memory cell. For example, in writing the information “1”, the write current I _W is set to be smaller than the write current at the rising point of the write characteristic curve 40 and larger than the write current at the rising point of the write characteristic curve 40. The information "1" can be written only when both the inrush current I _W and the Lorentz current I _L are given, and the Abrikosov magnetic flux quantum cannot be stored only by the write current I _W or the Lorentz current I _L. Can be Therefore, if the Lorentz current I _L and the write current I _W are respectively the write bit current (first write current) and the write word current (second write current) of the memory cell array, selective writing can be performed. It will be possible. Information "0" can be written in the same manner by reversing the directions of the write current I _W and the Lorentz current I _L.

Next, specific selective writing and selective reading in the memory cell array will be described.

In FIG. 5, the device of this embodiment is used as one memory cell, and 2
This is a 4-bit storage cell array of 2 rows. In the figure, AA, AB, BA, and BB are the memory cells in the 1st row, 1st column, 1st row, 2nd column, 2nd row, 1st column, and 2nd row, 2nd column, respectively. It corresponds to the terminals T1 to T8 shown in the figure. P1 and P1 '
Is a read bit current supply means, P2 and P2 'are sense circuits, P3 and P3' are write bit current supply means, P
Reference numerals 5 and P5 'are write word current supply means, and P6 and P6' are read word current supply means.

The selective write operation of the 4-bit storage cell array thus configured is performed as follows. As a specific example, the memory cell AA
The case of selecting will be described. The terminal T1 of the memory cell AA from the write bit current supply means P3 through the wiring W3.
When the above-mentioned Lorentz current I _L (first write current) in the negative direction is supplied to, the Lorentz current I _L causes the superconductor layer 2 constituting the information storage means 1 of the memory cells AA and AB to be ID. Direction, the write characteristics of both cells are changed from the curve 40 in FIG. 4 to the curve 41. At the same time, the write current I _W in the positive direction is applied from the write word current supply means P5 to the terminal T3 of the memory cell AA through the wiring W11.
When the (second write current) is supplied, this write current I _W flows in the IA direction in superconductor layer 12 which is the information writing means of memory cell AA and memory cell BA. Then, in the memory cell AA, the Lorentz current I _L flows in the superconductor layer 2 in the ID direction and the write current I _W flows in the superconductor layer 12 in the IA direction, so that the operating point is B in FIG. "1"
Is written. On the other hand, in the memory cell AB, the Lorentz current I _L flows in the superconductor layer 2 but the write current I _W does not flow in the superconductor layer 12, so that the operating point becomes A in FIG. 4 and no information can be written. Not done. Further, in the memory cell BA, the write current I _W flows in the superconductor layer 12 but the Lorentz current I _L does not flow in the superconductor layer 2, so that the operating point is C in FIG.
Next, information is not written. Since neither Lorentz current I _L nor write current I _W is supplied to the memory cell BB, no information is written.

In this way, when the write bit current (Lorentz current I _L ) supply means P3 and the write word current (write current I _W ) supply means P5 are selectively driven, selective writing is performed only to the memory cells AA. be able to.

Similarly, when the write bit current supply means P3 and the write word current supply means P5 'are selectively driven, the memory cell A
B, to the memory cell BA when the write bit current supply means P3 'and the write word current supply means P5 are selectively driven.
Further, when the write bit current supply means P3 'and the write word current supply means P5' are selectively driven, the memory cell BB
The information "1" is written in each of the fields. Note that the selective writing of the information “0” can be similarly performed by making the directions of the write current I _W and the Lorentz current I _L opposite to each other.

Next, the selective read operation will be described. In the threshold characteristic shown in FIG. 3, the bias current applied to the Josephson junction element 22 of the information reading means 21 overlaps the threshold characteristic curve 9 in the "1" storage state and the threshold characteristic curve 8 in the "0" storage state. Information reading means 2 is set to the value Is of the area.
The control current flowing through the control line 34 of 1 is set to the value Ics of the area within the threshold characteristic curve 8 of the "0" storage state and outside the threshold characteristic curve 9 of the "1" storage state.

Now, assume that the memory cell AA in the 4-bit memory cell of FIG. 5 is selectively read. Read bit current supply means P
When the bias current Is is supplied from 1 to the terminal T7 of the storage cell AA through the wiring W1 and at the same time the control current Ics is supplied from the read word current supply means P6 to the terminal T5 of the storage cell AA through the wiring W9, the Josephson in the storage cell AA. Since the bias current Is is applied to the Son junction element 22 and the control current Ics is applied to the control line 34, the operating point of the threshold characteristic is point A shown in FIG. At this time, when the memory state of the memory cell AA is “1”, the operating point A crosses the threshold characteristic curve 9 and goes out of the threshold characteristic curve 9, so that the Josephson junction element 22 changes from the zero voltage state to the voltage applied state. Transfer to. By detecting this voltage by the sense circuit P2, it can be detected that the memory cell AA is in the "1" memory state. Further, when the memory cell AA is in the “0” memory state, the operating point A remains within the threshold characteristic curve 8 of “0”, so that the Josephson junction element 22 is maintained in the zero voltage state. Therefore, the sense circuit P2 does not detect the voltage and can detect that the memory cell AA is in the "0" memory state. At this time, since the bias current Is from the read bit current supply means P1 does not flow in the memory cell BA, the Josephson junction element 22 does not enter the voltage state regardless of the memory state. Further, since the control current Ics from the read word current supply means P6 does not flow in the memory cell AB, the operating point is "1" and the threshold characteristic curve 9 in the memory state and "0".
The Josephson junction element 22 remains in the overlap region with the threshold characteristic curve 8 in the memory state and is in the zero voltage state regardless of the memory state. Further, since neither the bias current Is nor the control current Ics is supplied to the memory cell BB, the Josephson junction element 22 is in the zero voltage state regardless of the memory state. Therefore, the read bit current supply means P1
When the read word current supply means P6 is selectively driven, all the memory cells other than the memory cell AA are in the zero voltage state regardless of the memory state, and reading is not performed.

When reading the stored contents of the memory cell BA, the read word current supply means P6 and the read bit current supply means P1 '.
Is driven to detect by the sense circuit P2 ', and the memory cell A
When the stored content of B is read, the read word current supply means P6 'and the read bit current supply means P1 are driven for detection by the sense circuit P2, and when the stored content of the storage cell BB is read, the read word current supply is provided. It suffices to drive the means P6 'and the read bit current supply means P1' and perform detection by the sense circuit P2 '.

In the above embodiment, the sixth junction device is
A single tunnel barrier layer 2 as shown in the conceptual diagram of FIG.
Although the one having 5 is used, a so-called SQUID type having two tunnel barrier layers may be used as shown in FIG. In the figure, 26 and 26 'are upper electrodes, 25 and 25' are tunnel barrier layers, and 32 and 32 '.
Is a lower electrode, and when the SQUID type is used, the threshold characteristic can be changed by configuring the magnetic flux (broken line arrow) by the Abrikosov magnetic flux quantum to cross the central space.

〔The invention's effect〕

As described above, according to the superconductor layer device used in the present invention, the stored information "1" is stored in the superconductor layer of the information storage means according to the direction of the current applied to the superconductor layer forming the information writing means. Or, it selectively holds the upward or downward Abrikosov magnetic flux quanta corresponding to "0",
Whether holding the upward Abrikosov magnetic flux quanta or holding the downward Abrikosov magnetic flux quanta, the Abrikosov magnetic flux quanta in the memory state before writing are erased in pairs and written until the saturation state is reached. So
It is not necessary to confirm the stored contents each time at the time of writing and to eliminate the pair of flux quanta accurately and exactly like the conventional Abrikosov flux quantum storage device that uses the presence or absence of the Abrikosov flux quantum as an information element. Therefore, not only the writing operation becomes simple, but also the writing operation margin can be widened. Moreover, there is no malfunction due to accumulation of residual Abrikosov flux quanta.

According to the information writing method of the present invention, the first write current supplied to the superconductor layer forming the information storage means and the second write current supplied to the superconductor layer forming the information writing means. The Abrikosov magnetic flux quanta can be internally generated in the superconductor layer of the information storage means only when both write currents are applied, so that the storage cell matrix is configured with the superconducting storage device as one storage cell. In this case, selective writing can be easily realized.

[Brief description of drawings]

1 is a plan view showing an embodiment of the present invention, FIG. 2 is a sectional view taken along the line AA ', FIG. 3 is a view showing threshold characteristics of the Josephson junction element 22, and FIG. 4 is FIG. 5 is a write characteristic diagram showing the relationship between the threshold current transition amount of the Josephson junction element 22 and the write current, and FIG. 5 is a two-row structure in which the embodiment device of FIG. 1 is configured as one memory cell. FIG. 6 is a plan view of a 2-row 4-bit storage cell, FIG. 6 is a perspective view showing the concept of a Josephson junction element, and FIG. 7 is a perspective view showing a conventional Abrikosov magnetic flux quantum storage device. 1 ... Information storage means, 2 ... Superconductor layer, 4a, 4b ...
... current terminal, 11 ... information writing means, 12 ... superconductor layer, 21 ... information reading means, 22 ... Josephson junction element, 34 ... control line.

Claims (1)

[Claims] 1. An information storage means having a superconductor layer which internally generates an Abrikosov magnetic flux quantum when a magnetic field of a predetermined level or more is applied, and which self-holds the Abrikosov magnetic flux quantum even when the magnetic field is removed, and an electric current Information writing means having a superconductor layer for generating a magnetic field to be applied to the superconductor layer of the information storage means when energized, and Abrikosov magnetic flux quantum self-held in the superconductor layer of the information storage means Superconducting means comprising at least one Josephson junction element sensitive to the magnetic field produced by the device, and information reading means having a control line for generating a magnetic field capable of affecting the magnetic field due to the Abrikosov flux quantum in the Josephson junction element. In the storage device, a plurality of storage cells including the information storage means, the information writing means, and the information reading means are provided. When the memory cell is selected from the arrayed memory cell array for writing, a first write current corresponding to the write bit current is applied to the superconductor layer forming the information storage means of the selected memory cell. With supply
An information writing method in a superconducting memory device, comprising supplying a second write current corresponding to a write word current to a superconductor layer constituting the information writing means of the memory cell.