JPH0219982B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to reducing the operating current and stabilizing the operation of a superconducting memory element that utilizes magnetic flux quanta in a type II superconductor.

[Technical background of the invention and its problems] Superconductors containing impurities such as lead alloys, transition metal superconductors such as Nb, and compound superconductors such as Nb ₃ Ge are called type II superconductors. , which allows magnetic flux to enter in the form of localized flux quanta (called vortices or vortex threads, hereinafter referred to as vortex threads). The diameter of the vortex thread can be considered to be normally 500 nm or less, and if it can be used as a storage information carrier, it is possible to realize a large-capacity superconducting storage device.

Such a storage device stores and inputs vortices generated in a vortex generation section made of a second type superconductor thin film into an annular vortex transfer section also made of a second type superconductor thin film. A device configured to transmit information in accordance with information, move it sequentially through a ring transfer section using clock pulses, and detect it with a detection section consisting of a Josephson junction arranged in the ring is known as the ``IEEE
Transaction or Magnetics, Vol MAG−15,
No. 1, pp. 558-561 (1968), and the vortex generating portion thereof is disclosed in Japanese Patent Publication No. 56-53834.

However, such a superconducting memory device can only function as a serial memory. Furthermore, since the speed of the vortex is slow, the transfer clock cannot be made faster, and therefore the information readout speed is slow.

In addition, "Special Application No. 1983-65493" and the 42nd Japan Society of Applied Physics Academic Conference Proceedings P478 (7p-Y-
15, October 1981), a superconducting memory device was proposed in which the basic memory cell was a structure consisting of a vortex generating section, an accumulating section, and a detecting section. Although this configuration does not have the above-mentioned problem, in order to generate vortices in the second type superconductor thin film in a superconducting state, a high magnetic field higher than the first critical magnetic field must be generated. The current level for vortex generation is extremely high. The method of eliminating accumulated vortices is to generate vortices in the opposite direction or to release them out of the superconductor using a transport current. (irregular vortex traps) and therefore cancellation is incomplete. There was a drawback.

[Purpose of the invention]

In order to solve these drawbacks, the present invention temporarily transitions the second type superconductor thin film wire to a normal conduction state when writing the vortex thread, and erases the vortex to the normal conduction state of the second type superconductor thin film wire. This is done using transfer, and will be described in detail below with reference to the drawings.

[Embodiments of the invention]

FIG. 1 shows a first embodiment of a superconducting memory element according to the present invention, in which a is a plan view and b is a cross-sectional view taken along the line AA'. 1 is a type 2 superconductor thin film wire for information storage, and also serves as the lower electrode of the Josephson junction. Reference numeral 2 denotes an upper electrode superconductor thin film wire, which faces the lower electrode thin film wire 1 via a junction insulator layer 4 to form a Josephson junction. Reference numeral 3 denotes a resistor thin film wire, which is insulated from the information storage thin film wire 1 and the upper electrode thin film wire 2 via an insulator thin film 5. Further, these are all formed on a substrate 6.

Known Josephson circuit fabrication techniques can be used to fabricate this. That is, each layer of metal and insulator is formed by a vacuum evaporation method or a sputtering method, and is sequentially stacked while patterning is performed using a phosphorography technique.

A method of operating the memory element shown in FIG. 1 will be described.

(1) Writing vortices: From the thin film line 2 of the upper electrode of the Josephson junction to the thin film line 1 of the lower electrode (the direction may be reversed; in this case, the direction of the vortex produced is simply reversed, and no A bias current I _B (having the same effect) is applied. Next, a current I is applied to the resistor thin film wire 3, which is large enough to cause the temperature rise of the information storage thin film wire 1 due to the heat generated in this thin film to exceed the critical temperature and transition to a normal conduction state. Run _R. Then, the thin film wire 1 for information storage undergoes a normal conduction transition, and the Meissner effect that eliminates the magnetic field disappears, as shown in FIG. 2a.
This causes the magnetic field line B created by the bias current I _B to enter. Next, when the current flowing through the resistor thin film wire 3 is returned to zero while the bias current I _B remains flowing, the temperature of the information storage thin film wire 1 decreases because the heat generation source disappears, and the information storage thin film wire 1 enters a superconducting state. return. Since this transition to the superconducting state occurs in the magnetic field created by the bias current _IB , the magnetic field is incorporated into the information storage thin film wire 1 in the form of vortex threads A, as shown in FIG. 2b. This vortex thread A is a thin film wire 1 for information storage.
Because it is captured by many pins in
Even after the bias current is reduced to zero, it remains in the information storage thin film line 1, as shown in FIG. 2c. In this way, only the vortex is written and stored in the thin film wire 1 for information storage. In order to memorize the vortex threads more reliably, as shown in Figure 2d,
It is preferable that the film thickness of the thin film wire 1 for information storage is made thin near the junction. The energy possessed by the vortex threads decreases as the film thickness of the second type superconductor becomes smaller, so the vortex threads are confined in the thinner part of the film.

(2) Erasing the vortex: To erase the vortex once written, proceed as follows. With no bias current flowing, the temperature of the information storage thin film wire 1 due to the heat generated in this thin film rises to a critical temperature or higher and is large enough to transition to a normal conduction state in the resistor thin film wire 3. A current I _R is applied. That is,
Transforms a thin film for information storage into a normal conducting state.
The vortex thread is the eddy current in the superconductor (as mentioned above, the diameter
It is a circulating current that revolves around a circumference of about 500 nm or less, and it continues to flow forever because there is no resistance in the superconducting state) and surrounded by 2
It is a grain of magnetic flux with a size of ×10 ^-15 Vs. When the transition to normal conduction occurs, this eddy current is not maintained and is attenuated by resistance. Naturally, the accompanying magnetic flux also disappears. In this way, the vortex threads are eliminated.

(3) Detection of vortex threads: The device of the present invention uses vortex threads as storage information carriers. That is, the state in which there is a vortex in the information storage thin film wire 1 corresponds to, for example, the logical value "1" of binary logic storage information, and the state in which there is no vortex corresponds to "0". To detect the presence or absence of vortex threads, a Josephson junction consisting of 1, 4, and 2 is used. As is well known, the voltage versus current characteristic of a Josephson junction has a zero voltage current and hysteresis as shown in FIG. The maximum value of current that can flow in a state of zero voltage is called the critical current, and if a current greater than this is allowed to flow, a voltage will be generated across the junction.
Figure 3a is the IV characteristic without vortices, and its critical current is I _C1 . Thin film wire for information storage 1
If there is a vortex thread in the vortex thread, a part of the magnetic field of the vortex thread penetrates the bonding insulator layer 4, as shown in FIG. 2c.
As a result, the critical current of the Josephson junction is reduced, as is well known. The critical current value in this state is
I _C2 (Figure 3b). I _C1 to Josephson junction
＞I _B ＞I _C2 When a bias current I _B of a magnitude of If there are vortex threads, the junction voltage will be V _G (usually several mV). Thus, the presence or absence of stored information can be detected by passing a bias current of an appropriate magnitude through the Josephson junction.

The above is the operation of the element of the present invention shown in FIG. 1 as a superconducting memory element. FIG. 4 shows the experimental results of the writing characteristics of the vortex fibers, which were measured by manufacturing an element having the structure shown in FIG. In the figure, the horizontal axis is the bias current I _B that is applied to generate the magnetic field, and the vertical axis is the Josephson critical current I _C. Critical current without vortices
I _C is 2.2 mA and is shown by a straight line L1 parallel to the horizontal axis. The actual measured value of the critical current after the vortex is written by the method described in (1) is shown by a black circle, and although there is some variation, it is approximately below the curve shown by the broken line L2. The difference between the straight line L1 and the broken line L2 represents the change in critical current due to the writing of the vortex. As can be seen from the figure, even when the bias current is, for example, 2 mA, the vortex is sufficiently written. This value is
This value is approximately 1/80 of the current value required to write a vortex thread while maintaining the information storage thin film wire 1 in a superconducting state. In addition, the current flowing through the resistor thin film wire 3 necessary to transfer the information storage thin film wire 1 to normal conduction is approximately proportional to the width of the information storage thin film wire 1, and is actually measured to be 1 mA/μm. It was hot. Therefore, by narrowing the width, this current level can also be reduced to the order of several mA.

FIG. 5 shows a second embodiment of a superconducting memory element according to the present invention, in which, in addition to the structure shown in FIG. It is formed through a body thin film 5'. This newly added superconducting thin film wire 7 is for passing current for generating a magnetic field during vortex writing.
A current is passed through this line in place of the bias current that was passed during vortex writing in the first invention. The other operations are exactly the same as those in the first aspect of the invention. By adopting such a configuration, the information write line and read line can be separated, and the degree of freedom in circuit configuration can be increased.

〔Effect of the invention〕

As explained above, since the type 2 superconductor thin film wire for information storage is transferred to normal conductivity when writing the vortex thread, the write current level can be significantly reduced, and furthermore, the write current level can be significantly reduced. As a method of elimination, we use a method to transfer the type II superconductor thin film wire that stores vortices to normal conductivity, so the vortices can be completely eliminated regardless of the presence or absence of pins in the thin film. It has the advantage of being able to perform stable and repeated operations.

Since the device of the present invention cannot have a very high vortex write speed, it is considered difficult to apply it as a cache memory.However, since the device of the present invention uses the voltage transition of Josephson junction and is extremely high speed, the structure is simple and the cell With the advantages of reducing the area, it is effective to apply it to ultra-large capacity superconducting storage devices.

[Brief explanation of drawings]

FIG. 1a is a plan view showing a first embodiment of the present invention, FIG. 1b is a sectional view taken along the line AA' in FIG.
Figures a to d are explanatory diagrams of how vortices are generated according to the present invention, Figures 3a and b are voltage and current characteristic diagrams of Josephson junctions to explain the vortex detection operation according to the present invention, and Figure 4 FIG. 5 is a diagram showing the experimental results of the vortex writing characteristics according to the present invention, and FIG. 5 is a sectional view showing an example of the second embodiment of the present invention. 1... Type 2 superconductor thin film wire for information storage, 2...
...Superconductor upper electrode thin film wire, 3...Resistor thin film wire, 4...Josephson junction insulator layer, 5, 5'
...Insulator thin film, 6...Substrate, 7...Superconductor thin film wire for vortex writing.

Claims (1)

[Claims] 1. A thin film wire for information storage made of a second type superconductor;
A superconductor thin film wire facing the information storage thin film wire through an insulating layer having a thickness that can produce the Josephson effect, and a resistor insulated from the superconductor thin film wire and the information storage thin film wire. A superconducting memory element characterized by consisting of a thin film wire. 2. A thin film wire for information storage made of a second type superconductor,
A superconductor thin film wire facing the information storage thin film wire through an insulating layer having a thickness that can produce the Josephson effect, and a resistor insulated from the superconductor thin film wire and the information storage thin film wire. A superconducting memory element comprising a thin film wire, and a superconductor thin film wire insulated from the resistor thin film wire, the information storage thin film wire, and the superconductor thin film wire.