JPH077594B2 - Magnetic memory element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nonvolatile high density solid-state magnetic memory element.

(Prior Art) A solid-state magnetic memory has high reliability because it has no mechanical drive and is a non-volatile memory. The solid-state magnetic memory is roughly classified into a random access type memory and a serial access type memory. The core memory is typical of the former, and the bubble memory is typical of the latter. When aiming for a high-density storage element, the random access type needs to have a detector for each bit, and thus there is a limit to reducing the cell size. On the other hand, the serial access type is relatively easy to increase the density, but the increase in access time accompanying the increase in density is a serious problem. Further, an element such as a bubble memory that requires movement of a bubble, which is an information carrier, has a drawback that stability of information holding deteriorates with movement. Considering such a situation, as the nonvolatile solid-state magnetic memory, if the detector is solved, it is desirable that the random access memory realizes high density.

First, the basic operation of a random access memory using a magnetic material will be described by taking a representative core memory as an example.
For the core, a magnetic material whose magnetization curve has a square hysteresis as shown in Fig. 10 is used. Then, the material can stabilize the remanent magnetization in the + direction shown at the point A or in the-direction shown at the point B when the bias magnetic field H _Y = 0, so that the material has bistability. The respective directions can be used in the storage element by associating them with binary numbers "1" and "0".

In an actual device, a ring core 13 is made of this magnetic material as shown in FIG. 11, and three conductor wires 14, 15, 16 are formed in the core.
2 of them are stretched vertically and horizontally (referred to as Y-axis and X-axis, respectively), and the core 13 is placed at the intersection of the vertical and horizontal lines. The cores are arranged in a matrix and assembled into a memory element. The principle of information storage is shown in FIG. The direction of the magnetization 17 of the ring-shaped core 13 is changed to the 12th direction along the circumference of the ring core.
It is performed by turning it to the right as shown in FIG. 7A or turning it to the left as shown in FIG. For example, it is assumed that the magnetizations of all the ring cores are clockwise (-) in the initial state. Invert the direction of magnetization of the core at the specified address to make it counterclockwise (+). To do so, a pulsed current is sent to the Y- and X-rays that pass through the core, giving a magnetic field to the core.

Now, when this magnetic field is applied by the pulse current of either the X-ray or the Y-ray, the magnitude is insufficient to reverse the magnetization from the − state to the + state. For example,
Y ₀ - line, X ₀ - a pulse magnetic field lines is applied at the same time, the core of the magnetization inversion at the intersection of both lines is occurring, Y ₀ - line,
Alternatively, the magnetization reversal does not occur in the core to which the pulse magnetic field of only the X ₀ -line is applied. This is "1" at any coordinate of the matrix
Is a method of writing a signal.

In order to read the written information, a pulse for reversing the magnetization from + to − is applied to the X ₀ − line and the Y ₀ − line at the same time, and it is checked whether or not a signal appears on the read line 16. if,
If you get a signal, you know that the coordinate that sent the pulse was "1". However, in this method, once read, all become "0", which is so-called destructive read. If you are having trouble with this, you need to rewrite the read results as they were. The core memory has a problem that the bit cell size is large and it is difficult to increase the density.

There is a magnetic memory element in which this principle is applied to a magnetic thin film in order to realize high density memory. As the magnetic thin film, for example, an alloy of 19% Fe-81% Ni which is a soft magnetic material having a magnetostriction constant λ = 0 is used, and this is deposited in a disc shape on a substrate as shown in FIG. The film thickness is about 1000Å. A uniaxial anisotropy is provided in the film surface by applying a magnetic field during vapor deposition. In the present case, the easy axis of magnetization is set so as to be in the Y-axis direction.

When reversing the magnetization, a magnetic field H _Y parallel to the easy axis and in the direction opposite to the direction of the magnetization is given by the X ₀ -line, and at the same time, a magnetic field H _X is given by the Y ₀ -line in the direction perpendicular to it, and the It uses a magnetic moment simultaneous rotation mode that prevents reversal and allows magnetization reversal in a short time of the order of 10 ns. In contrast, the magnetic film pattern 18 in which only H _Y is added
Then, it tries to cause the magnetization reversal due to the domain wall movement, but since the reversal time is long, the reversal does not actually occur in the short width H _Y used for writing. That is, the magnetization reversal of the magnetic film pattern 18 'occurs only when a magnetic field is simultaneously applied by the X-direction conductor line and the Y-direction conductor line. magnetic field
H _X and H _Y are created by applying an electric current to the horizontal and vertical copper ribbons close to the deposited film.

(Problems to be solved by the invention) However, in this element, the reversed magnetization gradually returns to the original direction and the storage stability of information is not good, and when the magnetic film pattern is miniaturized. The detection output becomes small and it is difficult to read information.

(Means for Solving Problems) In the present invention, in order to solve these problems, a pattern formed of a superconducting material is introduced, and a diamagnetic current generated in a superconductor ring is utilized to store stored information. Provided is a high-density solid-state magnetic memory device which enhances stability and facilitates reading of stored information by utilizing the superconducting-normal transition of a superconductor ring or a Josephson junction attached to the superconducting ring.

The present invention provides a ferromagnetic film having uniaxial magnetic anisotropy in the film plane and two superconductor wires, a first superconductor line and a second superconductor line, which are arranged so as to intersect each other through the magnetic film. In the provided magnetic memory element, the third superconductor line is overlapped with the first superconductor line and the third superconductor line is sandwiched between the magnetic film pattern and the second superconductor line on the opposite side of the first superconductor line. Conductor lines are arranged, and the first and third superconductor lines form a superconductor ring surrounding the magnetic film and the second superconductor line at a position where the first and third superconductor lines intersect with the second superconductor line. The present invention relates to a magnetic memory element characterized by being formed. Still further, in this element, the fourth superconductor line is formed along the second superconductor film to form a magnetic memory element.

(Embodiment) FIG. 1 shows an example of the structure of a unit cell used for this memory element.

First, a superconducting layer is formed on a substrate, and this superconducting layer is patterned to form a third superconductor wire 3 extending in the X-axis direction. The second superconductor wire 2 is arranged thereon with an insulating layer interposed therebetween. A ferromagnetic film pattern 6 is formed thereon. The first superconductor wire 4 is again formed thereon with an insulating layer interposed therebetween. The superconductor wire 4 is processed so as to be in direct contact with the underlying superconductor wire 3 as shown in FIG. 1 in a region where the magnetic film pattern 6 and the second superconductor wire 2 are not present. As a result, a superconductor ring 5 is formed which surrounds the magnetic body and has the surface normal to the ring in the Y-axis direction. Since the unit cells shown in FIG. 1 are used as storage elements, they are arranged in a matrix form in FIG.

The portion surrounded by the dotted line indicated by A in FIG. 2 is the unit cell of FIG. The restrictions on the shape of the unit cell are that the film thickness of the superconducting film forming the ring is larger than the magnetic flux penetration depth, and that the inner wall of the ring is The area of the part around the intersection line is 0.25 μm ²
Can be made as small as possible. To further reduce this area, the magnitude of the magnetic field that confines one quantum magnetic flux is 10 ⁵ G
It is on the order of auss, and in order to realize this with a magnetic pattern, a new magnetic material with a large 4ΠMs is required. There is no direct limit to the width of the superconductor line, so set the width to 0.
It can be made as thin as 3 μm, and the size of the basic cell is 0.5 μm x 0.5
It can be reduced to about μm.

The material used for the element of the present invention is gold, aluminum, etc. as the conductor, and Pb-based, Nb-based, or Ba-Y-Cu-O as the superconductor.
SiO ₂ or the like can be used as the insulator, such as ceramics. For the magnetic film, a suitable material selected from a wide range of materials can be used. Known techniques can be used for the thin film forming technique. The ceramics may be formed by sputtering or laser deposition, and the etching may be dry etching using chlorine gas, for example.

Writing information will be described. As an initial state, the magnetization direction of the magnetic body is saturated in a predetermined direction. Now, let us say that the direction is a positive direction along the Y-axis direction shown by 9 in FIG.

The write operation is as follows (Fig. 3 (a),
(B)). A pulse current is applied to the superconducting wire 1 consisting of the first and third superconducting wires in the X-axis direction and the second superconducting wire 2 running in the Y-axis direction, and the magnetic field The direction of the magnetization 9 of the magnetic film pattern 6 existing at the position where the superconductors 1 and 2 intersect in the body pattern 6 is the Y-axis direction shown in FIG. -Reverse in the axial and negative directions. Here, when a current is applied to the superconductor wire 1, normally, no net magnetic field is generated in the ring 5, but in the present invention, by arranging the superconductor wire 2 so as to penetrate the ring. The magnetic field due to the current of the superconductor wire 4 is effectively applied to the magnetic film pattern 6 in the ring. In addition, the magnetic flux which is emitted from the magnetic film pattern and which tries to go to the superconductor wire 3 is shielded by the superconductor wire 2, so that most of the magnetic flux passes around the superconductor wire 4. In this magnetization reversal process, if the superconductor ring 1 part is temporarily transitioned from superconductivity to normal conduction, the magnetization state after the magnetization reversal will cause the newly induced diamagnetism in the superconductor ring 5. It is stabilized by electric current.

The 1-bit data write time when this method is used is on the order of 10 ns.

The reading method will be described. Two types of examples will be described as methods of reading the written information. One is that the direction of magnetization of the magnetic film pattern is tilted from the easy magnetization direction (Y-axis) by the current magnetic field of the superconductor wire 2, and the diamagnetic current induced in the superconductor ring along with this magnetization displacement. and read during a superconductor ring portion with a bias current I _b has given the superconductor wire 1 is transferred from the superconducting state to the normal conducting state, a method of reading voltage. The other is to attach a Josephson junction and detect the direction of the flow of magnetic flux from the magnetic film pattern.

First, the former will be described. At the time of reading, as shown in FIG. 4, a DC bias current I _b is applied to the superconducting ring in the X-axis direction and in the positive direction. Then, a pulse current is applied to the superconductor line 2 in the Y-axis direction to apply a magnetic field H _X in the in-plane difficult magnetization direction of the magnetic film pattern to temporarily tilt the magnetization 9 from the Y-axis direction. Then, depending on whether the stabilized direction of the magnetization is the positive direction or the negative direction along the Y-axis direction, the superconductor ring has a (a) or (() in FIG. 4, respectively. A diamagnetic current indicated by i _d is induced in b). In order to detect the direction of this diamagnetic current, the superconductor ring of the device of the present invention is formed by using two kinds of superconducting materials having different critical currents. Then, the rightward diamagnetic current and the leftward diamagnetic current are discriminated. In the example of FIG. 4, the superconductor ring 3 is formed of the superconductor wire 3 and a material having a larger critical current than that of the superconductor wire 3. _Let I _3c and I _4c be the critical currents of the superconductor wires 3 and 4, respectively.

The read operation will be described. Diamagnetic current I _d when the magnetization 9 of the bit with the magnetization reversed (FIG. 4 (b)) is directed in the X-axis direction by applying a pulse current to the superconductor line 2 in the Y-axis direction. And the direction of the bias current I _b are the same in the region (superconducting wire 3 in FIG. 4) where the critical current i _d is small in the superconductor ring.
In this state, set the bias current value I _b to a value within the specified range, and when the magnetization direction of the magnetic material is negative, 1/2 ・ I _b
+ I _d exceeds the critical current I _3c of the superconducting layer 3,
Causes normal transfer. Then 3
The bias current flowing through the superconducting wire 4 flows through the superconducting wire 4, and _Ib flows through the superconducting layer 4. If this I _b is set to be larger than the current value I _4c of the superconductor layer 4,
The ring becomes normal and a voltage develops across the superconductor ring. On the other hand, in the state where the magnetization is not reversed (FIG. 4 (a)), only (1/2 · I _b −i _d ) current flows in the superconducting layer 3 at this bias current value. The conduction-normal transition does not occur. Therefore, no voltage appears across the superconductor ring. That is, it is possible to discriminate the direction of magnetization in the magnetic film pattern.

The above is summarized as shown in FIG. Between the superconductor layer 3 of the critical current and 4 of the critical current, it is necessary _{I 4c> 1 / 2I b +} i d> I 3c> 1 / 2I b -i d> 0 the relationship is made up . Bias current I _b
Is set to satisfy 2I _3c > I _b > I _4c . From this, 1 / 2I _4c <I _3c <I _4c is obtained immediately.

Here the 1 / a bias current _{I b 2I b = I 3c -δ} (0 <δ <i d) If that condition 2I _3c with respect to the bias current value> I _b> I _4c are filled, and 1 / 2I _b + i _{The condition of d} > I _3c > 1 / 2I _b −i _b is also satisfied. In addition to the above, it is necessary to condition I _4c> I _3c + i _d is satisfied for _{I 4c> I 3c -δ + i} d> I 3c, I 3c as as large as possible to I _4c is I _3c = 1 / It is sufficient if 2I _4c + δ ′ (δ ′>0; minute amount) is satisfied.

For example, the bias current I _b is I _b = 17.5 × 10 ⁻³ A. When the superconducting ring undergoes a superconducting-normal transition due to the diamagnetic current due to the magnetization reversal of the ferromagnetic thin film, the induced current flowing in the superconducting ring is 0.25 × 10 ^-3 A, and the ring part in the normal conducting state The output voltage comes out. Moreover, the magnetization of the magnetic thin film returns to the state before reading when the current of the Y superconductor line is cut off.

Next, try to estimate the output voltage.

The specific resistance p of the superconductor is p = 200-250 μΩcm,
The conductor length e of the superconductor in the unit cell is assumed to be 4 μm. Then, the generated voltage V is on the order of 10 mV. This is an order of magnitude larger than in the case of bubble memory.

Next, a reading method using the Josephson junction will be described.

FIG. 6 shows a configuration example. The basic cell structure is shown in FIG. The superconductor wire 7 is arranged on the first superconductor wire 4. On the superconductor ring, a Josephson junction is formed with the superconductor wire 4 as shown by 8 in FIG. 7 (b). In the Josephson junction, as shown in FIGS. 8 (a) and 8 (b), the magnetic flux 10 flows in a direction depending on the magnetization direction of the magnetic film pattern. When this magnetic flux exists,
All junctions should give a sense current I _s between one end and the superconductor wire 1 of the superconductor line 7 so that the normal state. The following conditions must be satisfied for this sense current. Maximum value of superconducting current flowing in Josephson junction
I _max changes depending on the amount of magnetic flux penetrating the junction. An example thereof is shown in FIGS. 9 (a) and 9 (b). Φ on the horizontal axis is φ = H _W (2
can be written as λ + d). W is the width of the cross section orthogonal to the direction of the magnetic flux at the joint, 2λ + d is the magnetic flux penetration depth λ and the thickness d of the joint.
It is the effective thickness that the magnetic flux obtained from can pass. φ0 is a magnetic flux quantum. In the present case, H originates from the magnetic film pattern. The level of I _s of FIG. 9 for fixing the sense current of the superconductor wire 7. This I _s needs to be sufficiently lower than I _max when φ = 0 and higher than I _max when φ ≠ 0. In addition, in FIG. 8 (a), (b)
Reference numeral 11 indicates the direction of the current of the superconductor wire 4, and 12 indicates the magnetic flux due to this current.

Another kind of read current is supplied from the other end of the superconductor wire 1 to control the net magnetic flux amount at the junction. If the magnetization of the magnetic film pattern at the intersection of the superconductor lines 1 and 2 in FIG. 7 is downward, if I _r is caused to flow from the left to the right of the superconductor line 1, the Josephson junction at the intersection The effective magnetic flux of
It becomes zero. Therefore, the junction becomes superconducting, and the resistance between the one end of the superconductor wire 7 and the one end of the superconductor wire 1 becomes zero. On the other hand, when the magnetization of the magnetic film pattern is upward, the effective magnetic flux of the Josephson junction increases rather with respect to I _{r in the} same direction as described above, and the junction remains in the normal conduction state. Using this difference,
The magnetization direction of the magnetic film pattern is detected.

(Effects of the Invention) According to the present invention, high-performance high-density storage in which the instability of the magnetization state after the magnetization reversal and the instability of information reading accompanying the improvement of the storage density have been remarkably improved by the present invention. The device can be realized.

[Brief description of drawings]

FIG. 1 is an external view showing an example of the basic cell structure of the present invention.
The figure shows an example in which the basic cells are arranged in a matrix to form a memory element. FIGS. 3 (a) and 3 (b) are detailed structural diagrams of the basic cells, and FIGS. 4 (a) and 4 (b). Is a diagram showing an example of reading information, FIG. 5 is a diagram showing a relationship between critical currents of superconductor wires, and FIG. 6 is an example of another basic cell arranged in a matrix to form a memory element. FIG. 7 (a) and FIG. 7 (b) are diagrams showing a detailed structure example of a basic cell, and FIG.
The figure shows another example of information reading, FIG. 9 shows the relationship between the magnetic flux crossing the Josephson junction and the critical current flowing through the junction, and FIG. 10 shows the magnetic material with uniaxial magnetic anisotropy. Magnetization curve diagram when a magnetic field is applied in the easy magnetization direction, No. 11
Figure is a block diagram of the core memory, Figure 12 is a magnetization state diagram of the core,
FIG. 13 is a basic block diagram of a memory using a magnetic film pattern. 1; Superconductor wire, 2: Superconductor wire, 3,4: Superconductor wire, 5: Superconductor ring, 6: Magnetic film pattern, 7: Read superconductor wire, 8: Josephson Junction, 9: Magnetization in magnetic film pattern, 10: Magnetic flux due to magnetization of magnetic film pattern, 11: Direction of current of X-superconductor line, 12: Magnetic flux due to current 11, 13: Ring core, 14: X-conductor wire, 15: Y-conductor wire, 16: readout wire, 1
7: Magnetization in the core, 18: Magnetic film pattern, 18 ': Magnetic film pattern in which the magnetization is reversed.

Claims (2)

[Claims] 1. A ferromagnetic film having uniaxial magnetic anisotropy in a film plane, and first and second superconductor wires arranged so as to intersect each other through the magnetic film. In the provided magnetic memory element, the magnetic film pattern and the second superconductor line are sandwiched on the opposite side of the first superconductor line at the position corresponding to the magnetic film, and at the other positions, The third superconducting wire is arranged so as to overlap the first superconducting wire, and the first superconducting wire is placed at a position where the first and third superconducting wires intersect with the second superconducting wire. The body wire and the third superconductor wire are the magnetic film and the second superconductor wire.
A magnetic memory element characterized by forming a ring of a superconductor surrounding the superconductor wire of. 2. A ferromagnetic film having uniaxial magnetic anisotropy in the film plane, and first and second superconductor wires arranged so as to intersect each other through the magnetic film. In the provided magnetic memory element, the magnetic film pattern and the second superconductor line are sandwiched on the opposite side of the first superconductor line at the position corresponding to the magnetic film, and at the other positions, The third superconducting wire is arranged so as to overlap the first superconducting wire, and the first superconducting wire is placed at a position where the first and third superconducting wires intersect with the second superconducting wire. The body wire and the third superconductor wire are the magnetic film and the second superconductor wire.
Form a ring of superconductor surrounding the superconductor wire of
Also, a fourth superconductor wire is provided along the second superconductor wire and forms a Josephson junction with the first or third superconductor wire at a position corresponding to the magnetic film. Magnetic storage element characterized by.