JPH077593B2 - 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.
As the core, a magnetic material whose magnetization curve has a square hysteresis as shown in FIG. 6 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 10 is made of this magnetic material as shown in FIG. 7, and three conductor wires 11, 12, 13 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 10 is placed at the intersection of the vertical and horizontal lines. Arrange the cores in a matrix as shown in FIG.
Assemble into a memory element. The principle of information storage is shown in FIG. The magnetization 14 of the ring-shaped core 10 is turned clockwise along the circumference of the ring core as shown in FIG. 8 (a) or leftward as shown in FIG. 8 (b). For example,
In the initial state, the magnetizations of all ring cores are clockwise (-).
It is supposed to be. 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 13. 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 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 the simultaneous rotation mode of the magnetic moment that can prevent the reversal and can reverse the magnetization in a short time of the order of 10ns.
On the other hand, a magnetic film pattern in which only H _Y is added
In 15, an attempt is made to cause magnetization reversal due to domain wall movement,
Since the reversal time is long, the reversal does not occur with the short width H _Y that is actually used for writing. That is, the magnetization reversal of the magnetic film pattern 15 'occurs only when a magnetic field is simultaneously applied by the X-direction conductor line and the Y-direction conductor line. The magnetic fields H _X and H _Y are created by applying an electric current to the horizontal and vertical copper ribbons that are 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 relates to a ferromagnetic film having uniaxial magnetic anisotropy in the film plane, and first and second two superconductor wires arranged so as to intersect each other through the magnetic film. In the magnetic memory element including, the third superconductor line is arranged so as to overlap the first superconductor line and surround the magnetic film pattern and the second superconductor line. The first and third superconductor lines form a ring of the superconductor at a position intersecting with the second superconductor line, and the first and third superconductor lines are formed on the first superconductor line via the insulating layer. Another magnetic storage element is characterized in that another conductor wire or superconductor wire is arranged so as to overlap with the superconductor wire.

(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 4 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 3 is formed thereon again via the insulating layer. The superconductor line 3 is processed so that it is in direct contact with the underlying superconductor line 4 as shown in FIG. 1 in the region where the magnetic film pattern 6 and the second superconductor line 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. In order to use the unit cell shown in FIG. 1 as a memory element, another conductor wire (or superconductor wire) 7 of the present invention is added and arranged in a matrix form in FIG.

A portion surrounded by a dotted line indicated by A in FIG. 2 is a unit cell. 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 around the intersection line of can be reduced to about 0.25 μm ² by using a magnetic material that is currently in practical use. In order to further reduce this area, the magnitude of the magnetic field that confines one magnetic flux quantum is on the order of 10 ⁵ Gauss, and in order to realize this with a magnetic pattern,
New magnetic materials with large 4ΠMs are required. Since there is no direct limitation on the width of the superconductor line, the width can be reduced to 0.3 μm, and the size of the basic cell can be reduced to about 0.5 μm × 0.5 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, assume that the magnetization at that time is in the positive direction along the Y-axis direction indicated by 8 in FIG.

The write operation is as follows (Fig. 3 (a),
(B)). A pulse current is applied to the conductor wire 7 in the X-axis direction and the second superconductor wire 2 running in the Y-axis direction to cause the superconductor wires 1 and 2 to move in the magnetic material pattern 6 at each position. The direction of the magnetization 8 of the magnetic film pattern 6 existing at the position where is crossed is reversed from the positive direction to the negative direction in the Y-axis direction shown in FIG. In this magnetization reversal process, the superconductor ring 1
If the portion is temporarily made to transition from superconducting to normal conducting, the magnetization state after the magnetization reversal is stabilized by the newly induced diamagnetic current in the superconductor ring 5.

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

The reading method will be described. An example of a method of reading the written information will be described. First, the direction of magnetization of the magnetic film pattern is changed by the current magnetic field of the superconductor wire 2 in the easy magnetization direction (Y-axis).
Tilted from the superconductor ring portion and using the diamagnetic current induced in the superconductor ring along with this magnetization rotation and the bias current I _b given to the superconductor line 1 at the time of reading, the superconductor ring portion is brought into a superconducting state. This is a method of reading the voltage after the transition from the normal state to the normal state. Here, it is desirable to make a hole 9 in the superconductor wire 2 penetrating through the superconducting ring 5 as shown in FIG. The reason is that the magnetic field induced by the bias current I _{b at} the magnetic material pattern position in the superconductor ring is made almost zero to minimize the influence of I _b on the magnetization state of the pattern.

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, and a magnetic field H _X is applied in the direction of hard magnetization in the plane of the magnetic film pattern,
Temporarily tilt the magnetization away from the Y-axis. 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. The critical current of each of the superconductor wires 3 and 4
I _3c and I _4c .

The read operation will be described. Of the diamagnetic current I _d when the magnetization of the bit with the magnetization reversed (FIG. 4 (a)) is directed in the X-axis direction by applying a pulse current to the superconductor line 2 in the Y-axis direction. The direction and the direction of the bias current I _b are set to be 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, the bias current value I _b is set to a value within the specified range, and when the magnetization direction of the magnetic material is negative, 1/2 · I _b + i
_{The d} exceeds the critical current I _3c of the superconducting layer 3 so that the superconducting layer 3 is transformed into normal conduction. Then, the bias current flowing through 3 comes to flow through the superconducting wire 4, and _Ib flows through the superconducting layer 4. If this I _b is set to be larger than the critical current value I _4c of the superconducting 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 (b)), at this bias current value, only (1/2 · I _b −i _d ) flows in the superconducting layer 3, so 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 critical current and 4 of the critical current of the superconductor layer _{3, I 4c> 1 / 2I b} + i d> you 3c> 1 / 2I b -i d> 0 the relationship needs to have established. 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 bias current I _b to _{1 / 2I b = I 3c -δ} (0 <δ <i d) Taking the condition for the bias current 2I _3c> I _b> I _4c are filled, and 1 / 2I _b + i _{The condition of d} > I _3c > 1 / 2I _b −i _d 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 1 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.

(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 of an example of a basic cell structure of the present invention, FIG. 2 is a view showing an example in which basic cells are arranged in a matrix to form a memory element, and FIGS. 3 (a) and 3 (b) are Detailed structure diagram of the basic cell, FIGS. 4 (a) and 4 (b) are diagrams showing an example of information reading, FIG. 5 is a diagram showing the relationship with the critical currents I _b and I _d of the superconductor wire, FIG. 6 is a magnetization curve diagram when a magnetic field is applied in the easy magnetization direction of a magnetic substance having uniaxial magnetic anisotropy, FIG. 7 is a configuration diagram of a core memory, FIG. 8 is a magnetization state diagram of the core, and FIG. Is a basic configuration diagram of a memory using a magnetic film pattern. 1: Superconductor wire consisting of 3 and 4, 2: Superconductor wire, 3, 4: Superconductor wire, 5: Superconductor ring, 6: Magnetic film pattern, 7:
Writing conductor wire, 8: Magnetization in magnetic film pattern, 9: Hole formed in superconductor wire 2, 10: Ring core, 11: X-superconductor wire, 12: Y-superconductor wire, 13 : Readout line, 14: Magnetization in core, 15: Magnetic film pattern, 15 ': Magnetic film pattern in which magnetization is reversed.

Claims (1)

[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.
Forming a ring of a superconductor surrounding the superconductor wire of, and another conductor wire or a superconductor wire so as to overlap the first superconductor wire through the insulating layer. A magnetic memory element characterized by being arranged.