Domain wall motion element and magnetic random access memory

A domain wall motion element has a magnetic recording layer 10 that is formed of a ferromagnetic film and has a domain wall DW. The magnetic recording layer 10 has: a pair of end regions 11-1 and 11-2 whose magnetization directions are fixed; and a center region 12 sandwiched between the pair of end regions 11-1 and 11-2, in which the domain wall. DW moves. A first trapping site TS1 by which the domain wall DW is trapped is formed at a boundary between the end region 11-1, 11-2 and the center region 12. Furthermore, at least one second trapping site TS2 by which the domain wall DW is trapped is formed within the center region 12.

This application is the National Phase of PCT/JP2009/050287, filed Jan. 13, 2009, which is based upon and claims the benefit of priority from Japanese patent application No. 2008-031816, filed on Feb. 13, 2008, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a domain wall motion element and a magnetic random access memory (MRAM: Magnetic Random Access Memory) of a domain wall motion type.

BACKGROUND ART

A spin momentum transfer method recently proposed is one of promising data writing methods for an MRAM. According to spin transfer magnetization switching, a spin polarized current as a write current is injected into a magnetic recording layer and thereby a magnetization direction of the magnetic recording layer is switched. In a case of conventionally-known magnetization switching due to application of a current-induced magnetic field, a current required for the magnetization switching increases as a size of a memory cell becomes smaller. In contrast, in the case of the spin transfer magnetization switching, a current required for the magnetization switching decreases as a size of a memory cell becomes smaller. Therefore, the spin momentum transfer method is considered to be a promising method for achieving a high-capacity MRAM.

However, in a case where the spin momentum transfer method is applied to a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) element, it is necessary to overcome a problem of destruction of a tunnel barrier layer. When the magnetization switching is performed by the spin momentum transfer method, the spin polarized current is so supplied in the magnetic tunnel junction as to pass through the tunnel barrier layer. Here, as the case now stands, the spin polarized current as much as mA is required for the spin transfer magnetization switching. To supply such a large current through the magnetic tunnel junction may cause destruction of the tunnel barrier layer.

Moreover, in the case of the magnetic tunnel junction element of the spin momentum transfer type, both of a write current and a read current flow through the same path. Therefore, the so-called read disturb problem that data writing is performed at a time of data reading is caused. The read disturb problem becomes remarkable when the write current is reduced in order to prevent the above-mentioned destruction of the tunnel barrier layer.

In order to avoid the read disturb problem, it is necessary to reduce the read current. In this case, however, a read output becomes smaller and thus a special read circuit is necessary. As a result, it becomes difficult to achieve a high-speed data reading. Moreover, in a case of SoC embedded, circuit overhead is increased, which interferes with the embedded.

One approach to overcome the above-described problems is a technique that causes magnetization switching by supplying a spin polarized current in an in-plane direction of a magnetic recording layer. Such a technique is disclosed, for example, in Japanese Patent Publication JP-2005-191032A, Japanese Patent Publication JP-2005-123617A and U.S. Pat. No. 6,781,871. The spin polarized current is supplied in the in-plane direction of the magnetic recording layer, which causes a domain wall in the magnetic recording layer to move and thereby the magnetization in the magnetic recording layer is switched. From that viewpoint, this technique is called a “domain wall motion method”. An element based on the domain wall motion method is hereinafter referred to as a “domain wall motion element”. According to the domain wall motion method, it is not necessary to supply the spin polarized current so as to pass through the tunnel barrier layer, and thus the destruction of the tunnel barrier layer can be avoided effectively. Furthermore, since respective paths of the write current and the read current are different from each other, the read disturb problem can be avoided.

DISCLOSURE OF INVENTION

The domain wall motion element disclosed in the above-mentioned related documents is configured such that the domain wall is trapped at a position corresponding to data “0” or “1”, and the data “0” or “1” is determined based on variation in a resistance value depending on the position of the domain wall. That is to say, the domain wall motion element disclosed in the above-mentioned related documents does not support a multivalued operation and an analog operation.

An object of the present invention is to provide a domain wall motion element and an MRAM that are capable of a multivalued operation.

In a first aspect of the present invention, a domain wall motion element is provided. The domain wall motion element has a magnetic recording layer that is formed of a ferromagnetic film and has a domain wall. The magnetic recording layer includes: a pair of end regions whose magnetization directions are fixed; and a center region sandwiched between the pair of end regions, in which the domain wall moves. A first trapping site by which the domain wall is trapped is formed at a boundary between the end region and the center region. Moreover, at least one second trapping site by which the domain wall is trapped is formed within the center region.

In a second aspect of the present invention, a magnetic random access memory of a domain wall motion type having a plurality of memory cells that are arranged in an array form is provided. Each of the plurality of memory cells has a domain wall motion element. The domain wall motion element has a magnetic recording layer that is formed of a ferromagnetic film and has a domain wall. The magnetic recording layer includes: a pair of end regions whose magnetization directions are fixed; and a center region sandwiched between the pair of end regions, in which the domain wall moves. A first trapping site by which the domain wall is trapped is formed at a boundary between the end region and the center region. Moreover, at least one second trapping site by which the domain wall is trapped is formed within the center region.

The domain wall motion element and the MRAM according to the present invention are capable of a multivalued operation.

DESCRIPTION OF EMBODIMENTS

A domain wall motion element according to an exemplary embodiment of the present invention and an MRAM using the domain wall motion element as a memory cell will be described with reference to the attached drawings.

1. Summary of Domain Wall Motion Element

FIG. 1is a conceptual diagram for explaining principle of a domain wall motion element1according to an exemplary embodiment of the present invention. The domain wall motion element1has a magnetic recording layer10that is formed of a ferromagnetic film. The magnetic recording layer10has a domain wall DW, and the domain wall DW moves in the magnetic recording layer10.

More specifically, as shown inFIG. 1, the magnetic recording layer10includes a pair of end regions11(11-1,11-2) and a center region12. The end regions11-1and11-2are connected to both ends of the center region12, and the center region12is sandwiched between the end regions11-1and11-2. Respective magnetization directions of the end regions11-1and11-2are fixed in opposite directions to each other. On the other hand, magnetization in the center region12is reversible and can be parallel to the magnetization direction of the end region11-1or the magnetization direction of the end region11-2. Therefore, a domain wall DW is formed within the magnetic recording layer10. The domain wall DW can move in the center region12sandwiched between the end regions11-1and11-2whose magnetization directions are fixed.

InFIG. 1, a potential distribution that the domain wall DW feels in the magnetic recording layer10also is shown. The domain wall DW remains more stably at a position with lower potential, under a condition where no external force acts thereon. In other words, the domain wall DW can be trapped at a position where a potential difference exists, under the condition where no external force acts thereon. Such a position at which the domain wall DW can be trapped is hereinafter referred to as a “trapping site”. According to the present exemplary embodiment, at least two kinds of trapping site are formed in the magnetic recording layer10.

As shown in the potential distribution inFIG. 1, the potential in the end regions11-1and11-2is much higher than the potential in the center region12. Therefore, a first potential difference PA that is comparatively large exists at a boundary between the center region12and each of the end regions11-1and11-2. A “first trapping site TS1” is formed by the comparatively large first potential difference PA. That is, the first trapping site TS1by which the domain wall DW is trapped is formed at the boundary between the center region12and each of the end regions11-1and11-2. Furthermore, a second potential difference PB that is comparatively small exists within the center region12. A “second trapping site TS2” that is different from the first trapping site TS1is formed by the comparatively small second potential difference PB. That is, the second trapping site TS2by which the domain wall DW is trapped is formed within the center region12. The number of the second trapping site TS2in the center region12can be one or plural. At least one second trapping site TS2just needs to be formed within the center region12.

The domain wall DW exists between the end regions11-1and11-2whose potentials are high. The domain wall DW is trapped by any of the trapping sites (TS1, TS2) included between the end regions11-1and11-2, under a condition that no external force acts thereon. It should be particularly noted that the domain wall. DW can be trapped by the second trapping site TS2. This means that the domain wall DW can remain at a given position within the center region12.

It is preferable that the first potential difference PA for trapping the domain wall DW at the first trapping site TS1is larger than the second potential difference PB for trapping the domain wall DW at the second trapping site TS2. That is, it is preferable that the first trapping site TS1is “stronger” than the second trapping site TS2. The domain wall DW is trapped more stably at the first trapping site TS1than at the second trapping site TS2.

Various means are possible for forming the first trapping site TS1(first potential difference PA). For example, the first trapping site TS1can be formed by a difference in a shape (such as a film thickness and a width) or a difference in magnetic characteristics (such as saturation magnetization and magnetic anisotropy) between each end region11and the center region12.

Also, various means are possible for forming the second trapping site TS2(second potential difference PB). For example, as shown in a plan view ofFIG. 2, convexoconcave13formed on a lateral face of the center region12, namely, in-plane convexoconcave13of the center region12serves as the second trapping site TS2. Alternatively, as shown in a plan view ofFIG. 3, convexoconcave14formed on a surface of the center region12, namely, convexoconcave14in a film thickness direction of the center region12serves as the second trapping site TS2. Alternatively, as shown inFIG. 4, crystal grain boundary15in the ferromagnetic film forming the center region12serves as the second trapping site TS2.

A memory data of the domain wall motion element1varies depending on magnetization state of the center region12of the magnetic recording layer10, namely, the position of the domain wall DW. In order to write a desired data to the domain wall motion element1, the domain wall DW in the magnetic recording layer10just needs to be moved to a position corresponding to the desired data. The domain wall motion is achieved by the spin momentum transfer. More specifically, the domain wall DW is transferred by supplying the spin polarized current in the in-plane direction of the magnetic recording layer10.

FIG. 5is a conceptual diagram showing data writing to the domain wall motion element1according to the present exemplary embodiment. Let us consider a case where the domain wall DW is first positioned at the first trapping site TS1on the side of the end region11-2and then a write current is supplied from the end region11-1to the end region11-2through the center region12, as shown inFIG. 5. In this case, spin electrons of the write current move from the end region11-2to the center region12. In accordance with the moving direction of the spin electrons, the domain wall DW moves from the first trapping site TS1on the side of the end region11-2in a direction toward the first trapping site TS1on the side of the end region11-1.

Here, the movement distance of the domain wall DW can be variable controlled by adjusting the number of pulses or a pulse width of the write current. The reason is that at least one second trapping site TS2is formed within the center region12. In a case where the number of pulses or the pulse width of the write current is small, the domain wall DW does not reach to the first trapping site TS1on the side of the end region11-1and is trapped by the second trapping site TS2within the center region12. For example, when one short current pulse is injected, the domain wall DW moves from the first trapping site TS1on the side of the end region11-2to an adjacent second trapping site TS2. When another current pulse is further injected, the domain wall DW moves to the further adjacent second trapping site TS2. By repeating this procedure, the domain wall DW eventually reaches to the first trapping site TS1on the side of the end region11-1. In this manner, the movement distance of the domain wall DW varies depending on the number of pulses of the write current. The same applies to the case of the pulse width of the write current. The movement distance of the domain wall DW varies depending on the pulse width of the write current. By appropriately setting the number of pulses or the pulse width of the write current, it is possible to transfer the domain wall DW not only to the first trapping site TS1but also to the second trapping site TS2within the center region12. That is, it is possible to have the domain wall DW stop at a predetermined position within the center region12.

The same applies to a case of moving the domain wall DW in the opposite direction. In this case, the write current is supplied from the end region11-2to the end region11-1through the center region12. Therefore, spin electrons of the write current move from the end region11-1to the center region12. In accordance with the moving direction of the spin electrons, the domain wall DW moves in a direction toward the side of the end region11-2. Also in this case, by appropriately setting the number of pulses or the pulse width of the write current, it is possible to transfer the domain wall DW not only to the first trapping site TS1but also to the second trapping site TS2within the center region12.

As described above, the moving direction of the domain wall DW can be set based on the direction of the write current. Moreover, the movement distance of the domain wall DW can be set based on the number of pulses or the pulse width of the write current. Thus, it is possible to transfer the domain wall to a desired position (first trapping site TS1or second trapping site TS2) in the magnetic recording layer10.

The memory data of the domain wall motion element1can be read out by utilizing a magnetoresistance element. The magnetoresistance element is configured such that a resistance value thereof varies depending on the magnetization state (position of the domain wall DW) of the center region12of the magnetic recording layer10. It is therefore possible to sense the magnetization state of the center region12, namely, the memory data of the domain wall motion element1by detecting the resistance value of the magnetoresistance element. It can be said that the magnetic recording layer10constitutes a data write unit, while the magnetoresistance element constitutes a data read unit. Typically, a magnetic tunnel junction (MTJ) element is used as the magnetoresistance element.

FIG. 6shows an example of the data read unit (MTJ). In the example shown inFIG. 6, the domain wall motion element1has a tunnel barrier layer20being a nonmagnetic layer and a magnetization fixed layer30being a ferromagnetic layer, in addition to the magnetic recording layer10. The tunnel barrier layer20is an insulating layer and is sandwiched between at least the center region12of the magnetic recording layer10and the magnetization fixed layer30. The magnetization fixed layer30faces at least the center region12of the magnetic recording layer10across the tunnel barrier layer20. The center region12of the magnetic recording layer10, the tunnel barrier layer20and the magnetization fixed layer30form the MTJ.

A magnetization direction of the magnetization fixed layer30is fixed in one direction. On the other hand, the magnetization at each position in the center region12is parallel to or anti-parallel to the magnetization direction of the magnetization fixed layer30. Therefore, the resistance value of the MTJ varies depending on a relative angle between the magnetization direction of the magnetization fixed layer30and an average magnetization direction of the center region12. That is, the resistance value of the MTJ varies depending on the magnetization state (position of the domain wall DW) of the center region12of the magnetic recording layer10. At a time of data reading, a read current is so supplied in a perpendicular-to-plane direction as to pass through the MTJ. Based on the read current, it is possible to sense the resistance value of the MTJ, namely, the memory data.

FIG. 7shows another example of the data read unit (MTJ). In the example shown inFIG. 7, the domain wall motion element1has a conductive layer40, a magnetization free layer50being a ferromagnetic layer, a tunnel barrier layer60being a nonmagnetic layer and a magnetization fixed layer70being a ferromagnetic layer, in addition to the magnetic recording layer10. The tunnel barrier layer60is an insulating layer and is sandwiched between the magnetization free layer50and the magnetization fixed layer70. The magnetization free layer50, the tunnel barrier layer60and the magnetization fixed layer70form the MTJ. The conductive layer40is sandwiched between the magnetic recording layer10and the MTJ. It should be noted that the conductive layer40may be a nonmagnetic layer or may not be provided.

Moreover, a magnetization direction of the magnetization fixed layer70of the MTJ is fixed in one direction. Whereas, the magnetization free layer50of the MTJ is magnetically coupled to the center region12of the magnetic recording layer10. A magnetization state of the magnetization free layer50varies depending on the magnetization state of the center region12of the magnetic recording layer10.

For example, as shown inFIG. 7, the magnetic recording layer10is formed of a perpendicular magnetic film having perpendicular magnetic anisotropy, while the magnetization free layer50and the magnetization fixed layer70each is formed of an in-plane magnetic film having in-plane magnetic anisotropy. The magnetization direction at each position in the center region12of the magnetic recording layer10is the +Z direction or the −Z direction. The magnetization direction of the magnetization fixed layer70is fixed in the in-plane −Y direction. The magnetization free layer50and the center region12of the magnetic recording layer10are magnetically coupled to each other. Moreover, in the XY plane, a center of the magnetization free layer50is displaced in the Y direction from a center of the center region12. As a result, leakage magnetic flux from the perpendicular magnetization at each position in the center region12applies a magnetic component in the +Y direction or the −Y direction to the magnetization free layer50. The magnetization direction at each position in the magnetization free layer50is uniquely determined depending on the magnetization direction at each position in the center region12. That is to say, the magnetization state of the magnetization free layer50varies depending on the magnetization state of the center region12.

In the example shown inFIG. 7, the resistance value of the MTJ varies depending on a relative angle between the magnetization direction of the magnetization fixed layer70and an average magnetization direction of the magnetization free layer50. As mentioned above, the magnetization state of the magnetization free layer50depends on the magnetization state of the center region12. Therefore, it can be said that the resistance value of the MTJ varies depending on the magnetization state (position of the domain wall DW) of the center region12. At the time of data reading, a read current is so supplied in the perpendicular-to-plane direction as to pass through the MTJ. Based on the read current, it is possible to sense the resistance value of the MTJ, namely, the memory data.

FIG. 8conceptually shows an example of resistance values that the domain wall motion element1according to the present exemplary embodiment can take. As explained in the foregoingFIG. 5, the domain wall DW can be trapped not only by the first trapping site TS1at the end of the center region12but also by the second trapping site TS2within the center region12. It can be seen that the resistance value of the MTJ varies variously depending on the position of the domain wall DW, namely, the magnetization state of the center region12. In the example shown inFIG. 8, four kinds of resistance state (1) to (4) different from each other can be obtained. That is, the domain wall motion element1capable of storing multivalued data is achieved. The domain wall motion element1according to the present exemplary embodiment is capable of a multivalued operation.

To generalize, according to the domain wall motion element1of the present exemplary embodiment, at least three trapping sites are formed in the magnetic recording layer10in which the domain wall moves. By transferring the domain wall DW among the trapping sites, the multivalued operation can be achieved.

FIG. 9conceptually shows variation in the resistance value of the domain wall motion element1according to the present exemplary embodiment. A vertical axis represents the resistance value, and a horizontal axis represents the pulse length of the write current. Alternatively, the horizontal axis may be the number of pulses. As described above, it is possible to transfer the domain wall DW to a desired trapping site to achieve a desired resistance value (memory data), by controlling the pulse length (or the number of pulses) of the write current. Here, the resistance values that the domain wall motion element1can take depend on the number of the second trapping sites TS2within the center region12. In a case where the number of the second trapping sites TS is small and a pitch of the second trapping sites TS2is large, the resistance value varies in a stepwise manner. As the number of the second trapping sites TS2increases and the pitch of the second trapping, sites TS becomes smaller, variation in the resistance value becomes finer. When a large number of second trapping sites TS2are provided, the resistance value varies continuously (smoothly) and the domain wall motion element1behaves in an analog manner.

According to the present exemplary embodiment, as described above, the domain wall motion element1that is capable of the multivalued operation and further an analog operation can be achieved. By applying the domain wall motion element1to an MRAM, a domain wall motion type MRAM that is capable of the multivalued operation and further the analog operation can be achieved.

2. Examples of Domain Wall Motion Element

Examples of the domain wall motion element (domain wall motion type magnetoresistance element)1according to the present exemplary embodiment will be described below in more detail.

2-1. First Example

FIG. 10shows an example of the domain wall motion element1according to the present exemplary embodiment. The domain wall motion element1shown inFIG. 10has a structure similar to that shown in the foregoingFIG. 6, and provided with the magnetic recording layer10, the tunnel barrier layer20and the magnetization fixed layer30. The center region12of the magnetic recording layer10, the tunnel barrier layer20and the magnetization fixed layer30form the read unit (MTJ).

In the present example, the magnetic recording layer10and the magnetization fixed layer30each is formed of a perpendicular magnetic film having the perpendicular magnetic anisotropy. The magnetization direction of the magnetization fixed layer30is fixed, for example, in the +Z direction. The magnetization direction of the center region12of the magnetic recording layer10becomes parallel to or anti-parallel to the magnetization direction of the magnetization fixed layer30. The magnetization directions of the end regions11-1and11-2of the magnetic recording layer10are fixed in the opposite directions to each other along the Z axis.

The magnetic recording layer10is exemplified by a CoPt alloy film, a CoCrPt alloy film, a CoPd alloy film, a CoSm alloy film, a Co/Pt laminated film, a Co/Pd laminated film, a FePt alloy film, a FePd alloy film, a Fe/Pt laminated film, a Fe/Pd laminated film, a TbFeCo alloy film, a GdFeCo alloy film and so forth. The tunnel barrier layer20is exemplified by a Al—O film, a MgO film and so forth. The magnetization fixed layer30is exemplified by a CoPt alloy film, a CoCrPt alloy film, a CoPd alloy film, a CoSm alloy film, a Co/Pt laminated film, a Co/Pd laminated film, a FePt alloy film, a FePd alloy film, a Fe/Pt laminated film, a Fe/Pd laminated film, a TbFeCo alloy film, a GdFeCo alloy film and so forth.

InFIG. 10, a first magnetic film81and a second magnetic film82are so stacked as to be adjacent to the respective end regions11-1and11-2, in order to form the first trapping site TS1at the boundary between each end region11and the center region12. Saturation magnetization or crystal magnetic anisotropy of the first magnetic film81and the second magnetic film82is different from that of the magnetic film forming the magnetic recording layer10. Among a CoPt alloy film, a CoCrPt alloy film, a CoPd alloy film, a CoSm alloy film, a Co/Pt laminated film, a Co/Pd laminated film, a FePt alloy film, a FePd alloy film, a Fe/Pt laminated film, a Fe/Pd laminated film, a TbFeCo alloy film, a GdFeCo alloy film and so force, a film whose saturation magnetization or crystal magnetic anisotropy is different from that of the magnetic, recording layer10can be used as the first magnetic film81and the second magnetic film82. Due to the difference in the magnetic characteristics (saturation magnetization, magnetic anisotropy), the first trapping site TS1is formed at the boundary between the each end region11and the center region12.

As shown inFIG. 11andFIG. 12, the first trapping site TS1can also be formed by a difference in a shape (film thickness, width) between the each end region11and the center region12. Due to a step difference (film thickness difference, width difference) between the each end region11and the center region12, the first potential difference PA as shown inFIG. 1is caused and the first trapping site TS1is formed. InFIG. 11, the each end region11is formed such that the film thickness thereof is larger than that of the center region12. In this case, the film thickness of the magnetic recording layer10at the first trapping site TS1is larger on the side of the end region11than on the side of the center region12. InFIG. 12, the each end region11is formed such that an in-plane width (width in the XY plane) thereof is larger than that of the center region12. In this case, the in-plane width of the magnetic recording layer10at the first trapping site TS1is larger on the side of the end region11than on the side of the center region12.

A means for forming the second trapping site TS2within the center region12can be any of those explained in the foregoingFIGS. 2 to 4. In the example shown inFIG. 2, the convexoconcave13formed on a lateral face of the center region12, namely, the in-plane convexoconcave13of the center region12serves as the second trapping site TS2. The convexoconcave13can be formed by using a mask that has a pattern corresponding to the convexoconcave13, when patterning of the magnetic recording layer10is performed. In the example shown inFIG. 3, the convexoconcave14formed on a surface of the center region12, namely, the convexoconcave14in the film thickness direction of the center region12serves as the second trapping site TS2. The convexoconcave14can be formed by processing the film surface of the center region12by an ion beam etching method or the like. In the example shown inFIG. 4, a crystalline magnetic film is used and the crystal grain boundary15in the magnetic film serves as the second trapping site TS2. As the crystalline magnetic film, a CoPt alloy film, a CoCrPt alloy film, a CoPd alloy film, a CoSm alloy film, a Co/Pt laminated film, a Co/Pd laminated film, a FePt alloy film, a FePd alloy film, a Fe/Pt laminated film, a Fe/Pd laminated film can be used.

2-2. Second Example

FIG. 13shows another example of the domain wall motion element1according to the present exemplary embodiment. The domain wall motion element1shown inFIG. 13has a structure similar to that shown in the foregoingFIG. 7, and is provided with the magnetic recording layer10, the conductive layer40, the magnetization free layer50, the tunnel barrier layer60and the magnetization fixed layer70. The magnetization free layer50, the tunnel barrier layer60and the magnetization fixed layer70form the read unit (MTJ).

The magnetic recording layer10is the same as in the case of the above-described first example, and an overlapping description will be omitted. In the present example, the magnetization free layer50and the magnetization fixed layer70each is formed of an in-plane magnetic film having in-plane magnetic anisotropy. The magnetization direction of the magnetization fixed layer70is fixed, for example, in the −Y direction. The magnetization free layer50is magnetically coupled to the center region12of the magnetic recording layer10, and the magnetization state thereof varies depending on the magnetization state of the center region12.

As the conductive layer40, a Ta film and the like can be used. The magnetization free layer50is exemplified by a NiFe alloy film, a CoFe alloy film, a CoFeNi alloy film, a CoTaZr alloy film, a CoNbZr alloy film, a CoFeB alloy film and so forth. The tunnel barrier layer60is exemplified by an Al—O film, a MgO film and so forth. The magnetization fixed layer70is exemplified by a NiFe alloy film, a CoFe alloy film, a CoFeNi alloy film, a CoTaZr alloy film, a CoNbZr alloy film, a CoFeB alloy film and so forth. Alternatively, a laminated film of such an alloy film and an anti-ferromagnetic film (such as a PtMn alloy film and a IrMn alloy film) may be used as the magnetization fixed layer70.

2-3. Third Example

FIG. 14shows still another example of the domain wall motion element1according to the present exemplary embodiment. The domain wall motion element1shown inFIG. 14has a structure similar to that shown in the first example (FIG. 10). However, the magnetic recording layer10and the magnetization fixed layer30each is formed of an in-plane magnetic film having the in-plane magnetic anisotropy.

The magnetic recording layer10is exemplified by a NiFe alloy film, a CoFe alloy film, a CoFeNi alloy film, a CoTaZr alloy film, a CoNbZr alloy film, a CoFeB alloy film and so forth. The tunnel barrier layer20is exemplified by an Al—O film, a MgO film and so forth. The magnetization fixed layer30is exemplified by a NiFe alloy film, a CoFe alloy film, a CoFeNi alloy film, a CoTaZr alloy film, a CoNbZr alloy film, a CoFeB alloy film and so forth. Alternatively, a laminated film of such an alloy film and an anti-ferromagnetic film (such as a PtMn alloy film and a IrMn alloy film) may be used as the magnetization fixed layer30.

The magnetization direction of the magnetization fixed layer30is fixed, for example, in the +X direction. The magnetization direction of the center region12of the magnetic recording layer10becomes parallel to or anti-parallel to the magnetization direction of the magnetization fixed layer30. The magnetization directions of the end regions11-1and11-2of the magnetic recording layer10are fixed in the opposite directions to each other along the X axis. The center region12of the magnetic recording layer10, the tunnel barrier layer20and the magnetization fixed layer30form the read unit (MTJ).

InFIG. 14, a first magnetic film81and a second magnetic film82are so stacked as to be adjacent to the respective end regions11-1and11-2, in order to form the first trapping site TS1at the boundary between each end region11and the center region12. Saturation magnetization or crystal magnetic anisotropy of the first magnetic film81and the second magnetic film82is different from that of the magnetic film forming the magnetic recording layer10. A CoPt alloy film, a CoCrPt alloy film, a CoCrTa alloy film, a CoPd alloy film, a PtMn alloy film, a IrMn alloy film and so forth can be used as the first magnetic film81and the second magnetic film82.

As shown inFIG. 15andFIG. 16, the first trapping site TS1can also be formed by a difference in a shape (film thickness, width) between the each end region11and the center region12. Due to a step difference (film thickness difference, width difference) between the each end region11and the center region12, the first potential difference PA as shown inFIG. 1is caused and the first trapping site TS1is formed. InFIG. 15, the each end region11is formed such that the film thickness thereof is larger than that of the center region12. In this case, the film thickness of the magnetic recording layer10at the first trapping site TS1is larger on the side of the end region11than on the side of the center region12. InFIG. 16, the each end region11is formed such that an in-plane width (width in the XY plane) thereof is larger than that of the center region12. In this case, the in-plane width of the magnetic recording layer10at the first trapping site TS1is larger on the side of the end region11than on the side of the center region12.

A means for forming the second trapping site TS2within the center region12can be any of those explained in the foregoingFIGS. 2 to 4. In the example shown inFIG. 2, the convexoconcave13formed on a lateral face of the center region12, namely, the in-plane convexoconcave13of the center region12serves as the second trapping site TS2. The convexoconcave13can be formed by using a mask that has a pattern corresponding to the convexoconcave13, when patterning of the magnetic recording layer10is performed. In the example shown inFIG. 3, the convexoconcave14formed on a surface of the center region12, namely, the convexoconcave14in the film thickness direction of the center region12serves as the second trapping site TS2. The convexoconcave14can be formed by processing the film surface of the center region12by an ion beam etching method or the like. In the example shown inFIG. 4, a crystalline magnetic film is used and the crystal grain boundary15in the magnetic film serves as the second trapping site TS2. As the crystalline magnetic film, a NiFe alloy film, a CoFe alloy film and a CoFeNi alloy film can be used.

In either example, the multivalued operation and further the analog operation are possible (refer toFIG. 5,FIG. 8andFIG. 9).

A domain wall motion type MRAM according to the present exemplary embodiment uses the above-described domain wall motion element1as a memory cell and is capable of the multivalued operation.FIG. 17shows an example of a configuration of an MRAM100according to the present exemplary embodiment.FIG. 18briefly shows data writing/reading with respect to the MRAM100shown inFIG. 17.

The MRAM100has a memory cell array101in which a plurality of magnetic memory cells (domain wall motion elements)1are arranged in an array form. The memory cell array101includes not only the memory cells1used for data recording but also reference cells1rwhich are referred to at the time of data reading. A basic structure of the reference cell1ris the same as that of the memory cell1.

Each memory cell1(1r) is connected to a word line WL and a pair of bit lines BL1and BL2(BL1rand BL2r). More specifically, the end regions11-1and11-2of the magnetic recording layer10are respectively connected to a first bit line BL1(BL1r) and a second bit line BL2(BL2r) through a first transistor TR1and a second transistor TR2. Gates of the first transistor TR1and the second transistor TR2are connected to the word line WL. Moreover, one end of the MTJ as the data read unit is connected to a ground line.

A plurality of word lines WL are connected to an X selector102. The X selector102selects one word line WL connected to a target memory cell is as a selected word line WLs from the plurality of word lines WL. A potential of the selected word line WLs is set to “High”, and the first transistor TR1and the second transistor TR2of the target memory cell is are turned ON. A plurality of first bit lines BL1are connected to a Y-side current termination circuit104, and a plurality of second bit lines BL2are connected to a Y selector103. The Y selector103selects one second bit line BL2connected to the target memory cell is as a selected second bit line BL2sfrom the plurality of second bit lines BL2. The Y-side current termination circuit104selects one first bit line BL1connected to the target memory cell1sas a selected first bit line BL1sfrom the plurality of first bit lines BL1. In this manner, the target memory cell1sis selected.

A Y-side current source circuit105is a current source that supplies or draws a predetermined write current (IW1, IW2) with respect to the selected second bit line BL2s. The Y-side current source circuit105has a current selector section that determines a direction of the write current and a constant current source that supplies a constant current. A Y-side power source circuit106supplies a predetermined voltage to the Y-side current termination circuit104. As a result, the write current IW1, IW2by the Y-side current source circuit105flows into the Y selector103or flows out from the Y selector103, depending on the data to be written to the target memory cell1s.

For example, in a case of data writing to increase the resistance value of the MTJ, potentials of the first bit line BL1and the second bit line BL2are set to “High” and “Low”, respectively. As a result, the first write current IW1flows from the first bit line BL1to the second bit line BL2through the first transistor TR1, the magnetic recording layer10and the second transistor TR2. On the other hand, in a case of data writing to decrease the resistance value of the MTJ, potentials of the first bit line BL1and the second bit line BL2are set to “Low” and “High”, respectively. As a result, the second write current IW2flows from the second bit line BL2to the first bit line BL1through the second transistor TR2, the magnetic recording layer10and the first transistor TR1. It is possible to write a desired data by controlling the number of pulses or the pulse width of the write current IW1or IW2.

At the time of data reading, the first bit line BL1is set to “Open” and a potential of the second bit line BL2is set to “High”, for example. A read current load circuit107supplies a predetermined read current to the selected second bit line BL2s. The read current flows from the selected second bit line BL2sto the ground line through the second transistor TR2and the MTJ. Similarly, the read current load circuit107supplies the predetermined current to a reference second bit line BL2rwhich is connected to the reference cell1r. A sense amplifier108senses data of the target memory cell1sbased on a difference between a potential of the reference second bit line BL2rand a potential of the selected second bit line BL2s, and outputs the data.

The exemplary embodiments of the present invention have been described above with reference to the attached drawings. However, the present invention is not limited to the above-described exemplary embodiments and can be appropriately modified by a person skilled in the art without departing from the scope of the invention.