Magnetic storage device and method for producing the same

In the magnetic storage device, magnetization characteristics during write cycles are homogenized, and write cycles are carried out efficiently. In the magnetic storage device, the soft magnetic body is formed so as to cover the line either totally or partially, and the anti-ferromagnetic layer is formed on the outer surface of this soft magnetic body. Furthermore, the magneto-resistive element is disposed in the vicinity of the line. Suppose the case where the exchange coupling energy at the interface between the soft magnetic body and the anti-ferromagnetic layer is J (erg/cm2), the saturation magnetization of the soft magnetic body is Ms (emu/cc), and the coercive force of the soft magnetic body is Hc (Oe). Then, the thickness t (cm) of the soft magnetic body is selected to be such that t<J/(Hc·Ms).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic storage device or the like that stores data in magneto-resistive elements.

2. Description of the Related Art

MRAM (Magnetic Random Access Memory) has been receiving much attention recently as a memory device to be used in information processing equipment such as computers, communication equipment, and the like. Since MRAM stores data utilizing magnetism, it is capable of retaining the direction of magnetization without use of any electrical means, and hence, data are not inconveniently lost when power is cut, as is the case with DRAM (Dynamic Random Access Memory) or SRAM (Static RAM), which are generally referred to as volatile memories. Furthermore, when compared with conventional nonvolatile storing means such as flash EEPROM and hard disc drives, MRAM excels in performance in terms of access speed, reliability, power consumption, and the like. Because of these characteristics, it is a common belief that MRAM will be able to simultaneously realize all the advantages of not only volatile memories such as DRAMs and SRAMs but also of nonvolatile memories such as flash EEPROMs and hard disc drives.

Consider, for example, the case of developing information processing equipment that targets so-called ubiquitous computing in which information processing is available regardless of location. A requirement of such ubiquitous computing includes a memory device that must be capable of high speed processing but simultaneously have low power consumption. Such a memory device must also be able to avoid loss of information, even upon power shutdown. MRAM has the potential to meet both of these demands, and is expected to be employed in an increasing number of information processing equipment designs in the future.

In the case of tablets, mobile information terminals, and the like that are carried by a person in day-to-day living, it is often difficult to secure a sufficient power supply. Therefore, in order to be able to process a large amount of information in a very high usage environment, even MRAMs of low power consumption will require a further reduction in power consumption during information processing.

One example of the technology that improves the power consumption rate of an MRAM is a magnetic storage device described as follows. As shown inFIG. 21, this magnetic storage device500includes, in each of the storage areas (memory cells), a bit line502, a word line504disposed orthogonal to the bit line502, and a tunneling magneto-resistive (TMR) element506disposed at the intersection between the bit line502and the word line504. Each of the bit line502and the word line506is capable of creating a magnetic field whose strength is approximately half that necessary to invert the bit state of the TMR element506. When a current flows through the bit line502and the word line504that have been selected, the TMR element506at the cross-point inverts its magnetization configuration accordingly.

In this magnetic storage device500, the bit line502and the word line504both have a cladding structure where they are coated with a ferromagnetic film510which exhibits high magnetic permeability. Therefore, any leak of magnetic flux from the bit line502and the word line504can be reduced. Furthermore, when the bit line502or the word line504is energized, the ferromagnetic film510becomes magnetized, thereby creating a static magnetic field. Therefore, the sum of this static magnetic field and induced magnetic fields of the bit line502and the word line504are applied onto the TMR element506. As a result, even if the power supply is low, the magnetic field that is necessary to invert the magnetization configuration of the TMR element506can still be obtained.

Moreover, by coating three surfaces of the bit line502and the word line504, respectively, with the ferromagnetic film510but leaving the remaining surface facing the TMR element506side open, magnetic flux can be concentrated onto the TMR element506. This has an advantage in that a write cycle would require less time to be completed.

It should be noted that the TMR element in this instance includes a first magnetic layer (magnetic sensing layer) whose direction of magnetization changes according to an external magnetic field, a second magnetic layer whose direction of magnetization is fixed, and a nonmagnetic insulating layer interposed between the first magnetic layer and the second magnetic layer. This TMR element stores binary data by controlling the orientation of the magnetization directions of the first and second magnetic layers, so that the direction of magnetization is either parallel or antiparallel.

The technology of this magnetic storage device500is disclosed by the following documents.

According to further research carried out by the inventor of the present invention, however, although the coating of these bit lines502and word lines504with the ferromagnetic film510can reduce the current during write cycles, it is likely that it will make the strength of the resultant magnetic field uneven. In particular, it is difficult to evenly coat the bit line502or the word line504with the ferromagnetic film510along its longitudinal direction. In addition to this, a plurality of domains whose magnetization directions vary widely will be spontaneously formed within the ferromagnetic film510. These factors may contribute to the possibility that magnetization characteristics acting on respective TMR elements506during a write cycle may become uneven.

In addition, when a magnetic field is being inverted by switching the direction of current flowing through the bit line502or the word line504, the presence of the ferromagnetic film510causes a problem in that the strength or the rate of change of the magnetic field becomes uneven, depending on which way the current is flowing. As a result, each TMR element506experiences an unevenness in write speed, depending on the direction of the current, and it is a concern that control of the current or timing during the write cycle may become complicated.

Furthermore, if many domains are formed within the ferromagnetic film501, Barkhausen noise is produced when the magnetization configuration of the bit line502or the word line504changes, and this is also considered to contribute to the deterioration of the write cycle.

SUMMARY OF THE INVENTION

The present invention has been developed in consideration of the above problems, and it is thus an object of the present invention to control the unevenness of a writing magnetic field in a magnetic storage device, and to improve the writing performance thereof.

In order to achieve the above-mentioned objects, a magnetic storage device of the present invention includes: a line that is formed in an arbitrary direction; a soft magnetic body that is formed so as to cover the line either totally or partially; an anti-ferromagnetic layer that is formed on an outer surface of the soft magnetic body; and a magneto-resistive element that is disposed in the vicinity of the line. Then, the magnetic storage device satisfies the equation t<J/(Hc·Ms), where J (erg/cm2) is an exchange coupling energy at a boundary between the soft magnetic body and the anti-ferromagnetic layer, Ms (emu/cc) is a saturation magnetization of the soft magnetic body, Hc (Oe) is a coercive force of the soft magnetic body, and t (cm) is a thickness of the soft magnetic body.

Since the anti-ferromagnetic layer is formed on the outer surface of the soft magnetic body and, at the same time, the thickness of the soft magnetic body t is set to be within a prescribed range, the soft magnetic body can be securely pinned.

To achieve the foregoing object, the magnetic storage device of the present invention is characterized in that the thickness t of the soft magnetic body satisfies the equation t<J/(Hc2·Ms) where Hc2(Oe) is an applied magnetic field necessary for the soft magnetic body to reach 80% of the saturation magnetization Ms. As a result, the reliability of the pinning can further be improved. The coercive force Hc can be considered as the applied magnetic field necessary for the saturation magnetization Ms in the opposing direction to become zero. However, as is the case in the present invention, by setting the thickness t based on the applied magnetic field Hc2necessary to achieve 80% of the saturation magnetization Ms beyond zero saturation magnetization, the pinning can be achieved more securely.

To achieve the foregoing object, the magnetic storage device of the present invention is characterized in that a direction in which the line extends is almost perpendicular to a direction of easy axis of magnetization of the magneto-resistive element. If the pinning direction is almost perpendicular to the easy axis of magnetization of the magneto-resistive element, it will then be difficult to carry out annealing of the soft magnetic body to be pinned. However, by setting the thickness t of the soft magnetic body to be within a prescribed range as in the present invention, the pinning can be carried out by simply applying a magnetic field.

To achieve the foregoing object, the magnetic storage device of the present invention is characterized by an oxidation prevention cap layer that is provided on an outer circumference side of the anti-ferromagnetic layer. This oxidation prevention cap layer can prevent oxidation of the anti-ferromagnetic layer.

To achieve the foregoing object, the magnetic storage device of the present invention is characterized in that the soft magnetic body has a multi-layer structure which includes a first soft magnetic layer and a second soft magnetic layer, wherein these layers are made of different materials. By selecting appropriate materials, the pinning magnetic field obtained by the anti-ferromagnetic layer can be controlled.

To achieve the foregoing object, the magnetic storage device of the present invention is characterized in that the soft magnetic body includes an element side yoke that is disposed on the magneto-resistive element side of the line and a non-element side yoke that is disposed on the side of the line opposite to the magneto-resistive element, and is configured in an almost annular structure.

A method for producing a magnetic storage device of the present invention that achieves the above-mentioned objectives includes: an element formation step of forming a base material for magneto-resistive elements on a substrate; an element magnetic field application step of annealing a magnetic fixation layer of the base material for the magneto-resistive elements with an element magnetic field being applied in a prescribed direction to form magneto-resistive elements; a line formation step of forming lines almost perpendicular to the direction of the element magnetic field in the vicinity of the magneto-resistive elements; and a magnetic body formation step of forming a soft magnetic body, so as to cover the lines either partially or totally, and an anti-ferromagnetic layer on an outer surface of the soft magnetic body, with a pinning magnetic field formed in the same direction as the lines being applied. The method is also characterized in that a strength of the pinning magnetic field is selected to be greater than a coercive force of the soft magnetic body. With these characteristics, the magnetization configuration of the soft magnetic body can be oriented in the direction of the pinning magnetic field. Further to this, by depositing the anti-ferromagnetic layer thereover, pinning can be securely achieved.

To achieve the foregoing object, the method for producing a magnetic storage device of the present invention is characterized in that, in order to improve the reliability of the orientation of the soft magnetic body, the strength of the pinning magnetic field is selected to be greater than an applied magnetic field necessary for the soft magnetic body to reach 80% of the saturation magnetization Ms.

To achieve the foregoing object, the method for producing a magnetic storage device of the present invention is characterized in that, in the magnetic body formation step, the soft magnetic body and the anti-ferromagnetic layer are formed in a non-annealing environment.

According to the present invention, a magnetic field created by the line can be stabilized, and any change in such a magnetic field can be smoothed, thereby improving write performance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following paragraphs, various embodiments of the magnetic storage device according to the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same elements are designated with the same reference numerals, and as such, repetitive explanations therefor are omitted.

FIG. 1schematically illustrates a general view of a magnetic storage device1according to the first embodiment of the present invention. The magnetic storage device1includes a memory unit2, a bit selection circuit11, a word selection circuit12, a sense amplifier16, a plurality of bit lines13corresponding to first lines, and a plurality of word lines15corresponding to second lines. In the memory unit2, a plurality of memory cells3are disposed in a two-dimensional m×n formation (an array formation), where m and n are integers equal to or greater than 2. The bit lines13are arranged parallel to each other in a single plane, and, similarly, the word lines15are also arranged parallel to each other in another plane that is a prescribed distance away from the bit lines13. Since the extending directions of the bit lines13and the word lines15are perpendicular to each other, they form a grid pattern when viewed together in a general view. Electrical current that flows through the bit line13or the word line15creates a magnetic field that acts on the TMR element4when writing digital data. In this instance, the TMR (tunneling magneto-resistive) element4has a property that it changes its electrical resistance when the direction of magnetization is changed. The state of the electrical resistance following these changes determines which binary data is stored in the TMR element4.

The bit selection circuit11is capable of providing the bit line13of each memory cell3with a write current and a read current, which are either positive or negative. More specifically, the selection circuit11includes an address decoder circuit that selects a certain column from the array of memory cells3according to an address instructed either internally or externally and a current drive circuit that applies positive or negative voltage to the bit line13corresponding to the selected column to produce an electric current, thereby providing the TMR element4disposed at the bit line13of the selected column with a write magnetic field.

The word selection circuit12includes an address decoder circuit that selects a certain row from the array of memory cells3according to an address instructed either internally or externally and a current drive circuit that provides the word line15corresponding to the selected row with a prescribed voltage. Therefore, by providing an electric current to both the bit line13that has been selected by the bit selection circuit11and the word line15that has been selected by the word selection circuit12, a magnetic field can be applied to the TMR element4that is located at the cross-point of these two lines to write in binary data. Furthermore, the bit selection circuit11and the word selection circuit12are both capable of controlling a read current through an application of voltage small enough to not initiate a write cycle between the bit line13and the word line15. Specifically, the address decoder circuit of the bit selection circuit11selects a column corresponding to an address instructed either internally or externally, and a prescribed voltage is applied to the bit line13corresponding to the selected column. In addition to this, at the same time, the address decoder circuit of the word selection circuit12selects a row corresponding to the above-mentioned address, and a prescribed voltage is applied to the word line15corresponding to the selected row. Consequently, a read current flows through the TMR element4that is located at the cross-point of these bit line13and word line15. Then, based on this read current, a change of resistance of the TMR element4is detected by the sense amplifier16that is connected to the word line15.

As shown inFIG. 2under magnification, the TMR element4is disposed where the bit line13and the word line15cross each other (namely, the cross-point K), or more specifically, it is disposed at this cross-point K so as to beheld between the bit line13and the word line15. Therefore, one side of the TMR element4is in contact with the bit line13and the other side is in contact with the word line15. Such an arrangement of TMR elements4at the cross-points K that form the nodes of the grid means that, for each bit line13or word line15, there are a plurality of successively disposed TMR elements4. All other portions except the bit lines13, the word lines15, the TMR elements4, and the like, are electrically insulated. These insulated portions are made of insulating materials such as SiO2and are similar to insulated regions within a semiconductor layer. Materials such as W, Cu, and Al may be used to form the bit lines13or the word lines15.

Furthermore, the bit line13is partially covered with a bit line soft magnetic body40in the longitudinal direction, forming a so-called cladding structure. The bit line soft magnetic body40is disposed so as to directly cover the side of the bit line13which opposes the TMR element4(the non-element side) and the two other sides perpendicular thereto, forming a U-shaped cross-section. As a result, this bit line soft magnetic body40prevents any leakage of magnetic flux created by the bit line13.

Similarly, the word line15is partially covered with a word line soft magnetic body41in the longitudinal direction, forming a so-called cladding structure. The word line soft magnetic body41is disposed on the side of the word line15directly opposite to the TMR element4(the non-element side) and the two other sides perpendicular thereto, forming a U-shaped cross-section. As a result, this word line soft magnetic body41prevents any leakage of magnetic flux created by the word line15. These structures described above ensure that the magnetic flux created by the bit line13and the word line15is concentrated on the TMR element4. In this instance, it is preferable that the bit line soft magnetic body40and the word line soft magnetic body41be made, for example, of NiFe, CoFe, or the like.

A bit line anti-ferromagnetic layer26is disposed on the outer surface (opposite side to the bit line13) of the bit line soft magnetic body40. Exchange coupling at the joint between the bit line anti-ferromagnetic layer26and the bit line soft magnetic body40ensures that the magnetization direction of the bit line soft magnetic body40is stabilized. In particular, the pinning direction X of magnetization configuration of the bit line soft magnetic body40by the bit line anti-ferromagnetic layer26is selected to be almost the same as the extending direction of the bit line13, the cladding of which is provided by the bit line soft magnetic body40. The bit line anti-ferromagnetic layer26is made of an anti-ferromagnetic ordered alloy, an example of which includes materials having a CuAu—I structure. The anti-ferromagnetic ordered alloy has a high blocking temperature and an excellent corrosion-resistant property, but it requires annealing after deposition in order to convert it into an ordered alloy. Specific examples of materials used as an anti-ferromagnetic ordered alloy include NiMn, PtMn, and PdMn, and by applying a magnetic field in an annealing environment, an ordered alloy in the X direction mentioned above, namely, the extending direction of the bit line13, is achieved.

A word line anti-ferromagnetic layer27is disposed on the outer surface (opposite side to the word line15) of the word line soft magnetic body41. Exchange coupling at the joint between the word line anti-ferromagnetic layer27and the word line soft magnetic body41ensures that the magnetization direction of the word line soft magnetic body41is stabilized. In particular, the pinning direction Y of the magnetization of the word line soft magnetic body41by the word line anti-ferromagnetic layer27is selected to be almost the same as the extending direction of the word line15, the cladding of which is provided by the word line soft magnetic body41. The word line anti-ferromagnetic layer27is made of an anti-ferromagnetic random alloy, an example of which includes materials having an fcc structure. Although the anti-ferromagnetic random alloy has a low blocking temperature, it has a characteristic such that unidirectional anisotropy can be obtained without conducting heat treatment. Specific examples of materials used as an anti-ferromagnetic random alloy include IrMn, RhMn, and FeMn, and by applying a magnetic field along the intended pinning direction Y to be achieved in a non-annealing environment, namely, the extending direction of the word line15, a ordered alloy can be achieved. As a result, when there is no electric current, the magnetization configuration of the bit line soft magnetic body40(or the word line soft magnetic body41) is made of a single domain and parallel to the direction of the bit line13(or the word line15), the cladding of which is provided by the bit line soft magnetic body40(or the word line soft magnetic body41).

How the thickness t (cm) of the word line soft magnetic body41is determined will now be described. Suppose the case where the exchange coupling energy at the boundary between the word line soft magnetic body41and the word line anti-ferromagnetic layer27is J (erg/cm2), the saturation magnetization of the word line soft magnetic body41is Ms (emu/cc), and the coercive force of the word line soft magnetic body41is Hc (Oe). In this case, the thickness t of the word line soft magnetic body41is selected such that t<J/(Hc·Ms)

In particular, in the present embodiment, suppose the case where the externally applied magnetic field necessary for the word line soft magnetic body41to reach 80% of the saturation magnetization Ms is Hc2(Oe). In this case, the thickness t of the word line soft magnetic body41is selected such that t<J/(Mc2·Ms).

The reasons for these conditions are as follows. In order for the word line anti-ferromagnetic layer27to be able to sufficiently pin the word line soft magnetic body41, it is important that the pinning magnetic field Hua (Oe) acting on the word line soft magnetic body41due to the word line anti-ferromagnetic layer27is greater than the coercive force Hc of the word line soft magnetic body41. In this instance, the pinning magnetic field refers to an offset magnetic field where the word line anti-ferromagnetic layer27can act on the word line soft magnetic body41to provide the pinning effect. In other words, by maintaining the relationship Hua>Hc, unless another magnetic field is externally acted on, the magnetization configuration of the word line soft magnetic body41can be at least canceled out by the above-mentioned pinning magnetic field. Conversely, if Hua>Hc is not satisfied, the pinning magnetic field Hua will not be able to sufficiently pin the word line soft magnetic body41, and it will be hard to obtain a desired effect. Since the magnetic force Hua of the pinning magnetic field can be expressed as Hua=J/(Ms·t), by putting this relation into Hua>Hc, an equation that J/(Ms·t)>Hc can be obtained. On solving this equation for the thickness t of the word line soft magnetic body41, a relation t<J/(Hc·Ms) is obtained. Hence, by satisfying this equation for t, the word line anti-ferromagnetic layer27will be able to effectively pin the word line soft magnetic body41. If the thickness of the word line soft magnetic body41exceeds the above range, then the magnetic force Hua of the pinning magnetic field decreases, and sufficient pinning effect cannot be obtained.

For example, if NiFe is used for the word line soft magnetic body41and IrMn is used for the word line anti-ferromagnetic layer27, then the saturation magnetization Ms and the coercive force Hc of the word line soft magnetic body41made of NiFe becomes 780 (emu/cc) and 6 (Oe), respectively, and the exchange coupling energy J of NiFe/IrMn at a boundary between the word line soft magnetic body41and the word line anti-ferromagnetic layer27becomes 0.061 (erg/cm2). Therefore, substituting these figures into the equation t<J/(Hc·Ms), it will turnout that the thickness t of the word line soft magnetic body41needs to be less than 130 nm.

FIG. 3Aillustrates the magnetization curves of the word line soft magnetic body41made of NiFe when it is deposited to a thickness of 100 nm. As can be seen from the figure, the change in strength of magnetization when an external magnetic field H is acted on forms a hysteresis curve with the maximum magnetization in either direction being the saturation magnetization Ms. Therefore, considering the saturation magnetization Ms as a reference, the coercive force Hc, which is approximately 6 (Oe) in this case, is the strength of an external magnetic field required to bring the unidirectional magnetization back to zero and at the saturation magnetization Ms. In other words, if the coercive force Hc is externally applied to the word line soft magnetic body41, then the magnetization in the opposite direction can at least be canceled (can be brought to zero).

FIG. 3Billustrates magnetization curves when the word line soft magnetic body41made of NiFe is pinned by the word line anti-ferromagnetic layer27. In this instance, the word line anti-ferromagnetic layer27is made of IrMn, and its thickness is selected to be 10 nm. As can be seen from the figure, the magnetization curves have been offset due to the pinning effect. The amount of this offset corresponds to the pinning magnetic field Hua (Oe) by the word line anti-ferromagnetic layer27. In this instance, since the thickness of the word line soft magnetic body41is selected to be 100 nm and, at the same time, IrMn is used to form the word line anti-ferromagnetic layer27, the pinning magnetic field becomes Hua=6.5 (Oe). Consequently, since Hua is greater than Hc (=6 (Oe)), the magnetization configuration of roughly the whole word line soft magnetic body41can be oriented toward the pinning magnetic field Hua side.

Furthermore, in the present invention, it is preferable that the magnetic force Hua (Oe) of the pinning magnetic field formed by the word line anti-ferromagnetic layer27be larger than an externally applied magnetic field Hc2(Oe) at the word line soft magnetic body41. In this instance, this externally applied magnetic field Hc2can be defined as an external magnetic field that can be induced up to approximately 80% of the saturation magnetization Ms of the word line soft magnetic body41. The reason for this is that, if this externally applied magnetic field Hc2is exceeded by the magnetic force of the pinning magnetic field, the magnetization configuration of the word line soft magnetic body41can be made of a single domain more securely.

For example, an externally applied magnetic field Hc2necessary for the word line soft magnetic body41made of NiFe to reach approximately 625 (emu/cc), which is 80% of the saturation magnetization Ms, is 8 (Oe). If this value is put in the equation t<J/(Mc·Ms), then it turns out that the thickness t of the word line soft magnetic body41should preferably be selected to be less than 93 nm.

FIG. 4Aillustrates the magnetization curves of the word line soft magnetic body41made of NiFe when it is deposited to a thickness of 50 nm. As can be seen from the figure, the change in strength of magnetization when an external magnetic field H is acted on forms a hysteresis curve with the maximum magnetization in either direction being the saturation magnetization Ms. Therefore, the above-mentioned externally applied magnetic field Hc2can be considered as a magnetic field that can induce the saturation magnetization Ms in one direction to be approximately 80% of the saturation magnetization Ms in the opposite direction. Therefore, if the above-mentioned externally applied magnetic field Hc2is exceeded by the magnetic force Hua of the pinning magnetic field by the word line anti-ferromagnetic layer27, then the magnetization configuration of the word line soft magnetic body41is entirely oriented in the direction of the pinning magnetic field. In other words, by allowing a pinning magnetic field that exceeds the externally applied magnetic field Hc2to act on the word line soft magnetic body41, the magnetization configuration can be securely oriented.

FIG. 4Billustrates the magnetization curves when the word line soft magnetic body41made of NiFe is pinned by the word line anti-ferromagnetic layer27. In this instance, the word line anti-ferromagnetic layer27is made of IrMn, and its thickness is selected to be 10 nm. As can be seen from the figure, the magnetization curve has been offset due to the pinning effect, and the amount of this offset corresponds to the pinning magnetic field Hua (Oe) formed by the word line anti-ferromagnetic layer27. In this instance, since the thickness of the word line soft magnetic body41is selected to be 50 nm and, at the same time, IrMn is used to form the word line anti-ferromagnetic layer27, the pinning magnetic field becomes Hua=16.2 (Oe). As a result, since Hua is much larger than Hc, it turns out that the magnetization configuration of roughly the whole word line soft magnetic body41is oriented along the pinning magnetic field Hua more securely, compared to the above-mentionedFIGS. 3A and 3B.

FIG. 5illustrates the TMR element4under magnification. This TMR element4includes a first magnetic layer (free layer/magnetic sensing layer)4A whose direction of magnetization changes according to an external magnetic field, a second magnetic layer (pinned layer)4B whose direction of magnetization is fixed, a nonmagnetic insulating layer (insulating layer)4C interposed between the first magnetic layer4A and the second magnetic layer4B, and an anti-ferromagnetic layer4D that fixes (or pins) the direction of magnetization of the second magnetic layer4B. In this TMR element4, when it is acted on by an external magnetic field and the magnetization direction of the first magnetic layer4A changes, the value of the resistance between the first magnetic layer4A and the second magnetic layer4B changes. Consequently, by utilizing this change in the value of the resistance, binary data can be recorded. Examples of materials used to form the first magnetic layer4A include Co, CoFe, NiFe, NiFeCo, and CoPt.

The magnetization direction of the second magnetic layer4B is fixed by the anti-ferromagnetic layer4D. In other words, due to the exchange coupling at the joint between the anti-ferromagnetic layer4D and the second magnetic layer4B, the magnetization direction of the second magnetic layer4B is stabilized and oriented in one direction. The direction of the easy axis of magnetization of the second magnetic layer4B is selected to be along the direction of the easy axis of magnetization of the first magnetic layer4A. Ferromagnetic materials such as Co, CoFe, NiFe, NiFeCo, CoPt, and the like can be used to form the second magnetic layer4B. Furthermore, the anti-ferromagnetic layer4D can be made of IrMn, PtMn, FeMn, PtPdMn, NiO, or any combination of these materials.

The nonmagnetic insulating layer4C is made of materials that are nonmagnetic and have insulating properties, and is interposed between the first magnetic layer4A and the second magnetic layer4B, forming a structure where the tunneling magneto-resistive effect can manifest itself. More specifically, the nonmagnetic insulating layer4C has different values of electrical resistance, depending on the relative orientation (parallel or antiparallel) of the magnetization directions of the first magnetic layer4A and the second magnetic layer4B. The nonmagnetic insulating layer4C is preferably made of oxides or nitrides of metals such as Al, Zn, and Mg.

Although, in the present embodiment, the second magnetic layer4B is made of a single layer and its direction of magnetization is fixed by the anti-ferromagnetic layer4D, the present invention is not limited to such a structure. For example, although the illustration is omitted here, instead of this second magnetic layer, a layer having a synthetic structure made of three layers may be used, where the direction of magnetization of this synthetic structure is fixed by the anti-ferromagnetic layer4D. This synthetic structure may be made of one nonmagnetic layer disposed between two magnetic layers composed of a first magnetic layer side magnetic layer and an anti-ferromagnetic layer side magnetic layer. The magnetization directions of these two magnetic layers are selected to be always antiparallel. Therefore, by using the anti-ferromagnetic layer4D and fixing the magnetization direction of the anti-ferromagnetic layer side magnetic layer, the magnetization direction of the first magnetic layer side magnetic layer will be indirectly fixed to be antiparallel. Although there is no specific limitation to materials used to form the anti-ferromagnetic layer side magnetic layer of the synthetic structure, it is preferable that ferromagnetic materials such as Co, CoFe, NiFe, NiFeCo, CoPt, or the like be used alone or in combination. Furthermore, the nonmagnetic metal layer provided within the synthetic structure is preferably made of Ru, Rh, Ir, Cu, Ag, or the like. It is preferable that the thickness of the nonmagnetic metal layer be 2 nm or less because of a requirement that a strong exchange coupling be achieved with the magnetic layers that hold the nonmagnetic metal layer therebetween.

The anti-ferromagnetic layer4D of the TMR element4is electrically connected to the word line15. The first magnetic layer4A of the TMR element4is electrically connected to the bit line13. Because of this structure, a read current can flow from the bit line13to the word line15through the TMR element4, and the change in electrical resistance of the TMR element4can be detected by the sense amplifier16. The easy axis of magnetization of the first magnetic layer4A of the TMR element4is selected to be parallel to the longitudinal direction (extending direction) of the bit line13.

Returning toFIG. 2, in the TMR element4disposed at the cross-point K, the direction of magnetization fixation M of the second magnetic layer4B by the anti-ferromagnetic layer4D is parallel to the extending direction X of the bit line13that is formed in advance, before the TMR element4is formed. In other words, because of the exchange coupling at the joint between the anti-ferromagnetic layer4D and the second magnetic layer4B, the magnetization direction M of the second magnetic layer4B is oriented and stabilized along the extending direction X of the bit line13. The direction of easy axis of magnetization of the first magnetic layer4A is also selected to be in the extending direction of the bit line13. As a result, the pinning direction M of the second magnetic layer4B of the TMR element4and the pinning direction X of the bit line soft magnetic body40become parallel to each other, and the pinning direction M of the second magnetic layer4B and the pinning direction Y of the word line soft magnetic body41become perpendicular to each other.

A write cycle of this magnetic storage device1takes the following steps. First, based on the address requested, one bit line is selected from a plurality of bit lines13, and, based on the same address, one word line is selected from a plurality of word lines15. Then, which binary data (0 or 1) is to be written is determined, and a current based on this binary data is provided. As a result, circumferential magnetic fields are produced along the bit line13and the word line15, and because of these magnetic fields, magnetization directions X and Y of the bit line soft magnetic body40and the word line soft magnetic body41, respectively, are induced to make a smooth rotation and match the circumferential magnetic fields of the bit line13and the word line15, respectively. According to the cooperative effect of these magnetic fields of the bit line13and the word line15, the magnetization configuration of the first magnetic layer (not shown in the figure) of the TMR element4is selected to be in a designated direction, thereby completing the write cycle of the binary data.

In this magnetic storage device1, the magnetization configurations X and Y of the bit line soft magnetic body40and the word line soft magnetic body41, respectively, that are intended to prevent any leakage of magnetic flux are pinned in the extending directions of the bit line13and the word line15, respectively, to obtain a single domain. Therefore, the magnetization configuration when there is no current is neutral, and, moreover, the change of an induced magnetic field produced by the current is smooth so that the amount of write noise is reduced. Furthermore, with regard to the cladding structure employed for the bit line13and the word line15, if the characteristics of a plurality of TMR elements4that are disposed in the vicinity of the bit line13and the word line15(at the cross-point K) differ, the timing control during the write cycle or control of the current/voltage becomes complicated. However, in the magnetic storage device1, while retaining the cladding structure for the bit line13and the word line15, the magnetization configurations of the bit line soft magnetic body40and the word line soft magnetic body41are homogenized along the entire longitudinal extensions by the bit line anti-ferromagnetic layer26and the word line anti-ferromagnetic layer27, respectively. Therefore, any variations in characteristics are reduced, and the write precision can be improved accordingly.

In the magnetic storage device1, since an anti-ferromagnetic ordered alloy is used to form the bit line anti-ferromagnetic layer26, the pinning can be accomplished by the application of a magnetic field and the annealing. Conversely, since an anti-ferromagnetic random alloy is used to form the word line anti-ferromagnetic layer27, pinning can be accomplished by the application of a magnetic field in a non-annealing environment. Since the easy axis of magnetization of the TMR element4to be disposed after the formation of the bit line13is matched with the pinning direction of the bit line anti-ferromagnetic layer26, a change in the pinning configuration of the bit line anti-ferromagnetic layer26can be avoided when the TMR element4is subjected to the application of a magnetic field and annealing.

Furthermore, since, in the magnetic storage device1, the thickness t of the word line soft magnetic body41is selected to satisfy the equation t<J/(Hc·Ms), or preferably t<J/(Hc2·Ms), the pinning by the word line anti-ferromagnetic layer27can be securely realized. As will be described later, in the case where the word line soft magnetic body41and the word line anti-ferromagnetic layer27are disposed after the formation of the TMR element4, and, moreover, the pinning direction by the word line anti-ferromagnetic layer27is perpendicular to the pinning direction of the TMR element4, it is difficult to conduct annealing while depositing the word line anti-ferromagnetic layer27. However, if the thickness of the word line soft magnetic body41is selected as mentioned above, then, even if it is in the non-annealing condition, the pinning can be securely accomplished by applying a magnetic field.

Consequently, in the magnetic storage device1, both the bit line soft magnetic body40and the word line soft magnetic body41can be pinned in the directions of the bit line13and the word line15, respectively, while the stability of the magnetization configuration of the TMR element4is being maintained.

A method for producing the magnetic storage device1will now be described.

First, as shown inFIG. 6A, patterning to match the bit line13is conducted on a substrate70on which an insulating layer71made of material mainly consisting of SiO2is formed, and then a groove71A for the bit line13is formed in the insulating layer71using reactive ion etching (RIE). Next, as shown inFIG. 6B, base materials for the bit line anti-ferromagnetic layer26, the bit line soft magnetic body40, and the bit line13are deposited in this order over on a region which includes the groove71A. In this instance, an anti-ferromagnetic ordered alloy that has a high blocking temperature is used as the base material used to form the bit line anti-ferromagnetic layer26. Then, as shown inFIG. 6C, this layered structure is subjected to annealing within a magnetic field. The direction X of this magnetic field is taken to match the extending direction of the groove71A, namely, the extending direction of the bit line13to be created. As a result, the bit line soft magnetic body40is pinned by the bit line anti-ferromagnetic layer26. Next, as shown inFIG. 6D, part of the base materials for the bit line anti-ferromagnetic layer26, the bit line soft magnetic body40, and the bit line13that are covering regions other than the groove71A is removed using chemical-mechanical polishing (CMP) to obtain a smooth surface. Consequently, the bit line13that has a pinned cladding structure (bit line soft magnetic body40) is formed in the groove71A. In this instance, it is also preferable that the thickness t of the bit line soft magnetic body40satisfy the equation t<J/(Hc·Ms), or more preferably t<J/(Hc2·Ms).

Next, as shown inFIG. 7A, a base material for the TMR elements4is deposited. Specifically, base materials for the first magnetic layer4A, the nonmagnetic insulating layer (insulating layer)4C, the second magnetic layer (pinned layer)4B, and the anti-ferromagnetic layer4D are deposited in this order. In this instance, too, an anti-ferromagnetic ordered alloy that has a high blocking temperature is used for the anti-ferromagnetic layer4D. After the deposition of these base materials, as shown inFIG. 6B, this layered structure is subjected to annealing within a magnetic field so that the magnetization configuration of the anti-ferromagnetic layer4D becomes unidirectionally anisotropic and remains as such. The direction M of the magnetic field to be applied is selected to be parallel to the extending direction X of the bit line13, namely, the direction X of the magnetic field shown inFIG. 6C, and this step ensures that the magnetization direction M of the second magnetic layer4B will be pinned along the extending direction of the bit line13.

Then, by patterning to define regions corresponding to the TMR elements4, unwanted portions are removed, and, then, through refilling, the TMR elements4are formed. As mentioned previously, the direction of the easy axis of magnetization of this TMR element4and the pinning direction of the second magnetic layer4B concur with the extending direction of the bit line13.

Then, as shown inFIG. 8A, the word line15is formed by a liftoff process or a plating process, and, then, base materials for the word line soft magnetic layer41and the word line anti-ferromagnetic layer27are collectively deposited in this order so as to cover the word line15(seeFIG. 8B). As this is being done, the thickness t of the word line soft magnetic body41is adjusted to become t<J/(Hc·Ms), or preferably t<J/(Hc2·Ms), as already mentioned.

Furthermore, when depositing the word line soft magnetic body41and the anti-ferromagnetic layer27, a magnetic field for pinning in a non-annealing environment is applied. The direction Y of this pinning magnetic field is selected to be parallel to the extending direction of the word line15, and its strength is set to be greater than the coercive force Hc of the word line soft magnetic body41. More preferably, the strength of the pinning magnetic field is set to be greater than an applied magnetic field Hc2necessary for the word line soft magnetic body41to reach 80% of the saturation magnetization Ms.

As a result, the pinning magnetic field exceeds the coercive force Hc of the word line soft magnetic body41when the deposition of the word line soft magnetic body41is finished, and, therefore, the magnetization configuration of the word line soft magnetic body41is oriented along the pinning magnetic field and forms a single domain configuration. In particular, by selecting the strength of the pinning magnetic field to be greater than the above-mentioned applied magnetic field Hc2, the word line soft magnetic body41can be more securely oriented. Since the word line anti-ferromagnetic layer27can be deposited onto the word line soft magnetic body41that is in the oriented configuration, pinning can be securely achieved. For example, in the case where NiFe is used to form the word line soft magnetic body41, the strength of the pinning magnetic field is selected to be greater than the coercive force Hc=6 (Oe), or more preferably, greater than the above-mentioned applied magnetic field Hc2=8 (Oe).

As a result, the magnetization configuration of the word line soft magnetic body41is securely pinned in the direction Y. Then, unwanted portions of the word line soft magnetic body41and the word line anti-ferromagnetic layer27are removed, and the magnetic storage device1illustrated inFIG. 2is obtained.

According to the present production method, since the pinning direction of magnetization of the bit line soft magnetic body40and that of the TMR element4are made to concur each other, the annealing of the TMR element4acts to further strengthen the pinning action of the bit line soft magnetic body40. In other words, although the bit line anti-ferromagnetic layer26is annealed when the TMR element4is annealed, since the pinning directions are the same, it is possible to maintain the pinning configuration of the bit line anti-ferromagnetic layer26. Furthermore, since the thickness t of the word line soft magnetic body41as well as the strength of the pinning magnetic field are chosen to be within a prescribed range, the word line soft magnetic body41can be pinned simply by applying a magnetic field in a non-annealing environment, thereby avoiding adverse effects upon the magnetization configuration of the anti-ferromagnetic layer4D having a high blocking temperature or the bit line anti-ferromagnetic layer26. Therefore, the magnetic storage device1in which a stable magnetization configuration is achieved in all of the bit line soft magnetic body40, the word line soft magnetic body41, and the TMR element4can be produced.

A magnetic storage device101according to the second embodiment of the present invention will now be described with reference toFIG. 9. In the following description, the same or similar components as those appeared in the magnetic storage device1of the first embodiment will be designated with reference numerals whose last two digits are the same as in the first embodiment, and their descriptions will be omitted. Different parts will be mainly described.

In the magnetic storage device101, the bit line113and the word line115are provided with the bit line soft magnetic body140and the word line soft magnetic body141, respectively, each having a U-shaped cross-section and surrounding the bit line113or the word line115, and only the non-element sides of the bit line113and the word line115are covered with the bit line anti-ferromagnetic layer126and the word line anti-ferromagnetic layer127, respectively. The thickness t of the word line soft magnetic body141satisfies the equation t<J/(Mc·Ms) as in the first embodiment. According to this magnetic storage device101, it is possible that magnetization configurations of the bit line soft magnetic body140and the word line soft magnetic body141are securely pinned in the longitudinal directions of the bit line113and the word line115, respectively. As a result, in the present magnetic storage device101, too, the change in a write cycle magnetic field created by the bit line113and the word line115becomes smoothened, and variations of the magnetic field during the write cycle can be reduced.

Although, in this magnetic storage device101, both the bit line soft magnetic body140and the word line soft magnetic body141have a U-shaped cross-section, the present invention is not limited to such a structure. For example, as illustrated inFIG. 10, only the non-element sides of the bit line113and the word line115may be covered with the bit line soft magnetic body140and the word line soft magnetic body141, respectively, such that their cross-sections do not become U-shaped. Such an arrangement can also prevent any leakage of magnetic flux from the lines113and115.

As mentioned so far, in the present embodiment, the bit line113and the word line115have cladding structure along the entire extension in the longitudinal direction. However, the present invention is not limited to such a structure, and the cladding structure may be provided partially in the longitudinal direction thereof. Although the details will be described later, from the standpoint of improving the strength of the magnetic field to the TMR element, partial cladding over portions including at least the cross-point K of the bit line and the word line, which may be referred to as the yoke structure, may be available.

Furthermore, although the above embodiment illustrates an example where the TMR element makes direct contact with the bit line and the word line, the present invention is not limited to such an arrangement. For example, the TMR element may be disposed with some gaps to the bit line and the word line, and electrodes that make contact with the TMR element (read-only electrode) may be separately provided in those gaps.

A magnetic storage device201according to the third embodiment of the present invention will now be described with reference toFIG. 11and the like.FIG. 11is a conceptual illustration of the general view of a magnetic storage device201. This magnetic storage device201includes a memory unit202, a bit selection circuit211, a word selection circuit212, bit lines213and214, and word lines215and216. In the memory unit202, a plurality of memory cells203are disposed in a two-dimensional m×n formation (an array formation), where m and n are integers equal to or greater than 2. As illustrated under magnification inFIG. 12, each memory cell203includes a TMR element204, a write line205A, a read line205B, a write transistor206A, a read transistor206B, and a soft magnetic yoke220constituting the soft magnetic body. Since the write line205A and the read line205B are disposed so as to be drawn in from the bit line213, the write line205A, the read line205B, the soft magnetic yoke220, and the anti-ferromagnetic layer pinning this soft magnetic yoke220(details are to be described later) are independently disposed for each memory cell203.

In this magnetic storage device201, by separately disposing the write line205A and the read line205B, noise factors such as sneak current can be reduced.

Both ends of the write line205A are connected to the bit lines213and214, respectively, and the write transistor206A is disposed between the two ends. Therefore, by applying a voltage between the bit lines213and214and turning on the write transistor206A, a current can flow through the write line205A, thereby creating a magnetic field around the TMR element204disposed nearby. Moreover, both ends of the read line205B are also connected to the bit lines213and214, respectively, and the read transistor206B and the TMR element204are disposed between the two ends. Therefore, by applying a voltage between the bit lines213and214and turning on the read transistor206B, a current can flow through the read line205B, thereby detecting the change in a value of resistance of the TMR element204. Here, the write transistor206A is connected to the word line215, and the read transistor206B is connected to the word line216. Therefore, by utilizing voltages applied to these word lines215and216, the continuity of each of the transistors206A and206B is individually switched. As a result, a current can be made to flow from the bit lines213and214to the word line215if necessary. The write line205A disposed adjacent to the TMR element204extends in the direction of the arrayed surface (plane) of the memory cell203and is L-bent in this plane.

As illustrated under magnification inFIG. 13, the TMR element204includes a first magnetic layer (free layer/magnetic sensing layer)204A whose direction of magnetization changes according to an external magnetic field, a second magnetic layer (pinned layer)204B whose direction of magnetization is fixed, a nonmagnetic insulating layer (insulating layer)204C interposed between the first magnetic layer204A and the second magnetic layer204B, and an anti-ferromagnetic layer204D that fixes (or pins) the direction of magnetization of the second magnetic layer204B. Moreover, the anti-ferromagnetic layer204D of the TMR element204is electrically connected to one read line205B. In addition to this, the first magnetic layer204A of the TMR element204is electrically connected to the other read line205B. Because of this structure, by allowing a read current to flow through the read line205B, the change in a value of resistance of the TMR element204can be detected. In this instance, the easy axis of magnetization of the first magnetic layer204A of the TMR element204is selected to be in the direction that perpendicularly intersects the longitudinal direction of the write line205A, or, in other words, in the direction that perpendicularly intersects the direction of the write current.

As shown inFIG. 14, the soft magnetic yoke220includes an element side yoke220A disposed closely on the TMR element204side of the extending write line205A, and a non-element side yoke220B disposed closely on the side opposite to the TMR element204of the write line205A. Furthermore, there is provided a pair of yoke connecting sections220C and220C at both ends of the element side yoke220A as well as at both ends of the non-element side yoke220B so as to connect the two in an annular configuration. Therefore, when viewed from the TMR element204, the element side yoke220A is close to the TMR element204, and the non-element side yoke220B is far away from the TMR element204. Moreover, this soft magnetic yoke220partially covers the circumference of the write line205A, making almost annular structure. The non-element side yoke220B is made of a top region220T positioned over the write line205A, and inclined regions220S positioned on both sides of the top region220T, or equivalently, closely to the yoke connecting sections220C.

The element-side yoke220A has an opening220E at the middle of the annular configuration, where the TMR element204is disposed. Therefore, when the soft magnetic yoke220is viewed in the direction of into the figure, there are open ends220Ea and220Eb along its circumference, having an almost C-shaped configuration. These open ends220Ea and220Eb form tip ends of the element-side yokes220A and are disposed close to the lateral sides of the TMR element204.

Furthermore, a yoke anti-ferromagnetic layer226is formed on the outer circumference side of the soft magnetic yoke220. Therefore, the direction of magnetization of the soft magnetic yoke220is stabilized by the exchange coupling at the joint with the yoke anti-ferromagnetic layer226. The pinning direction of the magnetization configuration of the soft magnetic yoke220is selected to be almost equal to the extending direction of the write line205A, or equivalently the direction perpendicular to a magnetic field induced by the write line205A.

The thickness TZ of the top region220T of the non-element side yoke220B will now be described. If the exchange coupling energy at the boundary surface between the non-element side yoke220B and the yoke anti-ferromagnetic layer226is J (erg/cm2), the saturation magnetization of the non-element side yoke220B is Ms (emu/cc), and the coercive force of the non-element side yoke220B is Hc (Oe), then the thickness TZ (cm) of the non-element side yoke220B is selected such that TZ<J/(Hc·Ms). In particular, in the case of the present embodiment, if an externally applied magnetic field necessary for the non-element side yoke220B to reach 80% of the saturation magnetization Ms is Hc2(Oe), then the thickness TZ (cm) of the non-element side yoke220B is chosen such that TZ<J/(Hc2·Ms).

For example, in the case where the non-element side yoke220B is made of NiFe and the yoke anti-ferromagnetic layer226is made of IrMn, the saturation magnetization Ms of the non-element side yoke220B of NiFe is 780 (emu/cc), the coercive force Hc thereof is 6 (Oe), and the exchange coupling energy J of NiFe/IrMn at the boundary between the non-element side yoke220B and the yoke anti-ferromagnetic layer226is 0.061 (erg/cm2). Therefore, by putting these values into the equation TZ<J/(Hc·Ms), the thickness t of the non-element side yoke220B becomes less than 130 nm.

Furthermore, it is preferable that magnetic force Hua (Oe) of a pinning magnetic field by the yoke anti-ferromagnetic layer226be greater than the externally applied magnetic field Hc2(Oe) at the non-element side yoke220B. The reason for this is that, if the magnetic force of the pinning magnetic field Hua exceeds the externally applied magnetic field that is capable of inducing up to approximately 80% of the saturation magnetization Ms of the non-element side yoke220B, then the magnetization configuration of the non-element side yoke220B can be made to be a single domain more securely along the direction of the pinning magnetic field.

For example, an externally applied magnetic field necessary for the non-element side yoke220B made of NiFe to reach 80% of the saturation magnetization Ms is 8 (Oe). Hence, by putting this value into the equation TZ<J/(Hc2·Ms), it turns out that the thickness t of the non-element side yoke220B needs to be selected to be less than 93 nm.

Furthermore, the thickness TZ of the non-element side yoke220B is selected to be greater than the thickness BZ of the element side yoke220A. For example, the thickness TZ is selected to be 50 nm or more. Furthermore, the thickness BZ of the element side yoke220A that is thinner than the non-element side yoke220B is selected to be in the range from 10 nm to 30 nm. Moreover, the thickness SZ of the inclined region220S is selected to be in the range from 20 nm to 100 nm on the average. The maximum height of the non-element side yoke220B relative to the level of the TMR element204is, for example, selected to be 300 nm or less.

When forming the non-element side yoke220B of the soft magnetic yoke220, a pinning magnetic field is applied in a non-annealing environment. The direction Y of this pinning magnetic field is selected to be parallel to the extending direction of the write line205A, and the strength of the pinning magnetic field is selected to be greater than the coercive force Hc of the non-element side yoke220B. More preferably, the strength of the pinning magnetic field is selected to be greater than the applied magnetic field Hc2necessary for the non-element side yoke220B to reach 80% of the saturation magnetization Ms. Furthermore, in the non-annealing environment with the pinning magnetic field being applied, the non-element side yoke220B (the inclined region220S and the top region220T) and the yoke anti-ferromagnetic layer226are successively deposited in a series of processes. Consequently, the yoke anti-ferromagnetic layer226can be deposited while the magnetization configuration of the non-element side yoke220B is held oriented along the direction Y of the pinning magnetic field. Note that it is preferable that ferromagnetic materials constituting the soft magnetic yoke220be, for example, alloys containing at least one of Ni, Fe, and Co. In the present embodiment, NiFe is used.

Next, with reference toFIGS. 15,16, and17, a write cycle on the TMR element204of the magnetic storage device201of the present embodiment will be described.

As shown inFIG. 15, when there is no current flowing through the write line205A, no magnetic field is created by this write line205A. Therefore, the magnetization configuration X of the soft magnetic yoke220is under the influence of the pinning effect from the yoke anti-ferromagnetic layer226, and its direction is almost in agreement with the extending direction of the write line205A. Therefore, the soft magnetic yoke220is in the state of single domain with the magnetization direction of the entire portion being unified into a single direction. The magnetization direction B of the second magnetic layer204B and the magnetization direction A of the first magnetic layer204A within the TMR element204are the same. In this instance, it is defined that the binary data of 0 has been recorded when the magnetic directions A and B are the same.

As shown inFIG. 16, if a write current flows through the write line205A, a circumferential magnetic field F1is created around the write line205A. The magnetic field F1goes around and makes a closed loop within the soft magnetic yoke220that is provided around the write line205A. Then, the magnetization configuration X of the soft magnetic yoke220changes so that its magnetization direction is smoothly driven to turn 90° and approaches to the direction of the magnetic field F1so as to be induced by this magnetic field F1while resisting the influence of the pinning effect (see the dotted arrows in the figure) of the yoke anti-ferromagnetic layer226.

As a result, the magnetic field F1created by the write line205A and the magnetization configuration X produced in the soft magnetic yoke220combine to yield a strong magnetic field, which acts on the first magnetic layer204A in the TMR element204, inverting its magnetization direction A. Then, if the current I1in the write line205A is turned off while holding other conditions unchanged, the magnetization configuration X of the soft magnetic yoke220smoothly returns to its original state as shown inFIG. 15because of the pinning effect of the yoke anti-ferromagnetic layer226. In this case, however, the magnetization direction A of the TMR element204is maintained as being inverted as inFIG. 16. Since the magnetization directions A and B are maintained as being in the opposite direction, the binary data of 1 has been recorded here.

Next, as shown inFIG. 17, when a write current I2that is in the opposite direction to I1flows through the write line205A, a circumferential magnetic field F2is created around the write line205A. The magnetic field F2forms a closed loop that goes around within the soft magnetic yoke220that is provided around the write line205A. The magnetization configuration X of the soft magnetic yoke220changes so that its magnetization direction is smoothly driven to turn 90° and approaches to the direction of the magnetic field F2so as to be induced by this magnetic field F2while resisting the influence of the pinning effect (see the dotted arrows in the figure) of the yoke anti-ferromagnetic layer226.

As a result, the magnetic field F2produced by the write line205A and the magnetization configuration X produced in the soft magnetic yoke220combine to yield a strong magnetic field, which acts on the first magnetic layer204A in the TMR element204, inverting and making its magnetization direction A to be the same once again as the magnetization direction B of the second magnetic layer204B. Then if the current I2in the write line205A is turned off while holding other conditions unchanged, then the magnetization configuration X of the soft magnetic yoke220returns to its original state as shown inFIG. 15because of the pinning effect of the yoke anti-ferromagnetic layer226. Here, since, in the TMR element204, the magnetization directions A and B are the same, the binary data of 0 has been recorded again.

When reading the binary data stored in the TMR element204, a read current is allowed to flow through the read line205B, and a change in the value of the current or in the potential difference across the read line205B is detected. This reveals the value of the resistance, and which binary data is stored, or namely, whether the magnetization directions A and B of the first magnetic layer204A and the second magnetic layer204B, respectively, are parallel or antiparallel, can be determined. For example, when the magnetization direction A of the first magnetic layer204A is the same as the magnetization direction B of the second magnetic layer204B, the value of the resistance between the first magnetic layer204A and the second magnetic layer204B becomes relatively small due to the tunneling magneto-resistance (TMR) in the nonmagnetic insulating layer204C. On the other hand, when the magnetization directions A and B are in the opposite direction, the value of the resistance between the first magnetic layer204A and the second magnetic layer204B becomes relatively large due to the tunneling magneto-resistance.

According to the magnetic storage device201described above, since the thickness TZ of the soft magnetic yoke220is selected to be in a prescribed range, the magnetization configuration X of the soft magnetic yoke220is always of a single domain, and a smooth change of a magnetic field can be realized, compared to a case where a plurality of domains are formed spontaneously and irregularly. Therefore, Barkhausen noise during the change of magnetization configuration X can be reduced. Furthermore, in the case where a plurality of memory cells203are disposed in an arrayed formation as shown inFIG. 11, magnetic characteristics obtained by the write line205A and the soft magnetic yoke220can be homogenized, thereby facilitating the write cycle control.

Furthermore, in the case of the independent structure where the write line205A is drawn from the bit line213for each memory cell203, and the soft magnetic yoke220is formed for each write line205A, magnetization characteristics among a plurality of soft magnetic yokes220generally differ, and variations of magnetization characteristics of the memory cells203result easily. However, if the soft magnetic yoke220is made into a single domain as in the magnetic storage device201, the variations of magnetization characteristics can be reduced, and the writing speed can be homogenized among a plurality of memory cells203.

Furthermore, when the soft magnetic yoke220is provided to the write line205A, since the annular direction of the soft magnetic yoke220means the longitudinal direction, a plurality of domains will be formed mainly along the longitudinal direction, and annularly unbalanced magnetization configuration results unless pinning is carried out. In other words, during the write cycle of binary data, situations where a writing speed in one memory cell is different in another memory cell or where a value of voltage/current required in one memory cell is different in another memory cell may arise. Consequently, according to the third embodiment of the present invention, the magnetization configuration X of the soft magnetic yoke220is forced to orient in the direction of the write line205A. Therefore, the neutral magnetization configuration X is always maintained, and a write cycle does not depend upon the direction of the current through the write line205A. In addition to this, during the write cycle for either 0 or 1, values of current/voltage and magnetic field generation speeds (rate of rise/writing speed) can be homogenized.

Furthermore, according to this magnetic storage device201, since the soft magnetic yoke220is formed first and, then, the yoke anti-ferromagnetic layer226is deposited thereon during its production, the soft magnetic yoke220can be pinned easily. Moreover, when the magnetic storage device201has a complicated structure where the write line205A bends in front and behind of the soft magnetic yoke220, a magnetic field that can cause noise may be easily created from that line. Although the soft magnetic yoke220can be easily affected by these noise fields, by pinning the soft magnetic yoke220by the yoke anti-ferromagnetic layer226, a bias in the neutral direction can be provided to its magnetization configuration X, thereby reducing the influence of noises.

Since, in this magnetic storage device201, the write lines205A and the read lines205B are mutually independent, a current can be selected to flow only through the write line205A during the write cycle. Similarly, a current can also be selected to flow only through the read line205B during the read cycle. As a result, it is possible that a sneak current or the like be avoided without arranging any diodes or the like, and both write and read cycles can further be stabilized.

Furthermore, in the magnetic storage device201, the element-side yoke220A is severed and separated in the middle, and the TMR element204is disposed there with a certain gap being provided between the severed end of the element-side yoke220A and the end face of the TMR element204. As a result, it is possible that a magnetic field due to the soft magnetic yoke220be applied to the end face of the TMR element204, thereby improving the responsiveness during the write cycle. By the way, “the severed and separated element-side yoke” in the present embodiment refers to the shape of the finished soft magnetic yoke220and is not limited to the case where an unsevered element-side yoke220A is formed first and, then, severed and separated. For example, a pair of element-side yoke220A may be provided separately such that they appear to be severed and separated with the TMR element204being provided therebetween.

Next, a magnetic storage device301according to the fourth embodiment of the present invention will be described with reference toFIG. 18. In the following description, the same or similar components as those appeared in the magnetic storage device201of the third embodiment will be designated with reference numerals whose last two digits are the same as in the third embodiment and their descriptions will be omitted.

In this magnetic storage device301, the non-element side yoke320B made of a soft magnetic body has double layer structure including a first soft magnetic layer320Ba and a second soft magnetic layer320Bb. In addition to this, materials for the first soft magnetic layer320Ba and the second soft magnetic layer320Bb are different. Therefore, the yoke anti-ferromagnetic layer326is in contact with the second soft magnetic layer320Bb. The magnetic storage device301is further provided with an oxidation prevention cap layer350on the outer circumference side of the yoke anti-ferromagnetic layer326. This oxidation prevention cap layer350is preferably made of Ta or the like, which can prevent the oxidation of the yoke anti-ferromagnetic layer326.

In this magnetic storage device301, consider the case in which the thickness of the first soft magnetic layer329Ba is T1, the exchange coupling energy between the first soft magnetic layer320Ba and the yoke anti-ferromagnetic layer326is J1, the saturation magnetization of the first soft magnetic layer320Ba is Ms1, the thickness of the second soft magnetic layer320Bb is T2, the exchange coupling energy between the second soft magnetic layer320Bb and the yoke anti-ferromagnetic layer326is J2, and the saturation magnetization of the second soft magnetic layer320Bb is Ms2. Then, a pinning magnetic field Hua obtained by the yoke anti-ferromagnetic layer326can be expressed as Hua=J1/(Ms1·T1)+J2/(Ms2·T2). For example, when the first soft magnetic layer320Ba is made of NiFe, the saturation magnetization Ms1of NiFe is 780 (emu/cc) and the exchange coupling energy J1of NiFe/IrMn is 0.061 (erg/cm2). Furthermore, when the second soft magnetic layer320Bb is made of CoFe, the saturation magnetization Ms2of CoFe is 1500 (emu/cc), and the exchange coupling energy J2of CoFe/IrMn is 0.192 (erg/cm2). Since CoFe has the exchange coupling energy J2that is approximately three times that of NiFe, it acts in the direction of strengthening the pinning magnetic field Hua. Therefore, by providing the second soft magnetic layer320Bb made of CoFe as a thin film of, for example, approximately 2 nm, on the outer circumference side of the first soft magnetic layer320Ba made of NiFe, the pinning magnetic field Hua can be strengthened. The strengthened pinning magnetic field Hua allows in turn the thickness of the non-element side yoke320(T1+T2) to be large, thereby enhancing the shielding effect and reducing a leak of a magnetic field of the write line305A.

A plurality of memory cells203of the magnetic storage device201with the yoke structure according to the third embodiment of the present invention were prepared, and the write current needed at each memory cell was statistically analyzed. In this magnetic storage device201, the soft magnetic yoke220was made of NiFe, the yoke anti-ferromagnetic layer226was made of IrMn, and the thickness TZ of the non-element side yoke220B was selected to be 100 nm, so that TZ<J/(Hc·Ms)=130 nm was satisfied. As a comparative example, another magnetic storage device201with no yoke ferromagnetic layer226was prepared, and write currents were analyzed in a similar manner. The results are shown inFIG. 19. The average value of the write current calculated from bars designated by B in the graph, which serves as a comparative example, was 1.13 (mA). However, the average value of the write current calculated from bars designated by A in the graph was 0.90 (mA), and it turned out that the write current could be reduced. In other words, by pinning the soft magnetic yoke220, the magnetization characteristics of the memory cells203are homogenized, and the average current can be reduced.

Next, a plurality of memory cells203of the magnetic storage device201according to the third embodiment of the present invention were prepared, and the write current needed at each memory cell was statistically analyzed. In Example 2, a plurality of magnetic storage devices201were prepared, where those having the thickness TZ of the non-element side yoke220B of 100 nm were designated as sample A, and those having TZ of 50 nm as sample B. Since the soft magnetic yoke220was made of NiFe, and the yoke anti-ferromagnetic layer226was made of IrMn, sample A of the magnetic storage device201in which the thickness TZ of the non-element side yoke220B was selected to be 100 nm satisfied the equation TZ<J/(Hc·Ms)=130 nm. On the other hand, sample B of the magnetic storage device201in which the thickness TZ of the non-element side yoke220B was selected to be 50 nm also satisfied TZ<J/(Hc2·Ms)=93 nm. The results are shown inFIG. 20. With sample A, the mean value of the write current was 0.98 (mA), and the variations (variance) were 0.54 (mA). However, with sample B, the mean value of the write current was 0.54 (mA), and the variations (variance) were 0.27 (mA). Consequently, it was revealed that sample B was more capable of reducing the write current further and considerably reducing the variation of the write currents than sample A. In other words, it was revealed that, by selecting the magnetic force of the pinning magnetic field due to the yoke anti-ferromagnetic layer225to be greater than the externally applied magnetic field Hc2necessary for the non-element side yoke220B to reach 80% of the saturation magnetization Ms, the soft magnetic yoke220could be pinned more securely, and the magnetization characteristics of each memory cell203could be homogenized.

The magnetic storage device according to the present invention is not limited to those described in the above embodiments, but numerous variations thereof may be possible. For example, instead of using a TMR element as a tunneling magneto-resistive device as in the above-mentioned embodiments, a GMR element that utilizes giant magneto-resistance (GMR) may be used. GMR is a phenomenon that, depending on an angle formed with magnetization directions of two ferromagnetic layers with a nonmagnetic layer interposed therebetween, a value of resistance of the ferromagnetic layers in the direction perpendicular to that of lamination of layers changes. In other words, in a GMR element, the value of resistance of the ferromagnetic layers becomes minimum when the magnetization directions of the two ferromagnetic layers are parallel and maximum when the magnetization directions of the two ferromagnetic layers are antiparallel. By the way, TMR elements or GMR elements come in either a pseudo spin valve type in which a write/read cycle utilizes a difference in coercive force of the two ferromagnetic layers or a spin valve type in which the magnetization direction of one of the ferromagnetic layers is fixed by exchange coupling with the anti-ferromagnetic layer. Furthermore, data readout in a GMR element is accomplished by detecting the change in the value of resistance of the ferromagnetic layers in the direction perpendicular to that of the lamination of the layers. Furthermore, data writing in a GMR element is accomplished by inverting the magnetization direction of one of the ferromagnetic layers by a magnetic field created by a write current.

A magnetic storage device of the present invention is not limited to those described in the above-mention embodiments and may be practiced or embodied in still other ways without departing from the spirit thereof.

The present invention can be widely applied in a field where a variety of information is recorded and maintained in a tunneling magneto-resistive element.

The entire disclosure of Japanese Patent Application No. 2006-158142 filed on Jun. 7, 2006 including specification, claims, drawings, and summary are incorporated herein by reference in its entirety.