Abstract:
A spin-injection magnetoresistance effect element that can avoid use of a large writing current and allows use of a large reading current. The spin-injection magnetoresistance effect element includes layers that may exhibit a tunnel magnetoresistance effect and layers that may exhibit a giant magnetoresistance effect.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2005-087950 filed in the Japanese Patent Office on Mar. 25, 2005, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a spin-injection magnetoresistance effect element. 
     Magnetic random access memory (MRAM) has been known as a non-volatile magnetic memory element of high speed, high integration, and low power consumption (refer to U.S. Pat. No. 6,081,445, for example). The MRAM employing a tunnel magnetoresistance (TMR) effect includes tunnel magnetoresistive elements that are coupled to selection transistors formed of MOS-FETs. In addition, bit lines are provided above the tunnel magnetoresistive elements, and word lines extending perpendicularly to the bit lines are provided below the tunnel magnetoresistive elements. The tunnel magnetoresistive element has a multi-layer structure including from its bottom an antiferromagnetic layer, a magnetization fixed layer (referred to also as a pinned layer), a tunnel insulating film, and a recording layer (referred to also as a free layer) of which magnetization direction is readily rotated. In writing of information (data) to this MRAM, a current of the positive or negative direction is applied to the bit line while a current of a constant direction is applied to the writing word line. The synthetic magnetic field arising from these currents changes the magnetization direction of the recording layer, to thereby record a logical value of “1” or “0” in the recording layer. 
     In this MRAM employing the TMR effect, magnetization reversal is caused by a current magnetic field. However, the current amount required for this magnetization reversal increases in almost inverse proportion to the size of the tunnel magnetoresistive element. Therefore, construction of a high-capacity non-volatile magnetic memory element array involves a problem of large current consumption. 
     Spin-injection magnetization reversal is a novel magnetization reversal (magnetic information writing) method that does not depend on a current magnetic field and offers reduced power consumption (refer to U.S. Pat. No. 6,714,444, for example). In the spin-injection magnetization reversal, the current amount required for magnetization reversal decreases as the size of a magnetic element such as a non-volatile magnetic memory element becomes smaller. Therefore, the spin-injection magnetization reversal is suitable for achieving a high-capacity (e.g. giga-bit class) non-volatile magnetic memory element array. 
       FIG. 11A  is a conceptual diagram of a conventional non-volatile magnetic memory element that employs spin-injection magnetization reversal. The non-volatile magnetic memory element includes a magnetoresistance effect multi-layer film having a giant magnetoresistance (GMR) effect or a TMR effect. This multi-layer film is interposed between two electrodes  401  and  405 . Specifically, this multi-layer film is formed of a magnetization reversal layer (referred to also as a free layer)  404  having a function of recording information, a magnetization reference layer (referred to also as a pinned layer)  402  having a fixed magnetization direction and thus serving as a spin filter, and a nonmagnetic film  403  interposed between the layers  404  and  402 . A current flows perpendicularly to the planes of these films (see  FIG. 11A ).  FIG. 11B  schematically shows the planar shape of the magnetization reversal layer  404 . The size of the magnetization reversal layer  404  is typically about 200 nm or smaller in order to achieve single-domain magnetization and reduce the critical current I c  of spin-injection magnetization reversal, although depending on the magnetic material and thickness of the layer  404 . The magnetization reversal layer  404  can be magnetized in two or more directions (two lateral directions indicated by the arrowheads in  FIG. 11A , for example) due to its adequate magnetic anisotropy. Each magnetization direction corresponds to recorded information. In the example of  FIG. 11B , the magnetization reversal layer  404  is designed to have an elliptic planar shape so as to be provided with a shape magnetic anisotropy. The magnetization direction of the magnetization reference layer  402  is pinned typically by exchange coupling between the layer  402  and an antiferromagnetic layer  406  (see  FIG. 11C ). Additionally, a double spin filter structure is also known (see  FIG. 11D ). In this structure, in order to enhance the efficiency of spin-injection magnetization reversal, magnetization reference layers  402 A and  402 B are provided above and below the magnetization reversal layer  404  with the intermediary of nonmagnetic films  403 A and  403 B therebetween, respectively. This structure also includes antiferromagnetic layers  406 A and  406 B. In the examples of  FIGS. 11A ,  11 C and  11 D, sometimes the magnetization reversal layer  404  and the magnetization reference layer  402  (one of two magnetization reference layers  402 A and  402 B) are formed of a synthetic ferri-magnetic structure. The nonmagnetic films  403 ,  403 A and  403 B are composed of a metal material or an insulating material. In the examples of  FIGS. 11A and 11C , sometimes another structure is also employed in which the magnetization reference layer  402  is sufficiently larger than the magnetization reversal layer  404  in order to suppress a leakage magnetic field from the layer  402  to the layer  404 , in other words, in order to prevent the layers  402  and  404  from being magnetostatically coupled. In any structure, the conventional non-volatile magnetic memory element based on spin-injection magnetization reversal has a two-terminal spin transfer element structure in which a magnetoresistance effect multi-layer film is interposed between two electrodes. 
     In a high-capacity non-volatile magnetic memory element array of the giga-bit class, the widths of gate electrodes of CMOS-FETs for memory cell selection and various interconnects are 100 nm or smaller. The amount of current that can be applied to these gate electrodes and various interconnects is limited to about 100 μA or smaller. Therefore, when information is written to a magnetic element by spin-injection magnetization reversal, the writing current needs to be suppressed to about several tens of microamperes. Currently, however, the critical current (writing current) I c  of spin-injection magnetization reversal is on the order of several hundred microamperes to several milliamperes. This large critical current is a bottleneck in practical application of spin-injection magnetization reversal to a non-volatile magnetic memory element array. 
     The critical current (writing current) I c  of spin-injection magnetization reversal (sometimes referred to simply as the critical current I c , hereinafter) depends on the time period of writing information (i.e., current pulse width). Typically, the longer the information writing time period is, the smaller the critical current I c  may be. In practical MRAM, it is required that the writing time period be on the order of nanoseconds. Therefore, the following description is based on an assumption that the writing time period is one nanosecond, and the critical current I c  discussed in the following description is premised on this writing time period. 
     If a magnetic moment fluctuates due to heat, magnetization reversal is possibly caused stochastically. Therefore, the thermal fluctuation resistance directly affects the reliability of MRAM. The thermal fluctuation resistance is expressed as the ratio of the magnetic anisotropy energy of a magnetization reversal layer to the thermal energy thereof, (K u ·V/k B ·T). K u  denotes the magnetic anisotropy energy per unit volume of a magnetization reversal layer. V denotes the volume of the magnetization reversal layer. k B  denotes the Boltzmann constant. T denotes the absolute temperature of the magnetization reversal layer. Typically, the thermal fluctuation resistance needs to satisfy the relationship (K u ·V/k B ·T)&gt;40. The probability of thermal magnetization reversal is expressed by the Boltzmann distribution exp(−K u ·V/k B ·T). The time period τ of the occurrence of thermal magnetization reversal is expressed by the equation ln(τ/τ 0 )=K u ·V/k B ·T. τ 0  denotes the reversal trial time period (time period of one trial of magnetization reversal), and is about one nanosecond. 
     The basic configuration of a magnetic element (the structure of a multi-layer film) is based on a magnetoresistance effect film. The magnetoresistance effect film is formed of two magnetic films (a magnetization reversal layer and a magnetization reference layer) that are deposited with the intermediary of a nonmagnetic film therebetween. The magnetoresistance effect film is a multi-layer structure having a GMR effect when the nonmagnetic film is an electrically conductive film, while it is a multi-layer structure having a TMR effect when the nonmagnetic film is an insulating film. The magnetization reversal layer has a low resistance when the magnetization directions of two magnetic layers, in other words, the magnetization reversal layer and the magnetization reference layer, are parallel. In contrast, the magnetization reversal layer has a high resistance when the magnetization directions are antiparallel. This phenomenon (magnetoresistance effect) is used to record magnetic information of “0” or “1” in the magnetic element. Subsequently, an adequate current is passed through the magnetization reversal layer to thereby detect the resistance of the magnetization reversal layer, which allows nondestructive retrieval of the magnetic information recorded in the magnetization reversal layer. In a memory element, it is desirable to use a multilayer structure having a TMR effect, which offers larger outputs. 
     SUMMARY OF THE INVENTION 
     Conventional techniques have employed various features for reducing the critical current I c . However, these features cause other problems: lowering of the thermal fluctuation resistance; an increase of the reversal time period; and a complicated multi-layer structure of a magnetic element. Furthermore, when a magnetic element employs spin-injection magnetization reversal for writing information, a small current difference between the writing current and reading current leads to a possibility that the magnetization direction of the magnetization reversal layer is erroneously reversed at the time of reading information. Therefore, in order to prevent this erroneous writing when magnetic information is read out in a magnetic element, it is required that the reading current be suppressed to few tenths of the writing current (to about 10 μA). The suppression of the reading current however results in a small difference between output voltages from the magnetic element, corresponding to magnetic information of “0” and “1,” respectively (the difference is equivalent to the product between the resistance change of the magnetization reversal layer and the reading current). As a result, it is difficult to obtain a high S/N ratio. 
     An object of the present invention therefore is to provide a spin-injection magnetoresistance effect element that can eliminate a conventional problem that the critical current I c  of spin-injection magnetization reversal has a high current value on the order of several hundred microamperes to several milliamperes, and allows use of a large reading current. 
     In order to achieve the above-described object, a spin-injection magnetoresistance effect element according to one embodiment of the present invention includes (A) a magnetization reversal layer that has a first face and a second face opposite to the first face, and stores information, (B) a first magnetization reference layer (referred to also as a pinned layer) that is disposed near one end of the magnetization reversal layer in a manner of facing the first face of the magnetization reversal layer with intermediary of a first nonmagnetic film between the first magnetization reference layer and the magnetization reversal layer, and is magnetized in a first direction, (C) a second magnetization reference layer that is disposed near the other end of the magnetization reversal layer in a manner of facing the first face of the magnetization reversal layer and being separated from the first magnetization reference layer with intermediary of a second nonmagnetic film between the second magnetization reference layer and the magnetization reversal layer, and is magnetized in a second direction that is the opposite direction of the first direction, (D) a first electrode that is electrically coupled to the first magnetization reference layer, (E) a second electrode that is electrically coupled to the second magnetization reference layer, and (F) a third electrode that is disposed in a manner of facing the second face of the magnetization reversal layer with intermediary of an insulating film between the third electrode and the magnetization reversal layer. That is, a spin-injection magnetoresistance effect element according to one embodiment of the invention has a three-terminal spin transfer element structure. 
     In the spin-injection magnetoresistance effect element, the first magnetization reference layer, the first nonmagnetic film and the magnetization reversal layer may form a multi-layer structure having a tunnel magnetoresistance (TMR) effect. In addition, the second magnetization reference layer, the second nonmagnetic film and the magnetization reversal layer may form, although not limited to, a multi-layer structure having a giant magnetoresistance (GMR) effect. 
     Furthermore, in the spin-injection magnetoresistance effect element, a first antiferromagnetic layer may be formed between the first magnetization reference layer and the first electrode, and a second antiferromagnetic layer may be formed between the second magnetization reference layer and the second electrode. Alternatively, either one of the first and second magnetization reference layers may have a synthetic ferri-magnetic structure. Further alternatively, either one of the first and second magnetization reference layers may have a magnetostatically coupled structure. Yet further alternatively, the first magnetization reference layer may have a synthetic ferri-magnetic structure, and the second magnetization reference layer have a synthetic ferro-magnetic structure. 
     The multi-layer structure that is formed of the first magnetization reference layer, the first nonmagnetic film and the magnetization reversal layer, and has a TMR effect is equivalent to a structure in which the first nonmagnetic film serving as a tunnel insulating film is interposed between the first magnetization reference layer composed of a magnetic material and the magnetization reversal layer composed of a magnetic material. The multi-layer structure that is formed of the second magnetization reference layer, the second nonmagnetic film and the magnetization reversal layer, and has a GMR effect is equivalent to a structure in which the second nonmagnetic film composed of a conductive material is interposed between the second magnetization reference layer composed of a magnetic material and the magnetization reversal layer composed of a magnetic material. 
     The term synthetic ferri-magnetic structure indicates a structure that has a three-layer structure of a magnetic material layer, a ruthenium (Ru) layer, and a magnetic material layer, and in which interlayer exchange coupling between two magnetic material layers is antiferromagnetic coupling, for example. A structure that has a similar multi-layer structure, and in which interlayer exchange coupling between two magnetic material layers is ferromagnetic coupling is referred to as the synthetic ferro-magnetic structure. The thickness of the ruthenium layer determines whether antiferromagnetic or ferromagnetic coupling the interlayer exchange coupling is. In addition, referred to as the magnetostatically coupled structure is a structure in which two magnetic material layers are antiferromagnetically coupled due to leakage magnetic fields from end faces of the magnetic material layers. 
     Examples of materials of the magnetization reversal layer, the first magnetization reference layer and the second magnetization reference layer include following substances: ferromagnetic materials such as nickel (Ni), iron (Fe) and cobalt (Co); alloys of these ferromagnetic materials (e.g. Co—Fe, Co—Fe—Ni, and Ni—Fe); and alloys (e.g. Co—Fe—B) obtained by mixing into these alloys any of nonmagnetic elements (e.g. tantalum, boron, chromium, platinum, silicon, carbon, and nitrogen); oxides containing at least one of Co, Fe and Ni (e.g. ferrites such as Fe—MnO); a group of intermetallic compounds called half-metallic ferromagnetic materials (Huesler alloys such as NiMnSb, Co 2 MnGe, Co 2 MnSi and Co 2 CrAl); and oxides (e.g. (La, Sr)MnO 3 , CrO 2 , and Fe 3 O 4 ). The magnetization reversal layer, the first magnetization reference layer and the second magnetization reference layer may have any crystallinity essentially, and any of a polycrystalline layer, single-crystal layer and amorphous layer is available. Moreover, any of various magnetic semiconductor materials may be used. In addition, either material of a soft magnetic material (soft film) and a hard magnetic material (hard film) may be used. 
     Examples of material of the first nonmagnetic film in a multi-layer structure with a TMR effect include the following substances: aluminum oxides (AlO x ); an aluminum nitride (AlN), a magnesium oxide (MgO); magnesium nitrides, silicon oxides; silicon nitrides; and insulating materials such as TiO 2 , Cr 2 O 3 , Ge, NiO, CdO x , HfO 2 , Ta 2 O 5 , BN, and ZnS. Examples of material of the second nonmagnetic film in a multi-layer structure with a GMR effect include conductive materials such as Cu, Ru, Cr, Au, Ag, Pt, Ta and alloys of these metals. Alternatively, any non-metal material is also available as long as the material has high conductivity (resistivity lower than several hundred micro-ohm centimeters). It however is desirable to adequately select a material that hardly causes interfacial reaction with the magnetization reversal layer and the second magnetization reference layer. Note that the multi-layer structure formed of the second magnetization reference layer, the second nonmagnetic film and the magnetization reversal layer does not necessarily need to exhibit a GMR effect. Examples of material of the insulating film include insulating materials such as aluminum oxides (AlO x ), an aluminum nitride (AlN), a magnesium oxide (MgO), magnesium nitrides, silicon oxides, silicon nitrides, TiO 2  and Cr 2 O 3 , and ferroelectric materials such as BaTiO 3  (barium titanate, having a dielectric constant of about 2900), SrTiO 3  and PbTiO 3 . A material having a higher dielectric constant can store a larger amount of electrostatic energy, and therefore offers a higher effect of weakening the in-plane magnetic anisotropy in the magnetization reversal layer due to voltage application to the third electrode. Therefore, a ferroelectric material is a preferable insulating material in terms of reduction of the writing current (i.e., the critical current of spin-injection magnetization reversal) with use of voltage assist. 
     The first, second and third electrodes are formed of a single-layer film composed of Cu, Au, Pt or the like. Alternatively, these electrodes are composed of a multi-layer film including an underlayer made of Cr, Ti or the like and an upper layer formed thereon made of Cu, Au, Pt or the like. Examples of materials of the first and second antiferromagnetic layers include iron-manganese alloys, nickel-manganese alloys, platinum-manganese alloys, iridium-manganese alloys, rhodium-manganese alloys, cobalt oxides and nickel oxides. Examples of form of electrical coupling between the first magnetization reference layer and the first electrode are as follows: the first electrode is directly connected to the first magnetization reference layer; and the first electrode is coupled via the first antiferromagnetic layer to the first magnetization reference layer. Similarly, examples of form of electrical coupling between the second magnetization reference layer and the second electrode are as follows: the second electrode is directly connected to the second magnetization reference layer; and the second electrode is coupled via the second antiferromagnetic layer to the second magnetization reference layer. An underlayer film composed of Ta, Cr, Ti or the like may be formed between the first electrode and the first antiferromagnetic layer, and between the second electrode and the second antiferromagnetic layer, in order to improve the crystallinity of the first and second antiferromagnetic layers. 
     These layers can be formed by physical vapor deposition typified by sputtering, ion-beam deposition and vacuum deposition, or chemical vapor deposition typified by atomic layer deposition. 
     The first nonmagnetic film can be obtained by oxidizing or nitriding a metal film formed by sputtering for example. More specifically, when an aluminum oxide (AlO x ) is used as the insulating material of the first nonmagnetic film, any of the following methods is available for example: a method of oxidizing in an air atmosphere an aluminum film formed by sputtering; a method of plasma-oxidizing an aluminum film formed by sputtering; a method of oxidizing with IPC plasma an aluminum film formed by sputtering; a method of naturally oxidizing in an oxygen atmosphere an aluminum film formed by sputtering; a method of oxidizing with oxygen radical an aluminum film formed by sputtering; a method of naturally oxidizing in an oxygen atmosphere an aluminum film formed by sputtering in association with irradiation of the film with ultra-violet rays; a method of depositing an aluminum film by reactive sputtering; and a method of depositing an aluminum oxide (AlO x ) film by sputtering. 
     Examples of applications of the spin-injection magnetoresistance effect element according to one embodiment of the invention include MRAM, current sensors and voltage sensors. 
     The spin-injection magnetoresistance effect element according to one embodiment of the invention has a three-terminal spin transfer element structure unlike a conventional spin-injection magnetoresistance effect element having a two-terminal spin transfer element structure. If a voltage is applied to the third electrode when information is written to the magnetization reversal layer, electron distribution in the vicinity of the interface between the magnetization reversal layer and insulating film changes. Thus, induced in the magnetization reversal layer near the interface to the insulating film is the magnetic anisotropy in the perpendicular-to-plane direction (the thickness direction of the magnetization reversal layer, in other words, the Z-axis direction, hereinafter indicating the same direction), which therefore weakens the magnetic anisotropy (in-plane magnetic anisotropy) of the magnetization reversal layer in the in-plane direction (the direction perpendicular to the thickness direction of the magnetization reversal layer, in other words, the X-Y-axis direction, hereinafter indicating the same direction). Accordingly, the writing current (i.e., the critical current of spin-injection magnetization reversal) can be reduced to several tens of microamperes or smaller, for the writing time period on the order of nanoseconds. Thus, a non-volatile magnetic memory element array of ultra-low power consumption, high speed, and high capacity (over the giga-bit class) can be achieved. In addition, there is no need to suppress the reading current to few tenths of the writing current (to about 10 μA) in order to prevent erroneous writing at the time of retrieving magnetic information, and the reading current amount can be determined independently of the writing current amount. Thus, use of a large reading current is allowed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a conceptual diagram illustrating a spin-injection magnetoresistance effect element according to a first embodiment of the present invention; 
         FIG. 1B  is a schematic diagram showing the arrangement of a magnetization reversal layer, a first magnetization reference layer and a second magnetization reference layer; 
         FIG. 2  shows an equivalent circuit of MRAM including the spin-injection magnetoresistance effect element of the first embodiment; 
         FIG. 3  is a sectional view schematically illustrating part of the MRAM including the spin-injection magnetoresistance effect element of the first embodiment; 
         FIG. 4  is a conceptual diagram illustrating a modification of the spin-injection magnetoresistance effect element of the first embodiment; 
         FIG. 5A  is a conceptual diagram illustrating another modification of the spin-injection magnetoresistance effect element of the first embodiment; 
         FIG. 5B  is a schematic diagram showing the arrangement of the magnetization reversal layer, first magnetization reference layer and second magnetization reference layer shown in  FIG. 5A ; 
         FIG. 6A  is a conceptual diagram illustrating part of a spin-injection magnetoresistance effect element according to a second embodiment of the invention; 
         FIG. 6B  is an enlarged conceptual diagram illustrating a first magnetization reference layer and other layers in the spin-injection magnetoresistance effect element of the second embodiment; 
         FIG. 7  is an enlarged conceptual diagram illustrating a second magnetization reference layer and other layers in a modification of the spin-injection magnetoresistance effect element of the second embodiment; 
         FIG. 8A  is a conceptual diagram illustrating a spin-injection magnetoresistance effect element according to a third embodiment of the invention; 
         FIGS. 8B and 8C  are schematic diagrams illustrating a first magnetization reference layer and a second magnetization reference layer in the spin-injection magnetoresistance effect element of the third embodiment, and explaining a method for obtaining antiparallel magnetization directions of the first and second magnetization reference layers; 
         FIG. 9A  is a conceptual diagram illustrating part of a modification of a spin-injection magnetoresistance effect element of the third embodiment; 
         FIG. 9B  is an enlarged conceptual diagram illustrating a first magnetization reference layer and other layers in the modification of the spin-injection magnetoresistance effect element of the third embodiment; 
         FIG. 10A  is an enlarged conceptual diagram illustrating a first magnetization reference layer and other layers in a spin-injection magnetoresistance effect element according to a fourth embodiment of the invention; 
         FIG. 10B  is an enlarged conceptual diagram illustrating a second magnetization reference layer and other layers in the spin-injection magnetoresistance effect element of the fourth embodiment; 
         FIG. 11A  is a conceptual diagram illustrating a conventional non-volatile magnetic memory element employing spin-injection magnetization reversal; 
         FIG. 11B  is a schematic plan view of a magnetization reversal layer; 
         FIG. 11C  is a schematic diagram illustrating a conventional non-volatile magnetic memory element that employs spin-injection magnetization reversal and includes a magnetization reference layer of which magnetization direction is fixed due to exchange coupling to an antiferromagnetic layer; and 
         FIG. 11D  is a conceptual diagram illustrating a non-volatile magnetic memory element having a double spin filter structure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     First Embodiment 
     A first embodiment of the present invention relates to a spin-injection magnetoresistance effect element, and more specifically to a spin-injection magnetoresistance effect element applied to MRAM. A spin-injection magnetoresistance effect element  10  according to the first embodiment has a three-terminal spin transfer element structure that is referred to also as a voltage-assisted spin-injection magnetization reversal structure.  FIG. 1A  is a conceptual diagram illustrating the element  10 .  FIG. 1B  is a schematic diagram illustrating the arrangement of a magnetization reversal layer, a first magnetization reference layer and a second magnetization reference layer in the element  10 .  FIG. 2  illustrates an equivalent circuit of the MRAM.  FIG. 3  is a schematic sectional view illustrating part of the MRAM. Turning now to  FIG. 1A , the spin-injection magnetoresistance effect element  10  includes (A) a magnetization reversal layer (referred to also as a free layer)  20  that has a first face  20 A and a second face  20 B opposite to the first face  20 A, and stores information, (B) a first magnetization reference layer (referred to also a pinned layer)  32  that is disposed near one end of the magnetization reversal layer  20  in a manner of facing the first face  20 A of the layer  20  with the intermediary of a first nonmagnetic film  31  between the layers  20  and  32 , and is magnetized in a first direction (in the first embodiment, in the left direction as seen in  FIG. 1A ), (C) a second magnetization reference layer  42  that is disposed near the other end of the magnetization reversal layer  20  in a manner of facing the first face  20 A of the layer  20  and being separated from the first magnetization reference layer  32  with the intermediary of a second nonmagnetic film  41  between the layers  20  and  42 , and is magnetized in a second direction (in the first embodiment, in the right direction as seen in  FIG. 1A ) that is the opposite direction of the first direction, (D) a first electrode  34  that is electrically coupled to the first magnetization reference layer  32 , (E) a second electrode  44  that is electrically coupled to the second magnetization reference layer  42 , and (F) a third electrode  51  that is disposed in a manner of facing the second face  20 B of the magnetization reversal layer  20  with the intermediary of a nonmagnetic insulating film  50  between the electrode  51  and the layer  20 . 
     The spin-injection magnetoresistance effect element  10  is a memory device that stores one bit of data. The second electrode  44  (or the first electrode  34  in some cases) included in the spin-injection magnetoresistance effect element  10  is coupled to a selection transistor  11 . 
     Referring to  FIG. 3 , the selection transistor  11  is formed on a silicon semiconductor substrate  101 , and includes a gate electrode  104  and a source/drain region  105 . In  FIG. 3 , reference numerals  102  and  103  indicate an element isolation region and a gate insulating film, respectively. The gate electrode  104  is coupled to a row selection line. The selection transistor  11  is covered by a lower interlayer insulating film  106  and an upper interlayer insulating film  108 . Formed on the lower interlayer insulating film  106  is a writing/reading column selection line coupled to a writing/reading column selection circuit. One of the source and drain regions  105  of the selection transistor  11  is coupled via contact plugs  107  and  109 A to the second electrode  44 . The writing/reading column selection line is coupled via a contact plug  109 B to the first electrode  34 . 
     The first magnetization reference layer  32  and the second magnetization reference layer  42  (sometimes generically referred to as the magnetization reference layers  32  and  42 , hereinafter) have a function as an injection source of a spin-polarized current (spin filter). A first antiferromagnetic layer  33  for pinning the magnetization of the first magnetization reference layer  32  is provided in contact with the first magnetization reference layer  32 . In addition, a second antiferromagnetic layer  43  for pinning the magnetization of the second magnetization reference layer  42  is provided in contact with the second magnetization reference layer  42 . The first electrode  34  is in contact with the first antiferromagnetic layer  33 , and the second electrode  44  is in contact with the second antiferromagnetic layer  43 . However, the first electrode  34  can be positioned at any position as long as it can apply a current to the first magnetization reference layer  32 , and the second electrode  44  can be positioned at any position as long as it can apply a current to the second magnetization reference layer  42 . For example, the first and second electrodes  34  and  44  may be provided in contact with the first and second magnetization reference layers  32  and  42 , respectively. 
     In the first embodiment, from the first electrode  34  via the first magnetization reference layer  32 , or from the second electrode  44  via the second magnetization reference layer  42 , a spin-polarized current is injected into the magnetization reversal layer  20 . Thus, the magnetization direction in the magnetization reversal layer  20  is set to the first direction or second direction, which writes information in the magnetization reversal layer  20 . That is, writing of information to the magnetization reversal layer  20  (magnetization reversal) is implemented based on spin-injection magnetization reversal. The direction of current flowing between the first and second electrodes  34  and  44  determines the logical value of information written to the magnetization reversal layer  20 . The easy magnetization axis of the magnetization reversal layer  20  corresponds with the longitudinal direction of the magnetization reversal layer  20 . 
     Specifically, the selection transistor  11  is selected by the row selection circuit, and thus the selection transistor  11  enters the conductive state. In addition, electrons are injected from a first power supply  61  via the writing/reading column selection circuit to the first magnetization reference layer  32  (or the second magnetization reference layer  42 ). The injected electrons are subjected to spin polarization in the first magnetization reference layer  32  (or the second magnetization reference layer  42 ) serving as a spin filter. Thus, the spin-polarized electrons flow into the magnetization reversal layer  20  as a spin-polarized current. Due to interaction between conduction electrons (s-electrons) and d-electrons in the magnetization reversal layer  20  (sd-interaction), spin torque arises in the magnetic moment, and serves as a driving force for magnetization reversal. The spin torque in the magnetization reversal layer  20  is generated near the interface between the magnetization reversal layer  20  and the first magnetization reference layer  32  (or near the interface between the magnetization reversal layer  20  and the second magnetization reference layer  42 ). The spin torque reverses a magnetization in the magnetization reversal layer  20  above the first magnetization reference layer  32  (or the second magnetization reference layer  42 ), which produces a magnetic domain wall in the magnetization reversal layer  20 . This domain wall is pushed and moved by the spin-polarized current injected from the first magnetization reference layer  32  (or the second magnetization reference layer  42 ) to the magnetization reversal layer  20 . Thus, the whole magnetization in the magnetization reversal layer  20  is reversed eventually. When electrons flow from the first magnetization reference layer  32  (or the second magnetization reference layer  42 ) to the magnetization reversal layer  20 , the magnetization direction of the magnetization reversal layer  20  becomes parallel to the magnetization direction of the second magnetization reference layer  42  (or the first magnetization reference layer  32 ). Therefore, if the magnetization direction of the first magnetization reference layer  32  is set opposite (antiparallel) to that of the second magnetization reference layer  42 , magnetization reversal is allowed by a change of the direction of a current applied between the first and second electrodes  34  and  44 . One example of combinations of voltages V 1  and V 2  applied to the first and second electrodes  34  and  44 , respectively, is as follows: V 1 =+1 V and V 2 =0 V; and V 1 =−1 V and V 2 =0 V. 
     In addition, in writing of information to the magnetization reversal layer  20 , application of a voltage to the third electrode  51  allows control of a magnetic anisotropy in the magnetization reversal layer  20 , and thus can reduce the writing current (the critical current I c  of spin-injection magnetization reversal). 
     Specifically, when information is written to the magnetization reversal layer  20 , an adequate voltage (e.g. −1 V) is applied from a second power supply  62  to the third electrode  51 . The magnetization reversal layer  20  is grounded. Specifically, the magnetization reversal layer  20  is grounded via any of the conductive second nonmagnetic film  41 , the second magnetization reference layer  42 , the second antiferromagnetic layer  43 , and the second electrode  44 . Any of these layers and films may be grounded. The voltage application to the third electrode  51  applies a uniform electric field, in the direction perpendicular to the film planes, via the nonmagnetic insulating film  50  to the magnetization reversal layer  20  near the interface between the magnetization reversal layer  20  and the insulating film  50 . As a result, electron distribution near the interface between the magnetization reversal layer  20  and the insulating film  50  changes, and therefore a magnetic anisotropy in the perpendicular-to-plane direction is induced in the magnetization reversal layer  20  near the interface, which weakens the in-plane magnetic anisotropy in the magnetization reversal layer  20 . Consequently, the writing current (i.e., the critical current I c  of spin-injection magnetization reversal) can be decreased to several tens of microamperes or smaller, for the writing time period on the order of nanoseconds. Thus, an MRAM array of ultra-low power consumption, high speed, and high capacity (over the giga-bit class) can be achieved. If no voltage is applied to the third electrode  51  at the time of information writing to the magnetization reversal layer  20 , the writing current (i.e., the critical current I′ c  of spin-injection magnetization reversal) is on the order of several hundred microamperes to several milliamperes. 
     Passing a current between the first and second electrodes  34  and  44  allows the retrieval of information stored in the magnetization reversal layer  20 . Specifically, the selection transistor  11  is selected by the row selection circuit, and thus the selection transistor  11  enters the conductive state. In addition, a current is applied from the first power supply  61  via the writing/reading column selection circuit between the first and second electrodes  34  and  44 . Thus, a change of the resistance between the first and second electrodes  34  and  44  associated with a magnetization direction change of the magnetization reversal layer  20  is measured based on a potential change of the writing/reading column selection line. This measurement allows a determination as to whether “0” or “1” the information recorded in the magnetization reversal layer  20  is. 
     In the spin-injection magnetoresistance effect element  10  of the first embodiment, the writing current when no voltage is applied to the third electrode  51  (the critical current I′ c  of spin-injection magnetization reversal) is sufficiently large (several hundred microamperes to several milliamperes). Therefore, there is no limitation on the magnitude of the reading current I r  virtually, which allows the reading current I r  to be set sufficiently large. The reading current I r  can be equalized to the writing current I c , or alternatively can be set larger than the writing current I c . Specifically, the upper limit of the reading current I r  is the critical current I′ c , which is the critical current when no voltage is applied to the third electrode  51 . In conventional techniques, reduction of the writing current inevitably involves the need to correspondingly decrease the reading current. In contrast, such a restriction does not exist in embodiments of the present invention. Moreover, erroneous writing at the time of reading information is eliminated. Thus, the magnitude of outputs and S/N ratio are enhanced, which can ensure a low error rate required in a high-capacity storage device over the giga-bit class. 
     As described above, the provision of the third electrode  51  enables reading and writing currents to be controlled independently of each other. For example, reading and writing currents can be set to the same value. The equalization of the reading and writing currents can simplify control circuitry (drive circuitry) and control logic circuitry in the spin-injection magnetoresistance effect element  10 , and can reduce the area of the circuitry. As a result, peripheral circuitry can be reduced and the area occupation ratio of memory part to the entire chip can be increased, which contributes to increasing capacity. In addition, since there is no need to decrease the reading current to few tenths of the writing current unlike conventional techniques, high outputs can be obtained. It is desirable in terms of signal outputs that the reading current is large. The allowable maximum amount of a current passed between the first and second electrodes  34  and  44  is defined by the lower of two voltages: the breakdown voltage of the selection transistor  11 ; and the insulation breakdown voltage of the first nonmagnetic film  31 , which is an insulating barrier film. In step with reduction of interconnect width associated with capacity increase over the giga-bit class, the widths of the gate electrodes  104  of the selection transistors  11  and various interconnects become about 100 nm recently. For this width, the allowable maximum current corresponding to the breakdown voltage of the selection transistor  11  is 100 μA. In order to satisfy the above-described conditions, the first embodiment sets both the writing and reading currents to 50 μA. 
     In addition, since a small writing current (the critical current I c  of spin-injection magnetization reversal) can be used, there is no need to excessively reduce the writing current when no voltage is applied to the third electrode  51  (the critical current I′ c  of spin-injection magnetization reversal). As a result, the flexibility of design of the spin-injection magnetoresistance effect element can be enhanced. Specifically, there is no need to excessively reduce the volume V and the saturation magnetization M s  (the magnetic moment per unit voltage of the magnetization reversal layer  20 ) of the magnetization reversal layer  20 . Therefore, lowering of the thermal fluctuation resistance due to an attempt to excessively reduce the critical current I′ c  can surely be avoided. Furthermore, the flexibility of selecting the material of the magnetization reversal layer  20  is enhanced, and use of for example, a magnetic material with a high saturation magnetization M s  is allowed. Moreover, fabrication of a complicated multi-layer structure for enhancing the efficiency of spin-injection magnetization reversal can be simplified. Therefore, manufacturing of a spin-injection magnetoresistance effect element is facilitated, which improves the manufacturing yield thereof. 
     The control factors, excluding the temperature dependence, of the critical current I′ c0  of spin-injection magnetization reversal are represented by the equation I′ c0 =e(M s ·V/μ B )(α·γ·H eff )/g. e denotes the elementary charge. μ B  denotes the Bohr magneton. α denotes a damping coefficient (Gilbert damping coefficient). γ denotes the magnetic gyro constant. H eff  denotes an effective magnetic field (external magnetic field+anisotropic magnetic field in the easy magnetization axis+demagnetization field in the film thickness direction), and is a function representing the efficiency of spin-injection magnetization reversal. g is a function of a spin transfer efficiency. 
     In the first embodiment, the magnetization reversal layer  20 , the first and second magnetization reference layers  32  and  42  are composed of a magnetic material such as Co, Co—Fe, Ni—Fe, or Co—Fe—B. The first nonmagnetic film  31  and the nonmagnetic insulating film  50  are composed of an insulating material such as AlO x , MgO, SiO 2 , TiO 2 , or Cr 2 O 3 . The second nonmagnetic film  41  is composed of a conductive material such as Cu or Ru. The first, second and third electrodes  34 ,  44  and  51  are composed of Cu, Au or the like. Alternatively, these electrodes are composed of a multi-layer film including an underlayer made of Cr, Ti or the like and an upper layer formed thereon made of Cu, Au or the like. The first and second antiferromagnetic layers  33  and  43  are composed of Mn—Ir, Pt—Mn or the like. An underlayer film composed of Ta, Cr, Ti or the like may be formed between the first electrode  34  and the first antiferromagnetic layer  33 , and between the second electrode  44  and the second antiferromagnetic layer  43 . 
     The third electrode  51  may be provided for each spin-injection magnetoresistance effect element as shown in  FIG. 3 , or alternatively may be provide for a spin-injection magnetoresistance effect element array, which is an assembly of a plurality of spin-injection magnetoresistance effect elements. That is, the third electrode  51  may be provided in common to a plurality of spin-injection magnetoresistance effect elements. Note that the equivalent circuit diagram of  FIG. 2  illustrates the third electrode  51  provided in common to plural elements. When the third electrode  51  is provided for each spin-injection magnetoresistance effect element, each third electrode  51  is coupled via a switching element (formed of an FET, for example)  63  (see  FIG. 3 ) to the second power supply  62 . On that basis, when information is written to the magnetization reversal layer  20 , the switching element is turned on, to thereby apply an adequate voltage from the second power supply  62  to the third electrode  51 . 
     In the first embodiment, since the first nonmagnetic film  31  is composed of an insulating material, the first magnetization reference layer  32 , the first nonmagnetic film (insulating barrier film)  31 , and the magnetization reversal layer  20  form a multi-layer structure having a TMR effect. This multi-layer structure formed of the first magnetization reference layer  32 , the first nonmagnetic film  31 , and the magnetization reversal layer  20  will be referred to as a first multi-layer structure for convenience. Information stored in the magnetization reversal layer  20  can be retrieved as a difference of the resistance of the magnetization reversal layer  20 . There are two kinds of the information as logical values: the information “0” corresponding to the state in which the magnetization direction of the magnetization reversal layer  20  is parallel to that of the first magnetization reference layer  32 ; and the information “1” corresponding to the state in which the directions are antiparallel. In other words, the magnetization reversal layer  20  shows a low resistance when the magnetization direction of the magnetization reversal layer  20  is parallel to that of the first magnetization reference layer  32 , while the magnetization reversal layer  20  shows a high resistance when the magnetization direction of the magnetization reversal layer  20  is antiparallel to that of the first magnetization reference layer  32 . Since the second nonmagnetic film  41  is composed of a conductive material, the second magnetization reference layer  42 , the second nonmagnetic film  41 , and the magnetization reversal layer  20  form a multi-layer structure having a GMR effect, depending on the case. This multi-layer structure formed of the second magnetization reference layer  42 , the second nonmagnetic film  41 , and the magnetization reversal layer  20  will be referred to as a second multi-layer structure for convenience. The magnetization direction of the first magnetization reference layer  32  is opposite to that of the second magnetization reference layer  42 . Therefore, the second multi-layer structure having a GMR effect functions to counteract a resistance change of the first multi-layer structure having a TMR effect. Specifically, when the magnetization direction of the first magnetization reference layer  32  (the second magnetization reference layer  42 ) is parallel (antiparallel) to that of the magnetization reversal layer  20 , the magnetization direction of the second magnetization reference layer  42  (the first magnetization reference layer  32 ) is antiparallel (parallel) to that of the magnetization reversal layer  20 . However, a resistance change ratio based on a TMR effect (TMR ratio) is on the order of several tens of percents to several hundred percents, and this ratio is larger by a factor of several tens to several hundreds than a resistance change ratio based on a GMR effect (GMR ratio), which is on the order of several percents. This large difference permits neglect of the effect of the second multi-layer structure with a GMR effect on a resistance change in the first multi-layer structure with a TMR effect. The second multi-layer structure does not necessarily need to serve as a multi-layer structure having a GMR effect. Depending on the material (e.g. tungsten) of the second nonmagnetic film  41 , the second multi-layer structure serves as a multi-layer structure without a GMR effect in some cases. Such a configuration also yields no problem. However, if both the first and second multi-layer structures are a multi-layer structure having a TMR effect, the second multi-layer structure completely cancels a resistance change in the first multi-layer structure, which precludes the detection of the resistance change. 
     The length L 1  and the width W of the magnetization reversal layer  20  are, for example, 150 to 200 nm and 50 nm, respectively. The thickness thereof is about several nanometers. The planar shape of the magnetization reversal layer  20  is designed to have an adequate aspect ratio (the ratio of the major axis length L 1  to the minor axis length W, about 3 is adequate, for example), which provides the magnetization reversal layer  20  with a shape magnetic anisotropy (uniaxial magnetic anisotropy). In addition, the magnetization reversal layer  20  is miniaturized to have a size on the order of 100 nm, to thereby promote the single-domain magnetization thereof. These shape features allow achievement of two stable magnetization states corresponding to binary information of “0” and “1.” The magnetic stability is expressed by the magnetic anisotropy energy K u  per unit volume of the magnetization reversal layer  20 . The magnetic anisotropy energy of the spin-injection magnetoresistance effect element  10  is the product between the magnetic anisotropy energy K u  and the volume V of the magnetization reversal layer  20 . When the magnetization reversal layer  20  consists of a single magnetic domain and has a uniaxial anisotropy, the magnetic anisotropy energy K u  can be expressed by the equation K u =M s ·H c /2. M s  denotes the saturation magnetization of magnetic material of the magnetization reversal layer  20 . H c  denotes the coercivity of magnetic material of the magnetization reversal layer  20 . 
     Since the writing current (the critical current of spin-injection magnetization reversal) is proportional to the volume V of the magnetization reversal layer  20 , the length L 1  of the magnetization reversal layer  20  is decreased to the order of 100 nm in order to obtain a practical writing current value. However, too small a volume V of the magnetization reversal layer  20  causes a possibility that the magnetic anisotropy energy of the magnetization reversal layer  20  decreases, and therefore the requisite thermal fluctuation resistance for keeping the reliability as a memory element is not ensured. The first embodiment needs to ensure the above-described conditions and the state where the first and second magnetization reference layers  32  and  42  are positioned at the both ends of the magnetization reversal layer  20 . Therefore, the length L 1  of the magnetization reversal layer  20  is set to 150 to 200 nm in the first embodiment. 
     The thermal fluctuation resistance is defined as K u ·V/k B ·T as described above, and needs to be larger than about 40 (K u ·V/k B ·T&gt;40) for practical use. 
     The length L 2  of the magnetization reference layers  32  and  42  is 50 to 70 nm in the first embodiment. Both the layers  32  and  42  are processed so as not to protrude from the both ends of the magnetization reversal layer  20 . There is no problem even if, at the time of processing the magnetization reference layers  32  and  42 , the first antiferromagnetic layer  33  and the second antiferromagnetic layer  43  (sometimes generically referred to as the antiferromagnetic layers  33  and  43 , hereinafter) under the layers  32  and  42  are also processed collectively (refer to the regions indicated by arrows X in  FIG. 4 , which is a conceptual diagram illustrating a spin-injection magnetoresistance effect element as a modification of the first embodiment). The respective magnetization directions of the magnetization reference layers  32  and  42  are fixed to one direction due to exchange coupling across the interface between the first magnetization reference layer  32  and the first antiferromagnetic layer  33 , and due to exchange coupling across the interface between the second magnetization reference layer  42  and the second antiferromagnetic layer  43 . In addition, the magnetization reference layers  32  and  42  are designed to have a small size, and thus the single-domain magnetizations of the layers  32  and  42  are promoted. Due to these features, the magnetizations of the magnetization reference layers  32  and  42  are aligned in one direction, which allows the layers  32  and  42  to serve as a high-quality spin filter, in other words, a spin injection source. The magnetization reference layers  32  and  42  may have any shape and any size essentially as long as the magnetizations in the magnetization reference layers  32  and  42  overlapping with the magnetization reversal layer  20  are aligned in one direction. If the magnetization reference layers  32  and  42  are exchange-coupled to the antiferromagnetic layers  33  and  43 , respectively, with a sufficiently large coupling strength, shape anisotropies of the magnetization reference layers  32  and  42  are not necessarily required. The antiferromagnetic layers  33  and  43  are in contact with the entire surfaces of the magnetization reference layers  32  and  42 , and have sizes equal to or larger than the sizes of the magnetization reference layers  32  and  42 . 
       FIG. 5A  is a conceptual diagram illustrating another modification of the first embodiment.  FIG. 5B  is a schematic diagram illustrating the arrangement of the magnetization reversal layer  20 , the first magnetization reference layer  32 , the second magnetization reference layer  42 , and other layers in this modification. As shown in these drawings, end faces of the magnetization reference layers  32  and  42  do not need to be flush with end faces of the magnetization reversal layer  20 . The magnetization reference layers  32  and  42  may have any size and any shape essentially. However, it is required that the magnetizations in the magnetization reference layers  32  and  42  overlapping with the magnetization reversal layer  20  be aligned in one direction. Therefore, in order to allow the magnetization reference layers  32  and  42  to have any shape and any size, it is desirable for the magnetization reference layers  32  and  42  to be exchange-coupled to the antiferromagnetic layers  33  and  43 , respectively. End faces of the magnetization reference layers  32  and  42  do not need to be flush with end faces of the antiferromagnetic layers  33  and  43 . It should be noted that the example shown in  FIGS. 5A and 5B  can apply the features of the regions indicated by arrows X in  FIG. 4 . 
     When the magnetization reference layers  32  and  42  are formed of a single-layer film, it is desirable that the thicknesses thereof are about 10 nm. The area of the overlap region between the magnetization reference layers  32  and  42  and the magnetization reversal layer  20  is small, specifically 50 to 70 nm by 50 nm. Therefore, if the thicknesses of the magnetization reference layers  32  and  42  are several tens of nanometers or larger, the in-plane magnetic anisotropies thereof are lowered, and demagnetization fields at ends of the magnetization reference layers  32  and  42  increase. Accordingly, magnetization alignment at ends of the magnetization reference layers  32  and  42  is readily disturbed. Furthermore, the magnetic field strength of exchange coupling between the magnetization reference layers  32  and  42  and the antiferromagnetic layers  33  and  43  decreases in inverse proportion to the thicknesses of the magnetization reference layers  32  and  42 . Therefore, larger thicknesses of the magnetization reference layers  32  and  42  lead to more difficulty in aligning the magnetization inside the magnetization reference layers  32  and  42  (particularly at ends thereof) in one direction. In addition, disarrangement of the magnetization in the magnetization reference layers  32  and  42  due to a disturbance magnetic field is readily caused. In contrast, if the thicknesses of the magnetization reference layers  32  and  42  are too small, the function thereof as a spin filter is deteriorated. Therefore, when the magnetization reference layers  32  and  42  are formed of a single-layer film, it is desirable for the layers  32  and  42  to have a thickness equivalent to at least the spin diffusion length. The spin diffusion length differs depending on the magnetic materials of the magnetization reference layers  32  and  42 , and changes also depending on the materials of adjacent and close films. Therefore, the thicknesses of the magnetization reference layers  32  and  42  need to be adequately determined with consideration of the magnetic materials and layer structure, so that the above-described various conditions are satisfied. 
     The distance L 3  between the first and second magnetization reference layers  32  and  42  is about 50 to 60 nm in the first embodiment. The distance L 3  between the first and second magnetization reference layers  32  and  42  may take any value essentially as long as the first and second magnetization reference layers  32  and  42  are physically separated from each other. However, too small a distance L 3  possibly causes repulsion of magnetic poles at facing ends of the first and second magnetization reference layers  32  and  42 , which disturbs the magnetization directions of the first and second magnetization reference layers  32  and  42 . That is, it probably becomes difficult to ensure the magnetization directions that are uniaxially antiparallel. Pinning of magnetizations of the magnetization reference layers  32  and  42  by exchange coupling with the antiferromagnetic layers  33  and  43  can suppress the disturbance of the magnetizations due to magnetic pole repulsion to a large extent. As will be described in a second embodiment of the invention, if either one of the first and second magnetization reference layers  32  and  42  is formed of a synthetic ferri-magnetic structure based on antiferromagnetic coupling, the magnetization disturbance can be suppressed more effectively since the net magnetization of the synthetic ferri-magnetic structure is extremely small. 
     The resistance of the first multi-layer structure is 20 kΩ for example. Therefore, if a voltage of 1 V is applied between the first electrode  34  and the second electrode  44  for writing/reading information, the current flowing through the first multi-layer structure is 50 μA. Since the writing current I c  can be reduced to several tens of microamperes in the first embodiment, the area resistance R·A of the first multi-layer structure (more specifically, the first nonmagnetic film  31 ) can be enhanced to several tens of ohm-square micrometers (e.g. 50 Ω·μm 2 ). The area resistance R·A is the product between the resistance and area of the first nonmagnetic film  31 . This area resistance R·A is about ten times the area resistance R·A of a conventional TMR effect multi-layer structure that allows spin-injection magnetization reversal. Accordingly, there is no need to excessively decrease the thickness of the first nonmagnetic film  31 , which is an insulating barrier film. Therefore, the electrical reliability and breakdown voltage of the first nonmagnetic film  31  can be enhanced, and the reliability of the first multi-layer structure having a TMR effect can be improved. Furthermore, the formation of the first multi-layer structure having a TMR effect is facilitated, which improves the manufacturing yield of the spin-injection magnetoresistance effect element  10 . The insulation breakdown voltage of the first nonmagnetic film  31  that has an area resistance R·A of about 50 Ω·μm 2  and is composed of an insulating film is about one and several tenths of volts when the film  31  is composed of AlO x  for example, although depending on the insulating material and film quality of the film  31 . Therefore, the above-described application voltage of 1 V for writing/reading is a voltage that can sufficiently keep the reliability of the device as a writing/reading voltage for a high-capacity memory device. 
     The reliability of the first multi-layer structure having a TMR effect is determined dominantly by the extremely thin first nonmagnetic film  31 . Typically, the first nonmagnetic film  31  needs to be thin in order to obtain a low-resistance first multi-layer structure. The thickness reduction of the first nonmagnetic film  31  however leads to decrease of the insulation breakdown voltage of the film  31 , and significant deterioration of the first nonmagnetic film  31  due to repetition of current injection. In contrast, embodiments of the present invention allow use of a small writing current. Consequently, the first nonmagnetic film  31  is permitted to have a higher resistance, which allows use of the film  31  having a larger thickness. As a result, the insulation breakdown voltage of the first nonmagnetic film  31  increases, which also contributes to enhancement of the reliability of the first multi-layer structure with a TMR effect. 
     In addition, since a high resistance of the first nonmagnetic film  31  is allowed and the reading current I r  can be increased, the magnitude of outputs and S/N ratio can be enhanced. In a conventional first multi-layer structure with a TMR effect, the area resistance R·A needs to be about 10 Ω·μm 2  or smaller, in order to ensure spin-injection magnetization reversal with preventing the insulation breakdown of the first nonmagnetic film  31 . In contrast, the first embodiment allows use of writing current on the order of several tens of microamperes or smaller, which can set the area resistance R·A to several tens of ohm-square micrometers. This large area resistance R·A in combination with a large reading current I r  can increase output voltages and the S/N ratio. 
     The following method is available as one measure for ensuring the magnetization direction of the first magnetization reference layer  32  opposite (antiparallel) to that of the second magnetization reference layer  42 . Specifically, for example, a method is available that uses a combination between a first layer pair of the first magnetization reference layer  32  and the first antiferromagnetic layer  33  and a second layer pair of the second magnetization reference layer  42  and the second antiferromagnetic layer  43 , and the first and second layer pairs have different blocking temperatures. The blocking temperature is the temperature at which exchange coupling between a magnetization reference layer and an antiferromagnetic layer adjacent thereto is eliminated. More specifically, the first antiferromagnetic layer  33  is composed of Mn—Ir, while the second antiferromagnetic layer  43  is composed of Pt—Mn for example. In addition, by controlling the temperature and magnetic field direction in magnetic annealing for developing exchange coupling, directions of exchange coupling (unidirectional anisotropy) after cooling can be made opposite (antiparallel). Alternatively, a method is also available in which a synthetic ferri-magnetic structure is used as the structure of either one of the first magnetization reference layer  32  and the second magnetization reference layer  42 , like the second embodiment to be described later. The magnetization reference layer with the synthetic ferri-magnetic structure includes a first layer remote from the magnetization reversal layer  20 , and a second layer adjacent to the magnetization reversal layer  20 . The magnetization direction of the first layer is the same as the magnetization direction of the magnetization reference layer that does not have a synthetic ferri-magnetic structure, while the magnetization direction of the second layer is opposite to that. In this case, the antiferromagnetic layers  33  and  43  can be formed of the same antiferromagnetic material. 
     Second Embodiment 
     The second embodiment is obtained by modifying part of the first embodiment. A spin-injection magnetoresistance effect element of the second embodiment is different from an element of the first embodiment in that in the second embodiment, a synthetic ferri-magnetic structure is employed for either one of the first and second magnetization reference layers. In the following description, a first magnetization reference layer  132  has a synthetic ferri-magnetic structure.  FIG. 6A  is a conceptual diagram illustrating part of a spin-injection magnetoresistance effect element of the second embodiment.  FIG. 6B  is an enlarged conceptual diagram illustrating the first magnetization reference layer  132  and other layers in the spin-injection magnetoresistance effect element of the second embodiment. It should be noted that a structure in which the second magnetization reference layer  42  has a synthetic ferri-magnetic structure is also available. 
     In the second embodiment, the first magnetization reference layer  132  is formed of a multi-layer structure including a first ferromagnetic material layer (spin filter)  132 A, a nonmagnetic spacer layer  132 B, and a second ferromagnetic material layer  132 C. One surface of the first ferromagnetic material layer  132 A is in contact with the first nonmagnetic film  31 , and the other surface thereof is in contact with the nonmagnetic spacer layer  132 B. One surface of the second ferromagnetic material layer  132 C is in contact with the nonmagnetic spacer layer  132 B, and the other surface thereof is in contact with the first antiferromagnetic layer  33 . When the nonmagnetic spacer layer  132 B composed of Ru is designed to have a thickness of about 0.8 nm, the first ferromagnetic material layer  132 A and the second ferromagnetic material layer  132 C are coupled to each other via the nonmagnetic spacer layer  132 B by antiferromagnetic interlayer exchange coupling. This phenomenon is referred to as RKKY (Ruderman-Kittel-Kasuya-Yoshida) interaction. It is considered that Ru is the material offering the largest magnitude of the RKKY interaction. 
     The magnetization directions of the following layers are the same (parallel): the second ferromagnetic material layer  132 C, part of the first antiferromagnetic layer  33  near the interface to the second ferromagnetic material layer  132 C, the second magnetization reference layer  42 , and part of the second antiferromagnetic layer  43  near the interface to the second magnetization reference layer  42 . That is, unidirectional anisotropies due to exchange coupling across the interface between the ferromagnetic material layer and antiferromagnetic layer, and the interface between the magnetization reference layer and antiferromagnetic layer, are in the same direction. In contrast, the magnetization direction of the first ferromagnetic material layer  132 A is opposite (antiparallel) to that of the second ferromagnetic material layer  132 C. 
     If either one of the first and second magnetization reference layers is provided with a synthetic ferri-magnetic structure, the magnetization directions of the second ferromagnetic material layer  132 C and the first ferromagnetic material layer  132 A inevitably become opposite (antiparallel) after magnetic annealing. Accordingly, the magnetization direction of the first ferromagnetic material layer  132 A, which serves as a spin filter, becomes opposite (antiparallel) to that of the second magnetization reference layer  42 . In this configuration, the first and second antiferromagnetic layers  33  and  43  can be formed of the same antiferromagnetic layer. Although either layer of the first and second magnetization reference layers may have a synthetic ferri-magnetic structure, it is preferable that the first multi-layer structure having a TMR effect is provided with a synthetic ferri-magnetic structure, according to experience. 
     Since the adoption of a synthetic ferri-magnetic structure antiferromagnetically couples magnetizations of two magnetic layers (the second ferromagnetic material layer  132 C and the first ferromagnetic material layer  132 A), the net magnetization at ends of the second ferromagnetic material layer  132 C and the first ferromagnetic material layer  132 A is extremely small. Thus, disturbance of the magnetization (offset from the easy magnetization axis) at ends of the first ferromagnetic material layer  132 A virtually disappears, which allows the entire first ferromagnetic material layer  132 A to serve as a favorable spin filter. In addition, even if the distance L 3  between the first and second magnetization reference layers  132  and  42  is small (the layers  132  and  42  are close to each other), magnetic repulsion and magnetization disturbance due to the repulsion hardly occur since the magnetization of one of the layers is almost absent. Moreover, the first magnetization reference layer  132  as a whole virtually has an antiferromagnetic structure, and therefore is unsusceptible to the magnetic field from the second magnetization reference layer  42  and external magnetic fields. The magnetic field strength of interlayer exchange coupling is higher than the magnetic field strength of exchange coupling between the single-layer first magnetization reference layer  32  and the first antiferromagnetic layer  33  in the first embodiment. This high field strength enhances the tendency of the first ferromagnetic material layer  132 A to consist of a single magnetic domain. 
     Furthermore, the use of a synthetic ferri-magnetic structure allows the first ferromagnetic material layer  132 A and the second ferromagnetic material layer  132 C to have a small thickness of 2 to 3 nm. Even if the thickness is smaller than the spin diffusion length in a magnetic layer, the first ferromagnetic material layer  132 A can serve as a spin filter. It is also unnecessary that the second ferromagnetic material layer  132 C in contact with the first antiferromagnetic layer  33  has a thickness larger than the spin diffusion length unlike a single-layer magnetization reference layer (refer to the first embodiment). Therefore, the strength of the exchange coupling magnetic field applied to the second ferromagnetic material layer  132 C due to exchange coupling between the second ferromagnetic material layer  132 C and the first antiferromagnetic layer  33  can be maintained high. This is because the magnetic field strength of the exchange coupling between the second ferromagnetic material layer  132 C and the first antiferromagnetic layer  33  is in inverse proportion to the thickness of the second ferromagnetic material layer  132 C. However, too large thicknesses of the first and second ferromagnetic material layers  132 A and  132 C in a synthetic ferri-magnetic structure lead to a small polarization ratio of a spin-polarized current in some cases. It therefore is desirable for the first and second ferromagnetic material layers  132 A and  132 C to have a thickness of about 2 to 3 nm. 
     When the thicknesses of the first and second ferromagnetic material layers  132 A and  132 C are the same, these layers form a synthetic antiferro-magnetic structure, which is equivalent to a synthetic ferri-magnetic structure essentially. 
     A structure is also available in which either one of the first and second magnetization reference layers has a magnetostatically coupled structure. In the following description, the first magnetization reference layer has a magnetostatically coupled structure. The adoption of a magnetostatically coupled structure also allows the magnetization directions of the first and second ferromagnetic material layers  132 A and  132 C of the first magnetization reference layer  132  to be opposite (antiparallel) to each other. However, the use of a magnetostatically coupled structure requires that end faces of the first and second ferromagnetic material layers  132 A and  132 C be flush with each other. In the above-described RKKY interlayer exchange coupling, layers are coupled via the interfaces therebetween, and therefore the first and second ferromagnetic material layers  132 A and  132 C are coupled even if end faces thereof are not flush with each other. In a magnetostatically coupled structure, the nonmagnetic spacer layer  132 B does not need to be composed of Ru, but can be formed of any nonmagnetic material. Typically, an antiparallel arrangement based on a magnetostatically coupled structure is not referred to as a synthetic ferri-magnetic structure. When end faces of two magnetic layers are close to each other, a synthetic ferri-magnetic structure also encompasses antiparallel coupling based on magnetostatic coupling. 
       FIG. 7  is a conceptual diagram illustrating, in a magnified form, a second magnetization reference layer  142  and other layers in a spin-injection magnetoresistance effect element  10  according to a modification of the second embodiment. In this modification, the second magnetization reference layer  142  also has a synthetic ferri-magnetic structure. The first magnetization reference layer  132  has a three-layer structure of the first ferromagnetic material layer  132 A, the nonmagnetic spacer layer  132 B, and the second ferromagnetic material layer  132 C. In contrast, the second magnetization reference layer  142  has a five-layer structure of a first ferromagnetic material layer  142 A, a first nonmagnetic spacer layer  142 B, a second ferromagnetic material layer  142 C, a second nonmagnetic spacer layer  142 D, and a third ferromagnetic material layer  142 E. The adoption of this structure easily allows, through only one time of magnetic annealing, the magnetization direction of the first ferromagnetic material layer  132 A in the first magnetization reference layer  132  to be antiparallel to that of the first ferromagnetic material layer  142 A in the second magnetization reference layer  142 . That is, the magnetization directions of the first ferromagnetic material layers  142 A and  132 A can be rendered antiparallel to each other, while the magnetization direction of the second ferromagnetic material layer  132 C in the first magnetization reference layer  132  is kept parallel to that of the third ferromagnetic material layer  142 E in the second magnetization reference layer  142 . The numbers of layers in the first and second magnetization reference layers  132  and  142  are not limited to three and five, respectively, as long as the following conditions are satisfied: in the first and second magnetization reference layers, the ferromagnetic material layers adjacent to the magnetization reversal layer  20  with the intermediary of the first and second nonmagnetic films, respectively, have magnetization directions antiparallel to each other, and the ferromagnetic material layers in contact with the respective antiferromagnetic layers have magnetization directions parallel to each other. 
     The modifications of the first embodiment shown in  FIGS. 4 ,  5 A and  5 B can also be applied to the structures of the first and second magnetization reference layers in the second embodiment, third and fourth embodiments of the invention to be described later, and various modifications of these embodiments. 
     Third Embodiment 
     The third embodiment is also obtained by modifying part of the first embodiment. A spin-injection magnetoresistance effect element of the third embodiment is different from an element of the first embodiment in that in the third embodiment, first and second magnetization reference layers  32  and  42  have sufficiently high magnetic anisotropy energy (K u ·V), and therefore the first and second antiferromagnetic layers  33  and  43  are absent.  FIG. 8A  is a conceptual diagram illustrating a spin-injection magnetoresistance effect element according to the third embodiment. 
     The expression that the magnetic anisotropy energy (K u ·V) is “sufficiently high” means the following states:
     (1) The magnetization directions of the first and second magnetization reference layers  32  and  42  overlapping with the magnetization reversal layer  20  can be parallel to the easy magnetization axis of the magnetization reversal layer  20 . That is, the magnetization reference layers  32  and  42  maintain a single domain state, against demagnetization fields at ends of the magnetization reversal layer  20 ; and   (2) The thermal fluctuation resistances of the magnetization reference layers  32  and  42  satisfy the relationship K u ·V/k B ·T&gt;40.   

     Such magnetization reference layers  32  and  42  having sufficiently high magnetic anisotropy energy (K u ·V) can be achieved by any of the following methods:
     (a) enhancing the shape magnetic anisotropies and/or magnetocrystalline anisotropies of the magnetization reference layers  32  and  42 ;   (b) increasing the saturation magnetizations of magnetic materials of the magnetization reference layers  32  and  42 ; and   (c) setting the sizes of the magnetization reference layers  32  and  42  to the order of 100 nm.   

     In order to obtain antiparallel magnetization directions of the first and second magnetization reference layers  32  and  42  in the absence of the antiferromagnetic layers  33  and  43 , the following method is available, for example. Specifically, the magnetization reference layer  32  having a thickness t 1  and the second magnetization reference layer  42  having a thickness t 2  are formed. The thickness t 2  is not equalized to the thickness t 1 . More specifically, the thickness t 2  is set smaller than the thickness t 1 . The same magnetic material can be used for the first and second magnetization reference layers  32  and  42 . The coercivities H c1  and H c2  of the magnetization reference layers  32  and  42  are expressed by the products between the saturation magnetizations M 1s  and M 2s  of magnetic materials of the magnetization reference layers  32  and  42  and the thicknesses t 1  and t 2  of the magnetization reference layers  32  and  42 , respectively. When the coercivities have the relationship H c1 &gt;H c2 , a magnetic field having a field strength H large than H c1  (=M 1s ·t 1 ) is applied to the magnetization reference layers  32  and  42  in the left direction. Thus, the magnetization directions of the magnetization reference layers  32  and  42  become the left direction as seen in  FIG. 8B . Subsequently, as shown in  FIG. 8C , a magnetic field with a field strength H′ satisfying the relationship H c1 &gt;H′&gt;H c2  (=M 2s ·t 2 ) is applied to the magnetization reference layers  32  and  42 . Due to this application, the magnetization direction of the magnetization reference layer  42  is turned to the right direction as seen in  FIG. 8C , while the magnetization direction of the magnetization reference layer  32  is kept as the left direction as seen in  FIG. 8C . In this manner, the magnetization directions of the first and second magnetization reference layers  32  and  42  can be rendered antiparallel. 
     A modification of the third embodiment will now be described.  FIG. 9A  is a conceptual diagram illustrating part of a spin-injection magnetoresistance effect element of the modification.  FIG. 9B  is an enlarged conceptual diagram illustrating a first magnetization reference layer  232  and other layers in the spin-injection magnetoresistance effect element. In this modification, similarly to the second embodiment, either one of the first and second magnetization reference layers is provided with a synthetic ferri-magnetic structure (in the illustrated example, the first magnetization reference layer  232  has a synthetic ferri-magnetic structure). In this configuration, merely by applying to the first and second magnetization reference layers  232  and  42  a magnetic field that can saturate both the first and second magnetization reference layers  232  and  42 , the magnetization direction of a first ferromagnetic material layer (spin filter)  232 A of the first magnetization reference layer  232  can be made antiparallel to that of the second magnetization reference layer  42 . Note that the magnetization direction of a second ferromagnetic material layer  232 C is parallel to that of the second magnetization reference layer  42 . This modification example needs to satisfy the relationship M 1Cs ·t 1C &gt;M 1As ·t 1A , where M 1As  and t 1A  denote the saturation magnetization and thickness of the first ferromagnetic material layer (spin filter)  232 A, respectively, and M 1Cs  and t 1C  denote the saturation magnetization and thickness of the second ferromagnetic material layer  232 C, respectively. 
     The adoption of a synthetic ferri-magnetic structure can offer the same various advantages as those of the second embodiment. If the first ferromagnetic material layer (spin filter)  232 A and the second ferromagnetic material layer  232 C in the first magnetization reference layer  232  have flush end faces, an antiparallel arrangement based on magnetostatically coupling is allowed. In this case, the material of a nonmagnetic spacer layer  232 B is not limited to Ru, but any nonmagnetic material can be used. 
     Fourth Embodiment 
     The fourth embodiment is also obtained by modifying part of the first embodiment. A spin-injection magnetoresistance effect element of the fourth embodiment is different from an element of the first embodiment in that in the fourth embodiment, a first magnetization reference layer  332  has a synthetic ferri-magnetic structure (antiferromagnetic coupling) while a second magnetization reference layer  342  has a synthetic ferro-magnetic structure (ferromagnetic coupling).  FIG. 10A  is an enlarged conceptual diagram illustrating the first magnetization reference layer  332  and other layers in a spin-injection magnetoresistance effect element of the fourth embodiment.  FIG. 10B  is an enlarged conceptual diagram illustrating the second magnetization reference layer  342  and other layers. 
     The strength (polarity and strength) of RKKY interlayer exchange coupling changes in an oscillatory manner depending on the thickness of the nonmagnetic spacer layers  332 B and  342 B composed of Ru. Adjusting the thickness of the nonmagnetic spacer layer  332 B can control the coupling state between a first ferromagnetic material layer (spin filter)  332 A and a second ferromagnetic material layer  332 C in the first magnetization reference layer  332 . Adjusting the thickness of the nonmagnetic spacer layer  342 B can control the coupling state between a first ferromagnetic material layer (spin filter)  342 A and a second ferromagnetic material layer  342 C in the second magnetization reference layer  342 . Specifically, in order to obtain antiferromagnetic coupling, the nonmagnetic spacer layer composed of Ru is designed to have a thickness of 0.7 to 0.8 nm. In order to obtain ferromagnetic coupling, the nonmagnetic spacer layer composed of Ru is designed to have a thickness of 0.4 to 0.5 nm, or a thickness of one and several tenths of nanometers. Experiments can determine the more definite thicknesses. 
     Also by providing the first and second magnetization reference layers  332  and  342  with a synthetic ferri-magnetic structure (antiferromagnetic coupling) and a synthetic ferro-magnetic structure (ferromagnetic coupling), respectively, the magnetization direction of the first ferromagnetic material layer (spin filter)  332 A in the first magnetization reference layer  332  can be made antiparallel to that of the first ferromagnetic material layer (spin filter)  342 A in the second magnetization reference layer  342 . 
       FIGS. 10A and 10B  illustrate a structure in which the first and second electrodes  34  and  44  are directly connected to the first and second magnetization reference layers  332  and  342 , respectively. Such a structure can also be applied to the first to third embodiments. 
     Preferred embodiments of the present invention have been described above. The invention however is not limited to these embodiments. Various multi-layer structures, materials and so forth described in the embodiments are merely examples, and can be changed and modified adequately. Each of the embodiments employs a spin-injection magnetoresistance effect element in which a third electrode is provided as the uppermost layer. Alternatively, another spin-injection magnetoresistance effect element is also available in which the deposition order of layers is reversed and thus a third electrode is provided as the lowermost layer. Moreover, still another spin-injection magnetoresistance effect element is also available in which a third electrode is absent. Such a structure can improve the efficiency of spin-injection magnetization reversal compared with a conventional spin-injection magnetoresistance effect element shown in  FIG. 11A , for example. In the above-described embodiments, the first and second directions as the magnetization directions of the first and second magnetization reference layers, respectively, have separating vectors (i.e., the first and second directions are the left and right directions, respectively, as seen in the drawings). Alternatively, the first and second directions may have, vice verse, approaching vectors, which are relatively equivalent to the separating vectors in terms of magnetic alignment. 
     The above-described embodiments relate to a spin-injection magnetoresistance effect element applied to MRAM. In addition, a spin-injection magnetoresistance effect element according to embodiments of the invention can also be applied to current sensors and voltage sensors. 
     When a spin-injection magnetoresistance effect element of one embodiment of the invention is applied to a ternary-output current sensor for example, the first and second electrodes  34  and  44  are used as probes of the current sensor. Before current measurement, information “0” for example as a logical value is recorded in advance in the magnetization reversal layer  20  of the spin-injection magnetoresistance effect element. The amount of a current flowing between the first and second electrodes  34  and  44  (current to be detected by the current sensor) is defined as “i.” Furthermore, the writing current (i.e., the critical current of spin-injection magnetization reversal) when no voltage is applied to the third electrode  51  is defined as I′ c . The writing current (i.e., the critical current of spin-injection magnetization reversal) when a voltage is applied to the third electrode  51  is defined as I c  (&lt;I′ c ). When a current to be detected is passed between the first and second electrodes, the information stored in the magnetization reversal layer  20  changes from 0 to 1 or remains at 0, as shown in Table 1, depending on the current amount and the presence of voltage application to the third electrode  51 . The timing of voltage application to the third electrode  51 , the timing of retrieving information stored in the magnetization reversal layer  20 , and the timing of current detection are adjusted, which allows detection of the current value i. Specifically, which of three current ranges the current amount i exists in can be determined: i&lt;I c , I c ≦i&lt;I′ c , or I′ c ≦i. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 voltage application to 
                   
               
               
                   
                 third electrode 
               
             
          
           
               
                   
                 detected current i 
                 absence 
                 presence 
               
               
                   
                   
               
               
                   
                 i &lt; I c   
                 0 
                 0 
               
               
                   
                 I c  ≦ i &lt; I′ c   
                 0 
                 1 
               
               
                   
                 I′ c  ≦ i 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     When a spin-injection magnetoresistance effect element of one embodiment of the invention is applied to a binary-output voltage sensor for example, the third electrode  51  is used as a probe of the voltage sensor. Before voltage measurement, information “0” for example as a logical value is recorded in advance in the magnetization reversal layer  20  of the spin-injection magnetoresistance effect element. The voltage that is applied to the third electrode  51  and should be detected is defined as “v.” Furthermore, the writing current (i.e., the critical current of spin-injection magnetization reversal) when a voltage higher than “V 3 ” (&lt;0) is applied to the third electrode  51  is defined as I′ c . The writing current (i.e., the critical current of spin-injection magnetization reversal) when a voltage equal to or lower than “V 3 ” is applied to the third electrode  51  is defined as I c  (&lt;I′ c ). While a current I (I c &lt;I&lt;I′ c ) is passed between the first and second electrodes  34  and  44 , a voltage to be detected is applied to the third electrode  51 , and then the information stored in the magnetization reversal layer  20  is read out. If the read information is “0,” a determination can be made that the voltage v is larger than V 3 . If the read information is “1,” a determination can be made that the voltage v is equal to or lower than V 3 .