Patent Publication Number: US-2007096229-A1

Title: Magnetoresistive element and magnetic memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-315436, filed Oct. 28, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a magnetoresistive element and magnetic memory device and, for example, to a magnetoresistive element capable of recording information by supplying a current bidirectionally and a magnetic memory device using the same.  
      2. Description of the Related Art  
      There are recently proposed a number of solid-state memories that record information on the basis of a new principle. Among them all, a magnetoresistive random access memory (MRAM) using a tunneling magnetoresistive (TMR) effect is especially receiving a great deal of attention as a solid-state magnetic memory. As a characteristic feature, an MRAM stores data in accordance with the magnetization state of a magnetic tunnel junction (MTJ) element.  
      In a field-write-type MRAM, as the size of an MTJ element decreases, a coercive force Hc increases, and therefore, a current necessary for write increases. In fact, to manufacture an MRAM with a large storage capacity (256 Mbits or more), the chip size must be small. For this purpose, it is necessary to decrease the write current while suppressing size reduction of the MTJ element by increasing the cell array occupation ratio in the chip. However, the field-write-type MRAM cannot reduce the cell size for a larger capacity and is inapplicable to the manufacture of an MRAM with a large storage capacity.  
      To solve this problem, reference 1 (U.S. Pat. No. 6,256,223), reference 2 (C. Slonczewski, “Current-driven excitation of magnetic multilayers”, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, VOLUME 159, 1996, pp. L1-L7), and reference 3 (L. Berger, “Emission of spin waves by a magnetic multilayer traversed by a current”, PHYSICAL REVIEW B, VOLUME 54, NUMBER 13, 1996, pp. 9353-9358) propose a spin transfer MRAM using spin injection.  
      In a spin transfer MRAM, a magnetization switching current density Jc defines a magnetization switching current Ic. Hence, when an element area S decreases, the switching current Ic also decreases. The spin transfer MRAM is expected to have excellent scalability as compared to the field-write-type MRAM. However, the current spin transfer MRAM has a very high current density Jc on the order of 10 7  A/cm 2 .  
      In a spin transfer MRAM using a TMR film, hence, the tunnel barrier layer reaches a breakdown voltage Vbd and causes dielectric breakdown before obtaining a desired current density. Additionally, no operational reliability at a high voltage is ensured even without dielectric breakdown.  
      The switching current by spin injection is proportional to the volume of the recording layer. Hence, the magnetization switching current density is proportional to the thickness of the recording layer. As is generally known, the more the thickness increases, the larger the switching current becomes. On the other hand, to hold information recorded in the recording layer, its volume must generally be equal to or more than a desired value in consideration of the influence of heat (called thermal agitation).  
      An energy required to hold recorded information without magnetization switching by thermal agitation is defined by Ku·V=Ku·S·t (Ku is the magnetic anisotropy energy per unit volume of the recording layer, V is the volume of the recording layer, S is the area of the recording layer, and t is the thickness of the recording layer). “Ku·V” must be equal to or more than a desired value independently of the size. The magnetic anisotropy energy Ku is constant. For these reasons, when the element area decreases, the recording layer must be thick. As a result, the switching current density Jc becomes high.  
     BRIEF SUMMARY OF THE INVENTION  
      According to a first aspect of the present invention, there is provided a magnetoresistive element comprising: a magnetic recording layer which records information as a magnetization direction changes upon supplying a bidirectional current in an out-of-plane direction; a magnetic reference layer which has a fixed magnetization direction; and a nonmagnetic layer which is provided between the magnetic recording layer and the magnetic reference layer. The magnetic recording layer includes an interface magnetic layer which is provided in contact with the nonmagnetic layer and has a first magnetic anisotropy energy, and a magnetic stabilizing layer which has a second magnetic anisotropy energy higher than the first magnetic anisotropy energy.  
      According to a second aspect of the present invention, there is provided a magnetoresistive element comprising: a laminated structure including a first magnetic reference layer, a first nonmagnetic layer, a magnetic recording layer, a second nonmagnetic layer, and a second magnetic reference layer which are sequentially stacked, the first magnetic reference layer having a fixed magnetization direction, the magnetic recording layer recording information as a magnetization direction changes upon supplying a bidirectional current in an out-of-plane direction, and the second magnetic reference layer having a fixed magnetization direction. The magnetic recording layer includes a first interface magnetic layer and a second interface magnetic layer which are provided in contact with the first nonmagnetic layer and the second nonmagnetic layer and have a first magnetic anisotropy energy and a second magnetic anisotropy energy, respectively, and a magnetic stabilizing layer which is provided between the first interface magnetic layer and the second interface magnetic layer and has a third magnetic anisotropy energy higher than the first magnetic anisotropy energy and the second magnetic anisotropy energy.  
      According to a third aspect of the present invention, there is provided a magnetic memory device comprising a memory cell including the magnetoresistive element and a first electrode and second electrode which supply a current to the magnetoresistive element. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       FIG. 1  is a sectional view showing an MR element  10  according to the first embodiment;  
       FIG. 2  is a sectional view showing another arrangement example of a magnetic recording layer  13 ;  
       FIG. 3  is a sectional view showing still another arrangement example of the magnetic recording layer  13 ;  
       FIG. 4  is a sectional view showing the phase structure of a magnetic stabilizing layer  15 ;  
       FIG. 5  is a sectional view showing another example of the phase structure of the magnetic stabilizing layer  15 ;  
       FIG. 6  is a sectional view showing still another example of the phase structure of the magnetic stabilizing layer  15 ;  
       FIG. 7  is a sectional view showing another arrangement example of the MR element  10 ;  
       FIG. 8  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 9  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 10  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 11  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 12  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 13  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 14  is a sectional view showing still another arrangement example of the MR element  10 ;  
       FIG. 15  is a circuit diagram showing an MRAM according to the second embodiment; and  
       FIG. 16  is a sectional view of the MRAM shown in  FIG. 15 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The embodiments of the present invention will be described below with reference to the accompanying drawing. The same reference numerals denote element having the same functions and arrangements in the description, and a repetitive description will be done only if necessary.  
      (First Embodiment)  
       FIG. 1  illustrates the magnetoresistive element (MR element)  10  having, e.g., planar magnetization alignment. Each arrow in the drawings indicates a magnetization direction.  
      This embodiment uses spin momentum transfer. A bidirectional current in the out-of-plane direction (in the direction perpendicular to the plane) is supplied to the MR element  10  to switch the magnetization of a magnetic recording layer by the function of spin of electrons. That is, the MR element  10  is a spin transfer magnetoresistive element capable of switching magnetization by supplying spin-polarized electrons (spin injection).  
      The MR element  10  includes a magnetic recording layer (free layer)  13  having a laminated structure of an interface magnetic layer  14  and magnetic stabilizing layer  15 , a magnetic reference layer (pinned layer)  11 , and a nonmagnetic layer  12  sandwiched between the magnetic recording layer  13  and the magnetic reference layer  11 . The magnetization direction of the magnetic recording layer  13  switches. The magnetic reference layer  11  having a fixed magnetization direction is referred to in reading or writing information.  
      The direction of easy magnetization of the magnetic reference layer  11  and magnetic recording layer  13  is parallel to the film surface. The direction of easy magnetization is a direction that minimizes the internal energy of a certain ferromagnetic material with a macro size when its spontaneous magnetization turns to the direction without any external magnetic field. The direction of hard magnetization is a direction that maximizes the internal energy of a certain ferromagnetic material with a macro size when its spontaneous magnetization turns to the direction without any external magnetic field.  
      A magnetic material with a low magnetic anisotropy energy has a direction of easy magnetization depending on the shape of the magnetic material. Generally, the in-plane direction tends to be the direction of easy magnetization. A magnetic material to which induced magnetic anisotropy is applicable can decide the direction of easy magnetization to a field application direction by annealing in a magnetic field and film formation in a magnetic field. A material with a high magnetocrystalline anisotropy has the direction of easy magnetization in a crystallographically stable direction. Any magnetic material has a magnetocrystalline anisotropy which is regarded to be high at 5×10 5  erg/cm 2  or more.  
      An operation of writing information in the MR element  10  with the above-described arrangement will be explained from the physical viewpoint. In this embodiment, a current indicates a flow of electrons (e − ).  
      Phenomenologically, “1” data is written by supplying a current from the magnetic recording layer  13  to the magnetic reference layer  11 , and “0” data is written by supplying a current from the magnetic reference layer  11  to the magnetic recording layer  13 .  
      Referring to  FIG. 1 , when a current flows to the MR element  10  in the out-of-plane direction from the magnetic reference layer  11  to the magnetic recording layer  13 , spin-polarized electrons flow from the magnetic reference layer  11  to the magnetic recording layer  13  due to the spin accumulation effect in the magnetic reference layer  11 . In this case, polarized electrons with minority spin in the magnetic reference layer  11  are reflected by the magnetic reference layer  11 . Polarized electrons with majority spin in the magnetic reference layer  11  are transmitted through the magnetic reference layer  11  and enter the magnetic recording layer  13 .  
      The electrons with majority spin give a torque to the magnetic moment of the magnetic recording layer  13  and align its magnetization parallel to that of the magnetic reference layer  11 . The magnetization direction of the magnetic reference layer  11  becomes parallel to that of the magnetic recording layer  13 . The resistance value of the MR element  10  is minimum in this parallel alignment state. This state is defined as “0” data.  
      On the other hand, when a current flows to the MR element  10  in the out-of-plane direction from the magnetic recording layer  13  to the magnetic reference layer  11 , spin-polarized electrons flow from the magnetic recording layer  13  to the magnetic reference layer  11 . In this case, electrons with majority spin are transmitted through the magnetic reference layer  11 . Electrons with minority spin are reflected by the magnetic reference layer  11  and return to the magnetic recording layer  13  while keeping the spin angular momentum (without changing the spin direction).  
      The electrons with minority spin give a torque to the magnetic moment of the magnetic recording layer  13  and align its magnetization antiparallel to that of the magnetic reference layer  11 . The magnetization direction of the magnetic reference layer  11  becomes antiparallel to that of the magnetic recording layer  13 . The resistance value of the MR element  10  is maximum in this antiparallel alignment state. This state is defined as “1” data.  
      The MR element  10  can record information (“1” data and “0” data) in this way. Information is read out by supplying a read current to the MR element  10  and detecting a change in resistance value of the MR element  10 .  
      Detailed examples of the layers included in the MR element  10  will be described next. First, the arrangement of the nonmagnetic layer  12  will be explained. The nonmagnetic layer  12  can use an insulating material, a metal, or a mixed crystal thereof. An element using an insulating material is a tunneling magnetoresistive (TMR) element using a tunneling magnetoresistive effect. An element using a metal is a current perpendicular to plane (CPP)-giant magnetoresistive (GMR) element. These are collectively called magnetoresistive (MR) elements.  
      In a TMR element, a tunnel barrier layer serving as a nonmagnetic layer uses AlO x , MgO, CaO, EuO, SrO, BeO, MgO/Mg, AlO x /Al, TiO x , ZrO x , HfO x , and a laminated film thereof.  
      Especially, MgO with the highest MR ratio is preferable because it can achieve an MR ratio of 100% or more. MgO that has a very high tunneling probability for only majority spin is a representative oxide material having a spin filter effect. Hence, MgO can exhibit a TMR effect equal to or more than the spin polarizability of the magnetic recording layer  13  and magnetic reference layer  11 . MgO can achieve an MR ratio of 100% or more when the resistance and area (RA) product of the TMR element is 5 to 100 Ωμm 2 . MgO has an NaCl structure. A (001) plane orientation is most preferable from the viewpoint of MR ratio. However, a (110) or (111) plane orientation can also obtain a sufficiently high MR ratio of 100% or more.  
      Inserting an Mg layer with a thickness of 0.5 nm or less above or under the MgO layer is preferable from the viewpoint of MR ratio improvement. The MR ratio can further improve if Fe—Mg or Co—Mg bonds are dominant on the interface between the magnetic reference layer  11  and the MgO layer or on the interface between the interface magnetic layer  14  and the MgO layer. That is, an interface magnetic layer element hardly forms an oxide such as Fe—O, Co—O, Ni—O, Mn—O, or Cr—O on the interface between the magnetic reference layer  11  and the MgO layer or on the interface between the interface magnetic layer  14  and the MgO layer. To suppress deterioration of the TMR effect, the thickness of the inserted Mg layer is preferably a 3-atomic layer or less, i.e., about 0.5 nm or less.  
      The MgO layer serving as the nonmagnetic layer  12  is formed by sputtering using an MgO target or Mg target. The MgO layer may be formed by reactive sputtering in an O 2  atmosphere. The MgO layer may also be formed by forming an Mg layer and oxidizing it by oxygen radicals, oxygen ions, or ozone. The MgO layer may be epitaxially grown by molecular beam epitaxy (MBE) or electron beam evaporation using MgO.  
      In epitaxial growth, the orientation of MgO decides the orientation of a magnetic layer serving as an underlayer to be selected. The magnetic layer serving as an underlayer preferably has a bcc (body-centered cubic) structure (001), fcc (face-centered cubic) structure (111), and bcc structure (110) in correspondence with MgO (001), MgO (111), and MgO (110). The bcc structure is preferably made of Fe, Fe 100−x Co x  (0&lt;x&lt;70, at %), Co with the thickness of 1-nm or less, or a Co alloy material. The bcc structure may be made of Fe-rich Fe 100 (CoNi) 100−x  (0≦x&lt;50, at %).  
      When the magnetic reference layer  11  uses a 3-nm thick Co 40 Fe 20 B 20  (at %) layer, the nonmagnetic layer  12  uses a 1-nm thick MgO (001) layer, and the interface magnetic layer  14  uses a 1-nm thick Co 40 Fe 40 B 20  (at %) layer, a TMR element having an RA product of 10 Ωμm 2  and an MR ratio of 150% is obtained. More specifically, the TMR film structure is Ta5/Co 40 Fe 40 B 20 3/MgO0.75/Mg0.4/Co 40 Fe 40 B 20 3/Ru0.85/Co 90 Fe 10 2.5/PtMn15/Ta5//substrate.  
      To obtain an MRAM using spin injection magnetization switching, a magnetization switching current density Jc is preferably lower than 1×10 6  A/cm 2  because of the relationship between the withstand voltage and the MR ratio. Since the withstand voltage of MgO is about 1 V, the actual magnetization switching voltage must be 1 V or less. If the RA product is 2 to 100 Ωμm 2  (both inclusive), the MgO barrier film can ensure an MR ratio of 100% or more, i.e., an MR ratio that poses no problem for circuit operation. It is therefore essential that the upper value is 1 V, and the RA product is 100 Ωμm 2  or less. As a result, it is necessary to ensure the current density Jc of 1×10 6  A/cm 2 .  
      To achieve the RA product is 100 Ωμm 2  or less with the nonmagnetic layer  12  using MgO, the thickness of the MgO layer must be 1.5 nm or less. To set the RA product to 10 Ωμm 2  or less, the MgO layer must have a thickness of 1 nm or less.  
      The arrangement of the magnetic recording layer  13  will be described next. The magnetization direction of the magnetic recording layer  13  switches due to the spin injection effect or spin accumulation effect by an externally supplied current. The magnetic recording layer  13  includes the interface magnetic layer  14  and magnetic stabilizing layer  15 .  
      The interface magnetic layer  14  and magnetic stabilizing layer  15  ferromagnetically or antiferromagnetically exchange-couple with each other so that the magnetic recording layer  13  can have parallel alignment or antiparallel alignment at a portion adjacent to the nonmagnetic layer  12  with respect to the magnetic reference layer  11 . The interface magnetic layer  14  and magnetic stabilizing layer  15  exchange-couple with each other and therefore function as one magnetic layer. In the magnetic recording layer  13  shown in  FIG. 1 , the interface magnetic layer  14  and magnetic stabilizing layer  15  ferromagnetically exchange-couple with each other in a stable parallel alignment state.  
       FIGS. 2 and 3  show the magnetic recording layer  13  that is parallel to the magnetic reference layer  11  while the interface magnetic layer  14  and magnetic stabilizing layer  15  ferromagnetically and antiferromagnetically exchange-couple with each other. Referring to  FIGS. 2 and 3 , the magnetic reference layer  11  and nonmagnetic layer  12  have the same arrangements as in  FIG. 1 .  FIGS. 2 and 3  illustrate only the magnetic recording layer  13  of the MR element  10 .  
      To obtain the antiferromagnetic alignment state in  FIG. 2 , the magnetic stabilizing layer  15  uses a ferrimagnetic material. To obtain the antiferromagnetic alignment state in  FIG. 3 , a nonmagnetic layer  16  is formed on the interface magnetic layer  14 , and the magnetic stabilizing layer  15  is formed on the nonmagnetic layer  16 . In this case, the interface magnetic layer  14  and magnetic stabilizing layer  15  antiferromagnetically exchange-couple with each other through the nonmagnetic layer  16 . The nonmagnetic layer  16  can use, for example, Ru or Os.  
      As shown in  FIGS. 2 and 3 , the antiferromagnetic alignment state of the magnetic recording layer  13  cancels the saturation magnetization of the upper and lower layers. Hence, the apparent saturation magnetization amount in the residual magnetization state decreases, and the thermal stability and stability to an external magnetic field improve.  
      The magnetic recording layer  13  includes the interface magnetic layer  14  and magnetic stabilizing layer  15 . The interface magnetic layer  14  has a higher polarizability or smaller damping constant α than the magnetic stabilizing layer  15 . The polarizability and damping constant will be described later in detail. In this case, the spin torque generated by current supply to the MR element preferentially intensively acts on the interface magnetic layer  14 . More specifically, precession of magnetization of the interface magnetic layer  14  causes precession of magnetization of the entire magnetic recording layer  13 . To attain magnetization switching by causing the interface magnetic layer  14  to excite precession, design of the magnetic anisotropy energies and thicknesses of the interface magnetic layer  14  and magnetic stabilizing layer  15  is important. Material design to ensure them is also important.  
      The interface magnetic layer  14  has a lower magnetic anisotropy energy than the magnetic stabilizing layer  15 . Since the magnetic anisotropy energy of the interface magnetic layer  14  is low, its damping constant is also small. Hence, the damping constant is also smaller in the interface magnetic layer  14  than in the magnetic stabilizing layer  15 . The damping constant is obtained quantitatively by ferro-magnetic-resonance (FMR) measurement. The damping constant is expressed by “α”. The damping constant α of the interface magnetic layer  14  is preferably 0.05 or less. A material mainly containing Fe can suppress the damping constant to 0.01 or less. This is because the damping constant of Fe is as small as 0.002. On the other hand, the magnetic stabilizing layer  15  having a high magnetic anisotropy energy has a damping constant of 0.1 or more. The magnetic anisotropy energy is here regarded to be high at about 5×10 6  erg/cm 2  or more.  
      The interface magnetic layer  14  is mainly arranged to obtain the magnetoresistive effect. Hence, the interface magnetic layer  14  preferably has a high bulk polarizability of the material and a high interface polarizability to the nonmagnetic layer  12 . The interface magnetic layer  14  having a high polarizability can contribute improvement of the MR ratio. It is therefore possible to accurately read out information from the MR element  10  even when the read current decreases.  
      The thickness of the interface magnetic layer  14  needs to be 0.5 nm (inclusive) to 5 nm (exclusive). If thinner than 0.5 nm, it is impossible to obtain a sufficient material magnetic characteristic and crystallinity of the interface magnetic layer  14  and a sufficient MR ratio. At 5 nm or more, a current Ic necessary for magnetization switching largely increases, and magnetization switching may be impossible at a voltage equal to or lower than the breakdown voltage of the tunnel barrier layer.  
      Two detailed examples (1) and (2) of the material of the interface magnetic layer  14  will be described below. The interface magnetic layer  14  uses a magnetic material having a high polarizability. The polarizability of the interface magnetic layer  14  is obtained by Andrew reflection measurement or spectroscopy using X-ray magnetic circular dichroism (XMCD).  
      (1) Ferromagnetic materials containing Fe, Co, Ni, Mn, or Cr.  
      Detailed examples are a bcc-CoFe alloy or bcc-CoFeNi alloy such as Fe 50 Co 50  (at %) having a high bulk polarizability of 0.3 or more, an fcc-CoFe alloy or fcc-CoFeNi alloy such as Co 90 Fe 10  (at %) having a high polarizability, and an amorphous CoFe alloy or amorphous CoFeNi alloy such as (bcc-Co 0.5 Fe 0.5 ) 80 B20 (at %) having a high polarizability.  
      A bcc-CoFe alloy or bcc-CoFeNi alloy can adjust the damping constant to 0.01 or less by adjusting the composition. In this case, the Fe content must be 30 at % or more. However, (bcc-CoFe) 80 B 20  (at %) can achieve a damping constant of 0.01 or less when the Fe content of the Fe/Co composition ratio is 30 at % or more although the material has an amorphous structure.  
      (2) Mn-based ferromagnetic alloy, Mn-based ferromagnetic Heusler alloy, Cr-based ferromagnetic alloy, and oxide half-metal such as Fe 2 O 3 .  
      An Mn-based ferromagnetic Heusler alloy is a body-centered cubic system alloy represented by A 2 MnX having an ordered lattice. Examples of the “A” “element are Cu, Au, Pd, Ni, and Co. Examples of the “X” element are Al, In, Sn, Ga, Ge, Sb, and Si. Examples of an Mn-based ferromagnetic alloy are an MnAl alloy, MnAu alloy, MnZn alloy, MnGa alloy, MnIr alloy, MnPt 3  alloy. As a characteristic feature, these alloys have an ordered lattice. An example of a Cr-based ferromagnetic alloy is a CrPt 3  alloy which also has an ordered lattice. A half-metal indicates a ferromagnetic material in which electron spin in an electronic state at the Fermi level is 100% biased to one direction (only majority spin is present). An Mn-based ferromagnetic Heusler alloy can have a very small damping constant because it uses Mn. The damping constant of Mn as a single substance is theoretically 0.  
      The interface layer having ferromagnetism can also use FeRhX (X=Ir, Pt, or Pd) that causes magnetic transition from an antiferromagnetic state (AF) to a ferromagnetic state (FM). An FeRhX alloy has no Ms in the AF state and exhibits Ms in the FM state. The FeRhX alloy causes phase transition from the AF state to the FM state at a certain temperature. In the read mode at a low voltage, the MR element is in the AF state because the heat value is small, and write access by spin injection is impossible. In the write mode at a high voltage, the MR element changes to the FM state because the heat value increases, and write access by spin injection is possible.  
      The interface magnetic layer  14  contacts the nonmagnetic layer  12 . The interface magnetic layer  14  has a saturation magnetization Msf 1  and magnetic anisotropy energy Kaf 1 . The interface magnetic layer  14  has the magnetic stabilizing layer  15  on the surface opposite to the contact surface to the nonmagnetic layer  12 .  
      The interface magnetic layer  14  exchange-couples with the magnetic stabilizing layer  15 . The exchange coupling energy can range from 0.05 erg/cm 2  (inclusive) to 1.0 erg/cm 2  (exclusive). At an energy lower than 0.05 erg/cm 2 , magnetization rotation by spin injection magnetization switching does not occur in synchronism in the interface magnetic layer  14  and magnetic stabilizing layer  15 . That is, exchange coupling is actually lost due to, e.g., the influence of heat so that the magnetizations of the layers may rotate almost separately.  
      The magnetic stabilizing layer  15  has a saturation magnetization Msf 2  and high magnetic anisotropy energy Kaf 2 . When the magnetic recording layer  13  has the magnetic stabilizing layer  15  with the high magnetic anisotropy energy Kaf 2 , the thermal stability of the magnetic recording layer  13  improves. The magnetic anisotropy energy of the magnetic stabilizing layer  15  must be higher than that of the interface magnetic layer  14 .  
      Hence, the relationship between the magnetic anisotropy energies Kaf 1  and Kaf 2  is given by 
 
Kaf1&lt;Kaf2 
 
 The relationship between the saturation magnetizations Msf 1  and Msf 2  preferably satisfies 
 
Msf1≧Msf2 
 
      That is, an anisotropy magnetic field Ha is given by 
 
 Ha= 2 ·Ka/Ms  
 
 For this reason, an anisotropy magnetic field Hkf 1  of the interface magnetic layer  14  is smaller than an anisotropy magnetic field Hkf 2  of the magnetic stabilizing layer  15 . The anisotropy magnetic field Ha can generally be measured by an M-H curve or R-H curve in the direction of hard axis of the MR element  10 . 
 
      The product (Msf·tf) of a saturation magnetization Msf and thickness tf of the magnetic recording layer  13  is preferably 3.0×10 4  emu/cm 2  or less. This is because the present embodiment aims at a spin transfer MRAM with a large storage capacity by using the MR element  10  having a short side length of 100 nm or less, for which the magnetization switching current Ic by spin injection must be 0.1 mA or less. The restriction of the write current comes from the transistor size. When the minimum feature size (F) is 100 nm or less, it is difficult to drive a current larger than 0.1 mA.  
      From the viewpoint of saturation magnetization Ms, the saturation magnetization Msf of the magnetic recording layer  13  is preferably 600 emu/cc or less. This is based on the restriction of the switching current Ic of 0.1 mA or less.  
      In spin injection magnetization switching, the magnetic recording layer  13  has a thickness (characteristic length) for effective spin torque. This characteristic length is decided by a length (spin diffusion length) to conduct electrons while maintaining spin information and a length (decoherence length) to rotate spin and magnetization by precession almost one revolution.  
      The thickness tf of the magnetic recording layer  13  is preferably 10 nm or less because of the restriction of spin diffusion length. The thickness tf of the magnetic recording layer  13  is more preferably 5 nm or less in consideration of the restriction of precession of magnetization and the spin torque amount damping effect. Also considering the restriction of the interface magnetic layer  14  based on the above-described restriction of MR ratio, the thickness tf of the magnetic recording layer  13  satisfies 1 nm≦tf≦10 nm, and preferably, 1 nm≦tf≦5 nm. At this time, a thickness tf 2  of the magnetic stabilizing layer  15  satisfies 0.5 nm≦tf 2 ≦9.5 nm, and preferably, 0.5 nm≦tf 2 ≦4.5 nm. A magnetic stabilizing layer having a thickness of 0.5 nm less can exhibit no effective magnetic anisotropy energy.  
      The thickness ratio of the interface magnetic layer  14  to the magnetic recording layer  13  is preferably 1/20 to 1/2 (both inclusive). This is based on the fact that the thickness of the interface magnetic layer  14  to obtain a sufficient MR ratio is 0.5 nm. When the magnetic recording layer  13  is 10 nm thick, the thickness ratio is 1/20. When the magnetic recording layer  13  is 2 nm thick, the thickness ratio is 1/2. The thickness is 2 nm when taking the lower limit value of the thickness to obtain thermal stability into consideration.  
      When the magnetic recording layer has perpendicular magnetization while the interface magnetic layer has in-plane magnetization, and the magnetic stabilizing layer has perpendicular magnetization, the thickness of the interface magnetic layer is preferably 3 nm or less. In this case, Msf and Kaf of the entire magnetic recording layer must satisfy 
 
 Kaf− 4 πMsf   2 &gt;0 
 
      To obtain thermal stability and information holding stability, the magnetic anisotropy energy Kaf 2  of the magnetic stabilizing layer  15  needs to be 5×10 5  erg/cc or more. This is an empirical magnetic anisotropy energy Ka necessary for holding information recorded in the magnetic recording layer  13  for 10 years or more. Hence, the magnetic anisotropy energy Ka is preferably higher than it.  
      Especially when the magnetic anisotropy energy Kaf 1  of the interface magnetic layer  14  is low, and the anisotropy magnetic field Hkf 1  is smaller than 50 Oe, the magnetic anisotropy energy Kaf 2  of the magnetic stabilizing layer  15  is preferably 1×10 6  erg/cc or more, and the saturation magnetization Msf 2  is preferably 400 emu/cc or less. At this time, the anisotropy magnetic field Hkf 2  of the magnetic stabilizing layer  15  is Hkf 2 =2·Kaf 2 /Msf 2 =5000 Oe or more. To use perpendicular magnetization in the magnetic recording layer  13  and thermally stabilize it, the magnetic anisotropy energy Kaf 2  of the magnetic stabilizing layer  15  is preferably 1×10 6  erg/cc or more. At this time, the anisotropy magnetic field Hkf 2  is 1 kOe or more.  
      From the viewpoint of reduction of the switching current, it is preferable to set the thicknesses of the interface magnetic layer  14  and magnetic stabilizing layer  15  such that the coercive force of the magnetic recording layer  13  becomes 1 kOe or less. As described above, to form a spin transfer MRAM with a large storage capacity, an area Af of the magnetic recording layer  13  is preferably 0.005 μm 2  or less. Under these conditions, the size of the magnetic recording layer  13  is 0.1×0.05 μm 2  at an aspect ratio of 2 and almost 0.07×0.07 μm 2  at an aspect ratio of 1.  
      At this time, to prevent the switching current from varying, it is actually necessary to prevent the magnetization switching field from varying. Statistically examining, the magnetic characteristics of crystal grains do not even out unless the number of crystal grains of the magnetic recording layer  13  is at least about 100 per bit. This causes variations in magnetization switching of multiple bits when a cell array is formed.  
      Considering this, the crystal grain size is preferably 5 nm or less. From this viewpoint, at least one of the interface magnetic layer  14  and magnetic stabilizing layer  15  preferably has an amorphous phase. It is more preferable that the interface magnetic layer  14  should have an amorphous phase because control of the crystal grain is easier. It is preferable to use amorphous (CoFe) 100−x B x  (15&lt;x&lt;50, at %) or amorphous (NiFe) 100−x B x  (15&lt;x&lt;50, at %).  
      However, this does not apply to a perpendicular magnetization MR element using a high magnetocrystalline anisotropy. In the perpendicular magnetization MR element, the magnetic anisotropy almost aligns in the vertical direction. Hence, the perpendicular magnetization MR element has relatively small magnetic anisotropy dispersion as compared to an in-plane magnetization film that obtains an in-plane magnetic anisotropy by using shape magnetic anisotropy and magnetocrystalline anisotropy. Actually, in a perpendicular magnetization film having an hcp (hexagonal closest packing) structure, the crystal orientation of the (001) plane serves as an important index of magnetic anisotropy dispersion. The peak of the half-width of the locking curve in the hcp structure (001) is held at almost 5° or less. It is therefore possible to form a film having very small anisotropy dispersion, considering that magnetic anisotropy dispersion is almost equivalent to crystal orientation dispersion.  
      Three detailed examples (1) to (3) of the material of the magnetic stabilizing layer  15  will be described below.  
      (1) Ferrimagnetic materials containing at least one of Fe, Co, Ni, Mn, Cr, and rare-earth elements.  
      The rare-earth elements include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr. In this embodiment, Ho, Dy, Er, Nd, Gd, Sm, Tb, and Eu are particularly effective. A ferrimagnetic material containing a rare-earth element has an amorphous structure. The ferrimagnetic material can have a low saturation magnetization of 400 emu/cc or less and a high magnetic anisotropy energy of about 1×10 6  erg/cc by adjusting the composition. Some amorphous alloys containing a rare-earth element having a 3d element and 4f electron exhibit ferrimagnetism. These alloys readily cause perpendicular magnetization and are usable as a perpendicular magnetization film. Examples of amorphous materials having ferrimagnetism are CoFe—Tb and CoFe—Gd. CoFe—Tb has a high magnetic anisotropy energy and large spin-orbit interaction, the damping constant α is as large as 0.1 or more. However, an addition of Gd, Ho, or Dy can decrease the damping constant α.  
      A ferrimagnetic material has a composition point (composition compensation point) where the net Ms is 0 and can easily reduce Ms. A CoFe-RE (RE: rare-earth) alloy has a compensation point in the RE composition range of 15 to 40 at %. Since Ms 2  influences the magnetization switching current, Ms reduction is preferable for current reduction.  
      (2) Ferromagnetic materials containing at least one of Fe, Co, Ni, Mn, and Cr and at least one element selected from Pt, Pd, Ir, Rh, Re, Os, Au, Ag, Cu, B, C, Si, Al, Mg, Ta, Cr, Zr, Ti, V, Hf, Y, Sr, Ba, Sc, Ca, and rare-earth elements.  
      The rare-earth elements include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr. In this embodiment, Ho, Dy, Er, Nd, Gd, Sm, Tb, and Eu are particularly effective.  
      Representative materials of the magnetic stabilizing layer  15  are a CoCrPt alloy, CoCrTa alloy, and CoCrPtTa alloy having an hcp structure. The materials can have a magnetic anisotropy energy of 1×10 6  erg/cc or more.  
      In terms of a high magnetic anisotropy energy, an ordered Fe 50±10 Pt 50±10  (at %) alloy having an L 1   0  structure is preferable. FePt changes from an fcc structure to an fct structure after ordering. Since the axis of anisotropy runs along the [001] direction, the priority plane orientation is preferably (001). In this case, a FePt alloy has perpendicular magnetization.  
      (3) Ferromagnetic materials containing mixed crystal of metal magnetic phase and insulating phase.  
      The metal magnetic phase of the magnetic stabilizing layer  15  is made of a ferromagnetic material containing at least one of Fe, Co, Ni, Mn, and Cr and at least one of Pt, Pd, Ir, Rh, Re, Os, Au, Ag, Cu, Ta, and rare-earth elements. The insulating phase of the magnetic stabilizing layer  15  is made of an oxide, nitride, or oxynitride containing at least one element selected from B, C, Si, Al, Mg, Ta, Cr, Zr, Ti, V, Hf, Y, Sr, Ba, Sc, Ca, and rare-earth elements. The rare-earth elements include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr. In this embodiment, Ho, Dy, Er, Nd, Gd, Sm, Tb, and Eu are particularly effective.  
      FIGS.  4  to  6  are sectional views for explaining the phase structure of the magnetic stabilizing layer  15 . As described above, the magnetic stabilizing layer  15  is made of a ferromagnetic material containing a mixed crystal of a metal magnetic phase  17  and an insulating phase  18 .  
      In  FIG. 4 , the magnetic stabilizing layer  15  is separated into an insulating phases  18  and a plurality of metal magnetic phases  17  grown to a columnar shape and having a high magnetic anisotropy energy. Since a current concentrates to the metal magnetic phases  17 , the current density increases, and the substantial switching current becomes small. The magnetic stabilizing layers  15  shown in  FIGS. 15 and 16  also have the same effect. The magnetic stabilizing layer  15  shown in  FIG. 5  has a granular structure with a plurality of grain-shaped metal magnetic phases  17  in an insulating phase  18 . The magnetic stabilizing layer  15  shown in  FIG. 6  has an island grown structure having, in an insulating phase  18 , a plurality of metal magnetic phases  17  grown upward from the interface magnetic layer  14 .  
      Referring to  FIGS. 5 and 6 , since the current flows through a path with the smallest tunnel barrier, a current constriction effect is obtained, as in  FIG. 4 . In addition, electrons reflected by elastic scattering between the insulating phase  18  and the interface magnetic layer  14  and between the insulating phase  18  and metal magnetic phase  17  assist magnetization switching.  
      The ratio of the metal magnetic phase  17  to the insulating phase  18  depends on the volume and the degree of current constriction of the magnetic stabilizing layer  15  necessary for obtaining thermal agitation resistance. To sufficiently enhance the current constriction effect, the ratio of the metal magnetic phase  17  to the insulating phase  18  is preferably 0.5 or less. This is equivalent to an area ratio of 50% or less. Hence, the current density can be twice or more. As a result, it is possible to design a device capable of obtaining a current density enough for switching the portion having a high magnetic anisotropy energy.  
      The above-described columnar crystal grain, granular crystal grain, and island grown crystal grain have an appropriate grain size dispersion. When the short side length of the TMR element is 100 nm or less, microcrystallization is necessary. If 100 grains must be present in the TMR element to even out a variation between elements, the crystal grain must be about 1/10 the short side length of the element. If the short side length of the element is 100 nm, the crystal grain must be 10 nm or less. If the short side length of the element is 70 nm, the crystal grain must be 7 nm or less. If the short side length of the element is 45 nm, the crystal grain must be about 5 nm or less.  
      Consider a case wherein exchange coupling between crystal grains is not completely lost. Even small crystal grains exist which have a low apparent magnetocrystalline anisotropy energy. Hence, these crystal grains form a region that readily switches due to spin injection. Upon spin injection or magnetic field application, the crystal grains serve as a nucleus, and the magnetic recording layer causes magnetization switching at a small current or magnetic field.  
      On the other hand, since the crystal grains exchange-couple, crystal grains having a high magnetocrystalline anisotropy energy decide the thermal stability of the magnetic recording layer. Hence, the thermal stability is high. Generally, if such a phenomenon occurs, a coercive force Hcf, anisotropy magnetic field Hkf (or saturation magnetic field Hsf), the anisotropy energy Kaf, and the saturation magnetization Msf of the magnetic recording layer  13  of the TMR element satisfy 
 
 Hcf&lt; 2 ·Kaf/Msf  
 
 Hcf&lt;Hsf  (or  Hkf ) 
 
 as is empirically found. When the above relationships hold, the magnetic recording layer can do spin injection magnetization switching at a low current density. 
 
      The above-described columnar crystal structure, granular crystal structure, and island grown crystal structure can easily disperse the magnetocrystalline anisotropy energy of each crystal grain. Generally, no spin-transfer torque acts when the relative angle of magnetization is 0° and 180°. To cause magnetization switching, it is necessary to thermally activate the magnetic recording layer by using heat generated by supplying a large current. Hence, spin injection magnetization switching requires a large current.  
      However, if the magnetic recording layer has an appropriate magnetocrystalline anisotropy energy dispersion, spin injection magnetization switching occurs at a low current. The degree of magnetocrystalline anisotropy energy dispersion is in close relation to crystal orientation dispersion. When the crystal orientation dispersion is 5° to 45° (both inclusive), spin injection magnetization switching occurs at a low current as compared to a case wherein dispersion rarely exists. The crystal orientation dispersion is more preferably 5° to 15° (both inclusive).  
      In a Co alloy having an hcp structure or an FePt alloy having an fct (face-centered tetragonal) structure, spin injection magnetization switching can occur at a low current when the C-axis orientation or (001) plane orientation is controlled in the above range. The crystal orientation dispersion is effective in a spin transfer MR element including a magnetic recording layer and magnetic reference layer with an in-plane magnetic anisotropy or perpendicular magnetic anisotropy.  
      The magnetic reference layer  11  will be described next. The magnetic reference layer  11  having a uniaxial magnetic anisotropy or unidirectional magnetic anisotropy stabilizes in a predetermined magnetization direction. The magnetic reference layer  11  includes an interface magnetic layer made of a material with a high bulk polarizability and high surface polarizability on the interface to the nonmagnetic layer  12 .  
      The magnetic reference layer  11  includes a laminated film of an interface magnetic layer that has a saturation magnetization Msp 1  and magnetic anisotropy energy Kap 1  and is in contact with the nonmagnetic layer  12 , and a magnetic stabilizing layer that has a saturation magnetization Msp 2  and magnetic anisotropy energy Kap 2 . The magnetic anisotropy energies Kap 1  and Kap 2  are preferably 1×10 6  erg/cm 2  or more.  
      A product (Mps·tp) of a saturation magnetization Msp and thickness tp of the magnetic reference layer  11  is preferably larger than the product (Mpf·tf) of the saturation magnetization Msf and thickness tf of the magnetic recording layer  13 .  
      Other detailed arrangement examples of the spin transfer MR element  10  will be described next with reference to FIGS.  7  to  14 .  
      Referring to  FIG. 7 , a magnetic reference layer  20  includes the magnetic reference layer  11  and an antiferromagnetic layer  21 . The magnetization of the magnetic reference layer  20  is fixed in one direction by using exchange coupling of the antiferromagnetic layer  21  and ferromagnetic layer (magnetic reference layer  11 ). This layer will be called a magnetization fixing monolayer.  
      Referring to  FIG. 8 , the magnetic reference layer  20  has a synthetic antiferromagnetic (SAF) structure including the magnetic reference layer  11 , nonmagnetic layer  23 , magnetization fixing layer  22 , and antiferromagnetic layer  21 . The order of the layers included in the magnetic reference layer  20  is the order from the upper side. This also applies to the following description of a laminated structure.  
      The SAF structure is formed by stacking two ferromagnetic layers with reverse magnetization directions while inserting a nonmagnetic layer therebetween. In the SAF structure, the magnetic fields of the two ferromagnetic layers form a loop. Hence, the magnetic fields do not leak and influence the peripheral cells. In addition, the exchange-coupled ferromagnetic layers have an improved thermal agitation resistance as an effect of the increased volume.  
      Referring to  FIG. 8 , the magnetization fixing layer  22  exchange-couples with the antiferromagnetic layer  21  and therefore has a magnetization fixed in one direction. The magnetic reference layer  11  antiferromagnetically exchange-couples with the magnetization fixing layer  22  and therefore has a magnetization fixed in one direction.  
      Referring to  FIG. 9 , the magnetic reference layer  20  has a laminated structure including the magnetic reference layer  11 , nonmagnetic layer  23 , intermediate magnetic layer  25 , nonmagnetic layer  24 , magnetization fixing layer  22 , and antiferromagnetic layer  21 . The magnetic reference layer  20  with this structure has an improved thermal stability. That is, the sum of Ku·V (product of magnetic anisotropy energy and volume) of magnetic layers on the upper and lower sides of a nonmagnetic layer decides the thermal stability of an SAF structure. Hence, the thermal stability further improves in use of three magnetic layers and two nonmagnetic layers.  
      Referring to  FIG. 10 , the magnetic recording layer  13  has a laminated structure of a second magnetic stabilizing layer  32 , nonmagnetic layer  31 , first magnetic stabilizing layer  15 , and interface magnetic layer  14 . Of this structure, a magnetic stabilizing layer  30  has an SAF structure including the second magnetic stabilizing layer  32 , nonmagnetic layer  31 , and first magnetic stabilizing layer  15 . The magnetic recording layer  13  has a laminated structure of the magnetic stabilizing layer  30  and interface magnetic layer  14 .  
      Referring to  FIG. 10 , the sum of Ku·V (product of magnetic anisotropy energy and volume) of magnetic layers on the upper and lower sides of a nonmagnetic layer decides the thermal stability of the magnetic recording layer  13 . Ku·V also defines the thermal stability of a magnetic recording layer having no SAF structure. In an SAF structure, magnetic layers on the upper and lower sides of a nonmagnetic layer magnetostatically couple with each other. This cancels a magnetization with a reverse sign at the ends of the magnetic layers so that end magnetic domains formed at the ends of the magnetic layers disappear. As a result, the magnetic layers on the upper and lower sides of the nonmagnetic layer integrate, and the thermal stability of the magnetic recording layer  13  improves.  
      Since an external magnetic field resistance caused by the end magnetic domains and the effect of demagnetizing fields at the ends of the magnetic layers decrease, the magnetic and thermal stability of the entire magnetic recording layer improve. In this case, Mr·t (product of residual magnetization and thickness) of the magnetic layers formed on the upper and lower sides of the nonmagnetic layer are adjusted so that the absolute values of magnetization almost equal. Ideally, the absolute values preferably almost equal but actually have a slight shift in consideration of a problem of fabrication. However, a shift of Mr·t influences the magnetic field distribution. Hence, the shift amount is preferably 1 nmT (nanometer tesla) or less in terms of Mr·t.  
      Referring to  FIG. 11 , the magnetic recording layer  13  has an SAF structure including the second magnetic stabilizing layer  32 , nonmagnetic layer  31 , coupled magnetic layer  33 , first magnetic stabilizing layer  15 , and interface magnetic layer  14 . The coupled magnetic layer  33  assists antiferromagnetic exchange coupling between the first magnetic stabilizing layer  15  and the second magnetic stabilizing layer  32  through the nonmagnetic layer  31 . Insertion of the coupled magnetic layer  33  strengthens the antiferromagnetic coupling of the second magnetic stabilizing layer  32 , nonmagnetic layer  31 , and coupled magnetic layer  33 . This can completely integrate the motions of magnetizations of the magnetic layers on the upper and lower sides of the nonmagnetic layer  31 , i.e., the magnetization switching behaviors in the magnetic recording layer  13 , resulting in an improved thermal stability. Hence, the coupled magnetic layer  33  is made of, e.g., a CoFe alloy that firmly couples with Ru or Os.  
      Use of the magnetic recording layer having the SAF structure with antiferromagnetic exchange coupling enables to apparently reduce the residual magnetization amount, i.e., the product Mr·t of a residual magnetization Mr and thickness t of the magnetic recording layer  13 . This ensures the thermal stability and improves the external magnetic field resistance. Referring to  FIG. 2 , the apparent saturation magnetization amount Mr·t in the residual magnetization state is given by 
 
 Mr·t=|Mrf 1 ·tf 1 −Mrf 2 ·tf 2|
 
 where tf 1  is the thickness of the interface magnetic layer  14 , tf 2  is the thickness of the magnetic stabilizing layer  15 , Mrf 1  is the residual magnetization of the interface magnetic layer  14 , and Mrf 2  is the residual magnetization of the magnetic stabilizing layer  15 . 
 
      The MR element  10  shown in  FIG. 12  has a so-called dual-pin structure having two magnetic reference layers  20  and  40  having magnetizations fixed in different directions. The magnetic reference layer  40  has a laminated structure of an antiferromagnetic layer  46 , magnetization fixing layer  45 , nonmagnetic layer  44 , intermediate magnetic layer  43 , nonmagnetic layer  42 , and magnetic reference layer  41 . The magnetic recording layer  13  has a laminated structure of a second interface magnetic layer  14 B, magnetic stabilizing layer  15 , and first interface magnetic layer  14 A. A first nonmagnetic layer  12 A is inserted between the magnetic recording layer  13  and the magnetic reference layer  20 . A second nonmagnetic layer  12 B is inserted between the magnetic recording layer  13  and the magnetic reference layer  40 .  
      In this arrangement, supply of a current in the out-of-plane direction reduces the switching current because the spin injection effect and spin accumulation effect are available simultaneously. In the dual-pin structure, the magnetic reference layers  20  and  40  have magnetizations in reverse directions. For this reason, the current density necessary for magnetization switching of the magnetic recording layer  13  does not depend on the current direction, and “0” data and “1” data can be written with the same current value. Hence, the write circuit can be simple.  
      Even the MR element  10  having the dual-pin structure sets the magnetic anisotropy energy of the magnetic stabilizing layer  15  to be larger than that of the first interface magnetic layer  14 A. The MR element  10  also sets the damping constant of the magnetic stabilizing layer  15  to be larger than that of the first interface magnetic layer  14 A. The damping constant of the magnetic stabilizing layer  15  is 0.1 or more. The damping constant of the first interface magnetic layer  14 A is preferably 0.05 or less. Similarly, the MR element  10  sets the magnetic anisotropy energy of the magnetic stabilizing layer  15  to be larger than that of the second interface magnetic layer  14 B. The MR element  10  also sets the damping constant of the magnetic stabilizing layer  15  to be larger than that of the second interface magnetic layer  14 B. The damping constant of the second interface magnetic layer  14 B is preferably 0.05 or less. The magnetic anisotropy energy of the first interface magnetic layer  14 A may equal to or different from that of the second interface magnetic layer  14 B as long as they satisfy the above-described conditions. This also applies to the MR elements  10  with a dual-pin structure to be described later.  
      Each of the MR elements  10  shown in  FIGS. 13 and 14  has the two magnetic reference layers  20  and  40  with magnetizations fixed in the same direction. Each of the magnetic reference layers  20  and  40  has an SAF structure. The magnetic recording layer  13  also has an SAF structure. In  FIG. 13 , the structure including the coupled magnetic layer  33 , magnetic stabilizing layer  15 , and first interface magnetic layer  14 A is defined as a first magnetic recording layer. The second interface magnetic layer  14 B is defined as a second magnetic recording layer. Similarly in  FIG. 14 , the structure including a first coupled magnetic layer  33 A, first magnetic stabilizing layer  15 A, and first interface magnetic layer  14 A is defined as a first magnetic recording layer. The structure including the second interface magnetic layer  14 B, second magnetic stabilizing layer  15 B, and second coupled magnetic layer  33 B is defined as a second magnetic recording layer.  
      In these arrangements, the direction of magnetic field application in annealing uniquely decides the magnetization directions of the magnetization fixing layers  22  and  45  in the same direction. This also decides the magnetization directions of the magnetic reference layers  11  and  41  in the same direction.  
      On the other hand, the magnetic recording layer  13  also has the SAF structure, though the magnetic layers on the upper and lower sides of the nonmagnetic layer  31  have antiparallel magnetizations. Hence, if the magnetization direction of the first magnetic recording layer is parallel to that of the magnetic reference layer  20 , the magnetization direction of the second magnetic recording layer is antiparallel to that of the magnetic reference layer  40 . That is, use of the magnetic recording layer  13  with the SAF structure forms two, antiparallel and parallel states as in  FIG. 12  with respect to one magnetization direction of the dual-pin layer. Hence, supply of a current in the out-of-plane direction reduces the switching current because the spin injection effect and spin accumulation effect are available simultaneously.  
     EXAMPLES  
      A plurality of examples of the MR element  10  will be described below. First, the size and manufacturing method of the MR element  10  used as examples will be described.  
      An MR element  10  is formed between a lower electrode layer and an upper electrode layer. More specifically, an MTJ film is formed on the lower electrode layer by, e.g., DC magnetron sputtering. The lower electrode layer uses, e.g., Ta. The MTJ film is patterned to a size of about 0.1×0.15 μm 2  by photolithography using an excimer laser. At this time, a magnetic recording layer  13  has an aspect ratio (long axis/short axis) of 1.5. Then, the MR element  10  is fabricated by ion beam etching (IBE).  
      An interlayer insulating layer is formed next. The interlayer insulating layer is planarized by chemical mechanical polishing (CMP) to expose the upper surface of the MR element  10 . An upper electrode layer is formed on the MR element  10 . The upper electrode layer uses, e.g., Ta. Barrier formation conditions are adjusted such that the MR element  10  has a resistance R=5 kΩ in terms of element resistance. The RA product of the MR element  10  is set to about 15 Ωμm 2 .  
      The electric characteristic and magnetization switching characteristic of a thus formed MRAM were evaluated. Table 1 shows the layer structures of MR elements  10  of Comparative Example and Examples 1 to 8. A numerical value added to each layer represents a thickness. The unit of thickness is nm.  
                   TABLE 1                          Comparative Example   Ta5/NiFe6/CoFe0.5/AlO x 0.5/CoFe2.2/           Ru1/CoFe2.4/PtMn15/Ta5       Example 1   Ta5/CoCrPt2.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 2   Ta5/CoCrPt4.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 3   Ta5/CoCrPt9.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 4   Ta5/CoCrPt19.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 5   Ta5/Cu/CoCrTa4.5/a-FeCo0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 6   Ta5/Cu/CoCrPtTa—SiO 2 4.5/a-FeCoB0.5/           MgO0.8/a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 7   Ta5/Pt5/FeCoTb4.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5       Example 8   Ta5/Pt5/FeCoGd4.5/a-FeCoB0.5/MgO0.8/           a-FeCoB2.2/Ru1/FeCo2.5/PtMn15/Ta5                  
 
      In Table 1, the MR elements  10  of Examples 1 to 8 have the laminated structure shown in  FIG. 8 . In Table 1, a Ta layer corresponds to the upper or lower electrode layer. An a-FeCoB layer corresponding to an interface magnetic layer  14  or magnetic reference layer  11  is an amorphous layer with a composition of Fe 40 Co 40 B 20  (at %). The MR elements  10  underwent annealing at 380° C. MgO corresponding to a nonmagnetic layer  12  had a (001) plane orientation after this annealing.  
      The composition of CoCrPt corresponding to a magnetic stabilizing layer  15  was Co 72 Cr 20 Pt 8  (at %). The composition of CoCrTa was Co 76 Cr 20 Ta 4  (at %). The compositions of FeCoTb and FeCoGd were Fe 40 Co 40 Tb 20  (at %) and Fe 40 Co 40 Gd 20  (at %), respectively.  
      A switching current density Jc and MR ratio of each of the MR elements  10  were measured. Table 2 shows the measurement result. Jc(P-to-AP) in Table 2 indicates the switching current density Jc when the magnetization directions of the magnetic reference layer  11  and magnetic recording layer  13  change from the parallel state (P) to the antiparallel state (AP). On the other hand, Jc(AP-to-P) in Table 2 indicates the switching current density Jc when the magnetization directions of the magnetic reference layer  11  and magnetic recording layer  13  change from the antiparallel state (AP) to the parallel state (P).  
                               TABLE 2                                   Jc(P-to-AP)   Jc(AP-to-P)   MR ratio           (10 7  A/cm 2 )   (10 7  A/cm 2 )   (%)                                                            Comparative   2.5   1.2   41           Example           Example 1   0.092   0.048   153           Example 2   0.097   0.050   155           Example 3   0.097   0.050   150           Example 4   0.210   0.120   148           Example 5   0.097   0.050   157           Example 6   0.080   0.043   160           Example 7   0.091   0.050   148           Example 8   0.088   0.051   145                      
 
      As shown in Table 2, the switching current density Jc of each example largely decreases as compared to a comparative example. The MR ratio of each example also greatly improves.  
      As described above in detail, according to this embodiment, since the magnetic recording layer  13  has the magnetic stabilizing layer  15  with a high magnetic anisotropy energy, the thermal stability of the magnetic recording layer  13  can stabilize. In addition, the switching current density can largely decrease as the actual thickness decreases without lowering the thermal agitation resistance.  
      The magnetic recording layer  13  has the interface magnetic layer  14  with a high polarizability. The interface magnetic layer  14  with the high polarizability can contribute to improvement of the MR ratio of the MR element  10 . Hence, even when the read current is small, it is possible to accurately read out information from the MR element  10 .  
      In this embodiment, the layers included in the MR element  10  have in-plane magnetization alignment. However, the present invention is not limited to this. The layers may have perpendicular magnetization alignment. When the magnetocrystalline anisotropy dispersion of the magnetic stabilizing layer  15  is large, and the magnetocrystalline anisotropy in the 
          axis of a Co alloy having, e.g., a hcp structure is used as a magnetic anisotropy, the magnetic recording layer  13  forms a single magnetic domain to improve the spin injection efficiency in use of perpendicular magnetization alignment. Hence, the substantial switching current density can be reduced. 
 
 (Second Embodiment) 
       

      In the second embodiment, an MRAM is formed by using the MR element  10  described above.  
      As shown in  FIG. 15 , the MRAM shown  FIG. 1  comprises a memory cell array  50  having a plurality of memory cells MC arranged in a matrix. In the memory cell array  50 , a plurality of bit lines BL are arranged. The bit lines BL extend the column direction. In the memory cell array  50 , a plurality of word lines WL are arranged. The word lines WL extend the row direction.  
      The intersections of the bit lines BL and word lines WL have the above-described memory cells MC. Each memory cell MC includes the MR element  10  and a select transistor  51 . One terminal of each MR element  10  connects to the bit line BL. The other terminal of the MR element  10  connects to the drain of the select transistor  51 . The word line WL connects to the gate of the select transistor  51 . The source of the select transistor  51  connects to a source line SL.  
      A power supply circuit  53  connects to one end of the bit line BL. A sense amplifier circuit  54  connects to the other end of the bit line BL. A power supply circuit  52  connects to one end of the source line SL. A power supply  55  connects to the other end of the source line SL through a switching element (not shown).  
      The power supply circuit  53  applies a positive potential to one end of the bit line BL. The sense amplifier circuit  54  detects the resistance value of the MR element  10  and also applies, e.g., a ground potential to the other end of the bit line BL. The power supply circuit  52  applies a positive potential to one end of the source line SL. The power supply  55  turns on the switching element connected to it, thereby applying, e.g., a ground potential to the other end of the source line SL. Each power supply circuit includes a switching element to control electrical connection to a corresponding wiring layer.  
      Data write in the memory cell MC is done in the following way. First, to select the memory cell MC as a data write target, the word line WL connected to the memory cell MC is activated. This turns on the select transistor  51 .  
      A bidirectional write current Iw is supplied to the MR element  10 . More specifically, to supply the write current Iw to the MR element  10  from the upper side to the lower side, the power supply circuit  53  applies a positive potential to one end of the bit line BL. The power supply  55  turns on a switching element corresponding to it to apply a ground potential to the other end of the source line SL.  
      To supply the write current Iw to the MR element  10  from the lower side to the upper side, the power supply circuit  52  applies a positive potential to one end of the source line SL. The sense amplifier circuit  54  applies a ground potential to the other end of the bit line BL. The switching element corresponding to the power supply  55  is OFF. In this way, “0” data or “1” data is written in the memory cell MC.  
      Data read from the memory cell MC is done in the following way. First, the memory cell MC is selected. The power supply circuit  52  and sense amplifier circuit  54  supply, to the MR element  10 , a read current Ir flowing from the power supply circuit  52  to the sense amplifier circuit  54 . The sense amplifier circuit  54  detects the resistance value of the MR element  10  on the basis of the read current Ir. In this way, information stored in the MR element  10  can be read out.  
      The structure of the MRAM will be described next.  FIG. 16  is a sectional view of the MRAM.  FIG. 16  shows a portion of the MRAM corresponding to one memory cell MC.  
      The select transistor  51  serving as a switching element is formed on a p-type semiconductor substrate  61  (or a p-type well provided in a substrate). The p-type semiconductor substrate  61  has a shallow trench isolation (STI)  62  to electrically disconnect the select transistor  51  from neighboring elements.  
      The select transistor  51  includes, e.g., an NMOS transistor. More specifically, a gate insulating film  51 A is formed on the semiconductor substrate  61 . A gate electrode  51 B is provided on the gate insulating film  51 A. The gate electrode  51 B corresponds to the word line WL shown in  FIG. 15 . A source region  51 C and a drain region  51 D heavily doped with an N + -type impurity are provided on both sides of the gate electrode  51 B in the semiconductor substrate  61 .  
      A wiring layer  64  is formed on a contact plug  63  on the source region SiC. The wiring layer  64  corresponds to the source line SL shown in  FIG. 15 . A wiring layer  66  is formed on a contact plug  65  on the drain region  51 D. The wiring layer  66  electrically connects the MR element  10  to the drain region  51 D.  
      A lower electrode layer  67  is provided on the wiring layer  66 . The lower electrode layer  67  uses, e.g., Ta. The MR element  10  is provided on the lower electrode layer  67 . An upper electrode layer  68  is provided on the MR element  10 . The upper electrode layer  68  uses, e.g., Ta.  
      A wiring layer  69  is provided on the upper electrode layer  68 . The wiring layer  69  corresponds to the bit line BL shown in  FIG. 15 . An interlayer insulating layer  70  fills the space between the semiconductor substrate  61  and the wiring layer  69 .  
      As described above, a spin transfer MRAM can be formed by using the MR element  10  of the first embodiment. We confirmed the operation of the spin transfer MRAM shown in  FIG. 15  and that a current drivable by the transistor could cause magnetization switching in the MR element  10 . A bit yield of 99.9% or more was obtained.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.