Patent Publication Number: US-8530987-B2

Title: Magnetic memory including a magnetoresistive element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of PCT Application No. PCT/JP2009/066775, filed Sep. 28, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a magnetic memory. 
     BACKGROUND 
     In recent years, an MRAM (Magnetoresistive Random Access Memory) using an MTJ (Magnetic Tunnel Junction) element has received a great deal of attention. It can operate at a high speed and is therefore usable as a working memory, like a DRAM. In addition, the MRAM is highly advantageous in making a system power-thrifty and faster because it is a nonvolatile memory. 
     The MTJ element includes a reference layer made of a ferromagnetic material having a predetermined magnetization direction, a storage layer made of a ferromagnetic material having a variable magnetization direction, and a spacer layer sandwiched between them. The tunnel resistance between the reference layer and the storage layer takes a high resistance state or a low resistance state depending on the relative magnetization arrangement of the two ferromagnetic layers. For example, pieces of information (data) “0” and “1” are associated with the resistance states, respectively, thereby storing the information. Normally, the low resistance state is set when the magnetization arrangement is parallel, and the high resistance state is set when the magnetization arrangement is antiparallel. Note that pieces of information (data) “1” and “0” may be associated with the low resistance state and the high resistance state, respectively. 
     In general, an MRAM (to be also referred to as a magnetic memory hereinafter) includes at least one memory cell. The memory cell includes an MTJ element serving as a memory element, and a select transistor configured to select the MTJ element. 
     As a data write method, there exists a spin transfer torque method which changes the magnetization direction of the storage layer by a spin transfer torque. Magnetization reversal is caused by supplying a current with a predetermined value or more to the MTJ element. The current flowing direction is changed to change the storage layer and the reference layer from the parallel magnetization state to the antiparallel state or from the antiparallel state to the parallel state, thereby writing data. This MRAM is particularly called a spin transfer torque MRAM. To make the spin transfer torque MRAM more competitive than other memories such as a flash memory and a DRAM, it is necessary to reduce the area of one memory cell and thus reduce the area of the entire chip. To do this, it is most important to reduce the write current value. Generally, when the write current value decreases, the memory holding time shortens. That is, they have a tradeoff relationship. The memory holding time is largely affected by the thermal stability of the storage layer. Hence, reducing the write current while maintaining the thermal stability of the storage layer is the key to the technique. 
     In addition, to make the spin transfer torque MRAM have superiority over other memories, not only the above-described reduction of the write current but also suppressing the write, read, and memory holding error rates are very important. 
     Magnetization reversal caused by a spin transfer torque is a Poisson process that occurs with the aid of thermal energy of room temperature, that is, phonons. This is substantially a probabilistic phenomenon. A write current I c  of magnetization reversal has an amount that substantially fluctuates in each attempt due to the influence of phonons. The fluctuation width depends on the magnetization reversal process or the pulse width of a given current. The probability of magnetization reversal of the MTJ element caused by the spin transfer torque can supposedly be expressed as a simple thermal activation process represented by 
                     P   ⁡     (   τ   )       =     1   -     exp   ⁡     [         -   τ     /     τ   0       ⁢   exp   ⁢     {       -       Δ   ⁢           ⁢     E   a           k   B     ⁢   T         ⁢     (     1   -     I     I     c   ⁢           ⁢   0           )       }       ]                 (   1   )               
where τ (sec) is the pulse width of the current, τ 0  is an amount called an attempt time which is normally 1 (sec), ΔE a  is the anisotropy energy of the storage layer, whose ratio ΔE a /k B T to energy k B T of the phonons represents the thermal stability of the storage layer, I (A) is the magnitude of the supplied current, and I c0  (A) is the reversal current at absolute zero.
 
     In data read, data is discriminated using the resistance difference between the high resistance state and the low resistance state of the MTJ element. Hence, in the data read as well, the current (read current) is directly supplied to the MTJ element, as in the data write. In the spin transfer torque MRAM, the read current is normally set to be much smaller than the write current. However, as is apparent from equation (1), the reversal threshold fluctuates in each read attempt. For this reason, in the read performed, for example, 10 15  times, the probability that the magnetization reversal erroneously occurs cannot be neglected even if a small read current is supplied. To suppress the read error, it is necessary to suppress the fluctuation of the threshold. 
     The threshold of the write current also varies in a similar manner. Hence, the set value of the write current needs to be larger than an average threshold. It is therefore necessary to suppress the fluctuation of the threshold to make the write current smaller and prevent write errors. 
     As can be seen from equation (1), the fluctuation of the reversal threshold becomes smaller as the anisotropy energy ΔE a  of the storage layer increases. The memory holding time becomes longer as the anisotropy energy ΔE a  of the storage layer increases. Normally, the anisotropy energy of the storage layer and a write current I c0  at absolute zero are known to have a positive correlation. In particular, they have a proportional relationship when the storage layer has a perpendicular magnetic anisotropy. Hence, when the anisotropy energy of the storage layer is simply increased, the write current becomes large, although the fluctuation of the reversal threshold can be made smaller. To reduce the error rates and reduce the write current, reducing the write current while maintaining the thermal stability of the storage layer is a very important technical challenge. 
     As described above, since the magnetization reversal of a magnetic body is a probabilistic phenomenon, read errors, write errors, memory holding errors, and the like occur. It is possible to provide, in the nonvolatile memory, a circuit that corrects errors even if the above-described errors have occurred in a small amount. This processing is done using a method called ECC (Error Correction Coding). However, to prevent the operation errors of the MRAM, it is important to understand the probabilistic phenomenon of magnetization reversal of a magnetic body and control it to reduce error rates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing the result of numerical simulations of the reversal probability in spin transfer torque reversal. 
         FIG. 2  is a sectional view showing a magnetoresistive element used in the first embodiment. 
         FIG. 3  is a graph showing the dependence on the exchange coupling constant between magnetic layers concerning a write current. 
         FIG. 4  is a graph showing the elapse of time of magnetization Mz when Jex=10 erg/cm 2  and when Jex=2 erg/cm 2 . 
         FIG. 5  is a graph showing the external magnetic field-resistance curve of an MTJ element. 
         FIG. 6  is a sectional view showing a magnetoresistive element used in a modification of the first embodiment. 
         FIG. 7  is a sectional view showing a magnetoresistive element used in the second embodiment. 
         FIG. 8  is a graph showing the result of LLG simulations for proving reduction of the spin transfer torque reversal current. 
         FIG. 9  is a graph showing the time dependence of magnetization in spin transfer torque reversal of the magnetoresistive element. 
         FIG. 10  is a sectional view showing a magnetoresistive element used in the third embodiment. 
         FIG. 11  is a graph showing the result of LLG simulations of the magnetoresistive element of the third embodiment and a magnetoresistive element of a comparative example. 
         FIG. 12  is a graph that plots the a values of the logarithmic normal distribution of t-reverse as the function of a current density J. 
         FIG. 13  is a view showing the arrangement and characteristics of first to fourth magnetic memory. 
         FIG. 14  is a graph showing the result of LLG simulations of the diameter dependence of a storage layer concerning the fluctuation of a write current density Jc. 
         FIG. 15  is a circuit diagram showing a magnetic memory according to the fourth embodiment. 
         FIG. 16  is a sectional view showing a memory cell of the magnetic memory according to the fourth embodiment. 
         FIG. 17  is a graph showing the dependence of the write current density Jc on the exchange coupling constant between a low Ku layer and a high Ku layer. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a magnetic memory comprising a magnetoresistive element, the magnetoresistive element comprising a reference layer having an invariable magnetization direction; a storage layer having a variable magnetization direction; and a spacer layer provided between the reference layer and the storage layer, and wherein the storage layer has a multilayered structure including a first magnetic layer and a second magnetic layer, the second magnetic layer is provided between the first magnetic layer and the spacer layer and has a magnetic anisotropy energy lower than that of the first magnetic layer, and an exchange coupling constant Jex between the first magnetic layer and the second magnetic layer is not more than 5 erg/cm 2 . 
     The present inventors have found the condition to reduce a reversal current (write current), and details thereof will be described before an explanation of the embodiments. 
     The present inventors have found from actual experiments and numerical simulations that the reversal probability of spin transfer torque reversal cannot be represented by equation (1).  FIG. 1  shows the result of the numerical simulations (LLG (Landau-Liftshitz-Gilbert) simulations). As is apparent from  FIG. 1 , the process of magnetization reversal is divided by several characteristic times. The present inventors have found that when equation (1) is expanded, the process can approximately be represented by 
                     P   ⁡     (   τ   )       =     1   -     exp   ⁢     {         -     (     τ   /     τ   0       )       /     τ   0       ⁢     exp   ⁡     [       -       Δ   ⁢           ⁢     E   a           k   B     ⁢   T         ⁢       (     1   -     I     I     c   ⁢           ⁢   0           )     n       ]         }                 (   2   )               
where n is a constant of 1.5 to 2. As the parameters used in the calculation shown in  FIG. 1 , the storage layer has perpendicular magnetization, the magnetic anisotropy energy Ku=5.87 Merg/cm 3 , a saturation magnetization M s =1,000 emu/cm 3 , a film thickness t of the MTJ element=3.09 nm, and the MTJ diameter=φ30 nm. In this case, an energy barrier ΔE a  of magnetization reversal is 79 k B T. At a temperature of 300K, calculation was performed within the range of a current density J (the quotient of the current value divided by the area of the MTJ element) of the current supplied to the MTJ element=2 to 7 MA/cm 2 . Without supplying a current, a magnetization direction Mz perpendicular to the film surface of the storage layer is +1. After the spin transfer torque reversal, Mz=−1. The time during this will be defined as a switching time t sw . The present inventors have found that spin transfer torque reversal that is the write principle of the MRAM includes the following three stages, as is apparent from  FIG. 1 . Hence, the switching time t sw  is divided into three parts.
 
     In the first stage, the respective portions of the storage layer that are separately in a precession state start the precession coherently. This time will be referred to as t-coherent. Mz changes from 1 to about 0.95 during the time t-coherent. The time t-coherent largely fluctuates in each attempt because the precession starts to be coherent and then to be separate again due to the influence of phonons. 
     After the first stage, the coherent precession may be amplified accidentally. In the second stage, the magnetization direction Mz changes from 0.95 to about 0.8. This time will be referred to as t-amplify. The sum of the time t-coherent of the first stage and the time t-amplify of the second stage will be referred to as t-incubation-delay. The time t-incubation-delay is the time until the main stage of magnetization reversal starts, and is relatively long in the spin transfer torque reversal. As described above, since t-incubation-delay fluctuates, the write threshold also fluctuates. 
     In the third stage, the precession is further amplified, and magnetization reversal occurs with the aid of thermal agitation. This is the thermal activation process. The magnetization direction Mz changes from 0.8 to about −1. This time will be referred to as t-reverse. The fluctuation of t-reverse is small when the diameter of the MTJ element is smaller than a single magnetic domain diameter Ds. When the diameter of the MTJ element is larger than the single magnetic domain diameter Ds, t-reverse fluctuates as much as t-incubation delay.  FIG. 1  shows the times t-incubation-delay and t-reverse. 
     As is apparent from the above explanation, an effective strategy for reducing the write current when the anisotropy energy of the storage layer is large is to shorten t-incubation-delay and t-reverse. 
     In addition, suppressing the fluctuation of the write threshold which leads to a write error or a read error translates to suppressing the fluctuation of the times t-incubation-delay and t-reverse. 
     First Embodiment 
     A magnetic memory according to the first embodiment will be described. The magnetic memory of this embodiment includes at least one memory cell. This memory cell includes a magnetoresistive element (to be also referred to as an MTJ element hereinafter) serving as a memory element.  FIG. 2  illustrates the magnetoresistive element used in the magnetic memory of this embodiment. A magnetoresistive element  1  includes a storage layer  2  having a variable magnetization direction, a reference layer  6  having an invariable magnetization direction, and a spacer layer  4  provided between the storage layer  2  and the reference layer  6 . The storage layer  2  having a variable magnetization direction means that the magnetization direction of the storage layer  2  can change when the angular momentum of spin-polarized electrons generated by supplying a current in a direction perpendicular to the film surface (for example, upper surface) of the storage layer  2  is transmitted to the electrons in the storage layer  2 . The reference layer  6  having an invariable magnetization direction means that the magnetization direction of the reference layer does not change when a magnetization reversal current to reverse the magnetization direction of the storage layer  2  is supplied to the reference layer. 
     The storage layer  2  includes a layer (to be referred to as a high Ku layer hereinafter)  2   a  having a high magnetic anisotropy energy Ku and a layer (to be referred to as a low Ku layer hereinafter)  2   b  having a low magnetic anisotropy energy Ku. The low Ku layer is a magnetic layer with Ku of 10,000 erg/cm 3  or less. Ku is preferably lower than 1,000 erg/cm 3 . The high Ku layer is a magnetic layer with Ku higher than 10,000 erg/cm 3 . Ku is preferably higher than 1×10 5  erg/cm 3 . Note that the high Ku layer  2   a  used in this embodiment has Ku of, for example, 1×10 6  erg/cm 3 , and the low Ku layer  2   b  has Ku of 1,000 erg/cm 3 . An intermediate Ku layer used in the second embodiment to be described later need only have Ku lower than that of the high Ku layer. The Ku value is preferably ½ or less of that of the high Ku layer. As the material of the high Ku layer, for example, an alloy of a magnetic element such as Fe or Co and a rare-earth element such as Nd, Sm, Gd, Tb, or Dy, an artificial lattice or an alloy of a magnetic element such as Fe or Co and a precious metal element such as Pt, Pd, Ir, Ru, Au, or Rh, a CoCr alloy, or the like is used. Ku can be adjusted more finely by adding a nonmagnetic metal element such as Cu, Ta, B, Al, Ga, Zr, V, or Ti to these materials. As the material of the low Ku layer, a magnetic layer containing Co, for example, CoFe, CoB, CoFeB, CoFeNi, CoFeNiB, or the like is used. Note that as the material of the spacer layer, for example, MgO, Al 2 O 3 , Cu, or Au is used. 
     In general, reducing the write current is largely affected by an exchange coupling strength (exchange coupling constant) Jex between the stacking interfaces of the magnetic layers.  FIG. 3  shows a result of LLG simulations conducted while changing Jex. The diameter of the MTJ element was set to 30 nm. Calculation was performed for two cases in which the magnetic anisotropy energy Ku and volume of the high Ku layer  2   a  were changed so as to make the storage layer  2  attain the same average magnetic anisotropy energy. Full circles in  FIG. 3  represent the calculation result when Ku=1×10 6  erg/cm 3  and the film thickness=4 nm. Hollow triangles represent the calculation result when Ku=1×10 6  erg/cm 3  and the film thickness=1 nm. As can be seen from  FIG. 3 , the write current abruptly decreases when the exchange coupling constant Jex is 5 erg/cm 2  or less in both cases. This reveals that setting Jex to 5 erg/cm 2  or less is suitable for reducing the write current. According to the calculation results, the write current decreases to 38% when the value of the exchange coupling constant changes from 10 erg/cm 2  to 2 erg/cm 2 . It is important that at this time, only the values of the high Ku layer  2   a  and the low Ku layer  2   b  change, and the magnetic anisotropy energy of the storage layer  2  remains unchanged. 
     As is apparent from  FIG. 3 , the strength Jex of the exchange coupling between the high Ku layer  2   a  and the low Ku layer  2   b  is preferably 5 erg/cm 2  or less. More preferably, Jex is 2 erg/cm 2  or less, as will be described below. This arrangement allows to obtain a magnetoresistive element capable of guaranteeing a sufficiently long memory holding time and reducing the write current. 
       FIGS. 4(   a ) and  4 ( b ) show the elapse of time of magnetizations Mz of the high Ku layer  2   a  and the low Ku layer  2   b  when Jex=10 erg/cm 2  and Jex=2 erg/cm 2 . As is apparent from  FIGS. 4(   a ) and  4 ( b ), the time t-incubation-delay until the magnetization Mzs starts coherent precession is shorter when Jex=2 erg/cm 2  than that when Jex=10 erg/cm 2 . The probable reason is as follows. In spin transfer torque reversal, conduction electrons spin-polarized through the reference layer  6  transmit the spin torque to a portion of the storage layer  2  adjacent to the spacer layer  4 , thereby causing magnetization reversal. Hence, when Jex is small, magnetization of the low Ku layer  2   b  on the side of the spacer layer  4  starts coherent precession earlier. As a result, t-incubation-delay remarkably shortens. In fact, when Jex=2 erg/cm 2 , magnetization rotation of the low Ku layer  2   b  occurs earlier than the high Ku layer  2   a , as is apparent from  FIG. 4(   b ). 
     To know whether Jex is actually 5 erg/cm 2  or less, the resistance of the magnetoresistive element  1  is measured while applying an external magnetic field to the magnetoresistive element  1 .  FIG. 5  shows the external magnetic field-resistance curve obtained by this measurement. As is apparent from  FIG. 5 , when the exchange coupling constant Jex is larger than 5 erg/cm 2 , the resistance value behaves to maintain a predetermined value until it steeply changes. On the other hand, when Jex=5 erg/cm 2  or less, the resistance has a predetermined value within the range where the external magnetic field up to zero is applied in the same direction as the magnetization of the storage layer. When the external magnetic field is applied in the direction opposite to the magnetization of the storage layer, the resistance decreases. A state in which the resistance value immediately before the magnetization reversal decreases to 90% the resistance value for a magnetic field of 0 corresponds to Jex=5 erg/cm 2  or less. The write current decreases when Jex=5 erg/cm 2  or less because the low Ku layer causes magnetization reversal earlier, as described above. This can be regarded as the same as the phenomenon that the magnetization of the low Ku layer changes earlier upon magnetic field sweeping, and the resistance value immediately before reversal decreases. Hence, the value of the exchange coupling Jex between the high Ku layer and the low Ku layer can be known in the above-described way. 
     The value of the exchange coupling constant between adjacent magnetic bodies continuously formed in vacuum without inserting any intermediate layer is generally 10 erg/cm 2  or more. Hence, it is necessary to make the value of the exchange coupling constant smaller than 10 erg/cm 2  using some technique. To control the exchange coupling constant Jex, for example, a thin nonmagnetic layer or a magnetic metal layer  2   c  is inserted between the high Ku layer  2   a  and the low Ku layer  2   b , as in a modification of the embodiment shown in  FIG. 6 . Examples of the nonmagnetic metal to be used actually are single substances such as B, Mg, Al, Si, Ti, V, Cr, Mn, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os, Ir, Pt, and Au, and alloys, oxides, and nitrides thereof. Note that the nonmagnetic layer or metal layer  2   c  to adjust Jex to a desired value has a different film thickness. When the nonmagnetic layer or magnetic metal layer  2   c  is inserted between the high Ku layer  2   a  and the low Ku layer  2   b , interdiffusion between the high Ku layer  2   a  and the low Ku layer  2   b  can be suppressed, and a magnetic memory having high heat resistance can be attained. The magnetic metal layer to be inserted need only be an alloy layer containing Fe, Ni, and Co or at least one of them. 
     Jex may be adjusted by reforming the interface. Which one of the low Ku layer and the high Ku layer should be formed first is placed within the range of the magnetic memory design. When one of the layers is formed and exposed to O 2  or O 3  in an adjusted pressure or subjected to a plasma treatment in it, and the next layer is then stacked, an oxide layer with a well-controlled interface can be obtained, and Jex can be adjusted to a desired value. That is, the layer  2   c  inserted between the high Ku layer  2   a  and the low Ku layer  2   b  is an oxide layer. As a simpler method, the structure is put on standby in a stocker or the like for a predetermined time before formation of the magnetic layer. Since the surface is reformed by slight leakage in the film forming apparatus, Jex can be made smaller. The interface may be modified by exposing it to the plasma of an inert gas such as Ar, Kr, or Xe or performing annealing. 
     It is possible to set Jex to 5 erg/cm 2  or less by combining some of the above-described methods of adjusting Jex. 
     Generally, when a saturation magnetization Ms of the high Ku layer  2   a  and that of the low Ku layer  2   b  are different, the effect of reducing Jex is more conspicuous. To set Jex to 5 erg/cm 2  or less, the saturation magnetization Ms of the low Ku layer  2   b  is preferably larger than 1.2 times the saturation magnetization Ms of the high Ku layer  2   a.    
     As described above, according to this embodiment, even when the storage layer has a high magnetic anisotropy energy, setting the exchange coupling strength Jex between the high Ku layer  2   a  and the low Ku layer  2   b  to 5 erg/cm 2  or less allows to reduce the write current without deteriorating the thermal stability of the storage layer. 
     Second Embodiment 
     A magnetic memory according to the second embodiment will be described next. The magnetic memory of this embodiment includes at least one memory cell. This memory cell includes a magnetoresistive element serving as a memory element.  FIG. 7  illustrates the magnetoresistive element used in the magnetic memory of this embodiment. A magnetoresistive element  1  includes a storage layer  2 , a reference layer  6 , and a spacer layer  4  provided between the storage layer  2  and the reference layer  6 . The storage layer  2  includes a high Ku layer  2   a , a low Ku layer  2   b , and a magnetic layer (to be referred to as an intermediate Ku layer hereinafter)  2   d  provided between the high Ku layer  2   a  and the low Ku layer  2   b  and having Ku lower than that of the high Ku layer  2   a  and higher than that of the low Ku layer  2   b . This arrangement allows to reduce the write current. The mechanism will be described below. 
     In spin transfer torque reversal, conduction electrons spin-polarized through the reference layer  6  transmit the spin torque to a portion of the storage layer  2  adjacent to the spacer layer, thereby causing magnetization reversal. That is, the magnetization reversal starts from the low Ku layer  2   b  adjacent to the spacer layer  4  and gradually propagates to the high Ku layer  2   a . In the magnetoresistive element  1  shown in  FIG. 7 , the intermediate Ku layer  2   d  is inserted between the high Ku layer  2   a  and the low Ku layer  2   b . For this reason, the magnetization reversal that occurs from the low Ku layer  2   b  propagates to the high Ku layer  2   a  through the intermediate Ku layer  2   d . In a general magnetic body, the lower Ku is, the more easily the magnetization reversal occurs. Hence, as compared to the first embodiment in which the high Ku layer  2   a  is directly stacked on the low Ku layer  2   b , inserting the intermediate Ku layer  2   d  makes it possible to smoothly propagate the magnetization reversal through the low Ku layer  2   b , the intermediate Ku layer  2   d , and the high Ku layer  2   a  in this order so that the spin transfer torque reversal readily occurs. 
       FIG. 8  shows the result of LLG simulations that proved reduction of the spin transfer torque reversal current in the magnetoresistive element  1  used in this embodiment. In the calculation, a composite perpendicular magnetic anisotropy K ueff  of the high Ku layer  2   a  and the intermediate Ku layer  2   d  was kept constant. Letting K uh  be the magnetic anisotropy energy of the high Ku layer  2   a , V h  be the volume of the high Ku layer  2   a , K um  be the magnetic anisotropy energy of the intermediate Ku layer  2   d , and V m  be the volume of the intermediate Ku layer  2   d , K ueff  is given by 
     
       
         
           
             
               
                 
                   
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                           K 
                           uh 
                         
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                           V 
                           h 
                         
                       
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                           K 
                           um 
                         
                         ⁢ 
                         
                           V 
                           m 
                         
                       
                     
                     
                       
                         V 
                         h 
                       
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                         V 
                         m 
                       
                     
                   
                 
               
               
                 
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     In the above LLG simulations, the magnetic anisotropy energy of the intermediate Ku layer  2   d  and that of the high Ku layer  2   a  were adjusted to change K um /K uh  so that K ueff  became constant. In  FIG. 8 , a exchange coupling constant Jex between the low Ku layer and the intermediate Ku layer is set to 10 erg/cm 2 . As is apparent from  FIG. 8 , as K um /K uh  becomes smaller, the spin transfer torque reversal current from the parallel state to the antiparallel state clearly becomes smaller. K um /K uh =1 means that the intermediate Ku layer  2   d  and the high Ku layer  2   a  have the same magnetic anisotropy energy Ku so that they substantially form a single high Ku layer. 
       FIG. 9(   a ) is a graph showing the time dependence of magnetization in spin transfer torque reversal of the magnetoresistive element  1  used in this embodiment.  FIG. 9(   b ) is an enlarged view of the graph shown in  FIG. 9(   a ). As can clearly be confirmed in  FIG. 9(   b ), placing focus on the start of spin transfer torque reversal, when current supply starts, the low Ku layer  2   b  causes reversal first, and the magnetization reversal sequentially propagates to the intermediate Ku layer  2   d  and the high Ku layer  2   a . When the magnetization reversal smoothly propagates, t-incubation-delay shortens. 
     As described above, inserting the intermediate Ku layer  2   d  between the high Ku layer  2   a  and the low Ku layer  2   b  makes it possible to smoothly propagate the magnetization reversal and obtain the effect of reducing the write current. 
       FIG. 8  also shows a write current (switching current) that reverses the magnetization direction from parallel to antiparallel when the exchange coupling constant Jex between the low Ku layer and the intermediate Ku layer is changed from 10 erg/cm 2  to 2 erg/cm 2 . As described in the first embodiment, Jex=2 erg/cm 2  is more suitable than Jex=10 erg/cm 2 . Hence, in the second embodiment, the write current can further be reduced by setting the exchange coupling constant Jex between the low Ku layer  2   b  and the intermediate Ku layer  2   d  to 5 erg/cm 2  or less. In the first embodiment, actually, the write current decreases to 38% when the value of the exchange coupling constant changes from 10 erg/cm 2  to 2 erg/cm 2 , as shown in  FIG. 3 . In the arrangement of the second embodiment, when Kum/Kuh=0.3, as shown in  FIG. 8 , the write current for the change from parallel to antiparallel can be reduced to 20% by decreasing the value of the exchange coupling constant from 10 erg/cm 2  to 2 erg/cm 2  (see the arrow in  FIG. 8 ). In this case, needless to say, inserting a nonmagnetic layer or a magnetic layer or a plasma treatment, oxidation, heating, or the like is suitable for controlling the exchange coupling between the low Ku layer  2   b  and the intermediate Ku layer  2   d , as described in the first embodiment. 
     The thermal stability of a magnetic layer is represented by K ueff ×(V h +V m ). In the LLG simulations shown in  FIG. 8 , since K ueff  is constant, the thermal stability does not change. Hence, the result shown in  FIG. 8  represents that the reversal current is reduced while maintaining the thermal stability of the magnetic layer. It is also confirmed that the spin transfer torque reversal current reduction effect can also be obtained even when the storage layer includes a plurality of intermediate Ku layers, instead of having the three-layer structure as in this embodiment. Based on these results, using a multilayered structure as the storage layer allows to implement a magnetoresistive element capable of guaranteeing a sufficiently long memory holding time, that is, maintaining the thermal stability of the storage layer and reducing the write current and a magnetic memory using the same. 
     To actually implement the intermediate Ku layer  2   d , a nonmagnetic element or a magnetic metal element is added to the high Ku layer  2   a . Alternatively, a nonmagnetic layer or a magnetic metal layer may be inserted into the high Ku layer  2   a , and a nonmagnetic element from the nonmagnetic layer or a magnetic metal element from the magnetic metal layer may be added to the high Ku layer  2   a  by annealing to be performed later. Adding the magnetic metal element or the nonmagnetic element to the high Ku layer  2   a  allows to change the composition ratio and thus lower Ku. Since the composition ratio can be changed by adding at least one of the magnetic metal element and the nonmagnetic element, Ku can be lowered. As the nonmagnetic element, Ti, V, Cr, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os, Ir, Pt, Au, or the like is usable. As the magnetic metal element, Fe, Ni, Co, Mn, or the like is usable. Note that the same material as in the first embodiment can be used as the high Ku layer. For example, an alloy of a magnetic element such as Fe or Co and a rare-earth element such as Nd, Sm, Gd, Tb, or Dy, an artificial lattice or an alloy of a magnetic element such as Fe or Co and a precious metal element such as Pt, Pd, Ir, Ru, Au, or Rh, a CoCr alloy, or the like is usable. As the low Ku layer, the same material as in the first embodiment, that is, a magnetic layer containing Co, for example, CoFe, CoB, CoFeB, CoFeNi, CoFeNiB, or the like is used. 
     When the high Ku layer  2   a  is made of an intermetallic compound or an alloy of a nonmagnetic metal element and a magnetic metal element, that is, an intermetallic compound or an alloy containing at least one nonmagnetic metal element selected from the group consisting of Ti, V, Cr, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os, Ir, Pt, and Au and at least one magnetic metal element selected from the group consisting of Fe, Ni, Co, and Mn, the intermediate Ku layer  2   d  can be implemented by changing the composition ratio (content ratio) of the nonmagnetic metal element and the magnetic metal element in the high Ku layer  2   a . When the magnetic film is formed by, for example, sputtering, the content ratio of the atoms can continuously be changed very smoothly by continuously changing the discharge power of the metal whose content ratio changes during discharge. At this time, since the optimum atomic content ratio changes depending on the type of the nonmagnetic metal, adjustment is done for each substance, as needed. When such an intermediate Ku layer is used, and the composition ratio of the magnetic layer is analyzed on the low Ku layer or spacer side and on the electrode side, the magnetic layer has different compositions on these sides, as its characteristic feature. Note that the same material as in the first embodiment, for example, MgO, Al 2 O 3 , Cu, or Au is used as the spacer layer  4 . 
     As described above, according to this embodiment, it is possible to reduce the write current without deteriorating the thermal stability of the storage layer. 
     Third Embodiment 
     A magnetic memory according to the third embodiment will be described next with reference to  FIG. 10 . The magnetic memory of this embodiment includes at least one memory cell. This memory cell includes a magnetoresistive element serving as a memory element.  FIG. 10  illustrates the magnetoresistive element used in the magnetic memory of this embodiment. A magnetoresistive element  1  includes a storage layer  2 , a reference layer  6 , and a spacer layer  4  provided between the storage layer  2  and the reference layer  6 . The storage layer  2  includes a high Ku layer  2   a , a low Ku layer  2   b , and a low Ku layer  2   e . The low Ku layer  2   b  is provided between the high Ku layer  2   a  and the spacer layer  4 . The low Ku layer  2   e  is provided on a side opposite to the low Ku layer  2   b  with respect to the high Ku layer  2   a . Note that the same material as in the first embodiment can be used as the high Ku layer. For example, an alloy of a magnetic element such as Fe or Co and a rare-earth element such as Nd, Sm, Gd, Tb, or Dy, an artificial lattice or an alloy of a magnetic element such as Fe or Co and a precious metal element such as Pt, Pd, Ir, Ru, Au, or Rh, a CoCr alloy, or the like is usable. As the low Ku layers  2   b  and  2   e , the same material as in the first embodiment, that is, a magnetic layer containing Co, for example, CoFe, CoB, CoFeB, CoFeNi, CoFeNiB, or the like is used. When MgO is used as the spacer layer  4 , a magnetic layer containing Co and B is preferably used as the low Ku layer  2   b  on the side of the spacer layer  4 . When B is contained, the MR ratio of the magnetoresistive element can be made large. Note that the same material as in the first embodiment, for example, MgO, Al 2 O 3 , Cu, or Au is used as the spacer layer  4 . 
     As described in the first embodiment, since the low Ku layer  2   b  is provided adjacent to the spacer layer  4 , t-incubation-delay shortens as compared to a case in which the storage layer includes only the high Ku layer  2   a . This is probably because the magnetization of the low Ku layer  2   b  starts coherent precession first. In this embodiment, t-reverse shortens. This will be described with reference to  FIG. 11 .  FIG. 11  shows the result of LLG simulations of the magnetoresistive element  1  used in this embodiment shown in  FIG. 10  and a magnetoresistive element of a comparative example having the same arrangement as that of the magnetoresistive element shown in  FIG. 10  except that the storage layer is formed from the high Ku layer  2   a . When the simulation is repeated under the same condition, the times t-reverse are distributed in accordance with the logarithmic normal distribution. Outliers deviated from the median of t-reverse by 6σ are plotted along the ordinate as worst values, where σ represents the standard deviation. To shorten the outliers is important in reducing write current errors. As is apparent from  FIG. 11 , when the low Ku layer  2   e  is provided outside the high Ku layer  2   a , t-reverse shortens as compared to a single-layered storage layer having the same memory holding time. This is probably because the magnetization of the low Ku layer  2   e  rotates first, and the high Ku layer  2   a  follows then. 
       FIG. 12  is a graph that plots, as the function of a current density J, the σ values of the logarithmic normal distribution of t-reverse in magnetic memories having, as memory elements, the magnetoresistive elements according to this embodiment, the first embodiment in which the storage layer is formed from the high Ku layer  2   a  and the low Ku layer  2   b , and the comparative example in which the storage layer is formed from a single layer. As is apparent from  FIG. 12 , when the low Ku layer  2   b  is provided as in the first embodiment, or the low Ku layer  2   e  is provided as in the third embodiment, the standard deviation σ becomes smaller as compared to the comparative example having the storage layer formed from a single layer with the same memory holding time. In the comparative example in which the storage layer is formed from a single layer, the standard deviation σ is large because the magnetization reversal stays in the intermediate state between the two magnetic domains near Mz=0. However, when the low Ku layer  2   b  and the low Ku layer  2   e  are added, as in this embodiment, the standard deviation σ supposedly decreases because the domain wall width of the high Ku layer apparently increases, and the intermediate state between the two magnetic domains is not so stable. A domain wall width δW is given by δW=π(A/Ku) 1/2 . Ku of the low Ku layer  2   b  and the low Ku layer  2   e  is preferably 1/100 or less of that of the high Ku layer  2   a . This is because the domain wall width in the low Ku layer  2   b  and the low Ku layer  2   e  is 10 times or more of that in the high Ku layer  2   a  and becomes much larger than the diameter of the storage layer. When the domain wall width is much larger than the diameter of the storage layer, the intermediate state between the two magnetic domains is not stable and rarely appears. Note that A is the average exchange stiffness constant of the high Ku layer and the low Ku layer. Hence, using the magnetoresistive element  1  shown in  FIG. 10  as the memory element enables to reduce the fluctuation of the threshold and suppress write errors and read errors. 
     First to fourth magnetic memories are prepared. The first magnetic memory includes, as the memory element, the magnetoresistive element shown in  FIG. 10  in which the exchange coupling constant Jex between the low Ku layer  2   b  and the high Ku layer  2   a  is 10 erg/cm 2 , and the exchange coupling constant Jex between the high Ku layer  2   a  and the low Ku layer  2   e  is 10 erg/cm 2 . The second magnetic memory includes, as the memory element, a magnetoresistive element having the same arrangement as that of the magnetoresistive element shown in  FIG. 10  except that the storage layer  2  includes only the high Ku layer  2   a . The third magnetic memory includes, as the memory element, the magnetoresistive element shown in  FIG. 10  in which the exchange coupling constant Jex between the low Ku layer  2   b  and the high Ku layer  2   a  is 2 erg/cm 2 , and the exchange coupling constant Jex between the high Ku layer  2   a  and the low Ku layer  2   e  is 10 erg/cm 2 . The fourth magnetic memory includes, as the memory element, the magnetoresistive element shown in  FIG. 10  in which the exchange coupling constant Jex between the low Ku layer  2   b  and the high Ku layer  2   a  is 10 erg/cm 2 , and the exchange coupling constant Jex between the high Ku layer  2   a  and the low Ku layer  2   e  is 2 erg/cm 2 . 
     A reversal current density (write current density) Jc for reversing the magnetization of the storage layer is measured in each of the first to fourth magnetic memories.  FIGS. 13(   a ),  13 ( b ),  13 ( c ), and  13 ( d ) show the measurement results. 
     The first to fourth magnetic memories are set to have the same thermal stability of the storage layer  2 . In the first magnetic memory shown in  FIG. 13(   a ), the write current density Jc decreases to 0.6 times that of the second magnetic memory shown in  FIG. 13(   b ). In the first magnetic memory shown in  FIG. 13(   a ), the exchange coupling constant between the magnetic layers is as large as 10 erg/cm 2 . When the exchange coupling constant between the high Ku layer  2   a  and the low Ku layer  2   b  is decreased to 2 erg/cm 2 , as in the third magnetic memory shown in  FIG. 13(   c ), the write current density Jc decreases to 0.5 times, as compared to the memory shown in  FIG. 13(   b ). Reducing the exchange coupling between the low Ku layer  2   e  and the high Ku layer  2   a , as in the fourth magnetic memory shown in  FIG. 13(   d ), is not so effective for reducing the write current density Jc. 
     As is apparent from the above explanation, in this embodiment as well, the smaller exchange coupling between the high Ku layer  2   a  and the low Ku layer  2   b  on the side of the spacer layer  4  is suitable for reducing the write current. On the other hand, the exchange coupling between the high Ku layer  2   a  and the low Ku layer  2   e  arranged on the side opposite to the spacer layer  4  is suitably larger than that between the low Ku layer  2   b  and the high Ku layer  2   a . This is probably because when the exchange coupling constant Jex between the low Ku layer  2   e  and the high Ku layer  2   a  is as small as 2 erg/cm 2 , as shown in  FIG. 13(   d ), the fluctuation of t-reverse becomes large, and the effect of reducing the write current consequently becomes smaller.  FIG. 17  show the dependence of the write current density Jc on the exchange coupling constant Jex between the low Ku layer  2   e  and the high Ku layer  2   a . The write current abruptly increases when the exchange coupling constant Jex is lower than 5 erg/cm 2 . This reveals that Jex=5 erg/cm 2  or more is suitable for reducing the write current. Note that Jex can be controlled using the methods described in the first embodiment. 
     Another advantage of this embodiment using the magnetoresistive element shown in  FIG. 10  is the higher MR ratio. When the low Ku layer  2   b  and the low Ku layer  2   e  are placed adjacent to the spacer layer  4  together, the magnetization of the low Ku layer tilts, and the MR ratio consequently lowers. This problem can be avoided by dividing the low Ku layer into the low Ku layer  2   b  and the low Ku layer  2   e.    
       FIG. 14  shows the result of LLG simulations of the diameter dependence of the storage layer concerning the fluctuation of the write current density Jc when the storage layer has the two-layer structure including the low Ku layer and the high Ku layer (for example, first embodiment). The abscissa represents a diameter D of the storage layer normalized by Ds, and the ordinate represents the variation a (Jc) of the write current density Jc normalized by the write current density Jc. Ds represents the diameter of the MTJ element which ensures the same energy barrier in the process of causing reversal through two magnetic domains and the process of causing reversal along the axis of hard magnetization in a single magnetic domain in the magnetization reversal process. When the diameter of the storage layer is equal to or smaller than Ds, the storage layer can take the magnetization reversal process in the single magnetic domain. The present inventors have found that Ds of a multilayered storage layer formed from the high Ku layer  2   a  and one of the low Ku layer  2   b  and the low Ku layer  2   e  is given by 
                     D   s     =       6   ⁢       A   ⁢               ⁢     (           K     u   ⁢           ⁢   1         ⁢     t   1       +              K     u   ⁢           ⁢   2              ⁢     t   2         )           (         K     u   ⁢           ⁢   1       ⁢     t   1       +       K     u   ⁢           ⁢   2       ⁢     t   2         )     -     2   ⁢       π   ⁡     (     M   s   av     )       2     ⁢     (       N   a     -     1   2       )     ⁢     (       t   1     +     t   2       )                   (   4   )               
where Ku is the magnetic anisotropy energy density, and t is the thickness. Suffix “1” represents the high Ku layer, and “2”, the low Ku layer. A and M s   av  are the average exchange stiffness constant and the average saturation magnetization of the high Ku layer and the low Ku layer, and Na is the demagnetizing factor in the film thickness direction of a cylinder having the diameter Ds and a thickness t 1 +t 2 .
 
     Note that equation (4) can also be used for a three-layer structure (for example, third embodiment) in which the storage layer includes the low Ku layer  2   b , the high Ku layer  2   a , and the low Ku layer  2   e . In this case, K U2  represents the average value of K U  of the low Ku layer  2   b  and K U  of the low Ku layer  2   e , and t 2  represents the sum of the thickness of the low Ku layer  2   b  and the thickness of the low Ku layer  2   e.    
     As is apparent from  FIG. 14 , when D/Ds is 1.5 or more, the variation in the write current density Jc abruptly becomes large. When the fluctuation of Jc is large, write errors increase. To sufficiently reduce the write errors (for example, reduce the probability to 1×10 −10  or less), a large write current is necessary. Hence, making D smaller than 1.5 times of Ds is suitable for suppressing write errors and reducing the write current. In the single-layered storage layer, conventionally, D≦Ds is preferable. However, the present inventors have proved that D&gt;1.5Ds is preferable for the multilayered storage layer from the viewpoint of the fluctuation of the write current density Jc. The present inventors have found that when the low Ku layer (or outer low Ku layer) is stacked on the storage layer including only the high Ku layer, Ds increases in accordance with equation (4). If the diameter D is the same, the fluctuation of the write current density Jc is suppressed by the stacking. Equation (4) is different from both the equation of the single-layered storage layer and the equation obtained by averaging Ku of the low Ku layer and the high Ku layer and substituting it into the equation of the single-layered storage layer. Equation (4) represents the effect specific to the multilayered storage layer. 
     According to this embodiment, it is possible to reduce the write current without deteriorating the thermal stability of the storage layer. 
     As described above, according to the first to third embodiments, it is possible to reduce the write current without deteriorating the thermal stability of the storage layer. That is, it is possible to guarantee a sufficiently long memory holding time and reduce the write current. It is also possible to largely decrease the probability of operation errors caused by write, read, and memory holding errors which occur because the magnetization reversal of a magnetic body is a probabilistic phenomenon. 
     Fourth Embodiment 
     A magnetic memory according to the fourth embodiment will be described next with reference to  FIGS. 15 and 16 . 
       FIG. 15  is a circuit diagram of an MRAM according to this embodiment. The MRAM of this embodiment uses, as the memory element of a memory cell, a magnetoresistive element (MTJ element)  1  shown in  FIG. 2 ,  6 ,  7 , or  10  described in the first embodiment, the modification thereof, the second embodiment, and the third embodiment. This MRAM includes a memory cell array  30  including a plurality of memory cells MC arrayed in a matrix. A plurality of pairs of bit lines BL and /BL are formed on the memory cell array  30  so as to run in the column direction. In addition, a plurality of word lines WL are formed on the memory cell array  30  so as to run in the row direction. 
     The memory cells MC are arranged at the intersections between the bit lines BL and the word lines WL. Each memory cell MC includes the MTJ element  1 , and a select transistor  31  formed from an NMOS transistor. One terminal of the MTJ element  1  is connected to the bit line BL. The other terminal of the MTJ element  1  is connected to the drain terminal of the select transistor  31 . The gate terminal of the select transistor  31  is connected to the word line WL. The source terminal of the select transistor  31  is connected to the bit line /BL. 
     A row decoder  32  is connected to the word lines WL. A write circuit  34  and a read circuit  35  are connected to the pairs of bit lines BL and /BL. A column decoder  33  is connected to the write circuit  34  and the read circuit  35 . Each memory cell MC is selected by the row decoder  32  and the column decoder  33 . 
     Data write to the memory cell MC is performed in the following way. First, to select the memory cell MC for the data write, the word line WL connected to the memory cell MC is activated. The select transistor  31  is thus turned on. A bidirectional write current Iw is supplied to the MTJ element  1  based on the write data. More specifically, to supply the write current Iw to the MTJ element  1  from left to right in  FIG. 15 , the write circuit  34  applies a positive voltage to the bit line BL and the ground voltage to the bit line /BL. To supply the write current Iw to the MTJ element  1  from right to left in  FIG. 15 , the write circuit  34  applies a positive voltage to the bit line /BL and the ground voltage to the bit line BL. Data “0” or data “1” can thus be written to the memory cell MC. 
     Data read from the memory cell MC is performed in the following way. First, to select the memory cell MC for the data read, the word line WL connected to the memory cell MC is activated. The select transistor  31  of the selected memory cell MC is thus turned on. The read circuit  35  supplies a read current Ir flowing, for example, from right to left in  FIG. 15  to the MTJ element  1 . The read circuit  35  detects the resistance value of the MTJ element  1  based on the read current Ir. Data stored in the MTJ element  1  can thus be read. 
     The structure of the MRAM will be described next.  FIG. 16  is a sectional view showing the arrangement of the MRAM with focus on one memory cell MC. 
     An element isolation layer  41  is provided in the surface region of a p-type semiconductor substrate  40 . A surface region of the semiconductor substrate  40  where the element isolation layer  41  is not provided is the element region (active area) where the element is formed. The element isolation layer  41  is formed from, for example, STI (Shallow Trench Isolation). As the STI, for example, silicon oxide is used. 
     The select transistor  31  is formed in the element region of the semiconductor substrate  40 . The select transistor  31  is provided with a source region  42   a  and  a  drain region  42   b  which are spaced part from each other. Each of the source region  42   a  and the drain region  42   b  is made of an n + -type diffusion region formed by heavily doping the semiconductor substrate  40  with an n + -type impurity. A gate insulating film  51  is formed on the region of the semiconductor substrate  40  as a prospective channel  43  between the source region  42   a  and the drain region  42   b . A gate electrode  52  is provided on the gate insulating film  51 . 
     The gate electrode  52  functions as the word line WL. 
     An interconnection layer  63  is provided on the source region  42   a  via a contact  62 . The interconnection layer  63  functions as the bit line /BL. A lead  65  is provided on the drain region  42   b  via a contact  64 . The MTJ element  1  sandwiched between a lower electrode  71  and an upper electrode  72  is provided on the lead  65 . An interconnection layer  76  is provided on the upper electrode  72 . The interconnection layer  76  functions as the bit line BL. The space between the semiconductor substrate  40  and the interconnection layer  76  is filled with an interlayer dielectric film  67  made of, for example, silicon oxide. 
     According to this embodiment, the MRAM can be formed using the MTJ element  1  according to one of the first embodiment, the modification thereof, the second embodiment, and the third embodiment. 
     In this embodiment as well, it is possible to reduce the write current without deteriorating the thermal stability of the storage layer, as in the first embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.