Patent Publication Number: US-2023147268-A1

Title: Magnetoresistive element and magnetic memory device

Description:
TECHNICAL FIELD 
     The present invention relates to a magnetoresistive effect element and a magnetic memory device. 
     BACKGROUND ART 
     Non-volatile memory devices using a magnetoresistive effect element (MRAM) are drawing attention as next-generation logic integrated circuits. Among those MRAMs, a three-terminal SOT-MRAM (spin-orbit torque magnetic memory) is known to be suitable for nonvolatile memory that requires high-speed writing. For example, Japanese Patent Application Laid-open Publication No. 2018-157108 discloses a three-terminal SOT (spin-orbit torque)-MRAM (magnetic random access memory) having a multilayer structure of a channel layer, a recording layer, a barrier layer, and a reference layer. 
       FIG.  19    illustrate the basic configuration of the SOT-MRAM disclosed in Patent Literature 1. 
     As illustrated in the figure, this SOT-MRAM  1000  is constituted of a multilayer body made up of a channel layer  1111 , a recording layer  1112 , a barrier layer  1113 , and a reference layer  1114 , and includes three terminals T 11 , T 12 , and T 13 . 
     The channel layer  1111  is a conductive layer made of a heavy metal or the like, and a region where the spin orbit torque is generated when the write current flows. 
     The recording layer  1112  is a magnetic layer made of a magnetic body, and the magnetization M 1112  can be switched between the +X axis direction and the -X axis direction by the spin orbit torque generated by the write current that flows through the channel layer  1111 . 
     The barrier layer  1113  is a non-magnetic layer made of a tunnel insulating layer. 
     The reference layer  1114  is made of a magnetic body, and the direction of magnetization M 1114  is fixed. 
     When data is written to SOT-MRAM  1000 , a voltage corresponding to data to be written is applied between the terminal T 12  and the terminal T 13 , and the write current is made to flow through the channel layer  1111 . As a result, due to the effect of the spin orbit torque, the direction of magnetization M 1112  of the recording layer  1112  changes based on the flowing direction of the write current. 
     When the direction of magnetization M 1112  of the recording layer  1112  is the same as the direction of magnetization M 1114  of the reference layer  1114  (parallel state), resistance between the reference layer  1114  and the recording layer  1112  decreases. On the other hand, when the direction of magnetization M 1112  of the recording layer  1112  is opposite to the direction of magnetization M 1114  of the reference layer  1114  (antiparallel state), resistance between the reference layer  1114  and the recording layer  1112  becomes relatively large. Data is assigned to various resistance values. 
     When data is read out of the SOT-MRAM  1000 , a read voltage is applied between the terminal T 11  and the terminal T 12  or T 13  (in this example, T 13 ), and data corresponding to the size of the resultant read current I R  is outputted. 
     Citation List 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Laid-open Publication No. 2018-157108 
     SUMMARY OF THE INVENTION 
     Problems to Be Solved by the Invention 
     The channel layer  1111  is made of a heavy metal that is conductive. Thus, the write current that flows through the channel layer  1111  in data writing is large, which increases power consumption. 
     One way to reduce the write current is to increase the resistance of the channel layer  1111 . 
     However, in data reading, the read current I R  flows through a part of the channel layer  1111 , and therefore, if the resistance value of the channel layer  1111  is made larger, then the resistance component in that portion would become load resistance and the read current would be smaller. This would reduce the TMR ratio (Tunnel magnetic resistance ratio), and as a result, the reading speed would be slower. The more integrated the SOT-MRAM is, the more significant this problem becomes. 
     The TMR ratio is represented by the following formula where R P  is the resistance between the reference layer  1114  and the recording layer  1112  in the parallel state, R AP  is the resistance between the reference layer  1114  and the recording layer  1112  in the antiparallel state, and R L  is the load resistance between the terminal T 3  and the junction surface where the recording layer  1112  and the channel layer  1111  make contact with each other: 
     
       
         
           
             
               
                 TMR= 
                 
                   
                     
                       
                         
                           R 
                           
                             AP 
                           
                         
                         + 
                         
                           R 
                           L 
                         
                       
                     
                     − 
                     
                       
                         
                           R 
                           P 
                         
                         + 
                         
                           R 
                           L 
                         
                       
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       L 
                     
                   
                 
               
             
             
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 = 
                 
                   
                     
                       R 
                       
                         AP 
                       
                     
                     − 
                     
                       R 
                       P 
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       L 
                     
                   
                 
               
             
           
         
       
     
     As this formula indicates, the greater the load resistance R L , the smaller the TMR ratio. Also, the lower the TMR ratio, the longer the reading time (time required for data reading). This makes it difficult to perform a high-speed reading operation. 
     The present invention was made in view of the situations described above, and an object thereof is to provide a magnetoresistive effect element that can achieve low power consumption and high speed reading, and a magnetic memory device using such a magnetoresistive effect element. 
     To achieve the above object, a magnetoresistive effect element according to the present invention includes: a reference layer, a barrier layer, a recording layer, and a channel layer deposited on top of one another; and a first terminal connected to the reference layer, and a second terminal and a third terminal connected to the channel layer, wherein the channel layer includes a first channel layer and a second channel layer, wherein electrical resistance of the first channel layer is larger than electrical resistance of the second channel layer, wherein the second terminal is connected to the first channel layer, and the third terminal is connected to the second channel layer, wherein a write current flows between the second terminal and the third terminal via the first channel layer and the second channel layer, and wherein a read current flows between the first terminal and the third terminal. 
     For example, the reference layer is made of a ferromagnetic layer in which a magnetization direction is fixed, the barrier layer is made of a tunnel insulating layer, the recording layer is made of a ferromagnetic layer in which a magnetization direction is changed by a spin orbit torque generated by a write current that flows through the channel layer, the channel layer contains a heavy metal, the second terminal is connected to one end of the channel layer, and the third terminal is connected to the other end of the channel layer, the first channel layer includes an area of the channel layer between a junction surface of the channel layer and the recording layer and a connecting point of the second terminal, and the second channel layer includes an area of the channel layer between the joint surface and a connecting point of the third terminal. 
     The channel layer contains W, Ta, Pt, Pd, WOx, TaOx, PtOx, or PdOx. 
     The first channel layer is thinner than the second channel layer. 
     The first channel layer is longer than the second channel layer. The first channel layer is in a spiral shape. 
     The first channel layer is narrower than the second channel layer. 
     The first channel layer has a higher resistance rate than that of the second channel layer. 
     A circuit element connected to the second terminal has higher resistance than that of a circuit element connected to the third terminal. The circuit element includes a transistor, and ON resistance of a transistor constituting the circuit element connected to the second terminal is higher than ON resistance of a transistor constituting the circuit element connected to the third terminal. 
     A magnetic memory device according to the present invention includes, as a memory cell, the above magnetoresistive effect element. 
     According to the present invention, the channel layer has a first channel layer and a second channel layer having differing resistance values from each other. As a result, it is possible to adjust the overall resistance value of the channel layer, which is the combined resistance of the first channel layer and the second channel layer, to a value appropriate for suppressing the power consumption. On the other hand, the resistance value of the current path of the read current that flows between the first terminal and the third terminal can be suppressed by the resistance of the second channel layer, which makes it possible to increase the read current to a relatively large value to achieve high-speed reading. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  each illustrate the structure of a magnetoresistive effect element of an embodiment of the present invention;  FIG.  1 A  is a front view and  FIG.  1 B  is a plan view. 
         FIG.  2    a perspective view of a magnetoresistive effect element of an embodiment of the present invention. 
         FIG.  3    a diagram for explaining the structure of a channel layer of a magnetoresistive effect element of an embodiment of the present invention. 
         FIGS.  4 A- 4 C  are diagrams for explaining an operation to write “1” to a magnetoresistive effect element of an embodiment of the present invention where “0” is stored;  FIG.  4 A  is a diagram for explaining the write operation,  FIG.  4 B  is a waveform diagram of the write current, and  FIG.  4 C  is an equivalent circuit. 
         FIGS.  5 A- 5 C  are diagrams for explaining an operation to write “0” to a magnetoresistive effect element of an embodiment of the present invention where “1” is stored;  FIG.  5 A  is a diagram for explaining the write operation,  FIG.  5 B  is a waveform diagram of the write current, and  FIG.  5 C  is an equivalent circuit. 
         FIG.  6    is a diagram illustrating the relationship between the magnetic resistance and the write current in a magnetoresistive effect element of an embodiment of the present invention. 
         FIGS.  7 A and  7 B  are diagrams for explaining an operation to read data out of a magnetoresistive effect element according to an embodiment of the present invention;  FIG.  7 A  is a diagram for explaining the operation to read data out of a magnetoresistive effect element where data “0” is stored, and  FIG.  7 B  is an equivalent circuit. 
         FIGS.  8 A and  8 B  are diagrams for explaining an operation to read data out of a magnetoresistive effect element according to an embodiment of the present invention;  FIG.  8 A  is a diagram for explaining the operation to read data out of a magnetoresistive effect element where data “1” is stored, and  FIG.  8 B  is an equivalent circuit. 
         FIGS.  9 A and  9 B  are diagrams for explaining magnetoresistive effect elements of Comparison Examples 1 and 2. 
         FIGS.  10 A and  10 B  are circuit diagrams of a memory cells circuit using a magnetoresistive effect element of an embodiment of the present invention as a storage cell. 
         FIG.  11    is a block diagram of a magnetic memory device equipped with a plurality of the memory cell circuits of  FIG.  10   . 
         FIG.  12    is a diagram for explaining a wire driving time of the memory cell circuit of  FIG.  11   . 
         FIGS.  13 A and  13 B  are diagrams illustrating Configuration Example 1 of the first channel layer and the second channel layer of  FIG.  3   . 
         FIGS.  14 A and  14 B  are diagrams illustrating Configuration Example 2 of the first channel layer and the second channel layer of  FIG.  3   . 
         FIG.  15    is a diagram illustrating Configuration Example 3 of the first channel layer and the second channel layer of  FIG.  3   . 
         FIG.  16    is a diagram illustrating Configuration Example 4 of the first channel layer and the second channel layer of  FIG.  3   . 
         FIG.  17    is a diagram illustrating Configuration Example 5 of the first channel layer and the second channel layer of  FIG.  3   . 
         FIGS.  18 A and  18 B  are diagrams illustrating the structure of a modification example of a magnetoresistive effect element of an embodiment of the present invention. 
         FIG.  19    is a structural diagram illustrating a conventional three-terminal SOT-MRAM. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A magnetoresistive effect element according to an embodiment of the present invention and a magnetic memory device using the magnetoresistive effect element as a memory cell will be described with reference to the figures. 
     As illustrated in the front view of  FIG.  1 A , the plan view of  FIG.  1 B , and the perspective view of  FIG.  2   , a magnetoresistive effect element  100  of an embodiment of the present invention is a three-terminal SOT (spin orbit torque)-MRAM (magnetic random access memory), and has a configuration in which a channel layer  11 , a recording layer  12 , a barrier layer  13 , and a reference layer  14  are deposited in this order or a reversed order. This configuration is a basic configuration, and another layer may be added between any two layers, or any one layer may have multiple layers. Other components may also be added. 
     In the descriptions below, a XYZ cartesian coordinate system where the longitudinal direction (extending direction) of the channel layer  11  is the X axis direction, the traverse direction of the channel layer  11  is the Y axis direction, and the height direction along which the respective layers are deposited is the Z axis direction is set, and will be referenced to as necessary. 
     The channel layer  11  includes a heavy metal, and has a flat panel shape longer in the X axis direction. The channel layer  11  is a layer formed to be 0.5 to 40 nm-thick, 50 nm to 300 nm-long in the X axis direction, and 20 nm to 150 nm wide in the Y axis direction, for example. 
     When the write current is made to flow through the channel layer  11  in the longitudinal direction (± X axis direction), a spin orbit torque is generated, and this torque changes the direction of magnetization M 12  of the recording layer  12 . Thus, the channel layer  11  contains a heavy metal or an oxide of heavy metal with large spin-orbit interaction such as W, Ta, Pt, Pd, WOx, TaOx, PtOx, or PdOx, for example. The channel layer  11  may also contain Hf, Re, Os, Ir, Pb, or an alloy thereof. Alternatively, the channel layer  11  may be made of a material doped with those heavy metals and alloy. This material may be conductive or non-conductive. The channel layer  11  as a whole has conductivity. It is also possible to add another material such as B, C, N, O, Al, Si, P, Ga, or Ga as necessary, in order to attain a desired electrical characteristic or structure. 
     The recording layer  12  is a ferromagnetic layer made of a ferromagnetic body deposited on the channel layer  11 , and made of a ferromagnetic material containing such as CoFeB, FeB, Fe, Co, Ni, and Mn or their alloy. The magnetization M 12  of the recording layer  12  can be switched between the +X axis direction and the -X axis direction by the spin orbit torque generated by the write current that flows through the channel layer  11 . 
     In this embodiment, the recording layer  12  is formed in an oval shape in a plan view as one example, and the long axis is tilted by θ with respect to the X axis. θ is approximately ±2 to ±35 degrees, for example. Therefore, in this embodiment, external magnetic field is not needed. However, it is also possible to set θ to 0 degree and provide an external magnetic field. 
     The recording layer  12  is formed to have an aspect ratio of 2.5 to 5.0, and to be 1.30 to 1.60 nm-thick, for example. The aspect ratio is the ratio of the long axis La relative to the short axis Lb of the surface of the recording layer as illustrated in  FIG.  1 B . In order to attain this aspect ratio, the recording layer  12  is formed to be about 10 to 60 nm-wide in the short axis direction, and about 40 to 600 nm in the long axis direction. By having the aspect ratio of this range, the recording layer  12 , as a signal element, has the shape magnetic anisotropy in which the magnetization M 12  is orientated in the long axis direction of the recording layer  12 . 
     The barrier layer  13  is a tunnel insulating film made of a tunnel insulating material such as a material containing Mg and O, or more specifically, MgO, for example. The barrier layer  13  has the same planer shape as the recording layer  12 , for example, and the thickness thereof is 0.1 to 5 nm. 
     The reference layer  14  is a ferromagnetic layer made of a ferromagnetic body deposited on the barrier layer  13 . The direction of magnetization M 14  of the reference layer  14  is set to the ±X axis direction. The reference layer  14  is made of a material similar to that of the recording layer  12 . The reference layer  14  and the barrier layer  13  have the same outer shape. 
     On the reference layer  14 , a first terminal (electrode) T 1  is disposed and connected. At one end of the channel layer  11  in the extending direction, a second terminal (electrode) T 2  is disposed and connected, and at the other end of the channel layer  11  in the extending direction, a third terminal (electrode) T 3  is disposed and connected. 
     In this embodiment, the second terminal T 2  and the third terminal T 3  are used for the write operation. The write voltage is applied between the second terminal T 2  and the third terminal T 3 , and the write current flows between the second terminal T 2  and the third terminal T 3  via the channel layer  11 . For the read operation, the first terminal T 1  and the third terminal T 3  are used. The read volage is applied between the first terminal T 1  and the third terminal T 3 , and the read current flows between the first terminal T 1  and the third terminal T 3  via the reference layer  14 , the barrier layer  13 , the recording layer  12 , and the channel layer  11 . 
     As illustrated in  FIG.  3   , the channel layer  11  is logically or physically divided into a first channel layer  11 A and the second channel layer  11 B using the center of gravity CG of the oval junction surface with the recording layer  12 . The first channel layer  11 A is an area of the channel layer  11  where the write current flows, but the read current does not. The second channel layer  11 B is an area of the channel layer  11  where both the write and read currents flow. 
     The resistance value of electrical resistance of the first channel layer  11 A, or more specifically, the resistance value between the boundary area of the first channel layer  11 A and the second channel layer  11 B and the connecting point between the second terminal T 2  and the first channel layer  11 A is defined as R A . Also, the resistance value of electrical resistance of the second channel layer  11 B, or more specifically, the resistance value between the boundary area of the first channel layer  11 A and the second channel layer  11 B and the connecting point between the third terminal T 3  and the second channel layer  11 B is defined as R B . The resistance value R A  of the fist channel layer  11 A is α times as large as the resistance value R B  of the second channel layer  11 B (α is a real number greater than 1, such as 2 to 12 times, for example). 
     Generally, the resistance value between the second terminal T 2  and the third terminal T 3  is in a range of 200 to 500Q, and is set to 400Q, for example. Thus, the resistance value R A  is set within a range of 150 to 460Q, and the resistance value R B  is set within a range of 20 to 200Q. However, the values are not limited to those ranges. 
     The method to make the resistance value R A  of the first channel layer  11 A differ from the resistance value RB of the second channel layer  11 B will be described later. 
     Next, the operation of the magnetoresistive effect element  100  having the configuration described above will be explained. 
     First, the write operation will be explained with reference to  FIGS.  4  and  5   . 
     First, the second terminal T 2  and the third terminal T 3  of the magnetoresistive effect element  100  are connected to a write circuit  110 . 
     As illustrated in  FIG.  4 A , the direction of magnetization M 12  of the recording layer  12  is the same as the direction of magnetization M 14  of the reference layer  14 , or the parallel state is achieved. In this case, the magnetoresistive effect element  100  has data “0” saved therein. 
     In writing data “1” to the magnetoresistive effect element  100 , the write circuit  110  applies the write voltage +V W  between the second terminal T 2  and the third terminal T 3  for the pulse width T W  as illustrated in  FIG.  4 A . As a result, the write current I W  of the pulse width T W  flows through the channel layer  11  in the -X axis direction as illustrated in  FIG.  4 B . As illustrated in the equivalent circuit of  FIG.  4 C , the write current I W  flows through the series circuit of the resistance R A  and the resistance R B , and the size thereof is I W =V W /(R A +R B ). The write current I W  is equal to or greater than a threshold value IC1 of the write current I W  of the -X axis direction ( FIG.  6   ), and the pulse width T W  is set to a time period longer than the time required for writing. 
     As illustrated in  FIG.  4 A , when the write current I W  flows, a spin flow JS (the flow of spin angular momentum) is generated in the ±Z axis direction due to the spin hall effect. This causes uneven distribution of spin, which triggers the spin orbit torque, and because the direction of the magnetization M 12  of the recording layer  12  is switched to the -X axis direction, which is opposite to the magnetization M 14  of the reference layer  14 , the antiparallel state is created. As a result, the resistance value of the circuit from the reference layer  14  to the recording layer  12  becomes high resistance R AP . This way, the data “1” is written to the magnetoresistive effect element  100 . 
     As illustrated in  FIG.  5 A , in writing data “0” to the magnetoresistive effect element  100  that is in the antiparallel state and that has the data “1” stored therein, the write voltage +Vw is applied between the third terminal T 3  and the second terminal T 2  for the pulse width T W . As a result, the write current I W  of the pulse width T W  flows through the channel layer  11  in the +X axis direction as illustrated in  FIG.  5 B . As illustrated in the equivalent circuit of  FIG.  5 C , the write current I W  flows through the series circuit of the resistance R A  and the resistance R B , and the size thereof is I W =V W /(R A +R B ). The write current I W  is equal to or greater than a threshold value IC0 of the write current I W  of the +X axis direction ( FIG.  6   ), and the pulse width T W  is set to a time period longer than the time required for overwriting. 
     When the write current I W  flows, the spin flow JS is generated due to the spin hall effect and the like, causing the direction of the magnetization M 12  of the recording layer  12  to switch to the +X axis direction, which is the same as the magnetization M 14  of the reference layer  14  as illustrated in  FIG.  4 A . As a result, the resistance value of the circuit from the reference layer  14  to the recording layer  12  becomes low resistance R P . This way, the data “0” is written to the magnetoresistive effect element  100 . 
     Next, the operation to read data out of the magnetoresistive effect element  100  will be explained. 
     First, as illustrated in  FIGS.  7 A and  8 A , the first terminal T 1  and the third terminal T 3  of the magnetoresistive effect element  100  are connected to a read circuit  120 . 
     First, as illustrated in  FIG.  7 A , the parallel state where the direction of magnetization M 12  of the recording layer  12  is the same as the direction of magnetization M 14  of the reference layer  14  is created. In this case, the resistance of the circuit from the recording layer  12  to the reference layer  14  (current path) is low resistance R P , and the magnetoresistive effect element  100  has “0” stored therein. 
     The read circuit  120  applies a read voltage V R  between the third terminal T 3  and the first terminal T 1 , and finds the resultant read current I R . As shown in the equivalent circuit of  FIG.  7 B , the read current I R  is represented by the following formula: 
     
       
         
           
             
               I 
               R 
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   p 
                 
                 
                   
                     +R 
                   
                   B 
                 
               
             
           
         
       
     
     On the other hand, in the state of  FIG.  8 A , the direction of magnetization M 12  of the recording layer  12  is -X axis direction and the direction of magnetization M 14  of the reference layer  14  is the +X axis direction (antiparallel state). Thus, the resistance of the current path from the recording layer  12  to the reference layer  14  is high resistance R AP , and the magnetoresistive effect element  100  has “1” stored therein. 
     The read circuit  120  applies the read voltage V R  between the third terminal T 3  and the first terminal T 1 , and finds the resultant read current I R . As shown in the equivalent circuit of  FIG.  8 B , the read current I R  is represented by the following formula: 
     
       
         
           
             
               I 
               R 
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   
                     AP 
                   
                 
                 
                   
                     +R 
                   
                   B 
                 
               
             
           
         
       
     
     The read circuit  120  compares the read current I R  with a prescribed reference current Is that is set in advance, and if the read current I R  is greater than the reference current Is, it is determined that the read-out data is 0, and if the read current I R  is smaller than the reference current Is, it is determined that the read-out data is 1. 
     Here, as illustrated in  FIGS.  9 A and  9 B , MRAM  191  where both the first channel layer  11 A and the second channel layer  11 B have high resistance R A  (Comparison Example 1), and MRAM  192  where both the first channel layer  11 A and the second channel layer  11 B have both low resistance R B  (Comparison Example 2) will be discussed. Resistance R A  = α•RB, where α is a real number greater than 1. 
     The write current I W  will be compared between respective cases. 
     In an embodiment of the present invention, the write current I W  is represented by Formula (1): 
     
       
         
           
             
               I 
               w 
             
             = 
             
               V 
               w 
             
             / 
             
               
                 
                   R 
                   A 
                 
                 + 
                 
                   R 
                   B 
                 
               
             
             = 
             
               V 
               w 
             
             / 
             
               
                 
                   
                     α 
                     + 
                     1 
                   
                 
                 
                   R 
                   B 
                 
               
             
           
         
       
     
     In Comparison Example 1, the write current I W  is represented by Formula (2): 
     
       
         
           
             
               I 
               W 
             
             = 
             
               V 
               W 
             
             / 
             
               
                 
                   R 
                   A 
                 
                 + 
                 
                   R 
                   B 
                 
               
             
             = 
             
               V 
               W 
             
             / 
             
               
                 2 
                 ⋅ 
                 α 
                 ⋅ 
                 
                   R 
                   B 
                 
               
             
           
         
       
     
     In Comparison Example 2, the write current I W  is represented by Formula (3): 
     
       
         
           
             
               I 
               W 
             
             = 
             
               V 
               W 
             
             / 
             
               
                 
                   R 
                   A 
                 
                 + 
                 
                   R 
                   B 
                 
               
             
             = 
             
               V 
               W 
             
             / 
             
               
                 2 
                 ⋅ 
                 
                   R 
                   B 
                 
               
             
           
         
       
     
     Formula (1) indicates that, in the magnetoresistive effect element  100  of an embodiment of the present invention, the write current I W  can be set to an appropriate size that is not too large or too small by appropriately adjusting α. This makes it possible to prevent the write time T W  from getting longer. Thus, high-speed writing is achieved without increasing power consumption. 
     On the other hand, Formula (2) indicates that, in Comparison Example 1, the write current I W  is suppressed, but because the write current I W  is too small, data writing might fail, or it might take longer for data writing to complete. 
     Also, Formula (3) indicates that, in Comparison Example 2, the write current I W  cannot be suppressed, which makes it difficult to reduce the power consumption. 
     Next, the read current I R  will be compared between respective cases. 
     In an embodiment of the present invention, the read current I R  is represented by Formulas (4) and (5): 
     
       
         
           
             
               I 
               R 
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   
                     AP 
                   
                 
                 
                   
                     +R 
                   
                   B 
                 
               
             
             , where data is “1”  
             .... 
           
         
       
     
     
       
         
           
             
               I 
               R 
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   P 
                 
                 
                   
                     +R 
                   
                   B 
                 
               
             
             ,where data is “0” 
             .... 
           
         
       
     
     In Comparison Example 1, the read current IR is represented by Formulas (6) and (7): 
     
       
         
           
             
               I 
               R 
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   
                     AP 
                   
                 
                 
                   
                     +R 
                   
                   A 
                 
               
             
             
               
                 =V 
               
               R 
             
             / 
             
               
                 
                   R 
                   
                     AP 
                   
                 
                 +α 
                 ⋅ 
                 RB 
               
             
             , where data is “1” 
             .... 
           
         
       
     
     
       
         
           
             
               I 
               R 
             
             = 
             
               V 
               R 
             
             / 
             
               
                 
                   R 
                   P 
                 
                 + 
                 
                   R 
                   A 
                 
               
             
             = 
             
               V 
               R 
             
             / 
             
               
                 
                   R 
                   P 
                 
                 + 
                 α 
                 ⋅ 
                 RB 
               
             
             , where data is “0”  
             .... 
           
         
       
     
     In Comparison Example 2, the read current I R  is represented by Formulas (8) and (9): 
     
       
         
           
             
               I 
               R 
             
             = 
             
               V 
               R 
             
             / 
             
               
                 
                   R 
                   
                     AP 
                   
                 
                 + 
                 
                   R 
                   B 
                 
               
             
             , where data is “1”  
             .... 
           
         
       
     
     
       
         
           
             
               I 
               R 
             
             = 
             
               V 
               R 
             
             / 
             
               
                 
                   R 
                   P 
                 
                 + 
                 
                   R 
                   B 
                 
               
             
             , where data is “0”  
             .... 
           
         
       
     
     Formulas (4) to (9) suggest that an embodiment of the present invention and Comparison Example 2 have a relatively large read current I R , which allows for high-speed reading. On the other hand, in Comparison Example 1, the read current I R  is small, which may extend the reading time. 
     Next, the TMR ratio will be compared between respective cases. 
     In an embodiment of the present invention, the TMR ratio is represented by Formula (10): 
     
       
         
           
             
               
                 TMR= 
                 
                   
                     
                       
                         
                           R 
                           
                             AP 
                           
                         
                         + 
                         
                           R 
                           B 
                         
                       
                     
                     − 
                     
                       
                         
                           R 
                           P 
                         
                         + 
                         
                           R 
                           B 
                         
                       
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       B 
                     
                   
                 
               
             
             
               
                 = 
                 
                   
                     
                       R 
                       
                         AP 
                       
                     
                     − 
                     
                       R 
                       P 
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       B 
                     
                   
                 
               
             
           
         
       
     
     The TMR ratio of Comparison Example 1 is represented by Formula (11): 
     
       
         
           
             
               
                 TMR= 
                 
                   
                     
                       
                         
                           R 
                           
                             AP 
                           
                         
                         + 
                         
                           R 
                           A 
                         
                       
                     
                     − 
                     
                       
                         
                           R 
                           
                             AP 
                           
                         
                         + 
                         
                           R 
                           A 
                         
                       
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       A 
                     
                   
                 
               
             
             
               
                 = 
                 
                   
                     
                       R 
                       
                         AP 
                       
                     
                     − 
                     
                       R 
                       P 
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       A 
                     
                   
                 
                 = 
                 
                   
                     
                       R 
                       
                         AP 
                       
                     
                     − 
                     
                       R 
                       P 
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     α 
                     ⋅ 
                     
                       R 
                       B 
                     
                   
                 
               
             
           
         
       
     
     The TMR ratio of Comparison Example 2 is represented by Formula (12): 
     
       
         
           
             
               
                 TMR= 
                 
                   
                     
                       
                         
                           R 
                           
                             AP 
                           
                         
                         + 
                         
                           R 
                           B 
                         
                       
                     
                     − 
                     
                       
                         
                           R 
                           P 
                         
                         + 
                         
                           R 
                           B 
                         
                       
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       B 
                     
                   
                 
               
             
             
               
                 = 
                 
                   
                     
                       R 
                       
                         AP 
                       
                     
                     − 
                     
                       R 
                       P 
                     
                   
                 
                 / 
                 
                   
                     
                       R 
                       P 
                     
                     + 
                     
                       R 
                       B 
                     
                   
                 
                 … 
                 
                   
                     12 
                   
                 
               
             
           
         
       
     
     Formulas (10) to (12) suggest that an embodiment of the present invention and Comparison Example 2 have a relatively large TMR ratio, which makes it easier to identify data and allows for high-speed reading. On the other hand, in Comparison Example 1, since the TMR ratio is relatively small, it is difficult to identify data, which may increase the read time. 
     Based on the findings of the study, in the magnetoresistive effect element  100  of an embodiment of the present invention, by appropriately setting the resistance R A  of the first channel layer  11 A and the resistance R B  of the second channel layer  11 B, high-speed writing is achieved without increasing the write current I W  low, and high-speed reading is also possible. 
     The magnetoresistive effect element of this embodiment is not limited to the embodiment described above, and various modifications and applications are possible. 
     For example, in the embodiment described above, the first channel layer  11 A and the second channel layer  11 B were separated at the center of gravity CG of the junction area of the recording layer  12  and the channel layer  11 , but the dividing position may be appropriately modified as long as the following points are fulfilled: (i) overall, the channel layer  11  has an resistance value necessary for keeping the write current I W  low; and (ii) the resistance value of an area of the channel layer  11  where the read current I R  flows through is smaller than the resistance value of an area where the read current I R  does not flow through. 
     The resistance value (R A +R B ) of the channel layer  11  necessary to attain the appropriate write current I W  may be found through experiments or simulation. As for the resistance value of the area of the channel layer  11  where the read current I R  flows through, an appropriate resistance value R B  that results in appropriate read current I R  and TMR ratio may be found through experiments or simulation. 
     In the embodiment described above, the first terminal T 1  and the third terminal T 3  were used for data reading, but the second terminal T 2  may also be used for data reading. It is also possible to conduct data reading through the second terminal T 2  and data reading through the third terminal T 3  at the same time, i.e., dual port memory. However, in this case, in data reading through the second terminal T 2 , the read current I R  is smaller than that in data reading through the third terminal T 3 , and therefore, appropriate signal processing would be necessary. 
     Next, a configuration example of a memory circuit that uses as a memory cell the magnetoresistive effect element  100  having the configuration described above will be explained with reference to  FIG.  10 A . 
       FIG.  10 A  illustrates the configuration of a memory cell circuit  200  for one bit. 
     The memory cell circuit  200  includes a magnetoresistive effect element  100  constituting a memory cell for one bit, a pair of bit lines BL 1  and BL 2 , a word line WL, a source line SL, a first selector transistor T r   1 , and a second selector transistor T r   2 . 
     The first terminal T 1  of the magnetoresistive effect element  100  is connected to the source line SL. The second terminal T 2  is connected to the drain of the first selector transistor T r   1 , and the third terminal T 3  is connected to the drain of the second selector transistor T r   2 . The gate electrodes of the first selector transistor T r   1  and the second selector transistor T r   2  are commonly connected to the word line WL. The source of the first selector transistor T r   1  is connected to the first bit line BL 1 , and the source of the second selector transistor T r   2  is connected to the second bit line BL 2 . 
     In order to write data to the magnetoresistive effect element  100 , first, an active level signal is applied to the word line WL to turn on the selector transistors T r   1  and T r   2  to select the magnetoresistive effect element  100 . In this example, the selector transistors T r   1  and T r   2  are constituted of N-channel MOS transistors, and the word line WL is set to a high level. This turns on the first selector transistor T r   1  and the second selector transistor T r   2 . Then, depending on the data to be written, one of the first bit line BL 1  and the second bit lint BL 2  is set to the high level, and the other is set to the ground level. 
     Specifically, when data “1” is to be written, the first bit line BL 1  is set to the low level, and the second bit line BL 2  is set to the high level. This causes the write current I W  to flow in the -X axis direction as illustrated in  FIG.  4 A , and data “1” is written as a result. 
     On the other hand, when data “0” is to be written, the first bit line BL 1  is set to the high level, and the second bit line BL 2  is set to the low level. This causes the write current I W  to flow as illustrated in  FIG.  5 A , and data “0” is written as a result. Bit data is written to the magnetoresistive effect element  100  in this way, 
     On the other hand, in order to read information stored in the magnetoresistive effect element  100 , the word line WL is set to the active level to turn on the first selector transistor T r   1  and the second selector transistor T r   2 . The bit line BL 1  is set to an open state, and the bit line BL 2  is set to the low (ground) level. The read voltage V R  is applied to the source line SL. As a result, the read current I R  flows from the source line SL to the second bit line BL 2  via the first terminal T 1 , the reference layer  14  to the channel layer  11 , the third terminal T 3 , and the second selector transistor T r   2 . By comparing the read current I R  with the reference current I S  through a not-shown op-amplifier and the like, the stored data is obtained. 
     The circuit configuration and operation described above is one example, and may appropriately be modified. 
     For example, if the magnetoresistive effect element  100  is used as dual port memory, as illustrated in  FIG.  10 B , the gate of the first selector transistor T r   1  may be connected to the first word line WL 1 , and the gate of the second selector transistor T r   2  may be connected to the second word line WL 2 . In this configuration, by driving the first word line WL 1  and the second word line WL 2  independently of each other, data reading through the second terminal T 2  and data reading through the third terminal T 3  may be performed individually and in parallel. In this case, for data reading through the third terminal T 3 , relatively large read current I R  and TMR ratio are attained as described above. 
     On the other hand, in data reading through the second terminal T 2 , the load resistance R A  is relatively large. However, in dual port memory, it is common that each port has a different load resistance, and it is preferable to adopt a circuit configuration that can handle each load resistance. 
     Next, with reference to  FIG.  11   , a configuration of a magnetic memory device  300  that includes a plurality of memory cell circuits  200  illustrated in  FIGS.  10 A and  10 B  will be explained. 
     As illustrated in the figure, the magnetic memory device  300  includes a memory cell array  311 , an X driver  312 , a Y driver  313 , and a controller  314 . The memory cell array  311  has memory cell circuits  200  arranged in an array of N rows and M columns. The memory cell circuit  200  of each column is connected to a pair of first bit line BL 1  and second bit line BL 2  of the corresponding column. The memory cell circuit  200  of each row is connected to the word line WL and the source line SL of the corresponding row. The X driver  312  and the Y driver  313  are selector circuits for selecting a memory cell. 
     The X driver  312  is connected to a plurality of word lines WL and source lines SL, decodes a row address, and drives the word line of the row to be accessed to the active level. The X driver  312  also sets the voltage of the source line SL for required operations. For ease of illustration, there is only one word line WL illustrated, but in the configuration of  FIG.  10 B , two word lines are provided. 
     The Y driver  313  is connected to a plurality of first bit lines BL 1  and second bit lines BL 2 , decodes a column address, and brings the first bit line BL 1  and the second bit line BL 2  connected to the memory cell circuit  200  to be accessed to a data writing state or data reading state for desired data. Further, when reading data stored in the memory cell circuit  200 , the Y driver  313  sets the first bit line BL 1  to an open state and the second bit line BL 2  to the low level, compares the read current flowing through the bit lines BL 1  and BL 2  with the reference current to identify the resistance of the memory cell circuit  200  of each column, and retrieves stored data. 
     The controller  314  controls the X driver  312  and the Y driver  313  individually for data writing or data reading. 
     Each magnetoresistive effect element  100  has a small bit line driving capacity for data reading. Thus, as illustrated in  FIG.  11   , if a plurality of magnetoresistive effect elements  100  are connected to one bit line BL, it takes longer for one magnetoresistive effect element  100 , which is to be read, to drive the bit line BL (wire driving time) t cir , which might affect the access speed. This becomes more pronounced as the number of magnetoresistive effect elements  100  connected to one bit line BL increases. 
       FIG.  12    illustrates a typical relationship between the number of magnetoresistive effect elements  100  connected to one bit line, the total capacity, the wire driving time t cir , and the current I cell  that flows the magnetoresistive effect element  100 . 
     This graph is based on the following formula: 
     
       
         
           
             
               t 
               
                 cir 
               
             
             = 
             C 
             × 
             
               V 
               
                 cell 
               
             
             
               
                 /I 
               
               
                 cell 
               
             
               
             
               
                 C 
                   
                 represents 
                   
                 the 
                   
                 capacity 
                   
                 of 
                   
                 one 
                   
                 bit 
                   
                 line 
                   
                 
                   
                     wire 
                       
                     capacity 
                   
                 
               
             
           
         
       
     
     The wire capacity C can be found from “the wire capacity per unit length of a bit line” × “the length required to connect the magnetoresistive effect element 100”. In this example, the wire capacity per unit length of a bit line is 208aF(=208x10 -18 F)/µm. This is based on “International Roadmap for Devices and Systems (TRDS) 2018 Update”. Hypothetically, the cell size of the magnetoresistive effect element  100  is 4F × 4F = 16F 2 , F=40 nm. This represents a circuit that can accommodate two wires each in the X axis direction and Y axis direction per one memory cell, which coincides with the configuration illustrated in  FIG.  10   . The length of the bit line can be found by “4F × the number of connected memory cells”. 
     V cell  is the voltage of the bit line BL set by the magnetoresistive effect element  100  in the antiparallel state, and V cell  is 0.8 V in this case. I cell  is the value of read current that flows through the magnetoresistive effect element  100 . 
     As  FIG.  12    suggests, as long as the read current I cell  is 14 µA, even when the number of memory cells per bit line is approximately 500 and the bit size is approximately 1 Gbit, it is possible to achieve the wire driving time t cir  of about 0.5 ns. 
     The resistance R P  of a magnetoresistive effect element in the approximate parallel state is about 14,000 Q, and the resistance R AP  of a magnetoresistive effect element in the antiparallel state is about 34,000 Q. Thus, from 0.8/34,000, the read current I cell  is 23.5 µA. This means that according to an embodiment of the present invention, it is possible to achieve high-speed reading with the wire driving time t cir  being 0.5 ns or shorter as well as lower power consumption. 
     Next, an example of the specific structure to make the resistance value R A  of the first channel layer  11 A differ from the resistance value R B  of the second channel layer  11 B will be explained with reference to  FIGS.  13  to  17   . In  FIGS.  13  to  17   , the recording layer  12  to reference layer  14  will simply be referred to as MTJ. 
     Configuration Example 1 
     The length L, thickness t and width W of a thin film are defined as in  FIG.  13 A . 
     In this case, the resistance value R of a thin film is represented by R=ρ·L/(t·W), where ρ is a resistance rate (Ω·m), L is the length of thin film (m), t is the thickness of thin film (m), and W is the width of thin film (m). 
     This applies to other configuration examples. 
     As illustrated in  FIG.  13 B , by making the first channel layer  11 A thinner and the second channel layer  11 B thicker so that the thickness t differs from each other, it is possible to make the channel layer  11  have several regions with differing resistance values. 
     The first channel layer  11 A and the second channel layer  11 B may be formed to have different thicknesses by forming a base layer that have different surface levels, depositing a heavy metal on top of this base layer through sputtering, planarizing the surface of the deposited layer, and patterning the layer thereafter. 
     Configuration Example 2 
     As illustrated in  FIG.  14 A , by making the first channel layer  11 A longer and the second channel layer  11 B shorter so that the length L differs from each other, it is possible to make the channel layer  11  have several regions with differing resistance values. It is not necessary to make the first channel layer  11 A and the second channel layer  11 B in a linear shape, and as long as the necessary length L is secured, any appropriate shapes may be adopted such as a spiral shape as illustrated in  FIG.  14 B . 
     Configuration Example 3 
     As illustrated in  FIG.  15   , by making the first channel layer  11 A narrower and the second channel layer  11 B wider, it is possible to make the channel layer  11  having several different widths W. 
     Configuration Example 4 
     As schematically illustrated in  FIG.  16   , even when the outer shape of the first channel layer  11 A differs from that of the second channel layer  11 B, by using different materials for the first channel layer  11 A and the second channel layer  11 B, it is possible to make the channel layer  11  have differing resistance rates ρ. In this case, by adjusting the formulation of various heavy metals and additives, appropriate resistance rates are achieved. Examples of the manufacturing method for the channel layer  11  include: i) forming the first channel layer  11 A by forming the first heavy metal layer having the resistance rate ρ1 and patterning this layer; ii) forming a mask that covers the first channel layer  11 A; iii) forming the second channel layer  11 B by depositing the second heave metal having the resistance rate ρ2 to form a heavy metal layer and patterning this layer; and iv) removing the mask. 
     Configuration Example 5 
     Even when the first channel layer  11 A and the second channel layer  11 B are the same as each other, the channel layer  11  can be realized by increasing the resistance value of a circuit element connected to the second terminal T 2  and decreasing the resistance of a circuit element connected to the third terminal T 3 .  FIG.  17    illustrates an example where the selector transistors T r   1  and T r   2  of  FIGS.  10 A and  10 B , which are examples of the circuit element, are made to have differing ON resistances. As the method for having differing ON resistances, any appropriate method may be adopted such as making the size of the transistors (channel width or channel length) differ from each other, or doping the channel region with impurities. 
     Configuration Examples 1 to 5 may be combined appropriately. 
     In the descriptions above, the present invention was explained using an example of the magnetoresistive effect element  100  in which the direction of magnetization M 12  of the recording layer  12  and the direction of magnetization M 14  of the reference layer  14  are substantially parallel to the extending direction of the channel layer  11  (X axis direction) (for example, θ=0 to ±35°). However, the present invention is not limited to this. For example, as illustrated in  FIG.  18 A , the present invention may be applied to a magnetoresistive effect element  100 A in which the direction of magnetization M 12 A of the recording layer  12  and the direction of magnetization M 14 A of the reference layer  14  are substantially parallel to the plane of the recording layer  12  and the reference layer  14 , and substantially perpendicular to the extending direction of the channel layer  11  (X axis direction) (±Y axis direction). Also, as illustrated in  FIG.  18 B , the present invention may be applied to a magnetoresistive effect element  100 B in which the direction of magnetization M 12 B of the recording layer  12  and the direction of magnetization M 14 B of the reference layer  14  are substantially perpendicular to the plane of the recording layer  12  and the reference layer  14  (± Z axis direction). 
     As described above, according to an embodiment of the present invention, it is possible to provide a magnetoresistive effect element that can achieve high-speed writing and high-speed reading while keeping the write current I W  low for energy conservation, and a magnetic memory device provided with such a magnetoresistive effect element.