Abstract:
The present invention aims to reduce heat fluctuations of a memory cell and thereby provide a stable writing operation when a magnetization reversal process not involving a reversal magnetic field is used for writing into the memory cell. The magnetic memory cell has a structure where first and second magnetization pinned terminals are connected, with a space therebetween, to one surface of a non-magnetic region, and a magnetization free terminal is connected to the other surface. Magnetization directions of the first and second magnetization pinned terminals are anti-parallel to each other. Writing is performed by controlling a polarity of a current flowing between the first and second magnetization pinned terminals through the non-magnetic region and thus reversing magnetization of the magnetization free terminal. Reading is performed by detecting a magnetic resistance attributable to a change in relative magnetization direction between the first magnetization pinned terminal and the magnetization free terminal.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2007-002577 filed on Jan. 10, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a magnetic memory cell in which information writing and information reading are performed by the use of a magnetoresistance effect and also relates to a device using the same. 
         [0004]    2. Description of the Related Art 
         [0005]    The magnetoresistance effect is a phenomenon that an electric resistance is changed when a magnetic field is applied to a magnetic material or when a magnetization state of a magnetic material is changed. A magnetic head and a magnetic sensor have been known as a magnetoresistance effect device utilizing the magnetoresistance effect. Moreover, recently, a nonvolatile solid-state magnetic memory device (MRAM) and the like are being experimentally manufactured. 
         [0006]    A mainstream configuration of an MRAM which is now experimentally manufactured has a matrix structure in which tunnel magnetoresistance (TMR) elements are arranged at intersection points between bit lines and word lines. A magnetization direction of each of the TMR elements is reversed by a synthetic magnetic field formed by a current flowing through each line. Thus, the writing of information is performed. In this case, the TMR element serves as a memory cell. In order to read information from the memory cell, cell selection by a MOS transistor is required due to the presence of a leak current from the memory cell. As to the MRAM having the above structure, there have been pointed out, due to the structure, a drawback such as a complex process technology as well as the following three drawbacks that make the MRAM unfit for having higher density and larger capacity. The first drawback is that effective reversal magnetic field conditions are narrowed by reduction in size of the memory cell. The second is that a reversal magnetic field is increased by reduction in thickness of the magnetic material, and therefore a line current and power consumption are increased. The third is that, having a MOS transistor, the MRAM has a possible integration level only as high as a DRAM. 
         [0007]    In the meantime, there has recently been proposed a spin torque, which is a magnetization reversal process using no reversal magnetic field, and the occurrence of magnetization reversal by the spin torque has actually been confirmed. It has also been proposed that the spin torque be used for writing into a memory cell. However, the magnetization reversal by use of the spin torque currently has the following technical problem. The problem is specifically that a critical current density as high as approximately 10 8  A/cm 2  is required to generate the spin torque that causes the magnetization reversal. 
         [0008]    Meanwhile, spin torque magnetization reversal caused by a spin flow alone has been examined by use of a device structure called a non-local spin valve. However, it has only been confirmed that magnetization of a magnetization free layer is rotated to one direction from an anti-parallel state to a parallel state with respect to a magnetization pinned layer. Thus, writing into a memory cell with perfect control has not yet been realized; see Phys. Rev. Lett. 96, 037201-1-037201-4 (2006) (Non-patent Document 1). When only one direction of rotation was confirmed, the value of a critical current density in a non-magnetic portion that carries the spin flow became as high as approximately 2×10 8  A/cm 2 . 
         [0009]    Since the TMR element utilizes a high resistance of a tunnel barrier, such a high critical current density required for magnetization reversal leads to a concern over electrostatic breakdown of the tunnel barrier. In order to avoid the electrostatic breakdown, it has been proposed to separate a write current path from a read current path in a memory cell by making an element including three terminals in Japanese Patent Application Laid-Open Publication No. 2006-156477. However, since this three-terminal device structure includes three magnetic layers on a read current path, there arises a concern over a magnetic noise effect as the device is miniaturized. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to provide a magnetic memory cell which reduces heat fluctuations of the memory cell and performs a stable writing operation when a magnetization reversal process not involving a reversal magnetic field for writing into the memory cell is adopted as well as in the case of spin torque magnetization reversal. The present invention also aims to provide a device using the magnetic memory cells. 
         [0011]    In order to achieve the foregoing objects, a magnetic memory cell according to the present invention includes: first and second magnetization pinned terminals connected, with a space therebetween, to one surface of a non-magnetic region; and a magnetization free terminal connected to the other surface of the non-magnetic region. In the magnetic memory cell, magnetization directions of the first and second magnetization pinned terminals are anti-parallel to each other. Moreover, writing is performed by controlling a polarity of a current flowing between the first and second magnetization pinned terminals through the non-magnetic region, and thus reversing magnetization of the magnetization free terminal. Furthermore, reading is performed by detecting a magnetic resistance attributable to a change in the relative magnetization direction between the first magnetization pinned terminal and the magnetization free terminal. 
         [0012]    In the above configuration, reading from the magnetic memory cell, in other words, reading of the magnetization direction of the magnetization free terminal can be performed through a current path different from that in writing into the memory cell, in other words, at the time of magnetization reversal. Since only a current having a direct current density of not more than 10 4  A/cm 2  flows through the magnetization free terminal, heat fluctuations of the magnetization free terminal due to Joule heat can be suppressed. Moreover, for the reason described above, a relatively large current density can be used to perform writing; and this allows a writing operation to be carried out stably. 
         [0013]    According to the present invention, it is possible to realize a magnetic memory cell which enables stable writing with a low current density required for reading and low magnetic fluctuations of the memory cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a view showing an example of a planar structure of a magnetic memory cell according to the present invention. 
           [0015]      FIG. 2  is a view showing a simulation result of spin accumulation in a magnetic memory cell according to the present invention. 
           [0016]      FIGS. 3A to 3C  are schematic figures of spin accumulation in a magnetic memory cell according to the present invention. 
           [0017]      FIG. 4  is a view showing a simulation result of spin accumulation in a magnetic memory cell where the pinned terminal is replaced by a non-magnetic insulator. 
           [0018]      FIG. 5  is a view showing another example of a planar structure of a magnetic memory cell according to the present invention. 
           [0019]      FIG. 6  is a view showing an example of a solid state memory according to the present invention. 
           [0020]      FIG. 7  is a view showing a single solid state memory cell according to the present invention being mounted on a silicon substrate. 
           [0021]      FIG. 8  is a view showing an example of a solid state memory according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    With reference to the drawings, an embodiment of the present invention will be described below. 
       Application 1 to Solid State Memory Cell 
       [0023]      FIG. 1  is a view showing a planar structure of a magnetic memory cell  10  according to the present invention. An aluminum (Al) island (non-magnetic region)  11  having a width of 400 nm and a length of 400 nm is prepared by use of a standard electron-beam lithography technology, and a portion  12  of the Al island  11  is oxidized by use of a lift-off technique. Thereafter, a first magnetization pinned terminal  13  made of a Co thin wire having a width of 50 nm is connected to the oxidized portion  12  of the Al island so as to form a tunnel junction. Meanwhile, a second magnetization pinned terminal  14  made of a Co thin wire having a width of 100 nm and a magnetization free terminal  15  made of a permalloy (Py) thin wire having a width of 50 nm are connected to the Al island  11 . In this case, the Al island  11 , the magnetization pinned terminals  13  and  14 , and the magnetization free terminal  15  all have a thickness of 40 nm. After a sufficiently large external magnetic field is applied to align magnetization directions of the magnetization pinned terminals  13  and  14  and the magnetization free terminal  15  in one direction, an external magnetic field having an opposite direction to the above magnetic field is swept to reverse only the magnetization direction of the second magnetization pinned terminal  14 . The first magnetization pinned terminal  13  and the second magnetization pinned terminal  14  are provided with a space therebetween on one surface of the non-magnetic region  11 . The magnetization free terminal  15  is provided on a different surface of the non-magnetic region  11  from the surface on which the magnetization pinned terminals  13  and  14  are provided.  FIG. 1  shows the example where the magnetization free terminal  15  is provided at a position facing the first magnetization pinned terminal  13  across the non-magnetic region. However, the magnetization free terminal  15  does not always have to face the first magnetization pinned terminal  13 . 
         [0024]      FIG. 2  shows a simulation result obtained in the case where a current of 100 μA is applied in the direction from the first magnetization pinned terminal  13  to the second magnetization pinned terminal  14  in the above magnetization state. It should be noted that each of the magnetization terminals  13 ,  14 , and  15  is set to have a width of 100 nm for simplicity.  FIG. 2  shows a simulation result representing a spatial distribution of spin accumulation in the Al island  11  when an x-y coordinate system is introduced as shown in the left drawing in  FIG. 2 . ΔECP represents a difference in electrochemical potential between an upward spin and a downward spin. Specifically, this slope is proportional to the size of a spin flow flowing into and out of each of the magnetization terminals. This simulation is carried out at an absolute zero of temperature (T=0 K), and temperature dependence of the system is taken into consideration according to spin diffusion lengths of magnetic and non-magnetic materials. To be more specific, spin diffusion lengths of Co, Py and Al are set to be 50 nm, 5 nm and 600 nm, respectively, which have been estimated at low temperature. 
         [0025]    Here, for intuitive understanding of spin accumulation,  FIGS. 3  A,  3 B, and  3 C show how spins are accumulated. In  FIG. 3A , a dotted arrow indicates the direction of a current flow, and three thick arrows indicate the magnetization directions of the magnetization terminals. In  FIGS. 3A ,  3 B, and  3 C, the magnetization directions are indicated by the upward and downward arrows. Since only a parallel or anti-parallel relationship among the magnetization directions is important, a magnetization direction may be left and right, or frontward and backward with respect to the paper surface in addition to upward and downward as shown in  FIGS. 3A ,  3 B, and  3 C.  FIG. 3B  shows electrons flowing into the non-magnetic region  11  from the magnetization terminal  13  in a snapshot taken at certain time intervals. In this case, it is assumed here that six electrons have been injected. For simplicity, suppose that the spin polarizability of the magnetization terminal  13  is 66.6% (=⅔), and it is determined that four out of the six electrons have an upward spin and the remaining two have a downward spin.  FIG. 3C  shows electrons flowing into the magnetization terminal  14  from the non-magnetic region  11  in a snapshot taken at the same time intervals as in  FIG. 3B . According to the law of conservation of charge, the injected six electrons must flow out. Moreover, if suppose that the spin polarizability of the magnetization terminal  14  is also 66.6%, for simplicity, it is determined that two out of the six electrons to flow out have to have a upward spin and the remaining four have a downward spin. Therefore, two upward spins remain. 
         [0026]    Downward spins have to be carefully examined. There are only two downward spins in  FIG. 3B . Since the requirement is that the number of downward spins to flow out is four, two downward spins are taken out of the electrons in the non-magnetic material itself of the non-magnetic region  11 . These downward spins to be taken out are indicated by dotted circles in  FIG. 3C . As a result, disruption of the balance between upward spins and downward spins in the non-magnetic material causes two upward spins to appear. These upward spins to appear are indicated by upward spins connected with the dotted circles described above. It should be noted that these upward spins have a positive charge. As a result, the total charge becomes zero, and four upward spins are to be accumulated in the non-magnetic region  11 . Being spins in a nonequilibrium steady state, the spins accumulated in the non-magnetic region  11  are diffused as a spin flow into the magnetization terminal  15 . However, since there is no current flowing through the magnetization terminal  15 , the charges are required to be conserved. 
         [0027]    For comparison,  FIG. 4  shows a simulation result obtained in the case where only one of current injection terminals is made of a magnetic material and the other one is made of a non-magnetic material, as in the case of Non-patent Document 1. In relation to  FIG. 2 ,  FIG. 4  shows the case where the second magnetization pinned terminal  14  is made of a non-magnetic material. As is clear from the comparison between  FIGS. 2 and 4 , not only is the spin accumulation larger in  FIG. 2  but the slope in the vicinity of the magnetization free terminal  15  is also steeper in  FIG. 2 . Therefore, in the magnetic memory cell of the present invention, compared with the configuration corresponding to the device disclosed in Non-patent Document 1, the spin flow can be more efficiently injected into the magnetization free terminal  15 . Thus, the magnetization of the magnetization free terminal  15  can be reversed with a smaller current value. Moreover, in the case of reversal of current polarity, the symbols in the spin accumulation are merely reversed since the magnetization directions of the two magnetization pinned terminals are opposite to each other. As compared with the configuration corresponding to the device disclosed in Non-patent Document 1, the device structure is symmetrical in terms of reversal of current polarity. Thus, the magnetization direction of the magnetization free terminal  15  can be easily set parallel or anti-parallel to the magnetization direction of the first magnetization pinned terminal  13 . Hence, writing into the memory cell with perfect control can be easily realized. 
         [0028]    Electrodes which come into contact with the first and second magnetization pinned terminals  13  and  14  in the magnetic memory cell  10  are provided outside the memory cell, and an electrode is further provided on the magnetization free terminal  15 . First, a sense current of 0.1 μA is supplied in the direction from the first magnetization pinned terminal  13  to the magnetization free terminal  15 . A resistance was measured at this point. Next, a current of 10 μA is supplied with a pulse width of 1 ns in the direction from the first magnetization pinned terminal  13  to the second magnetization pinned terminal  14 . Subsequently, a sense current of 0.1 μA is supplied in the direction from the first magnetization pinned terminal  13  to the magnetization free terminal  15 . A resistance was measured at this point as well, and it was found that the resistance was increased by approximately 100Ω. This change in resistance is approximately equal to the change in resistance in the case where a sufficiently large external magnetic field is applied to align the magnetization directions of all the magnetization terminals in one direction, and then an external magnetic field having an opposite direction thereto is swept to reverse the magnetization of the magnetization free terminal  15 , as in the case described above. Specifically, supply of the current of 10 μA in the direction from the first magnetization pinned terminal  13  to the second magnetization pinned terminal  14  induces reversal of magnetization of the magnetization free terminal  15 . Therefore, the magnetization of the magnetization free terminal  15  is considered to be anti-parallel to that of the first magnetization pinned terminal  13 . 
         [0029]    Next, a current of 10 μA is supplied with a pulse width of 1 ns in the direction from the second magnetization pinned terminal  14  to the first magnetization pinned terminal  13 . In other words, a current having an opposite polarity to that of the current-induced magnetization reversal described above is supplied. Before and after this current supply, the resistance between the first magnetization pinned terminal  13  and the magnetization free terminal  15  was measured by following the same procedures as described above. It was found that the resistance was reduced by approximately 100Ω. Specifically, the magnetization of the magnetization free terminal  15  is considered to be in an initial state of being parallel to the magnetization of the first magnetization pinned terminal  13 . The above change in resistance is equivalent to approximately 100% when expressed in a magnetoresistance ratio. 
         [0030]    The device size is not limited to the value described above. Since the spin diffusion length of the Al island, which is a spin accumulation region, is 600 nm, the spin flow can be relatively efficiently transported as long as the device size is smaller than that value. Moreover, even when a different material is used for the spin accumulation region, the same effect can be achieved as long as the device size is set smaller than a spin diffusion length of that material. Having the device size larger than its spin diffusion length, the magnetization terminals can perform more effective injection and suction of the spins. In this case, Co is used for the first and second magnetization pinned terminals, and a Co-base alloy, such as CoFe and CoFeB, also has the same spin diffusion length. When an MgO insulator is used as a material of a tunnel barrier, CoFe and CoFeB are preferable from the viewpoint of formation of a matched interface. 
         [0031]    In terms of current density, the current in the magnetization reversal described above is equivalent to 5×10 5  A/cm 2  in the smaller Co terminal. This value is larger by about one digit than an ideal value if suppose the cells are integrated to form a memory. This is because, in the case where the cells operate as a memory, a current density of approximately 10 3  A/cm 2  is required to drive a decoder for specifying an address and a MOS-FET for selecting a cell. If the magnetization reversal can be performed at a value larger by one digit than the above value, the magnetization is to be reversed at low power consumption without accidentally causing the magnetization reversal by a current value for specifying the address. Moreover, a current value in measurement of the magnetic resistance described above is smaller by two digits than that in the magnetization reversal. However, if suppose the cells are accumulated to form a memory as well, a distinct difference of one digit or above is required between the two current values. 
         [0032]    The current in the magnetization reversal is not limited to the pulse width of 1 ns described above. Even if the shortest pulse width is set to 1 ns due to constraints on an experimental apparatus, the same change in resistance can also be confirmed with the pulse width of 100 ns. It should be noted, however, that prolonged current supply leads to a concern over magnetization instability due to an increase in temperature. 
       Application 2 to Solid State Memory Cell 
       [0033]      FIG. 5  is a view showing another example of a planar structure of a magnetic memory cell  20  according to the present invention. An Al island (non-magnetic region)  21  having a width of 400 nm and a length of 400 nm is prepared by use of an ordinary electron-beam lithography technology. Thereafter, a first magnetization pinned terminal  22  made of a Co thin wire having a width of 50 nm, a second magnetization pinned terminal  23  made of a Co thin wire having a width of 100 nm, and a magnetization free terminal  24  made of a permalloy (Py) thin wire having a width of 50 nm are connected to the Al island  21 . The first magnetization pinned terminal  22  and the second magnetization pinned terminal  23  are provided with a space therebetween on one surface of the non-magnetic region  21 . Moreover, the magnetization free terminal  24  is provided on a different surface of the non-magnetic region  21  from the surface on which the magnetization pinned terminals  22  and  23  are provided.  FIG. 5  shows the example where the magnetization free terminal  24  is provided at a position facing the first magnetization pinned terminal  22  across the non-magnetic region  21 . However, it is not necessarily meant that having the arrangement shown in  FIG. 5  is essential. 
         [0034]    An operation of the memory cell is the same as that of the magnetic memory cell  10  shown in  FIG. 1 . First, a sense current of 0.1 μA is supplied in the direction from the first magnetization pinned terminal  22  to the magnetization free terminal  24 . A resistance was measured at this point. Next, a current of 20 μA is supplied with a pulse width of 1 ns in the direction from the first magnetization pinned terminal  22  to the second magnetization pinned terminal  23 . Subsequently, a sense current of 0.1 μA is supplied in the direction from the first magnetization pinned terminal  22  to the magnetization free terminal  24 , and a resistance was measured at this point as well. As a result, it was found that the resistance was increased by approximately 1Ω. This change in resistance is approximately equal to a change in resistance in the case where an external magnetic field is swept to reverse magnetization of the magnetization free terminal  24 , as in the case of the magnetic memory cell  10  shown in  FIG. 1 . Then, a current of 20 μA is supplied with a pulse width of 1 ns in the direction from the second magnetization pinned terminal  23  to the first magnetization pinned terminal  22 . The resistances between the first magnetization pinned terminal  22  and the magnetization free terminal  24  before and after the current supply were measured, and it was found that the resistance was reduced by approximately 1Ω. 
       Application 1 to Solid State Memory 
       [0035]    Next, description will be given of an example of a solid state memory using the magnetic memory cell shown in  FIG. 1 .  FIG. 6  shows a solid state memory having two rows and two columns as an example that the magnetic memory cells  10  shown in  FIG. 1  are arranged in an X-Y matrix pattern. In  FIG. 6 , the magnetic memory cells  300   11 ,  300   12 ,  300   21  and  300   22  shown in  FIG. 1  are arranged at intersections where bit lines  311   1  and  311   2  cross with word lines  312   1 ,  312   2 ,  313   1  and  313   2 , respectively. Two word lines are provided for each memory cell. Reference numeral  318  denotes a decoder for the bit lines, and reference numerals  319   1  and  319   2  denote decoders for the word lines. In response to specification of a write or read address, the decoders  318  and  319  select one of the bit lines and one of the word lines, respectively. Thereafter, a gate is opened and a current is supplied to each of the magnetic memory cells  300  from a source. Incidentally, the word lines  313   1  and  313   2  are selectively connected to a data line  314  by opening and closing of the gates of MOS-FETs  317   12  and  317   22 . Reference numerals  315   1  and  315   2  denote negatively-biased and positively-biased power supply lines, respectively. 
         [0036]    First of all, description will be given of an operation by taking, as an example, writing into the magnetic memory cell  300   11 . First, current is applied to the bit line  311   1  by the decoder  318  which specifies a write address to open the gate near the magnetization terminal  13  in the magnetic memory cell  300   11 . Since the magnetization terminal  14  is grounded, the word line  312   1  is to be connected to the positively-biased power supply line  315   2  or the negatively-biased power supply line  315   1  by opening any of gates of MOS-FETs  316   11  and  316   12  by the decoder  319   2  which specifies a write address. As described above, being reversed or retained by a polarity of a supply current depending on the initial state, the magnetization direction of the magnetization free terminal  15  is uniquely determined. When the positively-biased power supply line  3152  is selected by opening the gate of the MOS-FET  316   11 , it is necessary to maintain the potential of the other word line  313   1  equivalent to that of the word line  312   1  by opening a gate of a MOS-FET  317   11  at the same time in order to prevent the current from flowing into the magnetization free terminal  15  while allowing a spin flow to flow through the magnetization free terminal  15 . When the negatively-biased power supply line  315   1  is selected, it is not necessary to connect between the power supply line and the word line  313   1 , since there is no current flow flowing into the magnetization free terminal  15 . 
         [0037]    Next, description will be given of an operation by taking, as an example, reading from the magnetic memory cell  300   11 . First, current is applied to the bit line  311   1  by the decoder  318  which specifies a read address to open the gate near the magnetization terminal  13  in the magnetic memory cell  300   11 . By opening the gate of the MOS-FET  317   12  by the decoder  319   1  which specifies a read address, a resistance connected to the data line  314  is read. 
         [0038]      FIG. 7  is a schematic view focusing on a single memory cell  110  to show an example where a solid state memory using the magnetic memory cells shown in  FIG. 1  described above is mounted on a silicon substrate. On a silicon substrate  120 , a MOS-FET is formed. Reference numerals  111  and  112  in  FIG. 7  denote a source and a drain of the MOS-FET, respectively. A first word line  312 , an oxide layer  113  of the MOS-FET, and a wiring  114  to the drain are formed. Reference numeral  311  in  FIG. 7  denotes a bit line. A non-magnetic region  11 , an oxide layer  12 , a magnetization pinned terminal  13 , a magnetization free terminal  15 , and a second word line  313  are formed. All of those described above are formed by use of a lithography technology commonly used in the field of semiconductor. The bit line  311  and the first and second word lines  312  and  313  are formed in an X-Y matrix pattern. Although  FIG. 7  shows only a single memory cell, a plurality of such memory cells are arranged in the X-Y matrix pattern on the silicon substrate  120 . In this example, Cu is used as a material of the bit line and the word lines. 
       Application 2 to Solid State Memory 
       [0039]      FIG. 8  shows a solid state memory having 2 rows and two columns as an example that the magnetic memory cells  10  shown in  FIG. 1  are arranged in an X-Y matrix pattern. The solid state memory shown in  FIG. 8  is different from that shown in  FIG. 6  in a point that memory cell selection is possible without using a MOS-FET, since a magnetoresistance ratio of the magnetic memory cell  10  reaches a value as large as approximately 100%. 
         [0040]    In  FIG. 8 , as in the case of the solid state memory shown in  FIG. 6 , the magnetic memory cells  400   11 ,  400   12 ,  400   21  and  400   22  shown in  FIG. 1  are arranged at intersections where bit lines  411   1  and  411   2  cross with word lines  412   1 ,  412   2 ,  413   1  and  413   2 . As in the case of the solid state memory shown in  FIG. 6 , two word lines are provided for each memory cell. Reference numeral  418  denotes a decoder for the bit lines, and reference numerals  419   1  and  419   2  denote decoders for the word lines. In response to specification of a read or write address, the decoders  418 ,  419   1  and  419   2  select one of the bit lines and one of the word lines, respectively, to supply a current to each of the magnetic memory cells  400 . The word lines  413   1  and  413   2  are selectively connected to a data line  414  by opening and closing gates of MOS-FETs  417   12  and  417   22 . Reference numerals  415   1  and  415   2  denote negatively-biased and positively-biased power supply lines, respectively. Incidentally, unlike the solid state memory shown in  FIG. 6 , having no MOS-FET for cell selection, the solid state memory shown in  FIG. 8  has the bit lines  411  connected to the two positive and negative power supply lines. Specifically, although not shown in  FIG. 8 , the same arrangement as that of MOS-FETs  416   11  and  416   12  and the power supply lines  415   1  and  415   2  provided in the word lines  412  is provided in the bit lines  411 . 
         [0041]    For example, when a current is supplied between the bit line  411   1  and the word line  412   1  selectively connected to the power supply lines  415   1  and  415   2 , the magnetization direction of the magnetization free terminal in  FIG. 1  is reversed or retained by a polarity of the supply current. Specifically, writing is performed by changing the magnetization direction of the magnetization free terminal in  FIG. 1 . In writing, when the bit line  411   1  is positively biased and the word line  412   1  is negatively biased or when both of the lines have their polarities reversed, it is necessary to apply a positive bias to the other word line  413   1  in order to prevent the current from flowing into the magnetization free terminal  15  while allowing only a spin flow to flow through the magnetization free terminal  15 . As described above, in writing, the word line  413   1  is always positively biased. Meanwhile, reading is performed by applying a voltage between the bit line  411   1  and the word line  413   1  selectively connected to the data line  4141  and by reading a resistance depending on a relative magnetization direction between the magnetization pinned terminal  13  and the magnetization free terminal  15  in  FIG. 1 .