Abstract:
A magnetic wire unit for storing information thereon includes a magnetic wire containing a material having an axis of easy magnetization, and extending in a first direction, the axis being switchable between the first direction and the second direction perpendicular to the first direction, the magnetic wire being capable of holding a plurality of magnetic domains representing information. The magnetic wire unit includes a current supply unit for applying an electric current to the magnetic wire so as to move magnetic domain walls defining the magnetic domains in the magnetic wire.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-232730, filed on Sep. 10, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a magnetic wire unit for storing information thereon, and a storage device having the magnetic wire unit. 
       BACKGROUND 
       [0003]    In recent years, the next generation of super-large nonvolatile memories has been studied and developed actively as alternatives to the current DRAMs (Dynamic Random Access Memory) or flash memories. The candidates may include an FeRAM (Ferroelectric Random Access Memory) applying a dielectric substance, a PRAM (Phase change RAM) applying the phase change of an insulator included in a memory, an MRAM (Magnetoresistive Random Access Memory) applying the TMR effect (tunnel magnetoresistance effect) and an RRAM (Resistive RAM) applying a giant resistance variation caused by the directions of application of the pulse current, the principle of which has not been clarified yet. However, all of these memory devices have advantages and disadvantages in performance and have not been developed to an extent as to be alternatives to the existing memories. 
         [0004]    Recently, the technology called racetrack memory has been proposed which attempts to implement a mass storage (memory) by using the magnetic domain wall motion phenomenon with spin injection and the TMR effect. Examples of arts related to the racetrack are discussed in U.S. Pat. No. 6,834,005, Specification. Examples of arts related to the magnetic domain wall motion phenomenon are discussed in A. Yamaguchi et al., Phys, Rev, Lett., 92, 077205 (2004), for example. Other examples of arts related to the storage and memory applying the magnetic domain wall motion phenomenon with spin injection and the TMR effect are discussed in Japanese Laid-open Patent Publications No. 2007-324269, No. 2007-324172 and No. 2007-317895. 
         [0005]    However, the development of the movement of the magnetic domain wall storage devices has several problems. It is preferable to reduce the current for driving magnetic domain walls of a magnetic wire (i.e. drive current for the magnetic domain walls). 
         [0006]    For example, M. Hayashi et al., Phys, Rev, Lett., 97, 207205 (2006), S. S. R Parkin et al., Science 320, 190 (2008), and M. Hayashi et al., Science 320, 209 (2008) disclose the fact that the current value has reached 3×10 12  A/m 2  as a result of the evaluation on the current for driving magnetic domain walls with pulse voltage as fast as nanoseconds by using a conventionally used magnetic wire, which is a magnetic wire being an in-plane magnetic anisotropic film and containing an NiFe single layer as a material, and a high heat-releasing substrate. Also, as a result of the similar experiment performed by the inventor et al., the similar result has been obtained as those in M. Hayashi et al., Phys, Rev, Lett., 97, 207205 (2006), S. S. R Parkin et al., Science 320, 190 (2008), and M. Hayashi et al., Science 320, 209 (2008). 
         [0007]    Accordingly, in order to obtain a magnetic domain wall motion storage device employing a magnetic wire and taking the heating by the wire itself and/or the vibration of a wire through which current is to be supplied to the wire, the value of the current for driving the magnetic domain walls is desirably reduced to at least one digit lower than the evaluation result. 
         [0008]    On the other hand, it has been recently known that, in order to reduce the drive current, it is effective to use of a perpendicular magnetic film as the magnetic wire. However, when a perpendicular magnetic film is used as the magnetic wire, minute current moves the magnetic domain walls. Therefore, the magnetic domain walls may rest unstably, which means that information may be held unstably. Furthermore, the use of a perpendicular magnetization film as the magnetic wire requires the TMR element, which detects the movement of the magnetic domain walls of the magnetic wire, in a perpendicular magnetization structure, which may be difficult to produce. 
       SUMMARY 
       [0009]    According to an aspect of the invention, a magnetic wire unit for storing information thereon includes a magnetic wire containing a material having an axis of easy magnetization, and extending in a first direction, the axis being switchable between the first direction and the second direction perpendicular to the first direction, the magnetic wire being capable of holding a plurality of magnetic domains representing information; and a current supply unit for applying an electric current to the magnetic wire so as to move magnetic domain walls defining the magnetic domains in the magnetic wire. 
         [0010]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a diagram schematically illustrating a part of a magnetic memory device according to a first embodiment; 
           [0013]      FIG. 2  is a perspective diagram illustrating a recording area and a reserve area. 
           [0014]      FIG. 3  is a section view of the magnetic memory device; 
           [0015]      FIG. 4  is a diagram illustrating the temperature dependencies of the axis of easy magnetization of the corresponding magnetic materials; 
           [0016]      FIG. 5  is a table illustrating the directions of the axis of easy magnetization and the ease of the movement of magnetic domain walls when the movement of magnetic domain walls is stopped and when the magnetic domain walls are moved according to the first embodiment; 
           [0017]      FIG. 6  is a diagram schematically illustrating a part of the magnetic memory device according to a second embodiment; 
           [0018]      FIG. 7  is a flowchart illustrating the control sequence by a control unit according to the second embodiment; 
           [0019]      FIG. 8  is a table illustrating the directions of the axis of easy magnetization and the ease of the movement of magnetic domain walls when the movement of magnetic domain walls is stopped, when they are heated and when the magnetic domain walls are moved according to the second embodiment; and 
           [0020]      FIG. 9  is a flowchart illustrating the control sequence by a control unit according to a variation example. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0021]    A magnetic memory device  100  being the storage device according to a first embodiment will be described with reference to  FIG. 1  to  FIG. 5  below. 
         [0022]      FIG. 1  is a perspective diagram schematically illustrating a part of the magnetic memory device  100  according to the first embodiment. The magnetic memory device  100  includes, as illustrated in  FIG. 1 , a magnetic wire  12 , a recording device  14 , a reproducing device  16  and a power supply  20  being current supply unit for supplying current to the magnetic wire  12 . Notably, the magnetic wire  12  and the power supply  20  are included to configure a magnetic wire unit. 
         [0023]    The magnetic wire  12  has plural magnetic domains  22 . Notably, forming physical slits on the magnetic domains  22  can increase the controllability over the positions of the magnetic domain walls. Depending on the direction of the magnetization (the direction of the arrow in  FIG. 1 ) at each of the magnetic domains  22 , the information “1” or “0” is recorded. Notably, the magnetic wire  12  has several hundreds to several tens of thousands of magnetic domains  22  in reality. On the magnetic wire  12 , when the magnetization directions of the magnetic domains  22  adjacent to each other are opposite, a magnetic domain wall  48  exists between those magnetic domains  22 . On the other hand, when the magnetization directions of the magnetic domains  22  adjacent to each other are the same, no magnetic domain walls  48  exist between those magnetic domains  22 . Notably, the opposite magnetization directions across the magnetic domain wall  48  is a general characteristic of a ferromagnetic substance. 
         [0024]    As illustrated in  FIG. 2 , the magnetic wire  12  is divided into a recording area  30  which is an area where information is to be recorded and a reserve area  40  which is the area excluding the recording area  30  in reality. Information is recorded within the magnetic domains  22  in the recording area  30 . The specific material and others of the magnetic wire  12  will be described later. 
         [0025]      FIG. 3  illustrates a section view of the specific configuration of the magnetic memory device  100  in  FIG. 1 . As illustrated in  FIG. 3 , the magnetic wire  12  is provided above a silicon substrate  52 , an inter-layer insulating layer  54  on the silicon substrate  52  and the upper sides of an inter-layer insulating layer  56  formed on the inter-layer insulating layer  54 . 
         [0026]    The silicon substrate  52  may have a transistor, for example, not illustrated, thereon as appropriately. 
         [0027]    The inter-layer insulating layer  56  has channels  56   a  and  56   b  thereon. Within the channels  56   a  and  56   b , a lower electrode  58   a  for of the recording device  14  and a lower electrode  58   b  of the reproducing device  16  are provided. The lower electrodes  58   a  and  58   b  are electrically connected to the transistors on the silicon substrate  52  appropriately. 
         [0028]    At the positions facing the lower electrodes  58   a  and  58   b  through the magnetic wire  12 , fixed magnetization layers  68   a  and  68   b  having a laminated ferromagnetic structure are provided through a barrier layer  66  containing MgO as a material are provided. 
         [0029]    The fixed magnetization layers  68   a  and  68   b  are configured by a laminated film formed by sequentially laminating a ferromagnetic layer  70  containing CoFeB as a material, a nonmagnetic layer  72  containing Ru as a material, a ferromagnetic layer  74  containing CoFe as a material, and an antiferromagnetic layer  76  containing IrMn as a material. On the upper sides of the fixed magnetization layers  68   a  and  68   b , connection electrodes  78   a  and  78   b  containing Ta as a material. 
         [0030]    On the inter-layer insulating layer  56  having the magnetic wire  12 , fixed magnetization layers  68   a  and  68   b  and connection electrodes  78   a  and  78   b  thereon, an inter-layer insulating layer  80  is provided with the top surface of the connection electrodes  78   a  and  78   b  exposed. The inter-layer insulating layer  80  has a pair of contact holes  82   a  and  82   b  across the magnetic wire  12 . Within the contact holes  82   a  and  82   b , contact plugs  84   a  and  84   b  are provided. 
         [0031]    On the inter-layer insulating layer  80 , an upper electrode  86   a , an upper electrode  86   b  and wires  88   a  and  88   b  are provided. On the inter-layer insulating layer  80 , an inter-layer insulating layer  90  containing the upper electrodes  86   a  and  86   b  and wires  88   a  and  88   b  is provided. 
         [0032]    Notably, the lower electrode  58   a , barrier layer  66 , fixed magnetization layer  68   a , connection electrode  78   a  and upper electrode  86   a  configure the recording device  14  for writing information to the magnetic domains  22  of the magnetic wire  12 . The lower electrode  58   b , barrier layer  66 , fixed magnetization layer  68   b , connection electrode  78   b  and upper electrode  86   b  configure the reproducing device  16  for reading information recorded on the magnetic domains  22  of the magnetic wire  12 . Notably, in the recording device  14 , the barrier layer  66  is formed appropriately. Considering the match between the forming processes of the recording device  14  and the reproducing device  16 , forming the barrier layer  66  is preferable from the viewpoint of the ease of the manufacturing process. The recording device  16  may be a lead wire such as a word line for use in an MRAM instead of the one having the structure illustrated in  FIG. 3 , and the current magnetic field by the current applied to the lead wire may be applied thereto. The reproducing device  16  may apply current between the upper and lower terminals ( 86   b  and  58   b ) of the TMR with the terminal  58   b  below the magnetic wire  12 . Alternatively, for example, without the lower terminal  58   b , the reproducing device  16  may apply current between the upper terminal  86   b  and the terminal  88   b  for applying the current for driving magnetic domain walls. 
         [0033]    The wires  88   a  and  88   b  are electrically connected to one end and the other end of the magnetic wire  12  through the contact plugs  84   a  and  84   b . The wires  88   a  and  88   b  are electrically connected to the power supply  20  illustrated in  FIG. 1 . 
         [0034]    In the magnetic memory device  100  configured as described above, the magnetic domain walls  48  can be moved appropriately by the spin torque caused when current (pulse current) is fed in the longitudinal direction of the magnetic wire  12 . With that, the information written on the magnetic domains  22  can be shifted appropriately. In other words, for example, when current is fed to the left in  FIG. 2 , the electron spin flows to the right. Thus, the magnetic domain walls  48  move to the right-hand side. When current is fed to the right in  FIG. 2 , the electron spin flows to the left. Thus, the magnetic domain walls  48  move to the left-hand side. 
         [0035]    Therefore, in order to write (record) or read (reproduce) information in the magnetic memory device  100 , the movement above moves the magnetic domains  22  from the recording area  30  to the reserve area  40  illustrated in  FIG. 2 , whereby the magnetic domains  22  on which the recording (or reproduction) are to be performed are moved to the position facing the recording device  14  (or the reproducing device  16 ). 
         [0036]    Then, information is written (or recorded) onto the magnetic domains  22  of the magnetic wire  12  by defining the magnetization direction of the magnetic domains  22  of the magnetic wire  12  to the same direction as (or in parallel with) the magnetization direction of the fixed magnetization layer  68   a  or in the opposite direction to (or in anti-parallel with) the magnetization direction of the fixed magnetization layer  68   a.    
         [0037]    More specifically, in order to invert the magnetization direction of the magnetic domains  22  of the magnetic wire  12  from the anti-parallel state to the parallel state, the potential of the lower electrode  58   a  is defined higher than the potential of the upper electrode  86   a . Thus, current flows from the magnetic wire  12  side to the fixed magnetization layer  68   a  side in the direction perpendicular to the film surface, and the spin polarized conduction electrons flow from the fixed magnetization layer  68   a  into the magnetic wire  12 , causing the exchange interaction with the electrons in the magnetic wire  12 . As a result, torque occurs between the electrons, and, when the torque is large enough, the magnetization direction of the magnetic domains  22  of the magnetic wire  12  is inverted from the anti-parallel state to the parallel state. 
         [0038]    On the other hand, in order to invert the magnetization direction of the magnetic domains  22  of the magnetic wire  12  from the parallel state to the anti-parallel state, the potential of the upper electrode  86   a  is defined higher than the potential of the lower electrode  58   a . Thus, the opposite effect to the one described above inverts the magnetization direction of the magnetic domains  22  of the magnetic wire  12  from the parallel state to the anti-parallel state. 
         [0039]    On the other hand, the information written (or recorded) on the magnetic domains  22  of the magnetic wire  12  is read (or reproduced) by detecting the value of resistance between the upper electrode  86   b  and the lower electrode  58   b  included in the reproducing device  16 . More specifically, when the magnetization direction of the fixed magnetization layer  68   b  and the magnetization direction of the magnetic domains  22  facing the fixed magnetization layer  68   b  are opposite (or anti-parallel), a high-resistance state is obtained between the lower electrode  58   b  and the upper electrode  86   b . On the other hand, when the magnetization direction of the fixed magnetization layer  68   b  and the magnetization direction of the magnetic domains  22  facing the fixed magnetization layer  68   b  are the same (or parallel), a low-resistance state is obtained between the lower electrode  58   b  and the upper electrode  86   b . Thus, because the high-resistance state and the low-resistance state exist, associating those two states with data “1” and “0” allows the identification of the information written in the magnetic domains  22  of the magnetic wire  12  as either “1” or “0”. 
         [0040]    Next, the materials of the magnetic wire  12  will be described. 
         [0041]    According to the first embodiment, an alloy containing Gd and Fe is adopted as a material of the magnetic wire  12 . More specifically, Gd 32 Fe 68  or Gd 32 Fe 58 Co 10  illustrated in  FIG. 4  is adopted (where the numerical subscript of each of the materials refers to the atomic percent (atm %)). As illustrated in  FIG. 4 , these materials Gd 32 Fe 68  and Gd 32 Fe 58 Co 10  exhibit a first state that the axis of easy magnetization is in the in-plane direction when the temperature is lower than approximately 170° C. to 180° C. Those materials exhibit a second state that the axis of easy magnetization is in the perpendicular direction when the temperature is higher than approximately 170° C. to 180° C. In other words, those materials have the state transition from the first state to the second state with the increase in temperature and, conversely, have the state transition from the second state to the first state with the decrease in temperature. The temperature that the state transition as described above occurs is called “transition temperature”. Notably, the relationship between the composition of the magnetic material and the temperature dependency of the axis of easy magnetization illustrated in  FIG. 4  is determined from the direction of the axis of easy magnetization based on the magnetization curve (M-H curve) in the in-plane and perpendicular direction by using, as the evaluation sample, the sample having GdFe(Co) 40 nm thick being protected vertically between a nonmagnetic material SiN. 
         [0042]    Next, the effects by the use of the material will be described in stopping the movement of the magnetic domain walls  48  and in moving the magnetic domain walls  48 . 
         [0043]    As illustrated in  FIG. 5 , in order to stop the movement of the magnetic domain walls  48 , the current supply is turned off from the power supply  20  to the magnetic wire  12 . Thus, the temperature of the magnetic wire  12  can be kept lower (than the transition temperature). Hence, the axis of easy magnetization is turns to the in-plane direction. Therefore, in order to stop the movement of magnetic domain walls, the axis of easy magnetization is defined in the in-plane direction with which the magnetic domain walls  48  is difficult to move. As a result, the positions of the magnetic domain walls  48  (that is, the information recorded on the magnetic domains  22 ) can be held in a stable manner. 
         [0044]    On the other hand, in order to move the magnetic domain walls  48 , the current supply is turned on from the power supply  20  to the magnetic wire  12 . Thus, the joule heat caused by the current increases the temperature of the magnetic wire  12  (more than the transition temperature). Hence, the axis of easy magnetization makes a transition to the perpendicular direction. Therefore, in order to move the magnetic domain walls  48 , the axis of easy magnetization is defined to the perpendicular direction with which the magnetic domain walls  48  is easy to move. As a result, the magnetic domain walls  48  can be moved easily, that is, the current for moving the magnetic domain walls  48  can be reduced. 
         [0045]    Notably, in the design stage, the magnetic wire  12  is desirably designed in consideration of the specific resistance and/or current density of the magnetic wire  12  such that the temperature of the magnetic wire  12  can be higher than the transition temperature when the current (which is the current for driving the magnetic domain walls) is supplied for the movement of the magnetic domain walls  48 . 
         [0046]    As described in detail above, according to the first embodiment, at the first state where the axis of easy magnetization of the magnetic wire  12  is in the in-plane direction, the current supply for moving the magnetic domain walls  48  is not performed. Only at the second state where the axis of easy magnetization is in the perpendicular direction with which the current for moving the magnetic domain walls  48  is small, the current supply for moving the magnetic domain walls  48  is performed. Performing the sequence allows the reduction of the current to be supplied for the movement of the magnetic domain walls. Furthermore, because it is easy to move the magnetic domain walls, the speed of the response of the movement of the magnetic domain walls to the current application to the magnetic wire  12  can be increased. In addition, because, when the magnetic domain walls  48  are not moved, the axis of easy magnetization is in the in-plane direction, the magnetic domain walls can rest stably, which means that information can be held stably. 
         [0047]    According to the first embodiment, because the sequence can be performed without performing any special control, the current to be supplied for moving the magnetic domain walls can be reduced easily. 
         [0048]    Notably, having described, according to the first embodiment the case that the direction of the axis of easy magnetization is changed by using the increase in temperature of the magnetic wire  12  due to the joule heat caused by the current when the magnetic domain walls  48  are moved, the present invention is not limited thereto. For example, a different mechanism for adjusting the temperature of the magnetic wire  12  may be provided near the magnetic wire  12  so as to adjust the temperature of the magnetic wire  12 . The example adopting such a mechanism is a second embodiment which will be described next. 
       Second Embodiment 
       [0049]    With reference to  FIG. 6  to  FIG. 8 , the second embodiment will be described below. The second embodiment is characterized in that a heater is used to actively perform the temperature control over the magnetic wire  12 , which has been described according to the first embodiment. 
         [0050]      FIG. 6  schematically illustrates a part of a magnetic memory device  100 ′ according to the second embodiment. As illustrated in  FIG. 6 , the magnetic memory device  100 ′ includes, in addition to the configuration (refer to  FIG. 1  to  FIG. 3 ) according to the first embodiment, a heater  110  provided near the magnetic wire  12 , a current supply portion  140  that supplies current to the heater  110 , and a control unit  120  that controls the operations by the current supply portion  140  and the power supply  20 . 
         [0051]    The heater  10  may contain a material having a larger specific resistance than those of a heating wire and/or the magnetic wire  12 , for example, and generates heat with the current supplied from the current supply portion  140  under the control of the control unit  120 . Notably, according to this embodiment, the heater  110 , control unit  120  and current supply portion  140  are included to configure transition means. 
         [0052]    Next, the processing by the control unit  120  will be described by following the flowchart in  FIG. 7 . 
         [0053]    The control unit  120  in step S 10  in  FIG. 7  determines whether the magnetic domain walls  48  are to be moved or not. In this case, the determination of this step results in YES if, for example, a command for recording information (data) or a command for reproducing information (data) on the magnetic wire  12  is issued by an external host. 
         [0054]    If the determination here results in YES, the control unit  120  in the next step S 12  instructs the current supply portion  140  to perform the current supply to the heater  110 . 
         [0055]    Next, the control unit  120  in step S 14  waits until current is supplied from the current supply portion  140  to the heater  110  for a predetermined period of time. The waiting for a predetermined period of time in step S 14  increases the temperature of the magnetic wire  12  over the transition temperature by receiving the heat by the heater  110 . 
         [0056]    Next, the control unit  120  in step S 16  starts the current supply from the power supply  20  to the magnetic wire  12  and starts moving the magnetic domain walls  48 . Then, when the control unit  120  in step S 18  determines that the movement of the magnetic domain walls  48  is finished, the control unit  120  returns to step S 10 . 
         [0057]    On the other hand, if in step S 10  the determination results in NO, that is, it is determined that the movement of the magnetic domain walls is not to be performed, the current supply from the current supply portion is stopped (or the current supply stop state is kept if the current supply has been already stopped) in step S 20 . 
         [0058]    By performing the processing in  FIG. 7 , the temperature of the magnetic wire  12  can be kept lower (than the transition temperature) as illustrated in step  1  in  FIG. 8  because the heater  110  does not generate heat (or is turned off) when the movement of magnetic domain walls is not to be performed. Therefore, the axis of easy magnetization turns to the in-plane direction. In this way, defining the axis of easy magnetization to the in-plane direction with which the movement of the magnetic domain walls  48  is difficult allows the positions of the magnetic domain walls  48  (that is the information recorded in the magnetic domains  22 ) to be held in a stable manner. 
         [0059]    On the other hand, the temperature of the magnetic wire  12  becomes higher (than the transition temperature) because the heater  110  generates heat as illustrated in step  2  in  FIG. 8  in the stage before the magnetic domain walls  48  are moved. Therefore, the axis of easy magnetization is defined to the direction (which is the perpendicular direction) with which the movement of the magnetic domain walls  48  is easy. 
         [0060]    Furthermore, because, in order to move the magnetic domain walls  48 , the current is supplied to the magnetic wire  12  by keeping the state in step  2  as illustrated in step  3  in  FIG. 8 , the magnetic domain walls  48  can be moved easily, that is, the current for moving the magnetic domain walls  48  can be reduced. 
         [0061]    As described above in detail, according to the second embodiment, like the first embodiment, at the first state that the axis of easy magnetization of the magnetic wire  12  is in the in-plane direction, the current supply is not performed for moving the magnetic domain walls  48 . At the second state that the axis of easy magnetization is in the direction (which is the perpendicular direction) with which the current for moving the magnetic domain walls  48  is small, the current supply is performed for moving the magnetic domain walls  48 . Therefore, the current to be supplied for the movement of magnetic domain walls can be reduced. Furthermore, because the movement of magnetic domain walls can be performed easily, the speed of the response of the movement of magnetic domain walls to the current application to the magnetic wire  12  can be increased. Still further, in order to stop the movement of magnetic domain walls, the axis of easy magnetization is defined to the direction (which is the in-plane direction) with which the magnetic domain walls  48  are difficult to move. Therefore, the positions of the magnetic domain walls  48  (that is, the information recorded in the magnetic domains  22 ) can be held in a stable manner. Notably, the second embodiment is particularly effective when only the current supply to the magnetic wire  12  does not cause the temperature of the magnetic wire  12  over the transition temperature, as in described according to the first embodiment. 
         [0062]    Having described, according the embodiments, the case where two kinds of materials Gd 32 Fe 68  and Gd 32 Fe 58 Co 10  are used among the materials illustrated in  FIG. 4 , the present invention is not limited thereto. The other materials (Gd 20 Fe 80 , Gd 20 Fe 68.2 Co 11.8 , Gd 26 Fe 47.6 Co 26.4  and Gd 26 Fe 63.1 Co 10.98 ) illustrated in  FIG. 4  may be adopted. In this case, as illustrated in  FIG. 4 , unlike Gd 32 Fe 68  or Gd 32 Fe 58 Co 10  used according to the embodiments, when the temperature is lower than the transition temperature, the axis of easy magnetization turns to the perpendicular direction. When it is higher than the transition temperature, the axis of easy magnetization is in the in-plane direction. 
         [0063]    Therefore, preferably in this case, the same configuration as that of the second embodiment (refer to  FIG. 6 ) is used, and then the sequence as illustrated in  FIG. 9  is adopted. 
         [0064]    In  FIG. 9 , if it is determined in step S 110  that the magnetic domain walls  48  are not to be moved (or if the determination in step S 110  results in NO), current is supplied from the current supply portion  140  to the heater  110  in step S 120 . If it is determined in step S 110  that the magnetic domain walls  48  are to be moved (or if the determination in step S 110  results in YES), the current supply from the current supply portion  140  is stopped. Notably, the processing in steps S 116  and S 118  is the same as that in steps S 16  and S 18  in  FIG. 7 . 
         [0065]    Performing the processing provides the same effects as those of the second embodiment even when a material (such as Gd 20 Fe 80 , Gd 20 Fe 68.2 Co 11.8 , Gd 26 Fe 47.6 Co 26.4 , and Gd 26 Fe 63.1 Co 10.98 ) is used which has the axis of easy magnetization in the perpendicular direction when the temperature is lower than the transition temperature and has the axis of easy magnetization in the in-plane direction when the temperature is higher than the transition temperature. 
         [0066]    Notably, according to this variation example, the current for the movement of magnetic domain walls is supplied to the magnetic wire  12  at the state that the current supply to the heater  110  is stopped. The current supplied to the magnetic wire  12  increases the temperature of the magnetic wire  12 , and the temperature of the magnetic wire  12  may possibly exceed the transition temperature. Therefore, assuming such a case, a cooling mechanism (such as a mechanism including a Peltier device) may be provided near the magnetic wire  12 . In supplying current to the magnetic wire  12 , the cooling mechanism may be used to cool the magnetic wire  12 . 
         [0067]    Having described, according to the embodiments and variation example, that the GdFe(Co)-based material is used as the material of the magnetic wire  12 , the present invention is not limited thereto. For example, a rare earth material similar to GdFe, such as TbFe, may also be used to obtain the same effects as those of the embodiments. Without limiting to the material, any magnetic material may be adopted which can cause a transition of the axis of easy magnetization according to various conditions. 
         [0068]    Having described, according to the embodiments, the temperature of the magnetic wire  12  is changed in order to cause a transition of the direction of the axis of easy magnetization, the invention is not limited thereto. For example, by controlling the pressure on the magnetic wire  12 , a transition of the direction of the axis of easy magnetization may be caused. 
         [0069]    Having described, according to the embodiments, the case where the magnetic wire unit at least including a magnetic wire and a power supply is adopted to the magnetic memory device as illustrating in  FIG. 1  and  FIG. 6 , the invention is not limited thereto. It is also applicable to various apparatus (such as a racetrack storage apparatus and an MRAM) in addition. 
         [0070]    The magnetic wire unit disclosed herein has advantages that the current for the movement of the magnetic domain walls can be reduced and that information can be held in a stable manner. The storage device disclosed herein has an advantage that the current consumption in recording or reproducing can be reduced. 
         [0071]    The embodiments described above are the preferred examples embodying the present invention. However, the invention is not limited thereto, but various changes and modifications may be made without departing from the spirit and scope of the present invention. 
         [0072]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.