Patent Publication Number: US-2015061393-A1

Title: Variable voltage circuit

Description:
BACKGROUND 
     The present invention relates to a variable voltage circuit. 
     BACKGROUND ART 
     In recent years, the widespread use of mobile telephones, smart phones, and other mobile terminals is significant. The resolution of liquid crystal displays and other display units used in these mobile terminals has been improved, and terminal bodies are being made compact and thin. The display unit disclosed in PTL 1 has a horizontal driving circuit that drives signal lines for each pixel row in the display unit. A digital/analog converter (referred to below as the D/A converter), into which a digital video signal is input and which converts it to an analog pixel driving signal, is incorporated in the horizontal driving circuit. 
     In conversion methods in D/A converters, a resistor ladder type, in which an operation function of an operational amplifier is used to turn on and off a resistor and obtain a target voltage from an applied voltage, and a resistor string type, in which resistors are connected in series and a connection is made at a necessary point with an analog switch, are often used. In the D/A converters described above, a voltage generating circuit that generates a plurality of output voltages is needed. This voltage generating circuit is formed separately from a display area, so together with other semiconductor circuits, the voltage generating circuit is desired to be compact and occupy less space. 
     In PTL 2, an example of the structure of a D/A converter of N-bit resistor string type is indicated. 
     V1 and V2 are first and second reference voltage sources, which are references for analog outputs. A resistor string formed with 2 N  resistors R 1 , R 2 , R 3 , . . . , R 2   N , which have the same resistance value, is connected in series depending on the resolution, is provided between the first and second reference voltage sources V1 and V2. 
     A total of 2 N  taps T 1 , T 2 , . . . , T 2   N  are placed from connection points between the first reference voltage source V1 and the resistors of the resistor string so that the voltage between the first and second reference voltage sources V1 and V2 is divided into 2 N . Decode switches formed with an output VO and 2 N  switches SW 1 , SW 2 , . . . , SW 2   N , which are used to connect the taps T 1 , T 2 , . . . , T 2   N , is provided. One ends of the switches SW 1 , SW 2 , . . . , SW 2   N  are connected to the output VO in common. 
     A desired analog output is obtained by receiving an N-bit digital input signal input from a digital signal input terminal VI, selecting one switch from the decode switches according to the digital value input by a control circuit, and closing (turning on) the selected switch. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2002-175021 
     [PTL 2] Japanese Unexamined Patent Application Publication No. 8-330964 
     SUMMARY 
     However, since the structure of the conventional D/A converter, disclosed in PTL 2, in a resistor string method is a structure in which when resolution is N bits, a desired output is obtained by using 2 N  resistors to fetch 2 N  taps and selecting one switch from a plurality of switches, the circuit size of a decoding area used for switch selection is increased as resolution is increased, which has been problematic in that the circuit area is enlarged. 
     The present invention addresses the above problem with the object of providing a variable voltage circuit that has a voltage output function equivalent to an N-bit D/A converter, the output function being achieved with few elements, and can implement a reduced circuit size. 
     To solve the above problem, the variable voltage circuit in the present invention is characterized in that a plurality of resistive elements are connected in series between a first voltage source and a second voltage source, at least one of the plurality of resistive elements is a magnetoresistance effect element that has a magnetization fixed layer and a magnetization free layer with a spacer layer interposed therebetween, a magnetic field supply mechanism that applies a magnetic field to the magnetoresistance effect element is provided in the vicinity of the magnetoresistance effect element, the magnetic field supply mechanism can vary the resistance value of the magnetoresistance effect element by varying the magnetic field, and one of nodes among the plurality of resistive elements is connected to an output terminal. 
     According to the variable voltage circuit characterized as described above, when the magnetic field supply mechanism varies the magnetic field to be applied to the magnetoresistance effect element, which is at least one of the plurality of resistive elements connected in series between the first voltage source and the second voltage source, a voltage obtained from the output terminal can be varied. 
     Since the resistance value of the magnetoresistance effect element is varied, the variable voltage circuit characterized as described above enables more types of output voltages to be obtained than with a conventional variable voltage circuit formed with the same number of fixed resistors. When the same types of output voltages as for the conventional variable voltage circuit are output, the number of resistive elements and the number of switches can be reduced when compared with the conventional variable voltage circuit, so a variable voltage circuit with a reduced circuit size can be provided. 
     The “first voltage source” and “second voltage source” indicated here includes a case in which any one of them is grounded. That is, it is only necessary that there is an electric potential difference between the “first voltage source” and “second voltage source”. The “node” means a point at which two resistive elements are connected. A connection to a switch or another network starts from this point. The node is the same as the tap indicated in PTL 2. The “switch” is an opened and closed device placed between a node and the output terminal. When the switch is closed (turned on), the node and output terminal are brought into conduction. Switches include mechanical switches that mechanically make a switchover between electric signals in response to an external force and semiconductor switches that use a CMOS transistor or the like to connect or disconnect a circuit. Selection of a certain node is to close the switch connected to the node and bring it into conduction with the output terminal; the node has the same electric potential as the output terminal. “Vicinity” means a distance up to which a magnetic field generated from the magnetic field supply mechanism can be applied to the magnetoresistance effect element. The distance from a side surface of the magnetoresistance effect element to the magnetic field generating end of the magnetic field supply mechanism is within a range of 5 [nm] to 1 [μm]. 
     The variable voltage circuit in the present invention is also characterized in that it is structured so that a plurality of resistance values are obtained as at least one of a first resistance value between the first voltage source and the output terminal and a second resistance value between the second voltage source and the output terminal and a plurality of types of output voltages are thereby obtained according to voltage division ratios obtained from the first resistance value and second resistance value. 
     The variable voltage circuit in the present invention is also characterized in that a plurality of the magnetoresistance effect elements are included as the plurality of resistive elements, and the magnetization free layers of at least two of the plurality of magnetoresistance effect elements have mutually different coercive forces. 
     According to the variable voltage circuit characterized as described above, even when the same magnetic field is applied to at least two magnetoresistance effect elements in which their magnetization free layers have mutually different coercive forces, types of the entire resistance value of the magnetoresistance effect elements connected in series can be increased by varying the magnetic field and types of obtained output voltages can thereby be increased. 
     The variable voltage circuit in the present invention is also characterized in that a plurality of the magnetoresistance effect elements are included as the plurality of resistive elements, and at least one magnetic field supply mechanism is provided in the vicinity of each of the magnetoresistance effect elements. 
     According to the variable voltage circuit characterized as described above, more types of output voltages can be obtained by controlling the magnetic field that is individually applied to each magnetoresistance effect element to vary its resistance value. 
     The variable voltage circuit in the present invention is also characterized in that the magnetic field supply mechanism can vary the resistance value of the magnetoresistance effect element by changing the magnetization direction in the magnetization free layer with the magnetic field. 
     According to the variable voltage circuit characterized as described above, when the magnetization direction in the magnetization free layer is changed to two states, a state substantially parallel to the magnetization direction in the magnetization fixed layer and a state substantially antiparallel to the magnetization direction, the resistance value of the magnetoresistance effect element can be largely varied, so differences among a plurality of types of obtained output voltages can be increased. When the angle of the magnetization direction in the magnetization free layer with respect the magnetization direction in the magnetization fixed layer is varied to an arbitrary angle, the resistance value of the magnetoresistance effect element can be changed to an arbitrary value, so many types of output voltages can be obtained. 
     The variable voltage circuit in the present invention is also characterized in that the magnetic field supply mechanism can vary the resistance value of the magnetoresistance effect element by varying the strength of a magnetic field to be applied to the magnetization free layer. 
     According to the variable voltage circuit characterized as described above, the resistance value of the magnetoresistance effect element can be continuously varied, so many types of output voltages can be obtained. 
     The variable voltage circuit in the present invention is also characterized in that two series resistor parts, in each of which the plurality of resistive elements are connected in series, are provided and the two series resistor parts are connected in parallel. 
     According to the variable voltage circuit characterized as described above, the range of the voltage division ratio can be widened, so the output voltage range can be widened. 
     The variable voltage circuit in the present invention is also characterized in that the magnetic field supply mechanism maintains the resistance value of the magnetoresistance effect element by applying the magnetic field to the magnetoresistance effect element. 
     According to the variable voltage circuit characterized as described above, the resistance value of the magnetoresistance effect element is stabilized, so a stable output voltage is obtained. 
     The variable voltage circuit in the present invention is also characterized in that the magnetization direction in the magnetization fixed layer of the magnetoresistance effect element and the magnetization direction in the magnetization free layer of the magnetoresistance effect element are maintained in a substantially parallel direction or substantially antiparallel direction even in a state in which the magnetic field is not generated from the magnetic field supply mechanism. 
     “Substantially parallel” and “substantially antiparallel” should not be interpreted as narrowly limiting only to cases in which the angle formed by the magnetization direction in the magnetization fixed layer and the magnetization direction in the magnetization free layer is 0° and 180° but are used to include cases in which the angle is about 0°±20° and about 180°±20°. 
     According to the variable voltage circuit characterized as described above, the resistance value of the magnetoresistance effect element is maintained even in a state in which the magnetic field is not generated from the magnetic field supply mechanism, so electric power used to drive the magnetic field supply mechanism to apply a magnetic field to the magnetoresistance effect element can be conserved. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a variable voltage circuit that has a voltage output function equivalent to an N-bit D/A converter, the output function being achieved with few elements, and can implement a reduced circuit size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a variable voltage circuit according to a first embodiment of the present invention. 
         FIG. 2  is a schematic cross sectional view of a magnetoresistance effect element. 
         FIG. 3  schematically illustrates a variable voltage circuit according to a second embodiment of the present invention. 
         FIG. 4  schematically illustrates a variable voltage circuit according to a third embodiment of the present invention. 
         FIG. 5  is a graph representing the strength of a magnetic field applied to a magnetoresistance effect element according to the third embodiment of the present invention and the resistance value of the magnetoresistance effect element. 
         FIG. 6  schematically illustrates a variable voltage circuit according to a fourth embodiment of the present invention. 
         FIG. 7  schematically illustrates an example in which one coil is connected in the schematic drawing of the variable voltage circuit according to the fourth embodiment of the present invention. 
         FIG. 8  schematically illustrates a variable voltage circuit according to a fifth embodiment of the present invention. 
         FIG. 9  schematically illustrates a variable voltage circuit according to a sixth embodiment of the present invention. 
         FIG. 10  schematically illustrates a variable voltage circuit according to a seventh embodiment of the present invention. 
         FIG. 11  schematically illustrates a variable voltage circuit according to an eighth embodiment of the present invention. 
         FIG. 12  illustrates the magnetic characteristics of a magnetoresistance effect element in an example. 
         FIG. 13  is a graph representing a relationship between a current flowing in a coil in an embodiment and a generated magnetic field. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Preferred embodiment of the present invention will be described below with reference to the drawings. The present invention is not limited to the embodiments below. Constituent elements described below include elements that a person having ordinary skill in the art can easily assume and elements substantially identical to these elements. In addition, the elements described below can be appropriately combined. 
     The structure of a variable voltage circuit  50  according to a first embodiment of the present invention will be described with reference to  FIG. 1 . 
     In the variable voltage circuit  50  illustrated in  FIG. 1 , a plurality of resistive elements are connected in series between a first voltage source  101  and a second voltage source  102 . More specifically, a resistive element  104  and a magnetoresistance effect element  108 , which is used as a resistive element, are placed in series on a wire that connects the first voltage source  101  and second voltage source  102  together. A node  106  is provided between the resistive element  104  and the magnetoresistance effect element  108 . The node  106  is connected to an output terminal  103 . The output terminal  105  has the same electric potential as the second voltage source  102 . An electric potential difference between the first voltage source  101  and the second voltage source  102  is divided by the resistive element  104  and magnetoresistance effect element  108 . Output voltages divided according to the voltage division ratio are output from the output terminals  103  and  105 . 
     The “first voltage source” and “second voltage source” indicated here include a case in which any one of them is grounded. That is, it is only necessary that there is an electric potential difference between the “first voltage source” and “second voltage source”. 
     The “node” means a point at which two resistive elements are connected. A connection to a switch or another network starts from this point. The node is the same as the tap indicated in PTL 2. 
     A magnetic field supply mechanism  121  is provided in the vicinity of the magnetoresistance effect element  108  in the variable voltage circuit  50  illustrated in  FIG. 1 . A magnetic field generated from the magnetic field supply mechanism  121  is applied to the magnetoresistance effect element  108 . The magnetic field supply mechanism  121  can vary the resistance value of the magnetoresistance effect element  108  by varying the magnetic field to be applied to the magnetoresistance effect element  108  to vary the magnetization direction in the magnetization free layer of the magnetoresistance effect element  108 . 
     “Vicinity” means a distance up to which a magnetic field generated from the magnetic field supply mechanism  121  can be applied to the magnetoresistance effect element  108 . The distance from a side surface of the magnetoresistance effect element to the magnetic field generating end of the magnetic field supply mechanism is within a range of 5 [nm] to 1 [μm]. 
     The variable voltage circuit  50  illustrated in  FIG. 1  is structured so that a plurality of resistance values are obtained as at least one of a first resistance value between the first voltage source  101  and the output terminal  103  and a second resistance value between the second voltage source  102  and the output terminal  103  (in this example, the first resistance value) and can thereby obtain a plurality of types of output voltages according to voltage division ratios obtained from the first resistance value and second resistance value. 
       FIG. 2  is a schematic cross sectional view of the magnetoresistance effect element  108 . The magnetoresistance effect element  108  is structured so that a magnetization free layer  11 , a spacer layer  12 , and a magnetization fixed layer  13  are laminated between an electrode  14  and an electrode  15 . A current flows in a lamination direction between the electrode  14  and the electrode  15 . The magnetization free layer  11  can have a lamination structure formed with, for example, a CoFe alloy and a CoFe alloy with a different composition or with a CoFeB alloy and a CoFe alloy. The spacer layer  12  can be made of, for example, MgO. The magnetization fixed layer  13  can have a lamination structure formed with, for example, a CoFe alloy and a CoFe alloy with a different composition or with a CoFeB alloy and a CoFeB alloy. 
     In the magnetoresistance effect element  108 , the magnetization free layer  11  is affected by an external magnetic field generated from the magnetic field supply mechanism  121 . Due to the external magnetic field from the magnetic field supply mechanism  121 , the direction in the magnetization of the magnetization free layer  11  is changed so that it matches the direction of the external magnetic field generated from the magnetic field supply mechanism  121 . At this time, the magnetization direction in the magnetization fixed layer  13  is not changed because the magnetization direction is determined by ferromagnetic coupling. When an angle formed by the magnetization direction in the magnetization free layer  11  of the magnetoresistance effect element  108  and the magnetization direction in the magnetization fixed layer  13  of the magnetoresistance effect element  108  is varied, the resistance value of the magnetoresistance effect element  108  is varied. 
     The magnetic field supply mechanism  121  has a magnetic core made of a magnetic material such as a permalloy and a coil with a conductor wound around the magnetic core, the coil being connected to a current generating source  107 . The magnetic field supply mechanism  121  is structured so that a magnetic field, which is generated by flowing a current from the current generating source  107  into the conductor, is applied through the magnetic core to the magnetoresistance effect element  108 . The magnetic field supply mechanism  121  changes the magnetization direction in the magnetization free layer  11  to two states, a state parallel to the magnetization direction in the magnetization fixed layer  13  and a state antiparallel to the magnetization direction. Even after the magnetization direction in the magnetization free layer  11  has been changed, the magnetic field supply mechanism  121  applies a magnetic field to the magnetoresistance effect element  108  to maintain the resistance value of the magnetoresistance effect element  108 . 
     The magnetic field supply mechanism  121  can be formed at the same time by using a thin-film process that forms the magnetoresistance effect element  108 . Therefore, the size of the magnetic field supply mechanism  121  can be restricted to abut the size of the magnetoresistance effect element  108 ; the size of the magnetic field supply mechanism  121  can be restricted to about 200 micrometer square. 
     As the structure of the magnetic field supply mechanism  121 , a structure can be considered in which part of a magnetic core in a closed magnetic circuit is cut and the magnetoresistance effect element  108  is placed therein. Since the magnetoresistance effect element  108  is placed in an inside of the closed magnetic circuit, in which a magnetic flux is easily transmitted, the inside being the part cut from the closed magnetic circuit, a leak magnetic field from the magnetic core can be efficiently applied to the magnetoresistance effect element  108 . The strength of the magnetic field can be varied by varying a current flowing from the current generating source  107  to the coil. 
     In the magnetoresistance effect element  108  illustrated in  FIG. 2 , the magnetization direction in the magnetization free layer  11  on the upper side can be changed. In the magnetization fixed layer  13  on the lower side, IrMn, which is an antiferromagnetic material, is laminated, making it impossible to change the magnetization direction. If the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  are parallel, the resistance value of the magnetoresistance effect element  108  is minimized. If the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  are antiparallel, the resistance value of the magnetoresistance effect element  108  is maximized. 
     Although the material of the spacer layer  12  has been MgO, it is also possible to use, as the material of the non-magnetic spacer layer  12 , an AlOx or TiO insulator, an alloy material that includes at least one of metallic elements Cu, Ag, Au, and Cr, or a zinc oxide, gallium oxide, tin oxide, indium oxide or indium tin oxide semiconductor. 
     The type of antiferromagnetic material that makes the magnetization direction in the magnetization fixed layer  13  unchangeable is not limited to IrMn. 
     Furthermore, the order in which the magnetization free layer  11  and magnetization fixed layer  13  are laminated between the electrode  14  and the electrode  15  may be vertically inverted. The magnetoresistance effect element  108  can have a structure similar to the structure of a magnetoresistance effect element used in a known MRAM or HDD read head or the like. 
     With the variable voltage circuit in the present invention, to obtain various voltage division ratios, the magnetoresistance effect element  108  is preferably a magnetoresistance effect element having a large magnetoresistance ratio (MR ratio). Magnetoresistance effect elements from which a large magnetoresistance ratio is obtained include a giant magnetoresistance effect (GMR) element, a tunnel magnetoresistance effect (TMR) element, and a current perpendicular in place GMR (CPP-GMR) element. In particular, for the TMR element and CPP-GMR element, a large magnetoresistance ratio can be obtained by laminating a magnetization fixed layer and a magnetization free layer with a non-magnetic interposed therebetween and applying a current in the lamination direction. 
     In the variable voltage circuit  50 , illustrated in FIG.  1 , in the first embodiment, when the resistance value of the magnetoresistance effect element  108  is the maximum value R 3max , a voltage division ratio is obtained from R 3max  and the resistance value R 2  of the resistive element  104 . An output voltage V OUTmax  obtained from the output terminal  103  in this case is represented by an electric potential difference V H −V L  between the first voltage source  101  and the second voltage source  102  and a voltage division ratio as in [Eq. 1]. 
     
       
         
           
             
               
                 
                   
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     When the resistance value of the magnetoresistance effect element  108  is the minimum value R 3min , a voltage division ratio is obtained from R 3min  and the resistance value R 2  of the resistive element  104 . An output voltage V OUTmin  obtained from the output terminal  103  in this case is represented as in [Eq. 2]. 
     
       
         
           
             
               
                 
                   
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     In the case of a conventional voltage circuit, in which two fixed resistors are connected in series, only one type of output voltage is obtained. As in the variable voltage circuit  50  illustrated in  FIG. 1 , however, if one of the two fixed resistors connected in series is changed to the magnetoresistance effect element  108 , the magnetic field supply mechanism  121  is placed in the vicinity of the magnetoresistance effect element  108 , the magnetization direction in the magnetization free layer  11  is changed by a magnetic field applied from the magnetic field supply mechanism  121 , the magnetization direction in the magnetization free layer  11  is changed to two states, a state parallel to the magnetization direction in the magnetization fixed layer  13  and a state antiparallel to the magnetization direction, and the resistance value of the magnetoresistance effect element  108  is varied from the maximum value to the minimum value, then two types of output voltages can be obtained. 
     With the variable voltage circuit  50  in the first embodiment, when the magnetic field supply mechanism  121  varies a magnetic field to be applied to the magnetoresistance effect element  108 , which is at least one of a plurality of resistive elements connected in series between the first voltage source  101  and the second voltage source  102 , to vary the resistance value of the magnetoresistance effect element  108 , the voltage obtained from the output terminal  103  can be varied. 
     When the resistance value of the magnetoresistance effect element  108  is varied, the variable voltage circuit  50  in the first embodiment enables more types of output voltages to be obtained than with a conventional variable voltage circuit formed with the same number of fixed resistors. When the same types of output voltages as for the conventional variable voltage circuit are output, the number of resistive elements and the number of switches can be reduced when compared with the conventional variable voltage circuit, so a variable voltage circuit with a reduced circuit size can be provided. 
     With the variable voltage circuit  50  in the first embodiment, the magnetic field supply mechanism  121  can vary the resistance value of the magnetoresistance effect element by changing the magnetization direction in the magnetization free layer  11  to two states, a state parallel to the magnetization direction in the magnetization fixed layer  13  and a state antiparallel to the magnetization direction, the resistance value of the magnetoresistance effect element  108  can be largely varied increased and differences among a plurality of types of obtained output voltages can thereby be increased. 
     With the variable voltage circuit  50  in the first embodiment, since the magnetic field supply mechanism  121  maintains the resistance value of the magnetoresistance effect element  108  by applying a magnetic field to the magnetoresistance effect element  108 , the resistance value of the magnetoresistance effect element  108  is stabilized, so a stable output voltage can be obtained. 
     Next, the structure of a variable voltage circuit  51  according to a second embodiment of the present invention will be described with reference to  FIG. 3 . 
     The magnetic field supply mechanism  121  indicated in the first embodiment cannot change the magnetization direction in the magnetization free layer  11  to an arbitrary direction. In the second embodiment, a magnetic field supply mechanism  122  is used instead of the magnetic field supply mechanism  121  in the first embodiment, as illustrated in  FIG. 3 . Other structures are the same as in the first embodiment. 
     As illustrated in  FIG. 3 , the magnetic field supply mechanism  122  has two magnetic cores with a coil wound around each magnetic core; the magnetic cores are placed so that their magnetic poles are oriented in different directions. Therefore, if magnetic fields applied from the two magnetic cores are combined, a magnetic field can be applied to the magnetoresistance effect element  108  in an arbitrary direction. 
     By using the magnetic field supply mechanism  122 , the magnetization direction in the magnetization free layer  11  can be changed to an arbitrary direction. Therefore, the resistance value of the magnetoresistance effect element  108  can be varied to an arbitrary value according to the angle formed by the magnetization direction in the magnetization fixed layer  13  and the magnetization direction in the magnetization free layer  11 . 
     When the direction of the magnetic field to be applied to the magnetoresistance effect element  108  is arbitrarily changed by varying currents to be flowed to the coils of the two magnetic cores and the resistance value of the magnetoresistance effect element  108  is thereby varied to an arbitrary value, a plurality of output voltages V OUT  can be obtained from the output terminal  103  between the output voltages V OUTmax  and V OUTmin  indicated in [Eq. 1] and [Eq. 2]. 
     As a result, more output voltages than the number of output voltages obtained in the first embodiment can be obtained. Although the number of output voltages obtained in the first embodiment has been  2 , it is possible in the second embodiment to obtain as many output voltages as the number of arbitrary values to which the resistance value of the magnetoresistance effect element  108  can be set, so more types of output voltages are obtained. 
     Next, the structure of a variable voltage circuit  52  according to a third embodiment of the present invention will be described with reference to  FIG. 4 . 
     The magnetic field supply mechanism  121  indicated in the first embodiment has a structure in which part of a magnetic core is cut and the magnetoresistance effect element  108  is placed therein. This structure is advantageous in that a magnetic field is efficiently applied, but may become complicated because the magnetic core needs to be place around the magnetoresistance effect element  108 . In the third embodiment, a magnetic field supply mechanism  125  is used in which a magnetic core  123  with a coil wound around it is placed on one of sides interposing the magnetoresistance effect element  108  and a permanent magnet  124  is placed on the opposite side. Other structures are the same as in the first embodiment. 
     The magnetic core  123  does not have a closed magnetic circuit structure. One of the pole ends of the magnetic core  123  is placed in the vicinity of the magnetoresistance effect element  108 . 
     The permanent magnet  124  is formed by using a thin-film process. Examples of thin-film permanent magnets include those formed by adding neodymium, samarium, or another rare-earth element to a material including FePt, Fe, Ni, Co, or the like. 
     The direction of a magnetic field applied from the magnetic core  123  and the direction of a magnetic field applied from the permanent magnet  124  are opposite to each other. 
       FIG. 5  illustrates a graph representing a relationship between a magnetic field applied to the magnetoresistance effect element  108  and the resistance value of the magnetoresistance effect element  108 . In  FIG. 5 , the horizontal axis indicates magnetic field H and the vertical axis indicates resistance value R. 
     A point  111  in  FIG. 5  indicates a case in which no magnetic field is applied from the magnetic field supply mechanism  123  and only a magnetic field from the permanent magnet  124  is applied to the magnetoresistance effect element  108 . At this time, the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  are parallel and the resistance value of the magnetoresistance effect element  108  is thereby minimized. If a magnetic field from the magnetic core  123  is applied in this state, the magnetic field from the permanent magnet  124  is weakened by the magnetic field from the magnetic core  123 . As the strength of the magnetic field from the magnetic core  123  is increased, a point at which the magnetic field from the magnetic core  123  and the magnetic field from the permanent magnet  124  are balanced appears as indicated by a point  112 . If the magnetic field applied from the magnetic core  123  is further strengthened, the resistance value of the magnetoresistance effect element  108  starts to continuously vary. If the magnetic field applied from the magnetic core  123  becomes adequately larger than the magnetic field applied from the permanent magnet  124 , the magnetization direction in the magnetization free layer  11  is inverted. Then, the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  become antiparallel, maximizing the resistance value of the magnetoresistance effect element  108 . 
     In the variable voltage circuit  52  indicated in the third embodiment, the magnetic field supply mechanism  125  can vary the resistance value of the magnetoresistance effect element by varying the strength of the magnetic field to be applied to the magnetization free layer  11  as described above, the resistance value of the magnetoresistance effect element  108  can be continuously varied. Therefore, many types of output voltages V OUT  can be obtained from the output terminal  103  between the output voltages V OUTmax  and V OUTmin  indicated in [Eq. 1] and [Eq. 2]. 
     According to the variable voltage circuit  52  indicated in the third embodiment, the structure around the magnetoresistance effect element  108  can be simplified when compared with a magnetic field supply mechanism having a structure in which part of a magnetic core is cut and the magnetoresistance effect element  108  is placed therein as in the first embodiment. Accordingly, the element size of a combination of the magnetoresistance effect element  108  and magnetic field supply mechanism  125  can be more reduced. 
     Next, the structure of a variable voltage circuit  53  according to a fourth embodiment of the present invention will be described with reference to  FIG. 6 . 
     In the fourth embodiment, the resistive element  104  in the variable voltage circuit  50  indicated in the first embodiment is replaced with a magnetoresistance effect element  109  and the magnetic field supply mechanism  121  is placed in the vicinity of the magnetoresistance effect element  109  as well, as illustrated in  FIG. 6 . The node  106  between the magnetoresistance effect elements  108  and  109  is connected to the output terminal  103 . A film structure as explained for the magnetoresistance effect element  108  can be used as the film structure of the magnetoresistance effect element  109 . 
     Magnetic fields to be applied to the magnetoresistance effect elements  108  and  109  are controlled by their respective magnetic field supply mechanisms  121 . The magnetization free layers  11  of the magnetoresistance effect elements  108  and  109  are affected by the magnetic fields applied from magnetic field supply mechanisms  121 . When the angle formed by the magnetization direction in the magnetization fixed layer  13  and the magnetization direction in the magnetization free layer  11  in the magnetoresistance effect elements  108  and  109  is varied, the resistance values of the magnetoresistance effect element  108  and  109  are varied. With the variable voltage circuit  53  illustrated in  FIG. 6 , an output voltage is obtained according to the voltage division ratio obtained from the resistance values of the magnetoresistance effect elements  108  and  109 . 
     In the variable voltage circuit  53  according to the fourth embodiment, the resistance values of the magnetoresistance effect elements  108  and  109  will be denoted R 3  and R 4 . Table 1 shows combinations of the maximum values and minimum values of R 3  and R 4  and also shows the output voltage V OUT  obtained from the output terminal  103  for each combination. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 R 3   
                 R 4   
                 VOUT 
               
               
                   
               
             
            
               
                 R 3max   
                 R 4max   
                 
                   
                     
                       
                         
                           V 
                           out 
                         
                         = 
                         
                           
                             
                               R 
                               
                                 4 
                                  
                                 max 
                               
                             
                             
                               
                                 R 
                                 
                                   3 
                                    
                                   max 
                                 
                               
                               + 
                               
                                 R 
                                 
                                   4 
                                    
                                   
                                       
                                   
                                    
                                   max 
                                 
                               
                             
                           
                            
                           
                             ( 
                             
                               
                                 V 
                                 H 
                               
                               - 
                               
                                 V 
                                 L 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 R 3max   
                 R 4min   
                 
                   
                     
                       
                         
                           V 
                           out 
                         
                         = 
                         
                           
                             
                               R 
                               
                                 4 
                                  
                                 min 
                               
                             
                             
                               
                                 R 
                                 
                                   3 
                                    
                                   max 
                                 
                               
                               + 
                               
                                 R 
                                 
                                   4 
                                    
                                   
                                       
                                   
                                    
                                   min 
                                 
                               
                             
                           
                            
                           
                             ( 
                             
                               
                                 V 
                                 H 
                               
                               - 
                               
                                 V 
                                 L 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 R 3min   
                 R 4max   
                 
                   
                     
                       
                         
                           V 
                           out 
                         
                         = 
                         
                           
                             
                               R 
                               
                                 4 
                                  
                                 max 
                               
                             
                             
                               
                                 R 
                                 
                                   3 
                                    
                                   min 
                                 
                               
                               + 
                               
                                 R 
                                 
                                   4 
                                    
                                   
                                       
                                   
                                    
                                   max 
                                 
                               
                             
                           
                            
                           
                             ( 
                             
                               
                                 V 
                                 H 
                               
                               - 
                               
                                 V 
                                 L 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 R 3min   
                 R 4min   
                 
                   
                     
                       
                         
                           V 
                           out 
                         
                         = 
                         
                           
                             
                               R 
                               
                                 4 
                                  
                                 min 
                               
                             
                             
                               
                                 R 
                                 
                                   3 
                                    
                                   min 
                                 
                               
                               + 
                               
                                 R 
                                 
                                   4 
                                    
                                   
                                       
                                   
                                    
                                   min 
                                 
                               
                             
                           
                            
                           
                             ( 
                             
                               
                                 V 
                                 H 
                               
                               - 
                               
                                 V 
                                 L 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     In Table 1, the maximum values of the resistance values of the magnetoresistance effect elements  108  and  109  are denoted R 3max  and R 4max  and their minimum values are denoted R 3min  and R 4min . The voltage division ratio is determined from the resistance values of the magnetoresistance effect elements  108  and  109 . The output voltage V OUT  according to the voltage division ratio is indicated in Table 1. With the variable voltage circuit  50  in the first embodiment, when the resistance value of the magnetoresistance effect element  108  is maximized and minimized, two types of output voltages have been obtained. With the variable voltage circuit  53  in the fourth embodiment, however, if the values of R 3max , R 4max , R 3min , and R 4min  are different, four types of output voltages can be obtained according to combinations of resistance values when the resistance values of the magnetoresistance effect elements  108  and  109  are maximized and minimized. Even in the case of R 3max =R 4max  and R 3min =R 4min , three types of output voltages can be obtained. 
     Since the variable voltage circuit  53  in the fourth embodiment includes a plurality of magnetoresistance effect elements  108  and  109  and at least one magnetic field supply mechanism  121  is provided for each magnetoresistance effect element as described above, when the magnetic field supply mechanisms  121  individually control magnetic fields to be applied to the magnetoresistance effect elements  108  and  109  to vary the resistance values of the magnetoresistance effect elements  108  and  109 , more types of output voltages can be obtained. 
     Although, with the fourth embodiment illustrated in  FIG. 6 , the magnetic field supply mechanisms  121  that apply magnetic fields to the magnetoresistance effect elements  108  and  109  are individually controlled, a method of controlling the magnetic field supply mechanisms  121  at one time by connecting their coils into one as in a variable voltage circuit  54  illustrated in  FIG. 7  can also be considered. In this method, the coil in the vicinity of the magnetoresistance effect element  108  and the coil in the vicinity of the magnetoresistance effect element  109  are wound around the relevant magnetic coils so that their winding directions are opposite. 
     With the structure of the variable voltage circuit  54  illustrated in  FIG. 7 , when a current is flowed from the current generating source  107  to the coil of the magnetic field supply mechanism  121 , magnetic fields can be applied to the magnetoresistance effect elements  108  and  109  in opposite directions. 
     Next, the structure of a variable voltage circuit  55  according to a fifth embodiment of the present invention will be described with reference to  FIG. 8 . 
     With the variable voltage circuit  55  in the fifth embodiment, a plurality of magnetoresistance effect elements  108  are connected in series, the node  106  is provided between each two magnetoresistance effect elements  108 , and a selector switch  126  is connected to each node  106 . A terminal, of each selector switch  126 , that is not connected to the node  106  is connected to the output terminal  103 . When one of the switches  126  is closed, the node  106  connected to the closed selector switch  126  and the output terminal  103  have the same electric potential. One magnetic field supply mechanism  121  is placed in the vicinity of each magnetoresistance effect element  108 . A magnetic field to be applied to the magnetoresistance effect element  108  is individually controlled by the relevant magnetic field supply mechanism  121 ; each magnetic field supply mechanism  121  can change the magnetization direction in the magnetization free layer  11  of the relevant magnetoresistance effect element  108 . 
     The “switch” indicated here is an opened and closed device placed between the node  106  and the output terminal  103 . When the switch  126  is turned on by being closed, the node  106  and output terminal  103  are brought into conduction. Switches include mechanical switches that mechanically make a switchover between electric signals in response to an external force and semiconductor switches that use a CMOS transistor or the like to connect or disconnect a circuit. Selection of a certain node is to close the switch connected to the node and bring it into conduction with the output terminal; the node has the same electric potential as the output terminal. 
     The magnetization direction in the magnetization free layer  11  of the magnetoresistance effect element  108  is changed by varying the magnetic field applied from the magnetic field supply mechanism  121 . Thus, an angle formed by the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  is varied. When the angle formed by the magnetization direction in the magnetization free layer  11  and the magnetization direction in the magnetization fixed layer  13  is varied, the resistance value of the magnetoresistance effect element  108  is varied. While the resistance values of the magnetoresistance effect elements  110  being maintained, when any one of the switches  126  is closed and its relevant node  106  and output terminal  103  thereby have the same electric potential, an output voltage obtained according to the voltage division ratio obtained by a first resistance value from the first voltage source to the node  106  connected to the closed selector switch  126  and a second resistance value from the second voltage source to the node  106  connected to the closed selector switch  126 . 
     If, in the variable voltage circuit  55  in the fifth embodiment, each magnetoresistance effect element  108  is assumed to maintain its maximum resistance value or minimum resistance value, then the number of magnetoresistance effect elements  108 , the number of switches  126 , and the number of obtained output voltages are as indicated in Table 2. In Table 2, the number of magnetoresistance effect elements is indicated by R N , the number of switches is indicated by S N , and the number of output voltages is indicated by V N . 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 R N   
                 S N   
                 V N   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 3 
                 2 
                 16 
               
               
                 4 
                 3 
                 48 
               
               
                 5 
                 4 
                 128 
               
               
                 6 
                 5 
                 320 
               
               
                 7 
                 6 
                 768 
               
               
                 8 
                 7 
                 1792 
               
               
                 9 
                 8 
                 4096 
               
               
                 10  
                 9 
                 9216 
               
               
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
               
               
                 N 
                 N − 1 
                 (N − 1) × 2 N   
               
               
                   
               
            
           
         
       
     
     The variable voltage circuit  55  in the fifth embodiment can obtain (N−1)×2 N  times more reference voltages than from 2 N  stages in the D/A converter in the resistor string type indicated in PTL 2. 
     More specifically, the eight-bit D/A converter indicated in PTL 2 needs 2 8  (=256) fixed resistors. With this converter, while a tap position is switched, an output voltage is obtained according to the voltage division ratio obtained from a resistance value from the first voltage source to the tap position and a resistance value from the second voltage source to the tap position. By contrast, with the variable voltage circuit  55  indicated in the fifth embodiment, due to combinations of two conditions, maintaining the resistance value of each magnetoresistance effect element  108  at its maximum value or minimum value and making a switchover among the switches  126 , when eight magnetoresistance effect elements  108 , the number of magnetoresistance effect elements  108  being the same as the number of resistive elements, are used, output voltages are obtained at (8−1)×2 8  (=1792) stages. 
     To obtain output voltages at about 256 stages as in the D/A converter in the resistor string type indicated in PTL 2 with variable voltage circuit  55  indicated in the fifth embodiment, the variable voltage circuit  55  is required to have six magnetoresistance effect elements. In this case, output voltages are obtained at (6−1)×2 6  (=320) stages. 
     Thus, when the variable voltage circuit  55  indicated in the fifth embodiment is used, functions similar to functions of a conventional variable voltage circuit are implemented and the number of resistive elements can be significantly reduced. 
     In comparison of the variable voltage circuit used in the D/A converter indicated in PTL 2 and the variable voltage circuit  55  according to the fifth embodiment of the present invention, when the variable voltage circuit  55  according to the fifth embodiment is used, a circuit can be structured with a small number of parts. When an eight-bit D/A converter is taken as an example, the number of resistive elements in PTL 2 is 256, but in the variable voltage circuit  55  according to the fifth embodiment, the number of magnetoresistance effect elements is 6, indicating that the variable voltage circuit  55  can have comparable functions with about 3% of the resistive elements and its circuit size can thereby be reduced. When the circuit size is reduced, the area of the circuit board can be reduced and a compact variable voltage circuit can be achieved. 
     Although the variable voltage circuit  55  in the fifth embodiment has magnetic field supply mechanisms  121 , even if magnetoresistance effect elements  108  and magnetic field supply mechanisms  121  are included, the variable voltage circuit  55  can be restricted to 0.2 [mm]×0.2 [mm] or smaller. This size is equal to or smaller than the size of the 0402-type fixed resistor, which is the smallest in the conventional surface-mounted fixed resistors. Therefore, the variable voltage circuit  55  indicated in the fifth embodiment can be structured with a smaller area than the conventional D/A converter in a resistance string method, so more output voltages can be obtained. 
     Next, the structure of a variable voltage circuit  56  according to a sixth embodiment of the present invention will be described with reference to  FIG. 9 . 
     In the variable voltage circuit  56  in the sixth embodiment illustrated in  FIG. 9 , magnetoresistance effect elements  111  and  112  are placed on a wire that connects the first voltage source  101  and second voltage source  102  together, and the magnetic field supply mechanism  121  is disposed in the vicinity of each of the magnetoresistance effect elements  111  and  112 . A film structure as explained for the magnetoresistance effect element  108  can be used as the film structures of the magnetoresistance effect elements  111  and  112 . 
     With the magnetoresistance effect elements  111  and  112 , the magnetization direction in the magnetization fixed layer  13  is parallel to the direction in which the magnetic field supply mechanism  121  applies a magnetic field. The cross section of the magnetization free layer  11  of the magnetoresistance effect elements  111  and  112 , the cross section being perpendicular to the lamination direction of the laminated layers in the magnetoresistance effect elements  111  and  112 , is an ellipse. Therefore, the magnetization free layer  11  of the magnetoresistance effect elements  111  and  112  has shape magnetic anisotropy. The major axis of the ellipse is parallel to the magnetization direction in the magnetization fixed layer  13 . 
     Thus, even if no magnetic field is applied from the magnetic field supply mechanism  121 , the magnetization direction in the magnetization free layer  11  of the magnetoresistance effect elements  111  and  112  is maintained, so the magnetization direction in the magnetization fixed layer  13  of the magnetoresistance effect elements  111  and  112  and the magnetization direction in their magnetization free layer  11  are maintained in a substantially parallel state or a substantially antiparallel state. 
     “Substantially parallel” and “substantially antiparallel” should not be interpreted as narrowly limiting only to cases in which the angle formed by the magnetization direction in the magnetization fixed layer  13  and the magnetization direction in the magnetization free layer  11  is 0° and 180° but are used to include cases in which the angle is about 0°±20° and about 180°±20°. 
     According to the variable voltage circuit  56  in the sixth embodiment, therefore, the resistance values of the magnetoresistance effect elements  111  and  112  are maintained even in a state in which the magnetic field is not generated from the magnetic field supply mechanism  121 , eliminating the need to continue to apply a magnetic field from the magnetic field supply mechanism  121 . Therefore, electric power used to drive the magnetic field supply mechanisms  121  to apply a magnetic field to the magnetoresistance effect elements  111  and  112  can be conserved. 
     Next, the structure of a variable voltage circuit  57  according to a seventh embodiment of the present invention will be described with reference to  FIG. 10 . 
     In the variable voltage circuit  57  in the seventh embodiment illustrated in  FIG. 10 , magnetoresistance effect elements  113  and  114  are placed on a wire that connects the first voltage source  101  and second voltage source  102  together, and a magnetic field supply mechanism  127  is disposed in the vicinity of the magnetoresistance effect elements  113  and  114 . A film structure as explained for the magnetoresistance effect element  108  can be used as the film structures of the magnetoresistance effect elements  113  and  114 . 
     The magnetic field supply mechanism  127 , which is placed in the vicinity of the magnetoresistance effect elements  113  and  114 , is connected to the current generating source  107 . The magnetic field supply mechanism  127  is a conductor made of Au, Cu, Al, or another metal or an alloy including them. When a current is applied to the conductor, a magnetic field is generated around the conductor. 
     The current generating source  107  can apply not only a constant current but also a pulse current to the magnetic field supply mechanism  127 . 
     The magnetic field supply mechanism  127  and the magnetoresistance effect elements  113  and  114  are fixed with a certain interval maintained. The magnetic field supply mechanism  127  is desirably placed at a position close to the magnetization free layers  11  of the magnetoresistance effect elements  113  and  114 . 
     When a current is applied from the current generating source  107  to the magnetic field supply mechanism  127 , a magnetic field is generated from the magnetic field supply mechanism  127 , varying the magnetization direction in the magnetization free layer  11  of the magnetoresistance effect elements  113  and  114 . Therefore, output voltages are obtained according to voltage division ratios obtained from the resistance values of the magnetoresistance effect elements  113  and  114 . 
     The magnetization direction in the magnetization fixed layer  13  of the magnetoresistance effect elements  113  and  114  and the direction in which the magnetic field supply mechanism  127  applies a magnetic field to the magnetoresistance effect elements  113  and  114  are parallel. 
     The magnetization free layers  11  of the magnetoresistance effect elements  113  and  114  have mutually different coercive forces. The cross section, of the magnetization free layer  11  of the magnetoresistance effect elements  113  and  114 , the cross section being perpendicular to the lamination direction of the laminated layers in the magnetoresistance effect elements  113  and  114 , is an ellipse. Therefore, the magnetization free layer  11  of the magnetoresistance effect elements  113  and  114  has shape magnetic anisotropy. The major axis of the ellipse is parallel to the magnetization direction in the magnetization fixed layer  13 . The magnetoresistance effect element  113  has a smaller ratio of the length of the major axis of the ellipse to the length of its minor axis than the magnetoresistance effect element  114 . That is, the shape magnetic anisotropy of the magnetization free layer  11  in the magnetoresistance effect element  113  and that in the magnetoresistance effect element  114  are different, and the magnetization free layer  11  of the magnetoresistance effect element  113  has a smaller coercive force than the magnetization free layer  11  of the magnetoresistance effect element  114 . 
     An exemplary case will be considered in which with the magnetization directions in the magnetization free layers  11  of the magnetoresistance effect element  113  and  114  being the same, a magnetic field is applied from the magnetic field supply mechanism  127  in a direction opposite to the magnetization directions in the magnetization free layers  11 . In this case, if the strength of the magnetic field to be applied is between the coercive force of the magnetization free layer  11  of the magnetoresistance effect element  113  and the coercive force of the magnetization free layer  11  of the magnetoresistance effect element  114 , the magnetization direction in the magnetization free layer  11  of the magnetoresistance effect element  113  is inverted, but the magnetization direction in the magnetization free layer  11  of the magnetoresistance effect element  114  is not inverted. If the strength of the magnetic field to be applied is larger than the coercive force of the magnetization free layer  11  of the magnetoresistance effect element  113  and the coercive force of the magnetization free layer  11  of the magnetoresistance effect element  114 , both the magnetization directions in the magnetization free layers  11  of the magnetoresistance effect elements  113  and  114  are inverted. 
     As described above, even if the same magnetic field is applied to the magnetoresistance effect elements  113  and  114 , when the magnetic field is varied, combinations of a plurality of types of magnetization states in the magnetization free layer  11  of the magnetoresistance effect elements  113  and  114  are obtained, so types of the entire resistance value of the magnetoresistance effect elements  113  and  114  connected in series can be increased. Accordingly, with the variable voltage circuit  57  in the seventh embodiment, types of obtained output voltages can be increased. 
     Since the magnetoresistance effect elements  113  and  114  also have a coercive force in the magnetization free layer  11  as with the magnetoresistance effect elements  111  and  112 , even if no magnetic field is applied from the magnetic field supply mechanism  127 , the magnetization direction in the magnetization free layer  11  is maintained, eliminating the need to continue to apply a magnetic field from the magnetic field supply mechanism  127 . Therefore, electric power used to drive the magnetic field supply mechanism  127  to apply a magnetic field to the magnetoresistance effect elements  113  and  114  can be conserved. 
     Although, in the seventh embodiment illustrated in  FIG. 10 , the magnetic field supply mechanism  127  connected to the current generating source  107  is formed with one conductor and controls magnetic fields applied to both the magnetoresistance effect elements  113  and  114  at the same time, it is also possible to place the magnetic field supply mechanism  127  in the vicinity of each of the magnetoresistance effect elements  113  and  114  and individually control the magnetic fields applied to both the magnetoresistance effect elements  113  and  114 . 
     Next, the structure of a variable voltage circuit  58  according to an eighth embodiment of the present invention will be described with reference to  FIG. 11 . 
     The variable voltage circuit  58  in the eighth embodiment has a series resistor part  130 , in which two magnetoresistance effect elements  115  and  116  are connected in series, and a series resistor part  131 , in which two magnetoresistance effect elements  117  and  118  are connected in series, the two series resistor part being connected in parallel. The node  106  between the magnetoresistance effect elements  115  and  116  is connected to the output terminal  103 , and the node  110  between the magnetoresistance effect elements  117  and  118  is connected to the output terminal  105 . The series resistor parts  130  and  131  connected in parallel are connected to the first voltage source  101  and second voltage source  102  between the first voltage source  101  and the second voltage source  102 . In the vicinity of each of the magnetoresistance effect elements  115 ,  116 ,  117 , and  118 , the magnetic field supply mechanism  121  that applies a magnetic field to the relevant magnetoresistance effect element  115 ,  116 ,  117 , or  118  is disposed. 
     Table 3 indicates results of comparison of examples of output voltages obtained when the variable voltage circuit  51  indicated in the second embodiment is operated and examples of output voltages obtained when the variable voltage circuit  58  indicated in the eighth embodiment is operated. The magnetoresistance effect elements  108 ,  115 , and  118  were the same element, and the magnetoresistance effect elements  109 ,  116 , and  117  were the same element. 
     The magnetoresistance effect elements  108 ,  115 , and  118  had an MR ratio of 100%, and the minimum value of their resistance value R 3  was 90 [Ω]. The magnetoresistance effect elements  109 ,  116 , and  117  had an MR ratio of 100%, and the minimum value of their resistance value R 4  was 200 [Ω]. Output voltages when a difference in electric potential between the first voltage source  101  and the second voltage source  102  is 1 [V], that is, differences in electric potential between the output terminal  103  and the output terminal  105 , and the difference between the maximum value and minimum value of the output voltages are indicated in Table 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Resistance value 
                 Output voltage [V] 
               
            
           
           
               
               
               
               
            
               
                 State of resistor 
                 [Ω] 
                 Second 
                 Eighth 
               
            
           
           
               
               
               
               
               
               
            
               
                 R 3   
                 R 4   
                 R 3   
                 R 4   
                 embodiment 
                 embodiment 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 R 3max   
                 R 4max   
                 180 
                 400 
                 0.69 
                 0.38 
               
               
                 R 3max   
                 R 4min   
                 180 
                 200 
                 0.53 
                 0.05 
               
               
                 R 3min   
                 R 4max   
                 90 
                 400 
                 0.82 
                 0.63 
               
               
                 R 3min   
                 R 4min   
                 90 
                 200 
                 0.69 
                 0.38 
               
            
           
           
               
               
               
            
               
                 ΔV OUT   
                 0.29 
                 0.58 
               
               
                   
               
            
           
         
       
     
     Table 3 indicates that, with the variable voltage circuit  51  indicated in the second embodiment, the difference ΔV OUT  between the maximum value and minimum value of the output voltages is 0.29 [V] and that, with the variable voltage circuit  58  indicated in the eighth embodiment, the difference ΔV OUT  between the maximum value and minimum value of the output voltages is 0.58 [V], indicating a difference of 0.29 [V]. 
     With the variable voltage circuit  58  in the eighth embodiment, in which two series resistor parts  130  and  131 , in each of which a plurality of resistive elements (magnetoresistance effect elements) are connected in series, are provided and the two series resistor parts  130  and  131  are connected in parallel as described above, the output voltage range can be expanded. 
     A large output voltage range indicates that more types of voltages can be output in that range. When differences among output voltages are large, error due to variations in output voltages can be suppressed. Accordingly, a larger output voltage range is desirable as functions of a variable voltage circuit. 
     Although the above fourth to sixth and eighth embodiments have been explained by using examples in which the magnetic field applying mechanism  121  indicated in the first embodiment is used as the magnetic field applying mechanism, the magnetic field applying mechanism  122  indicated in the second embodiment or the magnetic field applying mechanism  125  indicated in the third embodiment may be used instead of the magnetic field applying mechanism  121 . 
     EMBODIMENTS 
     Embodiment 1 
     A specific embodiment of the above sixth embodiment will be indicated. 
     As the magnetic field supply mechanism  121 , an Au coil was wound around a NiFe magnetic core by using a thin-film process. The Au coil is such that the width of Au is 10 [m], its thickness is 200 [nm], and the number of turns is 4. The magnetic field supply mechanisms  121  were placed so that their pole ends were at positions 50 nm distant from the magnetoresistance effect elements  111  and  112 . 
     The magnetization direction in the magnetization fixed layer  13  of the magnetoresistance effect elements  111  and  112  and the direction in which the magnetic field supply mechanism  121  applies a magnetic field are parallel. 
     The graph in  FIG. 12  indicates the magnetic characteristics of the magnetoresistance effect elements  111  and  112 ; the horizontal axis indicates magnetic field H applied to the magnetoresistance effect elements and the vertical axis indicates element resistance R of the magnetoresistance effect elements. The magnetoresistance effect element  111  has an MR ratio of 100%, and the minimum value of its resistance value is 200 [Ω]. The magnetic characteristics are as indicated by the dashed lines indicated in  FIG. 12 . A switching field in the magnetization direction in the magnetization free layer is at 400 [Oe]. When the strength of an external magnetic field applied by the magnetic field supply mechanism  121  is 400 [Oe] or higher, the resistance value of the magnetoresistance effect element  111  is maximized. The magnetoresistance effect element  112  has an MR ratio of 90%, and the minimum value of its resistance value is 150 [Ω]. The magnetic characteristics are as indicated by the solid lines indicated in  FIG. 12 . A switching field in the magnetization direction in the magnetization free layer is at 500 [Oe]. When the strength of an external magnetic field applied by the magnetic field supply mechanism  121  is 500 [Oe] or higher, the resistance value of the magnetoresistance effect element  112  is maximized. To invert the magnetization direction in the magnetization free layer of the magnetoresistance effect element  111 , a magnetic field at  400  [Oe] or higher needs to be applied to the magnetization free layer of the magnetoresistance effect element  111 . The magnetization direction in the magnetization free layer of the magnetoresistance effect element  111  can be inverted by applying a current at 15 [mA] or higher, as indicated by the graph indicated in  FIG. 13 , which represents the relationship between current flowing in the coil of the magnetic field supply mechanism  121  and the magnetic field generated from the end of the magnetic pole. 
     The graph in  FIG. 13  also indicates the performance of the magnetic field supply mechanism  121 ; the horizontal axis indicates current I flowing in the coil of the magnetic field supply mechanism and the vertical axis indicates the magnetic field H generated from the end of the magnetic pole. Since the switching field in the magnetization direction in the magnetization free layer of the magnetoresistance effect element  112  is at 500 [Oe], the magnetization direction in the magnetization free layer of the magnetoresistance effect element  112  can be inverted by applying a current at 19 [mA] or higher, as indicated by the graph indicated in  FIG. 13 , which represents the relationship between current flowing in the coil of the magnetic field supply mechanism  121  and the magnetic field generated from the end of the magnetic pole. 
     When a voltage V H  (=3 [V]) is applied to the first voltage source  101  and a voltage V L  (=0 [V]) is applied to the second voltage source  102 , if the resistance value R 8  of the magnetoresistance effect element  111  and the resistance value R 9  of the magnetoresistance effect element  112  are varied to the high-resistance state (H) and low-resistance state (L), four types of output voltages are obtained as the output voltage V OUT  obtained from the output terminal  103  as indicated in Table 4. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Output 
               
               
                   
                   
                   
                 voltage 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Resistance state 
                   
                 Element resistance [Ω] 
                 [V] 
               
            
           
           
               
               
               
               
               
            
               
                 R6a 
                 R7a 
                 R6a 
                 R7a 
                 Vout 
               
               
                   
               
               
                 H 
                 H 
                 400 
                 285 
                 1.25 
               
               
                 H 
                 L 
                 400 
                 150 
                 0.82 
               
               
                 L 
                 H 
                 200 
                 285 
                 1.76 
               
               
                 L 
                 L 
                 200 
                 150 
                 1.29 
               
               
                   
               
            
           
         
       
     
     REFERENCE SIGNS LIST 
     
         
         
           
               11  magnetization free layer 
               12  spacer layer 
               13  magnetization fixed layer 
               50 ,  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58  variable voltage circuit 
               101  first voltage source 
               102  second voltage source 
               103 ,  105  output terminal 
               104  resistive element 
               106 ,  110  node 
               107  current generating source 
               108 ,  109 ,  111 ,  112 ,  113 ,  114 ,  115 ,  116 ,  117 ,  118  magnetoresistance effect element 
               121 ,  122 ,  125 ,  127  magnetic field supply mechanism 
               130 ,  131  series resistor part