Patent Publication Number: US-2022238792-A1

Title: Magnetic memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of U.S. application Ser. No. 16/816,961, filed on Mar. 12, 2020, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-166174, filed Sep. 12, 2019, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a magnetic memory device. 
     BACKGROUND 
     A magnetic memory device (magnetoresistive random access memory (MRAM)) which adopts a magnetoresistive effect element as a memory element is known. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a magnetic memory device according to a first embodiment. 
         FIG. 2  is a circuit diagram showing a configuration of a memory cell array in the magnetic memory device according to the first embodiment. 
         FIG. 3  is a cross section showing a configuration of the memory cell array in the magnetic memory device according to the first embodiment. 
         FIG. 4  is a cross section showing a configuration of the memory cell array in the magnetic memory device according to the first embodiment. 
         FIG. 5  is a cross section showing a configuration of a magnetoresistive effect element in the magnetic memory device according to the first embodiment. 
         FIG. 6  is a schematic diagram showing a manufacturing method of the memory cell array of the magnetic memory device according to the first embodiment. 
         FIG. 7  is a schematic diagram showing a manufacturing method of the memory cell array of the magnetic memory device according to the first embodiment. 
         FIG. 8  is a schematic diagram showing a manufacturing method of the memory cell array of the magnetic memory device according to the first embodiment. 
         FIG. 9  is a schematic diagram showing a manufacturing method of the memory cell array of the magnetic memory device according to the first embodiment. 
         FIG. 10  is a schematic diagram showing a manufacturing method of the memory cell array of the magnetic memory device according to the first embodiment. 
         FIG. 11  is a schematic diagram showing effects of the first embodiment. 
         FIG. 12  is a schematic diagram showing a configuration of a magnetoresistive effect element in a magnetic memory device according to a first modification of the first embodiment. 
         FIG. 13  is a schematic diagram showing a configuration of a magnetoresistive effect element in a magnetic memory device according to a second modification of the first embodiment. 
         FIG. 14  is a schematic diagram showing a configuration of a magnetoresistive effect element in a magnetic memory device according to a second embodiment. 
         FIG. 15  is a schematic diagram showing a configuration of a magnetoresistive effect element in a magnetic memory device according to a first modification of the second embodiment. 
         FIG. 16  is a schematic diagram showing a configuration of a magnetoresistive effect element in a magnetic memory device according to a second modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a magnetic memory device includes a substrate, a first layer stack, and a second layer stack at a same side of the first layer stack relative to the substrate, and farther than the first layer stack from the substrate. Each of the first layer stack and the second layer stack includes a reference layer, a tunnel barrier layer provided in a direction relative to the reference layer, the direction being perpendicular to the substrate, a storage layer provided in the direction relative to the tunnel barrier layer, and a first nonmagnetic layer provided in the direction relative to the storage layer. A heat absorption rate of the first nonmagnetic layer of the first layer stack is lower than a heat absorption rate of the first nonmagnetic layer of the second layer stack. 
     The embodiments of the present invention will be explained with reference to the drawings. In the following explanation, components having the same functions and configurations will be referred to by the same reference symbol. In case structural components having the same reference symbols need to be distinguished from each other, letters or numerals may be added to the symbols. If the structural components need not be distinguished particularly from each other, only the common reference symbols are used, without additional letters or numerals. The additional letters or numerals are not limited to a superscript or subscript, but may be lower-case alphabetic characters attached to the end of a reference symbol, and indices indicating the arrangement order. 
     1. First Embodiment 
     A magnetic memory device according to a first embodiment will be explained. The magnetic device according to the first embodiment is, for example, a perpendicular magnetization-type magnetic memory device in which an element having a magnetoresistive effect provided by a magnetic tunnel junction (MTJ) (such an element may be called an MTJ element or a magnetoresistive effect element) as a resistance change element. 
     1.1 Configuration 
     First, a configuration of the magnetic memory device according to the first embodiment is explained. 
     1.1.1 Magnetic Memory Device 
       FIG. 1  is a block diagram showing a configuration of a magnetic memory device according to the first embodiment. A magnetic memory device  1  includes a memory cell array  10 , a row selection circuit  11 , a column selection circuit  12 , a decode circuit  13 , a write circuit  14 , a read circuit  15 , a voltage generator  16 , an input/output circuit  17 , and a controller  18 , as illustrated in  FIG. 1 . 
     The memory cell array  10  contains a plurality of memory cells MC, each of which is associated with a pair of one row and one column. In particular, the memory cells MC of the same row are coupled to the same word line WL, and the memory cells MC of the same column are coupled to the same bit line BL. 
     The row selection circuit  11  is coupled to the memory cell array  10  by way of word lines WL. The row selection circuit  11  receives decoding results (row address) of an address ADD from the decode circuit  13 . The row selection circuit  11  sets a word line WL corresponding to the row indicated by the decoding results of the address ADD to a selected state. Hereinafter, a word line WL that is set to a selected state will be referred to as a selected word line WL. Word lines WL other than the selected word line WL will be referred to as non-selected word lines WL. 
     The column selection circuit  12  is coupled to the memory cell array  10  by way of the bit line BL. The column selection circuit  12  receives decoding results (column address) of the address ADD from the decode circuit  13 . The column selection circuit  12  sets a column indicated by the decoding results of the address ADD to a selected state. Hereinafter, a bit line BL that is set to a selected state will be referred to as a selected bit line BL. Bit lines BL other than the selected bit line BL will be referred to as non-selected bit lines BL. 
     The decode circuit  13  decodes the address ADD received from the input/output circuit  17 . The decode circuit  13  supplies the decoding results of the address ADD to the row selection circuit  11  and column selection circuit  12 . The address ADD includes addresses of a column and row that are to be selected. 
     The write circuit  14  writes data to memory cells MC. The write circuit  14  may include a write driver (not shown). 
     The read circuit  15  reads data from memory cells MC. The read circuit  15  may include a sense amplifier (not shown). 
     The voltage generator  16  generates voltages for the operations of the memory cell array  10  using a power supply voltage supplied from the outside (not shown) of the magnetic memory device  1 . For example, the voltage generator  16  generates voltages required for a write operation and outputs them to the write circuit  14 . The voltage generator  16  also generates voltages required for a read operation and outputs them to the read circuit  15 . 
     The input/output circuit  17  transfers an address ADD received from the outside of the magnetic memory device  1 , to the decode circuit  13 . The input/output circuit  17  also transfers a command CMD received from the outside of the magnetic memory device  1 , to the controller  18 . The input/output circuit  17  transmits and receives various control signals CNT with respect to the outside of the magnetic memory device  1  and the controller  18 . The input/output circuit  17  transfers to the write circuit  14  the data DAT received from the outside of the magnetic memory device  1 , and outputs to the outside of the magnetic memory device  1  the data DAT transferred from the read circuit  15 . 
     The controller  18  controls the operations of the row selection circuit  11 , column selection circuit  12 , decode circuit  13 , write circuit  14 , read circuit  15 , voltage generator  16 , and input/output circuit  17  in the magnetic memory device  1 , in accordance with the control signal CNT and command CMD. 
     1.1.2 Memory Cell Array 
     Next, a configuration of the memory cell array of the magnetic memory device according to the first embodiment is explained with reference to  FIG. 2 . The circuit diagram of  FIG. 2  shows a configuration of the memory cell array of the magnetic memory device according to the first embodiment. In  FIG. 2 , the word lines WL are classified by a lower-case alphabetic character (“u” or “d”) and an angle-bracketed index (e.g., &lt;0&gt;). 
     Memory cells MC (MCu and MCd) are arranged to form a matrix in the memory cell array  10  as shown in  FIG. 2 , each of which is associated with a pair of one of the bit lines BL (BL&lt; 0 &gt;, BL&lt; 1 &gt;, . . . , BL&lt;N&gt;)) and one of the word lines WLd (WLd&lt; 0 &gt;, WLd&lt; 1 &gt;, . . . , WLd&lt;M&gt;) and WLu (WLu&lt; 0 &gt;, WLu&lt; 1 &gt;, . . . , WLu&lt;M&gt;) (where M and N are integers). In other words, a memory cell MCd&lt;i,j&gt; (0≤i≤M, 0≤j≤N) is coupled between the word line WLd&lt;i&gt; and bit line BL&lt;j&gt;, and the memory cell MCu&lt;i,j&gt; is coupled to the word line WLu&lt;i&gt; and bit line BL&lt;j&gt;. 
     The alphabetic characters “d” and “u” are attached simply for the sake of convenience to indicate a memory cell MC being arranged among a plurality of memory cells MC, for example, below or above a bit line BL. The three-dimensional configuration of the memory cell array  10  will be described later. 
     The memory cell MCd&lt;i,j&gt; includes a switching element SELd&lt;i,j&gt; and a magnetoresistive effect element MTJd&lt;i,j&gt; that are coupled in series with each other. The memory cell MCu&lt;i,j&gt; includes a switching element SELu&lt;i,j&gt; and a magnetoresistive effect element MTJu&lt;i,j&gt; that are coupled in series with each other. 
     The switching element SEL is provided with a function of switching, at the time of writing and reading data to and from its corresponding magnetoresistive effect element MTJ, to control the current supply to the magnetoresistive effect element MTJ. In particular, when the voltage applied to a memory cell MC is lower than the threshold voltage Vth, the switching element SEL of the memory cell MC serves as a highly resistive insulator and thereby interrupts the current (switching to an OFF state), while when the voltage exceeds the threshold voltage Vth, it serves as a low resistive conductor and thereby passes the current (switching to an ON state). That is, the switching element SEL is provided with a function of switching between interruption and passing of the current, in accordance with the voltage applied to the memory cell MC regardless of the direction of the current flow. 
     The switching element SEL may be a two-terminal type switching element. When the voltage applied between the two terminals is lower than the threshold voltage, this switching element is in a “high resistance” state, being electrically non-conductive, for example. When the voltage applied between the two terminals is higher than the threshold voltage, the switching element is in a “low resistance” state, being electrically conductive, for example. The switching element may be provided with this function, regardless of the polarity of the voltage. This switching element may contain one or more types of chalcogen elements selected from a group of tellurium (Te), selenium (Se) and sulfur (S). Alternatively, the switching element may contain chalcogenide, which is a compound containing any of the above chalcogen elements. The switching element may also contain one or more types of elements selected from a group of boron (B), aluminum (Al), gallium (Ga), indium (In), carbon (C), silicon (Si), germanium (Ge), tin (Sn), arsenic (As), phosphorus (P), antimony (Sb), titanium (Ti), and bismuth (Bi). In particular, this switching element may contain at least two elements selected from a group of germanium (Ge), antimony (Sb), tellurium (Te), titanium (Ti), arsenic (As), indium (In), and bismuth (Bi). In addition, this switching element may contain an oxide of at least one element selected from titanium (Ti), vanadium (V), chrome (Cr), niobium (Nb), molybdenum (Mo), hafnium (Hf), and tungsten (W). 
     With the current supply controlled by the switching element SEL, the resistance value of the magnetoresistive effect element MTJ can be switched between the low resistance state and high resistance state. The magnetoresistive effect element MTJ is designed to be data-writable in accordance with the change of the resistance state of the element, and functions as a readable memory element by storing the written data in a non-volatile manner. 
     Next, a cross-sectional configuration of the memory cell array  10  is explained with reference to  FIGS. 3 and 4 . To explain the configuration of the memory cell array in the magnetic memory device according to the first embodiment,  FIGS. 3 and 4  present exemplary cross sections thereof. The cross sections of the memory cell array  10  viewed from different directions that intersect each other are shown in  FIGS. 3 and 4 . 
     As illustrated in  FIGS. 3 and 4 , the memory cell array  10  is arranged on a semiconductor substrate  20 . In the following explanation, a plane parallel to the surface of the semiconductor substrate  20  will be referred to as an XY plane, and the axis perpendicular to the XY plane will be referred to as a Z-axis. A positive direction of the Z-axis may be a direction away or approaching from the surface of the semiconductor substrate  20 . On the XY plane, the axis extending along the word line WL will be referred to as an X-axis, and the axis extending along the bit line BL will be referred to as a Y-axis.  FIGS. 3 and 4  present the cross sections of the memory cell array  10  when viewed along the Y-axis and X-axis, respectively. 
     A plurality of conductors  21  are arranged on the top surface of the semiconductor substrate  20 . The conductors  21  are conductive and function as word lines WLd. The conductors  21  may be aligned along the Y-axis, each extending along the X-axis. The conductors  21  in  FIGS. 3 and 4  are arranged on the semiconductor substrate  20 , but the arrangement is not limited thereto. The conductors  21  may be arranged above the semiconductor substrate  20  without being in contact with the semiconductor substrate  20 . 
     A plurality of elements  22  are arranged on the top surface of a conductor  21  to individually function as a magnetoresistive effect element MTJd. The elements  22  on the conductor  21  may be aligned along the X-axis. In other words, the elements  22  are arranged along the X-axis on one conductor  21  and are commonly coupled thereto. The configuration of an element  22  will be described later in detail. 
     An element  23  is provided on the top surface of each of the elements  22  so as to function as a switching element SELd. The top surface of the elements  23  is coupled to one of the conductors  24 . The conductors  24  are conductive, and function as bit lines BL. The conductors  24  are aligned along the X-axis, each extending along the Y-axis. That is, the elements  23  aligned along the Y-axis are commonly coupled to one conductor  24 . In  FIGS. 3 and 4 , individual elements  23  are arranged on the elements  22  and on the conductor  24 , but the arrangement is not limited thereto. Each element  23  may be coupled to the elements  22  and conductor  24  with conductive contact plugs (not shown) interposed between. 
     A plurality of elements  25  are arranged on the top surface of the conductor  24  to individually function as a magnetoresistive effect element MTJu. The elements  25  on the conductor  24  may be aligned along the Y-axis. That is, the elements  25  aligned along the Y-axis are commonly coupled to one conductor  24 . The elements  25  may have a configuration similar to the elements  22 . 
     An element  26  is arranged on the top surface of each of the elements  25  to function as a switching element SELu. The top surface of the element  26  is coupled to one of the conductors  27 . The conductors  27  are conductive and function as word lines WLu. The conductors  27  may be aligned along the Y-axis, each extending along the X-axis. That is, the elements  26  aligned along the X-axis are commonly coupled to one conductor  27 . In  FIGS. 3 and 4 , individual elements  26  are arranged on the elements  25  and on the conductor  27 , but the arrangement is not limited thereto. The elements  26  may be individually coupled to the elements  25  and conductor  27  with conductive contact plugs interposed between (not shown). 
     With the above arrangement, a pair of word lines WLd and WLu corresponds to one bit line BL in the memory cell array  10 . Furthermore, in the memory cell array  10 , a memory cell MCd is arranged between a word line WLd and a bit line BL, and a memory cell MCu is arranged between a bit line BL and a word line WLu. This means that the memory cell array  10  has a structure of memory cells MC arranged at different levels along the Z-axis. In the cell structure of  FIGS. 3 and 4 , the memory cells MCd correspond to the lower level, and the memory cells MCu correspond to the upper level. Of the two memory cells MC coupled commonly to a single bit line BL, the memory cell MC in the upper level with respect to the bit line BL corresponds to a memory cell MCu with the alphabetic character “u” attached, while the memory cell MC in the lower level corresponds to a memory cell MCd with “d” attached. 
     1.1.3 Magnetoresistive Effect Element 
     Next, a configuration of magnetoresistive effect elements of the magnetic memory device according to the first embodiment is explained with reference to  FIG. 5 . In this drawing, a cross section of a configuration of the magnetoresistive effect elements in the magnetic memory device according to the first embodiment is presented.  FIG. 5  provides an exemplary cross section of the magnetoresistive effect elements MTJu and MTJd in  FIGS. 3 and 4 , taken along a plane perpendicular to the Z-axis (e.g., XZ plane). The magnetoresistive effect elements MTJu and MTJd correspond to (A) and (B), respectively, in  FIG. 5 . 
     As illustrated in  FIG. 5 , the magnetoresistive effect element MTJu may include a nonmagnet  31   u  functioning as a capping layer CAP 1 , a nonmagnet  32   u  functioning as a heat absorption layer HAL, a nonmagnet  33   u  functioning as a magnetic anisotropy enhancement layer (Hk enhancement layer) HkEL, a nonmagnet  34   u  functioning as a capping layer CAP 2 , a ferromagnet  35   u  functioning as a storage layer SL, a nonmagnet  36   u  functioning as a tunnel barrier layer TB, a ferromagnet  37   u  functioning as a reference layer RL, a nonmagnet  38   u  functioning as a spacer layer SP, a ferromagnet  39   u  functioning as a shift cancelling layer SCL, and a nonmagnet  40   u  functioning as a buffer layer BUF. The magnetoresistive effect element MTJd may include a nonmagnet  31   d  functioning as a capping layer CAP 1 , a nonmagnet  32   d  functioning as a heat absorption layer HAL, a nonmagnet  33   d  functioning as a Hk enhancement layer HkEL, a nonmagnet  34   d  functioning as a capping layer CAP 2 , a ferromagnet  35   d  functioning as a storage layer SL, a nonmagnet  36   d  functioning as a tunnel barrier layer TB, a ferromagnet  37   d  functioning as a reference layer RL, a nonmagnet  38   d  functioning as a spacer layer SP, a ferromagnet  39   d  functioning as a shift cancelling layer SCL, and a nonmagnet  40   d  functioning as a buffer layer BUF. 
     The magnetoresistive effect elements MTJu and MTJd have the same configuration with regard to the capping layers CAP 1  and CAP 2 , Hk enhancement layer HkEL, storage layer SL, tunnel barrier layer TB, reference layer RL, spacer layer SP, shift cancelling layer SCL, and buffer layer BUF. On the other hand, the magnetoresistive effect element MTJu and the magnetoresistive effect element MTJd have different configurations with regard to the heat absorption layers HAL. 
     In the magnetoresistive effect element MTJu, the nonmagnet  40   u , ferromagnet  39   u , nonmagnet  38   u , ferromagnet  37   u , nonmagnet  36   u , ferromagnet  35   u , nonmagnet  34   u , nonmagnet  33   u , nonmagnet  32   u , and nonmagnet  31   u  may be stacked in the order from the side of the bit line BL to the side of the word line WLu (in the Z-axis direction perpendicular to the semiconductor substrate  20 ). In the magnetoresistive effect element MTJd, the nonmagnet  40   d , ferromagnet  39   d , nonmagnet  38   d , ferromagnet  37   d , nonmagnet  36   d , ferromagnet  35   d , nonmagnet  34   d , nonmagnet  33   d , nonmagnet  32   d , and nonmagnet  31   d  may be stacked in the order from the side of the word line WLd to the side of the bit line BL (in the Z-axis direction). The magnetoresistive effect elements MTJu and MTJd may function as perpendicular magnetization type MTJ elements, in which the magnetization directions of the magnets that constitute the magnetoresistive effect elements MTJu and MTJd, respectively, are perpendicular to the film plane. 
     As described above, part of the layers of the magnetoresistive effect elements MTJu and MTJd have different configurations (i.e., materials and film thicknesses); however, the same number of layers may be included in the magnetoresistive effect elements MTJu and MTJd. The configuration is not limited to the above. As long as the number of layers is the same, the magnetoresistive effect elements MTJu and MTJd may further include layers (not shown) between any adjacent layers of the above layers  31   u  to  40   u  and between any adjacent layers of the above layers  31   d  to  40   d.    
     The nonmagnets  31   u  and  31   d  are non-magnetic conductive films, both functioning as an upper electrode that enhances the electric connectivity with the word line WLu and bit line BL, respectively. The nonmagnets  31   u  and  31   d  include high-melting-point metal. The high-melting-point metal may indicate a material that demonstrates a melting point higher than iron (Fe) and cobalt (Co), which is at least one element selected from zirconium (Zr), hafnium (Hf), tungsten (W), chrome (Cr), molybdenum (Mo), niobium (Nb), titanium (Ti), tantalum (Ta), vanadium (V), ruthenium (Ru), and platinum (Pt). 
     Nonmagnets  32   u  and  32   d  having different heat absorption rates are employed to adjust the amount of heat flowing into the magnetoresistive effect elements MTJu and MTJd, respectively, when manufacturing the memory cell array  10 . In particular, the heat absorption rate of the nonmagnet  32   u  is set higher than that of the nonmagnet  32   d . That is, when a certain amount of heat is applied to the memory cell array  10 , the nonmagnet  32   u  absorbs more heat than the nonmagnet  32   d  so that the temperature of the magnetoresistive effect element MTJu becomes higher than the temperature of the magnetoresistive effect element MTJd. The heat absorption rate is defined together with the optical reflectivity, given by (reflectivity)+(heat absorption rate)=1. From the viewpoint of preventing the resistances of the magnetoresistive effect elements MTJu and MTJd from increasing, the nonmagnets  32   u  and  32   d  should be formed of a material having approximately the same resistivity as the nonmagnets  31   u  and  31   d . For this reason, the nonmagnets  32   u  and  32   d  may each include at least one element selected from zirconium (Zr), hafnium (Hf), tungsten (W), chrome (Cr), molybdenum (Mo), niobium (Nb), titanium (Ti), tantalum (Ta), vanadium (V), ruthenium (Ru), and platinum (Pt). 
     When the nonmagnet  32   d  is formed n layers apart from the nonmagnet  36   d , the nonmagnet  32   u  is also formed n layers apart from the nonmagnet  36   u  (where n is an integer larger than or equal to 1). In the example of  FIG. 5 , the nonmagnets  32   u  and  32   d  are formed three layers apart from the nonmagnets  36   u  and  36   d  along the Z-axis, respectively. 
     The nonmagnets  33   u  and  33   d  may include at least one element selected from iridium (Ir), ruthenium (Ru), rhodium (Rh), molybdenum (Mo), iron (Fe), and cobalt (Co), having a function of enhancing the perpendicular magnetic anisotropy of the corresponding ferromagnets  35   u  and  35   d.    
     As described above, for the nonmagnets  32   u  and  32   d  designed to have different heat absorption rates, elements included in the film and film thicknesses are selected so as to be different from each other. Thus, nonmagnets  32   u  and  32   d  having different elements and/or thicknesses can be obtained. For example, if the nonmagnet  31   d , nonmagnet  32   d , and nonmagnet  33   d  mainly contain ruthenium (Ru), the nonmagnet  31   u , nonmagnet  32   u , and nonmagnet  33   u  may mainly contain ruthenium (Ru), tantalum (Ta) and ruthenium (Ru), respectively. With tantalum (Ta) having a greater heat absorption rate than ruthenium (Ru), the heat absorption rate of the magnetoresistive effect element MTJu can be set higher than the heat absorption rate of the magnetoresistive effect element MTJd. 
     When the nonmagnet  31   d , nonmagnet  32   d , and nonmagnet  33   d  mainly contain ruthenium (Ru), the compositions of the nonmagnet  31   d ,  32   d  and  33   d  of the magnetoresistive effect element MTJd become similar to each other, which means that the heat absorption layer HAL is indistinguishable from other layers (i.e., the capping layer CAP 1  and Hk enhancement layer HkEL in the above example) in the structure of the magnetoresistive effect element MTJd. In other words, the magnetoresistive effect element MTJd substantially may not contain a layer named “heat absorption layer HAL”. The present embodiment therefore includes a structure in which the magnetoresistive effect element MTJd does not contain a heat absorption layer HAL, while the magnetoresistive effect element MTJu does contain a heat absorption layer HAL. The present embodiment further includes a structure in which the magnetoresistive effect element MTJu does not contain a heat absorption layer HAL, while the magnetoresistive effect element MTJd does contain a heat absorption layer HAL. 
     The nonmagnets  34   u  and  34   d  each contain a nonmagnetic oxide, having functions of enhancing the perpendicular magnetic anisotropy of the ferromagnets  35   u  and  35   d , preventing the damping coefficient of the ferromagnets  35   u  and  35   d  from increasing, and preventing impurities from being diffused into the ferromagnets  35   u  and  35   d  during the crystallization process of the ferromagnets  35   u  and  35   d . In particular, the nonmagnets  34   u  and  34   d  contain magnesium oxide (MgO) or rare-earth oxide. The nonmagnets  34   u  and  34   d  containing magnesium oxide (MgO) are provided with a body-centered cubic (bcc) crystalline structure (a NaCl crystalline structure having a surface oriented in a (001) surface plane), and therefore serves as an excellent seed, or core material, for growing a crystalline film from the interface between the ferromagnet  35   u  and the nonmagnet  34   u , and the interface between the ferromagnet  35   d  and the nonmagnet  34   d , respectively. The nonmagnets  34   u  and  34   d  containing a rare-earth oxide each absorb boron (B) from the ferromagnets  35   u  and  35   d , thereby contributing excellent crystallization of the ferromagnets  35   u  and  35   d . In general, oxides have insulation properties. From the viewpoint of lowering the resistance, it is preferable that the film thickness of the nonmagnets  34   u  and  34   d  be no greater than several nanometers (nm). 
     The ferromagnets  35   u  and  35   d  have an easy magnetization axis in the direction perpendicular to the film plane, exhibiting ferromagnetism with the magnetization direction along the Z-axis toward either the bit line BL side or the word line WL side. The ferromagnets  35   u  and  35   d  contain at least one of iron (Fe), cobalt (Co) and nickel (Ni), and may further contain boron (B). Specifically, the ferromagnets  35   u  and  35   d  may contain iron cobalt boride (FeCoB) or iron boride (FeB), having a body-centered cubic crystalline structure. 
     The nonmagnets  36   u  and  36   d  are non-magnetic insulators, containing magnesium oxide (MgO), for example. In the crystallization process of the ferromagnets  35   u  and  35   d , the nonmagnets  36   u  and  36   d  each serve as a seed or a core for growing a crystalline film from the interface between the nonmagnet  36   u  and ferromagnet  35   u  and the interface between the nonmagnet  36   d  and ferromagnet  35   d . The nonmagnet  36   u  arranged between the ferromagnet  35   u  and ferromagnet  37   u  and the nonmagnet  36   d  arranged between the ferromagnet  35   d  and ferromagnet  37   d  each form a magnetic tunnel junction together with these adjacent ferromagnets. The ferromagnets  37   u  and  37   d  have an easy magnetization axis in the direction perpendicular to the film plane, exhibiting ferromagnetism having a magnetization direction along the Z-axis toward either the bit line BL side or the word line WL side. The ferromagnets  37   u  and  37   d  contain at least one of iron (Fe), cobalt (Co) and nickel (Ni), and may further contain boron (B). Specifically, the ferromagnets  37   u  and  37   d  may contain iron cobalt boride (FeCoB) or iron boride (FeB), having a body-centered cubic crystalline structure. The ferromagnets  37   u  and  37   d  have a fixed magnetization direction, as shown in  FIG. 5 , where the direction is indicated as being toward the ferromagnets  39   u  and  39   d  in this example. “Fixed magnetization direction” indicates the magnetization direction being unchanged even by a current (spin torque) large enough to reverse the magnetization direction of the ferromagnets  35   u  and  35   d.    
     Each of the ferromagnets  37   u  and  37   d  may be a layer stack including a plurality of layers, although they are not shown in  FIG. 5 . In particular, the layer stacks of the ferromagnets  37   u  and  37   d  may include the layer of the above discussed material as an interface layer between the ferromagnet  37   u  and the nonmagnet  36   u , and an interface layer between the ferromagnet  37   d  and the nonmagnet  36   d , and further include a ferromagnet with a non-magnetic conductor interposed between. The non-magnetic conductor may contain at least one metal selected from tantalum (Ta), hafnium (Hf), tungsten (W), zirconium (Zr), molybdenum (Mo), niobium (Nb) and titanium (Ti), or a nitride or oxide of such metals. The additional ferromagnet may include at least one selected from a multi-layer film (Co/Pt multi-layer film) containing cobalt (Co) and platinum (Pt), a multi-layer film (Co/Ni multi-layer film) containing cobalt (Co) and nickel (Ni), and a multi-layer film (Co/Pd multi-layer film) containing cobalt (Co) and palladium (Pd). 
     The nonmagnets  38   u  and  38   d  are nonmagnetic conductors, and may include at least one element selected from ruthenium (Ru), osmium (Os), iridium (Ir), vanadium (V), and chrome (Cr). 
     The ferromagnets  39   u  and  39   d  have an easy magnetization axis in the direction perpendicular to the film plane, and are ferromagnetic in a magnetization direction along the Z-axis toward either the bit line BL side or the word line WL side. In the same manner as the ferromagnets  37   u  and  37   d , the ferromagnets  39   u  and  39   d  have a fixed magnetization direction. In the example of  FIG. 5 , the magnetization direction is indicated as being toward the ferromagnets  37   u  and  37   d . The ferromagnets  39   u  and  39   d  include at least one alloy selected from cobalt platinum (CoPt), cobalt nickel (CoNi), and cobalt palladium (CoPd). In the same manner as the ferromagnets  37   u  and  37   d , the ferromagnets  39   u  and  39   d  may be a layer stack including a plurality of layers. If this is the case, the ferromagnets  39   u  and  39   d  may include at least one selected from a multi-layer film (Co/Pt multi-layer film) containing cobalt (Co) and platinum (Pt), a multi-layer film (Co/Ni multi-layer film) containing cobalt (Co) and nickel (Ni), and a multi-layer film (Co/Pd multi-layer film) containing cobalt (Co) and palladium (Pd). 
     The ferromagnets  37   u  and  39   u , and ferromagnets  37   d  and  39   d  are antiferromagnetically coupled to each other by the nonmagnets  38   u  and  38   d , respectively. In other words, the ferromagnets  37   u  and  39   u  are coupled so as to have magnetization directions that are anti-parallel to each other, and the ferromagnets  37   d  and  39   d  are coupled in the same manner. In the example of  FIG. 5 , the magnetization directions of the ferromagnets  37   u  and  39   u  face each other, and the magnetization directions of the ferromagnets  37   d  and  39   d  face each other. Such a structure of the ferromagnet  37   u , nonmagnet  38   u  and ferromagnet  39   u  coupled together and a structure of the ferromagnet  37   u , nonmagnet  38   u  and ferromagnet  39   u  coupled together are referred to as synthetic anti-ferromagnetic (SAF) structures. In this manner, the ferromagnet  39   u  can cancel the influence of the stray field of the ferromagnet  37   u  on the magnetization direction of the ferromagnet  35   u , and the ferromagnet  39   d  can cancel the influence of the stray field of the ferromagnet  37   d  on the magnetization direction of the ferromagnet  35   d.    
     The nonmagnets  40   u  and  40   d  are non-magnetic conductors, both functioning as an electrode that enhances the electric connectivity with the bit line BL and word line WLd. The nonmagnet  40   u  and  40   d  may include at least one element selected from zirconium (Zr), hafnium (Hf), tungsten (W), chrome (Cr), molybdenum (Mo), niobium (Nb), titanium (Ti), tantalum (Ta), vanadium (V), iridium (Ir), ruthenium (Ru), rhodium (Rh), and platinum (Pt), and may further include boron (B). 
     According to the first embodiment, a spin injection write scheme is adopted in which a write current flows directly into the magnetoresistive effect elements MTJ and a spin torque is injected by this write current into the storage layer SL and reference layer RL, thereby controlling the magnetization directions of the storage layer SL and the reference layer RL. The magnetoresistive effect elements MTJ may be either in the low resistance state or high resistance state, depending on whether the magnetization directions of the storage layer SL and reference layer RL are parallel or anti-parallel to each other. 
     When a certain write current Ic 0  is passed to the magnetoresistive effect element MTJ in the direction of arrow A 1  in  FIG. 5 , or in other words the direction from the storage layer SL toward the reference layer RL, the magnetization directions of the storage layer SL and of the reference layer RL have a parallel relationship. In a parallel state, the resistance of the magnetoresistive effect element MTJ takes the minimal value, which sets the magnetoresistive effect element MTJ to the low resistance state. This low resistance state is referred to as a “P (parallel) state”, defining for example a data “0” state. 
     On the other hand, when a write current Ic 1  greater than the write current Ic 0  is passed to the magnetoresistive effect element MTJ in the direction of arrow A 2  in  FIG. 5 , or in other words the direction from the reference layer RL toward the storage layer SL (direction opposite to arrow A 1 ), the magnetization directions of the storage layer SL and reference layer RL have an anti-parallel relationship. In the anti-parallel state, the resistance of the magnetoresistive effect element MTJ takes the maximum value, which sets the magnetoresistive effect element MTJ into the high resistance state. This high resistance state is referred to as an “anti-parallel (AP) state”, defining for example a data “1” state. 
     The following explanation is given in accordance with the above data defining method. However, the definition of data “1” and data “0” is not limited to the above. For instance, the P state may be defined as data “1”, and the AP state may be defined as data “0”. 
     1.2 Manufacturing Method of Magnetoresistive Effect Element 
     Next, the manufacturing method of a memory cell array of the magnetic memory device according to the first embodiment is explained. 
       FIGS. 6 to 10  are schematic diagrams showing the method of manufacturing the memory cell array of the magnetic memory device according to the first embodiment. 
     First, the conductors  21  extending along the X-axis above the semiconductor substrate  20  are formed to be aligned along the Y-axis, as illustrated in  FIG. 6 . After filling the gaps between the conductors  21  with an insulator, elements  22  are formed on the top surfaces of these conductors  21 . Each of the elements  22  may include stacked films corresponding to the magnetoresistive effect element MTJd, as explained by referring to (B) in  FIG. 5 . In the stacked film formed in the step corresponding to  FIG. 6 , the ferromagnet  35   d  is formed as an amorphous film. 
     Next, the elements  23  are formed on the top surfaces of the elements  22 , as illustrated in  FIG. 7 . Then, the conductors  24  are formed along the X-axis to extend along the Y-axis so that the top surfaces of the elements  23  aligned along the Y-axis can be electrically coupled to the conductors  24 . An insulator  28  is arranged suitably between the elements  22 , between the elements  23 , and between the conductors  24  to function as an interlayer insulating film INSd. 
     In the process explained with reference to  FIG. 7 , which is conducted after the formation of the magnetoresistive effect element MTJd and before the formation of the magnetoresistive effect element MTJu, the formation of other structural components referred to as a structure BEOLd is completed. In this process, a thermal budget T_BEOLd is input to the elements  22 . The “thermal budget” of a process represents the total amount of heat input to the target element from the start to the end of the process, where the unit of thermal budget may be expressed in joules (J). 
     Thereafter, elements  25  are individually formed on the top surfaces of the conductors  24 , as illustrated in  FIG. 8 . Each of the elements  25  includes a stacked film corresponding to the magnetoresistive effect element MTJu, as explained by referring to (A) in  FIG. 5 . In the stacked film formed in the process corresponding to  FIG. 8 , the ferromagnet  35   u  is formed as an amorphous film. 
     Next, the elements  26  are formed on the top surfaces of the elements  25 , as illustrated in  FIG. 9 . Then, the conductors  27  are aligned in the direction of the Y-axis to extend along the X-axis in a manner such that the top surfaces of the elements  26  aligned in the direction of the X-axis can be electrically coupled to the conductors  27 . Insulators  29  may be suitably arranged between the elements  25 , between the elements  26 , between the conductors  27 , and above the conductors  27  to function as an interlayer insulating film INSu. 
     In the process explained by referring to  FIG. 9 , which is conducted after the formation of the magnetoresistive effect element MTJu and before the annealing, the formation of the remaining structure BEOLu is completed. During this process, a thermal budget T_BEOLu is input to the elements  22  and elements  25 . 
     As explained before, the elements  22  and the elements  25  are formed to have different heat absorption rates. With regard to the thermal budget T_BEOLu (and T_BEOLd), however, unlike lamp annealing that is a heating technique in which a large amount of heat is applied rapidly in a short period of time, the layer stack on the semiconductor substrate  20  is heated over a long period of time relative to the lamp annealing. For this reason, in the formation of the structure BEOLu, the difference between the thermal budgets of the elements  22  and elements  25  observed in accordance with the difference between the heat absorption rates is negligible. Thus, the thermal budget T_BEOLu input to the elements  22  and to the elements  25  can be regarded as approximately the same, regardless of their different heat absorption rates. 
     Next, as illustrated in  FIG. 10 , the structure formed in the above process is heated by annealing, which transforms the ferromagnet  35   d  (and  37   d ) of the elements  22  and the ferromagnet  35   u  (and  37   u ) of the elements  25  from amorphous to crystalline. In this manner, the elements  22  and elements  25  attain the properties of the magnetoresistive effect elements MTJd and MTJu, respectively. 
     For the above annealing process, lamp annealing is conducted by providing a large amount of heat rapidly in a short period of time. For this purpose, thermal budgets T_postd and T_postu are input to the elements  22  and elements  25 , respectively, corresponding to their heat absorption rates. This means that the thermal budget T_postd input to the elements  22  is smaller than the thermal budget T_postu input to the elements  25 . 
     In view of the above, the total thermal budget Td (=T_BEOLd+T_BEOLu+T_postd) is input to the elements  22 , the total thermal budget Tu (=T_BEOLu+T_postu) is input to the elements  25 , and the process of manufacturing the memory cell array  10  is completed. 
     In the above example, annealing is conducted after forming the conductor  27 , but the process order is not limited thereto. For instance, annealing may be conducted after forming the elements  26  and before forming the conductors  27 . If this is the case, when inputting the thermal budgets T_postu and T_postd from above the magnetic memory device  1  to the elements  22  and  25  during the annealing, no conductor  27  would be present so that the input of the thermal budgets to the elements  22  and  25  would not be obstructed and the input would not be reduced from the certain targeted amount. 
     1.3 Effects of Present Embodiment 
     According to the first embodiment, variations can be suppressed in the properties of the magnetoresistive effect elements formed in different layers. The effects of the present embodiment will be described below. 
     The magnetoresistive effect element MTJd is arranged below the magnetoresistive effect element MTJu. This means that the magnetoresistive effect element MTJd receives more heat for the amount of the thermal budget T_BEOLd than the magnetoresistive effect element MTJu, where the thermal budget T_BEOLd is input during the process for preparing other films of the structure, after forming the magnetoresistive effect element MTJd and before forming the magnetoresistive effect element MTJu. If the magnetoresistive effect elements MTJu and MTJd are designed to have approximately the same heat absorption rate, the total thermal budgets Tu and Td will produce a difference expressed by the thermal budget T_BEOLd before the annealing, which may cause a difference between the properties of the magnetoresistive effect elements MTJu and MTJd. 
     According to the first embodiment, the nonmagnet  32   d  is designed to have a heat absorption rate smaller than that of the nonmagnet  32   u . In particular, if the nonmagnet  32   d  is formed to mainly contain ruthenium (Ru), the nonmagnet  32   u  is formed to mainly contain tantalum (Ta), which has a greater heat absorption rate than ruthenium (Ru). In this manner, under the application of a certain amount of heat for a short period of time by annealing, the thermal budget T_postd input to the magnetoresistive effect element MTJd becomes smaller than the thermal budget T_postu input to the magnetoresistive effect element MTJu (T_postd&lt;T_postu). By suitably setting the difference between the thermal budget T_postd and the thermal budget T_postu (for example, by bringing it close to T_BEOLd), the total thermal budget Td (=T_BEOLd+T_BEOLu+T_postd) input to the magnetoresistive effect element MTJd can be set to approximately the same value as the total thermal budget Tu (=T_BEOLu+T_postu) input to the magnetoresistive effect element MTJu when manufacturing the memory cell array  10 . 
     A further explanation is provided below with reference to  FIG. 11 . 
       FIG. 11  is a diagram showing the effects of the first embodiment. The horizontal axis of  FIG. 11  represents the temperature of the heat applied to the magnetoresistive effect element MTJ during annealing, while the vertical axis represents the properties of two different magnetoresistive effect elements MTJ having different heat absorption rates when the annealing temperature is changed from T 1  to T 3 . In particular, line Lu 1  represents the change of the magnetoresistance ratio TMRu of magnetoresistive effect element MTJu having a higher heat absorption rate, while line Ld 1  represents the change of the magnetoresistance ratio TMRd of magnetoresistive effect element MTJd having a lower heat absorption rate in (A) of  FIG. 11 . Line Lu 2  in (B) of  FIG. 11  represents the change of an antiferromagnetic coupling force Hexu of magnetoresistive effect element MTJu having a higher heat absorption rate, while line Ld 2  represents the change of an antiferromagnetic coupling force Hexd of magnetoresistive effect element MTJd having a lower heat absorption rate. In the example of  FIG. 11 , it is considered that the same amount of thermal budget T BEOL is input to the magnetoresistive effect elements MTJu and MTJd when forming films (or in other words, no thermal budget other than the thermal budget for annealing will be taken into consideration). 
     In (A) of  FIG. 11 , line Ld 1  representing the change of the magnetoresistance ratio TMRd is shifted toward the right of the drawing sheet (to the positive side of the vertical axis) with respect to line Lu 1  representing the change of the magnetoresistance ratio TMRu so that a response to the annealing temperature can be delayed. Specifically, the magnetoresistance ratio TMRd under the heat application at an annealing temperature T 2  is approximately the same as the magnetoresistance ratio TMRu under the heat application at an annealing temperature (T 2 −Δ), and the magnetoresistance ratio TMRd under the heat application at an annealing temperature (T 2 +Δ) is approximately the same as the magnetoresistance ratio TMRu under the heat application at the annealing temperature T 2  (where Δ&gt;0). 
     Furthermore, in (B) of  FIG. 11 , line Ld 2  representing the change of the antiferromagnetic coupling force Hexd is shifted toward the right of the drawing sheet (to the positive side of the vertical axis) with respect to line Lu 2  representing the change of the antiferromagnetic coupling force Hexu so that a response to the annealing temperature can be delayed. Specifically, the antiferromagnetic coupling force Hexd under the heat application at the annealing temperature T 2  is approximately the same as the antiferromagnetic coupling force Hexu under the heat application at the annealing temperature (T 2 −Δ), and the antiferromagnetic coupling force Hexd under the heat application at the annealing temperature (T 2 +Δ) is approximately the same as the antiferromagnetic coupling force Hexu under the heat application at the annealing temperature T 2 . 
     In this manner, by setting different values to the heat absorption rates of the magnetoresistive effect elements MTJu and MTJd, different thermal budgets can be input for the annealing. Thus, a difference expressed by T_BEOLd created between the heat budgets of the magnetoresistive effect elements MTJ formed in different layers can be compensated, bringing the total thermal budgets Tu and Td to approximately the same values. As a result, variations in the properties of the magnetoresistive effect elements MTJ can be reduced. 
     1.4 First Modification 
     In the above explanation of the first embodiment, the heat absorption layer HAL is formed above the storage layer SL in a top-free structure in which the storage layer SL is arranged above the reference layer RL, but the structure is not limited thereto. For example, the heat absorption layer HAL may be arranged below the storage layer SL in the top-free structure. In the following explanation, the points different from the first embodiment will be focused on for the sake of convenience. 
       FIG. 12  is a cross section of a configuration of a magnetoresistive effect element in a magnetic memory device according to a first modification of the first embodiment, which corresponds to the configuration of the first embodiment illustrated in  FIG. 5 . 
     In the magnetoresistive effect element MTJu as illustrated in  FIG. 12 , a plurality of films are arranged, namely, a nonmagnet  32   u A functioning as the heat absorption layer HAL, a nonmagnet  41   u  functioning as a crystal separation layer CSL, the nonmagnet  40   u , the ferromagnet  39   u , the nonmagnet  38   u , the ferromagnet  37   u , the nonmagnet  36   u , the ferromagnet  35   u , the nonmagnet  34   u , a nonmagnet  33   u , and a nonmagnet  31   u , in this order from the bit line BL side to the word line WLu side (in the direction of the Z-axis). In the magnetoresistive effect element MTJd, a plurality of films are arranged, namely, a nonmagnet  32   d A functioning as a heat absorption layer HAL, a nonmagnet  41   d  functioning as the crystal separation layer CSL, the nonmagnet  40   d , the ferromagnet  39   d , the nonmagnet  38   d , the ferromagnet  37   d , the nonmagnet  36   d , the ferromagnet  35   d , the nonmagnet  34   d , the nonmagnet  33   d , and the nonmagnet  31   d  in this order from the side of the word line WLd to the side of the bit line BL (in the direction of the Z-axis). 
     If the nonmagnet  32   d A is arranged n layers apart from the nonmagnet  36   d , the nonmagnet  32   u A is also formed n layers apart from the nonmagnet  36   u  (where n is an integer larger than or equal to 1). In the example of  FIG. 12 , the nonmagnets  32   u A and  32   d A are formed five layers apart, downwardly from the nonmagnets  36   u  and  36   d  along the Z-axis. The nonmagnets  32   u A and  32   d A are substantially the same as the nonmagnets  32   u  and  32   d  according to the first embodiment, except for the levels in which these components are positioned, and the explanation is therefore omitted. 
     The nonmagnets  41   u  and  41   d  are amorphous layers, having a function of dividing the crystalline structures in the layers immediately above and below. Preferably, the nonmagnets  41   u  and  41   d  maintain the amorphous state without becoming crystallized by annealing (at a temperature as high as 400 degrees Celsius, for example). For this purpose, the nonmagnets  41   u  and  41   d  may contain hafnium boride (HfB). 
     With the above configuration, even when the heat absorption layer HAL is arranged below the storage layer SL, the crystallization in each layer of the magnetoresistive effect element MTJ can be maintained. As a result, different heat absorption rates can be set for the upper magnetoresistive effect element MTJu and for the lower magnetoresistive effect element MTJd, while maintaining properties such as magnetic anisotropy and magnetoresistance ratio. 
     1.5 Second Modification 
     In the above explanation of the first embodiment, the capping layer CAP 1  is arranged on top of the heat absorption layer HAL in the top-free structure, but the arrangement is not limited thereto. For example, a capping layer CAP 1  may not necessarily be formed. In the following explanation, the points different from the first embodiment will be focused on for the sake of convenience. 
       FIG. 13  is a cross section of a configuration of a magnetoresistive effect element in a magnetic memory device according to a second modification of the first embodiment, which corresponds to the configuration of the first embodiment illustrated in  FIG. 5 . 
     As illustrated in  FIG. 13 , the nonmagnets  32   u  and  32   d  are formed as the topmost layers in the magnetoresistive effect elements MTJu and MTJd, respectively, according to the second modification example of the first embodiment. In this structure, different heat absorption rates can be set for the magnetoresistive effect element MTJu and for the magnetoresistive effect element MTJd, thereby obtaining effects similar to the first embodiment. 
     2. Second Embodiment 
     A magnetic memory device according to a second embodiment will be described below. The magnetoresistive effect element MTJ according to the first embodiment has a top-free structure. According to the second embodiment, the magnetoresistive effect element MTJ has a bottom-free structure. In the magnetoresistive effect element MTJ with a bottom-free structure, the storage layer SL is formed below the reference layer. 
     2.1 Magnetoresistive Effect Element A configuration of a magnetoresistive effect element according to the second embodiment is explained with reference to  FIG. 14 . A cross section of the configuration of the magnetoresistive effect element in the magnetic memory device according to the second embodiment is presented in  FIG. 14 , which corresponds to the configuration of the first embodiment illustrated in  FIG. 5 . 
     As illustrated in  FIG. 14 , the magnetoresistive effect element MTJu may include a nonmagnet  51   u  functioning as a capping layer CAP 1 , a nonmagnet  52   u  functioning as a heat absorption layer HAL, a ferromagnet  53   u  functioning as a shift cancelling layer SCL, a nonmagnet  54   u  functioning as a spacer layer, a ferromagnet  55   u  functioning as a reference layer RL, a nonmagnet  56   u  functioning as a tunnel barrier layer TB, a ferromagnet  57   u  functioning as a storage layer SL, a nonmagnet  58   u  functioning as an underlayer UL, a nonmagnet  59   u  functioning as an Hk enhancement layer HkEL, and a nonmagnet  60   u  functioning as a buffer layer BUF. The magnetoresistive effect element MTJd may include a nonmagnet  51   d  functioning as a capping layer CAP 1 , a nonmagnet  52   d  functioning as a heat absorption layer HAL, a ferromagnet  53   d  functioning as a shift cancelling layer SCL, a nonmagnet  54   d  functioning as a spacer layer, a ferromagnet  55   d  functioning as a reference layer RL, a nonmagnet  56   d  functioning as a tunnel barrier layer TB, a ferromagnet  57   d  functioning as a storage layer SL, a nonmagnet  58   d  functioning as an underlayer UL, a nonmagnet  59   d  functioning as an Hk enhancement layer HkEL, and a nonmagnet  60   d  functioning as a buffer layer BUF. 
     If the nonmagnet  52   d  is arranged n layers apart from the nonmagnet  56   d , the nonmagnet  52   u  is also formed n layers apart from the nonmagnet  56   u  (where n is an integer larger than or equal to 1). In the example of  FIG. 14 , the nonmagnets  52   u  and  52   d  are formed three layers apart from the nonmagnets  56   u  and  56   d  along the Z-axis. 
     The capping layer CAP 1 , shift cancelling layer SCL, spacer layer SP, reference layer RL, tunnel barrier layer TB, storage layer SL, Hk enhancement layer HkEL, and buffer layer BUF are the same as those in the first embodiment. In addition, these layers have the same structures between the magnetoresistive effect element MTJu and the magnetoresistive effect element MTJd. The underlayer UL is substantially the same as the capping layer CAP 2  according to the first embodiment, and has the same structure for the magnetoresistive effect elements MTJu and MTJd. On the other hand, the heat absorption layer HAL has a higher heat absorption rate in the magnetoresistive effect element MTJu than in the magnetoresistive effect element MTJd. 
     In the above bottom-free structure, the magnetoresistive effect element MTJ can be configured to include the heat absorption layer HAL above the storage layer SL in a similar manner to the top-free structure. In this manner, different heat absorption rates can be set for the upper magnetoresistive effect element MTJu and for the lower magnetoresistive effect element MTJd, which can reduce variations in the properties between the two elements. 
     2.2 First Modification 
     According to the second embodiment, the bottom-free structure in which the heat absorption layer HAL is arranged above the storage layer SL has been explained, but the structure is not limited thereto. For example, the heat absorption layer HAL may be arranged below the storage layer SL in the bottom-free structure. In the following explanation, the points different from the second embodiment will be focused on for the sake of convenience. 
       FIG. 15  is a cross section of a configuration of a magnetoresistive effect element in a magnetic memory device according to a first modification example of the second embodiment, which corresponds to the configuration of the second embodiment illustrated in  FIG. 14 . 
     In the magnetoresistive effect element MTJu as illustrated in  FIG. 15 , a plurality of films are arranged, namely, a nonmagnet  52   u A functioning as a heat absorption layer HAL, a nonmagnet  61   u  functioning as a crystal separation layer CSL, a nonmagnet  60   u , a nonmagnet  59   u , a nonmagnet  58   u , a ferromagnet  57   u , a nonmagnet  56   u , a ferromagnet  55   u , a nonmagnet  54   u , a ferromagnet  53   u , and a nonmagnet  51   u  in this order from the bit line BL side toward the word line WLu side (in the direction of the Z-axis). In the magnetoresistive effect element MTJd, a plurality of films are arranged, namely, a nonmagnet  52   d A functioning as a heat absorption layer HAL, a nonmagnet  61   d  functioning as a crystal separation layer CSL, a nonmagnet  60   d , a nonmagnet  59   d , a nonmagnet  58   d , a ferromagnet  57   d , a nonmagnet  56   d , a ferromagnet  55   d , a nonmagnet  54   d , a ferromagnet  53   d , and a nonmagnet  51   d , in this order from the word line WLd side toward the bit line BL side (in the direction of the Z-axis). 
     If the nonmagnet  52   d A is arranged n layers apart from the nonmagnet  56   d , the nonmagnet  52   u A is also formed n layers apart from the nonmagnet  56   u  (where n is an integer larger than or equal to 1). In the example of  FIG. 15 , the nonmagnets  52   u A and  52   d A are formed five layers apart downwardly from the nonmagnets  56   u  and  56   d  along the Z-axis. The nonmagnets  52   u A and  52   d A are substantially the same as the nonmagnets  52   u  and  52   d  according to the second embodiment, except for the levels in which these components are positioned, and the explanation thereof is omitted. 
     The nonmagnets  61   u  and  61   d  are amorphous, having a function of dividing the crystalline structures in the layers immediately above and below. Preferably, the nonmagnets  61   u  and  61   d  maintain the amorphous state, without becoming crystallized by annealing (under a temperature as high as 400 degrees Celsius, for example). For this purpose, the nonmagnets  61   u  and  61   d  may contain hafnium boride (HfB). 
     With the above configuration, even when the heat absorption layer HAL is arranged below the storage layer SL, the crystallization in each layer of the magnetoresistive effect elements MTJ can be maintained. As a result, while maintaining properties such as magnetic anisotropy and magnetoresistance ratio, different heat absorption rates can be set for the upper magnetoresistive effect element MTJu and for the lower magnetoresistive effect element MTJd. 
     2.3 Second Modification 
     In the second embodiment, the bottom-free structure in which the capping layer CAP 1  is arranged above the heat absorption layer HAL has been explained, but the arrangement is not limited thereto. For example, a capping layer CAP 1  may not need to be formed. In the following explanation, the points different from the second embodiment will be focused on for the sake of convenience. 
       FIG. 16  is a cross section of a configuration of a magnetoresistive effect element in a magnetic memory device according to a second modification example of the second embodiment, which corresponds to the configuration of the second embodiment illustrated in  FIG. 14 . 
     As illustrated in  FIG. 16 , the nonmagnets  52   u  and  52   d  are formed as the topmost layer in each of the magnetoresistive effect elements MTJu and MTJd, respectively, according to the second modification example of the second embodiment. In this structure, different heat absorption rates can be set for the magnetoresistive effect element MTJu and for the magnetoresistive effect element MTJd, thereby obtaining effects similar to the second embodiment. 
     3. Others 
     The first and second embodiments are not limitations, but various other modifications may be applied. 
     According to the first and second embodiments, different materials are adopted to attain different heat absorption rates for the upper heat absorption layer HAL and the lower heat absorption layer HAL, but this is not a limitation. For example, the upper heat absorption layer HAL may be configured to have a greater film thickness than the lower heat absorption layer HAL, regardless of whether or not the upper heat absorption layer HAL mainly contains the same element as that of the lower heat absorption layer HAL. In this manner, the heat absorption rate of the upper heat absorption layer HAL can be set higher than the heat absorption rate of the lower heat absorption layer HAL. This can set different heat budgets to be input during annealing, thereby compensating the difference expressed by T_BEOLd created between the heat budgets of the layers in the magnetoresistive effect elements MTJu and MTJd. As a result, the total thermal budgets Tu and Td can be brought to approximately the same value, reducing the variations in the properties of the magnetoresistive effect elements MTJu and MTJd. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit.