Patent Publication Number: US-2020294567-A1

Title: Magnetoresistive element and magnetic memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2019-048662, filed Mar. 15, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     An embodiment described herein relates to a magnetoresistive element and a magnetic memory device. 
     BACKGROUND 
     Magnetoresistive random-access memory (MRAM) is known as a type of semiconductor memory device. MRAM is a memory device that uses magnetoresistive elements, which have a magnetoresistive effect, in memory cells that store information. Spin-injection write technique is one of the writing techniques in MRAM. The spin-injection write technique is advantageous for high integration, low power consumption, and high performance, since the spin injection current required to reverse the magnetization decreases as the size of the magnetic body decreases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an MTJ element  10  according to the first embodiment. 
         FIG. 2  is a schematic diagram illustrating magnetic characteristics of a ferromagnetic layer to which a non-magnetic element is added. 
         FIG. 3  is a schematic diagram illustrating magnetic characteristics of a ferromagnetic layer to which another non-magnetic element is added. 
         FIG. 4  is a schematic diagram illustrating magnetic characteristics of a. ferromagnetic layer to which a rare-earth element is added. 
         FIG. 5  is a table illustrating characteristics of Comparative Examples 1-6 and Examples 1-3. 
         FIG. 6  is a cross-sectional view illustrating a stacked structure of Comparative Examples 1 and 2. 
         FIG. 7  is a cross-sectional view illustrating a stacked structure of Comparative Example 3. 
         FIG. 8  is a cross-sectional view illustrating a stacked structure of Comparative Examples 4-6. 
         FIG. 9  is a cross-sectional view illustrating a stacked structure of Examples 1-3. 
         FIG. 10  is a block diagram of an MRAM  100  according to the second embodiment. 
         FIG. 11  is a cross-sectional view of the MRAM  100  according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In generally, according to one embodiment, a magnetoresistive element includes a first magnetic layer having an invariable magnetization direction; a non-magnetic layer provided on the first magnetic layer; a second magnetic layer provided on the non-magnetic layer, having an invariable magnetization direction, and containing a rare-earth element; a third magnetic layer provided on the second magnetic layer and composed of cobalt; and an oxide layer provided on the third magnetic layer. 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. In the description that follows, components having the same functions and configurations will be denoted by the same reference symbols, and repeated descriptions will be given only where necessary. The drawings are schematic or conceptual, and the dimensions and ratios, etc. in the drawings are not always the same as the actual ones. The embodiments serve to give examples of apparatuses and methods that realize the technical concepts of the embodiments. The technical ideas of the embodiments are not intended to limit the materials, shapes, structures, arrangements, etc. of the components to those described herein. 
     First Embodiment 
     Hereinafter, a description will be given of a magnetoresistive element included in a magnetoresistive memory device. The magnetoresistive element is also called a magnetoresistive effect element, or a magnetic tunnel junction (MTJ) element. The magnetoresistive memory device (magnetic memory) is a magnetoresistive random access memory (MRAM). 
     [1] Configuration of MTJ Element 
       FIG. 1  is a cross-sectional view of an MTJ element  10  according to the first embodiment. The MTJ element  10  shown in  FIG. 1  is provided on a foundation structure (unillustrated) including a substrate. 
     As illustrated in  FIG. 1 , the MTJ element  10  includes a buffer layer (BL)  11 , a shift canceling layer (SGL)  12 , a spacer layer  13 , a reference layer (RL)  14 , a tunnel barrier layer (TB)  15 , a storage layer (SL)  16 , a cobalt layer (also referred to as a “magnetic layer”)  17 , an oxide layer (REO)  18 , and a cap layer (Cap)  19 , stacked in this order. The storage layer  16  is also referred to as a “free layer”. The reference layer  14  is also referred to as a “fixed layer”. The shift canceling layer  12  is also referred to as “a shift adjustment layer”. The planar shape of the MTJ element  10  is not particularly limited, and may be, for example, a circle or an oval. 
     The buffer layer  11  contains, for example, aluminum (Al), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), scandium (Sc), yttrium (Y), lanthanum (La), silicon (Si), zirconium ( 7 ,r), hafnium (Hf), tungsten (W), chromium (Cr), molybdenum (Mo), niobium (Nb), titanium (Ti), tantalum (Ta), or vanadium (V). The buffer layer  11  may contain a boride thereof. The boride is not limited to a binary compound consisting of two different elements, and may be a ternary compound consisting of three different elements. That is, the boride may be a mixture of binary compounds. For example, the buffer layer  11  may be composed of a hafnium boride (HfB), a magnesium aluminum boride (MgAlB), a hafnium aluminum boride (HfAlB), a scandium aluminum boride (ScAlB), a scandium hafnium boride (ScHfB), or a hafnium magnesium boride (HfMgB). The buffer layer  11  may be composed of more than one of these materials stacked upon one another. By using a high-melting-point metal or a boride thereof, it is possible to suppress diffusion of the material of the buffer layer into the magnetic layer, thereby preventing deterioration of the magnetoresistance (MR) ratio. The high-melting-point metal is a material having a melting point higher than iron (Fe) and cobalt (Co), and examples include zirconium (Zr), hafnium (Hf), tungsten (W), chromium (Cr), molybdenum (Mo), niobium (Nb), titanium (Ti), tantalum (Ta), and vanadium (V), as well as alloys thereof. 
     The shift canceling layer  12  has a function of reducing a leakage field from the reference layer  14 , suppressing the reduced leakage field from being applied to the storage layer  16 , and shifting the coercive force (or the magnetization curve) of the storage layer  16 . The shift canceling layer  12  is composed of a ferromagnetic material. The shift canceling layer  12  has, for example, perpendicular magnetic anisotropy, and its easy magnetization direction is approximately perpendicular to the film surface. The expression “approximately perpendicular” means that the direction of the remanent magnetization is within the range of 45°&lt;θ≤90°, with respect to the film surface. The magnetization direction of the shift canceling layer  12  is invariable and fixed to one direction. The magnetization directions of the shift canceling layer  12  and the reference layer  14  are set to be antiparallel. The shift canceling layer  12  is composed of, for example, the same ferromagnetic material as the reference layer  14 . The material of the reference layer  14  will be described later. Of the ferromagnetic materials that will be listed as example materials of the reference layer  14 , a material different from the reference layer  14  may be selected as the material of the shift canceling layer  12 . 
     The spacer layer  13  is composed of a non-magnetic material, and has a function of antiferromagnetically bonding the reference layer  14  and the shift canceling layer  12 . That is, the reference layer  14 , the spacer layer  13 , and the shift canceling layer  12  have a synthetic antiferromagnetic (SAF) structure. The reference layer  14  and the shift canceling layer  12  are antiferromagnetically bonded via the spacer layer  13 . The spacer layer  13  is composed of, for example, ruthenium (Ru) or an alloy of ruthenium (Ru). 
     The reference layer  14  is composed of a ferromagnetic material. The reference layer  14  has, for example, perpendicular magnetic anisotropy, and its easy magnetization direction is approximately perpendicular to the film surface. The magnetization direction of the reference layer  14  is invariable and fixed to one direction. The “invariable” magnetization direction means that the magnetization direction of the reference layer  14  does not change when a predetermined write current is allowed to flow through the MTJ element  10 . 
     The reference layer  14  is composed of a compound containing at least one of iron (Fe), cobalt (Co), and nickel (Ni). The reference layer  14  may further contain, as impurities, at least one of boron (B), phosphorus (P), carbon (C), aluminum (Al), silicon (Si), tantalum (Ta), molybdenum (Mo), chromium (Cr), hafnium (Hf) , tungsten (W), and titanium (Ti). More specifically, the reference layer  14  may contain, for example, a cobalt iron boron (CoFeB) or an iron boride (FeB). Alternatively, the reference layer  14  may contain at least one of cobalt platinum (Copt), cobalt nickel (Cori), and cobalt palladium (Coed). 
     The tunnel barrier layer  15  is composed of a non-magnetic material. The tunnel barrier layer  15  functions as a barrier between the reference layer  14  and the storage layer  16 . The tunnel barrier layer  15  is composed of, for example, an insulating material, and contains, in particular, a magnesium oxide (MgO). 
     The storage layer  16  is composed of a ferromagnetic material. The storage layer  16  has, for example, perpendicular magnetic anisotropy, and its easy magnetization direction is perpendicular or approximately perpendicular to the film surface. The magnetization direction of the storage layer  16  is variable and reversible. The “variable” magnetization direction means that the magnetization direction of the storage layer  16  may change when a predetermined write current is allowed to flow through the MTJ element  10 . The storage layer  16 , the tunnel barrier layer  15 , and the reference layer  14  form a magnetic tunnel junction. In  FIG. 1 , the magnetization directions of the storage layer  16 , the reference layer  14 , and the shift canceling layer  12  are denoted by arrows, as an example. The magnetization direction of each of the storage layer  16 , the reference layer  14 , and the shift canceling layer  12  is not limited to a perpendicular direction and may be an in-plane direction. 
     The storage layer  16  is composed of a compound containing a rare-earth element and at least one of iron (Fe), cobalt (Co), and nickel (Ni). Such a compound may further contain boron (B). In other words, the storage layer  16  may be composed of: Co and a rare-earth element; Fe and a rare-earth element; Ni and a rare-earth element; Co, Fe, and a rare-earth element; or one of these structures further containing B. The rare-earth elements include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Of these rare-earth elements, gadolinium (Gd), terbium (Tb), and dysprosium (Dy) are particularly effective. 
     The cobalt layer  17  is a magnetic layer consisting mainly of cobalt (Co). Specifically, the cobalt layer  17  is composed only of cobalt (Co). The cobalt layer  17  has a function of improving the magnetic characteristics of the storage layer  16 . 
     The oxide layer  18  is composed of a metal oxide, and contains a rare-earth element (RE). An oxide of a rare-earth element is also simply called a rare-earth oxide (REO). Examples of the rare-earth element contained in the oxide layer  18  include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The rare-earth element contained in the oxide layer  18  has a crystal structure in which the lattice of bonding (e.g. covalent bonding) has a large spacing, as compared to the other elements. Accordingly, when a ferromagnetic layer adjacent to the oxide layer  18  contains impurities and is noncrystalline (amorphous), the oxide layer  18  has a function of diffusing the impurities into itself in a high-temperature environment (e.g., during an annealing process). That is, the oxide layer  18  has a function of removing impurities from an amorphous ferromagnetic layer through an annealing process, and making the ferromagnetic layer in a highly-oriented crystallized state. 
     The cap layer  19  is a non-magnetic conductive layer, and contains, for example, platinum (Pt), tungsten (W), tantalum (Ta), or ruthenium (Ru). 
     The MTJ element  10  is capable of rewriting data using, for example, the spin-injection write technique. In the spin-injection write technique, a write current is allowed to directly flow through the MTJ element  10 , and the magnetization state of the MTJ element  10  is controlled by the write current. The MTJ element  10  may take either a low-resistance state or a high-resistance state, according to whether the relative relationship of magnetization between the storage layer  16  and the reference layer  14  is parallel or antiparallel. That is, the MTJ element  10  is a variable resistor element. 
     When a write current is allowed to flow through the MTJ element  10 , from the storage layer  16  to the reference layer  14 , the relative relationship of magnetization between the storage layer  16  and the reference layer  14  becomes parallel. In this parallel state, the MTJ element  10  has the lowest resistance value, and the MTJ element  10  is set to a low-resistance state. The low-resistance state of the MTJ element  10  is defined as, for example, data “0”. 
     On the other hand, when a write current is allowed to flow through the MTJ element  10 , from the reference layer  14  to the storage layer  16 , the relative relationship of magnetization between the storage layer  16  and the reference layer  14  becomes antiparallel. In this antiparallel state, the MTJ element  10  has the highest resistance value, and the MTJ element  10  is set to a high-resistance state. The high-resistance state of the MTJ element  10  is defined as, for example, data “1”. 
     This allows the MTJ element  10  to be used as a memory device capable of storing one-bit data (two-value data). The allocation of data to the resistance states of the MTJ element  10  may be suitably set. 
     When data is read from the MTJ element  10 , a read voltage is applied to the MTJ element  10 , and the resistance value of the MTJ element  10  is detected using a sense amplifier, etc., based on the read current flowing through the MTJ element  10  during the application of the read voltage. The read current is set to a value sufficiently lower than the threshold value at which the magnetization is reversed by spin injection. 
     [2] Structure of Storage Layer 
     Next, a description will be given of the structure of the storage layer. The storage layer is composed of a ferromagnetic layer. 
     To improve the write error rate (WER), it is desirable to decrease the saturation magnetization. Ms of the ferromagnetic layer. One way to decrease the saturation magnetization Ms is to add a non-magnetic element to the ferromagnetic layer. 
       FIG. 2  is a schematic diagram illustrating magnetic characteristics of a ferromagnetic layer to which a non-magnetic element is added. In the example of  FIG. 2 , a non-magnetic element having a relatively large mass is added to a ferromagnetic layer. Examples of the non-magnetic element having a relatively large mass include molybdenum (Mo), tungsten (W), and tantalum (Ta). The circles enclosing arrows shown in  FIG. 2  represent a plurality of ferromagnetic particles FM forming the ferromagnetic layer. The arrows in the ferromagnetic particles represent spins. The hatched circle shown in  FIG. 2  represents a non-magnetic element NM 1 . 
     The saturation magnetization Ms can be decreased in a ferromagnetic layer to which a non-magnetic element NM 1  having a relatively large mass is added, as shown in  FIG. 2 . However, the spins are disordered in the periphery of the non-magnetic element NM 1 . This causes deterioration of the thermal stability Δ of the ferromagnetic layer. In an MTJ element to be subjected to a high-temperature heat treatment in the manufacturing process, deterioration of the thermal stability Δ of the ferromagnetic layer is not preferable. 
     The disorder of the spins of the ferromagnetic layer causes an increase in the damping coefficient α. Since the write current is proportional to the damping coefficient α, it is desirable that the damping coefficient a be small to reduce the current. Moreover, the disorder in the spins of the ferromagnetic layer causes a decrease in the exchange stiffness constant Aex. The exchange stiffness constant Aex is a measure of the intensity of exchange interaction between particles. The decrease in the exchange stiffness constant Aex of the ferromagnetic layer causes deterioration of the thermal stability L. 
       FIG. 3  is a schematic diagram illustrating magnetic characteristics of a ferromagnetic layer to which another non-magnetic element is added. In the example of  FIG. 3 , a non-magnetic element having a relatively small mass is added to a ferromagnetic layer. Examples of the non-magnetic element having a relatively small mass include boron (B). The hatched circles shown in  FIG. 3  represent a non-magnetic element NM 2 . 
     The saturation magnetization Ms can be decreased in a ferromagnetic layer to which a non-magnetic element NM 2  having a relatively small mass is added, as shown in  FIG. 3 . However, the spins are disordered in the periphery of the non-magnetic element NM 2 , as in  FIG. 2 . This causes an increase in the damping coefficient a and a decrease in the exchange stiffness constant Aex. 
       FIG. 4  is a schematic diagram illustrating magnetic characteristics of a ferromagnetic layer to which a rare-earth element is added. The dashed circles shown in  FIG. 4  represent a rare-earth element RE. 
     As shown in  FIG. 4 , when a rare-earth element RE is added to a ferromagnetic layer, the magnetization direction of the rare-earth element RE becomes antiparallel to the magnetization direction of the ferromagnetic layer. That is, the rare-earth element RE is capable of partially canceling the saturation magnetization Ms of the ferromagnetic layer, thereby reducing the saturation magnetization Ms of the ferromagnetic layer. 
     In addition, since the rare-earth element RE and the ferromagnetic particles FM are magnetically bonded, the spins of the ferromagnetic layer are suppressed from being disordered. This suppresses a decrease in the exchange stiffness constant Aex of the ferromagnetic layer, thereby suppressing deterioration of the thermal stability Δ of the ferromagnetic layer. As the additive amount of the rare-earth element RE increases, the saturation magnetization Ms can be further decreased. 
     The storage layer  16  of the present embodiment has the configuration illustrated in.  FIG. 4 . A case will be described where the storage layer  16  is composed mainly of cobalt iron boron (CoFeB) to which a rare-earth element RE is added. 
     [3] Stacked Structure Including Storage Layer SL, Cobalt Layer Co, and Oxide Layer REO 
     Next, a description will be given of the stacked structure including the storage layer SL, the cobalt layer Co, and the oxide layer REO. 
       FIG. 5  is a table illustrating characteristics of Comparative Examples 1-6 and Examples 1-3.  FIG. 6  is a cross-sectional view illustrating a stacked structure of Comparative Examples 1 and 2.  FIG. 7  is a cross-sectional view illustrating a stacked structure of Comparative Example 3.  FIG. 8  is a cross-sectional view illustrating a stacked structure of Comparative Examples 4-6.  FIG. 9  is a cross-sectional view illustrating a stacked structure of Examples 1-3. In the cross-sectional views of  FIGS. 6-9 , the storage layer SL and its upper and lower layers are focused. 
       FIG. 5  illustrates the composition of the storage layer SL, the presence or absence of a cobalt layer Co, the thickness of the storage layer SL (nm), the anisotropy field Hk (kOe) of the storage layer SL, the saturation magnetization Ms (emu/cm 3 ) of the storage layer SL, the calculated value of the thermal stability Δ, the write error rate WER, and the annealing temperature. In  FIG. 5 , the composition of the storage layer SL is denoted as “SL composition”, the presence or absence of a cobalt layer is denoted as “Co insert”, the thickness of the storage layer SL is denoted as “SL THK”, the anisotropy field of the storage layer SL is denoted as “SL Hk”, the saturation magnetization of the storage layer SL is denoted as “SL Ms”, the calculated value of the thermal stability Δ is denoted as “Δcal.”, and the annealing temperature is denoted as “Anneal temp.” The write error rate WER is relatively expressed using two classifications, “Good” and “Bad”. The annealing temperature, is relatively expressed using three classifications, “high”, “middle”, and “low”. 
     As shown in  FIG. 6 , the MTJ element of Comparative Examples 1 and 2 has a stacked structure in which a tunnel barrier layer TB, a storage layer SL, and an oxide layer RED are stacked in this order. The tunnel barrier layer TB is composed of a magnesium oxide (MgO). The storage layer SL is composed of cobalt iron boron (CoFeB). The oxide layer RED is composed of a rare-earth oxide, such as a gadolinium oxide. As shown in  FIG. 6 , annealing (a thermal treatment) is performed after a. plurality of layers are stacked. In actuality, annealing is performed after all the layers forming the MTJ element  10  are stacked. Annealing is similarly performed in the comparative examples shown in  FIGS. 7-9 . 
     In Comparative Examples 1 and 2 shown in  FIG. 5 , the anisotropy field Hk is low, and the saturation magnetization Ms is high. Also, in Comparative Examples 1 and 2, the WER deteriorates. 
     As shown in  FIG. 7 , the MTJ element of Comparative Example 3 has a stacked structure in which a tunnel barrier layer TB, a storage layer SL, and an oxide layer RED are stacked in this order. The tunnel barrier layer TB is composed of a magnesium oxide (MgO). The storage layer SL is composed of cobalt iron boron (CoFeB) to which molybdenum (Mo) is added as a non-magnetic element. The CoFeB added with. molybdenum (Mo) is denoted “CoFeB—Mo”. The oxide layer RED is composed of a rare-earth oxide, such as a gadolinium oxide. 
     In Comparative Example  3  shown in  FIG. 5 , since the non-magnetic element, molybdenum (Mo), is added to the ferromagnetic layer (CoFeB), the saturation magnetization Ms is decreased. Also, the WER improves. However, the thermal stability Δ deteriorates in Comparative Example 3. 
     As shown in  FIG. 8 , the MTJ element of Comparative Examples 4-6 has a stacked structure in which a tunnel barrier layer TB, a storage layer SL, and an oxide layer REQ are stacked in this order. The tunnel barrier layer TB is composed of a magnesium oxide (MgO). The storage layer SL is composed of cobalt iron boron (CoFeB) to which a rare-earth element RE is added. The CoFeB added with a rare-earth element RE is denoted as “CoFeB-RE”. The rare-earth element RE is, for example, gadolinium (Gd). The CoFeB added with gadolinium (Gd) is denoted as “CoFeB—Gd”. 
     As shown in  FIG. 5 , the annealing temperatures in Comparative Examples 4, 5 and 6 are high, middle, and low, respectively. In Comparative Examples 4-6, the saturation magnetization Ms is further decreased. However, the thermal stability Δ deteriorates as the annealing temperature is higher, namely, in the order of Comparative Examples 6, 5, and 4. From Comparative Examples 4-6, it can be seen that the deterioration of the thermal stability Δ (decrease in Hk) occurs due to the low temperature resistance (low Neel temperature) of CoFeB—Gd. There are cases where annealing is performed at a high temperature in the process of manufacturing MTJ elements. Even during such high-temperature annealing, it is desirable for the magnetic characteristics of the MTJ elements not to deteriorate. 
     As shown in  FIG. 9 , the MTJ element of Examples 1-3 has a stacked structure in which a tunnel barrier layer TB, a storage layer SL, a cobalt layer Co, and an oxide layer REO are stacked in this order. The tunnel barrier layer TB is composed of a magnesium oxide (MgO). The storage layer SL is composed of CoFeB—RE, such as CoFeB—Gd. The storage layer SL, the cobalt layer Co, and the oxide layer REO in Examples 1-3 respectively correspond to the storage layer  16 , the cobalt layer  17 , and the oxide layer  18  shown in  FIG. 1 . 
     As shown in  FIG. 5 , the thickness of the cobalt layer Co is varied in Examples 1-3. Specifically, the thicknesses of the cobalt layer Co in Examples 1, 2 and 3 are 0.1 nm, 0.2 nm, and 0.3 nm, respectively. It is desirable that the thickness of the cobalt layer Co is equal to or greater than 0.1 nm, and equal to or less than 0.3 nm. The thermal stability Δ improves by inserting the cobalt layer Co between the storage layer SL and the oxide layer REO. In addition, the thermal stability Δ improves as the thickness of the cobalt layer Co increases, namely, in the order of Examples 1, 2 and 3. From Examples 1-3, it can be seen that Hk improves as the thickness of the cobalt layer Co increases, resulting in improvement in the thermal stability Δ. 
     [4] Advantageous Effects of First Embodiment 
     According to the first embodiment, a magnetoresistive element (MTJ element)  10  includes: (1) a reference layer  14  having an invariable magnetization direction; (2) a tunnel barrier layer  15  provided on the reference layer  14 ; (3) a storage layer  16  provided on the tunnel barrier layer  15 , having a variable magnetization direction, and containing a rare-earth element; (4) a magnetic layer  17  provided on the storage layer  16  and composed of cobalt; and (5) an oxide layer  18  provided on the magnetic layer  17  and containing a rare-earth element, as described above. 
     Thus, according to the first embodiment, the storage layer  16  is configured of a ferromagnetic layer to which a rare-earth element is added. Such a configuration reduces the saturation magnetization Ms of the storage layer  16 . This in turn results in a decrease in the write error rate WER. 
     Moreover, the MTJ element  10  includes an oxide layer  18  containing a rare-earth element. The oxide layer  18  is capable of removing impurities from an amorphous ferromagnetic layer through an annealing process. This improves the crystalline orientation of the storage layer  16 . 
     Furthermore, a cobalt layer  17  is inserted between the storage layer  16  and the oxide layer  18 . By inserting the cobalt layer  17 , the thermal stability Δ of the storage layer  16  improves. 
     That is, the storage layer  16  of the present embodiment is capable of suppressing deterioration of the thermal stability A, while reducing the saturation magnetization Ms. In addition, by inserting the cobalt layer  17 , the anisotropy field Hk improves, achieving both reduction in the saturation magnetization Ms and improvement in the thermal stability Δ while maintaining the exchange stiffness constant Aex. This results in realization of a magnetoresistive element with an improved performance. 
     Second Embodiment 
     The second embodiment is a configuration example of a magnetic memory device using the MTJ element  10  according to the first embodiment, namely, an MRAM. 
       FIG. 10  is a block diagram of an MRAM  100  according to the second embodiment. The MRAM  100  comprises a memory cell array  31 , a row decoder  32 , a column decoder  33 , column selection circuits  34 A and  34 B, write circuits  35 A and  35 B, a read circuit  36 , etc. 
     The memory cell array  31  includes a plurality of memory cells MC arranged in a matrix pattern. In the memory cell array  31 , a plurality of bit lines BL, a plurality of source lines SL, and a plurality of word lines WL are provided. The bit lines EL and the source lines SL extend in the column direction, and the word lines WL extend in the row direction intersecting the column direction. Each memory cell MC is coupled to one of the bit lines BL, one of the source lines SL, and One of the word lines WL. 
     Each memory cell MC includes one MTJ element  10  and one selective transistor  30 . The selective transistor  30  is composed of, for example, an n-channel MOS transistor. 
     One end of the MTJ element  10  is coupled to the bit line BL; the other end is coupled to the drain of the selective transistor  30 . The source of the selective transistor  30  is coupled to the source line SL, and the gate of the selective transistor  30  is coupled to the word line WL. 
     The row decoder  32  is coupled to the word lines WL. The row decoder  32  decodes an address signal received from the outside, and selects one of the word lines WL based on the decoded result. 
     The column decoder  33  decodes the address signal received from the outside, and generates a column selection signal. The column selection signal is transmitted to the column selection circuits  34 A and  34 B. 
     The column selection circuit  34 A is coupled to one set of ends of the bit lines BL and one set of ends of the source lines SL. The column selection circuit  34 B is coupled to the other set of ends of the bit lines BL and the other set of ends of the source lines SL. The column selection circuits  34 A and  34 B select one of the bit lines EL and one of the source lines SL, based on the column selection signal transmitted from the column decoder  33 . 
     The write circuit  35 A is coupled to the one set of ends of the bit lines BL and the one set of ends of the source lines SL, via the column selection circuit  34 A. The write circuit  35 A is coupled to the other set of ends of the bit lines BL and the other set of ends of the source lines SL, via the column selection circuit  34 A. The write circuits  35 A and  35 B allow a write current to flow through the memory cell MC, via the bit lines EL and the source lines SL, thereby writing data to the memory cell. The write circuits  35 A and  35 B include a source circuit, such as a current source or a voltage source that generates a write current, and a sink circuit that absorbs the write current. 
     The read circuit  36  is coupled to the bit line BL and the source line SL via the column selection circuit  34 B. The read circuit  36  reads data stored in the selected memory cell by detecting a current flowing through the selected memory cell. The read circuit  36  includes, for example, a voltage source or a current source that generates a read current, a sense amplifier that detects and amplifies the read current, and a latch circuit that temporarily stores data. 
     When data is written, the write circuits  35 A and  35 B allow a write current to bi-directionally flow through the MTJ element  10  in the memory cell MC, according to the data written into the memory cell MC. That is, the write circuits  35 A and  35 B supply the memory cell MC with either a write current flowing from the bit lines BL to the source lines SL, or a write current flowing from the source lines SL to the bit lines EL, according to the data written into the MTJ element  10 . The current value of the write current is set to be greater than the magnetization reversal threshold value. 
     When data is read, the read circuit  36  supplies the memory cell MC with a read current. The current value of the read current is set to be smaller than the magnetization reversal threshold value, in such a manner that the magnetization of the storage layer of the MTJ element  10  is not reversed by the read current. 
     The current value or the potential varies according to the magnitude of the resistance value of the MTJ element  10  to which the read current is supplied. The data stored in the MTJ element  10  is determined based on the amount of fluctuation (of a read signal or a read output) determined according to the magnitude of the resistance value. 
     Next, a description will be given of an example of the structure of the MRAM.  FIG. 11  is a cross-sectional view of the MRAM  100  according to the second embodiment. 
     The semiconductor substrate  40  is formed of a p-type semiconductor substrate. The p-type semiconductor substrate  40  may be a p-type semiconductor region (p-type well) provided in a semiconductor substrate. 
     A selective transistor  30  is provided in the semiconductor substrate  40 . The selective transistor  30  is composed of, for example, an n-channel MOS transistor. The selective transistor  30  is composed of a MOS transistor having, for example, a buried-gate structure. The selective transistor  30  is not limited to a buried-gate-type MOS transistor, and may be formed of a planar MOS transistor. 
     The selective transistor  30  includes a gate electrode  41 , a cap layer  42 , a gate insulation film  43 , a source region  44 , and a drain region  45 . The gate electrode  41  functions as a word line WL. 
     The gate electrode  41  extends in the row direction, and is buried in the semiconductor substrate  40 . An upper surface of the gate electrode  41  is below an upper surface of the semiconductor substrate  40 . The cap layer  42 , composed of an insulating material, is provided on the gate electrode  41 . The gate insulation film  43  is provided on the bottom surface and both side surfaces of the gate electrode  41 . The source region  44  and the drain region  45  are provided on both sides of the gate electrode  41  inside the semiconductor substrate  40 . The source region  44  and the drain region  45  are formed of an n+-type diffusion region, formed by introducing high-concentration n-type impurities into the semiconductor substrate  40 . 
     A pillar-shaped lower electrode  46  is provided on the drain region  45 , and an MTJ element  10  is provided on the lower electrode  46 . A pillar-shaped upper electrode  47  is provided on the MTJ element  10 . A bit line BL, extending in the column direction intersecting the row direction, is provided on the upper electrode  47 . 
     A contact plug  48  is provided on the source region  44 . A source line SL, extending in the column direction, is provided on the contact plug  48 . For example, the source line SL is composed of an interconnect layer provided below the bit line BL. An interlayer insulation layer  49  is provided between the semiconductor substrate  40  and the bit line BL. 
     According to the second embodiment, an MRAM can be configured using the MTJ element  10  described in the first embodiment. Also, an MRAM with an improved performance can be realized. 
     In the above-described embodiments, a case has been described where a three-terminal selective transistor is applied as a switching element; however, a two-terminal switching element with a switching function may be applied as a switching element. In addition, the architecture of the memory cell array may be freely selected, such as an array architecture including a plurality of structures stacked in Z direction, each structure being capable of selecting one memory cell MC by a combination of one bit line BL and one word line WL. 
     The embodiments described above are presented merely as examples and are not intended to restrict the scope of the invention/present disclosure. These novel embodiments may be realized in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention/present disclosure. Such embodiments and modifications are included in the scope and gist of the invention/present disclosure, and are included in the scope of the invention/present disclosure described in the claims and its equivalents.