Patent Publication Number: US-2019172513-A1

Title: Magnetoresistive element and electronic device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a magnetoresistive element, and more specifically relates to, for example, a magnetoresistive element that is included in a memory element and an electronic device having such a magnetoresistive element. 
     BACKGROUND ART 
     Various types of memory devices have been used in information processing systems of recent years as cache memories and storages. Development of non-volatile memories such as resistive RAMs (ReRAMs), phase-change RAMs (PCRAMs), and magnetoresistive RAMs (MRAMs) as memory devices of the next generation has been progressing. Among such non-volatile memories, an MRAM that uses a magnetoresistive element having a ferromagnetic tunnel junction (a magnetic tunnel junction (MTJ) element; which may also be referred to simply as a “magnetoresistive element” below) as a memory element has gained attention for the reason of being compact, achieving a high speed, allowing a virtually infinite number of rewrites, and the like, and a spin transfer torque-based magnetic random access memory (STT-MRAM) of a writing type using Spin-Momentum-Transfer (SMT) (a spin injection writing type) has been proposed. 
     A magnetoresistive element that stores information includes, for example, a magnetic material having perpendicular magnetic anisotropy. Such a magnetoresistive element includes a storage layer of which a magnetization direction is variable (which is also called a recording layer, a magnetization reversal layer, a magnetization free layer, a free layer, or a magnetic free layer), a magnetization fixed layer of which the magnetization is fixed (which is also called a pin layer or a magnetic pinned layer), and an intermediate layer including a tunnel insulation layer that is formed between the storage layer and the magnetization fixed layer. When a magnetization direction of the storage layer is parallel to a magnetization direction of the magnetization fixed layer (which is called a “parallel magnetization state”), the magnetoresistive element is in a low resistance state, and when the directions are anti-parallel (which is called an “anti-parallel magnetization state”), the magnetoresistive element is in a high resistance state. The difference in the resistance states is used to store information. Here, a larger amount of magnetization reversal current (which is also called a write current) is necessary at the time at which the state shifts from the parallel magnetization state (P state) to the anti-parallel magnetization state (AP state) than at the time at which the state shifts from the anti-parallel magnetization state (AP state) to the parallel magnetization state (P state). 
     However, structures of such magnetoresistive elements are classified into two types. That is, they are a bottom-pin structure in which a magnetization fixed layer is formed on a lower electrode and a storage layer is formed above the magnetization fixed layer with an intermediate layer interposed therebetween, and a top-pin structure in which a storage layer is formed on a lower electrode and a magnetization fixed layer is formed above the storage layer with an intermediate layer interposed therebetween. In addition, a magnetoresistive element is connected to a selection transistor, and an NMOS-type FET is normally used as a selection transistor. 
     At the time of information writing, a voltage and a current applied to a spin injection-type magnetoresistance effect element are determined depending on the driving ability of a selection transistor. Thus, asymmetry which is a difference in a value of a flowing drive current for a selection transistor between a case in which a current flows from a drain region to a source region and a case in which a current flows from a source region to a drain region exists. In a case in which an NMOS-type FET of which the drain region is connected to a spin injection-type magnetoresistance effect element is used as a selection transistor, when a current flowing from the drain region to the source region is denoted by I 1  and a current flowing from the source region to the drain region is denoted by I 2 , the relation I 1 &gt;I 2  is satisfied. 
     When a magnetization direction of a storage layer is reversed such that the magnetization direction of the storage layer and a magnetization direction of a magnetization fixed layer shift from a parallel magnetization state to an anti-parallel magnetization state (information is rewritten) as described above, a larger amount of magnetization reversal current is necessary. The bottom-pin structure is frequently employed for magnetoresistive elements. However, when such information is rewritten in the bottom-pin structure, a current I 2  flows from a selection transistor to a spin injection-type magnetoresistance effect element, and thus there may be some cases in which there is little tolerance regarding a current value of the NMOS-type FET and rewriting of information is difficult (refer to Non-Patent Literature 1). 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: Hiroki Koike, et al., “Wide operational margin capability of 1 Kbit spin-transfer-torque memory array chip with 1-PMOS and 1-bottom-pin-magnetic-tunnel-junction type cell,” Japanese Journal of Applied Physics 53, 04ED13 (2014) 
         Non-Patent Literature 2: Kay Yakushiji, et al., “High Magnetoresistance Ratio and Low Resistance-Area Product in Magnetic Tunnel Junctions with Perpendicularly Magnetized Electrodes,” Applied Physics Express 3 (2010) 053003 
       
    
     DISCLOSURE OF INVENTION 
     Technical Problem 
     Meanwhile, the problem of deficiency in the tolerance in a rewrite current value is resolved by employing the top-pin structure. However, in order to maintain perpendicular magnetic anisotropy of the storage layer formed on the lower electrode, it is necessary to form a ground layer between the lower electrode and the storage layer. For example, Non-Patent Literature 2 discloses a technology of forming a ground layer including Ru on a lower electrode and forming a magnetic ground layer including Co—Pt having perpendicular magnetic anisotropy between the Ru ground layer and a storage layer including Co—Fe—B. When the magnetic ground layer having perpendicular magnetic anisotropy is disposed adjacent to the storage layer as described above, the magnetic ground layer and the storage layer are magnetically coupled, and thus perpendicular magnetic anisotropy of the storage layer is strengthened and a coercive force of the storage layer is improved. However, when this is compared with a structure with no magnetic ground layer, there is a problem of a write current value increasing. 
     Therefore, the present disclosure aims to provide a magnetoresistive element that has a configuration and a structure that can avoid the problem of an increasing write current value even when a ground layer is formed and an electronic device having such a magnetoresistive element. 
     Solution to Problem 
     A magnetoresistive element according to a first aspect of the present disclosure to achieve the object described above is formed by laminating a lower electrode, a first ground layer including a non-magnetic material, a storage layer (which is also called a recording layer, a magnetization reversal layer, a magnetization free layer, or a free layer) having perpendicular magnetic anisotropy, an intermediate layer, a magnetization fixed layer, and an upper electrode. The storage layer includes a magnetic material including at least a 3d transition metal element and a boron element in a composition. A second ground layer is further included between the lower electrode and the first ground layer. The second ground layer includes a material including at least one kind of element among elements constituting the storage layer in a composition. 
     A magnetoresistive element according to a second aspect of the present disclosure to achieve the object described above is formed by laminating a lower electrode, a first ground layer including a non-magnetic material, a storage layer, an intermediate layer, a magnetization fixed layer, and an upper electrode. The storage layer has perpendicular magnetic anisotropy. A second ground layer is further included between the lower electrode and the first ground layer. The second ground layer has in-plane magnetic anisotropy or non-magnetism. 
     The electronic device of the present disclosure for achieving the above-described objectives has a magnetoresistive element according to the first and second aspects of the present disclosure. 
     Advantageous Effects of Invention 
     In the magnetoresistive element according to the first aspect of the present disclosure, the second ground layer included between the lower electrode and the first ground layer includes a material including at least one element among elements constituting the storage layer in a composition. In addition, in the magnetoresistive element according to the second aspect of the present disclosure, the second ground layer included between the lower electrode and the first ground layer has in-plane magnetic anisotropy or non-magnetism. In addition, by providing the second ground layer as described above, a crystal orientation of the first ground layer is improved, as a result, perpendicular magnetic anisotropy of the storage layer formed on the first ground layer can be improved, and thus, while a coercive force of the storage layer can be increased, the problem of a high write current value can be avoided. Note that the effects described in the present specification are merely illustrative, not limitative, and additional effects may be exhibited. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram of a magnetoresistive element according to Embodiment 1. 
         FIG. 2  is a schematic partial cross-sectional diagram of the magnetoresistive element according to Embodiment 1 including a selection transistor. 
         FIG. 3  is an equivalent circuit diagram of magnetoresistive elements and a memory cell unit according to Embodiment 1 including the selection transistors. 
         FIG. 4  is a conceptual diagram of a magnetoresistive element according to Embodiment 2. 
         FIG. 5A  is a graph showing a relation between a thickness (T 2 ) of a second ground layer and retention power of a storage layer for magnetoresistive elements according to Embodiment 1 and Comparative Example 1A, and  FIG. 5B  is a graph showing a relation between a thickness (T 1 ) of a first ground layer and retention power of the storage layer. 
         FIG. 6A  and  FIG. 6B  are a schematic perspective diagram illustrating a cut part of a composite magnetic head according to Embodiment 3 and a schematic cross-sectional diagram of the composite magnetic head according to Embodiment 3, respectively. 
         FIG. 7A  and  FIG. 7B  are conceptual diagrams of a spin injection-type magnetoresistance effect element to which spin injection magnetization reversal is applied. 
         FIG. 8A  and  FIG. 8B  are conceptual diagrams of a spin injection-type magnetoresistance effect element to which spin injection magnetization reversal is applied. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     The present disclosure will be described on the basis of embodiments with reference to the drawings, but, the present disclosure is not limited to the embodiments, and the various numerical values and materials of the embodiments are merely examples. Note that description will be provided in the following order. 
     1. Overall description of magnetoresistive element according to first and second aspects of present disclosure and electronic device of present disclosure
 
2. Embodiment 1 (magnetoresistive element according to first and second aspects of present disclosure and electronic device of present disclosure)
 
3. Embodiment 2 (modification of Embodiment 1)
 
4. Embodiment 3 (electronic device with magnetoresistive element described in Embodiment 1 or Embodiment 2)
 
     5. Others 
     &lt;Overall Description of Magnetoresistive Element According to First and Second Aspects of Present Disclosure and Electronic Device of Present Disclosure&gt; 
     In the magnetoresistive element according to the first aspect of the present disclosure and the magnetoresistive element according to the first aspect of the present disclosure included in an electronic device of the present disclosure, a second ground layer can have in-plane magnetic anisotropy or non-magnetism. 
     In the magnetoresistive element according to the first aspect of the present disclosure including the above-described preferred form, a magnetoresistive element according to first and second aspects of the present disclosure including the above-described preferred form included in an electronic device of the present disclosure, and a magnetoresistive element according to the second aspect of the present disclosure (which will be collectively referred to as a “magnetoresistive element of the present embodiment and the like” below), a storage layer includes Co—Fe—B, and the boron atom content of the second ground layer can be in the range of 10 atomic % to 50 atomic %. By regulating a lower limit value of the boron atom content of the second ground layer such that it is such a value, a crystal orientation of a first ground layer is further improved by the formation of the second ground layer, and as a result, perpendicular magnetic anisotropy of the storage layer can be more reliably improved. In addition, by regulating an upper limit value of the boron atom content of the second ground layer such that it is such a value, there is no concern of the problem that strength of a target material used to form the second ground layer using a sputtering method decreases. 
     In the magnetoresistive element of the present disclosure and the like including the above-described preferred forms, the second ground layer includes a Co—Fe—B layer, and the first ground layer can include one material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. This configuration will be called a “magnetoresistive element with a first configuration” for the sake of convenience. In addition, in the magnetoresistive element with the first configuration, when a thickness of the second ground layer is denoted by T 2  and a thickness of the storage layer is denoted by T 0 , it is possible to satisfy T 0 ≤T 2 , and furthermore, it is preferable to satisfy T 2 ≤3 nm, for example, 1 nm≤T 2 ≤3 nm. By setting T 0 ≤T 2 , the crystal orientation of the first ground layer is further improved, and as a result, perpendicular magnetic anisotropy of the storage layer can be further strengthened. Meanwhile, by setting T 2 ≤3 nm, the second ground layer exhibits appropriate in-plane magnetic anisotropy, and as a result, perpendicular magnetic anisotropy of the storage layer can be further strengthened, and a coercive force of the storage layer can be further improved. In addition, by regulating the thickness T 2  of the second ground layer as described above, in-plane magnetic anisotropy and non-magnetism of the second ground layer can be reliably achieved. Note that, when a magnetic field is applied to the Co—Fe—B layer in the normal direction, perpendicular magnetic anisotropy is exhibited when the thickness of the Co—Fe—B layer is greater than or equal to 1 nm and less than 1.5 nm, and in-plane magnetic anisotropy is exhibited when the thickness is greater than or equal to 1.5 nm in general. 
     Furthermore, in the magnetoresistive element with the first configuration including the above-described preferred forms, a third ground layer can be formed between a lower electrode and the second ground layer. Here, the third ground layer can include one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide, or the third ground layer can include the same material as the material that forms the first ground layer. By forming the third ground layer, a crystal orientation of the second ground layer can be improved, as a result, a crystal orientation of the first ground layer can be further improved, and perpendicular magnetic anisotropy of the storage layer can be further strengthened. 
     Alternatively, in the magnetoresistive element of the present disclosure including the above-described preferred forms and the like, the second ground layer can be formed by alternatively laminating first material layers and second material layers. This configuration will be referred to as a “magnetoresistive element with a second configuration” for the sake of convenience. In addition, in the magnetoresistive element with the second configuration, the first material layer includes a Co—Fe—B layer, and the second material layer can include a non-magnetic material layer. Furthermore, in the magnetoresistive element with the second configuration of the above-described configurations, the second material layer can include one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. Furthermore, in the magnetoresistive element with the second configuration of the above-described configurations, the material forming the first ground layer and the material forming the second material layer can be the same. Furthermore, in the magnetoresistive element with the second configuration of the above-described configurations, when a thickness of the second ground layer is denoted by T 2 ′, it is preferable to satisfy 3 nm≤T 2 ′, a crystal orientation of the first ground layer is further improved, and as a result, perpendicular magnetic anisotropy of the storage layer can be further strengthened. An upper limit of T 2 ′ and the number of first material layers and second material layers are not particularly limited, a thickness (height) of the laminated structure is defined on the basis of processability and thicknesses of various layers, and thus the value of T 2 ′ and the number of first material layers and second material layers may be determined in accordance with the thickness (height) of the laminated structure. In addition, when a thickness or the number of first material layers and second material layers increases, a processing time such as a film formation time of the first material layers and the second material layers is lengthened, and thus these values should be determined taking the processing time into consideration. For example, 10 nm can be exemplified as the upper limit of T 2 ′. When a thickness of the first material layer is denoted by T 2-A ′ and a thickness of the second material layer is denoted by T 2-B ′, although a relationship therebetween is not limited to the following, it is preferable to satisfy 
       0.2≤ T   2-A   ′/T   2-B ′≤5.
 
     In addition, the thickness T 2-A ′ of the first material layer may be thinner than a thickness T 0  of the storage layer, that is, it is preferable to satisfy 
         T   2-A   ′&lt;T   0 . 
     In the magnetoresistive element of the present disclosure including the above-described various preferred forms and configurations, the magnetoresistive element with the first configuration, the magnetoresistive element with the second configuration, and the like, when a thickness of the first ground layer is denoted by T 1 , it is preferable to satisfy 1 nm≤T 1 ≤4 nm. By satisfying 1 nm≤T 1 , for example, the influence of the in-plane magnetic anisotropy of the second ground layer exerted on perpendicular magnetic anisotropy of the storage layer decreases. Meanwhile, by satisfying T 1 ≤4 nm, a crystal orientation of the first ground layer is further improved, and as a result, perpendicular magnetic anisotropy of the storage layer can be more reliably improved. 
     In the magnetoresistive element of the present disclosure including the above-described various preferred forms and configurations, the magnetoresistive element with the first configuration, the magnetoresistive element with the second configuration, and the like, a magnetization direction of the storage layer changes in accordance with information to be stored, and an axis of easy magnetization of the storage layer is parallel to the laminating direction of the laminated structure including ground layers, the storage layer, an intermediate layer, and a magnetization fixed layer (i.e., of a perpendicular magnetization type). In addition, in this case, the magnetoresistive element can be a magnetoresistive element of a perpendicular magnetization type for writing and erasing information by reversing magnetization of the storage layer using spin torque (spin injection-type magnetoresistance effect element). Here, the ground layers include the first ground layer and the second ground layer, or include the first ground layer, the second ground layer, and the third ground layer. 
     In the magnetoresistive element of the present disclosure including the above-described various preferred forms, the magnetoresistive element with the first configuration, the magnetoresistive element with the second configuration, and the like (which may be referred to simply as an “element of the present disclosure” below), the crystallinity of the storage layer and the magnetization fixed layer is basically arbitrary, and may be poly-crystalline, mono-crystalline, or amorphous. 
     In the element of the present disclosure, although Co—Fe—B is exemplified as a material that forms the storage layer, broadly, the storage layer can include a metallic material (an alloy or a compound) including cobalt, iron, nickel, and boron. Specifically, for example, Fe—B or Co—B can be exemplified in addition to Co—Fe—B. Furthermore, in order to further increase perpendicular magnetic anisotropy, a heavy rare earth element such as terbium (Tb), dysprosium (Dy), holmium (Ho), or the like may be added to the alloy. A non-magnetic element may be added to the material forming the storage layer. In addition, due to the addition of the non-magnetic element, the effects of improved heat resistance attributable to prevention of diffusion, an increase in a magnetoresistance effect, an increase in a withstand voltage resulting from flattening, and the like are obtained. As non-magnetic elements to be added, C, N, O, F, Li, Mg, Si, P, Ti, V, Cr, Mn, Ni, Cu, Ge, Nb, Ru, Rh, Pd, Ag, Ta, Ir, Pt, Au, Zr, Hf, W, Mo, Re, and Os may be exemplified. 
     The storage layer can also have a single layer configuration, a laminated structure in which ferroelectric material layers having different compositions are laminated, or a laminated configuration in which a ferroelectric material layer and a non-magnetic layer are laminated. Alternatively, a ferroelectric material layer and a soft magnetic material layer may be laminated, or a plurality of ferroelectric material layers can be laminated having a soft magnetic material layer or a non-magnetic material layer interposed therebetween. In a case in which ferroelectric material layers are laminated having a non-magnetic material layer interposed therebetween, a relationship in magnetic strength between the ferroelectric material layers can be adjusted, and thus a magnetization reversal current of the spin injection-type magnetoresistance effect element can be prevented from increasing. Here, as ferromagnetic materials other than the above-described materials for forming the storage layer, a ferromagnetic material such as nickel (Ni), iron (Fe), or cobalt (Co), an alloy of these ferromagnetic materials (e.g., Co—Fe, Co—Fe—Ni, Fe—Pt, Ni—Fe, etc.), or an alloy obtained by adding gadolinium (Gd) to the aforementioned alloys, an alloy obtained by incorporating a non-magnetic element (e.g., tantalum, chromium, platinum, silicon, carbon, nitrogen, etc.) in these alloys, an oxide including one or more kinds of Co, Fe, and Ni (e.g., ferrite: Fe—MnO, etc.), a group of intermetallic compounds called half metallic ferromagnetic materials (Heusler alloys: NiMnSb, Co 2 MnGe, Co 2 MnSi, Co 2 CrAl, etc.), and oxides (e.g., (La, Sr)MnO 3 , CrO 2 , Fe 3 O 4 , etc.) can be exemplified. In addition, as a material of the non-magnetic material layer, Ru, Os, Re, Ir, Au, Ag, Cu, Al, Bi, Si, B, C, Cr, Ta, Pd, Pt, Zr, Hf, W, Mo, Nb, V, or an alloy thereof can be exemplified. 
     Furthermore, in the element of the present disclosure including the above-described various preferred forms, it is preferable for the intermediate layer to include a non-magnetic material. That is, the element of the present disclosure is a spin injection-type magnetoresistance effect element and exhibits a tunnel magnetoresistance (TMR) effect. That is, the element of the present disclosure has a structure in which the intermediate layer including a non-magnetic material that functions as a tunnel insulation layer is interposed between a magnetization fixed layer including a magnetic material and the storage layer including a magnetic material layer. The intermediate layer cuts the magnetic coupling between the storage layer and the magnetization fixed layer, is responsible for causing a tunnel current to flow, and is also called a tunnel insulation layer. 
     Here, as non-magnetic materials for forming the intermediate layer, magnesium oxide (MgO), magnesium nitride, magnesium fluoride, aluminum oxide (AlO X ), aluminum nitride (AlN), silicon oxide (SiO X ), silicon nitride (SiN), various insulating materials, dielectric materials, and semiconductor materials such as TiO 2 , Cr 2 O 3 , Ge, NiO, CdO X , HfO 2 , Ta 2 O 5 , Bi 2 O 3 , CaF, SrTiO 3 , AlLaO 3 , Mg—Al 2 —O, Al—N—O, BN, and ZnS can be exemplified. It is preferable for an area resistance value of the intermediate layer to be about several tens of Ω·μm 2  or lower. In a case in which the intermediate layer includes magnesium oxide (MgO), it is desirable for the MgO layer to be crystallized, and more desirable to have a crystal orientation in the (001) direction. In addition, in the case in which the intermediate layer includes magnesium oxide (MgO), it is desirable for a thickness thereof to be 1.5 nm or smaller. 
     The intermediate layer can be obtained by oxidizing or nitrifying, for example, a metal layer formed using a sputtering method. More specifically, in a case in which aluminum oxide (AlO X ) or magnesium oxide (MgO) is used as an insulating material to form the intermediate layer, for example, a method of oxidizing aluminum or magnesium formed using a sputtering method in the atmosphere, a method of plasma-oxidizing aluminum or magnesium formed using a sputtering method, a method of IPC-plasma-oxidizing aluminum or magnesium formed using a sputtering method, a method of naturally oxidizing aluminum or magnesium formed using a sputtering method in oxygen, a method of oxidizing aluminum or magnesium formed using a sputtering method with oxygen radicals, a method of radiating ultraviolet rays to aluminum or magnesium formed using a sputtering method when it is naturally oxidized in oxygen, a method of forming a film of aluminum or magnesium using a reactive sputtering method, or a method of forming a film of aluminum oxide (AlO X ) or magnesium oxide (MgO) using a sputtering method can be exemplified. 
     Since a magnetization direction of the magnetization fixed layer is a reference for information, the magnetization direction should not be changed by recording or reading of information, but it is not necessary for the direction to be fixed to a specific direction, and a configuration or a structure in which it is more difficult to change this magnetization direction than that of the storage layer may be provided by setting a greater coercive force than that of the storage layer, thickening the thickness, or increasing a magnetic damping constant. 
     In the element of the present disclosure including the above-described various preferred forms, the magnetization fixed layer can have a laminated ferromagnetic structure (which is also called a laminated ferri-pin structure) in which at least two magnetic material layers are laminated. The laminated ferromagnetic structure is a laminated structure with anti-ferromagnetic coupling, that is, a structure in which interlayer exchange coupling of two magnetic material layers (a reference layer and a fixed layer) is anti-ferromagnetic, which is also called synthetic anti-ferromagnetic coupling (Synthetic Antiferromagnet or SAF) indicating a structure in which interlayer exchange coupling of the two magnetic material layers (the reference layer and the fixed layer) is anti-ferromagnetic or ferromagnetic depending on a thickness of a non-magnetic layer provided between the two magnetic material layers, which is reported in, for example, Physical Review Letters, S. S. Parkin et. al, 7 May, pp. 2304 to 2307 (1990). A magnetization direction of the reference layer is a magnetization direction serving as a reference for information to be stored in the storage layer. One magnetic material layer (the reference layer) included in the laminated ferromagnetic structure is located on the storage layer side. By employing a laminated ferromagnetic structure for the magnetization fixed layer, it is possible to reliably cancel out asymmetry in thermal stability in an information writing direction and to improve stability in spin torque. In the laminated ferromagnetic structure, for example, a Co—Fe—B alloy can be exemplified as a material forming the reference layer, and a Co—Pt alloy can be exemplified as the fixed layer. Alternatively, the magnetization fixed layer can include a Co—Fe—B alloy layer, and a value in the range from 0.5 nm to 30 nm can be exemplified as a thickness of the magnetization fixed layer. 
     The above-described various layers can be formed using, for example, a sputtering method, an ion beam deposition method, a physical vapor deposition method (PVD method) exemplified as a vacuum evaporation method, or a chemical vapor deposition method (CVD method) typified by an atomic layer deposition (ALD) method. In addition, patterning of these layers can be performed using a reactive ion etching method (RIE method), or an ion milling method (ion beam etching method). It is preferable to continuously form various layers in a vacuum device and then to perform patterning thereon. 
     In the element of the present disclosure, if a magnetization reversal current flows from the storage layer to the magnetization fixed layer in an anti-parallel magnetization state, the magnetization of the storage layer is reversed due to spin torque acting due to electrons injected from the magnetization fixed layer to the storage layer, and thus the magnetization direction of the storage layer, the magnetization direction of the magnetization fixed layer (specifically, the reference layer), and the magnetization direction of the storage layer are arranged in parallel. On the other hand, if a magnetization reversal current flows from the magnetization fixed layer to the storage layer in a parallel magnetization state, the magnetization of the storage layer is reversed due to spin torque acting due to electrons flowing from the storage layer to the magnetization fixed layer, and thus the magnetization direction of the storage layer and the magnetization direction of the magnetization fixed layer (specifically, the reference layer) become in an anti-parallel magnetization state. 
     Although it is desirable for the three-dimensional shape of the storage layer to be a tubular shape (cylindrical shape) from the viewpoint of securing easy processability and uniformity in directions of the axis of easy magnetization of the storage layer, the disclosure is not limited thereto, and the shape can be a triangular cylinder, a square cylinder, a hexagonal cylinder, an octagonal cylinder, or the like (including one having rounded sides or side ridges), or elliptic cylinder. It is preferable for an area of the storage layer to be, for example, 0.01 μm 2  or smaller from the viewpoint of easy reversal of the direction of magnetization by a low magnetization reversal current. When a magnetization reversal current flows in the laminated structure from a lower electrode to an upper electrode or from the upper electrode to the lower electrode, the magnetization direction of the storage layer is in the parallel direction with or the opposite direction to the axis of easy magnetization, and thereby information is written in the storage layer. 
     The lower electrode can be connected to first wiring and the upper electrode can be connected to second wiring. The first wiring and the second wiring may have a single layer structure including Cu, Al, Au, Pt, Ti, or the like, or may have a laminated structure having an ground layer including Cr, Ti, or the like and a Cu layer, an Au layer, a Pt layer, or the like formed thereon. Furthermore, the wiring can have a single layer structure including Ta or the like or a laminated structure including Cu, Ti, and the like. The wiring, the lower electrode (a first electrode), and the upper electrode (with 2) can be formed using, for example, a PVD method exemplified as a sputtering method. 
     The storage layer has the selection transistor configured by an NMOS-type FET below the laminated structure, a projection image in the direction in which the second wiring (e.g., a bit line) extends can be orthogonal to a projection image in the direction in which a gate electrode (e.g., which also functions as a word line or an address line) included in the NMOS-type FET extends, and the direction in which the second wiring extends can also be parallel with the direction in which the gate electrode included in the NMOS-type FET extends. The selection transistor is connected to the lower electrode via the first wiring. 
     Although the preferred forms of the element of the present disclosure are as described above and the element has the selection transistor configured by an NMOS-type FET below the laminated structure, a more specific configuration thereof is, for example, not limited, and a configuration of the element including the selection transistor formed on a semiconductor substrate and an interlayer insulating layer covering the selection transistor, in which the first wiring connected to the lower electrode is formed on the interlayer insulating layer, an insulation material layer covering the laminated structure, the interlayer insulating layer, and the first wiring is formed, the second wiring connected to the upper electrode is formed on the insulation material layer, and the first wiring is electrically connected to one source/drain region of the selection transistor via a connection hole (or a connection hole and a landing pad part or lower layer wiring) provided in the interlayer insulating layer can be exemplified. The other source/drain region of the selection transistor is connected to a sense line. 
     The connection hole electrically connecting the first wiring and the selection transistor can include impurity-doped polysilicon, tungsten, a high melting-point metal such as Ti, Pt, Pd, Cu, TiW, TiNW, WSi 2 , or MoSi 2 , or a metal silicide, and can be formed using a CVD method or a PVD method that is exemplified as a sputtering method. The wiring can also include these materials. In addition, as materials to form the interlayer insulating layer and the insulation material layer, silicon oxide (SiO 2 ), silicon nitride (SiN), SiON, SOG, NSG, BPSG, PSG, BSG, LTO, and Al 2 O 3  can be exemplified. 
     As an electronic device (electronic apparatus) of the present disclosure, a portable electronic device such as a mobile apparatus, a game apparatus, a music apparatus, or a video apparatus or a fixed-type electronic device can be exemplified, and a magnetic head can be exemplified. In addition, a memory device (a memory cell unit) including a non-volatile memory element array in which magnetoresistive elements of the present disclosure (specifically memory elements, and more specifically non-volatile memory cells) are arrayed in a two-dimensional matrix shape can be exemplified. That is, a memory cell unit is formed such that a plurality of non-volatile memory cells is arrayed in a two-dimensional matrix shape in a first direction and a second direction that is different from the first direction, and the non-volatile memory cells include magnetoresistive elements of the present disclosure including the various preferred forms, magnetoresistive elements with the first configuration, and magnetoresistive elements with the second configuration. 
     Embodiment 1 
     Embodiment 1 relates to a magnetoresistive element of the present disclosure, specifically a magnetoresistive element with the first configuration, and more specifically a magnetoresistive element that is included in, for example, a memory element (a non-volatile memory cell), and to an electronic device of the present disclosure. A conceptual diagram of a magnetoresistive element  10  of Embodiment 1 is illustrated in  FIG. 1 . In the diagram, magnetization directions are denoted by outlined arrows. In addition, a schematic partial cross-sectional diagram of the magnetoresistive element of Embodiment 1 including a selection transistor is illustrated in  FIG. 2 , and an equivalent circuit diagram of magnetoresistive elements and a memory cell unit according to Embodiment 1 including selection transistors is illustrated in  FIG. 3 . 
     The magnetoresistive element  10  of Embodiment 1 has a top-pin structure in which a lower electrode (first electrode)  31 , a first ground layer  21 A including a non-magnetic material, a storage layer having perpendicular magnetic anisotropy (which is also called a recording layer, a magnetization reversal layer, or a free layer)  22 , an intermediate layer  23 , a magnetization fixed layer  24 , and an upper electrode (second electrode)  32  are laminated, and the storage layer  22  includes a magnetic material including at least a 3d transition metal element and a boron (B) element in a composition. In addition, a second ground layer  21 B is further included between the lower electrode  31  and the first ground layer  21 A, and the second ground layer  21 B includes a material including at least one kind of element among elements constituting the storage layer  22  in a composition. Here, the second ground layer  21 B has in-plane magnetic anisotropy or non-magnetism. 
     Alternatively, the magnetoresistive element  10  of Embodiment 1 is formed by laminating a lower electrode  31 , a first ground layer  21 A including a non-magnetic material, a storage layer  22 , an intermediate layer  23 , a magnetization fixed layer  24 , and an upper electrode  32 , the storage layer  22  has perpendicular magnetic anisotropy, a second ground layer  21 B is further included between the lower electrode  31  and the first ground layer  21 A, and the second ground layer  21 B has in-plane magnetic anisotropy or non-magnetism. 
     The electronic device of Embodiment 1 includes a magnetoresistive element  10  or  10 A of Embodiment 1 or Embodiment 2, which will be described below. Specifically, the electronic device of Embodiment 1 is a memory device (memory cell unit) including a non-volatile memory element array in which the magnetoresistive elements  10  or  10 A of Embodiment 1 or Embodiment 2, which will be described below, are arrayed in a two-dimensional matrix shape. That is, the memory cell unit includes a plurality of non-volatile memory cells arrayed in a first direction and a second direction which is different from the first direction in a two-dimensional matrix shape, and the non-volatile memory cells are constituted by the magnetoresistive elements  10  or  10 A of Embodiment 1 or Embodiment 2, which will be described below. 
     The magnetoresistive element  10  of Embodiment 1 is a magnetoresistive element  10  of a perpendicular magnetization type (spin injection-type magnetoresistance effect element) that performs writing and erasing of information when magnetization of the storage layer  22  is reversed due to spin torque. A magnetization direction of the storage layer  22  changes corresponding to information to be stored, and an axis of easy magnetization of the storage layer  22  is parallel with the laminating direction of a laminated structure  20  constituted by the first ground layer  21 A, the storage layer  22 , the intermediate layer  23 , and the magnetization fixed layer  24 . That is, the magnetoresistive element is a perpendicular magnetization type. A magnetization direction of a reference layer  24 A is a reference magnetization direction of information to be stored in the storage layer  22 , and relative angles formed by a magnetization direction of the storage layer  22  and a magnetization direction of the reference layer  24 A define information “0” and information “1.” 
     In the magnetoresistive element  10  or  10 A of Embodiment 1 or Embodiment 2, which will be described below, the storage layer  22  includes specifically a ferromagnetic material having a magnetic moment in which a magnetization direction freely changes in the laminating direction of the laminated structure  20 , more specifically a Co—Fe—B alloy [(Co 20 Fe 80 ) 80 B 20 ]. Although a three-dimensional shape of the storage layer  22  is set to a tubular shape (cylindrical shape) having a diameter of 60 nm, a shape thereof is not limited thereto. In addition, the boron atom content of the second ground layer  21 B is in the range of 10 atomic % to 50 atomic %. 
     However, although the second ground layer  21 B includes a material including at least one kind of element among elements constituting the storage layer  22  in a composition, the second ground layer  21 B includes, more specifically, one Co—Fe—B layer [specifically, (Co 20 Fe 80 ) 80 B 20 ] in the magnetoresistive element  10  of Embodiment 1. That is, in Embodiment 1, the second ground layer  21 B includes the same material as the storage layer  22 . In addition, the first ground layer  21 A includes one kind of material selected from a group consisting of high melting-point non-magnetic metals such as tantalum, molybdenum, tungsten, titanium, and magnesium, and magnesium oxide [more specifically, tantalum (Ta) in Embodiment 1]. Here, when a thickness of the second ground layer  21 B is denoted by T 2  and a thickness of the storage layer  22  is denoted by T 0 , T 0 ≤T 2  is satisfied, and T 2 ≤3 nm, more specifically, 1 nm≤T 2 ≤3 nm is satisfied. In addition, when a thickness of the first ground layer  21 A is denoted by T 1 , 1 nm≤T 1 ≤4 nm is satisfied. Specific values of T 0 , T 1 , and T 2  are exemplified in Table 1. 
     Furthermore, in the magnetoresistive element  10  of Embodiment 1, a third ground layer  21 C is formed between the lower electrode  31  and the second ground layer  21 B. Here, the third ground layer  21 C includes one kind of material selected from a group consisting of high melting-point non-magnetic metals such as tantalum, molybdenum, tungsten, titanium, and magnesium, and magnesium oxide, specifically, tantalum (Ta) in Embodiment 1. That is, the third ground layer  21 C includes the same material as the material that forms the first ground layer  21 A. Note that the first ground layer  21 A, the second ground layer  21 B, and the third ground layer  21 C are collectively denoted by a ground layer  21  in  FIG. 2 . 
     The magnetization fixed layer  24  has a laminated ferromagnetic structure in which at least two magnetic material layers are laminated. A non-magnetic layer  24 B is formed between one magnetic material layer (reference layer)  24 A constituting the laminated ferromagnetic structure and the other magnetic material layer (fixed layer)  24 C constituting the laminated ferromagnetic structure. An axis of easy magnetization of the reference layer  24 A is parallel with the laminating direction of the laminated structure  20 . That is, the reference layer  24 A includes a ferromagnetic material having a magnetic moment in which a magnetization direction changes in a direction parallel with the laminating direction of the laminated structure  20 , and more specifically, including a Co—Fe—B alloy [(Co 20 Fe 80 ) 80 B 20 ]. Furthermore, the fixed layer  24 C includes a Co—Pt alloy layer and has a laminated ferromagnetic structure in which the fixed layer is magnetically coupled with the reference layer  24 A via the non-magnetic layer  24 B that includes ruthenium (Ru). 
     The intermediate layer  23  that includes a non-magnetic material includes an insulating layer that functions as a tunnel barrier layer (tunnel insulating layer), specifically, a magnesium oxide (MgO) layer. By forming the intermediate layer  23  as an MgO layer, a magnetoresistance change ratio (MR ratio) can be increased, the effect of spin injection can be improved accordingly, and a density of a magnetization reversal current necessary for reversing a magnetization direction of the storage layer  22  can be reduced. 
     The lower electrode  31  is connected to first wiring  41 , and the upper electrode  32  is connected to second wiring  42 . In addition, information is stored in the storage layer  22  by causing a current (magnetization reversal current) to flow between the first wiring  41  and the second wiring  42 . That is, a magnetization direction of the storage layer  22  is changed when a magnetization reversal current flows in the laminating direction of the laminated structure  20 , and thereby information is recorded in the storage layer  22 . 
     The above-described layer configurations of the laminated structure  20  are exemplified together in the following table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   Upper electrode 32: Ru layer (upper layer) having a thickness of 
               
               
                 3 nm/Ta layer (lower layer) having a thickness of 5 nm 
               
               
                   Magnetization fixed layer 24 
               
               
                     Fixed layer 24C: Co—Pt alloy layer having a film thickness 
               
               
                     of 2.5 nm 
               
               
                     Non-magnetic layer 24B: Ru layer having a film thickness 
               
               
                     of 0.8 nm 
               
               
                     Reference layer 24A: (Co 20 Fe 80 ) 80 B 20  layer having 
               
               
                     a film thickness of 1.0 nm 
               
               
                     Intermediate layer 23: MgO layer having a film thickness 
               
               
                     of 1.0 nm 
               
               
                     Storage layer 22: (Co 20 Fe 80 ) 80 B 20  layer having a film 
               
               
                     thickness (T 0 ) of 1.25 nm 
               
               
                   Ground layers 
               
               
                     First ground layer 21A: Ta layer having a film thickness (T 1 ) 
               
               
                     of 1.0 nm 
               
               
                     Second ground layer 21B: (Co 20 Fe 80 ) 80 B 20  layer having 
               
               
                 a film thickness (T 2 ) of 2.0 nm 
               
               
                     Third ground layer 21C: Ta layer (a thickness of 5 nm) 
               
               
                   Lower electrode 31: TaN layer (a thickness of 5 nm) 
               
               
                   
               
            
           
         
       
     
     A selection transistor TR configured by an NMOS-type FET is provided below the laminated structure  20 . Specifically, the selection transistor TR formed on a semiconductor substrate  60  and an interlayer insulating layer  67  ( 67 A and  67 B) that covers the selection transistor TR are provided, the first wiring  41  (that also serves as the lower electrode  31 ) is formed on the interlayer insulating layer  67 , the laminated structure  20  is formed on the first wiring  41 , the insulation material layer  51  is formed on the interlayer insulating layer  67 , surrounding the laminated structure  20 , and the second wiring  42  connected to the upper electrode  32  is formed on the insulation material layer  51 . 
     In addition, the first wiring  41  (the lower electrode  31 ) is electrically connected to one source/drain region (drain region)  64 A of the selection transistor TR via a connection hole (or a connection hole and a landing pad part or lower layer wiring)  66  provided in the interlayer insulating layer  67 . 
     The selection transistor TR includes a gate electrode  61 , a gate insulating layer  62 , a channel formation region  63 , and source/drain regions  64 A and  64 B. The one source/drain region (drain region)  64 A and the first wiring  41  are connected via the connection hole  66  as described above. The other source/drain region (source region)  64 B is connected to a sense line  43  via a connection hole  66 . The gate electrode  61  functions as a so-called word line WL or an address line. In addition, a projection image in the direction in which the second wiring  42  (a bit line BL) extends is orthogonal to a projection image in the direction in which the gate electrode  61  extends or is parallel with a projection image in the direction in which the second wiring  42  extends. 
     It is assumed that information “0” stored in the storage layer  22  is to be rewritten to “1” as illustrated in the conceptual diagrams of  FIG. 7A  and  FIG. 8A . That is, a write current (magnetization reversal current) I 1  flows from the magnetization fixed layer  24  to the selection transistor TR via the storage layer  22  in a parallel magnetization state. In other words, electrons flow from the storage layer  22  to the magnetization fixed layer  24 . Specifically, for example, V dd  is applied to the second wiring  42 , and the source region  64 B of the selection transistor TR is grounded. Electrons having a spin in one direction that have reached the magnetization fixed layer  24  pass through the magnetization fixed layer  24 . On the other hand, electrons having a spin in another direction are reflected by the magnetization fixed layer  24 . In addition, when the electrons enter the storage layer  22 , torque is imposed on the storage layer  22 , and thus the state of the storage layer  22  is reversed to an anti-parallel magnetization state. Here, it may be thought that the magnetization direction of the magnetization fixed layer  24  is fixed and thus is not reversed and the state of the storage layer  22  is reversed to keep angular momentum of the whole system. 
     It is assumed that information “1” stored in the storage layer  22  is to be rewritten to “0” as illustrated in the conceptual diagrams of  FIG. 7B  and  FIG. 8B . That is, a write current I 2  flows from the selection transistor TR to the magnetization fixed layer  24  via the storage layer  22  in an anti-parallel magnetization state. In other words, electrons flow from the magnetization fixed layer  24  to the storage layer  22 . Specifically, for example, V dd  is applied to the source region  64 B of the selection transistor TR, and the second wiring  42  is grounded. The electrons that have passed through the magnetization fixed layer  24  are subject to spin polarization, that is, a difference is made between the numbers of upward and downward electrons. When a thickness of the intermediate layer  23  is sufficiently thin and electrons reach the storage layer  22  before the spin polarization relaxes and thus the layer returns to a non-polarization state (a state in which upward and downward electrons are the same in number) of a normal non-magnetic body, the sign at the time of spin polarization is reversed, and thus some electrons are reversed, that is, change the direction of spin angular momentum to lower the energy of the whole system. At this time, since the entire angular momentum of the system should be kept, a reaction whose amount is equivalent to the sum of a change in the angular momentum by the direction-changed electrons is given to the magnetic moment of the storage layer  22 . In a case in which a current, that is, the number of electrons passing through the magnetization fixed layer  24  in unit time is small, the total number of direction-changed electrons is small, thus the quantity of change of the angular momentum that occurs in the magnetic moment of the storage layer  22  is small accordingly, but if the current increases, a greater change of the angular momentum can be given to the storage layer  22  within unit time. The temporal change of the angular momentum is torque, and when the torque exceeds a certain threshold value, the magnetic moment of the storage layer  22  starts reversing and rotates 180 degrees due to uniaxial anisotropy of the layer, and at last the layer is stabilized. That is, the reversal from the anti-parallel magnetization state to the parallel magnetization state occurs, and thereby information “0” is recorded in the storage layer  22 . 
     When information written in the storage layer  22  is to be read, the selection transistor TR of the magnetoresistive element  10  from which information is to be read is in a conductive state. In addition, a current flows between the second wiring  42  (bit line BL) and the sense line  43 , and a potential appearing in the bit line BL is input to one input unit of a comparator circuit (not illustrated) constituting a comparison circuit (not illustrated). Meanwhile, a potential from a circuit (not illustrated) for obtaining a reference resistance value is input to the other input unit of the comparator circuit constituting the comparison circuit. Then, the comparison circuit compares whether the potential appearing in the bit line BL is high or low with reference to the potential from the circuit for obtaining the reference resistance value, and the comparison result (information 0 or 1) is output from the output unit of the comparator circuit constituting the comparison circuit. 
     An overview of a manufacturing method of the magnetoresistive element of Embodiment 1 will be described below. 
     [Step- 100 ] 
     First, an element isolation region  60 A is formed on the semiconductor substrate  60  including a silicon semiconductor substrate using a known method, and the selection transistor TR including the gate insulating layer  62 , the gate electrode  61 , the source/drain regions  64 A and  64 B is formed in a part of the semiconductor substrate  60  surrounded by the element isolation region  60 A. The part of the semiconductor substrate  60  located between the source/drain region  64 A and the source/drain region  64 B corresponds to the channel formation region  63 . Next, a lower layer  67 A of the interlayer insulating layer  67  is formed, a connection hole (tungsten plug)  65  is formed in a part of the lower layer  67 A on the one source/drain region (source region)  64 B, and further the sense line  43  is formed on the lower layer  67 A. Then, an upper layer  67 B of the interlayer insulating layer  67  is formed on the entire surface of the lower layer. In addition, the connection hole (tungsten plug)  66  is formed in parts of the upper layer  67 B and the lower layer  67 A on the other source/drain region (drain region)  64 A. In this way, the selection transistor TR covered by the interlayer insulating layer  67  can be obtained. In addition, after a conductive material layer for forming the first wiring  41  which also serves as the lower electrode  31  is formed on the interlayer insulating layer  67 , the conductive material layer is patterned, and thereby the first wiring  41  that also serves as the lower electrode  31  can be obtained. The first wiring  41  is in contact with the connection hole  66 . 
     [Step- 110 ] 
     Then, the third ground layer  21 C, the second ground layer  21 B, the first ground layer  21 A, the storage layer  22 , the intermediate layer  23 , the reference layer  24 A, the non-magnetic layer  24 B, the fixed layer  24 C, and the upper electrode  32  are sequentially formed on the entire surface of the lower electrode, the formed films are patterned, and thereby the laminated structure  20  can be obtained. Note that, the intermediate layer  23  including magnesium oxide (MgO) is formed by performing film formation of an MgO layer using an RF magnetron sputtering method. In addition, the other layers are formed using a DC magnetron sputtering method. 
     [Step- 120 ] 
     Next, the insulation material layer  51  is formed on the entire surface of the lower electrode. Then, a flattening process is performed on the insulation material layer  51  to level the top surface of the insulation material layer  51  with that of the upper electrode  32 . Thereafter, the second wiring  42  that is in contact with the upper electrode  32  is formed on the insulation material layer  51 . In this way, the magnetoresistive element  10  with the structure illustrated in  FIG. 2  (specifically, a spin injection-type magnetoresistance effect element) can be obtained. Note that patterning of each layer can be performed using an RIE method or an ion milling method (ion beam etching method). 
     As described above, the general MOS manufacturing process can be applied to manufacturing of the magnetoresistive element of Embodiment 1, and it can be applied as a universal memory. 
     How retention power of the storage layer  22  (unit: Oe) changes when the thickness of the second ground layer  21 B (T 2 ) is changed in the configuration shown in Table 1 was examined. The result is shown in  FIG. 5A . Note that a magnetic field from outside was applied to the magnetoresistive element after it was manufactured, an electrical resistance value of the manufactured magnetoresistive element was measured, and thereby coercive force of the storage layer  22  was calculated from a value of the magnetic field when the electrical resistance value was radically changed. The same applied to the following description as well. 
     In addition,  FIG. 5A  illustrates data of the magnetoresistive element with T 2 =0 (i.e., the magnetoresistive element in which the second ground layer  21 B is not formed) as Comparative Example 1A. In Comparative Example 1A, the ground layer includes one tantalum layer. 
     It is ascertained from  FIG. 5A  that, by setting the thickness of the second ground layer  21 B (T 2 ) to be 1 nm≤T 2 ≤3 nm, the coercive force of the storage layer  22  further increases and perpendicular magnetic anisotropy is further strengthened than the magnetoresistive element of Comparative Example 1A. 
     In addition, how retention power of the storage layer  22  (unit: Oe) changes when the thickness of the first ground layer  21 A (T 1 ) is changed in the configuration shown in Table 1 was examined. The result is shown in  FIG. 5B , and it is ascertained that satisfying 1 nm≤T 1 ≤4 nm is preferable. 
     A prototype of the magnetoresistive element for Comparative Example 1B was made in which a second ground layer formed by laminating a Pt layer, a Co layer, a Pt layer, and a Co layer and a first ground layer (having a film thickness of 0.4 nm) including Ta are formed on a third ground layer including Ta and a storage layer, an intermediate layer, and a magnetization fixed layer, which are similar to those of Embodiment 1, are formed on the first ground layer. 
     Write current values (unit: micro ampere), thermal stability, and thermal disturbance constants, which are an index of data retention (unit: dimensionless), of the magnetoresistive elements of Embodiment 1, Embodiment 2, which will be described below, Comparative Example 1A, and Comparative Example 1B were measured. The results are shown in Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Write 
                 Thermal 
               
               
                   
                 current value 
                 disturbance constant 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Embodiment 1 
                 70 
                 86 
               
               
                 Embodiment 2 
                 65 
                 80 
               
               
                 Comparative Example 1A 
                 20 
                 51 
               
               
                 Comparative Example 1B 
                 275 
                 94 
               
               
                   
               
            
           
         
       
     
     The coercive force of the magnetoresistive element of Comparative Example 1B was about 4370 (Oe), which is higher than that of the magnetoresistive element of Embodiment 1. That is, since the second ground layer formed by laminating a Pt layer, a Co layer, a Pt layer, and a Co layer is provided and the thin first ground layer having a thickness of 0.4 nm is provided in Comparative Example 1B, it is thought that the second ground layer is magnetically coupled with the storage layer via the thin first ground layer and the storage layer  22  exhibits greater perpendicular magnetic anisotropy than that of Embodiment 1. However, as shown in Table 2, the magnetoresistive element of Comparative Example 1B exhibited much higher write current value than that of Embodiment 1. 
     In addition, although the magnetoresistive elements of Embodiment 1 and Comparative Example 1B exhibited thermal disturbance constants of a similar degree as shown in Table 2, the magnetoresistive element of Comparative Example 1A exhibited a much lower thermal disturbance constant. That is, it is ascertained that thermal stability of a magnetoresistive element becomes low when the second ground layer is not provided. 
     As described above, in the magnetoresistive element of Embodiment 1, the second ground layer provided between the lower electrode and the first ground layer includes a material including at least one kind of element among elements constituting the storage layer in a composition, or has in-plane magnetic anisotropy or non-magnetism. In addition, by providing the second ground layer formed as described above, a crystal orientation of the first ground layer is improved, as a result, perpendicular magnetic anisotropy of the storage layer formed on the first ground layer can be improved, and thus coercive force of the storage layer can be increased. Moreover, the problem of a high write current value can be avoided. Furthermore, the magnetoresistive element of Embodiment 1 has high thermal stability. 
     In addition, the ground layers have simple structures and can be easily manufactured, and the storage layer can exhibit high perpendicular magnetic anisotropy and coercive force even if the storage layer is set to have a single layer configuration. Furthermore, the first ground layer can reliably prevent at least one kind of element (specifically, boron) of the elements constituting the storage layer from diffusing into the material forming the second ground layer. 
     Embodiment 2 
     Embodiment 2 is a modification of Embodiment 1 and relates to a magnetoresistive element with the second configuration. A conceptual diagram of the magnetoresistive element  10 A of Embodiment 2 is illustrated in  FIG. 4 . In Embodiment 2, a second ground layer  21 B is formed by alternately laminating first material layers  21 B 1  and second material layer  21 B 2 . The first material layer  21 B 1  includes a Co—Fe—B layer [specifically, a (Co 20 Fe 80 ) 80 B 20  layer]. That is, in Embodiment 2, the first material layer  21 B 1  includes the same material as the storage layer  22 . In addition, the second material layer  21 B 2  includes a non-magnetic material layer. The second material layer  21 B 2  includes one kind of material selected from a group consisting of a high melting-point non-magnetic metal such as tantalum, molybdenum, tungsten, titanium, and magnesium, and magnesium oxide, specifically, is made of tantalum (Ta) in Embodiment 2. In addition, a material included in the first ground layer  21 A and a material included in the second material layer  21 B 2  are the same (specifically, tantalum). Furthermore, when a thickness of the second ground layer  21 B is denoted by T 2 ′, 3 nm≤T 2 ′ is satisfied. Although the measurement results of a write current value and a thermal disturbance constant when T 2 ′=4 nm are shown in Table 2, they have substantially the same values as those of the magnetoresistive element of Embodiment 1. In addition, the coercive force of the magnetoresistive element of Embodiment 2 is about 2800 (Oe), which is a value of the same degree as that of Embodiment 1. 
     Since the configuration and the structure of the magnetoresistive element of Embodiment 2 are similar to the configuration and the structure of that of Embodiment 1 except the above-described points, detailed description thereof will be omitted. 
     Embodiment 3 
     Embodiment 3 relates to an electronic device having the magnetoresistive element  10  or  10 A described in Embodiment 1 or Embodiment 2, specifically, a magnetic head. Magnetic heads can be applied to various electronic apparatuses, electric apparatuses, and the like beginning from, for example, hard disk drives, integrated circuit chips, personal computers, mobile terminals, mobile telephones, and magnetic sensor apparatuses. 
     As an example,  FIG. 6A  and  FIG. 6B  illustrate an example in which a magnetoresistive element  101  is applied to a composite magnetic head  100 . Note that  FIG. 6A  is a schematic perspective diagram illustrating the composite magnetic head  100  of which a part has been cut to see the internal structure and  FIG. 6B  is a schematic cross-sectional diagram of the composite magnetic head  100 . 
     The composite magnetic head  100  is a magnetic head that is used in a hard disk device or the like, a magnetoresistance effect magnetic head with the magnetoresistive element  10  or  10 A described in Embodiment 1 or Embodiment 2 is formed on a substrate  122 , and an inductive magnetic head is further laminated and formed on the magnetoresistance effect magnetic head. Here, the magnetoresistance effect magnetic head operates as a head for reproduction, and the inductive magnetic head operates a head for recording. That is, a head for reproduction and a head for recording are combined in the composite magnetic head  100 . 
     The magnetoresistance effect magnetic head mounted in the composite magnetic head  100  is a so-called shield MR head, and includes a first magnetic shield layer  125  formed on the substrate  122  via an insulating layer  123 , a magnetoresistive element  101  formed on the first magnetic shield layer  125  via the insulating layer  123 , and a second magnetic shield layer  127  formed on the magnetoresistive element  101  via the insulating layer  123 . The insulating layer  123  includes an insulating material such as Al 2 O 3  or SiO 2 . The first magnetic shield layer  125  is a layer magnetically shielding the ground layer side of the magnetoresistive element  101 , and includes a soft magnetic material such as Ni—Fe. The magnetoresistive element  101  is formed on the first magnetic shield layer  125  via the insulating layer  123 . The magnetoresistive element  101  functions as a magneto-sensitive element that detects magnetic signals from a magnetic recording medium in the magnetoresistance effect magnetic head. A shape of the magnetoresistive element  101  is a substantially rectangular shape, and one side surface thereof is exposed as a surface facing a magnetic recording medium. In addition, bias layers  128  and  129  are disposed at both ends of the magnetoresistive element  101 . In addition, connection terminals  130  and  131  that are connected to the bias layers  128  and  129  are formed. A sense current is supplied to the magnetoresistive element  101  via the connection terminals  130  and  131 . The second magnetic shield layer  127  is provided above the bias layers  128  and  129  via the insulating layer  123 . 
     The inductive magnetic head laminated and formed on the magnetoresistance effect magnetic head includes a magnetic core including the second magnetic shield layer  127  and an upper layer core  132 , and thin film coils  133  formed to wind the magnetic core. The upper layer core  132  forms a closed magnetic path together with the second magnetic shield layer  127 , serves as a magnetic core of the inductive magnetic head, and includes a soft magnetic material such as Ni—Fe. Here, the second magnetic shield layer  127  and the upper layer core  132  are formed such that the front end parts thereof are exposed as surfaces facing the magnetic recording medium and the second magnetic shield layer  127  and the upper layer core  132  are in contact with each other at the rear end parts. Here, the front end parts of the second magnetic shield layer  127  and the upper layer core  132  are formed such that the second magnetic shield layer  127  and the upper layer core  132  are separated having a predetermined gap g with respect to the surface facing the magnetic recording medium. That is, in the composite magnetic head  100 , the second magnetic shield layer  127  magnetically shields the upper layer side of the magnetoresistive element  101  and also serves as a magnetic core of the inductive magnetic head, and the second magnetic shield layer  127  and the upper layer core  132  constitute the magnetic core of the inductive magnetic head. In addition, the gap g is a magnetic gap for recording of the inductive magnetic head. 
     In addition, the thin film coils  133  embedded in the insulating layer  123  are formed above the second magnetic shield layer  127 . The thin film coils  133  are formed to wind the magnetic coil including the second magnetic shield layer  127  and the upper layer core  132 . Although not illustrated, both end parts of the thin film coils  133  are exposed to outside, and terminals formed at both ends of the thin film coils  133  are external connection terminals of the inductive magnetic head. That is, when a magnetic signal is recorded into the magnetic recording medium, recording currents are supplied from the external connection terminals to the thin film coils  133 . 
     Although the composite magnetic head  100  described above has the magnetoresistance effect magnetic head mounted as a head for reproduction, the magnetoresistance effect magnetic head includes the magnetoresistive element  101  described in Embodiment 1 or Embodiment 2 as a magneto-sensitive element that detects magnetic signals from the magnetic recording medium. In addition, since the magnetoresistive element  101  exhibits very excellent characteristics as described above, the magnetoresistance effect magnetic head can achieve a higher recording density of magnetic recording. 
     Although the present disclosure has been described above on the basis of embodiments, the present disclosure is not limited to the embodiments. Various laminated structures, used materials, and the like described in the embodiments are merely examples, and can be appropriately modified. 
     Additionally, the present technology may also be configured as below. 
     [A01]&lt;&lt;Magnetoresistive Element: First Aspect&gt;&gt; 
     A magnetoresistive element formed by laminating a lower electrode, a first ground layer including a non-magnetic material, a storage layer having perpendicular magnetic anisotropy, an intermediate layer, a magnetization fixed layer, and an upper electrode, 
     in which the storage layer includes a magnetic material including at least a 3d transition metal element and a boron element in a composition, 
     a second ground layer is further included between the lower electrode and the first ground layer, and 
     the second ground layer includes a material including at least one kind of element among elements constituting the storage layer in a composition. 
     [A02] 
     The magnetoresistive element according to [A01], in which the second ground layer has in-plane magnetic anisotropy or non-magnetism. 
     [A03] 
     The magnetoresistive element according to [A01] or [A02], 
     in which the storage layer includes Co—Fe—B, and 
     a boron atom content of the second ground layer is in a range of 10 atomic % to 50 atomic %. 
     [A04]&lt;&lt;Magnetoresistive Element with First Configuration&gt;&gt; 
     The magnetoresistive element according to any one of [A01] to [A03], 
     in which the second ground layer includes one Co—Fe—B layer, and 
     the first ground layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [A05] 
     The magnetoresistive element according to [A04], in which, when a thickness of the second ground layer is denoted by T 2  and a thickness of the storage layer is denoted by T 0 , T 0 ≤T 2  is satisfied. 
     [A06] 
     The magnetoresistive element according to [A05], in which T 2 ≤3 nm is satisfied. 
     [A07] The magnetoresistive element described in any one of [A04] to [A06] in which a third ground layer is formed between a lower electrode and the second ground layer. 
     [A08] 
     The magnetoresistive element according to [A07], in which the third ground layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [A09] 
     The magnetoresistive element according to [A07], in which the third ground layer includes a same material as a material included in the first ground layer. 
     [A10]&lt;&lt;Magnetoresistive Element with Second Configuration&gt;&gt; 
     The magnetoresistive element according to any one of [A01] to [A03], in which the second ground layer is formed by alternately laminating a first material layer and a second material layer. 
     [A11] 
     The magnetoresistive element according to [A10], 
     in which the first material layer includes a Co—Fe—B layer, and 
     the second material layer includes a non-magnetic material layer. 
     [A12] 
     The magnetoresistive element according to [A10] or [A11], in which the second material layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [A13] 
     The magnetoresistive element according to any one of [A10] to [A12], in which a material included in the first ground layer and a material included in the second material layer are same materials. 
     [A14] 
     The magnetoresistive element according to any one of [A10] to [A13], in which, when a thickness of the second ground layer is denoted by T 2 ′, 3 nm T 2 ′ is satisfied. 
     [A15] The magnetoresistive element described in any one of [A10] to [A14] in which, when a thickness of the first material layer is denoted by T 2-A ′ and a thickness of the second material layer is denoted by T 2-B ′, 0.2≤T 2-A ′/T 2-B ′≤5 is satisfied.
 
[A16] The magnetoresistive element described in any one of [A10] to [A15] in which when a thickness of the first material layer is denoted by T 2-A ′ and a thickness of the storage layer is denoted by T 0 , T 2-A ′&lt;T 0  is satisfied.
 
     [A15] 
     The magnetoresistive element according to any one of [A01] to [A14], in which, when a thickness of the first ground layer is denoted by T 1 , 1 nm≤T 1 ≤4 nm is satisfied. 
     [B01]&lt;&lt;Magnetoresistive Element: Second Aspect&gt;&gt; 
     A magnetoresistive element formed by laminating a lower electrode, a first ground layer including a non-magnetic material, a storage layer, an intermediate layer, a magnetization fixed layer, and an upper electrode, 
     in which the storage layer has perpendicular magnetic anisotropy, 
     a second ground layer is further included between the lower electrode and the first ground layer, and 
     the second ground layer has in-plane magnetic anisotropy or non-magnetism. 
     [B02] 
     The magnetoresistive element according to [B01], 
     in which the storage layer includes Co—Fe—B, and 
     a boron atom content of the second ground layer is in a range of 10 atomic % to 50 atomic %. 
     [B03]&lt;&lt;Magnetoresistive Element with First Configuration&gt;&gt; 
     The magnetoresistive element according to [B01] or [B02], 
     in which the second ground layer includes one Co—Fe—B layer, and 
     the first ground layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [B04] 
     The magnetoresistive element according to [B03], in which, when a thickness of the second ground layer is denoted by T 2  and a thickness of the storage layer is denoted by T 0 , T 0 ≤T 2  is satisfied. 
     [B05] The magnetoresistive element described in [B04] in which T 2 ≤3 nm is satisfied.
 
[B06] The magnetoresistive element described in any one of [B03] to [B05] in which a third ground layer is formed between a lower electrode and the second ground layer.
 
     [B07] 
     The magnetoresistive element according to [B06], in which the third ground layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [B08] 
     The magnetoresistive element according to [B06], in which the third ground layer includes a same material as a material included in the first ground layer. 
     [B09]&lt;&lt;Magnetoresistive Element with Second Configuration&gt;&gt; 
     The magnetoresistive element according to [B01] or [B02], in which the second ground layer is formed by alternately laminating a first material layer and a second material layer. 
     [B10] 
     The magnetoresistive element according to [B09], 
     in which the first material layer includes a Co—Fe—B layer, and 
     the second material layer includes a non-magnetic material layer. 
     [B11] 
     The magnetoresistive element according to [B09] or [B10], in which the second material layer includes one kind of material selected from a group consisting of tantalum, molybdenum, tungsten, titanium, magnesium, and magnesium oxide. 
     [B12] 
     The magnetoresistive element according to any one of [B09] to [B11], in which a material included in the first ground layer and a material included in the second material layer are same materials. 
     [B13] 
     The magnetoresistive element according to any one of [B09] to [B12], in which, when a thickness of the second ground layer is denoted by T 2 ′, 3 nm≤T 2 ′ is satisfied. 
     [B14] 
     The magnetoresistive element according to any one of [B01] to [B13], in which, when a thickness of the first ground layer is denoted by T 1 , 1 nm≤T 1 ≤4 nm is satisfied. 
     [C01]&lt;&lt;Electronic Device&gt;&gt; 
     An electronic device including: 
     the magnetoresistive element according to any one of [A01 to [B14]. 
     [C02]&lt;Memory Cell Unit&gt; 
     A memory cell unit in which a plurality of non-volatile memory cells is arrayed in a first direction and a second direction that is different from the first direction in a two-dimensional matrix shape, and the non-volatile memory cells include the magnetoresistive element described in any one of [A01 to [B14]. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  10 A magnetoresistive element 
           20  laminated structure 
           21  ground layer 
           21 A first ground layer 
           21 B second ground layer 
           21 C third ground layer 
           22  storage layer 
           23  intermediate layer 
           24  magnetization fixed layer 
           24 A reference layer 
           24 B non-magnetic layer 
           24 C fixed layer 
           31  lower electrode (first electrode) 
           32  upper electrode (second electrode) 
           41  first wiring 
           42  second wiring 
           43  sense line 
           51  insulation material layer 
         TR selection transistor 
           60  semiconductor substrate 
           60 A element isolation region 
           61  gate electrode 
           62  gate insulating layer 
           63  channel formation region 
           64 A,  64 B source/drain region 
           65  tungsten plug 
           66  connection hole 
           67 ,  67 A,  67 B interlayer insulating layer 
           100  composite magnetic head 
           101  magnetoresistive element 
           122  substrate 
           123  insulating layer 
           125  first magnetic shield layer 
           127  second magnetic shield layer 
           128 ,  129  bias layer 
           130 ,  131  connection terminal 
           132  upper layer core 
           133  thin film coil