Abstract:
The CPPGMR element of the present invention has an orientation layer  12  formed on a substrate  11  to texture a Heusler alloy into a (100) direction, an underlying layer  13  that is an electrode for magneto-resistance measurement stacked on the orientation layer  12 , a lower ferromagnetic layer  14  and an upper ferromagnetic layer  16  each stacked on the underlying layer  13  and made of a Heusler alloy, a spacer layer  15  sandwiched between the lower ferromagnetic layers  14  and the upper ferromagnetic layers  16 , and a cap layer  17  stacked on the upper ferromagnetic layer  16  for surface-protection. This manner makes it possible to provide, inexpensively, an element using a current-perpendicular-to-plane giant magneto-resistance effect (CPPGMR) of a thin film having a trilayered structure of a ferromagnetic metal/a nonmagnetic metal/a ferromagnetic metal, thereby showing excellent performances.

Description:
TECHNICAL FIELD 
     The present invention relates to an element using a current-perpendicular-to-plane giant magneto-resistance effect (CPPGMR) of a thin film having a trilayered structure of a ferromagnetic metal/a nonmagnetic metal/a ferromagnetic metal, in particular, a current-perpendicular-to-plane giant magneto-resistance effect element using an ordinarily usable surface-oxidized Si substrate, silicon substrate, glass substrate, metal substrate or the like instead of an expensive MgO monocrystalline substrate. 
     BACKGROUND ART 
     Elements each using a current-perpendicular-to-plane giant magneto-resistance effect (referred to also as a CPPGMR hereinafter) of a thin film having a trilayered structure of a ferromagnetic metal/a nonmagnetic metal/a ferromagnetic metal have been expected for readout heads for magnetic disks. Researches have been made about elements each using a Heusler alloy, which is large in spin polarizability, as each of the ferromagnetic metals. A development has been made about a CPPGMR element using a polycrystalline thin film having a crystal orientation textured into a (110) direction as a layer of the Heusler alloy (for example, Patent Literatures 1 to 3). 
     By contrast, it is demonstrated that in a CPPGMR element, the use of a monocrystalline thin film textured into a (100) direction makes it possible to improve performances of the element (for example, Non Patent Literatures 1 and 2). However, for the production of the monocrystalline thin film, an expensive MgO monocrystalline substrate is required, and thus such methods are impracticable from the viewpoint of costs. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: JP 2010-212631 A 
         Patent Literature 2: JP 2011-35336 A 
         Patent Literature 3: JP 2005-116701 A 
       
    
     Non Patent Literatures 
     
         
         Non Patent Literature 1: Appl. Phys. Lett. 100, 052405 (2012). 
         Non Patent Literature 2: Appl. Phys. Lett. 101, 252408 (2012). 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention has been made in light of actual situations of the above-mentioned conventional techniques, and an object of the invention is to provide, without using any MgO monocrystalline substrates, a CPPGMR element more inexpensive and better in performances than CPPGMR elements each using a polycrystalline thin film having a crystal orientation textured to a (110) direction. 
     Solution to Problem 
     In order to solve the above-mentioned problems, the present invention provides a CPPGMR element having structural requirements described below. 
     For example, as illustrated in  FIG. 1 , the CPPGMR element of the present invention includes, an orientation layer  12  on a substrate  11  to texture Heusler alloy into a (100) direction, a lower ferromagnetic layer  14  and an upper ferromagnetic layer  16  that each includes a polycrystalline thin film of a Heusler alloy textured into a (100) direction, and that are each stacked on the orientation layer  12 , and a spacer layer  15  sandwiched between the lower ferromagnetic layer  14  and the upper ferromagnetic layer  16 . 
     In the CPPGMR element of the present invention, it is preferred that: the substrate  11  is at least one of a surface-oxidized Si substrate, a silicon substrate, a glass substrate, and a metal substrate; the orientation layer  12  includes at least one of MgO, TiN, and NiTa alloys; the lower ferromagnetic layer  14  and the upper ferromagnetic layer  16  each includes a Heusler alloy represented by a composition formula of Co 2 AB wherein A is Cr, Mn, or Fe, or a mixture obtained by mixing two or more of these elements with each other to set the total quantity of the mixed elements to 1, and B is Al, Si, Ga, Ge, In, or Sn, or a mixture obtained by mixing two or more of these elements with each other to set the total quantity of the mixed elements to 1; and the spacer layer  15  is at least one metal selected from the group consisting of Ag, Al, Cu, Au, and Cr, or any alloys of the selected metal(s). 
     In the CPPGMR element of the present invention, it is preferred that at least one of the orientation layer  12 , the lower ferromagnetic layer  14 , the upper ferromagnetic layer  16 , and the spacer layer  15  is formed by a sputtering method. 
     In the CPPGMR element of the present invention, it is further preferred that an underlying layer  13  for magneto-resistance measurement is laid to be sandwiched between the orientation layer  12  and the lower ferromagnetic layer  14 . The underlying layer  13  can be formed, using at least one metal selected from the group consisting of Ag, Al, Cu, Au, and Cr, or any alloys of the selected metal(s). It is advisable to form the underlying layer  13  by a sputtering method. 
     In the CPPGMR element of the present invention, it is also preferred that a cap layer  17  to be stacked on the upper ferromagnetic layer  16  for surface protection. The cap layer  17  may be formed, using at least one metal selected from the group consisting of Ag, Al, Cu, Au, Ru, and Pt, or any alloys of the selected metal(s). It is advisable to form the cap layer  17  by a sputtering method. 
     Advantageous Effects of Invention 
     In the present invention, an orientation layer is laid over a substrate including at least one of a surface-oxidized Si substrate, a silicon substrate, a glass substrate, and a metal substrate, which are inexpensive, without using any MgO monocrystalline substrates to produce a CPPGMR element including a (100)-textured polycrystalline thin film. It has been verified that this production makes the resultant element better in properties than any production using a (110)-textured polycrystalline thin film. Such a structure makes it possible to produce a CPPGMR element more inexpensive and higher in performances. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural view of a CPPGMR element according to an embodiment of the present invention. 
         FIG. 2  is a chart showing an X-ray diffraction pattern of a film obtained by stacking, onto an oxidized Si substrate, respective films of the following from below: MgO(10)/Cr(20)/Ag(50)/CFGG(10)/Ag(7)/CFGG(10)/Ag(5)/Ru(8). 
         FIG. 3  is a schematic sectional view of a CPPGMR element according to an embodiment of the present invention. 
         FIG. 4  is a graph demonstrating a change in the value of “the area of an element”×“the electric resistance thereof” versus a magnetic field applied thereto. 
         FIG. 5  is a graph demonstrating results of the value ΔRA of “the change of the magneto-resistance thereof”×“the area of an element” versus annealing temperature, in which black squares show results of a (110)-textured orientation film according to a conventional method. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present invention will be described, referring to the drawings. 
       FIG. 1  is a schematic structural view of a current-perpendicular-to-plane magneto-resistance effect (CPPGMR) element according to an embodiment of the present invention. In the figure, the CPPGMR element of the present embodiment is configured by a substrate  11 , an orientation layer  12 , an underlying layer  13 , a lower ferromagnetic layer  14 , a spacer layer  15 , an upper ferromagnetic layer  16 , and a cap layer  17  stacked in this order. 
     The substrate  11  is most preferably a surface-oxidized Si substrate from the viewpoint of costs, but may be a silicon substrate for semiconductor-production, or may be a glass substrate or a metal substrate. It is sufficient for the orientation layer  12  to be a layer having an effect of texturing a Heusler alloy into a (100) direction. Thus, the orientation layer  12  is preferably a layer containing at least one of MgO, TiN, and NiTa alloys. Of these components, MgO and TiN are crystalline. Such a crystalline orientation layer is textured into a (100) direction to grow easily, so that the layer itself undergoes (100) orientation to induce the (100) orientation of a Heusler alloy. Although NiTa is amorphous, NiTa induces the (100) orientation of a Heusler alloy growing on this component. When the substrate  11  has a crystal orientation, NiTa simultaneously has an effect of breaking off any effect of the crystal orientation. The underlying layer  13  is made of a metal or an alloy, and is to be an electrode for magneto-resistance measurement. For the underlying layer  13 , the following is usable: a metal containing at least one of Ag, Al, Cu, AuCr and others; or any alloys of one or more of these metal elements. A different underlying layer may be added below the orientation layer  12 . 
     The lower ferromagnetic layer  14  and the upper ferromagnetic layer  16  each contains a polycrystalline Heusler alloy textured to a (100) direction represented by a composition formula of Co 2 AB wherein A is Cr, Mn, or Fe, or a mixture obtained by mixing two or more of these elements with each other to set the total quantity of the mixed elements to 1, and B is Al, Si, Ga, Ge, In, or Sn, or a mixture obtained by mixing two or more of these elements with each other to set the total quantity of the mixed elements to 1. The Heusler alloy is in particular preferably a Co 2 FeGa 0.5 Ge 0.5  (CFGG) polycrystalline thin film, but may be a Co 2 FeAl 1-x Si x , Co 2 MnSi or Co 2 Fe 1-x Mn x Si polycrystalline thin film. For the upper ferromagnetic layer and the lower ferromagnetic layer, one Heusler alloy may be used. Alternatively, any combination of two or more Heusler alloys may be used, as well as any combination of one or more Heusler alloys with one or more different metals or alloys. 
     The spacer layer  15  is made of a metal or an alloy. The cap layer  17  is made of a metal or an alloy for surface-protection. For the spacer layer  15 , the following is usable: for example, a metal containing at least one of Ag, Al, Cu, Au, Cr, and others; or any alloys of one or more of these metal elements. For the cap layer  17 , the following is usable: for example, a metal containing at least one of Ag, Al, Cu, Au, Ru, Pt, and others; or any alloys of one or more of these metal elements. 
     For each of the orientation layer  12 , the underlying layer  13 , the spacer layer  15 , and the cap layer  17 , a single material may be used, or two or more materials stacked onto each other may be used. 
     It is preferred to form at least one of the orientation layer  12 , the lower ferromagnetic layer  14 , the upper ferromagnetic layer  16 , and the spacer layer by a sputtering method. It is also preferred to anneal the stacked-film at a temperature of 200 to 450° C. for about 15 to 60 minutes to be improved in crystal structure. 
     Examples 
     The following will describe examples of the present invention. 
       FIG. 3  is a schematic sectional view of a CPPGMR element according to an example of the present invention. In the figure, a surface-oxidized Si substrate is used as a substrate  11 ; MgO is used as an orientation layer  12 ; a stacked Cr layer  13   a  and Ag layer  13   b , the Cr layer  13   a  being positioned below, as an underlying layer  13 ; a polycrystalline (0001)-textured Heusler alloy, Co 2 FeGa 0.5 Ge 0.5  (CFGG), as an upper ferromagnetic layer  14  and a lower ferromagnetic layer  16 ; Ag as a spacer layer  15 ; and a stacked Ag layer  17   a  and Ru layer  17   b , the Ag layer  17   a  being positioned below, as a cap layer  17 . 
     The CPPGMR element of the present example is an element obtained by forming, onto the oxidized Si substrate, respective films of the following from below: MgO(10)/Cr(20)/Ag(50)/CFGG(10)/Ag(7)/CFGG(10)/Ag(5)/Ru(8). The number in each pair of parentheses represents the film thickness (nm). By a sputtering method, the film-formation of the layer structure is attained. 
       FIG. 2  is an X-ray diffraction pattern of the stack having the film structure illustrated in  FIG. 3 . According to the X-ray diffraction, the structure of the crystal was examined. As a result, it was understood from the results shown in  FIG. 2  that the layer of each of Cr, Ag, and CFGG was textured into a (100) direction. In order to improve the thin film in crystal structure, the sample was annealed at 400° C. for 30 minutes. Thereafter, to measure the electric resistance in the direction perpendicular to the plane of the film, the workpiece was finely worked, as illustrated in  FIG. 3 , and a silicon oxide (SiO 2 ) layer  19  was laid adjacently to the stack composed of the upper ferromagnetic layer  14 , the spacer layer  15 , the lower ferromagnetic layer  16 , and the cap layer  17 . Next, a Cu electrode layer  18  was attached onto the cap layer  17  and the silicon oxide layer  19 . A constant-current source  20  was connected between the underlying layer  13  and the Cu electrode layer  18 , and a voltmeter  21  was connected between the underlying layer  13  and the Cu electrode layer  18 . The constant-current source  20  and the voltmeter  21  were used to examine a change in the electric resistance of the CPPGMR element versus the magnetic field. Furthermore, while the temperature for annealing the sample was varied between 300° C. and 450° C., the variation ΔRA of the electric resistance per unit area of the element was examined. 
       FIG. 4  shows a change in the electric resistance of the CPPGMR element versus the magnetic field. When the magnetic field was in the range of about ±200 [Oe] (=1000/(4π)A)/m), the following variation of the electric resistance was obtained per unit area of the element: a variation ΔRA of 4.6 [mΩ·μm 2 ]. 
     For comparison, without using any MgOs (orientation layers), a sample was produced to have a film structure having, over an oxidized Si substrate, respective films of the following: Ta(5)/Cu(250)/Ta(5)/CFGG(5)/Ag(7)/CFGG(5)/Ag(5)/Ru(6), T(2) and Ru(2). The sample was measured in the same way. It was verified about this sample that the crystal orientation of CFGG was textured to (110). 
       FIG. 5  is a graph obtained by plotting the variation ΔRA of the electric resistance per unit area of each of the elements versus the annealing temperature T an . About the (110)-textured sample according to the conventional technique, the variation ΔRA was lowered in the case of T an &gt;400° C. By contrast, about the (100)-textured sample according to the present invention, at a T an  of 400° C., a variation ΔRA of 4.3 [mΩ·μm 2 ] was obtained as the average value, which was a value larger than the maximum value 3.5 [mΩ·μm 2 ] according to the conventional technique. 
     In a modified example of the present invention, a layer of an antiferromagnetic material may be further added, as a pinning layer, onto the upper ferromagnetic layer in the structure illustrated in  FIG. 3 . The antiferromagnetic material is, for example, any IrMn alloys or PtMn alloys. This layer structure, which has the upper ferromagnetic layer to which the pinning layer is added, makes it possible to restrain magnetization inversion in the upper ferromagnetic layer by exchange anisotropy to stabilize a state that the upper ferromagnetic layer and the lower ferromagnetic layer are magnetized in antiparallel to each other. The pinning layer may be inserted below the lower ferromagnetic layer. 
     In the above-mentioned embodiment, a case has been illustrated which has a film structure of MgO(10)/Cr(20)/Ag(50)/CFGG(10)/Ag(7)/CFGG(10)/Ag(5)/Ru(8). However, the present invention is not limited to this structure. Of course, the film material and the film thickness of each of the layers can be appropriately selected from scopes anticipated by those skilled in the art as far as the selected material and film thickness do not depart from the subject matters of the present invention. 
     INDUSTRIAL APPLICABILITY 
     current-perpendicular-to-plane magneto-resistance effect (CPPGMR), is suitable for being used for a read head for a magnetic disk, and is usable for detecting fine magnetic information pieces. 
     REFERENCE SIGNS LIST 
     
         
           11 : substrate 
           12 : orientation layer 
           13 : underlying layer 
           14 : lower ferromagnetic layer 
           15 : spacer layer 
           16 : upper ferromagnetic layer 
           17 : cap layer