Patent Abstract:
A magnetoresistive element (and method of fabricating the magnetoresistive element) that includes a free ferromagnetic layer comprising a first reversible magnetization direction directed substantially perpendicular to a film surface, a pinned ferromagnetic layer comprising a second fixed magnetization direction directed substantially perpendicular to the film surface, and a nonmagnetic insulating tunnel barrier layer disposed between the free ferromagnetic layer and the pinned ferromagnetic layer, wherein the free ferromagnetic layer, the tunnel barrier layer, and the pinned ferromagnetic layer have a coherent body-centered cubic (bcc) structure with a (001) plane oriented, and a bidirectional spin-polarized current passing through the coherent structure in a direction perpendicular to the film surface reverses the magnetization direction of the free ferromagnetic layer.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application No. 61/476,655, filed on Apr. 18, 2011 by the present inventor. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     SEQUENCE LISTING OR PROGRAM 
     Not Applicable 
     RELEVANT PRIOR ART 
     U.S. Pat. No. 7,313,013, Dec. 25, 2007—Sun et al. 
     U.S. Pat. No. 7,663,197, Feb. 16, 2010—Nagase et al. 
     U.S. Pat. No. 7,924,607, Apr. 12, 2011—Yoshikawa et al. 
     BACKGROUND 
     Magnetic (or magnetoresistive) random access memory (MRAM) is a non-volatile solid-state memory employing magnetoresistive (MR) elements or magnetic tunnel junctions (MTJs) to store and retrieve data. The MR element uses a tunnel (or giant) magnetoresistive effect in magnetic nultilayers. The MR element includes at least two magnetic layers separated from each other by a thin nonmagnetic insulator or semiconductor layer that serves as a tunnel barrier layer. One of the magnetic layers having a fixed direction of magnetization is called a pinned (or reference) layer. Another magnetic layer having a reversible direction of the magnetization is called a free (or storage) layer. Resistance of the MR element depends on a mutual direction of the magnetizations in the free and pinned layers. The resistance is low when the magnetization directions are parallel to each other and high when they are anti-parallel. Parallel configuration of the magnetizations corresponds to a logic “0”. The anti-parallel configuration of the magnetizations corresponds to logic “1”. A difference in the resistance between two logic states can exceed several hundred percents at room temperature. Theory predicts that MR ratio in MTJs made of FeCo/MgO/FeCo multilayer having body-centered cubic (bcc) structure with preferred (001) orientation can exceed 1000% at room temperature. The high TMR is a result of coherent tunneling of highly spin-polarized conductance electrons through the multilayer structure. 
     The magnetization direction in the free layer can be reversed by an external magnetic field or by a spin-polarized current running through the MR element in a direction perpendicular to its surface (substrate). Reversal of the magnetization direction in the MR element by a spin-polarized current (spin momentum transfer or spin torque transfer) are widely used in MRAM technology. The magnetization direction in the free layer can be controlled by a direction of the spin-polarized current running through the MR element, for instance, from the pinned layer to the free layer or vice-versa. 
     CoFeB/MgO/CoFeB multilayer has became a system of choice for manufacturing of MTJs. The multilayer having amorphous structure in as-deposited state can be crystallized into coherent body-centered cubic (bcc) structure with a (001) plane oriented. Moreover MgO can form flat and sharp interface with CoFeB layer. That is essential for coherent tunneling of spin-polarized electrons providing high tunneling magnetoresistance (TMR) and low density of switching current. 
     CoFeB layers typically have a substantial boron content (about 15-30 atomic %) to be amorphous in as-deposited state. MgO layer in the CoFeB/MgO/CoFeB system crystallizes first during annealing at a temperature about 250° C. or above into stable bcc structure with preferred (001) orientation. Then the crystalline MgO layer acts as a template during crystallization of the CoFeB layers because of a good lattice match between the bcc MgO and bcc CoFeB. Annealing can also promote an interfaces sharpness in the CoFeB/MgO/CoFeB multilayer. 
     Crystallization of CoFeB is a thermally activated process. Therefore annealing at lower (higher) temperatures for longer (shorter) periods may provide similar results. The crystallization of CoFeB requires a reduction in the boron (B) content in the layers. The boron diffuses through the multilayer structure forming other borides with another layers of MTJ stack. 
     The diffusion of the boron into the MgO tunnel barrier layer may lead to both degradation of TMR and increase of the spin-polarized switching current. In order to reduce accumulation of the boron in the MgO layer during annealing, boron “getter” layer (or layers) may be inserted into the MTJ structure. The reduction of the boron content reduces the crystallization temperature of the CoFeB layers and promotes their transformation from as-deposited amorphous into bcc (001) crystalline structure starting from MgO interfaces. 
     There are two types of magnetic materials used in the MRAM: materials with in-plane or perpendicular magnetization orientation (or anisotropy). MR elements with a perpendicular orientation of the magnetization (or perpendicular material) have excellent thermal stability and scalability. Moreover theory predicts that perpendicular MR element can have substantially lower density of the switching spin-polarized current than similar MR element using in-plane magnetic materials. 
     Perpendicular MR elements of MRAM require TMR about 100% or higher and a density of spin-polarized switching current about 1·10 6  A/cm 2  or lower. These parameters can be achieved in MR element wherein the tunnel barrier layer and adjacent magnetic free and pinned layers form a coherent bcc (001) texture with sharp and flat interfaces. This texture can be formed by annealing substantially amorphous in as-deposited state CoFeB/MgO/CoFeB multilayer at a temperature about 250° C. and/or above. 
     The pinned layer of the perpendicular MR element can have a multilayer structure for cancelling its fringing magnetic field produced in the vicinity of the free layer. The fringing filed can affect the thermal stability and switching current of the MR element. The pinned layer having a multilayer structure may comprise a layer of a ferrimagnetic material made of a rare earth-transition metal (RE-TM) alloy such as TbFeCo. Besides the pinned layer can comprise a synthetic antiferromagnetic (SAF) structure. 
     The pinned magnetic layers with the canceled fringing magnetic field can suffer from a poor thermal stability. For example, the RE-TM alloys may lose their perpendicular anisotropy at the annealing temperature above 200° C. The SAF structures frequently employ an ultrathin nonmagnetic spacer layer having a thickness less than 1 nm. The spacer layer is usually positioned between two magnetic layers forming the SAF structure to produce a substantial antiferromagntic exchange coupling between the magnetic layers. That is essential for stable perpendicular magnetization direction of the pinned layer made of CoFeB. Annealing of the SAF structure at the temperature about 250° C. and above may cause un uncontrollable diffusion through the ultrathin spacer layer and reduce the exchange coupling between the magnetic layers. That may lead to substantial reduction of TMR and increase of spin-polarized switching current. 
     Industry-wide efforts are underway to increase the TMR and to reduce the density of the spin-polarized switching current in the perpendicular MR elements for effective integration with CMOS technology. The present disclosure addresses to the above problems. 
     SUMMARY 
     Disclosed herein is a magnetoresistive element that comprises a free ferromagnetic layer comprising a first reversible magnetization direction directed substantially perpendicular to a film surface, a pinned ferromagnetic layer comprising a second fixed magnetization direction directed substantially perpendicular to the film surface, and a nonmagnetic insulating tunnel barrier layer disposed between the free ferromagnetic layer and the pinned ferromagnetic layer, wherein the free ferromagnetic layer, the tunnel barrier layer, and the pinned ferromagnetic layer have a coherent body-centered cubic (bcc) structure with a (001) plane oriented, and a bidirectional spin-polarized current passing through the coherent structure in a direction perpendicular to the film surface reverses the magnetization direction of the free ferromagnetic layer. 
     Also disclosed is a magnetoresistive element that comprises a free ferromagnetic layer comprising a first reversible magnetization direction directed substantially perpendicular to a film surface, a pinned ferromagnetic layer comprising a second fixed magnetization direction directed substantially perpendicular to the film surface, a nonmagnetic insulating tunnel barrier layer disposed between the free ferromagnetic layer and the pinned ferromagnetic layer, a first nonmagnetic conductive layer disposed contiguously to a side of the free ferromagnetic layer opposite to the tunnel barrier layer, and a pinning magnetic layer disposed adjacent to a side of the pinned ferromagnetic layer opposite to the tunnel barrier layer, and comprising a third fixed magnetization direction directed substantially perpendicular to the film surface, the pinning magnetic layer has a substantial antiferromagnetic exchange coupling with the pinned ferromagnetic layer, wherein the free ferromagnetic layer, the tunnel barrier layer, and the pinned ferromagnetic layer have a coherent body-centered cubic (bcc) structure with a (001) plane oriented, and a bidirectional spin-polarized current passing through the coherent structure in a direction perpendicular to the film surface reverses the magnetization direction of the free ferromagnetic layer. 
     Also disclosed is a method of fabricating a magnetoresistive element, the method comprising: forming on a substrate a first plurality of layers such that interfaces between the first plurality of layers are formed in situ; the first plurality of layers comprising: a first nonmagnetic conductive layer, a free ferromagnetic layer comprising an amorphous structure and a first magnetization direction, a nonmagnetic insulating tunnel barrier layer, a pinned ferromagnetic layer comprising an amorphous structure and a second magnetization direction, and a getter layer; annealing the first plurality of layers, removing the getter layer and a portion of the pinned ferromagnetic layer adjacent to the getter layer, forming above the pinned ferromagnetic layer a second plurality of layers such that interfaces between the pinned ferromagnetic layer and the second plurality of layers are formed in situ, the second plurality of layers sequentially comprising: a pinning magnetic layer comprising a third fixed magnetization direction directed substantially perpendicular to a film surface, and a second nonmagnetic conductive layer, wherein the free ferromagnetic layer, the tunnel barrier layer, and the pinned ferromagnetic layer crystallize during annealing in a coherent body-centered cubic (bcc) structure with (001) plane oriented, the first magnetization direction and the second magnetization direction are directed substantially perpendicular to the substrate. the second magnetization direction is fixed and directed antiparallel to the third magnetization direction, and the first magnetization direction is reversible in accordance with a direction of a spin-polarized current passing through the layers in a direction perpendicular to the substrate. 
     These and various other features and advantages will be apparent from a reading of the following detailed description. 
    
    
     
       BRIEF DISCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic sectional views showing a magnetoresistive element with spin-induced switching having different logic states according to an embodiment of the present disclosure. 
         FIG. 2  is a schematic sectional view showing a magnetoresistive element according to another embodiment of the present disclosure. 
         FIG. 3  is a schematic sectional view showing a magnetoresistive element according to an alternative embodiment. 
         FIG. 4  is a schematic sectional view showing a magnetoresistive element according to another alternative embodiment of the present disclosure. 
         FIGS. 5A-5C  illustrate different steps of fabricating method of a magnetoresistive element according to the embodiment of the present disclosure. 
         FIG. 6  illustrates a method of fabricating the magnetoresistive element shown in the  FIGS. 5A-5C . 
     
    
    
     EXPLENATION OF REFERENCE NUMERALS 
       10 ,  20 ,  30 ,  40  magnetoresistive element 
       12  free (or storage) ferromagnetic layer 
       14  pinned (or reference) ferromagnetic layer 
       16 A,  16 B pinning layer 
       18  tunnel barrier layer 
       22  seed layer (or underlayer) 
       24  cap layer (or overlayer) 
       26  interface layer 
       32  spacer layer 
       52  getter layer 
       60  method of fabricating the magnetoresistive element  10   
       61 ,  62 ,  63 ,  64 , steps of the fabricating method  60   
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be explained below with reference to the accompanying drawings. Note that in the following explanation the same reference numerals denote constituent elements having almost the same functions and arrangements, and a repetitive explanation will be made only when necessary. Since, however, each figure is an exemplary view, it should be noted that the relationship between the thickness and planar dimensions, the ratio of the thicknesses of layers, and the like are different from the actual ones. Accordingly, practical thicknesses and dimensions should be judged in consideration of the following explanation. 
     Note also that each embodiment to be presented below merely discloses an device or method for embodying the technical idea of the present disclosure. Therefore, the technical idea of the present disclosure does not limit the materials, shapes, structures, arrangements, and the like of constituent parts to those described below. The technical idea of the present disclosure can be variously changed within the scope of the appended claims. 
     Refining now to the drawings,  FIGS. 1-4  illustrate exemplary aspects of the present disclosure. Specifically, these figures illustrate MR element having a multilayer structure with a perpendicular direction of magnetization in magnetic layers. The direction of the magnetization in the magnetic layers are shown by solid or dashed arrows. The MR element can store binary data by using steady logic states determined by a mutual orientation of the magnetizations in the magnetic layers separated by a tunnel barrier layer. The logic state “0” or “1” of the MR element can be changed by a spin-polarized current running through the element in the direction perpendicular to layers surface (or substrate). The MR element may be included as a part of magnetic random access memory (MRAM). 
     The MR element herein mentioned in this specification and in the scope of claims is a general term of a tunneling magnetoresistance (TMR) element using a nonmagnetic insulator or semiconductor as a tunnel barrier layer. Although the following figures each illustrate the major components of the MR element, another layer may also be included as long as the arrangement shown in the figure is included. 
     In particular, as illustrated in  FIGS. 1A and 1B , an exemplary aspect of the present disclosure includes a MR element  10 , which comprises at least a free magnetic layer  12 , a pinned magnetic layer  14 , a pinning layer  16 A, a tunnel barrier layer  18  disposed in-between the free  12  and pinned  14  layers. The magnetic layers  12  and  14 , and the pinning layer  16 A have magnetization directions directed substantially perpendicular to the layers surface (or substrate, not shown) in their equilibrium state. Direction of the magnetizations in the layers  12 ,  14  and  16 A in the equilibrium state is indicated by arrows. The magnetization direction of the free layer  12  (shown by dashed arrow) can be reversed by a spin-polarized current I S  running through the element  10  in a direction perpendicular to the layers surface (substrate). The magnetization direction of the pinned layer  14  (solid arrow) is fixed. The pinned magnetic layer  14  may also have a coercive force substantially larger than that of the free layer  12 . 
     The magnetization direction of the pinned layer  14  is fixed by a substantial exchange coupling with the pinning layer  16 A. The pinned layer  14  has a direct contact with the pinning layer  16 A. The direction of the magnetizations in the layers  14  and  16 A are antiparallel to each other. The antiparallel orientation of the magnetizations cancels a fringing (or leakage) magnetic field produced by the layer  14  in the vicinity of the free layer  12 . For example the magnetization direction of the pinned layer  14  is directed downward (shown by a solid “down” arrow). Respectively, the magnetization direction of the pinning layer  16 A is directed “upward” (shown by the solid “upward” arrow). The direction of the magnetizations in the layers  14  and  16 A may have opposite configuration with the upward direction in the pinned layer  14  and downward direction in the pinning layer  16 A. The MR element  10  further includes a seed layer  22  and a cap layer  24 . Wafer and conductive electrodes are not shown. 
     The pinned layer  14  may produce a substantial fringing (leakage) magnetic field affecting the free layer  12 . The fringing magnetic field interferes with switching characteristics and thermal stability of the MR element  10 . Accordingly, a reduction of the fringing field produced by the pinned layer  14  is essential for performance of the MR element  10 . 
     One of the methods for reducing the leakage magnetic field from the pinned layer  14  having a perpendicular magnetization (anisotropy) is using a ferrimagnetic material. The pinning layer  16 A of the MR element  10  comprises the ferrimagnetic rear-earth-transition metal (RE-MT) alloy such as TbCoFe. Typical examples of RE are Tb, Gd, Dy, Ho, and Er that stabilize the perpendicular magnetization in RE-CoFe alloys. The Re-FeCo alloys can have an amorphous structure. When the RE composition of the RE-FeCo alloy is close to a compensation point, the saturation magnetization M S  becomes almost zero. At the compensation point the cause of the saturation magnetization M S  changes from FeCo to the RE element and vice-versa, and the coercive force H C  has a maxixmum. The pining layer  16 A made of the RE-rich alloy can produce a substantial antiferromagnetic exchange coupling with the pinned magnetic layer  14 . Accordingly, the total (net-M S ) saturation magnetization of the structure composed by the pinned layer  14  and pinning layer  16 A can be set almost zero by controlling the content of the RE-rich alloy or by controlling a thickness of the pinning layer  16 A. 
     A cancelation of the fringing magnetic field produced by the pinned layer in the vicinity of the free layer  12  is essential for improving the thermal stability and reduction of the switching current of the MR element  10 . The fringing magnetic field causes an unwanted bias of the free layer  12 . This results in different magnitudes of the spin polarized current required for switching of the MR element  10  from a logic state “0” to logic state “1” or vice-versa. Besides, the biased MR element  10  can have a reduced stability along the direction of the fringing filed. 
     In the given exemplarily embodiment the free layer  12  and the pinned layer  14  can be produced by a sputter deposition in a vacuum chamber from a target made of (Co 25 Fe 75 ) 85 B 15 (at. %) alloy. The thickness of the layers  12  and  14  can be about 1.8 nm and 2.5 nm, respectively. The tunnel barrier layer  18  can be made of MgO and have a thickness of about 1.1 nm. The tunnel barrier layer  18  can be made by a RF-sputter deposition in vacuum chamber from MgO target. 
     The free layer  12  can be deposited on the seed layer  22  made of the tantalum (Ta). Thickness of the seed layer  22  can be of about 5 nm. The CoFeB-based free layer  12  can be disposed between the seed  22  and tunnel barrier  18  layers. The layer  12  can exhibit a substantial perpendicular magnetic anisotropy due to a surface magnetic anisotropy at the Ta/CoFeB and CoFeB/MgO interfaces. Besides the Ta-based seed layer  22  can serve as a getter for the boron dissolved in the free layer  12 . During annealing the boron can diffuse from the free layer  12  into the seed layer  22 . Reduction of the boron concentration in the free layer  12  can promote its crystallization into bcc (001) texture at reduce annealing temperature. The alternative materials for formation of the seed layer  22  will be described later. 
     Similarly, the crystallization of the CoFeB pinned layer  14  usually starts from its interface with the MgO layer  18 . The MgO layer  18  having stable bcc (001) texture serves as a template and promotes formation of the coherent crystalline bcc (001) texture in the pinned layer  14 . Diffusion of the boron from the layer  14  from the MgO/CoFeB interface into a getter layer (not shown in the  FIGS. 1A and 1B ) can facilitate the crystallization. The alternative materials for formation of the free  12 , pinned  14  and tunnel barrier  18  layers will be described later. 
     Perpendicular magnetization (or anisotropy) in the pinned layer  14  can be provided by a surface magnetic anisotropy at the MgO/CoFeB interface along with a substantial exchange coupling between the pinned  14  and pinning  16 A layers. In the given exemplarily embodiment the pinning layer  16 A can be made of RE-rich alloy with uncompensated magnetic moment such as (Fe 90 Co 10 ) 72 Tb 28  and can have a thickness of about 30 nm. The ferrimagnetic RE-rich pinning layer  16 A can produce a substantial antiferromagnetic exchange coupling with the ferromagnetic pinned layer  14 . The direction of the magnetizations in the antiferromagnetically coupled layers  14  and  16 A layers are antiparallel as shown by solid arrows in  FIGS. 1A and 1B . Hence the net fringing magnetic field of the pinned  14  and pinning  16 A layers in the vicinity of the free layer  12  can be cancelled out. The pinning layer  16 A made of the ferrimagnetic RE-TM alloy can have a substantial perpendicular anisotropy. Therefore the crystallized CoFe(B) pinned layer  14  may have stable uniform perpendicular magnetization. The alternative materials for formation of the pinning layer  16 A will be described later. 
     Furthermore, a cap layer  24  may also be formed on the pinning layer  16 A. In the given exemplarily embodiment the cap layer  24  can be made of platinum (Pt). The cap layer  24  functions as a protective layer that prevents an oxidation of the RE-TM-based pinning layer  16 A. Moreover, the cap layer  24  made of Pt can stabilize the perpendicular anisotropy in the pinning layer  16 A. The cap layer  24  can be about 5 nm thick. The alternative materials for formation of the cap layer  24  will be described later. 
     In the MTJ element  10  the magnetization directions (shown by arrows) of the free  12  and pinned  14  layers may be are arranged in parallel ( FIG. 1A ) or antiparallel ( FIG. 1B ) configuration. Resistance of the MR element  10  depends on mutual direction of the magnetizations in the layers  12  and  14 . The MR element  10  with parallel direction of the magnetizations has a lowest resistance that corresponds to a logic “0”. On the contrary, the MR element with the antiparallel direction of the magnetizations exhibits the highest resistance that corresponds to a logic “1”. A spin-polarized current I S  is used to reverse the direction of the magnetization in the free layer  12  and to change the logic state of the MR element  10 . 
     More specifically, when the spin-polarized current I S  flows from the free layer  12  to the pinned layer  14  through the tunnel barrier layer  18 , as shown in  FIG. 1A  (conductance electrons run in the opposite direction), the electrons storing a spin information of the pinned layer  14  are injected into the free layer  12 . A spin angular momentum of the injected electrons is transferred to electrons of the free layer  12  in accordance with the law of spin angular momentum conservation. This forces the magnetization direction of the free layer  12  to be oriented in parallel with the magnetization direction of the pinned layer  14  (logic “0”). 
     On the other hand, when the spin-polarized write current I S  flows from the pinned layer  14  to the free layer  12 , as shown in  FIG. 1B , the spin-polarized electrons run in the opposite direction from the free layer  12  to the pinned layer  14 . The electrons having spin oriented in parallel to the magnetization direction of the pinned layer  14  are transmitted. The electrons having spin antiparallel to the magnetization direction of the pinned layer  14  are reflected. As a result, the magnetization orientation of the free layer  12  is forced to be directed antiparallel to the magnetization direction of the pinned layer  14  (logic “1”). 
     Reading of the data stored in the MR element  10  is provided by measuring its resistance and comparing it with the resistance of the reference element (not shown). 
       FIG. 2  illustrates another exemplarily embodiment of the of the present disclosure. MR element  20  having a structure similar to the MR element  10 , may further comprise an interface layer  26 . The layer  26  is disposed between the pinned  14  and pinning  16 A layers. The layer  26  provides a smooth interfaces between the layers  14  and  16 A. Besides the layer  26  may reduce the crystalline lattice misfit at the interlace between the substantially crystalline CoFe(B) pinned layer  14  and amorphous TbCoFe pinning layer  16 A. The layer  26  can be made of CoFeB alloy and may have a thickness of about 0.5 nm. The interface layer  26  can promote an exchange coupling between the substantially crystalline pinned  14  and amorphous pinning  16 A layers. Increase of the exchange coupling between the layers  14  and  16 A can improve the perpendicular anisotropy in the pinned layer  14  that can result in the increase of TMR and reduction of the spin-polarized switching current. 
       FIG. 3  shows an alternative exemplary embodiment of the present disclosure. MR element  30  distinguishes from the MR element  10  shown in  FIGS. 1A and 1B  by a structure of the pinning layer. The pinning layer  16 A of the MR element  10  made of the RE-rich alloy can be replaced by a bilayer structure comprising a ferromagnetic pinning layer  16 B and a nonmagnetic spacer layer  32 . The spacer layer  32  is positioned in-between the pinned  14  and pinning  16 B layers and provides a substantial antiferromagnetic exchange coupling between the ferromagnetic layers. 
     The pinning layer  16 B can be made of a ferromagnetic material having a substantial perpendicular anisotropy, for example, of a multilayer [Co(0.2 nm)/Pt(0.2 nm)] 10 , where 10 is a number of Co/Pt bilayers. The spacer layer  32  can be made of a thin layer of ruthenium (Ru) having a thickness of about 0.85 nm. In the given structure a substantial antiferromagnetic exchange coupling between the layers  14  and  16 B can be observed. The direction of the magnetizations (shown by solid arrows) in the pinned  14  and pinning  16 B layers is antiparallel. The antiparallel configuration of the magnetization directions cancels the fringing magnetic field produced by the pinned layer  14  in the vicinity of the free layer  12  that is essential for a high thermal stability and low switching current of the MR element  30 . Alterative materials for the pinning layer  16 B and spacer layer  32  will be described latter. 
     As shown in  FIG. 4 , the interface layer  26  may also be inserted in-between the pined  14  and the spacer  32  layers. The layer  26  can be made of a ferromagnetic material having a substantial spin-polarization and providing a smooth interface between the substantially crystalline CoFe(B) pinned layer  14  and the thin spacer layer  32 . For instance, the interface layer  26  can be made of amorphous (Co 25 Fe 75 ) 85 B 15  alloy having thickness of about 0.5 nm. The sharpness and flatness of the interface between the layers  26  and  32  is required for providing a substantial exchange coupling between the pinned  14  and pinning  16 B layers. The pinning layer  16 B can be made of a multilayer (Co(0.2 nm)/Pt(0.2 nm) 12 . The alternative materials for formation of the interface layer  26  will be disclosed latter. 
     The layer  26  has a substantial ferromagnetic exchange coupling with the ferromagnetic pinned layer  14 . Therefore the layers  14  and  26  can have similar direction of the magnetization and behave like one magnetic volume. The CoFe(B) pinned layer  14  can have a substantially crystalline texture after annealing and may have a rough top surface after etching that facing the spacer layer  32 . The roughness can deteriorate a strength of the antiferromagnetic exchange coupling between the layers  14  and  16 B through ultrathin spacer layer  32  resulting in the degradation of perpendicular anisotropy of the layer  14 . To provide a smooth interface between the layers  14  and  32  the layer  26  can be used. As a result, the magnetization directions of the pinned  14  and pinning  16 B layers are oriented antiparallel to each other. This configuration of the magnetizations leads to a cancelation of the fringing field in the vicinity of the free layer  12 . 
     Another aspect of the present disclosure includes a method of fabricating an MR element.  FIGS. 5A-5C  illustrate exemplary steps of the method of fabricating a spin-current switchable MR element  10  shown in  FIGS. 1A and 1B . 
     As shown in  FIG. 5A , a layer  22  made of tantalum (Ta) and having a thickness of about 5 nm is formed on a substrate (not shown) as a seed layer. A free layer  12  made of amorphous CoFeB having a thickness of about 1.5 nm is formed on the seed layer  22 . An MgO layer having a thickness of about 1.1 nm is formed on the free layer  12  as a tunnel barrier layer  18 . An amorphous CoFeB layer having a thickness of about 2.5 nrn is formed on the tunnel barrier layer  18  as a pinned layer  14 . To promote a crystallization of the as-deposited amorphous pinned layer  14  into desirable bcc (001) texture during following annealing, a getter layer  52  made of Zr and having a thickness of about 3 nm is formed on the pinned layer  14 . All the above deposited layers can be formed by a sputter deposition in a vacuum chamber without breaking the vacuum. Then annealing is performed at a temperature about 350° C. for about 1 hour. A magnetic field about 100 Oe or above may be applied during annealing in parallel or perpendicular direction relatively to a substrate surface. 
     Crystallization of the CoFeB requires a reduction of the boron content in the layers. Excessive amount of the boron diffuses into adjacent layers of the MTJ. Diffusion of the boron into MgO barrier layer is undesirable since it may cause a destruction of coherent tunneling resulting in TMR reduction and switching current increase. In order to reduce the accumulation of the boron in the MgO barrier during annealing, boron “getter” layer  52  can be used. 
     Tantalum seed layer  22  serves both as a getter layer for the boron dissolved in the layer  12  and as a promoter of perpendicular magnetization direction in the free layer. Surface magnetic anisotropy at Ta/CoFe(B) interface can serves as a source of the perpendicular anisotropy in the CoFe(B) free layer  12 . Moreover, the perpendicular magnetization direction in the free layer  12  can be promoted by the surface magnetic anisotropy at the interface CoFe(B)/MgO formed by free  12  and tunnel barrier  18  layers. Similarly, the MgO/CoFe(B) interface between the tunnel barrier  18  and pinned  14  layers promotes the perpendicular anisotropy in the pinned layer made of CoFe(B). 
     The getter layer  52  can be made of zirconium (Zr) and may have a thickness of about 3 nm. The getter layer can absorb an excess of the boron in the pinned layer  14  and facilitate the crystallization of the layer  14  in desirable bcc (001) texture. After annealing the getter layer  52  and adjacent portion of the pinned layer  14  can be removed by etching in a vacuum chamber ( FIG. 5B ). The getter layer  52  may cause a non-uniform distribution of the boron across the thickness of the pinned layer  14  during annealing with the boron concentration decreasing from the MgO/CoFeB interface towards CoFeB/Zr interface. The crystallization of the as-deposited amorphous CoFeB pinned layer  14  into bcc (001) texture may start from the MgO/CoFeB interface. The remote portion of the pinned layer  14  adjacent to the getter layer  52  may remain amorphous due to a substantial concentration of the boron. This portion of the pinned layer  14  alone with the getter layer  52  may be removed by post-annealing etching. The remaining after etching crystallized CoFe(B) layer  14  may have the desirable bcc (001) texture that is coherent with a crystalline texture of the MgO layer  18 . 
     Then without breaking the vacuum after etching an (Fe 90 Co 10 ) 72 Tb 28  layer having a thickness of about 30 nm can be formed on the etched surface of the pinned layer  14  as a pinning layer  16 A. Further, a platinum (Pt) layer having a thickness of about 5 nm can be formed on the pinning layer  16 A as a cap layer  24 . The cap layer  24  can stabilize the perpendicular anisotropy in the pinning layer  16 A and protect the pinning layer made of the RE-TM alloy from the oxidation during following processing. 
       FIG. 6  illustrates an exemplary aspect of a method  60  for fabricating a spin-current switchable MR element  10  shown in  FIGS. 5A-5B  according to the exemplary aspects of the present disclosure. As illustrated in  FIG. 6 , the method  60  includes: ( 61 ) forming on a substrate a first plurality of layers such that interfaces between the first plurality of layers are formed in situ, the first plurality of layers comprising: a first nonmagnetic conductive layer, a free ferromagnetic layer comprising an amorphous structure, a nonmagnetic insulating tunnel barrier layer, a pinned ferromagnetic layer comprising an amorphous structure, and a getter layer; ( 62 ) annealing the first plurality of layers; ( 63 ) removing the getter layer and a portion of the pinned ferromagnetic layer adjacent to the getter layer; ( 64 ) forming above the pinned ferromagnetic layer a second plurality of layers such that interfaces between the pinned ferromagnetic layer and the second plurality of layers are formed in situ, the second plurality of layers sequentially comprising: a pinning magnetic layer comprising a fixed magnetization direction directed substantially perpendicular to a film surface, and a second nonmagnetic conductive layer. 
     It should be noted that the layers of the exemplary MR elements  10 ,  20 ,  30 , and  40  disclosed above can be made of several alternative materials and may have different thickness. For example, the free magnetic layer  12  can be made of ferromagnetic material containing at least one element of the group consisting of Fe, Co and Ni, their based alloys such as CoFe, CoFeB, FeCoNi, FePt, FePd, FeB, FeNi and similar, and their based laminates such as CoFeB/Fe, CoFeB/CoFe, CoFeB/NiFe, CoFe/Fe, Co/Ni and similar. Thickness of the free layer  12  can be in a range from about 0.5 nm to about 3 nm. The free magnetic layer should have a substantial spin polarization and provide a required thermal stability factor Δ=K U V/k B T, where K U  is a crystal magnetic anisotropy, V is a volume of the free magnetic layer, k B  is a Boltzmann constant, and T is a temperature. Usually the thermal stability factor Δ is about 60·k B T. 
     The pinned magnetic layer  14  can be made of the ferromagnetic materials similar to the materials used for free layer formation. Thickness of the pinned layer  14  can be in a range from about 0.5 nm to about 10 nm. 
     The pinning layer  16 A can be made of ferrimagnetic material having a perpendicular magnetic anisotropy and containing at least one element selected from the group consisting of Fe, Co, and Ni, and at least one element selected from the group consisting from Gd, Tb, Dy, Ho, and Er such as GdFe, TbFe, DyFe, TbCo, GdCo, TbCoFe, GdFeCo and similar. To provide an antiferromagnetic exchange coupling with the pinned ferromagnetic layer  14  the pinning  16 A layer can have an RE-rich composition. Thickness of the pinning ferrimagnetic layer  16 A may be in a range from about 5 nm to about 75 nm. 
     The ferromagnetic pinning layer  16 B can be made of a ferromagnetic material having a substantial crystal perpendicular anisotropy and containing at least one element selected from the group consisting of Fe, Co and Ni such as CoPt, CoCrPt, CoPtTa, FePt, FePd and similar, their based laminates such as Co/Pt, CoFe/Pt, CoFe/Pd, Fe/Pt, Fe/Pd, Ni/Co and similar. Thickness of the pinning ferromagnetic layer  16 B may be in a range from about 2.5 nm to about 25 nm. 
     The tunnel barrier layer  18  can be made of an oxide or semiconductor such MgO X , AlO X , TaO X , TiO X , NbO X , Si, Ge, C, SiC, SiGe and similar, and their based laminates. Thickness of the tunnel barrier layer  18  may be in a range from about 0.5 nm to about 3 nm. 
     The seed layer  22  can be made of material containing at least one element selected from the group consisting of Ta, W, V, Cr, Nb, Mo, Cu, and Ag, their based alloys and laminates. The thickness of the seed layer  22  can be in a range from about 0.5 nm to about 50 nm. 
     The cap layer  24  can be made material containing at least one element selected from the group consisting of Pt, Pd, Au, Ta, Ru, Ti, Mo, and W, their based alloys and laminates. Thickness of the cap layer  24  can be in a range from about 0.5 nm to about 25 nm. 
     The interface layer  26  can be made of a ferromagnetic material containing at least one element selected from the group consisting of Fe, Co, and Ni, and at least one element selected from the group consisting of B, P, and Zr, such as FeB, CoFeB, FeNiB, CoFeSiB, FeNiP and similar. The thickness of the interface layers  26  can be in range from about 0.25 nm to about 2.5 nm. 
     The spacer layer  32  can be made of material containing at least one element selected from the group consisting of Ru, Ir, Rh, Re, Cu, Cr, V and W, their base alloys such as RuRh and/or laminates such as Ru/Rh. Thickness of the spacer layer  32  can be in a range from about 0.3 nm to about 2.5 nm. 
     The getter layer  52  can be made of material containing at least one element selected from the group consisting of Zr, Ti, Hf, Ta, and Nb, based alloys and laminates. Thickness of the getter layer  52  may be in a range from about 1 nm to about 10 nm. 
     Note that this specification mainly refers to an MR element using magnetic materials having perpendicular anisotropy. However, the present invention is also applicable to an MR element using a magnetic layer having in-plane magnetization direction. 
     While the specification of this disclosure contains many specifics, these should not be construed as limitations on the scope of the disclosure or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     It is understood that the above embodiments are intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the embodiments should be, therefore, determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     While the disclosure has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the disclosure can be practiced with modification within the spirit and scope of the appended claims. Specifically, one of ordinary skill in the art will understand that the drawings herein are meant to be illustrative, and the spirit and scope of the disclosure are not limited to the embodiments and aspects disclosed herein but may be modified.

Technology Classification (CPC): 6