Abstract:
A magnetic tunnel junction (MTJ) includes a magnetic free layer, having a variable magnetization direction; an insulating tunnel barrier located adjacent to the free layer; a magnetic fixed layer having an invariable magnetization direction, the fixed layer disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy; and one or more of: a composite fixed layer, the composite fixed layer comprising a dusting layer, a spacer layer, and a reference layer; a synthetic antiferromagnetic (SAF) fixed layer structure, the SAF fixed layer structure comprising a SAF spacer located between the fixed layer and a second fixed magnetic layer; and a dipole layer, wherein the free layer is located between the dipole layer and the tunnel barrier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/093,287, filed Apr. 25, 2011, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates generally to the field of magnetoresistive random access memory (MRAM), and more specifically to spin momentum transfer (SMT) MRAM. 
         [0003]    MRAM is a type of solid state memory that uses tunneling magnetoresistance (MR) to store information. MRAM is made up of an electrically connected array of magnetoresistive memory elements, referred to as magnetic tunnel junctions (MTJs). Each MTJ includes a magnetic free layer having a magnetization direction that is variable, and a magnetic fixed layer having a magnetization direction that is invariable. The free layer and fixed layer are separated by an insulating non-magnetic tunnel barrier. An MTJ stores information by switching the magnetization state of the free layer. When the magnetization direction of the free layer is parallel to the magnetization direction of the fixed layer, the MTJ is in a low resistance state. When the magnetization direction of the free layer is anti-parallel to the magnetization direction of the fixed layer, the MTJ is in a high resistance state. The difference in resistance of the MTJ may be used to indicate a logical ‘1’ or ‘0’, thereby storing a bit of information. The MR of an MTJ determines the difference in resistance between the high and low resistance states. A relatively high difference between the high and low resistance states facilitates read operations in the MRAM. 
         [0004]    The magnetization state of the free layer may be changed by a spin torque switched (STT) write method, in which a write current is applied in a direction perpendicular to the film plane of the magnetic films forming the MTJ. The write current has a tunneling magnetoresistive effect, so as to change (or reverse) the magnetization state of the free layer of the MTJ. In STT magnetization reversal, the write current required for the magnetization reversal is determined by the current density. As the area of the surface in an MTJ on which the write current flows becomes smaller, the write current required for reversing the magnetization of the free layer of the MTJ becomes smaller. Therefore, if writing is performed with fixed current density, the necessary write current becomes smaller as the MTJ size becomes smaller. 
         [0005]    MTJs that include material layers that exhibit perpendicular anisotropy (PMA) may be switched with a relatively low current density as compared to MTJs having in-plane magnetic anisotropy, also lowering the total necessary write current. However, MTJs made using PMA materials may have a relatively low MR because of structural and chemical incompatibility between the various material layers that comprise a PMA MTJ. A relatively low MR may result in difficulty with read operations in the STT MRAM, as the difference in resistance between the high and low resistance states of the MTJs will also be relatively low. In a PMA MTJ, the fixed layer and the free layer may be magnetized in directions that are parallel or anti-parallel to one another, and the fixed layer may apply a relatively strong dipolar field to the free layer. The fixed layer dipolar field may offset the free layer loop by about 1000 oersteds (Oe) or more. The free layer H c  needs to be higher than the offset field from the fixed layer; otherwise, there is only one stable resistance state instead of the two stable resistance states (referred to as bistability) needed to store information in the MTJ. 
         [0006]    MRAM is formed from a layered sheet comprising a magnetic stack of the various MTJ layers that is patterned to form individual MTJs. The MTJs may take the form of relatively small cylinders, each comprising the layered magnetic stack. In a sheet film, there is a Neel coupling between the various layers, and the fixed layer dipolar field is not present. The fixed layer dipolar field becomes apparent after the MTJs are patterned, as the dipolar field originates at the edge of the MTJ device. The fixed layer dipolar field becomes stronger as the MTJs are made smaller and is non-uniform across an MTJ. The fixed layer dipolar field creates a number of issues in MTJ devices, including increasing the free layer loop offset and the minimum required free layer H c  needed to ensure bistability of the MTJ. The minimum free layer H c  must be maintained across the full temperature operating range of the device. The fixed layer dipolar field may also change the switching mode of an MTJ, and the impact on device functionality increases as MTJ size is scaled down. 
       BRIEF SUMMARY 
       [0007]    In one aspect, a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes a magnetic free layer, having a variable magnetization direction; an insulating tunnel barrier located adjacent to the free layer; a magnetic fixed layer having an invariable magnetization direction, the fixed layer disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy; and one or more of: a composite fixed layer, the composite fixed layer comprising a dusting layer, a spacer layer, and a reference layer, wherein the spacer layer is located between the reference layer and the tunnel barrier, and wherein the dusting layer is located between the spacer layer and the tunnel barrier; a synthetic antiferromagnetic (SAF) fixed layer structure, the SAF fixed layer structure comprising a SAF spacer located between the fixed layer and a second fixed magnetic layer, wherein the fixed layer and the second fixed magnetic layer are anti-parallely coupled through the SAF spacer; and a dipole layer, wherein the free layer is located between the dipole layer and the tunnel barrier. 
         [0008]    In another aspect, a method of forming a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes forming a magnetic free layer having a variable magnetization direction; forming a tunnel barrier over the free layer, the tunnel barrier comprising an insulating material; forming a magnetic fixed layer having an invariable magnetization direction over the tunnel barrier, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy; and forming one or more of: a composite fixed layer, the composite fixed layer comprising a dusting layer, a spacer layer, and a reference layer, wherein the spacer layer is located between the reference layer and the tunnel barrier, and wherein the dusting layer is located between the spacer layer and the tunnel barrier; a synthetic antiferromagnetic (SAF) fixed layer structure, the SAF fixed layer structure comprising a SAF spacer located between the fixed layer and a second fixed magnetic layer, wherein the fixed layer and the second fixed magnetic layer are anti-parallelly coupled through the SAF spacer; and a dipole layer, wherein the free layer is located between the dipole layer and the tunnel barrier. 
         [0009]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0011]      FIG. 1  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer on top of the magnetic stack. 
           [0012]      FIG. 2  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer on the bottom of the magnetic stack. 
           [0013]      FIG. 3  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer and an SAF structure on top of the magnetic stack. 
           [0014]      FIG. 4  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer and an SAF structure on the bottom of the magnetic stack. 
           [0015]      FIG. 5  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer and an SAF structure on top of the magnetic stack, and a dipole layer on the bottom of the magnetic stack. 
           [0016]      FIG. 6  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer and an SAF structure on the bottom of the magnetic stack, and a dipole layer on top of the magnetic stack. 
           [0017]      FIG. 7  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer on top of the magnetic stack, and a dipole layer on the bottom of the magnetic stack. 
           [0018]      FIG. 8  is a cross sectional view illustrating an embodiment of a magnetic stack with a composite fixed layer on the bottom of the magnetic stack, and a dipole layer on top of the magnetic stack. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments of PMA magnetic stacks for STT MRAM are provided, with exemplary embodiments being discussed below in detail. The PMA magnetic stacks form MTJs that exhibit a high MR and a reduced fixed layer dipolar field thus commensurately reduced free layer loop offset through inclusion of one or more of a composite fixed layer, a synthetic antiferromagnetic (SAF) structure in the fixed layer, and a dipole layer. A composite fixed layer includes three layers: a dusting layer, a spacer layer, and a reference layer. A fixed layer SAF structure includes a SAF spacer located between two layers of magnetic material that are anti-parallelly coupled through the SAF spacer. The magnetization of the two layers of magnetic material in the SAF structure may be adjusted such that they are aligned opposite to one another, reducing the overall fixed layer dipole field. A dipole layer is located on the opposite side of the free layer from the tunnel barrier, and may be magnetized in a direction opposite to the fixed layer to cancel out the fixed layer dipole field. A PMA MTJ may be formed with one or more of the composite fixed layer, the SAF structure, and the dipole layer, in any combination, to reduce the fixed layer dipole field and free layer loop offset of the PMA MTJ. 
         [0020]    Referring initially to  FIG. 1 , there is shown a cross sectional view of a PMA magnetic stack  100  with a composite fixed layer  107 . The composite fixed layer  107  includes dusting layer  104 , spacer  105 , and reference layer  106 . The MTJ  100  also includes a free layer  102  that is grown on a seed layer  101 . Seed layer  101  may include tantalum (Ta), or tantalum magnesium (TaMg) with a percentage of Mg that is less than 20%, in some embodiments. The thickness of seed layer  102  may be about 0.5 nanometers (nm) or more, and from about 1 nm to about 3 nm in some exemplary embodiments. Free layer  102  may include cobalt-iron-boron (CoFeB) with various compositions; the Co composition may be less than 90%, and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  102  may also comprise CoFeB|Fe or Fe|CoFeB. The thickness of the free layer  102  may be from about 0.6 nm to about 2 nm, depending on the composition of free layer  102 . Tunnel barrier  103  is located on free layer  102 , and comprises an insulating material such as magnesium oxide (MgO). An MgO tunnel barrier  103  can be formed by natural oxidation, radical oxidation or radiofrequency (RF) sputtering. 
         [0021]    In the embodiment shown in  FIG. 1 , composite fixed layer  107  is located on top of tunnel barrier  103 . The dusting layer  104  and the reference layer  106  are magnetically coupled through the spacer  105  and have PMA. The dusting layer  104  may be pure CoFeB, CoFe, Fe, or bilayers of Fe|CoFeB, CoFe|CoFeB, CoFeB|Fe or CoFeB|CoFe in various embodiments. The thickness of the dusting layer  104  may be from about 0.5 nm to about 2 nm. The spacer  105  comprises a non-magnetic material, such as chromium (Cr), ruthenium (Ru), titanium nitride (TiN), titanium (Ti), vanadium (V), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), magnesium (Mg) or oxides such as MgO in various embodiments. The thickness of the spacer  105  may be from about 0.1 nm to about 1 nm in some embodiments, or thicker than 1 nm in other embodiments; the thickness of spacer  105  must allow dusting layer  104  and reference layer  106  to be magnetically coupled together through spacer  105 . The spacer layer  105  may alternately have a tri-layer structure, with a relatively thin center magnetic layer (which may comprise CoFeB, Fe, or CoFe) sandwiched by two non-magnetic layers (which may comprise Cr, Ru, TiN, Ti, V, Ta, TaN, Al, Mg or oxides such as MgO) in some embodiments. For a tri-layer spacer  105 , the thickness of the center magnetic layer may be from about 0.1 nm to about 0.5 nm. Reference layer  106  may comprise cobalt-platinum (Co|Pt) or cobalt-palladium (Co|Pd), in multilayers or a mixture, in various embodiments. Additional embodiments of MTJs that may comprise a composite fixed layer including a dusting layer, spacer, and reference layer are shown in  FIGS. 2-8 ; composite fixed layers  204 ,  307 ,  406 ,  507 ,  606 ,  707 , and  804  may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1 . 
         [0022]      FIG. 2  shows an embodiment of a PMA magnetic stack  200  with the composite fixed layer  204  located on the bottom. The composite fixed layer  204  includes dusting layer  203 , spacer  202 , and reference layer  201 . Magnetic stack  200  further includes tunnel barrier  205  and free layer  206 . Tunnel barrier  205  comprises an insulating material such as MgO. An MgO tunnel barrier  205  can be formed by natural oxidation, radical oxidation or RF sputtering. Free layer  206  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  206  may also comprise CoFeB|Fe or Fe|CoFeB. 
         [0023]    A composite fixed layer may be incorporated into a SAF fixed layer structure, as shown in PMA magnetic stacks  300  and  400  of  FIGS. 3 and 4 . By adjusting the magnetic moments and the magnetic anisotropy of the magnetic material located on either side of the SAF spacer in the SAF structure such that the magnetic moments are aligned opposite to one another, a reduced fixed layer dipole field and a centered (or zero offset) free layer loop can be obtained. In  FIG. 3 , composite fixed layer  307 , including dusting layer  304 , spacer  305 , and reference layer  306 , is anti-parallelly coupled to a top reference layer  309  through an SAF spacer  308 . Top reference layer  309  may comprise cobalt-nickel (Co|Ni), Co|Pd or Co|Pt, in multilayers or a mixture. SAF spacer  308  may comprise Ru, and may be from about 8 angstroms to about 10 angstroms thick in some embodiments. MTJ  300  further includes seed layer  301 , free layer  302 , and tunnel barrier  303 . Seed layer  301  may include Ta, or TaMg with a percentage of Mg that is less than 20%, in some embodiments. The seed layer  301  thickness may be about 0.5 nm or more, and from about 1 nm to about 3 nm in some exemplary embodiments. Free layer  302  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  302  may also comprise CoFeB|Fe or Fe|CoFeB. The thickness of the free layer  302  may be from about 0.6 nm to about 2 nm depending on the free layer composition. Tunnel barrier  303  is located on free layer  302 , and comprises an insulating material such as MgO. An MgO tunnel barrier  303  can be formed by natural oxidation, radical oxidation or radiofrequency (RF) sputtering. Composite fixed layer  307  may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of an MTJ with a fixed layer SAF structure, composite fixed layer  307  may be replaced by a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  304  and spacer  305 . 
         [0024]    In  FIG. 4 , PMA magnetic stack  400  includes a composite fixed layer  406 , with dusting layer  405 , spacer  404 , and reference layer  403 , anti-parallelly coupled through SAF spacer  402  to bottom reference layer  401 . Bottom reference layer  401  may comprise Co|Ni, Co|Pd or Co|Pt, in multilayers or a mixture. SAF spacer  402  may comprise Ru, and may be from about 8 angstroms to about 10 angstroms thick in some embodiments. PMA magnetic stack  400  further includes tunnel barrier  407  and free layer  408 . Tunnel barrier  407  comprises an insulating material such as MgO. An MgO tunnel barrier  407  can be formed by natural oxidation, radical oxidation or RF sputtering. Free layer  408  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  408  may also comprise CoFeB|Fe or Fe|CoFeB. Composite fixed layer  406  may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of a PMA magnetic stack with a fixed layer SAF structure, composite fixed layer  406  may be replaced by a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  405  and spacer  404 . 
         [0025]    In the embodiments of PMA magnetic stacks  300  and  400  with fixed layer SAF structures shown in  FIGS. 3 and 4 , because top reference layer  309 /bottom reference layer  401  is further away from the free layer  302 / 408  than the composite fixed layer  307 / 406 , the magnetic moment of top reference layer  309 /bottom reference layer  401  has to be larger than the magnetic moment of composite fixed layer  307 / 406  in order to cancel the dipole field from composite fixed layer  307 / 406 . In the case that both reference layer  306 / 403  and top reference layer  309 /bottom reference layer  401  comprise multilayers, top reference layer  309 /bottom reference layer  401  must include more layer repeats, and therefore be thicker, than reference layer  306 / 403 . As top reference layer  309 /bottom reference layer  401  is made thicker, the compensation effect of additional layer repeats becomes weaker, as the additional layer repeats are further away from the free layer  302 / 408 . Thicker multilayer stacks may also lead to longer deposition times during magnetic stack fabrication and more materials expense. Therefore, a dipole layer that is magnetized in a direction that is the same as that of top reference layer  309 /bottom reference layer  401  and opposite to that of the reference layer  306 / 403  may be incorporated into the PMA magnetic stack in addition to the fixed layer SAF structure so as to reduce the thickness of the top reference layer  309 /bottom reference layer  401  that is necessary to cancel out the dipole field from the composite fixed layer  307 / 406 . Magnetic stacks including a composite fixed layer, a fixed layer SAF structure, and a dipole layer are shown in  FIGS. 5 and 6 . Alternately, the dipole layer may be incorporated into a PMA magnetic stack that omits the fixed layer SAF structure, as shown below with respect to  FIGS. 7 and 8 , and the dipole layer may be magnetized in a direction that is opposite to the fixed layer. 
         [0026]    The PMA magnetic stack  500  of  FIG. 5  includes a fixed layer SAF structure with composite fixed layer  507  anti-parallelly coupled to a top reference layer  509  through an SAF spacer  508 . Top reference layer  509  may comprise Co|Ni, Co|Pd or Co|Pt, in multilayers or a mixture. SAF spacer  508  may comprise Ru, and may be from about 8 angstroms to about 10 angstroms thick in some embodiments. The dipole layer  510  may comprise cobalt chromium platinum (CoCrPt), Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To increase the magnetic moment and reduce the thickness of the dipole layer  510 , a CoFeB dusting layer (not shown), which has relatively large saturation moment, may be grown directly on top of the CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole layer  510  in some embodiments. Dipole layer  510  is magnetized in a direction that is the same as that of top reference layer  509  and opposite to that of the reference layer  506  to cancel out the fixed layer dipole field and reduce the loop offset of the free layer  502 . A seed layer  501  located between the free layer  502  and dipole layer  510  provided magnetic separation between the dipole layer  510  and free layer  502 . Seed layer  501  may include Ta, or TaMg with a percentage of Mg that is less than 20%, in some embodiments. The seed layer  501  thickness may be 0.5 nm or more, and from about 1 nm to about 3 nm in some exemplary embodiments. Free layer  502  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  502  may also comprise CoFeB|Fe or Fe|CoFeB. The thickness of the free layer  502  may be from about 0.6 nm to about 2 nm depending on the free layer composition. Tunnel barrier  503  is located on free layer  502 , and comprises an insulating material such as MgO. An MgO tunnel barrier  503  can be formed by natural oxidation, radical oxidation or RF sputtering. Composite fixed layer  507 , including dusting layer  504 , spacer  505 , and reference layer  506 , may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of a PMA magnetic stack with a fixed layer SAF structure and a dipole layer, composite fixed layer  507  may be a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  504  and spacer  505 . 
         [0027]    In  FIG. 6 , PMA magnetic stack  600  includes a fixed layer SAF structure including composite fixed layer  606  anti-parallelly coupled through SAF spacer  602  to bottom reference layer  601 . Bottom reference layer  601  may comprise Co|Ni, Co|Pd or Co|Pt, in multilayers or a mixture. SAF spacer  602  may comprise Ru, and may be from about 8 angstroms to about 10 angstroms thick in some embodiments. The dipole layer  609  may comprise CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To increase the magnetic moment and reduce the thickness of the dipole layer  609 , a CoFeB dusting layer (not shown), which has relatively large saturation moment, may be grown directly on the bottom of the CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole layer  609  in some embodiments. Dipole layer  609  is magnetized in a direction that is the same as that of bottom reference layer  601  and opposite to that of the reference layer  603  to cancel out the fixed layer dipole field and reduce the loop offset of the free layer  608 . A cap layer  610  located between the free layer  608  and dipole layer  609  provides magnetic separation between the dipole layer  609  and free layer  608 . Tunnel barrier  607  comprises an insulating material such as MgO. An MgO tunnel barrier  607  can be formed by natural oxidation, radical oxidation or RF sputtering. Free layer  608  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  608  may also comprise CoFeB|Fe or Fe|CoFeB. Composite fixed layer  606 , with dusting layer  605 , spacer  604 , and reference layer  603 , may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of a PMA magnetic stack with a fixed layer SAF structure and a dipole layer, composite fixed layer  606  may be a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  605  and spacer  604 . 
         [0028]    The dipole fields generated by top reference layer  509 /bottom reference layer  601  and dipole layers  510  and  609  in  FIGS. 5 and 6  together compensate the dipole field generated by the reference layer  506 / 603 . As long as the H c  of composite fixed layer  507 / 606  is either greater or less than the H c  of both the top reference layer  509 /bottom reference layer  601  and the dipole layer  510 / 609 , the magnetization of the three layers can be adjusted to reduce the offset field of free layer  502 / 608 . 
         [0029]    A dipole layer that is magnetized in a direction opposite to the fixed layer may also be used to cancel out the fixed layer dipole field and center the magnetization loop of the free layer in the absence of a fixed layer SAF structure, as shown in  FIGS. 7 and 8 . In  FIG. 7 , PMA magnetic stack  700  includes a composite fixed layer  707  and a dipole layer  708 . The dipole layer  708  may comprise CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To increase the magnetic moment and reduce the thickness of the dipole layer  708 , a CoFeB dusting layer (not shown), which has relatively large saturation moment, may be grown directly on top of the CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole layer  708  in some embodiments. A seed layer  701  located between the free layer  702  and dipole layer  708  provided magnetic separation between the dipole layer  708  and free layer  702 . Seed layer  701  may include Ta, or TaMg with a percentage of Mg that is less than 20%, in some embodiments. The seed layer  701  thickness may be 0.5 nm or more, and from about 1 nm to about 3 nm in some exemplary embodiments. Free layer  702  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from about 10% to about 100% (pure Fe) in various embodiments. Free layer  702  may also comprise CoFeB|Fe or Fe|CoFeB. The thickness of the free layer  702  may be from about 0.6 nm to about 2 nm depending on the free layer composition. Tunnel barrier  703  is located on free layer  702 , and comprises an insulating material such as MgO. An MgO tunnel barrier  703  can be formed by natural oxidation, radical oxidation or RF sputtering. Composite fixed layer  707 , with dusting layer  704 , spacer  705 , and reference layer  706 , may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of a PMA magnetic stack with a dipole layer, composite fixed layer  707  may be a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  704  and spacer  705 . 
         [0030]    In  FIG. 8 , PMA magnetic stack  800  includes composite fixed layer  804  and a dipole layer  807 . The dipole layer  807  may comprise CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To increase the magnetic moment and reduce the thickness of the dipole layers  807 , a CoFeB dusting layer (not shown), which has relatively large saturation moment, may be grown directly on the bottom of the CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole layer  807  in some embodiments. A cap layer  808  located between the free layer  806  and dipole layer  807  provides magnetic separation between the dipole layer  807  and free layer  806 . Tunnel barrier  805  comprises a insulating material such as MgO. An MgO tunnel barrier  805  can be formed by natural oxidation, radical oxidation or radiofrequency (RF) sputtering. Free layer  806  may include CoFeB with various compositions; the Co composition may be less than 90% and the Fe may be from 10% to about 100% (pure Fe) in various embodiments. Free layer  806  may also comprise CoFeB|Fe or Fe|CoFeB. Composite fixed layer  804 , with dusting layer  803 , spacer  802 , and reference layer  801 , may comprise any of the materials, structures, and thicknesses listed above with respect to composite fixed layer  107  of  FIG. 1  in some embodiments. In other embodiments of an PMA magnetic stack with a dipole layer, composite fixed layer  804  may be a simple fixed layer comprising an appropriate magnetic material, omitting dusting layer  803  and spacer  802 . 
         [0031]    In the PMA magnetic stacks  700  and  800  of  FIGS. 7 and 8 , the dipole layer  708 / 807  and the reference layer  706 / 801  are magnetized to opposite directions, such that their dipole fields cancel each other. The H c  of the dipole layer  708 / 807  and the H c  of the reference layer  706 / 801  may not be equal. A wide resetting field window can be achieved compared to the SAF/dipole PMA magnetic stacks  500  and  600  of  FIGS. 5 and 6 . However, a relatively thick dipole layer  708 / 807  is required in PMA magnetic stacks  700  and  800  of  FIGS. 7 and 8 , as compared to the SAF/dipole PMA magnetic stacks  500  and  600  of  FIGS. 5 and 6 . 
         [0032]    The technical effects and benefits of exemplary embodiments include PMA magnetic stacks for MTJs having a relatively high magnetoresistance and a relatively low fixed layer dipolar field and free layer offset loop. 
         [0033]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0034]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.