Patent Publication Number: US-2020286508-A1

Title: Read head including semiconductor spacer and long spin diffusion length nonmagnetic conductive material and method of making thereof

Description:
FIELD 
     The present disclosure relates generally to the field of hard disk drives, and particularly to a read head with a semiconductor spacer layer and methods of manufacturing the same. 
     BACKGROUND 
     Magnetic heads are employed to operate hard disk drives. A magnetic head can include a reading (i.e., read) head and a recording (i.e., writing or write) head. General structures and method of manufacture for prior art magnetic heads are disclosed, for example, in U.S. Patent Application Publication Nos. 2004/0097173 A1; 2007/0230063 A1; 2011/0294398 A1; and 2015/0260757 A1 and U.S. Pat. Nos. 8,291,743 B1; 8,361,541 B1; 8,443,510 B1; 8,717,709 B1; 8,735,565 B2; 8,964,333 B1; 9,153,261 B1; 9,321,146 B2; and 9,390,733 B2 the entire contents of which are incorporated herein. 
     SUMMARY 
     According to an aspect of the present disclosure, a read head includes a first ferromagnetic layer, a second ferromagnetic layer, a first diffusion-assist nonmagnetic metallic layer located between the first ferromagnetic layer and the second ferromagnetic layer, a second diffusion-assist nonmagnetic metallic layer located between the first ferromagnetic layer and the second ferromagnetic layer, and a semiconductor spacer layer located between the first diffusion-assist nonmagnetic metallic layer and the second diffusion-assist nonmagnetic metallic layer. 
     According to another aspect of the present disclosure, a method of forming read head is provided, which comprises: forming a first magnetic shield over a substrate; forming a sensor layer stack including, in order, a first ferromagnetic layer, a first diffusion-assist nonmagnetic metallic layer, a semiconductor spacer layer, a second diffusion-assist nonmagnetic metallic layer, and a second ferromagnetic layer; forming a read sensor stripe by patterning the sensor layer stack; and forming a second magnetic shield over the read sensor stripe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top schematic view of a disk drive including a slider including read head of an embodiment of the present disclosure. 
         FIG. 2  is a side schematic view of the disk drive of  FIG. 1 . 
         FIG. 3  is an in-track vertical cross-sectional view of an exemplary magnetic head incorporating the read head of the present disclosure. 
         FIG. 4A  illustrates a top-down view of an exemplary structure for forming a magnetic head after formation of a sensor layer stack according to an embodiment of the present disclosure. 
         FIG. 4B  is a vertical cross-sectional view of a sensor region of the exemplary structure along the plane B-B′ of  FIG. 4A . 
         FIG. 5A  illustrates a top-down view of the exemplary structure after formation of a read sensor stripe by patterning the sensor layer stack according to an embodiment of the present disclosure. 
         FIG. 5B  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane B-B′ of  FIG. 5A . 
         FIG. 6A  illustrates a top-down view of the exemplary structure after formation of an electrical isolation dielectric layer, a nonmagnetic spacer material layer, and a pair of nonmagnetic spacers according to an embodiment of the present disclosure. 
         FIG. 6B  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane B-B′ of  FIG. 6A . 
         FIG. 7A  illustrates a top-down view of the exemplary structure after formation of a pair of side shields according to an embodiment of the present disclosure. 
         FIG. 7B  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane B-B′ of  FIG. 7A . 
         FIG. 8A  illustrates a top-down view of the exemplary structure after application and patterning a sensor backside edge, after deposition of a dielectric fill material layer and a second magnetic shield according to an embodiment of the present disclosure. 
         FIG. 8B  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane B-B′ of  FIG. 8A . 
         FIG. 8C  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane C-C′ of  FIG. 8A . 
         FIG. 9A  illustrates a top-down view of the exemplary structure after performing a lapping process to form an air bearing surface according to an embodiment of the present disclosure. 
         FIG. 9B  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane B-B′ of  FIG. 9A . 
         FIG. 9C  is a vertical cross-sectional view of the sensor region of the exemplary structure along the plane C-C′ of  FIG. 9A . 
         FIG. 10  is a magnified vertical cross-sectional view of a first exemplary sensor layer stack according to an embodiment of the present disclosure. 
         FIG. 11  is a magnified vertical cross-sectional view of a second exemplary sensor layer stack according to an embodiment of the present disclosure. 
         FIG. 12  is a magnified vertical cross-sectional view of a third exemplary sensor layer stack according to an embodiment of the present disclosure. 
         FIG. 13  is a graph of a magnetoresistance for various sensor layer stacks as a function of a resistance-area (RA) product according to embodiments of the present disclosure. 
         FIG. 14  is an R-H graph of a resistance of a sensor layer stack of an embodiment of the present disclosure as a function of an external magnetic field. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, the present disclosure is directed to a read head including a semiconductor spacer and long spin diffusion length nonmagnetic conductive interlayers, (e.g., charge carrier diffusion-assist interlayers) located between two ferromagnetic layers and methods of manufacturing the same, the various aspects of which are described below in detail. The charge carrier diffusion-assist interlayers include nonmagnetic metal layers, such as Cu, Ag, Au or Ti that have a relatively long electron diffusion length. 
     The drawings are not drawn to scale. Multiple instances of an element may be duplicated where a single instance of the element is illustrated, unless absence of duplication of elements is expressly described or clearly indicated otherwise. Ordinals such as “first,” “second,” and “third” are employed merely to identify similar elements, and different ordinals may be employed across the specification and the claims of the instant disclosure. The same reference numerals refer to the same element or similar element. Unless otherwise indicated, elements having the same reference numerals are presumed to have the same composition. As used herein, a first element located “on” a second element can be located on the exterior side of a surface of the second element or on the interior side of the second element. As used herein, a first element is located “directly on” a second element if there exist a physical contact between a surface of the first element and a surface of the second element. 
     As used herein, a “layer” refers to a material portion including a region having a thickness. A layer may extend over the entirety of an underlying or overlying structure, or may have an extent less than the extent of an underlying or overlying structure. Further, a layer may be a region of a homogeneous or inhomogeneous continuous structure that has a thickness less than the thickness of the continuous structure. For example, a layer may be located between any pair of horizontal planes between, or at, a top surface and a bottom surface of the continuous structure. A layer may extend horizontally, vertically, and/or along a tapered surface. A substrate may be a layer, may include one or more layers therein, or may have one or more layer thereupon, thereabove, and/or therebelow. 
       FIG. 1  is a top schematic view of a hard disk drive  300  including a slider  308  with a read head of an embodiment of the present disclosure.  FIG. 2  is a side schematic view of the slider  308  of  FIG. 1  and illustrates the magnetic head  600  of the embodiments of the present disclosure. The disk drive  300  may include one or more of the disks/media  302  configured to store data. The disks/media  302  reside on a spindle assembly  304  that is mounted to a drive housing  306 . Data may be stored along tracks  307  in the magnetic recording layer of disk  302 . The reading and writing of data is accomplished with the magnetic head  600  that incorporates both the read head (i.e., a reader)  610  and a recording head  660  (i.e., a writer or a writing head). The slider  308  is part of a head-gimbal assembly (HGA) supported by one end of an actuator arm  309 . The opposite end of the actuator arm  309  is connected to a head stack assembly (HSA) which may include a carriage and a voice coil motor  311 . The recording head  660  is used to alter the properties of the magnetic recording layer of disk  302  and thereby write information thereto. The read head  610  is used to read information stored on the magnetic recording layer of the disk  302 . 
     The read head  610  and the recording head  660  are disposed along an air bearing surface ABS of the slider  308 . The ABS is the bottom surface of the slider  308 , which is the slider surface that is the most proximate to the media  302 . The separation distance between the ABS and the media  302  is self-limiting through the air flow between the ABS and the read head  610  and/or the writing head  660 . In operation, a spindle motor (not shown) rotates the spindle assembly  304 , and thereby rotates the disk  302  to position the magnetic head  600  containing the read head  610  and the writing head  660  at a particular location along a desired disk track  307 . The position of the read head  610  and/or the recording head  660  relative to disk  302  may be controlled by a position control circuitry  310  which controls the HSA to move the actuator arm  309 . 
     Referring to  FIG. 3 , an in-track vertical cross-sectional view of an exemplary magnetic head  600  of an embodiment the present disclosure is illustrated. The magnetic head  600  is positioned over a recording track  307  on a disc media  302 . The magnetic head  600  comprises, from the leading side of the head, a read head  610  and a recording (i.e., writing) head  660 . The reading head comprises a lower reading shield  102 , a read sensor  650  (i.e., a reading element), and an upper reading shield  104 . The read sensor  650  can include a sensor layer stack  110  (e.g., magneto-resistive (MR) device) of the embodiments of the present disclosure, such as a giant magneto-resistive (GMR) stack (also referred to as a current perpendicular to plane (CPP) spin valve or CPP-GMR spin valve). The recording head  660  can comprise an optional auxiliary pole  402 , a magnetic coil  425  that is wound around a main pole  420 , a record element  450 , and a trailing shield  480  which may be integrated with an upper pole  482 . The record element  450  is formed between the main pole  420  and the trailing shield  480 . An insulating material portion  470  is provided around the magnetic coil  425  between the main pole  420  and the trailing shield  480 . 
     Referring to  FIGS. 4A and 4B , an exemplary structure for forming a read head of the embodiments of the present disclosure is illustrated. The exemplary structure includes a substrate  101 , which can be, for example, an aluminum titanium carbide substrate. A first magnetic shield  102  is formed within a sensor region of the exemplary structure. The first magnetic shield  102  includes a soft magnetic material, and may have a thickness in a range from 200 nm to 2,000 nm, although lesser and great thicknesses can also be employed. The first magnetic shield  102  can be subsequently patterned to provide the lower reading shield  102  of a magnetic head  600  in a finished product. In one embodiment, the first magnetic shield  102  can comprise, or consist essentially of, NiFe, NiCo, CoFe, NiFeCo, CoB, CoFeB, and/or combinations thereof. 
     A sensor layer stack (e.g., CPP-GMR spin valve)  110  can be deposited over the first magnetic shield  102  in the sensor region by sequential deposition of material layers. The sensor layer stack  110  can include a first ferromagnetic layer  112 , a barrier spacer stack  114 , and a second ferromagnetic layer  116 . In one embodiment, the sensor layer stack  110  can further include a nonmagnetic seed layer  111  underneath the first ferromagnetic layer  112 , and a nonmagnetic cap layer  118  above the second ferromagnetic layer  116 . The nonmagnetic seed layer  111  is also referred to as a backside nonmagnetic conductive layer. The nonmagnetic cap layer  118  is also referred to as a front-side nonmagnetic conductive layer. 
     The nonmagnetic seed layer  111  can include a material layer or a layer stack that facilitates growth of subsequently layers. For example, the nonmagnetic seed layer  111  can include materials such as a graded nickel iron alloy and/or ruthenium, and can have a thickness in a range from 1 nm to 10 nm, such as 1-2 nm, although lesser and greater thicknesses can also be employed. In one embodiment, the nonmagnetic seed layer  111  can include at least one material selected from ruthenium, silver, and titanium. The nonmagnetic seed layer  111  is interposed between the first magnetic shield  102  and the first ferromagnetic layer  112  and functions as template for crystalline growth of grains of the first ferromagnetic layer  112 . The crystallographic grains of the first ferromagnetic layer  112  can be aligned to crystallographic grains of the nonmagnetic seed layer  111 . 
     The first ferromagnetic layer  112  can include at least one ferromagnetic alloy layer. In one embodiment, the first ferromagnetic layer  112  includes a single ferromagnetic alloy layer, such as Heusler alloy layer. The Heusler alloy layer may include any suitable ferromagnetic alloy having a formula M 2 TX, where M is a first transition metal, T is a second transition metal different from the first transition metal and X is an element from Groups 13 to 17 of the Periodic Table of elements. For example, M may be Co, Ni, Fe, Pd and/or Mn, T may be Fe, Mn and/or V and X may be Si, Al, Ge, Sb, Ga and/or Sn. For example, the ferromagnetic alloy may consist essentially a cobalt-iron-aluminum (e.g., Co 2 FeAl) alloy or a cobalt-manganese-germanium (Co 2 MnGe) alloy. 
     In another embodiment, the first ferromagnetic layer  112  includes a ferromagnetic layer stack including at least two ferromagnetic material sublayers configured to tune the magnetostriction. For example, the multi-layer stack can include a stack of a NiFe5% layer and an amorphous CoFeBTa layer and/or a stack of a Ta layer and a CoFeB layer. The above described Heusler alloy layer can be deposited on the multi-layer stack in one embodiment. In yet another embodiment, the first ferromagnetic layer  112  can comprise a layer or a layer stack including various materials such as NiFe, NiCo, CoFe, Fe, NiFeCo, CoB, CoFeB, and/or combinations thereof. 
     The thickness of the first ferromagnetic layer  112  can be in a range from 0.8 nm to 3 nm, such as 1 nm to 2 nm, although lesser and greater thicknesses can also be employed. In one embodiment, the first ferromagnetic layer  112  can be a first free layer having a first magnetization having at least two preferred magnetization directions. The at least two preferred magnetization directions can be an up direction and a down direction if the first ferromagnetic layer  112  has positive axial magnetic anisotropy. 
     In another embodiment, the first ferromagnetic layer  112  can be a pinned layer having a fixed magnetization direction that does not change during operation of the device. In this case, a synthetic antiferromagnetic stack (not shown) can be interposed between the first magnetic shield  102  and the first ferromagnetic layer  112 . For example, the synthetic antiferromagnetic stack can be located between the nonmagnetic seed layer  111  and the first ferromagnetic layer  112 . The fixed magnetization of the first ferromagnetic layer  112  can be is pinned to a magnetization within the synthetic antiferromagnetic stack. 
     The barrier spacer stack  114  includes, from bottom to top, a first diffusion-assist nonmagnetic metallic layer  114 A, a semiconductor spacer layer  114 B, and a second diffusion-assist nonmagnetic metallic layer  114 C. 
     In one embodiment, the diffusion-assist nonmagnetic metallic layers  114 A and  114 C comprise an electrically conductive, nonmagnetic layers which have a relatively long electron diffusion length, and which optionally can function as diffusion barriers which prevent or reduce diffusion of atoms therethrough. In one embodiment, each of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C comprises an elemental metal having a face-centered-cubic (FCC) lattice structure. The FCC lattice structure of layers  114 A and  114 C, which when rotated 45 degrees, can lattice match the Heusler alloy of the first ferromagnetic layer  112  (e.g., a Heusler alloy having a L21, B2 lattice structure) and the semiconductor material of the semiconductor spacer layer  114 B (e.g., CIGS semiconductor material having a chalcopyrite structure). 
     In one embodiment, each of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C can have a thickness in a range from 1 monolayer of the elemental metal to 3 monolayers of the elemental metal. In one embodiment, each of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C comprises a metal selected from Ag, Au, Cu, or Ti. 
     The semiconductor spacer layer  114 B comprises semiconductor material, such as a compound semiconductor material. In one embodiment, the semiconductor spacer layer  114 B includes a compound semiconductor material containing copper and selenium. In one embodiment, the semiconductor spacer layer  114 B includes a material selected from a copper-indium-gallium-selenide (CIGS) material, a copper-indium-selenide (CIS) material, or a copper-gallium-selenide (CGS) material. Other suitable semiconductor materials may also be used. In an illustrative example, the semiconductor spacer layer  114 B can include a material selected from Cu(In x Ga 1-x )Se 2  where 0&lt;x&lt;1, CuInSe 2 , or CuGaSe 2 . 
     In an illustrative example, a copper-indium-gallium-selenide material in a bulk form has a chalcopyrite crystal structure or zincblende crystal structure. In one embodiment, the semiconductor spacer layer  114 B has a thickness in a range from 1 nm to 3 nm, such as 1.5 nm to 2.5 nm. 
     The second ferromagnetic layer  116  can include at least one ferromagnetic alloy layer. In one embodiment, the second ferromagnetic layer  116  include the same or different material from the first ferromagnetic layer  112 . In one embodiment, the second ferromagnetic layer  116  includes a single ferromagnetic alloy layer, such as Heusler alloy layer. The Heusler alloy layer may include any suitable ferromagnetic alloy having a formula M 2 TX, where M is a first transition metal, T is a second transition metal different from the first transition metal and X is an element from Groups 13 to 17 of the Periodic Table of elements. For example, M may be Co, Ni, Fe, Pd and/or Mn, T may be Fe, Mn and/or V and X may be Si, Al, Ge, Sb, Ga and/or Sn. For example, the ferromagnetic alloy may consist essentially a cobalt-iron-aluminum (e.g., Co 2 FeAl) alloy or a cobalt-manganese-germanium (Co 2 MnGe) alloy. 
     In another embodiment, the second ferromagnetic layer  116  includes a ferromagnetic layer stack including at least two ferromagnetic material sublayers configured to tune the magnetostriction. For example, the multi-layer stack can include a stack of a NiFe5% layer and an amorphous CoFeBTa layer, and/or a stack of a Ta layer and a CoFeB layer. The above described Heusler alloy layer can be deposited under the multi-layer stack in one embodiment. In yet another embodiment, the second ferromagnetic layer  116  can comprise a layer or a layer stack including various materials such as NiFe, NiCo, CoFe, Fe, NiFeCo, CoB, CoFeB, and/or combinations thereof. 
     The thickness of the second ferromagnetic layer  116  can be in a range from 0.8 nm to 3 nm, such as 1 nm to 2 nm, although lesser and greater thicknesses can also be employed. In one embodiment, the second ferromagnetic layer  116  can be a free layer having a magnetization having at least two preferred magnetization directions. The at least two preferred magnetization directions can be an up direction and a bottom direction if the second ferromagnetic layer  116  has positive axial magnetic anisotropy. 
     In one embodiment, the first ferromagnetic layer  112  can be a first free layer having a first free magnetization and the second ferromagnetic layer  116  can be a second free layer having a second free magnetization. In this case, the first ferromagnetic layer  112  and the second ferromagnetic layer  116  can have the same type of axial anisotropy, i.e., positive axial anisotropy or negative axial anisotropy. In another embodiment, the second ferromagnetic layer  116  can be a pinned layer having a fixed magnetization direction that does not change during operation of the device. In this case, a synthetic antiferromagnetic stack (not shown) can located over the second ferromagnetic layer  116 . 
     Thus, in one embodiment, one of the first ferromagnetic layer  112  and the second ferromagnetic layer  116  may be a free layer and the other can be a pinned (e.g., reference) layer. In another embodiment both the first and the second ferromagnetic layers  112  and  116  may be free layers. 
     The nonmagnetic cap layer  118  can comprise, or consist essentially of, Ag, Ru, Ta, Ti, and/or combinations thereof. The thickness of the nonmagnetic cap layer  118  can be in a range from 1 nm to 10 nm, such as 1-2 nm, although lesser and greater thicknesses can also be employed. The nonmagnetic cap layer  118  can be interposed between the second ferromagnetic layer  116  and a second magnetic shield to be subsequently formed. 
     The sensor layer stack  110  can be deposited by a series of layer deposition processes such as chemical vapor deposition, atomic layer deposition, and/or physical vapor deposition. In other embodiments, other suitable materials known in the art can be used for any layer within the sensor layer stack  110 . 
     Thermal processing steps can be used during formation of the sensor layer stack  110  to increase the grain size and/or to stabilize the crystal structure in the various material layers of the sensor layer stack  110 . For example, an in-situ post-deposition thermal anneal process can be performed after deposition of the first ferromagnetic layer  112 . A cryogenic cooling treatment (for example, down to at least the boiling point of liquid nitrogen) can be subsequently performed to reduce surface roughness and to enhance film uniformity for the deposited material layers up to the first ferromagnetic layer  112 . The anneal may also cause discontinuities in the first ferromagnetic layer  112 . The cryogenic cooling may remove the discontinuities and may the first ferromagnetic layer  112  continuous. 
       FIGS. 5A to 9C  illustrate optional patterning steps and additional optional layers used to form the read head  610 . It should be noted that other patterning steps may be used and that some of the optional layers may omitted or replaced with other layers. 
     Referring to  FIGS. 5A and 5B , the sensor layer stack  110  is patterned to provide a read sensor stripe  110 S between a pair of recess cavities. The read sensor stripe  110 S can have a substantially uniform vertical cross-sectional view along planes parallel to the air bearing surface to be subsequently formed, which are parallel to the vertical cross-sectional plane of  FIG. 5B . The read sensor stripe  110 S can have a tapered profile such that upper layers within the patterned sensor layer stack  110  have lesser areas than lower layers within the patterned sensor layer stack  110 . 
     The patterning of the sensor layer stack  110  can be performed, for example, by applying a photoresist layer  119  over the blanket (unpatterned) sensor layer stack  110 , lithographically patterning the photoresist layer  119  to form a pair of openings separated by a rectangular area having parallel edges that are perpendicular to the air bearing surface to be subsequently formed, and performing a continuous ion milling process on the layers of the sensor layer stack  110  to provide a pair of openings through the sensor layer stack  110  with tapered sidewalls. The photoresist layer  119  can protect covered regions of the sensor layer stack  110  during the continuous ion milling and subsequent processes. The taper angle on the sidewalls of the patterned sensor layer stack  110  provides continuous reduction of the width of the layers in the sensor layer stack  110  within the read sensor stripe  110 S. 
     Referring to  FIGS. 6A and 6B , an electrical isolation dielectric layer  120  can be formed on the physically exposed top surfaces of the first magnetic shield  102  and on the sidewalls of the sensor layer stack  110 , which include the sidewalls of the read sensor stripe  110 S. The electrical isolation dielectric layer  120  includes a dielectric material that provides electrical isolation, and may be formed by a conformal deposition process. For example, the electrical isolation dielectric layer  120  can comprise, or consist essentially of, aluminum oxide, magnesium oxide, silicon nitride, silicon oxide, and/or combinations or stacks thereof. 
     A nonmagnetic spacer material layer can be deposited on the electrical isolation dielectric layer  120 . In one embodiment, the nonmagnetic spacer material layer can comprise, or consist essentially of, NiFeCr, NiCr, Ta, Ru, Cr, oxides thereof, and/or combinations thereof. 
     An angled milling process can be performed to remove vertical and tapered portions of the nonmagnetic spacer material layer. Specifically, the vertical and tapered portions of the nonmagnetic spacer material layer can be removed along the angled sides of the sensor layer stack  110 . In one embodiment, the angled milling process removes portions of the nonmagnetic spacer material layer along the angled sides of the sensor layer stack  110  located at and above the second ferromagnetic layer  116 . Each remaining portion of nonmagnetic spacer material layer underlying the horizontal plane including the bottom surface of the second ferromagnetic layer  116  constitutes a nonmagnetic spacer  122 . A pair of nonmagnetic spacers  122  is formed on the sidewalls of the read sensor stripe  110 S over planar (horizontal) portions of the electrical isolation dielectric layer  120  that contact the first magnetic shield  102 . Each nonmagnetic spacer  122  has a respective top surface below the horizontal plane including the bottom surface of the second ferromagnetic layer  116 . The pair of nonmagnetic spacers  122  is laterally spaced from the read sensor stripe  110 S by tapered portions of the electrical isolation dielectric layer  120 . A remaining portion of the nonmagnetic spacer material layer overlying the photoresist layer  119  constitutes a nonmagnetic material layer  122 ′. 
     Referring to  FIGS. 7A and 7B , a ferromagnetic side shield material layer can be anisotropically deposited. The ferromagnetic side shield material layer can include iron, cobalt, or a cobalt-iron alloy. A top surface of the ferromagnetic side shield material layer overlying the nonmagnetic spacers  122  can be at the level of the interface between the electrical isolation dielectric layer  120  and the photoresist layer  119 . 
     An angled milling process can be performed to selectively remove vertical and tapered portions of the ferromagnetic side shield material layer. Specifically, the vertical and tapered portions of the ferromagnetic side shield material layer can be removed along the angled sides of the sensor layer stack  110 . In one embodiment, the angled milling process removes portions of the ferromagnetic side shield material layer along the angled sides of the sensor layer stack  110  located above the nonmagnetic cap layer  118 . Each remaining portions of ferromagnetic side shield material layer underlying filling a pair of cavities in the sensor layer stack  110  constitutes a pair of side shields  130 . The pair of side shields  130  is formed on the sidewalls of the electrical isolation dielectric layer  120  over the horizontal plane including the bottom surfaces of the second ferromagnetic layer  116 . The pair of side shields  130  is laterally spaced from the read sensor stripe  110 S by the electrical isolation dielectric layer  120 , and overlies the pair of nonmagnetic spacers  122 . 
     Vertical portions of the electrical isolation dielectric layer  120  that overlie the pair of side shields  130  can be removed by the angled milling process. The photoresist layer  119 , the nonmagnetic material layer  122 ′, and a remaining portion of the ferromagnetic side shield material layer overlying the photoresist layer  119  can be subsequently removed, for example, by a lift-off process that lifts off the photoresist layer  119 . For example, a wet etch process employing a solvent that dissolves, and/or lifts off, the photoresist layer  119  may be employed. The pair of side shields  130  is formed on the electrical isolation dielectric layer  120  on both sides of the read sensor stripe  110 S. 
     The pair of side shields  130  is spaced from the first magnetic shield  102  by a planar (horizontal) portion of the electrical isolation dielectric layer  120  having a planar surface that is parallel to an interface between the first magnetic shield  102  and the sensor layer stack  110 . The pair of side shields  130  can be formed directly on the pair of nonmagnetic spacers  122 . Further, the pair of side shields  130  can be formed directly on a respective tapered sidewall of the electrical isolation dielectric layer  120 . A top surface of the nonmagnetic cap layer  118  can be physically exposed, which may be coplanar with, raised above, or recessed below, top surfaces of the pair of side shields  130 . The pair of side shields  130  provides a magnetic bias to the second ferromagnetic layer  116  and/or in the first ferromagnetic layer  112  along the horizontal direction, which is the cross-track direction during operation of the magnetic head  600 . 
     Referring to  FIGS. 8A-8C , a photoresist layer (not shown) for patterning the backside edge of each magnetic sensor is applied and patterned over the exemplary structure. The photoresist layer is applied and patterned to form an opening having a straight edge that is parallel to the air bearing surface. The straight edge can overlie a back side of the read sensor stripe  110 S and back sides of the pair of side shields  130 . As used herein, a “backside” or “back side” refers to a side that is distal from the air bearing surface to be subsequently formed, and a “front side” refers to a side that is proximal to the air bearing surface to be subsequently formed. In one embodiment, the opening in the photoresist layer can have a substantially rectangular shape. 
     Unmasked portions of the material layers overlying the first magnetic shield  102  are patterned by transferring the pattern of the photoresist layer therethrough. In one embodiment, a first ion milling process can be performed employing the photoresist layer as an ion milling mask layer. A backside edge of the sensor layer stack  110  (e.g., the read sensor stripe  110 S) is formed, which is herein referred to as a sensor backside edge SBE. The sensor backside edge SBE is formed at a periphery of a recess cavity that underlies the opening in the photoresist layer. 
     A dielectric material, such as aluminum oxide, tantalum oxide, silicon oxide or silicon nitride is deposited in the recessed region and over the patterned read sensor stack  110 . Excess portions of the dielectric material is removed from above the horizontal plane including the top surface of the patterned read sensor stack  110 . A remaining portion of the dielectric material forms a dielectric fill layer  280  behind patterned read sensor stack  110 . The dielectric fill layer  280  includes a dielectric material such as aluminum oxide, tantalum oxide, silicon oxide, silicon nitride, or combinations thereof. 
     A second magnetic shield  104  is then formed on the sensor layer stack  110  and the pair of side shields  130 . The second magnetic shield  104  includes a soft magnetic material, and may have a thickness in a range from 200 nm to 2,000 nm, although lesser and great thicknesses can also be employed. The second magnetic shield  104  can be subsequently patterned to provide the upper reading shield  104  of a magnetic head  600  in a finished product. 
     Referring to  FIGS. 9A-9C , the writing head  660  is then formed over the read head  610 . A lapping process is then performed on the exemplary structure to provide an air bearing surface (ABS). 
     Referring to  FIG. 10 , a first exemplary sensor layer stack  110  according to an embodiment of the present disclosure is illustrated, which includes, and in one embodiment may consist only of, from bottom to top, a nonmagnetic seed layer  111 , a first ferromagnetic layer  112 , a barrier spacer stack  114 , a second ferromagnetic layer  116 , and a nonmagnetic cap layer  118 . The barrier spacer stack  114  includes, from bottom to top, a first diffusion-assist nonmagnetic metallic layer  114 A, a semiconductor spacer layer  114 B, and a second diffusion-assist nonmagnetic metallic layer  114 C. The first ferromagnetic layer  112  can be a first free layer, and the second ferromagnetic layer  116  can be a second free layer. 
     Referring to  FIG. 11 , a second exemplary sensor layer stack  110  according to an embodiment of the present disclosure is illustrated, which is configured to provide tunable magnetostriction. At least one magnetostriction modulation layer stack ( 113 ,  117 ) may be included within the sensor layer stack  110 . Specifically, the sensor layer stack  110  can include at least one of a seed-side magnetostriction modulation layer stack  113  and/or a cap-side magnetostriction modulation layer stack  117 . Thus, the sensor layer stack  110  can include, from bottom to top, a nonmagnetic seed layer  111 , an optional seed-side magnetostriction modulation layer stack  113 , a first ferromagnetic layer  112 , a barrier spacer stack  114 , a second ferromagnetic layer  116 , an optional cap-side magnetostriction modulation layer stack  117 , and a nonmagnetic cap layer  118 . The barrier spacer stack  114  includes, from bottom to top, a first diffusion-assist nonmagnetic metallic layer  114 A, a semiconductor spacer layer  114 B, and a second diffusion-assist nonmagnetic metallic layer  114 C. The first ferromagnetic layer  112  can be a first free layer, and the second ferromagnetic layer  116  can be a second free layer. 
     Each of the first ferromagnetic layer  112  and the second ferromagnetic layer  116  can have large positive magnetostriction. The seed-side magnetostriction modulation layer stack  113  and/or the cap-side magnetostriction modulation layer stack  117  can include a negative magnetostriction material layer ( 113 A,  117 A) that can provide negative magnetostriction to reduce the positive magnetostriction provided by the first ferromagnetic layer  112  and the second ferromagnetic layer  116 . The negative magnetostriction material layers ( 113 A,  117 A) can be spaced from the first ferromagnetic layer  112  and the second ferromagnetic layer  116  by a combination of an amorphous nonmagnetic material layer ( 113 B,  117 B) and an amorphous magnetic material layer ( 113 C,  117 C). In one embodiment, the seed-side magnetostriction modulation layer stack  113  can include, from bottom to top (i.e., in a direction toward the barrier spacer stack  114 ), a seed-side negative magnetostriction material layer  113 A, a seed-side amorphous nonmagnetic material layer  113 B, and a seed-side amorphous magnetic material layer  113 C. The cap-side magnetostriction modulation layer stack  117  can include, from bottom to top (i.e., in a direction away from the barrier spacer stack  114 ), a cap-side amorphous magnetic material layer  117 C, a cap-side amorphous nonmagnetic material layer  117 B, and a cap-side negative magnetostriction material layer  117 A. In an illustrative example, the negative magnetostriction material layers ( 113 A,  117 A) can include NiFe5%, the amorphous nonmagnetic material layers ( 113 B,  117 B) can include amorphous tantalum, and the amorphous magnetic material layers ( 113 C,  117 C) can include amorphous CoFeB alloy. 
     Referring to  FIG. 12 , a third exemplary sensor layer stack  110  according to an embodiment of the present disclosure is illustrated, which can be derived from the second exemplary sensor layer stack  110  by inserting a synthetic antiferromagnetic (SAF) structure ( 212 ,  214 ,  216 ) between the nonmagnetic seed layer  111  and the first ferromagnetic layer  112 . The first ferromagnetic layer  112  becomes a pinned (i.e., reference) layer having a fixed magnetization direction, and sensing of magnetization can be performed by the second ferromagnetic layer  116  that becomes the only free layer within the third exemplary sensor layer stack  110 . 
     The SAF structure ( 212 ,  214 ,  216 ) can include an antiferromagnetic pinning layer  212 , a ferromagnetic pinned layer  214 , and a nonmagnetic spacer layer  216 . The anti-ferromagnetic pinning layer  212  can comprise, or consist essentially of, IrMn, IrMnCr, and/or combinations thereof. The ferromagnetic pinned layer  214  can comprise CoFe, CoB, CoFeB, and/or combinations thereof. The nonmagnetic spacer layer  216  includes a nonmagnetic material such as ruthenium. The SAF structure ( 212 ,  214 ,  216 ) fixes the direction of magnetization of the first ferromagnetic layer  112 , causing the first ferromagnetic layer  112  to function as a pinned magnetization layer. 
     In one embodiment, at least one of the seed-side magnetostriction modulation layer stack  113  and/or the cap-side magnetostriction modulation layer stack  117  may be present within the third exemplary sensor layer stack  110 . If the seed-side magnetostriction modulation layer stack  113  is present within the third exemplary sensor layer stack  110 , then the SAF structure ( 212 ,  214 ,  216 ) can be provided between the nonmagnetic seed layer  111  and the seed-side magnetostriction modulation layer stack  113 . 
     In another embodiment, if the cap-side magnetostriction modulation layer stack  117  is present within the third exemplary sensor layer stack  110 , then the negative magnetostriction material layer  113 A such as NiFe5% can be replaced with a cobalt layer. 
     Referring to  FIG. 13 , the relationship between a resistance-area (RA) product (e.g., in units of Ohm-microns square) and MR ratio is calculated and plotted for three sensor layer stacks. The first curve  1310  represents the relationship between RA and MR ratio for a first comparative example sensor layer stack that is derived from the first exemplary sensor layer stack  110  of  FIG. 10  by removing the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C. In other words, only the semiconductor spacer layer  114 B remains out of the barrier spacer layer stack  114  of the first exemplary sensor layer stack  110  of  FIG. 10 . 
     The second curve  1320  represents the relationship between RA and MR ratio for a second exemplary sensor layer stack  110  of  FIG. 10  which includes copper first diffusion-assist nonmagnetic metallic layer  114 A and copper second diffusion-assist nonmagnetic metallic layer  114 C. The third curve  1330  represents the relationship between RA and MR ratio for a third exemplary sensor layer stack  110  of  FIG. 10  which includes a silver first diffusion-assist nonmagnetic metallic layer  114 A and a silver second diffusion-assist nonmagnetic metallic layer  114 C. 
     The use of copper layers as the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C within a barrier spacer layer stack  114  improves MR ratio compared to the comparative example, especially for low RA values. The use of silver layers as the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C within a barrier spacer layer stack  114  improves the MR ratio for low RA values even more than use of the copper layers. Thus, MR can be detected (e.g., has a value between 15 and 39%) for low RA values (e.g., below 0.2, such as between 0.1 and 0.15 Ohm-microns square) for the second and third exemplary sensor layer stacks, but essentially cannot be detected (e.g., MR value below 3%) for the comparative example sensor layer stack. 
     Without being bound by any particular theory, the long electron diffusion length of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C is believed to be a factor in increasing the MR ratio in the barrier spacer layer stack  114  of the embodiments of the present disclosure by reducing interlayer coupling between the first and the second ferromagnetic layers ( 112 ,  116 ). Thus, the long spin diffusion length of the diffusion-assist nonmagnetic metallic layers ( 114 A,  114 C) can keep the high spin polarization of the ferromagnetic layers, such as the Heusler alloy ferromagnetic layers ( 112 ,  116 ) and reduce the interlayer coupling to achieve ultra-low RA with reasonable MR value. 
     Further, it is believed that lattice matching between the crystal structure of the semiconductor spacer layer  114 B and the thin films of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C may contribute to improved semiconductor spacer layer  114 B grain size and interface properties by improving grain growth during deposition of the semiconductor spacer layer  114 B and/or by reducing or preventing interdiffusion between the semiconductor spacer layer  114 B and the ferromagnetic layers ( 112 ,  116 ). 
     Referring to  FIG. 14 , an R-H curve of resistance (in Ohms) as a function of an applied magnetic field (in Oe) is plotted for a sensor layer stack  110  of an embodiment of the present disclosure that includes silver first diffusion-assist nonmagnetic metallic layer  114 A and silver second diffusion-assist nonmagnetic metallic layer  114 C. A resistance-area product of 0.098 Ohm-μm 2  with MR ratio of 39% is used in  FIG. 14 . 
     Referring to all embodiments described above, a read head  610  includes a first ferromagnetic layer  112 , a second ferromagnetic layer  116 , a first diffusion-assist nonmagnetic metallic layer  114 A located between the first ferromagnetic layer and the second ferromagnetic layer, a second diffusion-assist nonmagnetic metallic layer  114 C located between the first ferromagnetic layer and the second ferromagnetic layer, and a semiconductor spacer layer  114 B located between the first diffusion-assist nonmagnetic metallic layer and the second diffusion-assist nonmagnetic metallic layer. 
     In one embodiment, the semiconductor spacer layer  114 B directly contacts both the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C. In one embodiment, the first ferromagnetic layer  112  directly contacts the first diffusion-assist nonmagnetic metallic layer  114 A, and the second ferromagnetic layer  116  directly contacts the second diffusion-assist nonmagnetic metallic layer  114 C. 
     In one embodiment, each of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C comprises a metal selected from Ag, Au, Cu, or Ti. The semiconductor spacer layer  114 B comprises a material selected from copper-indium-gallium-selenide, copper-indium-selenide or copper-gallium-selenide. At least one of the first ferromagnetic layer  112  and the second ferromagnetic layer  116  comprises a ferromagnetic Heusler alloy layer, such as a Co 2 FeAl alloy or a Co 2 MnGe alloy layer 
     In one embodiment, each of the first diffusion-assist nonmagnetic metallic layer  114 A and the second diffusion-assist nonmagnetic metallic layer  114 C comprises an elemental metal selected from Ag, Cu, Au or Ti which has a thickness in a range from 1 monolayer of the elemental metal to 3 monolayers of the elemental metal, the semiconductor spacer layer  114 B has a thickness in a range from 1 nm to 3 nm, and each of the first ferromagnetic layer  112  and the second ferromagnetic layer  116  has a thickness in a range from 0.8 nm to 3 nm. 
     In another embodiment shown in  FIG. 11 , at least one of the first ferromagnetic layer and the second ferromagnetic layer comprises a layer stack comprising a negative magnetostriction material layer ( 113 A,  117 A), an amorphous nonmagnetic material layer ( 113 B,  117 B), an amorphous magnetic material layer ( 113 C,  117 C) and a Heusler alloy magnetic material layer ( 112 ,  116 ). In one embodiment, the negative magnetostriction material layer comprises a NiFe5% alloy, the amorphous nonmagnetic material layer comprises amorphous tantalum which directly contacts the negative magnetostriction material layer, the amorphous magnetic material layer comprises an amorphous CoFeB alloy which directly contacts the amorphous nonmagnetic material layer, and the Heusler alloy magnetic material layer comprises a Co 2 FeAl alloy or a Co 2 MnGe alloy which directly contacts the amorphous magnetic material layer. 
     In some embodiments shown in  FIGS. 10 and 11 , the first ferromagnetic layer  112  is a first free layer having a first magnetization having at least two preferred magnetization directions, and the second ferromagnetic layer  116  is a second free layer having a second magnetization having at least two preferred magnetization directions. 
     In another embodiment shown in  FIG. 12 , a synthetic antiferromagnetic stack ( 212 ,  214 ,  216 ) is provided, such that one of the first and the second ferromagnetic layers ( 112 ,  116 ) comprises a free layer and the other one of the first and the second ferromagnetic layers ( 112 ,  116 ) comprises a pinned reference layer. 
     In one embodiment the read head further comprises a first magnetic shield  102  and a second magnetic shield  104 . A sensor layer stack  110  comprising the first ferromagnetic layer, the second ferromagnetic layer, the first diffusion-assist nonmagnetic metallic layer, the second diffusion-assist nonmagnetic metallic layer and the semiconductor spacer layer is located between the first magnetic shield  102  and the second magnetic shield  104 . 
     The various sensor layer stacks  110  of the embodiments of the present disclosure can be included in a read head of a magnetic head of a hard disk drive, such as a hard disk drive  300  shown in  FIGS. 1 and 2 . As described above, the hard disk drive  300  can include a slider  308  supporting the magnetic head  600 , an actuator arm  309  supporting the slider, a motor  310  configured to control the actuator arm, and a magnetic disk  302 . The first diffusion-assist nonmagnetic metallic layer  114 A and/or the second diffusion-assist nonmagnetic metallic layer  114 C can significantly increase magnetoresistance for a layer stack having a low resistance-area product, thereby increasing the sensitivity of the read head. 
     Although the foregoing refers to particular preferred embodiments, it will be understood that the disclosure is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the disclosure. Where an embodiment employing a particular structure and/or configuration is illustrated in the present disclosure, it is understood that the present disclosure may be practiced with any other compatible structures and/or configurations that are functionally equivalent provided that such substitutions are not explicitly forbidden or otherwise known to be impossible to one of ordinary skill in the art. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.