Abstract:
A tunneling barrier for a spin dependent tunneling (SDT) device is disclosed that includes a plurality of ferromagnetic particles. The presence of such particles in the tunneling barrier has been found to increase a magnetoresistance or ΔR/R response, improving the signal and the signal to noise ratio. Such an increased ΔR/R response also offers the possibility of decreasing an area of the tunnel barrier layer and/or increasing a thickness of the tunnel barrier layer. Decreasing the area of the tunnel barrier layer can afford improvements in resolution of devices such as MR sensors and increased density of devices such as of MRAM cells. Increasing the thickness of the tunnel barrier can afford improvements in manufacturing such as increased yield.

Description:
TECHNICAL FIELD 
     The present application relates to spin-dependent tunneling (SDT) devices. Such devices may be employed in many applications, including information storage and retrieval devices (e.g., electromagnetic transducers), solid-state memory for computers and digital processing systems (e.g., MRAM) and measurement and testing systems (e.g., magnetic field sensors). 
     BACKGROUND 
     Spin-dependent tunneling (SDT) effects are believed to depend upon a quantum mechanical probability of electron tunneling from one ferromagnetic (FM) electrode to another through a thin, electrically nonconductive layer, with the probability of tunneling depending upon the direction of magnetization of one electrode versus the other. SDT effects have many potential applications in magnetic field sensing devices, such as magnetic field sensors and information storage and retrieval devices. Read transducers for magnetic heads used in disk or tape drives, which may be termed magnetoresistive (MR) sensors, and solid-state memory devices such as magnetic random access memory (MRAM), are potential commercial applications for spin tunneling effects. 
     Elements of SDT devices include two FM electrodes and an electrically insulating tunneling barrier. One of the electrodes may include a pinned ferromagnetic layer and the other may include a free ferromagnetic layer. The pinned layer typically consists of a FM layer that has its magnetic moment stabilized by a pinning structure. The pinning structure may be an antiferromagnetic (AFM) layer that adjoins the pinned layer. The magnetic stabilization of the pinned layer may also be accomplished with a synthetic AFM structure that includes a transition metal such as ruthenium (Ru) in a sandwich between two FM layers, in which the transition metal layer has a precisely defined thickness that is typically less than 10 Å. The magnetization direction of the pinned FM layer is set upon deposition and annealing in a magnetic field. The free layer is typically a magnetically soft FM layer. 
     The free layer is designed to be magnetically decoupled from the pinned layer, so that the pinned layer does not hinder the response of the free layer to a magnetic field signal that is to be detected. The nonmagnetic tunneling barrier provides the magnetic decoupling between the pinned and free layers. The tunneling barrier is made of a thin dielectric layer, such as Al 2 O 3  or AlN, which has a thickness typically in a range between 0.5 nm and 2 nm. 
     The tunnel barrier layer is designed to be a uniform and pinhole free dielectric film at the atomic scale, in order to avoid electrical shorting and ferromagnetic coupling through the pinholes. For applications involving tunneling magnetoresistive (TMR) heads, it is also desirable for the device resistance to be relatively low, in order to achieve a wide bandwidth and high frequency operation. The probability of electron tunneling through a tunnel barrier increases exponentially as the barrier is made thinner, however, for thicknesses gless than 10 Å electrical shorting between the electrodes becomes increasingly problematic. 
     For example, a media-facing surface of MR sensors may be formed by lapping or polishing in a direction that traverses the tunnel barrier layer, which can cause dislodged electrode particles to bridge across a thin barrier. Similarly, conventional solid-state memory processing requires annealing at a relatively high temperature after formation of memory cells, which could in the case of MRAM devices cause diffusion of electrode materials into a tunnel barrier. 
     For a tunnel barrier material having a uniform specific resistance at each point, the overall resistance of the barrier layer is an exponential function of the thickness of the layer and inversely proportional to the area of the layer. For MR heads the area of the tunnel barrier layer is constrained, however, by the desired resolution of the head. Similarly, for MRAM applications the area of the tunnel barrier layer is constrained by the desired density of the memory cells. 
     The resistance and area product (RA product) is a figure of merit for SDT films, and is sensitively dependent upon the barrier thickness. Given the constraints upon the area of the devices, tunnel barrier layers may be as thin as several atomic layers. Another figure of merit for a SDT device is the magnetoresistance, which is the change in resistance divided by the resistance (ΔR/R) of the device in response to a change in applied magnetic field. Since the noise of the device is related to the resistance, the magnetoresistance is also a measure of the signal to noise ratio (SNR) of the device. 
     SUMMARY 
     In accordance with an embodiment of the present disclosure, a tunneling barrier for a spin dependent tunneling (SDT) device includes a plurality of ferromagnetic particles. The presence of such particles in the tunneling barrier has been found to increase the magnetoresistance, also known as the ΔR/R response to an applied magnetic field, improving the signal and the signal to noise ratio. Such an increased ΔR/R response also offers the possibility of decreasing an area of the tunnel barrier layer. Decreasing the area of the tunnel barrier layer can afford improvements in resolution of devices such as MR sensors and increased density of devices such as of MRAM cells. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross-section of a SDT device having a plurality of magnetic particles disposed in a tunnel barrier layer that separates ferromagnetic layers having easy axes of magnetization substantially parallel to each other. 
     FIG. 2 is a cross-section of a SDT device having a plurality of magnetic particles disposed in a tunnel barrier layer that separates ferromagnetic layers having easy axes of magnetization substantially perpendicular to each other. 
     FIG. 3 is a cross-section of a SDT device having a plurality of magnetic particles disposed in a tunnel barrier layer and disposed between conductive leads. 
     FIG. 4 is a plot of the magnetorestistance (ΔR/R) of SDT devices having different amounts of magnetic particles contained in an alumina tunnel barrier layer. 
     FIG. 5 is a plot of the magnetorestistance (ΔR/R) versus a resistance-area product (RA) of SDT devices having different amounts of magnetic particles contained in an alumina tunnel barrier layer. 
     FIG. 6 is a schematic perspective view of plural SDT devices used as part of a solid-state memory such as an MRAM device. 
     FIG. 7 is a cutaway cross-sectional view of an SDT device used in an information storage system such as a hard disk drive. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a cross-section of a SDT device  20  having a plurality of magnetic particles  30  disposed in a tunnel barrier layer  26 . The device  20  includes a first ferromagnetic (FM) layer  22  and second FM layer  24  separated by the tunnel barrier layer  26 . A pinning structure  28  constrains the direction of magnetization of the second FM layer  24 . In response to an applied magnetic field, the magnetic moment of the first FM layer  22  changes direction, as shown by arrow  27 , while the magnetic moment of the second FM layer  24  is pinned, as shown by arrow  29 . In this embodiment, which may for example be used in a MRAM device, an easy axis of magnetization of the first FM layer may be substantially parallel to that of the second FM layer  24 , so that the second FM layer switches between substantially parallel and substantially antiparallel states. As long as the easy axes of magnetization of the first and second FM layers are more parallel than perpendicular, switching between two states is facilitated. 
     The pinning structure  28  may be an antiferromagnetic (AFM) layer that adjoins the pinned layer. The magnetic stabilization of the pinned layer may also be accomplished with a synthetic AFM structure involving a pair of FM layers exchange coupled across a thin precious metal layer. For example ruthenium (Ru) can be used in a sandwich of FM/Ru/FM in which the Ru layer has a thickness less than 10 Å. The magnetization direction of the pinned FM layer may be set upon deposition and annealing in a magnetic field. 
     Either or both of the first and second FM layers can be made primarily or entirely of metals such as iron (Fe), cobalt (Co), nickel (Ni) or alloys of such metals. Either or both of the first and second FM layers may also be made of half-metallic magnets such as CrO 2 , Fe 3 O 4 , PtMnSb, NiMnSbCo 2 MnSi or Sr 2 FeMoO 6 . 
     The tunnel barrier layer  26  can be made primarily of dielectric materials such as Al 2 O 3 , AlN, SiO 2 , Si 3 N 4 , TaO, TaO 2 , Ta 2 O 5 , HfO 2 . A preferred tunnel barrier material in one embodiment is Al 2 O 3 . The magnetic particles  30  can include Co, Ni or Fe atoms or molecules, or compound molecules such as CoFe, NiFe, CoNi or NiFeCo, for example. The magnetic moment of the particles is not fixed. A related disclosure of magnetic particles contained in a tunnel barrier layer can be found in the U.S. Patent Application entitled “Spin Dependent Tunnel Barriers Formed With A Magnetic Alloy,” invented by the same inventors and filed on the same day as the present application, and incorporated by reference herein. 
     The thickness of the tunnel barrier layer  26  can vary significantly for different embodiments, and for one embodiment the thickness is in a range between about 4 Å and about 15 Å. The tunnel barrier layer  26  may itself be composed of plural dielectric layers with the magnetic particles disposed in at least one of the dielectric layers. Although the particles in that embodiment do not form a continuous layer that separates the dielectric layers, the average thickness of the layer of magnetic particles may be in a range between about 0.1 Å and about 3 Å. Individual magnetic particles in one embodiment may range between single atoms and molecules containing up to about sixteen ferromagnetic atoms. Care must be taken during fabrication to avoid having the magnetic particles act as conductors between the first and second FM layers  22  and  24 , despite an overall tunnel barrier thickness that may be as little as several atomic layers. 
     FIG. 2 shows a cross-section of a SDT device  60  having a plurality of magnetic particles  70  dispersed in a tunnel barrier layer  66 . The device  60  includes a first ferromagnetic (FM) layer  62  and second FM layer  64  separated by the tunnel barrier layer  66 . A pinning structure  68  constrains the direction of magnetization of the second FM layer  64 . In response to an applied magnetic field, the magnetic moment of the first FM layer  62  changes direction, as shown by arrow  67 , while the magnetic moment of the second FM layer  64  is pinned, as shown by cross marks, indicating a moment directed away from the viewer. In this embodiment, which may for example be used in a magnetic sensing device, an easy axis of magnetization of the first FM layer may be substantially perpendicular to that of the second FM layer  64 , so that the second FM layer switches between low resistance and high resistance states. As long as the easy axes of magnetization of the first and second FM layers are more perpendicular than parallel, a linear change in resistance is facilitated. 
     FIG. 3 shows a cross-section of a portion of a SDT sensor  100  in accordance with one embodiment of the invention, such as may be employed in an information storage and retrieval device. Formation of SDT sensor  100  may occur on a wafer substrate such as AlTiC, SiC or Si, not shown, upon which a seed layer  101  of Ta has been deposited. A first or bottom lead  102  of electrically conductive material such as Cu or Au has been formed, and capped with another Ta seed layer  104 . An electrically conductive layer  106  of AFM material was then formed on the seed layer  104 . The AFM material may, for example, include IrMn, FeMn, NiMn, PdPtMn, NiFeCr/PtMn or NiFe/PtMn. A magnetically pinned structure  110  can then be formed of a sandwich of FM layers  112  and  116  that surround a very thin coupling layer  114  of Ru or similar elements. The FM layers  112  and  116 , which may for example be formed of CoFe, are magnetically coupled with moments directed in opposite directions about coupling layer  114 , with the moment of FM layer  112  additionally pinned by AFM layer  106 . A single pinned layer may be alternatively employed instead of the three layer magnetically pinned structure  110 . 
     A first SDT film  120  that may be between a single atomic layer and several or more nanometers in thickness, depending upon the desired application, is then deposited on the FM layer  116 . The first SDT film  120  may be formed by depositing an initial layer, such as Al, Si, Mg, Ta or Hf on the FM layer  116 , and then oxidizing or nitridizing the initial layer. Magnetic particles  122  are then deposited on and in some cases in the first SDT film  120 , by sputtering or ion beam depoition. The magnetic particles  122  may range in size between individual atoms and clusters of atoms having a diameter of about 10 angstroms. In order to have properties of individual magnetic particles  122  it is desirable for this embodiment that the particles do not form a continuous layer atop the first SDT film  120 . A second SDT film  124  that may be between a single atomic layer and several or more nanometers in thickness, depending upon the desired application, is then deposited on the magnetic particles  122  and the first SDT film  120 . The second SDT film  124  may be formed by depositing an initial layer, such as Al, Si, Mg, Ta or Hf, and then oxidizing or nitridizing the initial layer. 
     Together the first SDT film  120 , magnetic particles  122  and second SDT film  124  form a tunnel barrier layer  121  between pinned FM layer  116  and a free FM layer  126 . The thickness of tunnel barrier layer  121  for a SDT head implementation may be between 5 Å and 5 nm, the additional thickness made possible by the increased magnetoresistive effect of the novel tunnel barrier layer  121 . The layers  120  and  124  may be oxidized or nitridized by the same or different processes, as desired for a particular application. Depending upon the process and materials employed, layers  120  and  124  may be distinct or may be merged into the single layer  121 . The atomic concentration of magnetic particles in a tunnel barrier layer such as layer  121  may range between less than one percent and about ten percent. The amount of dopants can be measured by various means, such as x-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), parallel electron energy loss spectroscopy (PEELS), secondary ion mass spectroscopy (SIMS) or x-ray fluorescence (XRF). 
     After the tunneling barrier has been made, a free FM layer  126  is formed of materials such as CoFe, NiFe, CoNiFe, or multilayers of these materials. A capping layer  130  of Ta, Ru, Cr or NiFeCr may be used to prevent the oxidation of the free layer  126 . Edges of the SDT device are then defined, for example by ion beam milling into a desirable structure, and a top lead  133  is formed, e.g., of Au or Cu. For an embodiment to be used as a magnetic sensor, for example in a read-write head, a hard bias material may be formed adjacent to edges of the free layer  126  to reduce Barkhausen noise. A similar technique can be applied to form a top SDT structure, i.e., by creating a pinned layer or layers after a SDT barrier, which is formed atop a free layer. Other variations of the sensor stack structure are also possible that include plural magnetic particles in a tunnel barrier region. For instance, some portions of a pinned or free layer may be intentionally oxidized or nitridized to form a nanooxide or nanonitride that can enhance the sensitivity. 
     FIG. 4 shows some hysteresis plots of the magnetorestistance (ΔR/R) of SDT devices having different amounts of magnetic particles contained in an alumina tunnel barrier layer such as layer  121  of FIG.  3 . In this example, curve  202  is a plot of (ΔR/R) versus applied field H for a SDT device having no magnetic particles in the barrier layer. A maximum (ΔR/R) for this device can be seen to be about 31%. Curve  204  is a plot of (ΔR/R) versus applied field H for a SDT device having CoFe magnetic particles contained in a layer averaging 0.3 Å in thickness within the alumina barrier layer. A maximum (ΔR/R) of about 33% for this device is higher than that of the SDT device having no magnetic particles. Curve  206  is a plot of (ΔR/R) versus applied field H for a SDT device having CoFe magnetic particles contained in a layer averaging 0.5 Å in thickness within the barrier layer. A maximum (ΔR/R) of about 35% for this device is significantly higher than that of the SDT device having no magnetic particles. 
     FIG. 5 is a plot of (ΔR/R) versus a resistance-area product (RA) of SDT devices having different amounts of magnetic particles contained in an alumina tunnel barrier layer such as layer  121  of FIG.  3 . The diamond-shaped data points  250  indicate ΔR/R and RA values for SDT devices having no magnetic particles in an alumina tunnel barrier layer such as layer  121  of FIG.  3 . The triangle-shaped data points  252  indicate ΔR/R and RA values for SDT devices having CoFe magnetic particles contained in a layer averaging 0.3 Å in thickness within the alumina barrier layer. Both the ΔR/R and RA values show significant improvement over the values of the undoped barrier layer. The circle-shaped data points  255  indicate ΔR/R and RA values for SDT devices having CoFe magnetic particles contained in a layer averaging 0.5 Å in thickness within the alumina barrier layer. Even higher ΔR/R and RA values are evident for this group. Magnetic particles may also be made to diffuse through a dielectric tunnel barrier layer so that, for example, they can be found at various thicknesses of the layer. 
     FIG. 6 shows one embodiment using the SDT devices as part of a solid-state memory such as an MRAM device  300 . In this example, four memory bits or cells  303 ,  305 ,  307  and  309  are shown as stacks of layers between conductive lines arranged to write and read data to and from each cell. Lines  311  and  313 , which may be called bit lines, are used for both reading and writing and are in electrical as well as magnetic communication with the cells. Lines  315  and  317 , which may be called digit lines, are used only for writing, and are in magnetic communication with but electrically isolated from the cells. Word lines  321  and  323  are in electrical communication with the cells via transistors, and are used only for reading. 
     Each of the cells  303 ,  305 ,  307  and  309  includes a pinned FM layer, a free FM layer and a tunnel barrier layer containing magnetic particles. Focusing on cell  305  provides an example for the operation of various cells. Cell  305  has a pinned structure or layer  330 , a free layer  333  and a tunnel barrier layer  335  containing magnetic particles. A transistor  331  controlled by word line  323  is coupled to a conductive lead  337  adjoining the pinned structure  330 . The conductive lead  337  is electrically isolated from digit line  317  by insulation layer  339 . 
     Pinned layer  330  has a magnetic moment indicated by arrow  340 , and free layer  333  has a magnetic moment indicated by arrow  343 . A magnetic state is written to cell  305  by flowing current through lines  313  and  317  in an amount sufficient to switch the magnetic moment of the free layer  333 . Current in line  313  creates a magnetic field along a hard axis of free layer  333  and current in line  317  creates a magnetic field along an easy axis of free layer  333 , the combined fields being sufficient to switch the magnetic moment of the cell, whereas either field alone is insufficient to switch the cell. Arrow  343  is antiparallel to arrow  340 , indicating that free layer  333  has a magnetic moment antiparallel to that of pinned layer  330 . Turning on transistor  331  to sense the state of cell  305  would result in a high voltage state (when a constant current is applied) through the transistor, which may indicate for instance that a value of zero has been stored in cell  305 . 
     Cell  303 , on the other hand, has a free layer with a magnetic moment parallel to that of its pinned layer, as indicated by arrows  350  and  355 . Turning on transistor  351  to sense the state of cell  305  would result in a low voltage state (when a constant current is applied) through the transistor, which may indicate for instance that a value of one has been stored in cell  305 . It may also be possible for such SDT devices to have more than two distinct states or levels of resistance, allowing more information to be stored in each cell or MRAM module. 
     FIG. 7 shows one embodiment using an SDT device as part of an information storage system such as a hard disk drive  400 . A rigid disk  402  spins rapidly in a direction shown by arrow  408  relative to a head  404  containing a transducer for reading and writing magnetic patterns on a media layer  406  of the disk. A hard coating layer  410  forms a surface of the disk  402  protecting the media layer  406  from the head  404 , and a similar hard coating  412  forms a surface of the head  404  to protect the transducer from the disk, while another hard coating  411  forms a trailing end of the head. The disk includes a substrate  414  upon which the media layer  406  may be formed, and the disk may include additional layers, not shown. The head also includes a substrate  420  upon which the transducer has been formed, and may include additional layers, not shown. 
     A first shield  422  has been formed over the substrate, the shield including ferromagnetic material for shielding a SDT sensor  424  from magnetic signals that are not directly opposite the sensor  424  in the media layer  406 , as well as conductive material for providing electrical current to the sensor  424 . A second shield  426  is separated from the first shield  422  by a dielectric layer  428 , the shields connected by the sensor  424 . The second shield  426  also includes ferromagnetic material for shielding the sensor  424  from magnetic signals that are not directly opposite the sensor  424  in the media layer  406 , as well as conductive material for providing electrical current to the sensor  424 . As the media layer  406  travels past the sensor  424  in the direction of arrow  408 , the sensor reads magnetic signals from the media layer. 
     First and second ferromagnetic yoke layer  430  and  433  are magnetically coupled together in a loop that is inductively driven by current in coil winding sections  435 . The magnetic loop is broken by a nonferromagnetic gap  438 , so that some magnetic flux propagating around the loop is diverted to the media layer  406  of the disk  402 , writing a magnetic signal to the media layer. This magnetic signal can later be read by the MR sensor, which may have an increased resolution due to subnanometer magnetic particles contained in a tunnel barrier layer. 
     Although the above description has focused on illustrating SDT devices for use with information storage systems and solid-state memory, other devices can be formed in accordance with the present invention. Moreover, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. Therefore, this invention is limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.