Abstract:
In a magnetic tunnel junction (MTJ) device having a pinned layer and upper and lower antiparallel-coupled free sublayers, to avoid loss in tunnel magnetoresistance, etching or milling of the free sublayer layer materials is stopped in the lower free sublayer. The total thickness of the free sublayers may be large to ease manufacture because the effective magnetic thickness of the free layer combination may be as small as desired by appropriately establishing a small difference between the thicknesses of the AP-coupled free sublayers. A contiguous hard bias material is centered on the free sublayers for stabilization.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to magnetoresistive devices, such as magnetic tunnel junction (MTJ) devices for, e.g., disk drive read heads. 
     BACKGROUND OF THE INVENTION 
     In magnetic disk drives, data is written and read by magnetic transducers called “heads.” The magnetic disks are rotated at high speeds, producing a thin layer of air called an air bearing surface (ABS). The read and write heads are supported over the rotating disk by the ABS, where they either induce or detect flux on the magnetic disk, thereby either writing or reading data. Layered thin film structures are typically used in the manufacture of read and write heads. In write heads, thin film structures provide high areal density, which is the amount of data stored per unit of disk surface area, and in read heads they provide high resolution. 
     The present invention is directed generally to devices that can be used, in some implementations, as heads for disk drives, and more particularly the present invention is directed to magnetic tunnel junction (MTJ) devices. An MTJ device has at least two metallic ferromagnetic layers separated by a very thin nonmagnetic insulating tunnel barrier layer, wherein the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. The high magnetoresistance at room temperature and generally low magnetic switching fields of the MTJ renders it effective for use in magnetic sensors, such as a read head in a magnetic recording disk drive, and nonvolatile memory elements or cells for magnetic random access memory (MRAM). 
     In a MTJ device, one of the ferromagnetic layers has its magnetization fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the field of the other ferromagnetic layer is “free” to rotate in the presence of an applied magnetic field in the range of interest of the read head or memory cell. 
     To increase both sensitivity and output, the free layer may be established by two sublayers that are separated from each other by an antiparallel coupling. (AP-coupling) layer. Since the magnetic fields of the two free sublayers are antiparallel to each other, the effective magnetic thickness of the overall free layer that is established by the two sublayers is the difference in thickness between the sublayers. 
     To make an AP-coupled MTJ device, a lower free sublayer is established on the barrier that overlays the above-mentioned pinned stack, then the AP-coupling layer is deposited on the first free sub-layer, then an upper free sublayer is established on the AP-coupling layer. Next, the free layers are formed into a stack by protecting only the area of the free layer sought to be maintained and ion milling the remainder away, down to the barrier covering the pinned stack. 
     As critically recognized herein, during the above process the barrier might be unintentionally eroded because it is difficult to stop removing material exactly as the last of the free layer intended to be removed is indeed milled away. This results in a deleterious loss of tunnel magnetotresistance between the free and pinned stacks from shunting caused by a breakdown in the barrier and/or by redeposited material. 
     The present invention makes the additional critical observations. As understood herein, it is necessary for stabilization purposes to provide stabilization structure in MTJ devices, and one way to do this is to surround the free stack with a hard bias material. The present invention recognizes that for optimum stabilization, when so doing the hard bias layer ideally is centered around the free stack. 
     With these recognitions in mind, the invention herein is provided. 
     SUMMARY OF THE INVENTION 
     A magnetic tunnel junction device includes a pinned ferromagnetic layer that has a magnetization direction substantially prevented from rotation in the presence of an applied magnetic field. An insulating tunnel barrier layer is on the pinned layer. Also, a free ferromagnetic stack is on the tunnel barrier layer and has a magnetization direction that is substantially free to rotate in the presence of an applied magnetic field. The stack includes a lower free sublayer on the barrier layer, an upper free sublayer, and an antiparallel (AP)-coupling layer between the sublayers. According to this particular aspect of the present invention, a skirt extends away from the stack against the barrier. From another aspect, no portions of the upper free sublayer or AP-coupling layer extend beyond vertical edges of the free stack, whereas the skirt, which extends radially away from the free stack, is integral to the lower free sublayer and is of the same material as the lower free sublayer, or is an oxide thereof. 
     In some implementations a hard bias material may be over the skirt and may surround the stack. The hard bias material is substantially centered on the free stack. The AP-coupling layer can be nonmagnetic and the free sublayers can be ferromagnetic and can be antiparallel to each other. 
     In another aspect, a magnetic tunnel junction device includes a ferromagnetic layer having a magnetic field pinned from rotation and upper and lower ferromagnetic sublayers not having their magnetic fields pinned from rotation and being AP-coupled together. The lower sublayer includes a skirt extending radially beyond the upper sublayer. A barrier layer is between the lower sublayer and ferromagnetic layer. 
     In another aspect, a method for making a MTJ device includes forming a barrier layer on a pinned stack, and forming a lower free ferromagnetic sublayer on the barrier layer. The method also includes forming an AP-coupling layer on the lower free ferromagnetic sublayer and forming an upper free ferromagnetic sublayer on the AP-coupling layer. The upper free ferromagnetic sublayer and the AP-coupling layer are etched or milled completely through, but the lower free ferromagnetic sublayer is etched or milled only part way through before stopping the etching or milling process to thereby establish a skirt on the lower free ferromagnetic sublayer that extends radially beyond the upper free ferromagnetic sublayer. 
     The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a hard disk drive, showing one non-limiting environment for the present invention; 
         FIG. 2  is an elevational view of a non-limiting MTJ device made in accordance with the present invention; and 
         FIG. 3  is a flow chart of the method for making the device shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a magnetic disk drive  30  includes a spindle  32  that supports and rotates a magnetic disk  34 . The spindle  32  is rotated by a spindle motor that is controlled by a motor controller which may be implemented in the electronics of the drive. A slider  42  has a combined read and write magnetic head  40  and is supported by a suspension  44  and actuator arm  46  that is rotatably positioned by an actuator  47 . The head  40  may be a GMR or MR head or other magnetoresistive head. It is to be understood that a plurality of disks, sliders and suspensions may be employed. The suspension  44  and actuator arm  46  are moved by the actuator  47  to position the slider  42  so that the magnetic head  40  is in a transducing relationship with a surface of the magnetic disk  34 . When the disk  34  is rotated by the spindle motor  36  the slider is supported on a thin cushion of air known as the air bearing that exists between the surface of the disk  34  and an air bearing surface (ABS) of the head. The magnetic head  40  may then be employed for writing information to multiple circular tracks on the surface of the disk  34 , as well as for reading information therefrom. To this end, processing circuitry  50  exchanges signals, representing such information, with the head  40 , provides spindle motor drive signals for rotating the magnetic disk  34 , and provides control signals to the actuator for moving the slider to various tracks. The components described above may be mounted on a housing  55 . 
     Now referring to  FIG. 2 , the head  40  which is manufactured using the process of the present invention includes a pinned stack  60 , it being understood that the pinned stack  60  is formed on a substrate. In non-limiting implementations the pinned stack  60  may include, in order from the substrate, a first seed layer that may be, without limitation, Ta that is twenty Angstroms thick, an optional “template” ferromagnetic layer that may be, without limitation, NiFeCr or Ru of twenty Angstroms thickness on the seed layer, a layer of antiferromagnetic material that may be, without limitation, a seventy five Angstrom thick layer of IrMn on the template layer, and a “pinned” ferromagnetic layer formed on and exchange coupled with the underlying antiferromagnetic layer. The ferromagnetic layer is called the pinned layer because its magnetization direction is prevented from rotation in the presence of applied magnetic fields in the desired range of interest for the MTJ device. Without limitation, the pinned ferromagnetic layer may be, e.g., Co 75 Fe 25  (forty Angstroms thick)/Ru (4.5 Angstroms)/CoFeB (forty Angstroms), where the thicknesses of magnetic layers are scaled to Ni 81 Fe 19  moment, and CoFeB is next to the barrier layer. 
     In addition, other conducting underlayers may without limitation include Ta, Cu and Au. Other CoFe and NiFe alloys may be used for the ferromagnetic layers and other antiferromagnetic materials may include NiMn, PtMn and IrMn. The substrate may be a silicon wafer if, for instance, the device is a memory cell, and ordinarily would be the bottom electrically conductive lead located on either the alumina gap material or the magnetic shield material on the trailing surface of the head carrier if the device is a read head. 
     Formed on the pinned stack  60  is a barrier layer  62  that is made of an insulating tunnel barrier material. By way of non-limiting example, the barrier layer  62  may be five to fifteen Angstroms thick and may and may be made by depositing Aluminum on the pinned stack  60  and then oxidizing it to create an Al 2 O 3  insulating tunnel barrier layer  62 . While Al 2 O 3  may be used, a wide range of other materials may be used, including MgO, AlN, aluminum oxynitride, oxides and nitrides of gallium and indium, and bilayers and trilayers of such materials. 
     A free ferromagnetic stack, generally designated  64 , is formed on the barrier layer  62  as shown. The free stack  64  is surrounded by an insulating layer  66  of, e.g., Al 2 O 3  and may be covered by a protective cap  68 . 
     In accordance with present principles, the free stack  64  includes, from the barrier layer  62 , a lower free ferromagnetic sublayer  70 , a non-magnetic AP-coupling layer  72  that may be made of, e.g., Ru, and an upper free ferromagnetic sublayer  74 . The sublayers  70 ,  74  are magnetically coupled together by the AP-coupling layer  72 , and are magnetically antiparallel to each other. By “free.” is meant that the magnetization direction of the free stack  64  is not pinned by exchange coupling, and is thus free to rotate in the presence of applied magnetic fields in the range of interest. The free sublayers may be, without limitation, alloys of Fe and one or more of Co and Ni, or combinations thereof or may contain CoFeB. 
     As shown in  FIG. 2 , after the manufacturing process described below, a skirt portion  76  of the lower free sublayer  70  that extends radially away from the free stack  64  remains after etching, i.e., the lower free sublayer  70  is only partially etched through. In, contrast, after etching/milling no portions of the upper free sublayer  74  or AP-coupling layer  72  extend beyond the vertical edge of the free stack  64  as shown. In any case, the skirt is integral to the lower free sublayer and is of the same material as the lower free sublayer, or, as set forth further below, may be an oxide thereof. 
     In one non-limiting implementation, the free sublayers  70 ,  74  may be relatively thick (e.g., one hundred and one hundred thirty Angstroms, respectively), to render easier the stopping of the etch/mill process before completely removing the skirt  76 , because the effective magnetic thickness, which is the difference in thicknesses between the sublayers  70 ,  74 , may be made as small as desired by appropriately establishing the thicknesses of the sublayers  70 ,  74 . Other thicknesses can be used, e.g., the lower free sublayer  70  may be thirty Angstroms thick and the upper free sublayer  74  may be sixty Angstroms thick. 
     The insulating layer  66  thus is deposited both around the free stack  64  and on top of the skirt  76 . Further, a stabilizing hard bias material  78  is formed over the insulating layer  66 , substantially centered on the free stack  64  as shown. 
     Now referring to  FIG. 3 , at block  80  the pinned stack  60  and barrier  62  are formed on a substrate in accordance with principles known in the art, e.g., by sputtering. Proceeding to block  82 , lower and upper free sublayers  70 ,  74  are formed with the AP-coupling layer  72  between them, likewise by sputtering or other deposition technique. Then, at block  84  the entire portions of the upper free sublayer  74  and AP-coupling layer  72  outside the free stack  64 , along with only portions of the low free sublayer  70 , are removed by etching (e.g., reactive ion etching) or milling (e.g., ion milling) to leave the skirt  76  of the lower free sublayer  70  as shown. The skirt  76  may be oxidized at block  86  and then the insulating layer  66  and hard bias material  78  are formed at block  88 , with the hard bias material  78  substantially centered on the free stack  64  as shown. 
     While the particular MTJ DEVICE WITH PARTIALLY MILLED ANTI-PARALLEL COUPLED BOTTOM FREE LAYER as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and is thus representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. It is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be, dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. Absent express definitions herein, claim terms are to be given all ordinary and accustomed meanings that are not irreconcilable with the present specification and file history.