Abstract:
The performance of an MR device has been improved by inserting one or more Magneto-Resistance Enhancing Layers (MRELs) into approximately the center of one or more of the magnetic layers such as an inner pinned (AP1) layer, spin injection layer (SIL), field generation layer (FGL), and a free layer. An MREL is a layer of a low band gap, high electron mobility semiconductor such as ZnO or a semimetal such as Bi. The MREL may further comprise a first conductive layer that contacts a bottom surface of the semiconductor or semimetal layer, and a second conductive layer that contacts a top surface of the semiconductor or semimetal layer.

Description:
[0001]    This is a Divisional application of U.S. patent application Ser. No. 12/799,468, filed on Apr. 26, 2010, which is herein incorporated by reference in its entirety, and assigned to a common assignee. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the general field of magneto-resistance (MR) devices with particular reference to improving their performance. 
       BACKGROUND OF THE INVENTION 
       [0003]    A typical example of a spintronic device is a magneto-resistance sensor. As shown schematically in  FIG. 1 , this includes seed layer  11 , an antiferromagnetic (AFM) pinning layer  12 , outer pinned layer  13  (anti-parallel 2 or AP2), AFM coupling layer  14 , typically, but not limited to, Ru, inner pinned layer  15  (AP1), spacer layer  16 , free layer (FL)  17 , and capping layer  18 . Depending on whether spacer layer  16  is conductive or insulating (i.e. a barrier layer), the sensor is either a Giant Magneto-Resistance (GMR) device or a Tunneling Magneto-Resistance (TMR) device. 
         [0004]    As recording densities increase, device sizes become correspondingly smaller. Consequently the sensor (or other spintronic device) has to continue having a high enough magneto-resistance (MR) ratio, as well as a low enough resistance×area product (RA), for it to still have sufficient output amplitude and also for it to be suitable for future extendibility. It is, however, becoming increasingly more difficult for MgO-based TMR devices to maintain their current high MR ratio in the lower RA region. Current Perpendicular to the Plane (CPP) GMR or Confined Current Path (CCP) GMR devices have not yet been able to produce high enough MR ratios for them to take over as the future read head technologies. There is thus an urgent need for sensors that have higher output amplitude and/or a higher MR ratio. 
         [0005]    The present invention, as will be detailed below, discloses some novel methods for further enhancing the MR ratio. 
         [0006]    A routine search of the prior art was performed with the following references of interest being found: 
         [0007]    U.S. Pat. No. 7,596,015 (Kitigawa et al) shows a free layer including an intermediate layer of Cu and/or Zn or Sn. (See col. 5, lines 58-63 free layer is the same as recording layer 12, and see col. 11, lines 1-15 for materials added to the recording layer—at least one of the materials including Cu, Zn, and Sn). 
         [0008]    U.S. Pat. No. 7,223,485 (Yuasa et al) teaches an insertion layer in the free layer. (See col. 2-3 where the insert layer contains at least one element from Cu, Zn, O). K. Zhang et al, HT06-055 “A novel CPP device with enhanced MR ratio” and the reference therein. Y. Chen et al, “Spin-dependent CPP transport properties of ZnO/ferromagnet Heterostructures” Physics Letters A 303 (2002) 91-96. 
       SUMMARY OF THE INVENTION 
       [0009]    It has been an object of at least one embodiment of the present invention to provide a method for enhancing the performance, particularly the MR and R.A ratios, of magneto-resistance devices. 
         [0010]    Another object of at least one embodiment of the present invention has been to provide a process for manufacturing the invention. 
         [0011]    A further object of at least one embodiment of the present invention has been that said process be fully compatible with existing processes for manufacturing spintronic devices. 
         [0012]    Still another object of at least one embodiment of the present invention has been to provide a detailed description thereof. 
         [0013]    These objects have been achieved by inserting one or more Magneto-Resistance Enhancing Layers (MRELs) into approximately the center of one or more of the active layers (such as the magnetic AP1, spin induction layer or SIL, and free layers). An MREL is a layer of a low band gap, high electron mobility semiconductor such as ZnO or a semimetal such as Bi. Optionally, the interface(s) between the MREL and the magnetic layer into which it has been inserted may be bridged by a thin layer of a highly conductive metal such as copper so as to ensure an ohmic contact across the full interface between the MREL and the magnetic layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional view of a typical MR device of the prior art. 
           [0015]      FIG. 2  is a perpendicular RH curve for a Ta10/Ru20/Cu20/[Co2/Ni6]×15/Cu20/FeCo150/Ru10/Ta40/Ru30 structure of the prior art. 
           [0016]      FIG. 3  is a perpendicular RH curve for the structure shown in  FIG. 2  modified to become Ta10/. . . ×15/Cu20/FeCo75/Cu3/ZnO15/Cu3/FeCo75/. . . /Ta40/Ru30. 
           [0017]      FIG. 4  is a cross-sectional view of a typical MREL. 
           [0018]      FIG. 5  illustrates a typical MR device after insertion of MRELs in both the AP1 and free layers. 
           [0019]      FIG. 6  shows the three main component sections of a spin torque oscillator (STO), including the field generation layer (FGL) into which has been inserted an MREL. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    We have found that when a trilayer of a low band gap semiconductor such as ZnO, sandwiched between layers of a conductive metal such as Cu (to form Cu3/ZnO15/Cu3 for example), is inserted into a spintronic device the MR ratio of that device is increased significantly. An early example of this was discovered in the course of experiments with a spin torque oscillator (STO), patterned to around 45×45 nm, whose structure was the following: Ta10/Ru20/Cu20/[Co2/Ni6]×15/Cu20/FeCo150/Ru10/Ta40/Ru30. This device had almost no MR ratio for the in-plane RH measurement. However, when the applied field was close to the perpendicular direction (specifically, H was tilted 7 degrees from the perpendicular direction), the observed MR ratio was typically about 1.5% with an RA of about 0.06. A typical RH curve is shown in  FIG. 2 . 
         [0021]    After insertion of the above-specified trilayer (MREL) into the ferromagnetic (FM) FeCo layer, the resulting full structure became Ta10/Ru20/Cu20/[Co2/Ni6]×15/Cu20/FeCo75/Cu3/ZnO15/Cu3/FeCo75/Ru10/Ta40/Ru30). The MR ratio of the MREL modified structure was found to have increased significantly (from about 1.3% to as high as 17%) with little change in the RA product. A typical curve is shown in  FIG. 3 . This measurement was performed at an angle of 7 degrees off vertical so it is likely that if the applied field used for the measurement is made to be precisely normal to the plane of the film, a much higher MR value will be obtained. 
         [0022]    In  FIG. 4  we show a schematic cross-section of a MREL in which a central semiconductor (or semimetal) layer  41  is seen sandwiched between two (optional) metal layers  42  and  43 . Some additional findings about the MREL structure were the following. First, the thickness chosen for the Cu3/ZnO15/Cu3 trilayer must ensure that the two 75 Angstrom thick FeCo layers on either side of it are strongly exchange coupled with each other. The fulfillment of this condition was confirmed when a MH loop measured on a Ta20/Ru20/IrMn70/FeCo75/Cu3/ZnO15/Cu3/FeCo75 film stack showed only one hysteresis loop for the two FeCo layers, indicating that said layers are strongly coupled together. 
         [0023]    Second, an in-plane measurement showed almost no MR ratio, which is very similar to what was observed for the reference case. These facts indicate that the high MR ratio cannot be coming from the magnetic switching of the [FeCo75/Cu31ZnO15/Cu3/FeCo75] stack of layers, but rather from the [Co2/Ni6]×15/Cu20/[FeCo75/Cu3/ZnO15/Cu3/FeCo75] switching across the Cu spacer. Thus, the [FeCo75/Cu3/ZnO15/Cu3/FeCo75] stack behaves as though it is a single layer (like the original FeCo150 layer that it replaced). 
         [0024]    These findings confirm that the Cu3/ZnO15/Cu3 trilayer acts as an MR enhancing layer. It is thought that the spin scattering coefficient of the [FeCo75/Cu3/ZnO/Cu3/FeCo75] stack is significantly enhanced by the insertion of the Cu3/ZnO15/Cu3 trilayer. This is in accordance with the Schmitt theory, which shows that when spin is injected from a FM metal (FeCo) into a semiconductor (e.g. ZnO), the spin coefficient is proportional to the ratio of the conductivity of the semiconductor to that of the FM metal. 
         [0025]    It is also possible that the Cu3/ZnO/Cu3 MREL acts as a spin filtering layer, possibly even one operating under a ballistic scattering regime. Regardless of the precise mechanism at work here, the MR enhancement that results from inserting the Cu/ZnO/Cu (or similar layer) can be realized in a wide variety of applications that we will detail below. 
         [0026]    Before describing several applications for these MRELs we note the following. In the general case, the MREL&#39;s structure is M1/low band gap, high mobility, N-type semi-conductor/M2, where (a) M1 and M2 are high conductivity metals such as (but not limited to) Cu, Ag, Au, C (including Graphene, nano-tubes etc), Zn, Ti, Sn, Cr, Al, Mg, and Ru. M1 and M2 will generally be the same material and will have the same thickness (ranging from 0 to 50 Å), with about 3 Å Cu for both M1 and M2 being preferred. However, M1 and M2 may be different materials and they may have different thicknesses without significantly affecting the effectiveness of the invention. 
         [0027]    (b) Additionally, the basic M1/semiconductor/M2 trilayer structure that makes up the MREL may be repeated in situ one or more times, each such additional trilayer not necessarily comprising the same materials and/or individual thicknesses as other trilayers in the same MREL. 
         [0028]    (c) The band gap of the semiconductor portion of the MREL should be in a range of from 0.3 to 8 eV, with from 1 to 6 eV being preferred. 
         [0029]    (d) The electron mobility of the semiconductor portion of the MREL should be in a range of from 10 to 2,000,000 —  cm 2 ·sec −1 ·V −1  with from 50 to 50,000 cm 2 ·sec −1 ·V −1  being preferred. 
         [0030]    Examples of suitable semi-conductor materials include (but are not limited to) ZnO, ZnS, Zn x Mg (1−x) O (x ranging from 0 to 0.99), ZnCuO, ZnCdO, ZnAlO, ZnSe, ZnTe, Si, Ge, TiO 2 , AlN, GaN, InN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, ZnS, CdS, CdTe, HgTe, PbS, PbSe, PbTe, SnO, SnTe, Cu 2 O, FeSi 2 , CrMnSi, Mg 2 Si, RuSi 3 , and Ir 3 Si 5 . 
         [0031]    The semiconductor portion may be undoped or its conductivity may be adjusted by the inclusion of a dopant selected from the group consisting of Si, B, Mg, Mn, Al, Cu, Cd, Cr, Zn, Ti, Sn, Zr, Hf, Ru, Mo, Nb, Co, Fe, and. Ni. Moreover, the semiconductor portion may be replaced by a semi-metal such as Sb, Bi, CoSi, Co x Fe (1−x) Si, Co x Ni (1−x) Si, Co x Mn (1−x) Si, FeSi, or Co x Cr (1−x) Si. The semiconductor/semimetal layer should have a thickness in a range of from 1 to 50 Å, with from 3 to 20 Å being preferred. 
         [0032]    We now describe several examples of how these MR enhancing layers may be used to improve the performance of existing MR devices. These include (but are not limited to) GMR devices (including CIP, CPP, and CPP/CCP versions), TMR devices, spin injection devices, magnetic random access memories (MRAMs) and microwave assisted magnetic recording (MAMR) devices (see item 4 below). 
         [0033]    It will be understood that those skilled in the art may be able to identify additional spintronic devices not listed above (such as, for example, sensors of all types (including biosensors), thermally assisted magneto-resistance (TAMR) devices, or dual spin valve devices to which the principles disclosed by the present invention would obviously be applicable. 
       1. GMR Devices: 
       [0034]    A prior art structure is seed/AFM/AP2/Ru/AP1/conductive spacer/FL/cap. Possible structures based on the invention (see  FIG. 4 ) are the following: 
         [0000]    seed/AFM/AP2/Ru/[FM1/MREL/FM2]/conductive spacer/FL/cap [enhanced AP1];
 
seed/AFM/AP2/Ru/AP1/conductive spacer/[FL1/MREL/FL2]/cap [enhanced free layer];
 
and seed/AFM/AP2/Ru/[FM1/MREL/FM 2]/conductive  spacer/[FL1/MREL/FL2]/cap wherein both of the AP1 layer and free layer comprise a MREL.
 
         [0035]    The spacer layer could be a uniform metallic layer (CPP devices) or it could be in the form of conductive islands embedded in an insulating matrix such as, for example, Cu in Al 2 O 3  (CPP/CCP devices). 
       2. TMR Devices: 
       [0036]    A prior art structure is seed/AFM/AP2/Ru/AP1/barrier layer/FL/cap. Possible structures based on the invention (see  FIG. 4 ) are the following: 
         [0000]    seed/AFM/AP2/Ru/[FM1/MREL/FM2]/barrier layer/FL/cap [enhanced AP1];
 
seed/AFM/AP2/Ru/AP1/barrier layer/[FL1/MREL/FL2]/cap [enhanced free layer];
 
and seed/AFM/AP2/Ru/[FM1/MREL/FM2]/barrier layer/[FL1/MREL/FL2]/cap. The barrier layer could be MgO, AlO, TiO, ZnO, Zn/ZnO, or Zn/ZnO/Zn (as a laminate or as ZnO with a stoichiometric excess of Zn).
 
         [0037]      FIG. 5  is a schematic view of a GMR/TMR device after insertion of MRELs into both the AP1 layer (split into sub-layers  15 A and  15 B) and the free layer (split into sub-layers  17 A and  17 B). 
         [0000]    3. Spin Injection Layer (SIL) Devices that Operate Without an AFM/AP2/Ru/AP1 Pinning Stack: 
         [0038]    A prior art structure is seed/FM/spacer/FL/cap (including an H field normal to the layers). In other words, a field (H) is applied in a direction normal (perpendicular) to the layers after the SIL stack of layers is completed in order to magnetize the FM layer in a direction perpendicular to the top and bottom surfaces of the FM layer. Possible structures based on the invention (see  FIG. 5 ) are the following: 
         [0000]    seed/[FM1/MREL/FM2]spacer/FL/cap;
 
seed/FM/spacer/[FL1/MREL/FL2]cap];
 
and seed/FM/[FM1/MREL/FM2]/spacer/[FL1/MREL/FL2]/cap.
 
       4. Spin Torque Oscillator (STO) Devices (Particularly the FGL or Field Generating Layer): 
       [0039]    A prior art structure is seed/SIL/spacer/FGL/cap (including an H field normal to the layers). Possible structures based on the invention (see  FIG. 5 ) are the following: 
         [0000]    seed/[FM1/MREL/FM]/spacer/FGL/cap;
 
seed/SIL/spacer/[FL1/MREL/FL2]cap];
 
and seed/[FM1/MREL/FM2]spacer/[FL1/MREL/FL2]/cap wherein a first MREL is inserted in the SIL and a second MREL is inserted in the FGL.
 
         [0040]      FIG. 6  shows the three main component sections of a spin torque oscillator (STO), including SIL  62 , inter-layer  63 , and the FGL comprising the sub-layers  64 A and  64 B which are separated from one another by MREL  51 . 
         [0041]    For the examples listed in the above four categories, the seed layer could be Ta/Ru, Ta/Cu, Ta/NiFe, Ta/NiCr, Ta/NiFeCr, Ta/Ti, Ta/Ti/Cu, Ta/Ti/Ru/Cu, etc, the AFM layer (when present) could be IrMn, PtMn, PtPdMn etc, and AP2 (where present) could be CoFe, CoFe/FeTa/CoFe, or CoFe/CoFeB/CoFe, for example.