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Perpendicular Recording at High Areal Densities

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					Perpendicular Recording at
   High Areal Densities

      Eric Meloche

      February 14th, 2008
Eric Meloche   February 14, 2008
Areal Density=(bpi) X (tpi)

       Eric Meloche   February 14, 2008
Perpendicular Recording
                                                                  -    Magnetized
                                                                       head (“single
                                                                       pole head”)

                                                                  +
                                                                            storage
                                                                            layer
                                                                            interlayer




                                                                           SUL




                                      • A soft magnetic layer is used in double
          BL
                                      layer perpendicular media to “image” the head.

                                      • Transition is recorded by Deep Gap field


                        δP



   Eric Meloche   February 14, 2008
Eric Meloche   February 14, 2008
                                                                                                   Perpendicular hcp(0001)
 CoCrPt Crystal Structure and                                                                           basal planes
 Magnetization Orientation

                        Grain magnetic anisotropy, Hk,
                        lies along the crystalline c-axis


Saturation magnetization of Co100-x-yCrxPty
decreases with increasing Pt concentration Magnetocrystalline anisotropy of CoCrPt




         Eric Meloche       February 14, 2008   T. Shimatsu, H. Sato, T. Oikawa, Y. Inaba, O. Kitakami, S. Okamoto, H. Aoi, H. Muraoka,
                                                Y. Nakamura, IEEE Trans. Magn. 41, 566 (2005).
Eric Meloche   February 14, 2008
 Limits on conventional perpendicular recording

         Large field or low anisotropy
                                              Write-ability
CoFe largest moment
Material Ms=2.4T



                 Thermal
                 Stability                                    SNR (grain size)

   Large grains or high anisotropy                             Small bits requires small grains




           Eric Meloche   February 14, 2008
  Stoner-Wolfarth Particle Switching Field


                                                                             Hk
                                                     H sw (θ ) =
                                                                   (cosθ 2 3 + sin θ 2 3 ) 3 2




     H                                           Energy Barrier View



 For a S-W particle, the switching field is a
 function of the crystalline anisotropy energy
 & the angle θ between the applied field &
  the particle easy axis direction:
Hk: effective anisotropy field


            Eric Meloche   February 14, 2008
                  Modern Perpendicular Writer
                                                Top Return Pole

Front Shields

                                             Main Pole            3 turn coil

                                               Bottom Pole Top




                                          Cross sectional view
                                                           Top reader element
                                        Magneto Resistive (MR) Return Pole
                                                         Front Shields


                  Main pole tip with                 Tip of main pole
   Eric Meloche   Trapezoidal shape
                    February 14, 2008
               Why the Trailing Shield Shield ?

                     Shields




                              Pole
                                                                                  Switching field/Hk
                      Perpendicular    Downtrack
             16000              no FS
                               with FS

             12000
Field (Oe)




             8000


             4000


                 0
                                                                             Better field angle=> helps to switch grain
             -4000                                                  Large field gradient give sharper transition- helps SNR
                           -100      0        100       200   300
                           Downtrack Direction (nm)

                     Eric Meloche        February 14, 2008
     How a writer works?
                                               Main Pole
    Pole tip material: CoFe 2.4T
                                          δP


                                          tK
                                                           Media Soft Under Layer

        • Coil will generate about 200 Oe
        • Fields in the write gap ~ 20 000 Oe
        • 100 X increase in field
        • Pole material is magnetically soft – easily aligns
         to magnetic field                                           M
                                                                                    MH loop
Soft Magnetic Material means that M can                                         H   of soft material
easily align with the applied field H.
           Eric Meloche   February 14, 2008
FEM modeling (Finite Element Method)
  • commercial software – look at write head fields
    • Not micromagnetics – solves Maxwell’s equations on a mesh
   No exchange interactions. Moments are induced by external fields only (M=0 if Hext=0).
    • Useful for large structures like the Write Element (~ 10 um).

                  Top Return Pole
                                                                  Main Pole
                             SUL




                             3 Turn Coil




                         Side View
          Eric Meloche   February 14, 2008
              Bubble Trailing Edge
                                        Write bubble defined by the pole tip shape




 Skew=0o



   2D magnetic field footprint in media plane

Skew=18o



                                                    The perpendicular track width
At skew, the side of the writing bubble              increases more rapidly with skew
produced by the pole field is projected
                                                    Skew effects can be suppressed
along the disc radial direction.
                                                    With trapezoidal pole
The effect causes an increase in the track
pitch and a reduction in the track density.
             Eric Meloche February 14, 2008
Written tracks and Skew
 -ADJ                    +ADJ                                                -ADJ                   +ADJ
 track                   track                                               track                  track
                         wrt Px                                                                     wrt Px
 wrt Nx
          BGI
                                                      Drive                  wrt Nx   BGI



                                                       OD             ID
                                                      Skew>0     Skew<0
                                                      High DR High Skew



                                                      Z0              Z16
Reader                   Reader
            Data
                                                            MD               Rea                      Rea
                                                                                                         der
                                                                                der
            track                                          Skew=0
                                                                                            Data
                                                           High BPI
                                                                                            track



                                               KTPI
                                       Kilotracks/inch

                                                                            Track pitch larger at ID and OD than
                                                                            at MD due to larger erase-bands.

                                                                            TPI capability (TPIC) vs. zone
                                                                            ~ skew is “TPI WARP” curve



          Eric Meloche            February 14, 2008
                                                            Zone (radius)
Micromagnetics                       Landau-Lifschitz-Gilbert equation
                  r
                                    αγ r
                                                      (         )
              2 dM
                        r r                r r
        (1 + α )    = −γM × H eff −    M × M × H eff                                   Damping
                 dt                 M
                                                                                      α ~ 0.1 – 0.01
                                                                                   Spin-wave interactions,
                                              Interacting bar magnets              magnetoelastic coupling,
                                                                                   impurities,…


Each grain M(r) is acted upon by an effective field with numerous contributions: Heff (r)= -δE/δM(r)
 r             r           r              r            r
 H effective = H applied + H anisotropy + H exchange + H magnetosta tic
                  E.g., Writer        Very strong in     NN only                       Long-Range
                  head field          Media. Very                           NN         demagnetization
                                      weak in Head H ex (ri ) = − JS / M s ∑ M j
                                                                    2     2

                                                                             j
                                                                                       fields increase
                                      materials.                                       computational
                                                                                       demands.



          Eric Meloche    February 14, 2008
                                                                Werner Scholz
 Micromagnetic Model
    Parallel finite element micromagnetics
    code "magpar"
    Head model imported from Flux3D,
    current density distribution and magnetic
    field of coils calculated with Flux3D
                                                Partitioning for 5 processors
    Contributions to the total energy:
      • exchange energy
      • magnetocrystalline anisotropy
      • Zeeman energy
      • magnetostatic energy
    Magnetization dynamics obtained by
    integration of LLG
    Material parameters:
    Pole tip: Bs=2.4 T, HK=50 Oe                Field of coil
    yoke:     Bs=1.0 T, HK=20 Oe
    SUL:      Bs=2.0 T, HK=30 Oe
    Zero temperature, no eddy currents
http://magnet.atp.tuwien.ac.at/scholz/magpar/

         Eric Meloche   February 14, 2008
               Movie of magnetization dynamics for

                        α=1.0 Large damping



Main Pole




Soft Underlayer
    Eric Meloche   February 14, 2008
     Erasure in Perpendicular Recording
•      Erasure sources:
                •   write pole erasure
                • On-track erasure (erase-after-write)                                           RP
                • Adjacent track erasure                                                 Erasure induced by
                                                                                          RP’s interacting
Bottom shield




                • Side erasure (further away)
                                                                                         with SUL response
Top shield




                                                                                                                                Main
                                                                                          to off-track field
                                    p o le                   main
                                n                       iter le                                                                 pole
                           retur                      Wr p o
                    Writer
                                                                                                                               RP
                                                                                                               erased region
                                                                 Wr
                                                                    itt   en                                   skew
                           Return flux in SUL                                  tra
                                                                                  ck
SIDE-TRACK                                        ADJACENT
 ERASURE                                        TRACK ERASURE
                                                                                       •In perpendicular recording the write
                                                                                       fields must be controlled by proper
    MFM image                                                                          head and media design

    showing STE



                         Eric Meloche        February 14, 2008
Overshoot Effect & Field Dynamics
                                                                                    Test Point
 A full size writer & media SUL were used to                                                     Return Pole
 study the field dynamics & overshoot effects
                          100mA                                                                  Main Pole

                                                              Linear Response:         Return Pole
                                                          Field overshoot is seen
                                                 50mA
                                                             in the return pole
                                                                                             Fields in the media
                                                                                              at the test point
                                  Main pole write field                                     near the return pole




                     Overshoot magnitude=100%




  Overshoot helps increase write field
  switching speed (reduced rise time)                     Results from MagPar
                                                             using a = 0.1
  Return pole fringe field is more sensitive
  to overshoot (Linear response)                                                      Non-linear Response:

  The data shown here only represents fringe field at                                 No overshoot is seen
  a point in time & in a specific location                                            in the main pole field



           Eric Meloche      February 14, 2008
SUL Media Domains                                                        H                                            Return Pole
                                                                                                                                            Test Line

                                                                   recording layer
    Iw                                       Pole Location
             Current with overshoot                                                                                                       Main Pole
                                                                                     M in SUL
                       t         Domains in SUL
                                                                Writer Flux                                                                Return Pole
                                                                 in SUL Pole Location
                                                              Mx            1000
                                                                                                                                              Field in the media
                                                                                                                                 Hx_RP1        at the test point
                                                                                                                                 Hy_RP1      near the return pole
x                                                                                                         500
                                                                                                                                 Hz_RP1




                                                                                     Fringe field (Oe)
         z              Pole                                  My                                            0

                      location
                                              M in SUL                                                    -500



                                                                                                         -1000

                                                              Mz
                                                                                                         -1500
                                                                                                                 -5      -3         -1        1           3   5

                                                   Steady state + overshoot = 100mA                                           cross track distance (um)

                                                   Steady state writer current = 50mA (0-p)
             Experiment                            Overshoot writer current = 50mA

                                                        The MagPar micro-magnetic model was used to
                                                         investigate the writer fields & SUL magnetization
                                                         behavior (LLG α was 0.1)
                                                        A domain was formed in the SUL in an area where the writer
                                                        pole field opposed the SUL magnetization

J. Fernandez-de-Castro et al., IEEE Trans Magn (2007)
Exp. Ref: J. Heidmann et al., “Direct Measurement of Magnetodynamics in a Perpendicular
Recording System”, IEEE Trans. Magn. (2006)
                 Eric Meloche      February 14, 2008
More on Write Field Dynamics                                                      160 nm




                                                                          25 nm
Need to switch magnetization
very fast!
How can we make the writer
faster?
                                                Servo Actuator

Transfer rates ~ 3Gbit/sec

  Relative head-disc linear velocity exceeds 50 nm/ns in high-performance disc drives.
  Flux reversal time in the writer must be faster than 200 ps to write 25 nm long bits!

             Eric Meloche   February 14, 2008
                                                      Perpendicular field at trailing edge of pole
               Use Overshoot waveform
                                                                                 Control of the damping
                                         t_dur=0.15
                      80
                                         t_s=0.15
                                         total=1.0

                      40
Current (mA)




                       0

                                                      Overshoot
                      -40                                  0
                                                           50
                      -80                                  100


                            0    1            2          3          4
                                         Time (ns)
                      1.2
                                                          OS_0
H_Perpendicular (T)




                                                          OS_50
                      0.8                                 OS_100

                      0.4

                      0.0

                      -0.4                                                                  α<0.1: fast response,
                      -0.8                                                                  slow approach to saturation
                      -1.2                                                                  α>0.4: slow response,
                          3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
                                        Time (ns)
                                                                                             slow approach to saturation
Overshoot helps increase write
field switching speed (reduced rise time)

                                     Eric Meloche            February 14, 2008
    Increasing the data rate                                          current waveform:
          Micromagnetic Simulation                                    - 0.2 ns rise time
       steady state                               0.8     0.4 0.1 0   - decreasing steady
       period          3 ns      2 ns         1         0.6 0.2 0       state period
                                                                      - 0.2 ns decay time




                                                                      Maximum field
                                                                      decreases with
                                                                      increasing data-
                                                                      rate



W. Scholz, S. Batra, IEEE Trans. Magn. 41 (2005) 702-706.

             Eric Meloche     February 14, 2008
     How to get to 1Tb/in2 and beyond
 X Assisted Magnetic Recording
            WAMR- Wire Assisted Magnetic Recording
            MAMR- Microwave Assisted Magnetic Recording
            HAMR- Heat Assisted Magnetic Recording
                     All based on the same concept

enable higher anisotropy (Hk)                 smaller grain size   better SNR.


 Composite Media- ECC
            Exchange Coupled Composite Media


 Patterned Media
            DTR – Discrete Track Recording
            BPM – Bit Patterned Media



      Eric Meloche     February 14, 2008
  WAMR - Wire Assisted Magnetic Recording
Basic Physics: Use the field produced by a simple wire to give a
“boost” to the total field produced by the main pole.

                         • use Perpendicular recording head   +
                           Field from wire

                         Hwire ~ 1/r

                                                                           Side view
                                                                  Front Shield

      Main pole
                                                                  Assist wire

                                                                   Main pole
                   Assist wire
          Eric Meloche      February 14, 2008
                                             Recessed Wire – Wire Height
                                                               SUL                                                                     SUL
                             Front shield                                                              Front shield


                                                                                                     Recessed wire
                      Recessed wire



                                                                                            Write pole
        Write pole
                                        Wire height 100nm                                                            Wire height 50nm



                      7000                                                                           7000
                                                         wc 0 cc 50                                                              wc 0 cc 50
                      6000                                                                           6000




                                                                               In-plane field (Oe)
In-plane field (Oe)




                                                         wc m20 cc 50                                                            wc m20 cc 50
                      5000                                                                           5000
                                                         wc p20 cc 50                                                            wc p20 cc 50
                      4000                                                                           4000
                      3000                                                                           3000
                      2000                                                                           2000
                      1000                                                                           1000
                         0                                                                              0
                         400        450            500       550         600                            400    450        500        550     600
                                            Ontrack (nm)                                                              Ontrack (nm)




                                    Eric Meloche         February 14, 2008
    MAMR – Microwave Assisted Recording
       • Novel method for recording at a head field below coercivity


  Basic Theory: use Ferromagnetic Resonant Phenomena
   • Microwave induced precession of media leads to improved writeability

   But:     Precessional switching may lead to longer
   media grain switching times which can result in transition broadening




                                                        This is bad for SNR!!


                                                 • We have to control location of
                                                 RF field and gradient


• Trajectory of magnetization in Presence of AC field

              Eric Meloche   February 14, 2008
                                                                  J. Zhu, IEEE Trans. Mag. 2008
Summary of MAMR Effect
 Proposed by J Zhu in 2003
 Presence of MW field in media leads to Sub-Stoner Wohlfarth switching
 Up to 3X AD gain projected (Zhu modelled >1.5Tb/in2)
  Media experiences conventional write pole field plus an
     in-plane magnetic field in the 10 – 40GHz range

                               Write Pole
                             Writer
                                 Field




                             In-Plane Assist F
                                MW Assist
      Media
                                  Field
                               (500 – 1500Oe



     Switching field at optimal frequency
      At 300 the switching field is about
      1/3 of the Stoner Wohlfarth value
              Eric Meloche     February 14, 2008
     • Assist field (2 kOe downtrack, 20 GHz)

                                     Micromagnetic simulation
                                                                       same media but with
                                                                       tuned RF frequency




Assist field is applied
uniformly within
region delineated by
white lines.                No assist            RF edge at trailing edge
                            Can barely write     Sharp transitions and less
                            down track center    curvature – Good for SNR
                            Media is very
                            difficult to write
                            without assistance
             Eric Meloche   February 14, 2008
  HAMR – Heat Assisted Magnetic Recording
 As track widths narrow and media anisotropy increases, max head field attainable will
 soon become insufficient to write the disc. What then? Researchers at Seagate are
 developing a solution where the medium is locally heated to above its Currie temperature,
 oriented, and then quenched to preserve the desired orientation.

Principle of                                                                   Drive
                                                                            Temperature




                                                       C oercivity
HAMR
                                                                                                    Heat
                                                                          Store                     Media
                                                                          Here
                                                                                            Cool
                                                                                            Media



                                                                     Available Head Field                   Write
                                                                                                            Here

                                                                                                    Temperature

  • The challenge in making HAMR work is in creating sufficient thermal gradients that will prevent
  interference between adjacent bits.

            Eric Meloche   February 14, 2008
 ECC - Exchange Coupled Composite Media
Victora, Shen (2004)
  • ECC media design offer design space to provide
                                                        Happl
  better writeability, Better thermal stability, and                       Soft: low Hk
  SNR improvement
 • Magnetic soft and hard regions within the
 same grain
                                                                           Hard: high Hk
 • Can decrease the switching field of the
 media
                                                                Hexc
                                                       Switching Field vs Field Angle
 • Magnetic hard layer (high Hk) provides
 the thermal stability


 • Magnetic soft layer (low Hk) functions
 as a ‘lever’ to help switch the hard layer
 during writing.




             Eric Meloche   February 14, 2008
Experiments (Shen et al. 2005)



                                                           PdSi interlayer controls
                                                           exchange coupling

                                                     Remanent Coercivity of ECC media and
                                                     Single Hard layer versus field angle




                                         C

         A         B

 Exchange strength            A- uniform switching           C
                              B-dynamic tilted switching
           Eric Meloche   February 14, 2008
                              C- exchange decoupled case
    DTR - Discrete Track Recording
                                          Advantages
     On track: Reduce track edge noise → SNR improvement
     Off track: Reduces ATI and ATE exposure → High TPI

   Patterned region


                            Discrete Tracks



Recording track

Experiment




             Eric Meloche   February 14, 2008
                                       Micromagnetic Modelling J. Zhu, Carnegie Mellon
BPM- Bit Patterned Media
  Lithographically Patterned Bits, single magnetic grain
to decrease transition jitter and increase SNR.
  Self-Assembled Magnetic Nanoparticle Arrays




       Eric Meloche   February 14, 2008
         BPM- Bit Patterned Media
Timing errors: fluctuations of the write current phase
caused by electronics, spindle speed variations etc...                                write bubble
          requires "timing synchronization" solution                                dot position
                                                                      write field   fluctuations

                             target dot                v
                                                                        sD


                                                W      ideal timing
                                                            head does no longer catch dot
                                                    head catches wrong dot




              Eric Meloche      February 14, 2008
Let’s get creative: 10 Tb/in2 design:
"nail" writer
                                         Down-track


                                           Main pole


                                       40 nm 10 nm

                                          HMS=2nm


                                                      Thickness=16nm
                                       IL=1nm

                                                SUL



    Eric Meloche   February 14, 2008
    Staggered Bits


                                                         Hk=9 kOe
                               8nm
                                                   8nm   Ms=1400 emu/cc
                                                         Aexc=2·10-6 erg/cm
    TP=16nm
                                                       Hk=80 kOe
                                                   8nm Ms=1000 emu/cc
                                                       Aexc=1·10-6 erg/cm
                       BL=4nm
                                           4.5nm


              AD=10Tb/in2: LD=6400kfci, TD=1600ktpi, BAR=4

Same lithography feature sD, but extreme head dimensions
avoided

        Eric Meloche   February 14, 2008
                  Thanks for listening




Eric Meloche   February 14, 2008
Magnetic Materials and Fields
                                      • Carbon 3-4nm
                                      • Magnetic layer ~20nm (CoCrPt)


                                      • Interlayers


                                      • SUL 80-200nm




   Eric Meloche   February 14, 2008
Writer Risetimes
• Goals: Determine the most important factors that limit
flux rise times throughout the entire pole structure
                                    Simple Picture
                                           • Perpendicular component of magnetization in the
                                           interior of the writer has a fast response to coil field
  AB C
                                           Points B and C - Fast response time
                                          • Magnetization response slows down near the ABS
                                                Point A - Slow response time
                                                   Time resolved optical measurement of
                                                   Pole region magnetization
                                                         Coil current


       Eric Meloche   February 14, 2008
                                          M.R. Freeman et al., J. Appl. Phys. 81 (1997)

				
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