Perpendicular Magnetic Recording Dmitri Litvinov and Sakhrat Khizroev Seagate Research
Acknowledgments Leon Abelmann (U Twente) James Bain (CMU) Chunghee Chang Roy Chantrell Roy Gustafson Kent Howard Earl Johns Jack Judy (U Minnesota) Mark Kryder Andreas Lyberatos Terry McDaniel Ed Murdock Kevin O Grady (U York) Rajiv Ranjan Thomas Roscamp Robert Rottmayer Michael Seigler Erik Svedberg Dieter Weller Jason Wolfson (CMU/Maxtor) November 5, 2002 Page 2
Outline Overview Superparamagnetic limit and the need for a new technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 3
Scaling: Primary Technology Approach Magnetizing Coil MR Reader Inductive Ring Writer S N N S S N N S S N N S Scaling S N N S S N N S S N N S Write field Recording Media Longitudinal recording has been the underlying technology in the disk drive industry for the past several decades November 5, 2002 Page 4
From RAMAC to Seagate Cheetah Drive 4.4 MB 73.4 GB 70 kbit/s IBM RAMAC 1955 2 kbits/in 2 50x24 dia disks 888 Mbit/s Seagate Cheetah 15K.3 34 Gbits/in 2 4 x 2.5 dia disk November 5, 2002 Page 5
Progress in Magnetic Data Storage 10000 Areal Density (Gbit/in 2 ) 1000 Seagate 100 Gbit/in 100 2 Fujitsu Hitachi Seagate (32.6 Gbit/in 2 ) Read-Rite IBM (25.7 Gbit/in 10 IBM 2 ) 1 1 Tbit/in 2 LAB DEMOS Products Demos: ~100 Gbpsi long ~90 Gbpsi perp Products: ~33 Gbpsi long 40 GB per disk 0.1 Historical 60% CGR line 1990 1995 2000 2005 2010 Year November 5, 2002 Page 6
Superparamagnetic limit Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 7
Media Microstructure, Scaling, and SNR Magnetic grains SNR ~ log(n), N - number of grains per bit Bit transition While scaling, need to preserve number of grains per bit to preserve SNR Grain size is reduced for higher areal densities: a ~ 1 Areal Density November 5, 2002 Page 8
Superparamagnetism Probability of magnetization reversal due to thermal fluctuations: E ± f± = f0exp k BT f 0 K V U ~ 10 9-10 12, E anisotropy energy grain volume ± K V Energy, ev 2.0 1.5 1.0 0.5 0.0-0.5 Thermally stable media: U θ E + E - -45 0 45 90 135 180 225 K U V T k B H Magnetization angle > 40 60 November 5, 2002 Page 9
Media Writability Limit K U V 1 1 Stable media: ~ 40 60 KU ~ or KU ~ 3 T V a k B H write > H = α 0 2K M U S N eff M S ~ 1 a 3 ~ Areal Density 3/ 2 Higher areal density media requires higher write fields!!! H write ~ M S of the head material Highest 4πM S (=B S ) available today is ~26 kgauss (2.6Tesla) In longitudinal recording, the highest write field possible to generate is ~2πM S!!! November 5, 2002 Page 10
Perpendicular vs. Longitudinal Magnetizing Coil MR Reader Inductive Ring Writer Perpendicular S N N S S N N S S N N S Magnetizing Coil Inductive SPH Writer MR Reader Write field Recording Media Longitudinal Write field Recording layer SUL November 5, 2002 Page 11
Gap versus Fringing Field Writing Real head Coil Yoke Coil Gap fields Physical Gap Effective Gap SUL boundary Fringing fields Recording medium Transition Written moment in media Image head In perpendicular recording the write process effectively occurs in the gap (Write Field ~ 4πM S ) In longitudinal recording the write process is done with the fringing fields (Write Field ~ 2πM S ) November 5, 2002 Page 12
Advantages of Perpendicular Recording Higher write field amplitude - can use higher anisotropy media, better thermal stability Higher write field gradients and well aligned recording layers - thicker media, better thermal stability Zero demag at transitions - sharp bit transitions, more stable recorded data Decrease of demag with areal density increase - improved media stability at higher areal densities Higher playback amplitude - improved playback performance at higher areal densities November 5, 2002 Page 13
Soft Underlayer Challenges Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 14
Domain Noise and SUL Biasing Playback (uv) (with amplification) 80000 60000 40000 20000 0-20000 -40000-60000 Single Transition ---- Non-biased soft underlayer ---- Biased soft underlayer -80000 175 200 225 250 275 300 0.15 Time (nsec) +++ ----- 2.00E-01 head Hard layer Soft underlayer Magnets ----- +++ Playback 0.1 0.05 0 0.00E+00 5.00E-07 1.00E-06 1.50E-06-0.05 Biasing Playback 0.00E+00 5.00E-07 1.00E-06 1.50E-06-0.1-0.15-2.00E-01 Time (s) Time (s) November 5, 2002 Page 15
Recording Layer and SUL Magnetics Magnetization (a.u.) 4 2 0-2 -4 20nm SUL Magnetization (a.u.) 4 2 0-2 -4 20nm SUL + RL -30-20 -10 0 10 20 30 Field (Oe) -50-25 0 25 50 Field (Oe) TABLE: Coercivity of SUL in the presence of a Recording Layer Permalloy FeAlN Ni45Fe55 CoCrPtTa 25 Oe 20 Oe 7 Oe Multilayer 15 Oe 11 Oe 3 Oe Higher moment/anisotropy SUL + lower moment RL minimizes the effect November 5, 2002 Page 16
M (a.u.) M (a.u.) Dynamic Kerr Microscopy 300 200 100 0-100 -200-300 0 2 4 6 8 10 12 14 16 18 20 300 200 100 0-100 -200 Time (ns) -300 0 2 4 6 8 10 12 14 16 18 20 Time (ns) 20nm SUL 20nm SUL + RL Writer Media Stack Glass Substrate Microscope Presence of a RL can dramatically affect the dynamics of a SUL November 5, 2002 Page 17
Soft Underlayer and Playback Resolution PW50 (nm) 29 28 27 26 25 Sensitivity Function Amplitude PW50 0 20 40 60 80 100120140 SUL-to-ABS distance (nm) 1.0 0.9 0.8 0.7 0.6 Sensitivity Function Amplitude (normalized units) The underlayer boundary line Real head Image head Recording layer Buffer layer Soft underlayer introduces asymmetry into the playback system. November 5, 2002 Page 18
Soft Underlayer Micromagnetics Playback (dbm) CoCrPtTa recording layer 0 FeAlN soft underlayer No soft underlayer -10-20 -30-40 -50-60 -70 250 500 750 1000 1250 t Linear Density (kfci) Perfect imaging CoCrPtTa alloy based recording layer is capable of recording densities well in excess of 600kfci. Further development is necessary to minimize noise and/or distortions caused by the presence of the soft underlayer 2t Soft underlayer Distorted imaging November 5, 2002 Page 19
Skew Angle Sensitivity Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance via Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 20
Skew angle Zero skew P2 Non-zero skew P2 Trailing edge Trailing edges Track direction Skew angle (±15 degrees) November 5, 2002 Page 21
Skew Angle Sensitivity Zero skew Trailing edge P2 Non-zero skew P2 Trailing edges Track direction 25kfci Skew = 0 degrees Skew = 15 degrees Effective track width increase Loss in areal density Playback Signal (mv) 2.5 2.0 1.5 250kfci 250kfci -50-25 0 25 50 Offset across the track (µin) November 5, 2002 Page 22
Narrow Gap Single Pole Heads H MAX (koe) 1.8 1.6 1.4 1.2 1.0 Gap 0.2 0.4 0.6 Trailing Pole Thickness (µm) Trailing Pole Thickness Conventional SPH Narrow Gap SPH Hz (koe) 1.5 1.0 0.5 0.0 Trailing pole Trailing edge 0nm 30nm 70nm 150nm 300nm 700nm 0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (µm) In a narrow gap single pole heads, the write field is reduced towards the leading edge, thus, minimizing the skew angle sensitivity Can minimize the loss in track density from 25% to less than 5% Gap Thickness: November 5, 2002 Page 23
Track width (µm) Skew Sensitivity of a Gapless Writer 1.8 1.6 1.4 1.2 1µm Gap Gapless -15-10 -5 0 5 10 15 Skew angle (degrees) 6G Gapless Writer P1 P2 W P2 = 1.2µm t P2 = 3.3µm Gapless writer has substantially reduced sensitivity to non-zero skew angles November 5, 2002 Page 24
Flux Conductance in Nanoscale Writers Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 25
Nanoscale Writers H z (koe) 3.5 3.0 2.5 2.0 1.5 500 nm pole (I up) 1.0 500 nm pole (I down) 0.5 1000 nm pole (I up) 1000 nm pole (I down) 0.0 0 200 400 600 800 1000 1200 I, ma x turn Vertical field component above the center of the ABS at a 20nm spacing vs. write current for a single-pole head Normalized M z 1.0 0.8 0.6 0.4 0.2 0.0 0 200 400 600 800 1000 L z (nm) Remanent magnetization after reversal for a 170nm x 85nm single pole tip. For L z > 432nm, the pole tip has non-zero remanence. Can writers with nanometer scale dimensions smaller than the domain wall thickness conduct flux efficiently? What can be done to reduce remanence in nanoscale writers? November 5, 2002 Page 26
FIB-trimmed Nano Writers Tilted ABS View ABS View Void 40nm A singularity has been formed at the ABS to alter micromagnetic behavior of the pole tip November 5, 2002 Page 27
Nanoscale Writers MFM signal (a.u.) 1.0 0.5 0.0-0.5-1.0 750 Zero remanence Non-zero remanence 1000 L=500-500 -250 0 250 500 Drive current (ma turn) MFM signal (a.u.) 1.5 1.0 0.5 0.0-0.5-1.0 L=200 Non-zero remanence 400 300-500 -250 0 250 500 Drive current (ma turn) Void formation at the ABS substantially reduces remanence in nanoscale writers November 5, 2002 Page 28
Towards Optimum Reader Design Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 29
Perpendicular vs. Longitudinal Playback Perpendicular + + H stray M + + charges in the transition longitudinal Playback Signal H stray + + + + + + + + - - - - - - - - - - - - - - - - - - - - + + + + + + + perpendicular Time Longitudinal If a conventional reader is used, the channel sees the playback signal of different shape Can differentiate, however, part of the information is lost Playback Signal Time November 5, 2002 Page 30
Parallels between Perpendicular and Longitudinal Recording Reciprocity Principle: M - media magnetization H - sensitivity field S shield ~ MR Sensor 1 I shield M H r MR Sensor MR Sensor shield MR Sensor MR Sensor shield 6 Recording Medium 1.0 Recording Medium Recording Medium Sensitivity Field (a.u.) 4 2 0-2 -4 Shielded Reader (Hx) Diff Reader (Hz) Shield Diff (Hz) -6-200 -100 0 100 200 Distance along the track (nm) Sensitivity Field (a.u.) 0.5 0.0-0.5-1.0 Shielded, Hx Differential, Hz Shielded Diff, Hz -200-100 0 100 200 Distance along the track (nm) November 5, 2002 Page 31
Performance Comparison Playback (a.u.) 35 30 25 20 15 10 5 Shielded (perpendicular) Differential (perpendicular) Shield Diff (perpendicular) 0 0 1000 2000 Linear Density (kfci) Normalized Playback 1.0 0.8 0.6 0.4 0.2 Shielded Differential Shielded Differential 0.0 0 1000 2000 Linear Density (kfci) Differential reader provides highest amplitude playback Shielded differential reader provides best spatial resolution November 5, 2002 Page 32
Equivalent Perpendicular Reader Conventional shielded reader shield Differential reader MR elements yoke shield MR element Longitudinal media Perpendicular media Playback Signal Playback Signal Time Time November 5, 2002 Page 33
Materials Issues Overview of magnetic recording Superparamagnetic Limit and the Need for a New Technology Soft Underlayer Challenges Skew Angle Sensitivity Flux Conductance in Nanoscale Writers Towards Optimum Reader Design Materials Issues November 5, 2002 Page 34
Perpendicular Media Materials STORAGE LAYER : Seed/Exchange de-coupling layer SOFT UNDERLAYER : Overcoat High squareness Exchange de-coupling Grain size control Efficiency of the recording system Soft underlayer noise Buffer layer Substrate Composition & Microstructure Magnetic Properties Performance November 5, 2002 Page 35
Recording layers: Higher K u Materials Alloy System Material Anisotropy Saturation Magnetization Anisotropy Field Minimum stable grain size K u (10 7 erg/cc) M s (emu/cc) H k (koe) a (nm) CoCrPtX 0.20 200-300 15-20 8-10 Co-alloy Co 0.45 1400 6.4 8.0 Co 3 Pt 2.00 1100 36 4.8 FePd 1.8 1100 33 5.0 L1 0 -phase FePt 6.6-10 1140 116 2.8-3.3 CoPt 4.9 800 123 3.6 MnAl 1.7 560 69 5.1 Rare Earth Nd 2 Fe 14 B 4.6 1270 73 3.7 SmCo 5 11-20 910 240-400 2.2-2.7 Minimum thermally stable grain size: a 3 60 k T B K u November 5, 2002 Page 36
Microstructure of Recording layers TEM 80000 70000 60000 CoCrPtTa on Ti FWHM=6.3 0 X-ray rocking curve ideal 50000 40000 30000 20000 10000 0 0 10 20 30 40 ω non-ideal Average grain size ~ 13nm (00_2) fiber-like texture with texture spread of 6.3 0 CoB/Pd multilayer on ITO Co Pd Average column size ~ 20nm Randomly oriented November 5, 2002 Page 37
Media Grain Size Engineering x ln A xc y = y + 0 exp 2 2π wx 2w 2 Frequency (count) 35 30 25 20 15 10 5 0 Grain Size Distribution thermal instabilities media noise 0 5 10 15 20 25 30 35 Grain Diameter (nm) Narrowing the grain size distribution improves SNR and media stability November 5, 2002 Page 38
Seagate Perpendicular Recording Demo Intermag 02 NAPA DEMO SOFTWARE DEMO PW50 (nm(uin)) 64.6 2.54 64.6 2.54 ACSN (db) @ Tc=84nm 17.6 17.6 @Channel Bit Density 14 12 Data Rate (Mbps) 277 368 Reader Width (nm(uin)) 101.6 4 101.6 4 Writer Width (nm(uin)) 165.1 6.5 165.1 6.5 Channel Bit Density (kfci) Normalized Density 491 1.3 650 1.7 Tp (nm(uin)) (10% OTC 1e-4) 178 7 178 7 ktpi 142.9 142.9 Channel Channel Density (Gb/in 2 ) NAPA 70.2 Software 92.9 Target [4 6-3 -5-2] GPR6 Code 96/102 Multiple Parity Bits(d,k)RLL (0,7) TR57 Code Rate 0.94 0.90 Post Processor on and correcting User density (Gb/in 2 ) 66.0 83.9 On Track BER 2.4E-05 4.8E-05 Spared Sectors 5/100 November 5, 2002 Page 39