Perpendicular Magnetic Recording. Dmitri Litvinov and Sakhrat Khizroev Seagate Research

Transcription

1 Perpendicular Magnetic Recording Dmitri Litvinov and Sakhrat Khizroev Seagate Research

2 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

3 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

4 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

5 From RAMAC to Seagate Cheetah Drive 4.4 MB 73.4 GB 70 kbit/s IBM RAMAC 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

6 Progress in Magnetic Data Storage Areal Density (Gbit/in 2 ) 1000 Seagate 100 Gbit/in 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 Year November 5, 2002 Page 6

7 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

8 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

9 Superparamagnetism Probability of magnetization reversal due to thermal fluctuations: E ± f± = f0exp k BT f 0 K V U ~ , E anisotropy energy grain volume ± K V Energy, ev Thermally stable media: U θ E + E K U V T k B H Magnetization angle > November 5, 2002 Page 9

10 Media Writability Limit K U V 1 1 Stable media: ~ 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

11 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

12 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

13 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

14 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

15 Domain Noise and SUL Biasing Playback (uv) (with amplification) Single Transition ---- Non-biased soft underlayer ---- Biased soft underlayer Time (nsec) E-01 head Hard layer Soft underlayer Magnets Playback E E E E Biasing Playback 0.00E E E E E-01 Time (s) Time (s) November 5, 2002 Page 15

16 Recording Layer and SUL Magnetics Magnetization (a.u.) nm SUL Magnetization (a.u.) nm SUL + RL Field (Oe) 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

17 M (a.u.) M (a.u.) Dynamic Kerr Microscopy Time (ns) 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

18 Soft Underlayer and Playback Resolution PW50 (nm) Sensitivity Function Amplitude PW SUL-to-ABS distance (nm) 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

19 Soft Underlayer Micromagnetics Playback (dbm) CoCrPtTa recording layer 0 FeAlN soft underlayer No soft underlayer 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

20 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

21 Skew angle Zero skew P2 Non-zero skew P2 Trailing edge Trailing edges Track direction Skew angle (±15 degrees) November 5, 2002 Page 21

22 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) kfci 250kfci Offset across the track (µin) November 5, 2002 Page 22

23 Narrow Gap Single Pole Heads H MAX (koe) Gap Trailing Pole Thickness (µm) Trailing Pole Thickness Conventional SPH Narrow Gap SPH Hz (koe) Trailing pole Trailing edge 0nm 30nm 70nm 150nm 300nm 700nm 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

24 Track width (µm) Skew Sensitivity of a Gapless Writer µm Gap Gapless 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

25 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

26 Nanoscale Writers H z (koe) nm pole (I up) nm pole (I down) nm pole (I up) 1000 nm pole (I down) 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 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

27 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

28 Nanoscale Writers MFM signal (a.u.) Zero remanence Non-zero remanence 1000 L= Drive current (ma turn) MFM signal (a.u.) L=200 Non-zero remanence Drive current (ma turn) Void formation at the ABS substantially reduces remanence in nanoscale writers November 5, 2002 Page 28

29 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

30 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

31 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.) Shielded Reader (Hx) Diff Reader (Hz) Shield Diff (Hz) Distance along the track (nm) Sensitivity Field (a.u.) Shielded, Hx Differential, Hz Shielded Diff, Hz Distance along the track (nm) November 5, 2002 Page 31

32 Performance Comparison Playback (a.u.) Shielded (perpendicular) Differential (perpendicular) Shield Diff (perpendicular) Linear Density (kfci) Normalized Playback Shielded Differential Shielded Differential Linear Density (kfci) Differential reader provides highest amplitude playback Shielded differential reader provides best spatial resolution November 5, 2002 Page 32

33 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

34 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

35 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

36 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 Co-alloy Co Co 3 Pt FePd L1 0 -phase FePt CoPt MnAl Rare Earth Nd 2 Fe 14 B SmCo Minimum thermally stable grain size: a 3 60 k T B K u November 5, 2002 Page 36

37 Microstructure of Recording layers TEM CoCrPtTa on Ti FWHM=6.3 0 X-ray rocking curve ideal ω non-ideal Average grain size ~ 13nm (00_2) fiber-like texture with texture spread of CoB/Pd multilayer on ITO Co Pd Average column size ~ 20nm Randomly oriented November 5, 2002 Page 37

38 Media Grain Size Engineering x ln A xc y = y + 0 exp 2 2π wx 2w 2 Frequency (count) Grain Size Distribution thermal instabilities media noise Grain Diameter (nm) Narrowing the grain size distribution improves SNR and media stability November 5, 2002 Page 38

39 Seagate Perpendicular Recording Demo Intermag 02 NAPA DEMO SOFTWARE DEMO PW50 (nm(uin)) ACSN Tc=84nm 17.6 Bit Density Data Rate (Mbps) Reader Width (nm(uin)) Writer Width (nm(uin)) Channel Bit Density (kfci) Normalized Density Tp (nm(uin)) (10% OTC 1e-4) ktpi Channel Channel Density (Gb/in 2 ) NAPA 70.2 Software 92.9 Target [ ] GPR6 Code 96/102 Multiple Parity Bits(d,k)RLL (0,7) TR57 Code Rate Post Processor on and correcting User density (Gb/in 2 ) On Track BER 2.4E E-05 Spared Sectors 5/100 November 5, 2002 Page 39