perpendicular magnetic recording - thic · 02 46 8101214161820-300-200-100 0 100 200 300 ... p1 p2...

PerpendicularMagnetic RecordingDmitri Litvinov and Sakhrat KhizroevSeagate Research

Perpendicular Magnetic RecordingNovember 5, 2002

Acknowledgments

Leon Abelmann (U Twente)James Bain (CMU)Chunghee Chang

Roy ChantrellRoy GustafsonKent HowardEarl Johns

Jack Judy (U Minnesota)Mark Kryder

Andreas Lyberatos

Terry McDanielEd Murdock

Kevin O’Grady (U York)Rajiv Ranjan

Thomas RoscampRobert RottmayerMichael SeiglerErik SvedbergDieter Weller

Jason Wolfson (CMU/Maxtor)


Outline

OverviewSuperparamagnetic limit and the need for a new technologySoft Underlayer ChallengesSkew Angle SensitivityFlux Conductance in Nanoscale WritersTowards Optimum Reader DesignMaterials Issues


Scaling: Primary TechnologyApproach

S N N S N SS N S NN S

Inductive“Ring” Writer

MR ReaderMagnetizingCoil

Write field Recording Media

Longitudinal recording has been the underlying technology in the diskdrive industry for the past several decades

S N N S N SS N S NN SScaling


70 kbit/sIBM RAMAC 19552 kbits/in2

50x24” dia disks

4.4 MB

From RAMAC to Seagate Cheetah Drive

888 Mbit/sSeagate Cheetah 15K.334 Gbits/in2

4 x 2.5” dia disk

73.4 GB


Progress in Magnetic Data Storage

1990 1995 2000 2005 20100.1

1

10

100

1000

10000

1 Tbit/in2

100 Gbit/in2

Seagate (32.6 Gbit/in2)IBM (25.7 Gbit/in2)

SeagateFujitsuHitachi

Read-RiteIBM

Historical 60% CGR line

Area

l Den

sity

(Gbi

t/in2

)

Year

Demos:~100 Gbpsi long~90 Gbpsi perpProducts:~33 Gbpsi long40 GB per disk

Products

LAB DEMOS


Overview of magnetic recordingSuperparamagnetic Limit and the Need for a New TechnologySoft Underlayer ChallengesSkew Angle SensitivityFlux Conductance in Nanoscale WritersTowards Optimum Reader DesignMaterials Issues

Superparamagnetic limit


Media Microstructure, Scaling, andSNR

Magneticgrains

Bit transitionSNR ~ log(N), N - number of grains per bitWhile scaling, need to preserve number of grains per bit to preserve SNRGrain size is reduced for higher areal densities:

DensityAreala

1~


-45 0 45 90 135 180 225

-0.5

0.0

0.5

1.0

1.5

2.0

E+

E-

Ener

gy, e

VMagnetization angle

Superparamagnetism

H

θ

megrain voluenergy anisotropyE ,10-10~

kEexp

1290

B0

−−

≅∆

∆−=

±

±±

VK

VKf

Tff

U

U

Probability ofmagnetization reversaldue to thermalfluctuations:

6040kB

−>TVKUThermally stable media:


Media Writability Limit

6040~kB

−TVKUStable media: 3

1~or 1~a

KV

K UU

2/330 ~1~2 DensityAreala

MNMKHH SeffS

Uwrite −=> α

rial head mate theof ~ Swrite MH

Highest 4πMS (=BS) available today is ~26 kGauss (2.6Tesla)

In longitudinal recording, the highest write fieldpossible to generate is ~2πMS !!!

Higher areal density media requires higher write fields !!!


Perpendicular vs. Longitudinal

S N N S N SS N S NN S

Inductive“Ring” Writer

MR ReaderMagnetizingCoil

Write field Recording Media

Inductive“SPH” Writer

MR Reader

MagnetizingCoil

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Recordinglayer

SUL

Writefield

Longitudinal

Perpendicular


Gap versus Fringing Field WritingCoil Yoke

Fringingfields

Recordingmedium

TransitionWritten moment in media

In perpendicular recording the write process effectively occurs inthe gap (Write Field ~ 4πMS)In longitudinal recording the write process is done with the fringingfields (Write Field ~ 2πMS)

“Gap” fields

Real head

Image head

Coil

SULboundary

Physical Gap Effective Gap


Advantages of PerpendicularRecording

Higher write field amplitude - can use higher anisotropy media, betterthermal stabilityHigher write field gradients and well aligned recording layers - thickermedia, better thermal stabilityZero demag at transitions - sharp bit transitions, more stable recordeddataDecrease of demag with areal density increase - improved media stabilityat higher areal densitiesHigher playback amplitude - improved playback performance at higherareal densities


Soft Underlayer Challenges



175 200 225 250 275 300-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

Play

back

(uV)

(with

am

plifi

catio

n)

Time (nsec)

Domain Noise and SUL Biasing

---- Non-biased soft underlayer---- Biased soft underlayer

Single Transition

Soft underlayerHard layer

+++-----

-----+++

Magnets

head

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.00E+00 5.00E-07 1.00E-06 1.50E-06

Time (s)

Play

back

-2.00E-01

0.00E+00

2.00E-01

5.00E-07 1.00E-06 1.50E-06

Time (s)Pl

ayba

ckBiasing


Recording Layer and SUL Magnetics

Higher moment/anisotropy SUL + lower moment RLminimizes the effect

Permalloy FeAlN Ni45Fe55CoCrPtTa 25 Oe 20 Oe 7 OeMultilayer 15 Oe 11 Oe 3 Oe

TABLE: Coercivity of SUL in the presence of a Recording Layer

-30 -20 -10 0 10 20 30-4

-2

0

2

4M

agne

tizat

ion

(a.u

.)

Field (Oe)-50 -25 0 25 50

-4

-2

0

2

4

Mag

netiz

atio

n (a

.u.)

Field (Oe)

20nm SUL 20nm SUL + RL


0 2 4 6 8 10 12 14 16 18 20-300-200-100

0100200300

M (a

.u.)

Time (ns)

0 2 4 6 8 10 12 14 16 18 20-300-200-100

0100200300

M (a

.u.)

Time (ns)

Dynamic Kerr Microscopy20nm SUL

20nm SUL + RL

Presence of a RL can dramatically affect the dynamics of a SUL

Glass SubstrateMedia Stack

Microscope

Writer


Soft Underlayer and PlaybackResolution

Soft underlayer introduces asymmetry into the playback system.

Realhead

Imagehead

Recording layer

Buffer layer The underlayerboundary line

0 20 40 60 80 100120140

25

26

27

28

29

PW50

PW50

(nm

)

SUL-to-ABS distance (nm)

0.6

0.7

0.8

0.9

1.0 Sensitivity Function Amplitude

Sens

itivi

ty F

unct

ion

Ampl

itude

(n

orm

aliz

ed u

nits

)


Soft Underlayer MicromagneticsCoCrPtTa alloy based recordinglayer is capable of recordingdensities well in excess of 600kfci.

Further development is necessaryto minimize noise and/or distortionscaused by the presence of the softunderlayer

250 500 750 1000 1250-70-60-50-40-30-20-10

0 FeAlN soft underlayer No soft underlayer

Play

back

(dBm

)

Linear Density (kfci)

CoCrPtTa recording layer

⇒t2t

Soft underlayer

⇒Perfect imaging

Distortedimaging


Skew Angle Sensitivity

Overview of magnetic recordingSuperparamagnetic Limit and the Need for a New TechnologySoft Underlayer ChallengesSkew Angle SensitivityFlux Conductance via Nanoscale WritersTowards Optimum Reader DesignMaterials Issues


Skew angle

P2Zero skew

Non-zero skew

Trailing edge

Trailing edges

Track direction

P2

Skew angle (±15 degrees)


Skew Angle Sensitivity

P2Zero skew

Non-zero skew

Trailing edge

Trailing edgesTrack direction

-50 -25 0 25 50

1.5

2.0

2.5

Skew = 0 degrees

Play

back

Sig

nal (

mV)

Offset across the track (µin)

Skew = 15 degrees

250kfci 250kfci

25kfci

Effective track width increase ⇓

Loss in areal density

P2


Narrow Gap Single Pole HeadsTrailing

pole

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

Hz

(kO

e)Distance down the track (µm)

0nm 30nm 70nm 150nm 300nm 700nm

Trailingedge

In a narrow gap single pole heads, the write field is reduced towards the leadingedge, thus, minimizing the skew angle sensitivityCan minimize the loss in track density from 25% to less than 5%

0.2 0.4 0.6

1.0

1.2

1.4

1.6

1.8

HM

AX (k

Oe)

Trailing Pole Thickness (µm)

Conventional SPH Narrow Gap SPH

Trailing PoleThickness

Gap Gap Thickness:


Skew Sensitivity of a Gapless Writer

P1

P2

WP2 = 1.2µm tP2 = 3.3µm

6G Gapless Writer

-15 -10 -5 0 5 10 151.2

1.4

1.6

1.8 1µm Gap Gapless

Trac

k w

idth

(µm

)

Skew angle (degrees)

Gapless writer has substantially reduced sensitivity to non-zeroskew angles


Flux Conductance in NanoscaleWriters



0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Mz

Lz (nm)

Nanoscale Writers

Can writers with nanometer scale dimensions smaller than the domainwall thickness conduct flux efficiently?What can be done to reduce remanence in nanoscale writers?

Vertical field component above the center ofthe ABS at a 20nm spacing vs. write current fora single-pole head

Remanent magnetization after reversal for a170nm x 85nm single pole tip. For Lz > 432nm,the pole tip has non-zero remanence.

0 200 400 600 800 1000 12000.00.51.01.52.02.53.03.5

Hz (

kOe)

I, mA x turn

500 nm pole (I up) 500 nm pole (I down) 1000 nm pole (I up) 1000 nm pole (I down)


FIB-trimmed Nano Writers

A “singularity” has been formed at the ABS to altermicromagnetic behavior of the pole tip

Tilted ABS View ABS View

Void

∅ 40nm


Nanoscale Writers

Void formation at the ABS substantially reduces remanencein nanoscale writers

-500 -250 0 250 500

-1.0

-0.5

0.0

0.5

1.0

1.5

400

300L=200

Non-zeroremanence

MFM

sig

nal (

a.u.

)

Drive current (mA turn)-500 -250 0 250 500

-1.0

-0.5

0.0

0.5

1.0

1000750

L=500

Drive current (mA turn)

MFM

sig

nal (

a.u.

)

Zero remanence

Non-zero remanence


Towards Optimum Reader Design



If a conventional reader is used, the channel seesthe playback signal of different shapeCan differentiate, however, part of the informationis lost

Perpendicular vs. LongitudinalPlayback

++

charges in the transition

++

+ + + + + + + + - - - - - - - - - - - - - - - - - - - - + + + + + + +

Hstray

Hstray

M

Play

back

Sig

nal

Time

longitudinal

perpendicular

Play

back

Sig

nal

Time

Perpendicular

Longitudinal


Parallels between Perpendicular andLongitudinal Recording

-200 -100 0 100 200-6

-4

-2

0

2

4

6

Sens

itivi

ty F

ield

(a.u

.)

Distance along the track (nm)

Shielded Reader (Hx) Diff Reader (Hz) Shield Diff (Hz)

-200 -100 0 100 200

-1.0

-0.5

0.0

0.5

1.0

Sens

itivi

ty F

ield

(a.u

.)

Distance along the track (nm)

Shielded, Hx Differential, Hz Shielded Diff, Hz

∫ ∂⋅ rHMI

S 1~Reciprocity Principle: M - media magnetizationH - sensitivity field

Recording Medium

MR

Sen

sor

shield shield

Recording Medium

MR

Sen

sor

MR

Sen

sor

Recording Medium

MR

Sen

sor

MR

Sen

sor

shield shield


Performance Comparison

0 1000 200005

101520253035

Play

back

(a.u

.)


Shielded (perpendicular) Differential (perpendicular) Shield Diff (perpendicular)

Differential reader provides highest amplitude playbackShielded differential reader provides best spatial resolution

0 1000 20000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Pla

ybac

k


Shielded Differential Shielded Differential


Play

back

Sig

nal

Time

Equivalent Perpendicular Reader

Play

back

Sig

nal

Time

Perpendicularmedia

Longitudinalmedia

shield shield

MR element

Conventional shielded reader

MR elements

yokeDifferentialreader


Materials Issues



Perpendicular Media Materials

SOFT UNDERLAYER :Efficiency of the recordingsystemSoft underlayer noise

Seed/Exchange de-coupling layer

STORAGE LAYER :High squareness Exchange de-couplingGrain size control

Overcoat

Substrate

Buffer layer

Composition&

Microstructure

MagneticProperties

Performance


Alloy System Material Anisotropy Saturation Magnetization Anisotropy Field Minimum stable grain sizeKu (107erg/cc) Ms (emu/cc) Hk (kOe) a (nm)

CoCrPtX 0.20 200-300 15-20 8-10Co-alloy Co 0.45 1400 6.4 8.0

Co3Pt 2.00 1100 36 4.8FePd 1.8 1100 33 5.0

L10-phase FePt 6.6-10 1140 116 2.8-3.3CoPt 4.9 800 123 3.6MnAl 1.7 560 69 5.1

Rare Earth Nd2Fe14B 4.6 1270 73 3.7SmCo5 11-20 910 240-400 2.2-2.7

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Recording layers: Higher Ku Materials

360

auK

TBk⋅≅Minimum thermally stable grain size:


Microstructure of Recording layers

Average column size ~20nmRandomly oriented

Average grain size ~ 13nm(00_2) fiber-like texture withtexture spread of 6.30

CoB/Pd multilayer on ITO

CoCrPtTa on Ti

0 1 0 2 0 3 0 4 00

1 0 0 0 0

2 0 0 0 0

3 0 0 0 0

4 0 0 0 0

5 0 0 0 0

6 0 0 0 0

7 0 0 0 0

8 0 0 0 0

FWHM=6.30

ω

X-ray rocking curve TEMideal

non-ideal

CoPd


0 5 10 15 20 25 30 350

5

10

15

20

25

30

35Grain Size Distribution

Freq

uenc

y (c

ount

)

Grain Diameter (nm)

thermalinstabilities

Media Grain Size Engineering

−+= 2

2

0 2

lnexp

2 wxx

wxAyy c

π

Narrowing the grain sizedistribution improves SNRand media stability

media noise


PW50 (nm(uin)) 64.6 2.54 64.6 2.54ACSN (db) @ Tc=84nm @Channel Bit Density 14 12Data Rate (Mbps) 277 368Reader Width (nm(uin)) 101.6 4 101.6 4Writer Width (nm(uin)) 165.1 6.5 165.1 6.5Channel Bit Density (kfci)Normalized DensityTp (nm(uin)) (10%OTC 1e-4) 178 7 178 7ktpiChannel Channel Density (Gb/in2)TargetCodeCode RatePost ProcessorUser density (Gb/in2)On Track BER

Spared Sectors 5/1004.8E-05

[4 6 -3 -5 -2]96/102 Multiple Parity Bits

0.94

2.4E-05

Software

on and correcting

92.9GPR6(d,k)RLL (0,7) TR57

0.90

70.2

NAPA DEMO SOFTWARE DEMO

142.9

17.6

66.0 83.9

17.6

4911.3

6501.7

142.9NAPA

Seagate Perpendicular RecordingDemo – Intermag ‘02

perpendicular magnetic recording - thic · 02 46 8101214161820-300-200-100 0 100 200 300 ... p1 p2...

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