the physics of perpendicular recording

The Physics of Perpendicular RecordingH.J. Richter

Seagate Technology, Fremont

Overview

Introduction

Perpendicular Recording and the “tri-lemma”

Microstructure of perpendicular media

The role of intergranular exchange

Recording geometry and angle effects

A way to postpone the tri-lemma: composite (ECC) media

Summary

Slide 2Perp. RecordingIdema Dec 2006 H.J. Richter

Areal Density ProgressResearch frontier:≥500 Gbits/in2

Commercial products:≤130 Gbits/in2, 80-130 GB/3.5” Platter

Demonstrations: up to 421 Gbit/in2

Technology optionsSingle Particle Superparamagnetic Limit

Slide 3Perp. RecordingIdema Dec 2006 H.J. Richter

1990 1995 2000 2005 2010 2015 2020 20250.1

1

10

100

1000

10000 10 Tbit/in2

INSIC

||⊥

1 Tbit/in2

100 Gbit/in2

Products

Max

. Are

al D

ensi

ty (G

bit/i

n)

Availability Year

Demos

HAMR & BPM

BPM or HAMR

Perpendicular(tilted, composite)

Longitudinal

2

421

BPM = bit patterned mediaHAMR = heat assisted magnetic recording

What gives perpendicular recording the lead over longitudinal recording?

Slide 4Perp. RecordingIdema Dec 2006 H.J. Richter

High SNR Requires Small GrainsDue to its size, each grain is a “single domain particle” and cannot be cut magnetically.Hence, the boundary between the bits can at best follow the grain structure.

Slide 5Perp. RecordingIdema Dec 2006 H.J. Richter

Superparamagnetic Limit

Increasing SNR requires smaller grains Sm

aller grain

A Dilemma: a situation requiring a choice between equally undesirable alternatives (Webster)

s

KV too small –media are ther-mally unstable

Head fields are too weak to write media

Higher K (HA)

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"Tri-Lemma" In Magnetic Recording

The “Superparamagnetic limit”

Is it postponed by perpendicular recording?

SNR

write-ability thermal decay

conflicting requirements

H.J. Richter

Slide 7Perp. RecordingIdema Dec 2006 H.J. Richter

Perpendicular Recording GeometryHeads and media are different!

Longitudinal Recording Perpendicular Recording

Slide 8Perp. RecordingIdema Dec 2006 H.J. Richter

Demagnetization

In longitudinal recording increasing the layer thickness increases demagnetization and causes poorer SNRVery mild SNR penalty in perpendicular recording – can make small grains and thick media and store volume “in the depth”

Slide 9Perp. RecordingIdema Dec 2006 H.J. Richter

Perpendicular Media: Microstructure

Slide 10Perp. RecordingIdema Dec 2006 H.J. Richter

Cross-section

Plan view

CoPt-TiO2 magnetic layer

Ru – interlayerSeedlayerSUL

Medium Magnetics: Hysteresis Loop

Hysteresis loop along film normal is sheared due to demagnetizing effects

Increasing exchange makes the hysteresis loop squarer

Exchange coupling leads to square hysteresis loops

Square hysteresis loops are beneficial, but collective reversal

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Optimized Exchange

Slide 12Perp. RecordingIdema Dec 2006 H.J. Richter

Wide transitions & small clusters

Narrower transitions & big clusters

optimum

More on Exchange Interaction

magnetostatics destabilize Miexchange field aligned with Mi

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grain i

Less field required to switch the media

Exchange helps to stabilize magnetization in the bit

Thermal Decay

For amplitude decay, longitudinal recording and perpendicular recording are mirror images.

Slide 14Perp. RecordingIdema Dec 2006 H.J. Richter

Perpendicular Oxide Media

• Longitudinal media use CoPtXCr with Cr as segregant for decoupling; perpendicular media use CoPt + oxide to decouple grains– Less tendency of the grains to grow wider for thick films

– Decoupling by oxide, not Cr Segregation – narrower SFD

• Very high orientation ratio - a FWHM of 3° corresponds to an orientation ratio of more than 40!

• High magnetization, high anisotropy

• Dual layer media to control intergranular exchange (CGC)

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High Orientation: Angles Matter!

It is important to realize that the angle dependence matters a lot for perpendicular recording. A tilt of the head field helps to switch the media => hence a mild sensitivity to interlayer thickness

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Shielded HeadsTrailing shield cuts the tail of the head field => high field gradient, shaper transitions

Fields of shielded heads are lower, but angle effects mitigate write-ability.

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A Way to Postpone the Tri-Lemma

magnetically soft (low anisotropy)δ2

H

magnetically hard (high anisotropy)δ1

Exchange Coupled Composite (ECC) media

The soft layer is a “switching assist” for the hard layer

R.H. Victora and X. Shen, IEEE Trans. Magn. vol. 41, pp 537-542 (2005)

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How Do Composite Media Work?

hard

soft

Reversal process:

1. Nucleation

2. Domain wall formation and compression of wall

3. Switch: wall moves through hard layer

Applied field compresses wall

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ECC Media: Reducing Switching Field at Will?

( )

hardhard

softsoft

hardSW

AMAM

HH

≡

+=

γ

γ2

1Theoretically” yes!

Graded anisotropies even more efficient! (Suess APL, 2006)

Infinite layers: Kronmüller, Goll(2002)

switc

hing

fiel

dthickness of soft layer

1/4

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Composite Media Gains• Gain must consider the

constraint: δconv = δcomp

(and similar Mrδ)

• Absolute limit*: Gmax = 4Using graded anisotropy app. K~x2 and non-existing materials

• G = 2…2.5 (estimate for a hard material with an anisotropy field of ~ 100kOe)

δ

Note: gains are in grain size limit.

*P. Visscher, U. of Alabama

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Summary: Perpendicular Recording1. Perpendicular media have better microstructure

High orientation (orientation factor >40)Smaller grains with narrower distributions=> sharper transitions

2. Can use thicker magnetic layer without SNR penalty => can reduce grain size and maintain KV for thermal stability

3. Some intergranular exchange is desirable for SNR4. Intergranular exchange helps to stabilize magnetization5. Can use composite media (ECC)6. Higher output

Slide 22Perp. RecordingIdema Dec 2006 H.J. Richter