Magnetic Recording Media Evolution and Future Challenges

Magnetic Recording Media Evolution and Future Challenges

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Description: Evolution Of Perpendicular Magnetic Recording Media, PMR Media, Evolution by Product Generation, Limitations, Recording Technology Options, Microwave Assisted Magnetic Recording, Heat Assisted Magnetic Recording, HAMR Media Challenges, Bit Pattern Media, ASTC Industry Consortium.

 
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Contents:
IEEE Santa Clara Valley Chapter
Oct. 25, 2011

EVOLUTION OF PERPENDICULAR MAGNETIC RECORDING MEDIA AND UPCOMING CHALLENGES
Gerardo Bertero Collaborators: Mike Mallary, Ram Acharya, Kumar Srinivasan, Mrugesh Desai, Alex Chernyshov, Hua Yuan, David Treves, Mike Madison, Bogdan Valcu, Eric Champion

Western Digital Corporation, San Jose, California, USA



Outline
Introduction PMR Media
Evolution by Product Generation Limitations Going Forward

Recording Technology Options



Microwave Assisted Magnetic Recording
Heat Assisted Magnetic Recording HAMR Media Challenges Bit Pattern Media (not covered here)

ASTC Industry Consortium
Summary and Discussions



2

Introduction
Introduced commercially in 2005, conventional perpendicular recording can now be considered a mature technology in its 6th (or 7th) product generation. Alternative technologies such as Energy Assisted Recording and Bit Patterned Media Recording are not yet ready for productization. Here we will address the key technical challenges regarding media extendibility for CMR and will also discuss the main issues in the development of media for the next likely technologies.



3

Cross-Section TEM of Typical PMR Media Structure
Overcoat Graded CoCrPtX-Oxide NL 2 NL 1 Seed

SUL 2 Ru SUL 1 � Advanced DS mag structure design � Graded anisotropy � Varying degrees of inter-granular exchange coupling through the thickness of grains

Adhesion


4

PMR Media Design By Generation
2005 Cap 2010

Cap
Mag 2

Cap

Cap

Bottom
Mag 1 Seed 1st 2nd Seed 5th, 6th

Hard Magnetic Layers Only
From the simple 2-layer capping layer media to the latest 7-8 layer magnetic structures, PMR media has increased significantly in complexity since it was first adopted.


5

ECC and Exchange-Spring Media

Inspired by the "tilted media" concept Incoherent switching through column of grain Compared to "Stoner-Wahlfarth" media, in its simplest rendition it provides twice the energy barrier with same switching field.
R.H. Victora and X. Shen, IEEE Transactions on Magnetics, 41, NO. 2, 537-542, (2005). D. Suess et al., J. Magn. Magn. Mat, 551, 290-291, (2005).



6

Dynamic-Spring Media (Media Write Field Boost)
Effective field to reverse storage layer is a combination of: 1) Head Field 2) Assist Layer Ms*t 3) Interface Exchange
Assist layer high Ms/low Ku Storage layer low Ms/high Ku



Dynamic-spring media is an intrinsic magnetic self-assisted switching stack
The high Ku portion of the media provides for the required thermal stability By Courtesy of D. Suess The softer portion of the media is used to adjust (lower) the switching field This technology is currently being introduced into high volume manufacturing


7

Graded Anisotropy Media

Micro-magnetic simulations predict further gains in performance by extending the single ECC or Exchange-Spring concept to a system where the anisotropy is graded. Caveat is that switching has to take place by incoherent rotation for advantage to be realized.



Advanced ECC Media
Concept:
Low Ku

Practice:
Two-Spin Model

1 2K H Hc 4 J hard
High Ku

Figure of merit = 2
Graded Anisotropy

Single EBL

Low Ku

Three-Spin Model

Hc

1 2K H C J hard


High Ku

Figure of merit > 4

Double EBL

C>4

9

Hs and KuV/kT as function of EBL 1/2
8800 8600 8400 8200

EBL 1&2 = 0 nm

Single-EBL
Dual-EBL
EBL 1 = 0 nm EBL 2 = 0.7 nm EBL 1 = increasing EBL 2 = 0.7 nm

Hs (Oe)

8000 7800 7600 7400 7200 7000 6800

150

200

250

300

350

400

KuV/kT

Double ECC structures allow for higher degree of freedom when optimizing Hs and KuV/kT (i.e., writability and thermal stability)


10

ECC PMR Media
Double exchange-break layer media is latest generation media structure It allows higher degree of incoherent switching through the grain column.

Provides higher design and performance optimization flexibility
Allows higher anisotropy (Ku) media to be used yet maintaining good writability Can result in thinner overall media stacks It should be a key enabler for smaller media grains which is required for extending conventional PMR recording past the 1 Tb/in2 mark.



11

Magnetic Grains and Bit Size
100 Gb/in2 ~90 Grains/Bit

250 Gb/in2

~40 Grains/Bit

500 Gb/in2

~20 Grains/Bit

800 Gb/in2

~11 Grains/Bit
15 nm

To maintain SNR, number of grains per bit should ideally be kept constant


12

12

Magnetic Grains and Bit Size
Presented at TMRC 2010

Grain Size (Center-To-Center), (nm)

16 15 14 13 12 11 10 9 8 7 6

Longitudinal

Perpendicular

1

10

100

1000

Areal Density (Gb/in2)

To maintain SNR, number of grains per bit should ideally be kept constant


13

13

Simple Picture of Grain Size Scaling in Media
0.31 PW50 B W SNR a2 S
2
W: Read Width (MRW) B: Bit Length S: Cross-Track Corr. Length ~ Grain Size a: Transition Parameter

PW50 B W SNR 0.31 2 B a S
~ constant over generations of products
Grain size will limit ultimately transition width

Decreasing with product generation
Presented at TMRC 2010

Grain Size (Center-To-Center), (nm)

16 15 14 13 12 11 10 9 8 7 6

Longitudinal

Perpendicular

Bit Length

GS a 0.7 n


1 10 100 1000

Areal Density (Gb/in2)

Simple Picture of Grain Size Scaling in Media
Grain Size Limited Transitions
8.00 7.00 Grain Size Limited B/a 6.00

10 nm GS 9 nm GS 8 nm GS 7 nm GS

5.00
4.00 3.00 2.00 1.00 Areal Density, Tb/in2


Grain Size Progression vs. Areal Density
Grain Size (Center-To-Center), (nm) 16 15 14 13 12 11 10 9 8 7 6 1 10 100 1000

Longitudinal

Perpendicular

Areal Density (Gb/in2)

ECC media structure was expected to yield higher areal density capability by enabling smaller grain size in media. In practice, grain size has remained practically unchanged since PMR introduction. Instead, the trade-off of choice was to use ECC media to improve writability, maintain acceptable adjacent track erasure, and optimize transition parameter with utilization of higher Ku alloys and optimization of lateral exchange coupling through � the various layers.

Grain Size and Ku Scaling
120 110 CoCrPt-O (Ms=700 emu/cc) FePt L10 (Ms=1100 emu/cc) 90 80 100

FePt(L10) based
Hk [KOe]

70

700-900 Gb/in2
120-500 Gb/in2

60 50

1-1.5 Tb/in2

40 30 20 10 0

CoCrPt-O based

9.5

8.5

7.5

6.5

5.5

4.5

3.5

2.5


Core Grain Size [nm]

17

HDD Industry Historical Areal Density Trend
10 1E+6
10 1E+5
6 5

Areal Density Perspective

Perpendicular Recording

40% CGR
PR Projection

TuMR Heads
4

Areal Density Megabits/in2

Are we slowing down ?

1E+4 10

GMR Heads

AFC Media 100% CGR

10 3 1E+3

10 1E+2
10 1E+1 1E+0 1 1E-1 10 10 1E-2 10 1E-3

2

AMR Head Thin Film Head 25% CGR

60% CGR
200 Million X Increase

PRML Channels

Thin Film Media
-1 -2 (First Hard Disk Drive) -3

HDD Products Industry Lab Demos

60

70

Production Year 80 90

100 2000

110 10



18

Current Technology Is Reaching Maturity
2 Disk Mobile Historical Announces With Volume Ship of > 5M/Qtr
1000 900 800 700 600 purple cross show when industry shipped more than 5M/Qtr

HDD Capacity (GBytes)

500 400 300
80% CAGR enabled by large head-media SNR gains System features enable growth

Company A Company B Company C More than 5M/Qtr

200

Colored boxes show the press announcement dates

100

Aug-07

Dec-06

Dec-08

Sep-09

Jan-11

Sep-11

May-10

May-12

Feb-13

Oct-13

Apr-08



19

Month-Year

Technology Options
Reader Shingle Writer Reader Laser Spin-Torque Oscillator Reader Write Pole

Progressive Scan Head Motion

Near Field

Write Field

1 Bit = 1 Island

SMR/TDMR

Energy Assisted Recording

Bit Patterned

� Continued scaling requires innovations in systems technologies, materials science and process engineering to advance areal density


20

(Illustrations courtesy of Y. Shiroishi, Intermag 2009)

Technology Options for > 1.5 Tb/in2
10.00
TDMR MAMR

Areal Density (Tb/in2)

EAMR+BPM

1.00
HAMR

PMR Extendibility
(Conventional & Shingle)

Conventional PMR (CM)

0.10
200 6 200 7 200 8 200 9 201 0 201 1 201 2 201 3 201 4 201 5 201 6 201 7 201 8 201 9


Date (Year)

21

MAMR: Spin-Torque Oscillator Physics

The spin-transfer torque opposes
the damping torque
When both are equal, precession becomes stable (condition "d" above)

Damping is energy loss per cycle
(into lattice waves, spin waves, electron excitations, etc.)


Reduction of 30- 40% of media switching field Effective gradients of 1500-3000 Oe/nm


22

Hitachi/NEDO STO Demonstration

Perpendicular Bias



23

WD Confidential

MAMR Technology and Media
MAMR can extend the use of conventional PMR media. Higher Ku in
CoPtX-Oxide alloys must be realized.

With the higher Ku materials, media grain size must be decreased
aggressively for this technology to show its promise.

If MAMR is extendible to >2Tb/in2, new high Ku material systems will
be required



...but could be simpler than HAMR media as Curie temperature is not part of the design criteria.

Companies now assessing MAMR's potential. Not much energy has
been devoted to it across industry.



24

Grain Size Expectations
4.5
Anisotropy Field [KOe]
65 55 45 35 25

Areal Density (Tb/in2)

4 3.5 3 2.5 2
10

HAMR
(granular media)

High Ku
(e.g., L10)

Conventional
9 8 7 6 5 4

15 5

1.5 1 0.5

Core Grain Size [nm]

BPM
0
4 4.5 5

High Ku
5.5 6 6.5 7

CoCrPt-Oxide
7.5 8 8.5 9 9.5

Grain Size CTC (nm)
25

Grain size must come down to enable higher areal densities. HAMR has potential to switch extremely high Ku materials.

Hk [KOe]


9 nm hard mag layer Thickness 70 KuV/kT

HAMR Recording Fields
Pole
Hc(x)

Y. Shiroishi, Intermag 2009)

T(x)

Happ(x)

V
Media direction of Travel

dx V dt dT dT dx dx

Transition Location

By knowing T(x), M(x), Ku(x), Hk(x) and Happ (x) we can make simple observations of the recording process for grains of different size.


26

Cross-sectional TEM Image of Experimental EAMR Media
Protective cap for FIB prep

FePt-X Seed

Heat-Sink Layer Structure

Seed Plan-View TEM Soft Magnetic Underlayer





Heat Assisted Media Challenges
Grain Size and Morphology
Grain Size Uniformity HK Uniformity Grain-to-grain L10 atomic ordering uniformity Grain-to-grain compositional uniformity Intergranular Magnetic Exchange Uniformity E.g., grain boundary width uniformity

Roughness Thermal Design

Heat-sink and thermal barriers Tc tailoring

Overcoat/Lubricant


28

Magnetization and Anisotropy as Function of Temperature
1200 4.E+07

1000

3.E+07

3.E+07

MS(emu/cc)

800

2.E+07
600

Ms (emu/cc)
400

2.E+07

Ku (erg/cc)

1.E+07

200

5.E+06

0
0 100 200 300 400 500 600 700 800

0.E+00

Temperature (K)

29

Knowing Ms(T) and Ku(T) allow us to estimate many aspects of the writing process

Mangetocrystalline Energy, Ku (erg/cc)



EAMR Recording: Grain Size Effect on Transition Location
Hc(x) Big Grain Hc(x) Small Grain Happ(x)

Same orientation

Direction of applied field
(consistent with magnetization in bit to be written)

Opposite orientation
Media direction of Travel

T>TB TB: Blocking Temperature

Transition delta for big vs. small grains



30

Energy Balance for Grains at Either Side of Transition
Magnetocrystalline: Zeeman : Demagnetizing:
Applied Field

Ku x Sin2() x Vol M.H= M x H x (Cos ) x Vol 2 x x N x M2 x Vol
Applied Field

M

M

Magnetocrystalline + Zeeman - Demag

Magnetocrystalline - Zeeman - Demag

For a particle (grain) to freeze magnetically its Energy Barrier/KbT must be larger than a certain value.


31

Energy Barriers During Writing
180 160 140 120
Eb/kT for written pattern Upper Bound E Barrier/kT

1T
2.M(T).H

Eb/kT

100 60 40 20 0 -20 0 10 20 30 40 50
2.M(T).H

2.M(T).H

Lower Bound E Barrier/kT

1T

80

2T

Media motion

60

70

80


Distance from NFT Center (nm)

32

What Energy Barrier to Freeze Grains in Their State?
1

P
Probability of Switching 0.1

f0 e v x0



Eb ( x ) k T ( x) B

dx

f 0 1011

1 s

0.01

0.001 0 1 2 3 4 5 Energy Barrier, Eb/kT 6 7

Contrary to conventional recording in HAMR, grains freeze magnetically once their energy barrier is as low as 1.5-4 times kBT. Media grain volume difference effects are magnified by this fact. Thermal gradients must be steep enough to guarantee the freezing of all grains within a short distance from the intended transition location.



33

Grain Size Sensitivity to Magnetization Freezing
630 600 570 540 510 480

Temperature at Eb/kT=2 Crossing, K

EB E B (Volume ) Ku Ku (T ) Ms Ms (T )
2 4

Range (DD)

6

8 10 Grain size

12

14

450 1 3 5 7 9 11 13 15

Grain Size (Diameter), nm

As grain mean size is reduced, grain size distribution sigma must be reduced significantly for linear density capability. Smaller grain size does not automatically result in better performance.



34

FePt-L10 Coherency Length as Function of Temperature
At high temperature the domain wall width is significantly larger than at RT.
Increasing thickness beyond 10 nm may not necessarily increase energy barrier based on this estimate of coherency length for FePt.
30.00

25.00

Coherency Length, d l, nm

EAMR writing occurs here

A(T ) d l (T ) 4 K u (T )
M (T )kbT0 0.117 0 B A(T ) S 2 g 0 B M s (0)
2/3

20.00

Cont.M s (T )

15.00

T. Bublat and D. Goll, J. Appl. Phys. 108, 113910 (2010)

10.00

5.00

0.00 500 550 600 Temperature, K 650 700


35

HAMR Media Grain Size Observations
Grains of smaller size require lower temperatures to magnetically freeze. Under typical HAMR recording conditions freezing takes place near
EB/kBT~1-3.

Grain size effects are magnified under EAMR (high T) conditions. For grains in a given distribution to freeze within a distance
corresponding to a fraction of the bit length, they must meet very tight size sigma requirements.

Sigma requirements for grain sizes in the 2-4 Tb/in2 are very stringent if
Eb/kBT>>2 . What is the correct energy barrier to use ? (strongly dependent on media design).

Physics of high speed switching near Tc have not been sufficiently
studied.


36

Hk Fluctuations near Tc

The promise of heat assisted recording is rooted on the premise of extremely high thermal gradients to write transitions much sharper than possible by just magnetic means. recording linear density,

In order to capitalize on the high switching effective gradients and achieve high

The magnetic anisotropy of the media must be as high as possible The Curie point of the media must be as low as possible (but safely higher than RT for archivability) FePt is an nearly ideal system for satisfying both conditions above.

However, we assume time averaged properties for anisotropy vs. temperature of the
media.

There are some suspicions that temporal

fluctuations in Hk near Tc can be large and relevant during the writing process. conjecture, with particle size being a big factor. minimize the negative effects through novel layer structure designs will be needed.


Modeling by R.Victora, UMN, support this

If experiments confirm this behavior, ways to

37

Ultrafast Measurements at LCLS

Experimentally, these fluctuations could be time resolved by new tools such as the LCLS facility at Stanford with femto-second resolution.


38

New Industry Consortium

"Advanced Storage Technology Consortium"
New consortium created to expand and enhance the power of R&D
funding
Acceleration of technology development by targeted collaborations between storage industry
participants, suppliers, universities, laboratories, and institutes

Mission: member-directed, scalable R&D organization to address
fundamental technology challenges

Supply chain involvement HDD technology roadmap


Why Do We Need Something Different ?
Pace of technology transitions and scope of
change required means business as usual approach won't work

Need to collimate and focus entire industry R&D to
be successful

Need coordinated transitions in supply base components, equipment, and materials



Expected Outputs from ASTC
Focused, collaborative research projects that will
enable better understanding of key scientific challenges

Shared, realistic roadmap for the Industry Solutions � science to engineering to

manufacturing options � that will shorten time from invention to productization



41

ASTC Funding Allocations
29 projects preselected for 1st year of funding By System By Area

HAMR

BPM

MAMR

TDMR

ADV. PMR

Systems

Media

Heads

Signal Proc.

HDI

Tools�

42

Summary And Conclusions
Conventional perpendicular media will continue to evolve to higher
Ku and smaller grain size as we extend CPR technology.

Magnetic head fields and Ku limits of CoPtX-Oxide media will limit
densities to 1-1.5 Tb/in2 range. Shingling may help extend that range.

Media for heat assisted recording based on FePt has improved
significantly but big challenges remain.

Reducing grain size and size distributions while preserving columnar morphology remain Optimum heat-sink design of media to enhance thermal gradients is limited somewhat by
other functional requirements such as low surface roughness. challenging given the high processing temperatures required for L10 ordering.

Pace of development needs to increase in order to productize these
new technologies as per roadmaps.

New ASTC consortium provides a forum for more open
collaboration in addressing the most challenging industry problems.


43

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