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Future Magnetic Storage Media

Jim Miles

Electronic and Information Storage Systems Research Group

2/35

Future Magnetic Storage Media

1. Media requirements for very high density

2. Model description

3. Predicted effects of grain size distribution

4. Patterned media: possible routes

5. Conclusions

3/35

Granular or Patterned Media?

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5/35

Granular Media Limitations

WD

The transition from one bit to another follows the grains…

(or maybe clusters of grains).

Jitter

Small grains are needed for low noise.

WD

6/35

Writing to Media

Anisotropy Ku

Magnetisation MS

0MS

Field H > HK = 2KU

A sufficiently large field is needed to overcome the anisotropy of the material, which keeps magnetisation aligned along one axis

7/35

Thermal Stability of Media

• Energy barrier EB = KUV

• Thermal energy ~ KBT

• Spontaneous switching

when EB < 70KBT

• Require EB ~70 KBT

8/35

To Increase the Density:

• Decrease the bit length: Jitter must decrease• Decrease the track width W: Jitter must not

increase.• Jitter , grain diameter D must fall

• Volume V = D2t/4 Volume falls KU must rise to keep EB = KUV high enough bigger write field H > 2KU/0MS is needed.

• Density can only rise by increasing write field.

WD

9/35

Perpendicular Recording

Increases write field, but only by ~ x2…

10/35

Other Problems of Granular Media

• Media are granular.• Grains are not equal-sized.• Typically D ~ 0.2<D>, V ~ 0.4<V>• Hypothesis - Irregularity in media structure

produces noise:– Big grains give big transition deviations;– Different grain volumes switch more or less easily;– Different grains see different local interaction fields.

11/35

0 0.5 1 1.5 2

x 10-7

0

0.5

1

1.5

2

x 10-7

Perpendicular Media Modelling

Real Storage Medium Model Storage Medium (not to identical scale)

12/35

•Landau-Lifshitz dynamic and M-C thermal solvers.

•Arbitrary sequences of uniform vector applied fields

•Recording simulation with FEM or analytical head fields.

•Soft underlayer by perfect imaging

•Microstructural clustering and texturing.

•Fully arbitrary grain positions and shapes.

•Full account of grain shape in interaction fields

•Allows vertical sub-division and tilted columns (MET like)

Manchester MicroMagnetic Multilayer Media Model (M6)

13/35

Magnetostatic Interaction - Pairs of Grains

Magnetostatic interaction tensors D are computed numerically

‘Field’ grain experiences a field that varies through the volume.

Surface charge from each polygon face of the source generates field. Typically 48 faces per polygon.

Top and bottom faces computed similarly by division into strips.

Interaction Field: Hj = Dij Mi

Integrate over the surface charge of i and the volume of j.

Underlayer included by incorporating images into Dij

Mi

Hj

ji

s ij

xixji

v

xyij dvds

ij

3

ˆ.

4

1

rr

rryMD

14/35

Exchange Interaction - Pairs of GrainsExchange interaction factors are computed numerically

Integral term computed numerically from polygon geometry

iijjsj

iex xd

dx

v

t

M

AH m

)(

2

0, jiex

NN

iexE MH .,

10

x

x

dij

diji (source)

j (field)

Grain j experiences an exchange field

due to grain i

15/35

Varying Grain Size• Voronoi seed positions randomised • Minimum grain boundary width 0.7nm fixed• Number of grains/m2 and packing fraction fixed• Mean grain volume remains constant Hex remains constant

5.5 6 6.5 7 7.5 8 8.5 9x 10-8

4.5

5

5.5

6

6.5

x 10-8

Downtrack (m)

Cro

sstr

ack

(m)

6 6.5 7 7.5 8 8.5 9 9.5x 10-8

4

4.5

5

5.5

6

6.5x 10-8

Downtrack (m)

Cro

sstr

ack

(m)

1.15 1.2 1.25 1.3 1.35 1.4 1.45x 10-7

6

6.5

7

7.5

8

8.5

x 10-8

Downtrack (m)

Cro

sstr

ack

(m)

σv/<v> = 0% σv/<v> = 15% σv/<v> = 39%

16/35

0 5 10 15 20 25 300

100

200

300

400

500

600

700

800

Area (nm2)

Freq

uenc

yGrain Size Distributions

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

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0.6 0.8 1 1.2 1.40

20

40

60

80

100

Hey/<He>

% o

f gra

ins

Exchange Field Distributions

Average exchange field does not change as the microstructure changes.

HE = 0.5 HD

A = 1.85x10-13

for all structures

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

18/35

Exchange Interaction Between Pairs of Grains Width of line Hex

Uniform grains, perfect hexagonal lattice. Exchange field is identical between all pairs.

Thermally decayed from DC saturated

σv/<v> = 0

HE/HD = 0.5

4 5 6 7 8 9

x 10-8

5

5.5

6

6.5

7

7.5

8

8.5

9

x 10-8

19/35

Exchange Interaction Between Pairs of GrainsWidth of line Hex

0.9 1 1.1 1.2 1.3 1.4

x 10-7

0.5

1

1.5

2

2.5

3

3.5

4

4.5

x 10-8

Large volume distribution:

σv/<v> = 39%

Irregular structure, Large variation in HE

<HE>/<HD> = 0.5

20/35

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50

20

40

60

80

100

Hdy/<Hd>

% o

f gra

ins

Magnetostatic (Demag) Field Distributions

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

21/35

10 20 30 40 50 60 700

20

40

60

80

100

Eb/KbT

% o

f Mag

net

ic M

ate

rial

Energy Barrier Distributions

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

22/35

2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

x 10-7

0

2

4

6

8

10

x 10-8

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

x 10-7

2

4

6

8

10

x 10-8

Recorded Transitions, b=20nm, Tp = 80nm, 411 Gb/in2

σv/<v> = 39%

σv/<v> = 0%

23/35

1 1.5 2 2.5 3 3.5 4x 10

6

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

kfrci

Fu

nd

ma

me

nta

l/Ms

Effect of Irregularity on Data Signal

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

24/35

1 1.5 2 2.5 3 3.5 4

x 106

0

0.1

0.2

0.3

0.4

0.5

kfrci

sigm

aMp/

<Mp>

Effect of Irregularity on Noise

σv/<v> = 0% σv/<v> = 4.7% σv/<v> = 10.2% σv/<v> = 15.5% σv/<v> = 22.6% σv/<v> = 29.4% σv/<v> = 38.7%

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Grain Microstructure Conclusions

• Grain size distributions give rise to decreased signal and increased noise (BAD)

• Media with small grain size distributions are needed

• Patterned media are needed

• Additional advantage: switching volume is the bit size, not the grain size lower switching field is possible.

26/35

Tom Thomson

27/35

Tom Thomson

28/35

Direct Write e-beam

1. Form master by direct write e-beam on resist layer

2. Evaporate gold coating

3. Lift-off gold from unexposed areas

4. Etch to remove magnetic layer except where protected by gold

50 nm diameter islandsB. Belle et. al.University of Manchester

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Patterned Media Potential

• Provides a route to regular arrays of thermally stable low noise

• 1Tb/in2 requires 12.5nm lithography

• Not feasible using semiconductor manufacturing technology for some years to come…

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Self-Organised Magnetic Assembly (SOMA Media)

1. FePt nanoparticles manufactured in aqueous suspension.

2. Very narrow size distribution.

3. Deposited onto substrate.

4. Self-Assemble into ordered structure.

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FePt Particle Growth

32/35

FePt problemsFePt manufactured in solution has low Ku.

Very high Ku can be developed by annealing:

Much ongoing research in low temperature formation of high coercivity FePt…

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Other Potential Technologies

Electro-chemical deposition in self-ordered templates: University of Southampton.

Electroplating into self-ordered pores in Alumite: R. Pollard et. al, Queens University Belfast.

Vacuum deposition through self-assembled nanosphere templates: Paul Nutter, Ernie Hill, University of Manchester.

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Self-Assembly – Long Range Order

40nm diameter CoCrPt nanoparticles. Mask made from a diblock co-polymer (polystyrene/PMMA), self-assembled in nanoimprinted grooves.

(Naito et al, Toshiba, IEEE Trans. Magn 38 (5) (2002)

Self-assembled pattern using a diblock co-polymer (in nanoimprinted grooves.

(C. Ross et al, MIT, 2002)

Self-assembly produces only local order. Over long ranges order breaks down at dislocations.

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Conclusions

• Conventional media can only be extended so far.• Patterned media overcome thermal stability issues.• Higher stability granular materials could be used

with heat assisted recording (HAMR)• …but patterned media might still be needed to

avoid excessive transition noise.• Patterned media are likely to be necessary in ~5

years