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Future Magnetic Storage Media Jim Miles Electronic and Information Storage Systems Research Group

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Future Magnetic Storage Media

Jim Miles

Electronic and Information Storage Systems Research Group

Media requirements for very high density

Model description

Predicted effects of grain size distribution

Patterned media: possible routes

Conclusions

Thetransition from one bit to another follows the grains…

(or maybe clusters of grains).

Jitter

Small grains are needed for low noise.

W

D

Granular Media Limitations

Field H > HK = 2KU

0MS

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

Anisotropy Ku

Magnetisation MS

Energy barrier EB = KUV

Thermal energy ~ KBT

Spontaneous switching

when EB < 70KBT

Require EB ~70 KBT

- 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.

Increases write field, but only by ~ x2…

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.

Real Storage Medium Model Storage Medium

(not to identical scale)

Manchester MicroMagnetic Multilayer Media Model (M6)

Mmmmmm...

- 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)

Hj

Mi

Magnetostatic Interaction - Pairs of Grains

Interaction Field: Hj = DijMi

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.

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

Underlayer included by incorporating images into Dij

x

j (field)

dij

dij

x

i (source)

Exchange Interaction - Pairs of Grains

Exchange interaction factors are computed numerically

Grain j experiences an exchange field due to grain i

Integral term computed numerically from polygon geometry

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

σv/<v> = 0% σv/<v> = 15% σv/<v> = 39%

Grain 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%

100

80

% of grains

60

40

20

0

0.6

0.8

1

1.2

1.4

Hey/<He>

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%

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

Exchange Interaction Between Pairs of GrainsWidth of line Hex

Large volume distribution:

σv/<v> = 39%

Irregular structure, Large variation in HE

<HE>/<HD> = 0.5

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%

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%

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

σv/<v> = 39%

σv/<v> = 0%

0.5

0.45

0.4

0.35

Fundmamental/Ms

0.3

0.25

0.2

0.15

1

1.5

2

2.5

3

3.5

4

kfrci

6

x 10

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%

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%

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.

Tom Thomson

Tom Thomson

- Form master by direct write e-beam on resist layer
- Evaporate gold coating
- Lift-off gold from unexposed areas
- Etch to remove magnetic layer except where protected by gold

50 nm diameter islands

B. Belle et. al.

University of Manchester

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…

FePt nanoparticles manufactured in aqueous suspension.

Very narrow size distribution.

Deposited onto substrate.

Self-Assemble into ordered structure.

FePt 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…

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.

Self-Assembly – Long Range Order

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

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

(C. Ross et al, MIT, 2002)

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)

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