Transcript Manchester MicroMagnetic Media Model

Future Magnetic Storage Media
Jim Miles
Electronic and Information Storage
Systems Research Group
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Future Magnetic Storage Media
1.
2.
3.
4.
5.
Media requirements for very high density
Model description
Predicted effects of grain size distribution
Patterned media: possible routes
Conclusions
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Granular or Patterned Media?
3/35
4/35
Granular Media Limitations
The transition from
one bit to another
follows the grains…
(or maybe clusters of
grains).
D


Jitter
W
W
D
Small grains are
needed for low noise.
5/35
Writing to Media
Anisotropy Ku
Magnetisation MS
A sufficiently large field is
needed to overcome the
anisotropy of the material,
which keeps
magnetisation aligned
along one axis
Field H > HK = 2KU
0MS
6/35
Thermal Stability of Media
• Energy barrier EB = KUV
• Thermal energy ~ KBT
• Spontaneous switching
when EB < 70KBT
• Require EB ~70 KBT
7/35
To Increase the Density:
• Decrease the bit length: Jitter must decrease
• Decrease the track width W: Jitter must not
increase.
• Jitter   D W ,  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.
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Perpendicular Recording
Increases write field, but only by ~ x2…
9/35
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.
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Perpendicular Media Modelling
-7
x 10
2
1.5
1
0.5
0
0
0.5
1
1.5
Real Storage Medium
Model Storage Medium
(not to identical scale)
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2
-7
x 10
Manchester MicroMagnetic Multilayer
Media Model (M6)
•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)
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Magnetostatic Interaction - Pairs of Grains
Interaction Field:
Hj = Dij Mi
Magnetostatic interaction tensors D are computed numerically
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.
‘Field’ grain
experiences a
field that varies
through the
volume.
Hj
Mi
1
D 
4
xy
ij

vj
Mi .yˆ rxj  rxi 
si
Integrate over the surface charge of i and the volume of j.
Underlayer included by incorporating images into Dij
13/35
r j  ri
3
dsi dv j
Exchange Interaction - Pairs of Grains
Exchange interaction factors are computed numerically
dij
i (source)
H ex,i
x
j (field)
dij
x
 2 A t
dx 

mi

 0 M sj v j d ij ( x ) 
Grain j experiences
an exchange field
due to grain i
 NN

Eex     0Hex,i .M j
 i 1

Integral term computed numerically from polygon geometry
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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
-8
-8
x 10
6.5
6.5
x 10
8.5
6
5.5
5
8
5.5
Crosstrack (m)
Crosstrack (m)
6
Crosstrack (m)
-8
x 10
5
4.5
4.5
6
6.5
7
7.5
Downtrack (m)
8
σv/<v> = 0%
8.5
9
-8
x 10
7
6.5
4
5.5
7.5
6
6
6.5
7
7.5
8
Downtrack (m)
8.5
9
σv/<v> = 15%
15/35
9.5
-8
x 10
1.15
1.2
1.25
1.3
1.35
Downtrack (m)
1.4
σv/<v> = 39%
1.45
-7
x 10
Grain Size Distributions
800
700
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%
Frequency
600
500
400
300
200
100
0
0
5
10
15
20
Area (nm2)
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25
30
Exchange Field Distributions
% of grains
100
80
Average exchange field
does not change as the
microstructure
changes.
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%
60
40
20
0
HE = 0.5 HD
A = 1.85x10-13
for all structures
0.6
0.8
1
1.2
Hey/<He>
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1.4
Exchange Interaction Between Pairs of Grains
x 10
Width of line  Hex
-8
9
Uniform grains,
perfect hexagonal
lattice. Exchange
field is identical
between all pairs.
8.5
8
7.5
Thermally decayed
from DC saturated
7
σv/<v> = 0
6.5
HE/HD = 0.5
6
5.5
5
4
5
6
7
8
9
x 10
18/35
-8
Exchange Interaction Between Pairs of Grains
x 10
Width of line  Hex
-8
Large volume
distribution:
4.5
4
σv/<v> = 39%
3.5
Irregular structure,
Large variation in HE
3
<HE>/<HD> = 0.5
2.5
2
1.5
1
0.5
0.9
1
1.1
1.2
1.3
1.4
x 10
19/35
-7
Magnetostatic (Demag) Field Distributions
100
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%
% of grains
80
60
40
20
0
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Hdy/<Hd>
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Energy Barrier Distributions
% of Magnetic Material
100
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%
80
60
40
20
0
10
20
30
40
Eb/KbT
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50
60
70
Recorded Transitions, b=20nm, Tp = 80nm, 411 Gb/in2
x 10
σv/<v> = 39%
-8
10
8
6
4
2
0
3
2.5
x 10
3.5
4
4.5
5
5.5
6
6.5
7
σv/<v> = 0%
-8
7.5
x 10
-7
10
8
6
4
2
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
x 10
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Effect of Irregularity on Data Signal
0.5
Fundmamental/Ms
0.45
0.4
0.35
0.3
0.25
0.2
0.15
1
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%
1.5
2
2.5
kfrci
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3
3.5
4
6
x 10
Effect of Irregularity on Noise
0.5
sigmaMp/<Mp>
0.4
0.3
0.2
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%
0.1
0
1
1.5
2
2.5
kfrci
24/35
3
3.5
4
6
x 10
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.
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Tom Thomson
26/35
Tom Thomson
27/35
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
islands
B. 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.
2.
3.
4.
FePt nanoparticles
manufactured in aqueous
suspension.
Very narrow size
distribution.
Deposited onto substrate.
Self-Assemble into
ordered structure.
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FePt Particle Growth
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FePt problems
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…
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Other Potential Technologies
Electro-chemical deposition in selfordered 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
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)
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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
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