arrays of nanomagnets from nanoporous polymer templates · deepak singh (physics) mustafa bal...
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Arrays of Nanomagnets FromNanoporous Polymer Templates
Mark TuominenDepartment of Physics
University of Massachusetts Amherst
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Acknowledgements
Thomas Russell (PSE)Robert Krotkov (Physics)Andrei Ursache (Physics)Qijun Xiao (Physics)Ozgur Yavucetin (Physics)Deepak Singh (Physics)Mustafa Bal (Physics)Jorg Schotter (Physics)Gerd Kästle (Physics)Cheol Soo Yang (Physics)Thomas Thurn-Albrecht (PSE)Ting Xu (PSE)James Goldbach (PSE)Matt Misner (PSE)Kyusoon Shin (PSE)
SUPPORTNational Science Foundation: NIRT, MRSEC, NSECKeck FoundationSeagateDraper Laboratories
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Outline
1. Motivation2. Nanoscopic templates from block copolymer films3. Electrochemically deposited magnetic "nanowires"4. Hierarchical assembly5. Nanoscopic magnetic rings6. Interacting magnetic clusters
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Perpendicular Magnetic Recording (PMR)• At least three companies (Seagate, Hitachi, Toshiba)
have recently introduced commercial PMR hard drives• SOA is 100-200 Gbit/in2
Granular Media
PerpendicularWrite Head
Soft Magnetic UnderLayer (SUL)
coil
Y. Sonobe, et al., JMMM (2006)
Also, what lies beyond perfect media?
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Nanomagnet Patterning
“self-assembled”nanoporous
template
electrodepositionphysical or reactive
etching
physical deposition & liftoff
Target:density > 1012 elements/in2
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MICROPHASE SEPARATION OF DIBLOCK COPOLYMERS
Block “B”Block “A”PSPMMA
~10 nmScale set by molecular size
Ordered Phases
10% A 30% A 50% A 70% A 90% A
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CORE CONCEPT FOR NANOFABRICATION
DepositionTemplate
EtchingMask
Remove polymerblock within cylinders(expose and develop)
Target: Versatile, self-assembling, nanoscale lithographic system
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PROCESSING OF NANOPOROUS TEMPLATE
PMMAPS
Diblock Copolymer in Solvent
Science 290, 2126 (2000) Adv. Mat. 12, 787 (2000)Spin-coat
Thick film Thin film Annealing and electric field alignment Self-alignment by
controlled interfacialaluminum
Kaptonor silicon
PMMAV
Kapton
gold
PS
interactions
Thin templatenanopores
PMMA removal by UV/ebeamdegradation & chemical rinse
Thick template100nm -10 µm
10 -100 nm
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TEMPLATE CHARACTERIZATION
SAXS
SEM
100 cpp
Example:Array Period = 24 nm Pore Diameter = 14 nmMW = 42,000
PS/PMMA
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ALIGNMENT AND ORDERING BY SLOW SOLVENT EXTRACTION
AFM image
PS/PEO
(large χ)
2 µm T. Russell, et al.UMass Amherst
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Patterned Nanomagnet Arrays
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Electrodeposited Magnetic Nanowires
Potentiostat
WE REF
electrolyte
CE
WaveformGenerator
DC, or
1.2 x 1012 wires/in2
Three-electrode electrochemical cell
Structural and magnetic properties depend on electrodeposition conditions.
Science 290, 2126 (2000)
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Tuning Magnetic Properties by Bath pHhcp Co
c-axisoriented
X-RAY DIFFRACTION
polycrystal
hcp/fcc
Able to create preferred crystalline orientationand perpendicular magneto-crystalline anisotropy
A. Ursache, et al., Mat. Res. Soc. Proc. 721, 2002
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Monitoring via Electrochemical Quartz Crystal Microbalance
nanowires periodicmultilayered
nanowires
non-periodicheterostructures
1 nm resolution
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Pulse-Reverse Electrodeposition
Using Pulse-Reverse Electrodeposition andin situ quartz crystal monitoring to achievehigh-crystal-quality, c-axis oriented hcp cobalt nanowires
A. Ursache, et al. J. Appl. Phys. 97, 10J322 (2005)
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DC DC
pulse-rev. pulse-rev.
300 K 5 K
H
H
Improved Perpendicular Anisotropy
• Larger coercivity• Larger perpendicular magnetic anisotropy• No exchange bias
A. Ursache, et al. J. Appl. Phys. 97, 10J322 (2005)
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Magnetization Reversal in Magnetic NanowiresMeasured by AMR
Sweep downSweep up
θω
Field direction
Nanowire axis(current direction)
M
H Sharp Magnetization Reversal Transitions
M. Bal et al. 2003Important design implications for media and devices
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Hierarchical Self-Assembly
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Vacuum Deposited Nanomagnet Arrayslow aspect ratio nanomagnets
cobalt nanodots polymer template
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Hierarchical PatterningUsing Block Copolymer Films
2 µm
Combining conventional and BCP Patterning
1012 bit/in2 density PS-PEO
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Nanoscopic Magnetic Rings
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Nanoscopic Magnetic Rings
"0" "1"
• Stable binary states• Non-interacting
OnionState
VortexState
SDState
M
H
SD
Onion
Vortex
largering
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Nanoscopic Cobalt Rings
Ferromagnetic cobalt rings as small as 15 nm OD D. Singh, et al. 2006
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Multistate Clusters
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Concept: Multistate ClustersAn end-run solution to increasing storage density?
Media
PerpendicularWrite Head
coil
“0” “2” “1” “3”
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Interacting Magnetic Clusters: Multistate Magnetization
For example, a 3 nanomagnet cluster:
• Each nanomagnet interacts with the applied field — and with its neighbors
• A interacting cluster of nanomagnets has several (N+1) stable states, each with distinct net magnetization. • The whole cluster can be treated as a multilevel data storage element. It is larger than a single nanomagnet and can be addressed more easily with a suitable R/W head.
• Each level can serve as a stable remanent state• In this example, 4 stable remanent states
Mz
H
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Landau-Lifshitz-Gilbert (LLG) Simulations of 3D Nanomagnets
M
dm dt
=γ 0
1+α2
m × (
H Total − α m ×
H Total )
HTotal = applied field + interaction field + anisotropy field + thermal field
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Case Study: Seven-Magnet Cluster of 3D Nanomagnets with Perpendicular Anisotropy
15nm
10nm
Axis z
Axis x
30nm
Co3Pt• K1= 2 x 106 erg/cm3 along the z axis
Steady state magnetization can be along the z axis only• Exchange length is 42 nm
The nanodots can be treated as single domain particles.
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Switching Field Distribution
single switching trace
Single dot
multiple traces
histogram
Mz
H
(sanity check)
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Seven-Magnet Cluster
Q. Xiao, et al. J. of Appl. Phys. 99, 08G305 (2006)
8 states
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Using Asymmetry to Tune Switching StepsSimulation maps help to assess ‘favorable’ cluster designs
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Summary
• Nanoscopic templates from block copolymer filmsprovides a possible route to patterned media
• Considerable work is still needed to fully implement this as a nanomanufacturing technology for PMR
• BCP self assembly is a natural choice for hierarchicalpatterning schemes