Arrays of Nanomagnets FromNanoporous Polymer Templates

Mark TuominenDepartment of Physics

University of Massachusetts Amherst

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

Outline

1. Motivation2. Nanoscopic templates from block copolymer films3. Electrochemically deposited magnetic "nanowires"4. Hierarchical assembly5. Nanoscopic magnetic rings6. Interacting magnetic clusters

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?

Nanomagnet Patterning

“self-assembled”nanoporous

template

electrodepositionphysical or reactive

etching

physical deposition & liftoff

Target:density > 1012 elements/in2

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

CORE CONCEPT FOR NANOFABRICATION

DepositionTemplate

EtchingMask

Remove polymerblock within cylinders(expose and develop)

Target: Versatile, self-assembling, nanoscale lithographic system

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

TEMPLATE CHARACTERIZATION

SAXS

SEM

100 cpp

Example:Array Period = 24 nm Pore Diameter = 14 nmMW = 42,000

PS/PMMA

ALIGNMENT AND ORDERING BY SLOW SOLVENT EXTRACTION

AFM image

PS/PEO

(large χ)

2 µm T. Russell, et al.UMass Amherst

Patterned Nanomagnet Arrays

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)

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

Monitoring via Electrochemical Quartz Crystal Microbalance

nanowires periodicmultilayered

nanowires

non-periodicheterostructures

1 nm resolution

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)

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)

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

Hierarchical Self-Assembly

Vacuum Deposited Nanomagnet Arrayslow aspect ratio nanomagnets

cobalt nanodots polymer template

Hierarchical PatterningUsing Block Copolymer Films

2 µm

Combining conventional and BCP Patterning

1012 bit/in2 density PS-PEO

Nanoscopic Magnetic Rings

Nanoscopic Magnetic Rings

"0" "1"

• Stable binary states• Non-interacting

OnionState

VortexState

SDState

M

H

SD

Onion

Vortex

largering

Nanoscopic Cobalt Rings

Ferromagnetic cobalt rings as small as 15 nm OD D. Singh, et al. 2006

Multistate Clusters

Concept: Multistate ClustersAn end-run solution to increasing storage density?

Media

PerpendicularWrite Head

coil

“0” “2” “1” “3”

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

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

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.

Switching Field Distribution

single switching trace

Single dot

multiple traces

histogram

Mz

H

(sanity check)

Seven-Magnet Cluster

Q. Xiao, et al. J. of Appl. Phys. 99, 08G305 (2006)

8 states

Using Asymmetry to Tune Switching StepsSimulation maps help to assess ‘favorable’ cluster designs

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