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FABRICATION OF PATTERNED FERROMAGNETIC SHAPE MEMORY THIN

FILMS

Pablo Álvarez-Alonso

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Group of Magnetism and Magnetic Materials I

http://gmmmt.net

Objetives: New magnetic materials: from preparation and characterization to applications.

Research lines:MagnetoelasticityMagnetoimpedance and magnetoresistanceMagnetocaloric materials Nanopatterned magnetic materialsFerromagnetic Shape Memory Alloys

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BCMaterialsII

http://www.bcmaterials.net/

Objetives: Functional Materials with advanced Mechanical, Thermal, Electric, Magnetic, and Optical properties- from basic aspects to applications.

Research lines:Active (smart) materials

Advanced functional materials

Nanopatterned magnetic materials

Ferromagnetic Shape Memory AlloysSmart Polymers and compositesHybrid multiferroics (magnetoelectric) materials

Materials for EnergyMaterials for Sensors and Bio-Sensors.Materials for Particle accelerators

Magnetic Nanoparticles-Biomedical and Industrial Applications.Magnetic Nanostructures

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OUTLINE

1.INTRODUCTION

2.ANTIDOTS FABRICATION

2.1 High temperature method

2.2 Low temperature method

3. RESULTS

3.1 Microstructure

3.2 Phase transitions

3.3 Magnetic properties

4. CONCLUSIONS

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INTRODUCTION1

MINIATURIZATION

MEMS (Micro-Electro-Mechanic System)

Driving force of technological and social changeMoor’s law: the number of transistors in a circuit doubles each two years

* Inkjet-printer cartridges* Accelerometers* Micromirrors

* Microtransmissions* Chemical, pressure, and flow sensors* Microactuators

Multifunctional materials Weight-efficient Volume-efficient performance flexibility Less maintenance

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INTRODUCTION1

Magnetic Shape memory Alloys

Austenite

Martensite

y-variant

x-variant

Martensitic transformation

(1st order) Direct (A M) Indirect (M A)

SMA (T)MSMA (T,H)

Changes in shape

Variant Equivalent structures with different orientations

Magnetic field-induced strain

(MFIS) in Ni-Mn-Ga

1-10% Single crystal Sensors and Actuators! <0.01% Polycrystalline (suppression of the twin-boundaries motion)

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INTRODUCTION1

Ni-Mn-Ga micropillars [3]

[1] M. Chmielus et al., Nat. Mater. 8 (2009) 854 - 855[2] N. Scheerbaum et al., Acta Mater. 55 (2007) 2707[3] M. Reinhold et al., J. Appl. Phys. 106 (2009) 053906[4] M. Schmitt et al., Microelectron. Eng. 98 (2012) 536–539

Ni-Mn-Ga microfibers [2]

Ni-Mn-Ga grain size ~ Characteristic sample size

Increase of the free space Increase of the MFIS

Fabrication of Ni–Mn–Ga nanostructures is a challenging task

Ni-Mn-Ga foams [1]

Ni-Mn-Ga cantilevers [4]

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ANTIDOTS FABRICATION2

Fast (parallel) fabrication process Large areas (~1cm2) Low cost technique

APPROACHES

Mix of “Bottom-up” and “Top-down” techniques

* Sputtering* Self-assembled spheres (polystyrene, latex, silica,…)

* Reactive Ion Etching

Nanospheres lithography

4 main stages:

a) Deposit of the spheres (monolayer)

b) Reduction of the spheresc) Metal depositiond) Removal of the spheres

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ANTIDOTS FABRICATION2

Reflux at 60ºC / 18h

N2

Atmosphere

Polymerization

+

Dry in vacuum oven

(12h)

Ingredients:

2g of S (estyrene) monomer 0.04g of AIBN (azobisisobutironitrile)

initiator 0.1g of PVP (polivynilpyrrolidone)

stabilizer 20g of methanol dissolvent

Substrate: Si (100)

COMMON STEPS

PS spheres recipe [1]

[1] J. Lee et al., J. Colloid Interface Sci. 298 (2006) 663–671

Directions:

Centrifuged

Milli-Q water

20 mm

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ANTIDOTS FABRICATION2

Drop-coating [1]

Large-scale monolayered particle mask High hexagonal order Short preparation time

4 stages:

Deposit a dropPS-5% + DI-95%

Vol DI = Vol Ethanol Glass with DI water Consolidation

Triton 2% Liftoff

COMMON STEPS

[1] J. Rybczynski et al., Colloids Surf. Physicochem. Eng. Asp. 219 (2003) 1–6

0.5cm

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ANTIDOTS FABRICATION2

PS reduction: Reactive-Ion Etching

RIE

Conditions for the dry etch

GasesFlow

(sccm)ICP/RF

(W)Pressure

(Torr)Temperature

(oC)Time (min)

PS sphere reduction

O2 120/100 0.1 10 3

Ar 5

Ions and Radicals Physical etch

Chemical (selective) etch

COMMON STEPS

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ANTIDOTS FABRICATION2.1

Conditions for the dry etch

ROUTE 1

GasesFlow

(sccm)ICP/RF

(W)Pressure

(Torr)Temperature

(oC)Time (min)

SiO2 SF6 30150/15

00.1 10 0.3

Si CHF3 10 0/50 0.02 10 15

PS spheres removal with a dissolvent

(Tetrahidrofurane, THF)

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ANTIDOTS FABRICATION2.1

Magnetron DC Sputtering

ROUTE 1

Sputtering conditions

Ar pressure(mbar)

Power(W)

DTarget-Substrate

(cm)Temperature

(ºC)Time(min)

2.6·10-2 150 9 500 5

Ni-Mn-Ga Thickness≈250nm

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ANTIDOTS FABRICATION2.1 ROUTE 1

Si wet etching

KOH 20% 60-100ºC / 1-10min

Potassium hydroxide (KOH) Si and SiO2 etchant

[1] H. Seidel et al., J. Electrochem. Soc. 137 (1990) 3612-3632

KOH (20%) [1]Si (100) SiO2

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ANTIDOTS FABRICATION2.2

Sputtering conditions

PS melting point≈100ºC

ROUTE 2

Dissolvent (THF)

Infrared furnace (P = 10-5 torr) 500ºC / 4h 800ºC / 1h

Enhance the structural order degree [1]

Goal

[1] V.A. Chernenko et al., Mater. Trans. 47 (2006) 619

Ar pressure(mbar)

Power(W)

DTarget-Substrate

(cm)Temperature

(ºC)Time(min)

2.6·10-2 150 9 RT 10

Ni-Mn-Ga Thickness≈500nm

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2.2

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-40

-20

0

20

40

Deposited at 500ºC

Deposited at RT

Annealed at 500ºC

Annealed at 800ºC

Unpatterned films

M (

Am

2 /Kg

)

0H (T)

T = 0ºC

Formation of Ni agglomeratesPre-heated substrate Higher atomic ordering degree

Annealing at 500ºC Enhancement of the magnetic properties

(RT)

Ms =46 Am2/Kg

Ms =13 Am2/Kg

ANTIDOTS FABRICATIONROUTE 2

0 100 200 300 400-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Unpatterned film annealed at 800oC

M(A

m2 /K

g)

Temperature (ºC)

0H=10mT

1710µm

3.1

RESULTS

30µm

Mean diameter: 1.35 µmSt. Deviation: 0.04 µm

Mean diameter: 1.00 µmSt. deviation: 0.06 µm

After RIE

Drop-Coating

MICROSTRUCTURE

* PS diameter homogeneity * Large domains size

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3.1

RESULTS

Elements % at

Ni 48 ± 1

Mn 32 ± 1

Ga 20 ± 1

AFM

Height of the Si dots

~250 nm-thick Ni-Mn-Ga Film

* Sidewall deposition of Si dots * Large Ni-Mn-Ga crystalline grains

ROUTE 1 (High temperature)

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100ºC

Film removed in <3 min

Partial Si removal

60ºC

3.1

RESULTS

Colapse of Ni-Mn-Ga

layer

ROUTE 1 (High temperature)

Wet Etching KOH (20%)

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3.1

RESULTS

Continuous film (RT)

Patterned film (RT)

Elements % at % at

Ni 49 ± 1 50 ± 1

Mn 27 ± 1 27 ± 1

Ga 24 ± 1 23 ± 1

Mean diameter: 1.16 µmSt. Deviation 0.07 µm

Similar mean composition but inhomogeneity ~1-2%

2µm

Room temperature

500ºC / 4h

ROUTE 2 (Room temperature)

~500 nm-thick Ni-Mn-Ga patterned and continuous films

* Small crystalline size* Slight increase of the grain size* Deformation of the antidots shape

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3.2

RESULTS

VSM and Four-probe method

0.020

0.025

0.030

Res

iste

nci

e (

)

0

10

20

301

0.008

0.010

0.012

0.014

0.016

Res

iste

nci

e (

)

0

2

4

6

0H = 10mT

2

-100 -50 0 50 100 150

0.015

0.020

0.025

0.030

Re

sis

ten

cie

()

Temperature (ºC)

0.0

0.2

0.4

0.6

0.8

1.0

0H = 10mT

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PHASE TRANSITIONS

1. Unpatterned Ni-Mn-Ga film deposited at 500ºC2. Unpatterned Ni-Mn-Ga film annealed at 500ºC3. Patterned Ni-Mn-Ga film annealed at 500ºC

Route 2

Sharp decrease of magnetization TC ≈100ºC TM≈ -50/25 ºC

TC ≈50ºC. No martensitic transformation

Crystal disorder

Multiple drops of the magnetization TC

≈100ºC TM <-30ºC

1.

2.

3.

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3.2

RESULTS

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

T = 50ºC T= -173ºC

Ni-Mn-Ga antidots (Route 2)

M/M

s

0H (T)

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Unpatterned Ni-Mn-Ga film (deposited at 500ºC)

T = 50ºC T = -173ºC

M/M

s

0H (T)

Martensite Higher anisotropy Larger coercive field HC

Sample µ0Hc (mT) at -173ºC µ0Hc (mT) at 50ºC µ0ΔHc (mT)

Unpatterned film deposited at 500ºC 120 49 71

Patterned film annealed at 500ºC/4h 84 17 67

MAGNETIC PROPERTIES

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CONCLUSIONS

1 Two ways for Ni-Mn-Ga thin-films micropatterning have been developed by using self-assembled polystyrene spheres and reactive ion etching.

  Route 1: Si sacrificial layer to deposit Ni-Mn-Ga at

500ºC.

Route 2: Large area of 2D-arrays of Ni-Mn-Ga antidots at room temperature and subsequent annealing in a high-vacuum furnace at 500ºC for 4 hours.

2 Route 1 proved to be promising (optimization is need)

3 Antidots synthesized by Route 2 present functional characteristics: Ferromagnetisms (TC~100ºC) and a spread martensitic transformation.