Free-standing Single-crystal Ni2MnGa Thin Films: A New Functional Material for MEMS Actuators

Jianwei Dong, J. Q. Xie, J. Lu, C. Adelmann, A. Ranjan, S. McKernanand C. J. Palmstrøm

Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA

Q. Pan, J. Cui, and R.D. James

Dept. of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA

Supported by AFOSR-MURI, ONR, DARPA and NSF-MRSEC

AVS 49th International Symposium

Denver, Colorado

Nov. 7th, 2002

Hybrid Material Epitaxy Center University of Minnesota

0

0.01

0.02

0.03

0.04

0.05

0 100 200 300 400 500

Mag

netic

Mom

ent (

emu)

Temperature (K)

Tc

Ms

Requires both ferromagnetic transition (Tc) and martensitic phase transformation (Ms)

Ferromagnetic Shape Memory Effect in Ni2MnGa

• Magnetostriction: 9.5% in bulk single crystal (Sozinov et al., 2002)

• Frequency of response : > 5000 Hz in bulk (Ezera et al., 1999)

Hybrid Material Epitaxy Center University of Minnesota

Martensite

Weak anisotropy

Strong anisotropy

H

L

H

Variant 1 Variant 2

Variant 3

Austenite

c

c

c

Single-crystal Free-standing Thin Films of Ni2MnGa: New Concepts for MEMS Actuators

Hybrid Material Epitaxy Center University of Minnesota

- MEMS applications require thin films.

- Single-crystal thin film supports:• single variant martensite

• compatibility between austenite and single variant martensite along special

directions• well-defined martensite twinning structures enable MEMS design

H(t)

picture drawn with measured lattice parameters of Ni2MnGa

Variant 2

Variant 1

[100]

e3 e

n

[001] [010]

[100]

H

Released martensite film of Ni2MnGa

A AM

GaMn

Ni

Bulk Properties and Crystal Structures of Ni2MnGa

Structural Properties

Structure Lattice Parameter (Å) Mismatch to GaAs

(%)

HT Austenite (cubic) 5.825 3.0

LT Martensite (tetragonal) a =5.920 c = 5.566 4.7 (a), -1.5 (c)

Hi-T (Austenite)

L21 Heusler structure

Hybrid Material Epitaxy Center University of Minnesota

Shape Memory Alloy

Cubic Tetragonal

Ms ~200K (Ni2MnGa), ~340K (Ni50Mn30Ga20)

Magnetic Properties

Ms~700 emu/cm3 Tc~373 K

Ku ~ 2 106 erg/cm3 for martensite

K1 ~ 104 erg/cm3 for austenite

Ga

Mn

Ni

Sc,Er

As

Ga

As

MBE Growth of Single-crystal Ni2MnGa:Sample Structure

Materials with NaCl crystal structures should be good templates for Ni2MnGa

growth. In particular, Sc0.3Er0.7As is ideal because it is epitaxial, lattice matched,

and thermodynamically stable on GaAs. Hybrid Material Epitaxy Center University of

Minnesota

6 ML-thick Sc0.3Er0.7As

Ni/Mn+Ga/Ni/…

(5 ML’s)

(001) GaAs substrate

GaAs Buffer layer

AlGaAs Etch stop

Ni2MnGa (900 Å)

Sc0.3Er0.7 As

NaCl structure

a = 5.653 Å

GaAsZincblende structure

a = 5.653 Å

Ni2MnGa

NaCl-like structure

a = 5.825 Å

MBE Growth of Ni2MnGa Thin Films on GaAs (001)

Hybrid Material Epitaxy Center University of Minnesota

E-beam // [110]

E-beam // [100]

In situ RHEED after 900 Å-thick Ni2MnGa growth

100

101

102

103

104

105

106

107

10 20 30 40 50 60 70 80In

ten

sity

2(deg.)

Ga

As

(00

4)

Ni 2

Mn

Ga

(0

04

)

Ga

As

(00

2)

Ni 2

Mn

Ga

(0

02

)

Ex situ XRD -2 scan

The film is epitaxial with an out-of-plane lattice constant of 6.18 Å.

High quality epitaxial growth with smooth surface .

Hybrid Material Epitaxy Center University of Minnesota

GaAs

Sc0.3Er0.7As

Ni2MnGa

Cross-section TEM Study: Ni2MnGa(900 Å) / Sc0.3Er0.7As(17 Å) / GaAs (001)

Spot splitting

Ga

Mn

Ni

Sc,Er

As

Ga

As

Pseudomorphic growth of the

Ni2MnGa films:(a = 5.65 Å, c = 6.18 Å)

<110> zone

-300

-200

-100

0

100

200

300

-1500-1000 -500 0 500 1000 1500

Mag

netiz

atio

n (e

mu/

cm3 )

Magnetic Field (Oe)

Ms ~ 250 emu/cm3

Hc ~ 50 Oe

Magnetic field applied along in-plane [110]

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350

M/M

0

Temperature (K)

Tc ~ 340 K

Cool down without field, then warm in a field of 1000 Oe

Magnetic Properties of Epitaxial Ni2MnGa Films

Hybrid Material Epitaxy Center University of Minnesota

No phase transformation is observed in unreleased films!

Ni2MnGaGaAs

RT Hysteresis loop M vs. T measurement

The softness implies Austenite-like behavior of the epitaxial films.

Patterning and Processing of Free-standing Films

Hybrid Material Epitaxy Center University of Minnesota

GaAs

Ni2MnGaPhotoresist

Photolithography of film side

Ar/Cl2 Plasma

RIE of Ni2MnGa film

After RIEAfter selective chemical etching

Backside IR alignment and photolithography

Free-standing Cantilever

Mechanical polishing substrate to 100 m-thick

Free-standing Ni2MnGa bridges

Released bridges form tent-shape features at RT and show strong 180 magnetic domain structures. Hybrid Material Epitaxy Center University of

Minnesota

Free-standing bridge

<11

0>

Open area

Optical Microscope Image Tapping Mode MFM Images

400 m

100 m Topographic Magnetic

10 m 10 m

Attached film

Magnetic Characterization: SQUID Measurements on Partially Released Ni2MnGa Films

Hybrid Material Epitaxy Center University of Minnesota

0

20

40

60

80

0 50 100 150 200 250 300 350

Mom

ent (

em

u)

Temperature (K)

Cool down without field, then warm/cool/warm with 100 Oe field applied in-plane

1. Initial warm up

2 & 3. Cool/Warm

overlapped Free-standing films

After the film is partially released from the substrate, there is a phase transformation ~ 300 K

Two-way Shape Memory Effect in Free-standing Films

Hybrid Material Epitaxy Center University of Minnesota

(a) RT (b) 100C (c) 120C (d) 150C

(e) <150C (f) ~120C(g) 100C(h) 60C

Heating

Cooling

Cooling

<11

0>Polarized light optical microscope images

Magnetic Field Induced Strain in Ni2MnGa Films

Hybrid Material Epitaxy Center University of Minnesota

T = 135 K

Shape change is saturated at an applied field of ~1.2 T.

<110>

H = 0 T

H = 0.96 T

H = 0.24 T H = 0.48 T

H = 0.72 TH = 1.20 T

H

Optical microscope images

Magnetic Field Induced Strain in Ni2MnGa Films

Hybrid Material Epitaxy Center University of Minnesota

H = 0 T

H = 1.2 T

T = 135 K T = 200 K

<110>

Observational evidence confirms: Ms ~ 200 K, As ~ 180 K.

H

Optical microscope images

T = 250 K

Summary

•First pseudomorphic MBE growth of 900 Å-thick Ni2MnGa films on GaAs

(001) with a unique tetragonal structure (a = 5.65 Å, c = 6.18 Å). The films are

ferromagnetic at RT and has a Curie temperature ~ 340 K.

•Control of martensitic phase transformation temperatures through

composition control.

•Developed sub-millimeter free-standing Ni2MnGa thin films in bridge and

cantilever forms.

•First observation of two-way shape memory effect and magnetic field induced

strain in the released films.

Hybrid Material Epitaxy Center University of Minnesota