free-standing single-crystal ni 2 mnga thin films: a new functional material for mems actuators...
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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