1. 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. McKernan
and 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

2. Ferromagnetic Shape Memory Effect in Ni2MnGaTc Ms
Requires both ferromagnetic transition (Tc) and martensitic phase transformation (Ms) 
Austenite
Weak anisotropy c 
Variant 3 H  c
Variant 2 c
Variant 1
Strong anisotropy
Martensite L  H
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

3. Single-crystal Free-standing Thin Films of Ni2MnGa: New Concepts for MEMS Actuators
- 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 M A A
Released martensite film of Ni2MnGa
Hybrid Material Epitaxy Center
University of Minnesota

4. Bulk Properties and Crystal Structures of Ni2MnGa
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
Hi-T (Austenite)
L21 Heusler structure
Structural Properties
Structure Lattice Parameter (Å) Mismatch to GaAs (%)
HT Austenite (cubic)
LT Martensite (tetragonal) a =5.920 c = (a), -1.5 (c)
Hybrid Material Epitaxy Center
University of Minnesota

5. MBE Growth of Single-crystal Ni2MnGa:Sample Structure
Sc,Er As
Ni2MnGa
NaCl-like structure
a = Å
Ni2MnGa (900 Å)
Ni/Mn+Ga/Ni/…
(5 ML’s)
6 ML-thick Sc0.3Er0.7As
(001) GaAs substrate
GaAs Buffer layer
AlGaAs Etch stop
Sc0.3Er0.7 As
NaCl structure
a = Å
GaAs
Zincblende structure
a = Å
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. MBE Growth of Ni2MnGa Thin Films on GaAs (001)
In situ RHEED after 900 Å-thick Ni2MnGa growth
Ex situ XRD  -2 scan
E-beam // [110]
E-beam // [100]
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

7. Cross-section TEM Study: Ni2MnGa(900 Å) / Sc0. 3Er0
Cross-section TEM Study: Ni2MnGa(900 Å) / Sc0.3Er0.7As(17 Å) / GaAs (001)
Spot splitting
Pseudomorphic growth of the Ni2MnGa films:
(a = 5.65 Å, c = 6.18 Å) Ga Mn Ni
Sc,Er As
GaAs
Sc0.3Er0.7As
Ni2MnGa
<110> zone
Hybrid Material Epitaxy Center
University of Minnesota

8. Magnetic Properties of Epitaxial Ni2MnGa Films
Ms ~ 250 emu/cm3
Hc ~ 50 Oe
Magnetic field applied along in-plane [110]
Ni2MnGa
GaAs
RT Hysteresis loop
M vs. T measurement
Tc ~ 340 K
Cool down without field, then warm in a field of 1000 Oe
The softness implies Austenite-like behavior of the epitaxial films.
No phase transformation is observed in unreleased films!
Hybrid Material Epitaxy Center
University of Minnesota

9. Patterning and Processing of Free-standing Films
Ar/Cl2 Plasma
RIE of Ni2MnGa film
Photoresist
Photolithography of film side
GaAs
Ni2MnGa
Mechanical polishing substrate to 100 m-thick
Backside IR alignment and photolithography
After selective chemical etching
After RIE
Free-standing Cantilever
Hybrid Material Epitaxy Center
University of Minnesota

10. Free-standing Ni2MnGa bridges
Optical Microscope Image
Tapping Mode MFM Images
100 m
Topographic
Magnetic
10 m
Open area
<110>
Attached film
Free-standing bridge
400 m
Released bridges form tent-shape features at RT and show strong 180 magnetic domain structures.
Hybrid Material Epitaxy Center
University of Minnesota

11. Magnetic Characterization: SQUID Measurements on Partially Released Ni2MnGa Films
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
Hybrid Material Epitaxy Center
University of Minnesota

12. Two-way Shape Memory Effect in Free-standing Films
Polarized light optical microscope images
Heating
(a) RT
(b) 100C
(c) 120C
(d) 150C
<110>
Cooling
(h) 60C
(g) 100C
(f) ~120C
(e) <150C
Cooling
Hybrid Material Epitaxy Center
University of Minnesota

13. Magnetic Field Induced Strain in Ni2MnGa Films
Optical microscope images
T = 135 K
H = 0 T
H = 0.24 T
H = 0.48 T H
<110>
H = 1.20 T
H = 0.96 T
H = 0.72 T
Shape change is saturated at an applied field of ~1.2 T.
Hybrid Material Epitaxy Center
University of Minnesota

14. Magnetic Field Induced Strain in Ni2MnGa Films
Optical microscope images
<110>
H = 0 T H
H = 1.2 T
T = 135 K
T = 200 K
T = 250 K
Observational evidence confirms: Ms ~ 200 K, As ~ 180 K.
Hybrid Material Epitaxy Center
University of Minnesota

15. 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