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

Ferromagnetic Shape Memory Effect in Ni2MnGa Tc 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

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

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) 5.825 3.0 LT Martensite (tetragonal) a =5.920 c = 5.566 4.7 (a), -1.5 (c) Hybrid Material Epitaxy Center University of Minnesota

MBE Growth of Single-crystal Ni2MnGa:Sample Structure Sc,Er As Ni2MnGa NaCl-like structure a = 5.825 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 = 5.653 Å GaAs Zincblende structure a = 5.653 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

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

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

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

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

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

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

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

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

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

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