AVS 52nd International Symposium
    Thin Films Wednesday Sessions
       Session TF-WeA

Paper TF-WeA9
Microstructural Characterization of Single Crystal Ferromagnetic Shape Memory Films

Wednesday, November 2, 2005, 4:40 pm, Room 306

Session: Fundamentals of Thin Films
Presenter: T.C. Shih, University of Minnesota
Authors: T.C. Shih, University of Minnesota
S. McKernan, University of Minnesota
S.K. Srivastava, University of Minnesota
J.Q. Xie, University of Minnesota
R.D. James, University of Minnesota
T.W. Shield, University of Minnesota
C.J. Palmstrom, University of Minnesota
Correspondent: Click to Email
Microstructure and martensitic phase formation behavior are known to be important for magnetic-field-induced-strain in ferromagnetic shape memory alloys. Transmission electron microscopy (TEM) has been used to examine the microstructure and martensitic transformation behavior of non-stoichiometric Ni-Mn-Ga and Co-Ni-Ga post-growth-released single crystal films grown by molecular beam epitaxy (MBE) on GaAs (001). The twin and precursor tweed structures observed in the TEM images are of particular interests. The film compositions were determined using Rutherford backscattering spectrometry (RBS), particle-induced X-ray emission (PIXE), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS). For a thin film with nominal composition Co@sub 50@Ni@sub 25@Ga@sub 25@, convergent beam electron diffraction patterns indicated that the austenite exhibited primarily B2 ordering with additional weak L2@sub 1@ ordering at room temperature. Sharp satellite reflections appeared at 1/6 the spacing of the {220} reflections around the {220} spots along the <220> directions at ~250 K, with the evidence of high-density striations or tweed contrast observed in the dark-field images. The tweed contrast became more pronounced on cooling, accompanied by increasing intensity of the four additional reflections. On further cooling and stabilizing at 100 K, no twin structure was observed. For thin films with nominal composition Ni@sub 50@Mn@sub 25@Ga@sub 25@, Ni@sub 50@Mn@sub 30@Ga@sub 20@, and Ni@sub 53@Mn@sub 25@Ga@sub 23@, the martensitic transformation temperatures showed linear dependence with increasing average valence electron concentration per atom (e/a). Each specimen transformed into different microstructures with the microtwins ranging from 10 nm to 80 nm. This presentation will emphasize the influence of composition and epitaxial growth conditions on the resulting microstructure of the FSMA films before and after being released from the substrates.