Microstructure and atomic configuration of the (001)-oriented surface of epitaxial Ni–Mn–Ga thin films

Abstract

The (001)-oriented surface of epitaxial off-stoichiometric
\nNi–Mn–Ga ferromagnetic shape memory alloys was studied in both austenitic
\nand martensitic phases. Scanning tunneling microscopy (STM) imaging of
\nthe austenitic surface reveals a well-ordered and reconstruction-free surface
\nexhibiting predominantly Mn–Ga termination. We found that only one of the
\ntwo atomic species (Ga or Mn) is visible in STM, which is attributed to a
\npronounced geometric corrugation of the surface layer. After a transformation
\nof the sample from the initial austenitic phase to the martensitic phase upon
\na high-temperature annealing step, a thorough investigation of the martensitic
\nsurface was conducted. On a larger scale, pronounced corrugation lines arise
\nfrom the macroscopically twinned surface. A second corrugation feature is
\nfound on a distinctly smaller scale and is shown to originate from the modulated
\nnature of the martensitic film structure. The irregularly spaced corrugation lines
\nsupport the model of adaptive martensites.