Growth and Martensitic Transformation of Ferromagnetic Co-Cr-Ga-Si Epitaxial Films

Martensitic transformations are important for many materials exhibiting magneto-, elasto- and barcaloric effects and
often the functional properties benefit from additional magnetic transformations. For caloric applications, thin films
are of particular interest due to their fast heat exchange, which promises a high cooling power. In particular epitaxial
films are a model system to understand the formation of the martensitic microstructure.[1] However, commonly
transformations from austenite to martensite only occur during cooling. The recent observation of a so-called reentrant
transformation obtained much interest, as an additional transformation from martensite to austenite was observed
during further cooling.[2,3] A reentrant transition increases both, the number of new physical effects and the possible
applications. However, reports are currently very few and only on bulk materials. Will this phenomenon occur also in
thin films? Will the martensitic transformation and microstructure differ when induced by heating or cooling? Also
for caloric applications, reentrant martensite is interesting as it may enable bidirectional caloric effects driven by
deformation and recovery of the ferromagnetic shape memory alloys (FSMA) through both, heating and cooling.
As a model system, epitaxial Co-Cr-Ga-Si ferromagnetic shape memory thin films were grown by DC magnetron
sputtering. To obtain epitaxial growth, films were deposited on different substrates as well as orientations. We identify
MgO(100) as the optimum substrate. Films were grown at different deposition temperatures and the influence of an
additional postannealing process was examined to understand the influence on elemental composition, structure,
microstructure and the transformation behavior. As a kind of summary, Fig.1-a depicts a phase diagram of all samples
prepared. Under optimum conditions, we obtain epitaxial growth, as evident from pole figure measurements of an
austenitic film (Fig. 1-b). When films are subjected to an additional heat treatment at 800°C, films are martensitic at
room temperature, which is evident the peak splitting in pole figure measurements (Fig. 2-a) and agree well with pole
figures calculated by the phenomenological theory of martensite with a c/a ratio of 1.37 (Fig. 2-b). Furthermore, the
microstructure becomes granular and twin boundaries become visible. Though annealing improves the martensitic
transformation, we observe a degradation of the ferromagnetic transition. To sum up, we could demonstrate epitaxial
growth of Co-Cr-Ga-Si films, which are martensitic and ferromagnetic at room temperature. This is a key step to
utilize the additional possibilities of reentrant martensite also for thin films.