Tuning macro-twinned domain sizes and the b-variants content of the adaptive 14-modulated martensite in epitaxial Ni–Mn–Ga films by co-sputtering

Abstract

In order to obtain modulated martensite in our epitaxial Ni-Mn-Ga films, we tuned the composition by using a co-sputtering process.Here we present how the composition affects the variant distribution of the 14-modulated martensite at room temperature. The nature of such modulated martensites is still strongly debated for magnetic shape memory alloys. It has been very recently demonstrated that the modulated martensites in Ni-Mn-Ga are adaptive phases. The results presented here corroborate this theory for the first time, for three different compositions. Moreover, we demonstrate with the help of the adaptive modulations theory that b-variants of the 14-modulated martensite form close to the free surface of the film to release the stress induced by branching of macro-twinned domains during the martensitic transformation on a rigid substrate. At room temperature, the content of such b-variants is found to strongly decrease when the macro-twinned domain sizes increase.

Insight into variant selection of seven-layer modulated martensite in Ni–Mn–Ga thin films grown on MgO(0 0 1) substrate

“…Indeed, the character of ultrafine microstructural constituents poses a major challenge for crystallographic characterization by conventional methods. As previously examined with optical microscopy (OM) [8], transmission/scanning electron microscopy (TEM/SEM) [7,20], and atomic force microscopy (AFM) [4,23], two distinct kinds of martensite plate colonies may exist in epitaxial Ni-Mn-Ga thin films. One refers to long and straight plates with undetectable surface corrugations (low surface corrugation zones), and the other to short and bent plates with clear surface corrugations (high surface corrugation zones).…”

“…Epitaxial thin films of Ni-Mn-Ga ferromagnetic shape memory alloys have attracted considerable attention during the past decade [1][2][3][4][5][6][7][8], due to their potential application as new materials for micro-actuators and sensors in microelectro-mechanical systems [9][10][11][12][13]. To obtain qualified thin films, the choice of an appropriate substrate is important.…”

“…One refers to long and straight plates with undetectable surface corrugations (low surface corrugation zones), and the other to short and bent plates with clear surface corrugations (high surface corrugation zones). By combining the surface corrugation detection using AFM and the global orientation analysis on X-ray pole figures [4,[6][7][8]22,23] crystallographic orientations of seven-layer modulated (named 7M or 14M) martensite plates, the orientation relationships between adjacent plates and the inter-plate interface planes were deduced.…”

“…Efforts have been constantly directed to reveal the microstructural features and crystallographic correlations of martensite variants in epitaxial Ni-Mn-Ga films [4,7,8,[20][21][22]. Indeed, the character of ultrafine microstructural constituents poses a major challenge for crystallographic characterization by conventional methods.…”

Magnetic Shape Memory Microactuators

“…The resulting film structure depends on various parameters including substrate, deposition temperature, sputtering power, and annealing conditions. Consequently, as-prepared films can exhibit polycrystalline structure [18][19][20][21][22][23][24] or an epitaxial relation to the substrate [11][12][13][14][25][26][27][28].…”