De nos jours, l’essor de la miniaturisation est un paramètre clé pour la réalisation de microsystèmesde plus en plus complexes. Les recherches sur l’élaboration de matériaux « intelligents », onttoujours suscité un grand intérêt. Dans ce cadre, on se propose d’étudier l’alliage à mémoirede forme magnétique Ni2MnGa sous la forme de couche mince. Ce matériau a la propriétéparticulière de répondre aux sollicitations mécaniques, thermiques et magnétiques. Le couplage deces effets permettrait l’élaboration de micro-actionneurs usuellement réalisés à partir d’assemblagescomplexes. Cette étude pluridisciplinaire traite de l’influence des paramètres d’élaboration sur lespropriétés fonctionnelles des couches minces. L’originalité de ce travail de thèse réside dansl’emploi de substrats en silicium dans la perspective d’une transposition technologique. Un procédéd’élaboration par PVD a été qualifié afin d’obtenir un film aux propriétés AMF magnétique / Nowadays, the miniaturization development is a key parameter in order to fabricate increasinglycomplex microsystems. Research on smart materials aroused a great interest. In this context, westudy the magnetic shape memory alloy Ni2MnGa as a thin layer. This material can be activatedby mechanical, thermal and magnetic stresses. The coupling of these effects would allow thedevelopment of micro-actuators usually made from complex assemblies. This multidisciplinary studyfocuses on the impact of the process on the functional properties of thin films. The originality of thiswork lies in the use of silicon substrates in the context of a technological implementation. A methoddeveloped by PVD was qualified to obtain a film with magnetic shape memory alloy properties.

In this study, the shape memory response of Ni2MnGa and NiMnCoIn magnetic shape memory alloys was observed under compressive stresses. Ni2MnGa is a magnetic shape memory alloy (MSMA) that has been shown to exhibit fully reversible, stressassisted magnetic field induced phase transformation (MFIPT) in the I X-phase transformation because of a large magnetostress of 7 MPa and small stress hysteresis. The X-phase is a recently discovered phase that is mechanically induced, however, the crystal structure is unknown. To better understand the transformation behavior of Ni2MnGa single crystal with [100] orientation, thermal cycling and pseudoelasticity tests were conducted with the goal of determining the Clausius-Clapeyron relationships for the various phase transformations. This information was then used to construct a stresstemperature phase diagram that illustrates the stress and temperature ranges where MFIPT is possible, as well as where the X-phase may be found. NiMnCoIn is a recently discovered meta-magnetic shape memory alloy (MMSMA) that exhibits unique magnetic properties. The ferromagnetic parent phase and the paramagnetic martensite phase allow the exploitation of the Zeeman energy. To gain a better understanding of the transformation behavior of NiMnCoIn, thermal cycling and pseudoelasticity tests were conducted on single crystals from two different batches with crystallographic orientations along the [100](011), [087], and [25 7 15] directions. A stress-temperature phase diagram was created that illustrates the Clausius- Clapeyron relationships for each orientation and batch. SQUID tests revealed the magnetic response of the alloy as well as the suppression of the martensite start temperature with increasing magnetic field. Pseudoelasticity experiments with and without magnetic field were conducted to experimentally quantify the magnetostress as a function of magnetic field. For the first time, it has been shown that NiMnCoIn is capable of exhibiting magnetostress levels of 18-36 MPa depending upon orientation, as well as nearly 6.5% transformation strain in the [100] direction. The results of this study reveal increased actuation stress levels in NiMnCoIn, which is the main limitation in most MSMAs. With this increased blocking stress, NiMnCoIn is a strong candidate for MFIPT.

Pomocou COHP analýzy sme študovali chemickú väzbu v diboridoch prechodných kovov ako aj v zliatine Ni2MnGa. Elektrónová štruktúra študovaných materiálov bola vypočítaná použitím výpočtov z prvých princípov pomocou metódy PAW. V prípade diboridov tranzitných kovov z výsledkov vyplýva, že sila väzby bór-bór je silne závislá od transferu elektrónov na atómy bóru. Zvyšujúci sa počet valenčných elektrónov v kove spôsobuje destabilizáciu alfa štruktúry kvôli vzájomnej interakcii elektrónov prislúchajúcich jednému atómu (on-site interakcie) v blízkosti Fermiho hladiny. Pre zliatinu s tvarovou pamäťou Ni2MnGa bola použitá metóda DFT+ U upravujúca popis lokalizácie elektrónov. V prípade použitia parametra U na Ni dochádza k destabilizácii kubického austenitu a k stabilizácii tetragonálneho nemodulovaného martenzitu. Naopak, zvýšenie lokalizácie elektrónov mangánu martenzit destabilizuje. Analýza väzieb ukázala, že najsilnejšou väzbou je Ni-Ga s kovalentným charakterom. Zvýšená lokalizácia valenčných elektrónov Mn zvyšuje podiel kovalentnosti Mn-Ni väzby a kovový charakter Ni-Ni väzby. Vplyvom zvýšenej lokalizácie Ni elektrónov sa Mn-Ni väzba stáva viac kovovou.

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This Master‘s thesis is focused on theoretical study of twinning in magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concentration of manganese at the expense of gallium was studied on total energy and stress profiles along different deformation paths in the (10-1)[101] shear system of non-modulated martensite. Further, this work deals with the effect of the concentration of manganese on the energy of planar fault caused by presence of partial dislocation due to motion of twin boundary. The results show that the shear modulus in studied shear system increases with the increasing concentration of manganese as well as energy barrier and deformation characteristics along shear deformation paths increases, which makes the shear more difficult in Mn-rich alloys. Increasing concentration of manganese also leads to rising the planar fault energy. All these effects can be responsible for lower mobility of twin boundaries in alloys with higher concentration of manganese.