Synthesis and Functional Properties of Ni-Mn-Ga Ferromagnetic Shape Memory Thin Films

Abstract

<p> The purpose of this work was to develop a growth pathway for synthesis of high-quality ferromagnetic Ni-Mn-Ga shape memory thin films and to understand their functional properties. </p> <p>Two groups of Ni-Mn-Ga films were prepared using the pulsed laser deposition technique. One group was grown on (100) YSZ ((Zr,Y)O2 with ZrO2:Y2O2=92:8) and was used to study the thermal, magnetic, and magnetocaloric properties. The other group was deposited on (100) MgO. These films were fabricated into free-standing micro-bridges.</p> <p>These Ni-Mn-Ga films were synthesized in a previously unexplored high temperature deposition regime. The temperatures employed encouraged the development of the desired micrometer-size highly twinned martensite grain structure while the general problem of preferential evaporation of the volatile elements at elevated temperatures was solved by using a manganese and gallium enriched target.</p> <p> The films grown on (100) YSZ exhibit a self-reversible, magnetically induced reorientation of the martensite variants (MIR), which is temperature dependent. The mechanism associated with the self-activated reversibility in the MIR effect was established through a detailed characterization of the texture, microstructure, and magnetic domain structure of the films. </p> <p> The synthesized films also show a large magnetocaloric effect (MCE), which is particularly strong at low magnetic fields. The effect is associated with an overlapped ferromagnetic and martensitic phase transition. Detailed characterization of these transitions allow for an understanding of the role each phase transition plays in determining the level of enhancement to a standard MCE governed only by the ferromagnetic phase transition.</p> <p> Ni-Mn-Ga free-standing micro-bridges were fabricated using photolithography followed by wet chemical etching. Microstructural evolution of the martensitic variants resulting from changes to the stress field in the detached areas was studied in the context of potential film applications for micro-actuators.</p>