Development of a Ni-Mn-Ga Microdevice for Actuation and Sensing

This paper reports on the development of a smart microdevice of a Ni-Mn-Ga thin film, which shows actuation and sensing capabilities. A typical application is an adaptive microoptical deflection system, which is a key component for optical information technology.

For layout and optimization, a simulation tool has been developed. A fully coupled model for the magneto-thermo-mechanical behaviour of the material is implemented in a program package for coupled finite element simulations. The layout of the microdevice consists of a Ni-Mn-Ga double-beam cantilever placed in the inhomogeneous field of a miniature permanent magnet. As a first step, the magnetic field is calculated along the trajectory of the cantilever. From the resulting magnetic force components and their gradients, the deflection of the cantilever in martensitic state is determined. Then, the increase of temperature due to electrical heating is calculated. From the resulting martensite and austenite phase fractions and magnetization data, the actual position of the cantilever is derived. An empirical model for the temperature- and magnetic-field-dependent resistance and magnetization is included in the simulation. Stress-strain and magnetization experiments are performed to establish an empirical database. The simulation results are complemented by deflection and resistivity measurements as a function of heating power. For this purpose, test devices are fabricated from Ni-Mn-Ga thin films by microtechnology processes consisting of photolithography and wet etching. Different geometries are considered to investigate actuation properties like forces and displacements. In addition, the options to use the intrinsic material characteristics of temperature-dependent resistance and magneto resistance for position sensing are discussed.