Ferromagnetic shape memory thin film Actuators
Final Report Abstract
The FACTS project has been a collaborative effort between the University of Karlsruhe (IMT) and partners from the North Carolina State University (NCSU), USA, who simultaneously submitted their counterpart to the National Science Foundation (NSF). The FACTS project addresses the development of the first fully coupled thermo-magneto-mechanical model for ferromagnetic shape memory alloy (FSMA) materials and the implementation of the model in a finite element tool to simulate the performance of FSMA devices with arbitrary geometry. In particular, the specific properties of devices based on FSMA thin films and foils are addressed to explore their potential for microsystems applications. The modeling approach follows the previous work of the project partner NCSU on conventional SMAs to construct Gibbs free energy expressions for a representative lattice element and to apply the theory of thermally activated processes to derive evolution equations for martensite phase fractions. In order to validate the model, the magneto-mechanical and magnetization performance of single crystalline bulk Ni-Mn- Ga actuators are reproduced first. Subsequently, the model is extended step-wise to take into account further coupling effects in the Ni-Mn-Ga material system, e.g., thermal coupling and heat exchange effects. In its final version, the simulation tool is capable to describe the following properties: • The magnetic-field-induced reorientation (MIR) effect including its dependence on stress, anisotropy energy and actuator geometry • The magnetic-field-induced martensite (MIM) effect including its dependence on stress, anisotropy energy and actuator geometry • The formation of single/multiple martensite variants upon cooling in a magnetic field including its dependence on field strength, anisotropy energy and stress • Ferromagnetic attraction and Lorentz forces • The temperature-dependent magneto-electrical resistance In parallel to the simulation efforts, micromachining and integration technologies are developed to fabricate various Ni-Mn-Ga test actuators and, finally, a functional optical deflection system as a demonstrator. Due to lack of availability of single crystalline Ni-Mn-Ga films and foils, polycrystalline Ni-Mn-Ga films are used for fabrication. The FSMA microactuator makes use of the intrinsic ferromagnetic deflection and thermal shape memory effect for actuation. The use of the intrinsic magneto-resistance effect for position sensing of a FSMA microactuator is investigated for the first time. The performance of the demonstrator is summarized as follows: • The demonstrator is designed for a maximum deflection of 200 //m • For a maximum available magnetic fleld of a miniature permanent magnet of about 0.7 Tesla, a negative in-plane magneto resistance of 0.26% occurs at room temperature • For increasing heating power, the negative in-plane magneto resistance increases to a maximum of 0.77% before it drops to zero when the average temperature reaches Tc of 370 K. • For a maximum magnetic field of 0.7 Tesla, the magneto-resistance effect allows position sensing with an accuracy of about 10 pm. The deveioped model is considered to be a versatile tool to describe the rather complicated thermo-magneto-mechanical coupling effects in FSMA materials encountered under various physical conditions, which is very useful for future engineering applications. So far, single crystalline Ni-Mn-Ga bulk materials and polycrystalline Ni-Mn-Ga films have been investigated. In future work, other FSMA material systems (e.g. FePd, FePt, CoNiGa) will be considered being of interest to actuator and sensor applications. Thus, also more complex FSMA actuators and sensors may be studied consisting, for instance, of several different FSMA materials. The simulation tool will be helpful to study thermo-magneto-mechanical training effects required to improve the performance of FSMA devices. Furthermore, the simulation tool will allow deducing design rules for the layout of FSMA sensors and actuators. Additional coupling effects such as the magneto-caloric effect can be implemented in a straightfonward manner. From the microsystems engineering perspective, it will be important to investigate the impact of fabrication technology on the thermo-magneto-mechanical performance of micro-scale devices. The success of the modeling approach will be stimulating to other related research topics, for instance, the development of a coupled simulation tool for multiferroic materials or other multifunctional material systems. Eventually, the simulation and technology results may become important for nano science and technology. In particular at the interface between micro-/ nanotechnologies on the one hand and (F)SMA materials development on the other, a new realm for research and development is currently emerging. Nanomachined FSMA structures are, for instance, interesting test systems for the study of fundamental scientiflc questions on the coupling of structural and magnetic properties and their size dependence. The gained knowledge about the technical feasibility and limits of micro-scale FSMA actuators and sensors will help in the ongoing efforts in further miniaturization. We expect that the development of novel FSMA nano actuators and sensors may come in reach within the next years opening up a new class of technical systems with unprecedented performance properties, which would hardly be possible by classical actuation and sensing concepts.
