Alfred Ludwig

Development of multifunctional thin films using high-throughput experimentation methods

Abstract This paper describes the use of thin film high-throughput experimentation methods for the efficient development of multifunctional materials, using Ni – Ti – X and ferromagnetic shape memory alloys as examples. The thin films were fabricated in the form of binary, ternary, and quaternary materials libraries by special magnetron sputter deposition processes. These materials libraries were subsequently processed and characterized by high-throughput experimentation methods in order to relate compositional information with structural and functional properties. For this, appropriate visualization of the data is necessary. Results show that the martensitically transforming regions in ternary thin films are generally larger than was known from literature. Within these regions, the variation of the functional properties can be mapped with respect to the composition and microstructure, and thus the most suitable materials for applications can be effectively selected.

Giant magnetostrictive thin films for applications in microelectromechanical systems (invited)

Sputter-deposited giant magnetostrictive thin films allow the realization of microactuators and sensors which can be addressed by remote control operation. Applications reviewed in this article are in the field of microelectromechanical systems (MEMS), e.g., microfluidic devices, micromotors, laser scanner mirrors etc. In general, all these MEMS applications of magnetostrictive thin films require a well defined uniaxial in-plane anisotropy and are operated at or above room temperature. In previous investigations it was found that (Tb40Fe60/Fe50Co50) multilayers represent the most promising thin film material with respect to their unique combination of soft magnetic and giant magnetostrictive properties. Consequently, this material was used to show the possibility to control the orientation of the magnetic easy axis by magnetic field annealing. Furthermore data about the temperature dependence of these magnetostrictive films are given.

Giant magnetostrictive spring magnet type multilayers

Thin film magnetostrictive materials can be extremely useful as the active material in microactuators. In this article, some results on a novel type of multilayer structure which combines exchange coupled giant magnetostrictive materials and materials with large magnetic polarizations are presented for the first time. Giant magnetostrictions are achieved at low fields, due to the polarization enhancement in such multilayers. Therefore such composite materials should be much more appropriate for applications than the simple homogeneous alloy films studied up to now.

MEMS tools for combinatorial materials processing and high-throughput characterization

Using the combinatorial material synthesis approach, materials libraries can be produced in one experiment that contain up to several thousand samples on a single substrate. In order to identify optimized materials in an efficient way using screening methods, adequate automated material characterization tools have to be designed and applied. Microsystems (micro-electromechanical systems: MEMS) offer powerful tools for the fabrication and processing of materials libraries as well as for accelerated material characterization on planar substrates such as Si wafers. MEMS can be used for parallel materials processing, either as passive devices such as shadow mask structures, or as active devices such as micro-hotplates. Microstructured wafers, which incorporate sensor or actuator structures such as electrode or cantilever arrays, can be used to identify materials properties in an efficient way.

Giant magnetostrictive multilayers (invited)

Multilayers combining exchange-coupled giant magnetostrictive materials (amorphous TbFe) and materials with large magnetic polarizations (FeCo or FeCoBSi) exhibit significant improvements in terms of low field magnetostriction and magnetostrictive susceptibility compared to state-of-the-art giant magnetostrictive single layer materials, due to the polarization enhancement and the anisotropy reduction in such multilayers. The magnetic, magnetostrictive, and mechanical properties of these multilayers can be engineered by varying their layer thickness ratios. With TbFe/FeCo multilayers saturation magnetoelastic coupling coefficients exceeding 25 MPa in combination with a magnetic saturation field of 20 mT and a coercive field of 2 mT can be obtained, while in TbFe/FeCoBSi multilayers the hysteresis almost vanishes but at the expense of the magnetostriction. Furthermore these multilayered materials show ΔE and magnetoresistance effect, which should allow the realization of smart thin film actuators.

Structure-related antibacterial activity of a titanium nanostructured surface fabricated by glancing angle sputter deposition

The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs).Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography.

Bistable Thin-Film Shape Memory Actuators for Applications in Tactile Displays

Bistable shape memory actuators were fabricated by microsystem technology processes and characterized with regard to their use in tactile graphic displays. The actuators were realized as sputter-deposited buckled metallic thin-film carriers, having structured Ti-Ni-Cu and Ti-Ni-Hf shape memory alloys on the top and at the bottom, respectively. They were sputtered on wavy-structured substrate and had lateral dimensions ranging from 2.2 to 3.5 mm in width and from 1 to 3 mm in length. The actuators were switched with voltages in the range of 0.2 to 0.8 V and with currents in the range of 0.2 to 0.8 A. A force of 2.2 mN with a displacement of 0.7 mm was reached. To improve the performance further, a special test setup was developed. The bistable actuators in it were sputtered on a planar substrate with lateral dimensions ranging from 6 to 8 mm in width and from 3 to 6 mm in length. These actuators were switched with actuation voltages in the range of 0.6 to 1.6 V and with currents in the range of 0.6 to 1.8 A. Thus, a force of 16 mN with a displacement of 1.2 mm was reached.

Integration of two degree-of-freedom magnetostrictive actuation and piezoresistive detection: application to a two-dimensional optical scanner

A novel two-dimensional (2-D) optical-scanner device is presented. This device incorporates a highly magnetostrictive thin film with anisotropic properties, so that it can produce 2-D-actuation corresponding to bending and torsion vibrations. The magnetostrictive material is a TbFe-CoFe multilayer film, which has optimized properties for micro-actuators operating at low excitation magnetic fields. The new scanner also integrates an original 2-D piezoresistive detector realized in an easy fabrication process using integrated circuit (IC)-compatible technology. The detectors are able to selectively measure bending and torsional vibrations. This new device enables the synchronization of actuation and sensing for 2-D position control.

Magnetic properties and microstructure of giant magnetostrictive TbFe/FeCo multilayers

Magnetostrictive multilayer films which combine exchange coupled giant magnetostrictive materials (amorphous Tb0.4Fe0.6) and soft magnetic materials with large polarizations and considerable magnetostriction (crystalline Fe0.5Co0.5) were prepared by magnetron sputtering. The microstructure and the magnetic properties of these multilayers were investigated as a function of the annealing temperatures and the corresponding film stresses. Giant magnetoelastic coupling coefficients (or magnetostrictions) are achieved at low fields, due to the magnetic polarization enhancement in such multilayers, the optimized stress state, and a suitable microstructure. For these optimized Tb0.4Fe0.6(7 nm)/Fe0.5Co0.5(9 nm) multilayers a saturation magnetoelastic coupling coefficient of 27.5 MPa at 20 mT and a coercive field of 2 mT has been achieved.

Novel strain sensors based on magnetostrictive GMR/TMR structures

Summary form only given. While there has been considerable research devoted to the use of GMR (giant magnetoresistance) and TMR (tunnel magnetoresistance) layered structures-these studies have focused mainly on data storage applications (e.g., MRAM, Read-write heads) and magnetic field sensors. Comparatively few studies have been devoted to the exploitation of these materials in broader industrial applications for use such as stress, strain or pressure sensors. This study presents an investigation into various magnetic layer structures suitable for devices sensing mechanical responses such as stress, strain and pressure.