In-situ Straining of Epitaxial Freestanding Ferroic Films through a MEMS Device

Abstract

Mechanical strain can be used to control physical properties in materials. The experimental investigation of strain-induced effects at the nanoscale is of importance not only for its fundamental aspect, but also for the development of device applications. Transmission X-ray microscopy is a particularly well-suited technique for the nanoscale imaging of magnetic materials, but its compatibility with in-situ mechanical straining of samples is limited. In this work, we present a setup for applying tailored in-situ mechanical strains to freestanding thin films by means of a micro electromechanical system (MEMS) actuator. We then present a proof-of-concept experiment where a freestanding 80 nm thick (001) BiFeO$_3$ multiferroic thin film is strained with the MEMS device, allowing us to control the coupled ferroelectric/spin cycloidal configuration.

Figures (7)

• Figure 1: (a) Sketch of the gas cell-based setup used to generate strain through the bending of a Si$_3$N$_4$ membrane. Image adapted from Finizio2016. (b) XMCD-STXM images of a Co$_{40}$Fe$_{40}$B$_{20}$ microstructured square fabricated on a rectangular Si$_3$N$_4$ membrane under different pressure differences between the two sides of the membrane (measured by gauges G1 and G2 in (a)), generating a uniaxial tensile strain of different magnitudes. The red arrows mark the direction of the in-plane magnetization in the microstructured squares. Image adapted from Finizio2017.
• Figure 2: Schematic representation of the MEMS strainer (a) before and (b) after applying a voltage; (c-d) Cross-sectional view of the same device highlighting the different materials. The positions of the contacts used for the voltage poling are sketched by the red lines in (c).
• Figure 3: SEM image of a 80 nm thick (001) BFO lamella bridging the gap between the two cantilevers on the MEMS device. The BFO lamella was attached by ion beam assisted carbon deposition in the two points marked by the orange arrows.
• Figure 4: (a) Amplitude contrast XLD-ptychography image of a 80 nm (001) BFO freestanding film. Both the ferroelectric mosaic domains (larger domains) and the spin cycloid (smaller "waves”) can be observed. The grayscale bar depicts the direction of the ferroelectric polarization. The ROI marked in yellow corresponds to the region shown in Fig. \ref{['fig:cycloid']}. (b) Linescan across the region marked by the red dotted line in (a), showing the change in XLD contrast caused by the spin cycloid (oscillating part) and by the coupled ferroelectric domain (constant offset from 0).
• Figure 5: (a) Variation of the distance between the two MEMS cantilevers as a function of the applied voltage difference between the cantilevers' top and bottom surfaces. The recorded displacement is independent of the size of the gap between the two cantilevers; (b) Maximum tensile strain generated by the displacement of the cantilevers.