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

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.

💡 Research Summary

The paper introduces a novel in‑situ mechanical straining platform for X‑ray transparent freestanding ferroic thin films, based on a micro‑electromechanical system (MEMS) actuator. Conventional approaches—piezoelectric crystal stages or Si₃N₄ membrane bending—are limited by low strain amplitudes (≤0.5 %), incompatibility with epitaxial growth, and restricted geometry. To overcome these constraints, the authors employ a commercially available PεTRA thin‑film piezo technology (STMicroelectronics) to fabricate a MEMS device consisting of two 10 µm‑thick Si cantilevers linked by a central bridge. Each cantilever carries four 2 µm‑thick Pb(Zr₀.₅₂Ti₀.₄₈)O₃ (PZT) patches in parallel. Applying a unipolar voltage (0–40 V) across the PZT layers induces contraction, causing the cantilevers to deflect upward and generate a uniaxial tensile strain in any material bridging the gap.

To make the device compatible with transmission X‑ray microscopy, the central bridge is removed by plasma focused ion beam (PFIB) cutting, creating a 15–25 µm gap while preserving electrical contacts. A freestanding 80 nm‑thick (001) BiFeO₃ (BFO) film, previously released from a substrate, is extracted as a lamella using a Ga‑FIB/micromanipulator. The lamella orientation is pre‑determined by X‑ray linear dichroism (XLD) ptychography, ensuring cuts along the