Opportunities and challenges in freestanding multiferroic membranes

Multiferroic materials have two or more ferroelectric orders, such as ferroelectricity and ferromagnetism, at the same time. The coupling and interaction between these ferroelectric orders can give rise to intriguing magnetoelectric effects, which can be used in information storage, energy conversion, sensing, etc. At present, there are still many important scientific questions in basic science and electronic applications, such as realizing ferroelectric/ferromagnetic multiferroic materials with strong magnetoelectric coupling above room temperature, device integration with semiconductors, etc. The rapid development of freestanding oxide membranes in recent years provides new opportunities for studies on multiferroics. Compared with bulk materials and epitaxial thin films clamped on rigid substrates, freestanding membranes have an excellent degree of freedom in lattice tuning. They can experience unprecedented extreme one-dimensional and two-dimensional strains and strain gradients, and can also be utilized to construct artificial heterostructures and devices, providing new material systems and opportunities to advance the development of multiferroic materials. In this article, we focus on the recent progress of research on multiferroic materials based on freestanding membranes, and attempt to forecast the opportunities and challenges that may be faced in this direction in the future.