Magnetically frustrated materials offer a playground for realizing exotic magnetic ground states such as quantum spin ices and spin liquids that have been proposed as building blocks in quantum computing and as potential hosts for unconventional superconductivity. The ability to synthesize such materials in thin-film form is necessary for their integration into the proposed device architectures and also allows further tuning of the magnetic properties with dimensionality and epitaxial strain. However, thin-film realizations of quantum spin liquid candidate materials remain scarce. Here, we use reactive oxide molecular beam epitaxy to synthesize the first thin films of hexagonal TbInO3, a magnetically frustrated rare-earth system that was recently proposed as a spin liquid candidate. In bulk, TbInO3 exhibits geometrically driven improper ferroelectricity similar to the multiferroic hexagonal manganites. The underlying lattice distortion causing this domain pattern imposes a stuffed honeycomb geometry on the quasi-two-dimensional Tb sublattice which harbors the magnetic frustration in TbInO3. Here, we investigate the ferroelectric distortion in our epitaxial TbInO3 thin films using in-situ RHEED and post-deposition HAADF-STEM. We furthermore use SQUID magnetometry and X-ray spectroscopy to investigate the magnetic behavior. Our work constitutes one of the few thin-film realizations of a quantum spin liquid candidate, and opens up for the use of epitaxy to further explore and manipulate the spin liquid physics in this material.

Nano ferroelectricity presents promising technological applications, particularly in energy-saving and miniaturized electronic devices. Using tunnel junctions and transistors with ferroelectric gates exemplifies its potential significance in advancing memory devices research. These applications offer enhanced efficiency and reduced footprint, driving innovation in memory storage technologies.

BaTiO₃ (BTO), a lead-free ferroelectric, shows fascinating properties when interfaced with electrodes, including a “positive dead-layer” or interface ionic relaxation. This makes BTO a compelling candidate for integration into multifunctional structures with nano-scale dimensions. Understanding the physical and chemical properties of BTO-based interfaces and ultrathin layers is thus essential for controlling their properties effectively.

In this study, we investigate ultrathin BTO films grown by industrially scalable magnetron sputtering on conductive Nb-doped SrTiO₃ substrates and on structurally compatible conductive layers of SrRuO₃ (SRO) and La₀.₆₇Sr₀.₃₃MnO₃ (LSMO), to explore the electronic structure and chemical reconstruction within the films and at the different interfaces.
The average structure and strain of ultrathin BTO films ranging from 1 to 16 nm were investigated using X-ray diffraction. A comprehensive spectroscopic analysis was conducted to investigate the relationship between strain and materials, as well as processing parameters.

Through synchrotron X-ray absorption spectroscopy and laboratory X-ray photoelectron spectroscopy, we probed the electronic structure under different ultra-high vacuum (UHV) annealing conditions, examining the generation of electronic and ionic defects such as oxygen vacancies.
Structural and polarisation states of strained BTO films were analysed through X-ray natural linear dichroism at the Ti-L₂,₃ edges, supporting the experimental evidence of the contribution of the strain to the polarisation of the films as a function of thickness. Finally, scanning transmission electron microscopy and electron energy-loss spectroscopy offer atomic-scale insights into the electronic structure across the films and at the interfaces.

When combined with phase field modelling, these results are essential for understanding and controlling the fundamental physical mechanisms underlying the ferroelectric properties of BTO-based devices.

The reversable polarization and magnetization of a ferroelectric-ferromagnetic multiferroic imbues materials with two exciting new degrees of freedom, stimulating the exploration of new fundamental physics and opening new pathways in the design of novel devices. Thin-film, room temperature multiferroics such as the perovskite R3c BiFeO3 (BFO) have created great excitement for these reasons. Furthermore, the non-centrosymmetric R3c symmetry has been successfully engineered in a multitude of low tolerance perovskites without lone pairs, although the switching dynamics of these materials generally results in a competition between large polarizations and low energy barriers. Using first-principles simulations, we have identified an alternative orthorhombic (Pna21) polar phase that can be stabilised in a range of magnetic ABO3 perovskites under the application of epitaxial strain. Such a phase would exhibit low switching barriers and, owing to its out-of-plane polarization direction, would be amenable to ferroelectric switching in the thin-film geometry. A prediction of a strain stabilized Pna21 phases has previously been made for the fluorites and some non-magnetic oxides. Here we extend the analysis to magnetic oxides, unravel and generalise the origin of the Pna21 phase and demonstrate the stability of novel multiferroic phases displaying strong spin-phonon coupling and intriguing magnetoelectric properties.