AVS 72 Session TF-WeA: Far from Equilibrium Thin Films and Processes

Magnetoelectric multiferroics - materials in which electric polarization and magnetic order are intrinsically coupled - offer a platform for ultralow-power switching, nonvolatile memory, and energy-efficient transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a metastable multiferroic phase of BiFeO₃ (BFO) that not only sustains both order parameters at room temperature with no dead layer in the four unit cell, ultrathin limit, but also exhibits signatures of emergent altermagnetism. This second-order, thickness-driven phase transition gives rise to new multiferroic topological textures, where electric and magnetic symmetries become intertwined in previously unobserved ways. Additionally, we show pathways to design altermagnetism and topology at thicker length scales, where at domain boundaries, we uncover a rich hierarchy of noncollinear topological magnetoelectric structures, including polar bi-merons, polar vertices coupled to magnetic cycloid disclinations, and topological cycloidal knots, whose stability and dynamics are intimately linked to the underlying crystal symmetry. These findings establish a pathway to stabilize multi-order topology at device-relevant thicknesses and reveal emergent magnetic symmetry. Collectively, they reframe scaling limits in oxide multiferroics and open the door to emergent topological phenomena and ultralow-energy functionalities in correlated quantum materials.

Among rare-earth oxide derivatives, hexagonal manganites h-RMnO₃ (where R = Sc, Y, Dy-Lu) stand out as particularly interesting for exploring their geometrically driven improper ferroelectricity by exhibiting a polar phase with a space group of P6₃cm below the Curie temperatures of approximately 1250 K. These materials further demonstrate multiferroic behavior below 100 K, with antiferromagnetic order occurring in the Mn sublattice. In this study, we report the synthesis of a high-entropy rare-earth manganite (Y_0.167Gd_0.167Dy_0.167Ho_0.167Er_0.167Yb_0.167MnO₃) in both bulk and thin-film forms, with X-ray diffraction (XRD) confirming that both forms stabilized as a hexagonal single-phase structure. Thin films with a targeted thickness of 80-100 nm were deposited by pulsed laser deposition (PLD) onto YSZ substrates and conductive bottom electrodes of Pt and ITO films for ferroelectric testing. Polarization-electric field measurements confirmed robust ferroelectric hysteresis loops at room temperature with a remanent polarization of ~2 µC/cm². Structural characterization via scanning transmission electron microscopy (STEM) reveals a polar structure accompanied by the rare-earth displacements and captures the domain structures. Preliminary magnetic measurements indicate non-negligible saturation magnetization values both in-plane and out-of-plane below 50 K. Furthermore, we investigate the influence of the growth atmosphere and discuss the associated defect chemistry in relation to the improper ferroelectricity. Ultimately, this work provides a detailed discussion of potential A-sites (e.g., Gd and Eu, which are traditionally orthorhombic-stabilizing species) and B-site disorder (e.g., Fe and Ga, for potential magnetic ordering tuning), and presents the first thorough investigation of improper ferroelectricity in a complex high-entropy system. These results demonstrate that the geometrically driven improper ferroelectricity remains remarkably robust against extreme local chemical disorder and defects, leading to a new design principle for novel multiferroic materials.

Since the discovery of high-temperature superconductivity in cuprates, there have been long-standing efforts to understand and engineer the underlying electronic and crystal structure in superconducting materials. A new family of high-temperature superconductors was recently discovered in the n=2,3 Ruddlesden-Popper (RP) nickelates [1,2]. When bulk crystals of these nickelates are placed under large hydrostatic pressures, superconductivity is stabilized concomitant with suppression of correlated density wave states, and also concomitant with crystallographic symmetry raising [1,2]. Thus far, compressive epitaxial strain has been demonstrated to mimic effects of pressure in n=2 RP nickelates, leading to ambient-pressure superconductivity and equivalent structural changes in compressively-strained thin films [3,4]. However, ambient-pressure superconductivity and other analogous effects of strain have not yet been demonstrated in the n=3 compound. Here, we use reactive oxide molecular beam epitaxy (MBE) to synthesize a series of n=3 RP nickelate (La₄Ni₃O₁₀) thin films across a wide range of epitaxial strains [5]. Combining electronic transport, picoscale electron microscopy, and synchrotron X-ray diffraction techniques, we develop a strain-dependent electronic and structural phase diagram for n=3. Critically, we identify a structural distortion and corresponding crystal symmetry not present in n=3 (or n=2) bulk crystals, highlighting key distinctions within the RP nickelate family towards engineering more robust superconducting materials, and demonstrating the unique capability of epitaxial synthesis to stabilize new metastable states in complex oxides.

[1] Sun, H. et al. Nature 621, 493-498 (2023)

[2] Zhu, Y. et al. Nature 631, 531-536 (2024)

[3] Ko, E.K. et al. Nature 638, 935-940 (2025)

[4] Bhatt, L., Abarca Morales, E., Jiang, A.Y. et al. Nature 653, 76-82 (2026)