Sajid Husain

Topological insulators have been explored extensively for spin-charge interconversion via magnetic interfaces, yet the true response of their spin-charge conversion, particularly in the absence of an external magnetic field, remains to be studied. Here, we report electric field control of spin-charge conversion in the topological insulator Bi ₂ Te ₃ with the antiferromagnetic multiferroic BiFeO ₃ , using a nonlocal spin transport device. A systematic thickness dependence of the spin transport across the interface between Bi ₂ Te ₃ and BiFeO ₃ reveals a signature of topological surface-state–dominated spin transport in the bilayer system. The spin-charge conversion remains robust for thicknesses above 10 nanometers but falls rapidly with reducing thickness and vanishes at 5 nanometers. This is consistent with hybridization-induced emergence of a trivial insulating phase, which is supported by the coherency factor estimated from the magnetoconductance of Bi ₂ Te ₃ . These results establish that spin-momentum–locked surface states dominate interfacial spin transport in the decoupled regime. Beyond presenting efficient spin-charge interconversion at an entirely insulating magnetic interface, this work also highlights sputter-deposited Bi ₂ Te ₃ as a high-quality and scalable platform for integrating quantum materials onto the devices. The nonlocal spin transport approach presented here provides a simple and unprecedented direct evidence of spin-charge conversion and opens an efficient and practical pathway toward designing energy-efficient spin-based devices.

Ferroelectric materials exhibit a spontaneous electric polarization that can be reversed by an electric field¹, a property central to non-volatile memories^2,3, sensors⁴ and actuators^5,6. In most conventional ferroelectrics, the polarization originates from a softening of a polar mode⁷. The amplitude of the mode along a given direction couples with the applied field along the same direction⁸. Ferroelectrics with a predominant in-plane polarization are harder to use in standard device geometries and therefore the field has mostly focused on out-of-plane ferroelectrics. Developing approaches that enable manipulation of the in-plane polarization component with an out-of-plane field would therefore provide new opportunities for device design and functionality. Here we have discovered that a trilinear coupling between the in-plane and out-of-plane polarization, mediated by means of the octahedral tilts and rotations in the layered ferroelectric Bi₄Ti₃O₁₂, naturally fulfils this challenge and enables perpendicular switching of the polarization state. In its bulk monoclinic phase, this material hosts a large in-plane polarization (about 50 μC cm^-2) driven by a proper ferroelectric instability, together with smaller out-of-plane polarization (about 5 μC cm^-2) of improper origin, induced by oxygen octahedral distortions. We demonstrate that, in c-axis-oriented epitaxial films, the in-plane polarization switches deterministically under an out-of-plane electric field. This cross-coupling between orthogonal polarization components provides a route to transverse manipulation of ferroic order parameters and establishes layered ferroelectrics as a platform for capacitive computing concepts.

Liquid crystals (LCs) provide a canonical framework to understand intermediate phases between liquids and crystals, such as nematic and smectic order, and the rich phenomenology that emerges when such orders are subjected to boundaries, confinement, and elastic frustration. Here, we reveal a hidden mesoscale liquid-crystal-like organization of spin cycloids in a room-temperature magnetoelectric antiferromagnet BiFeO3. It is observed that ferroelectric domain walls act as anchoring surfaces, locally selecting cycloid propagation vectors analogous to surface induced alignment in LCs. Interestingly, confinement imposed by multiple domain walls stabilizes coherent smectic-like cycloidal order in between domain walls through “order by confinement”, while geometrical frustration at domain boundaries induces non-local Helfrich-Hurault–type elastic instabilities in cycloid smectics. Building on these insights, we show that surface-energy engineering dramatically enhances the long range orientational coherence of spin cycloids, enabling robust non-local magnon transport in the ultra-thin limit. These findings establish self-organization as a guiding principle for engineering novel magnetic textures and emergent spin functionalities, opening new opportunities for spintronic and magnonic technologies.

The stabilization of ferroelectricity in non‐polar oxides remain a key challenge in the design of next‐generation functional materials, as existing approaches typically rely on chemical substitution, or metastable phases. Here, we report a universal and experimentally realizable route to emergent ferroelectricity based on interfacial symmetry frustration in oxide superlattices composed of dissimilar crystal symmetries. We design superlattice thin films heterostructures integrating cubic (CeO ₂ , YSZ) and monoclinic (HfO ₂ , ZrO ₂ ) oxide layers showing spontaneous polarization that are non‐polar in their bulk phases. High‐resolution structural characterization confirms coherent heterointerfaces, while piezoresponse force microscopy and macroscopic electrical measurements establish switchable polarization. First‐principles calculations reveal that the crystalline symmetry mismatch across the interfaces breaks inversion symmetry and induces polar distortions through interface‐driven structural frustration, giving rise to emergent ferroelectricity without the need for chemical modification or metastable phase control. Our findings identify interfacial crystalline symmetry incompatibility as a general design principle for engineering ferroelectricity in superlattice heterostructures, opening new pathways for functional oxide electronics beyond conventional ferroelectric material systems.

BiFeO ₃ is a model multiferroic in which the ferroelectric polarization is coupled to ferroelastic lattice distortions, yet deterministic control of its domain structure remains limited by high switching fields and competing polarization variants. Here, we identify a mechanically assisted polarization switching pathway in epitaxial BiFeO ₃ thin films that fundamentally alters the switching energetics. Using just out-of-plane electric fields, polarization reversal requires voltages of approximately 4 volts and stabilizes coexisting polarization states. In contrast, when mechanical pressure is applied concurrently, the positive coercive voltage can be substantially reduced, even to 0 volts, resulting in spontaneous switching. Piezoresponse force microscopy measurements reveal that applied mechanical pressure suppresses ferroelastic domain competition, indicating a decrease in the required electrical energy barrier associated with polarization rotation and domain wall motion. This frames the strain field from the AFM tip directly as an effective voltage that assists in ferroelectric switching. By directly coupling lattice distortions to polarization reversal, mechanically assisted switching provides a general framework for controlling coupled order parameters in multiferroic oxides, which can be directly applied in the device-level architecture, where a small mechanical pressure can help in achieving a lower switching energy of ferroelectric polarization. This work advances the fundamental understanding of electromechanical coupling in complex ferroics and establishes mechanical energy as a powerful tool for probing and manipulating ferroelastic-ferroelectric interactions.

Nanoscale topological polar textures promise new functionalities for ferroelectric memories and logic, yet their three-dimensional structure and mesoscale organization remain experimentally inaccessible. Here we introduce depth-resolved electron diffraction imaging (DREDI), a fast, non-destructive, method that maps polarization with 3 films, DREDI reveals a hidden depth evolution of polar textures: surface 71˚ stripes evolve into subsurface flux-closure vortices that bifurcate into three-fold vertices near the bottom interface. Cross-sectional multi-slice electron ptychography and phase-field modeling confirm these buried configurations and attribute them to strain heterogeneity and ferroelastic twinning in the SrRuO₃ electrode. Large-area analysis further shows that vertex-like frustration forms a mesoscale percolating network above a critical length scale of 4 µm. DREDI enables real-time, volumetric studies of buried topological textures in ferroic nanomaterials.

The coexistence of ferroelectric and antiferromagnetic order in BiFeO ₃ makes it promising for next-generation magnetoelectric devices. But, single-phase multiferroics with robust room-temperature polarization and magnetization are rare. Here, enhanced, room-temperature ferroelectric polarization (≈ 120 µC cm ^–2 ), saturation magnetization (≈ 40 emu cm ^–3 ), and strong magnetoelectric coupling (≈ 400 mV cm ^–1 Oe ^–1 ) are observed in epitaxial (1– x )BiFeO ₃ –( x )BaTiO ₃ thin films. These values of magnetization and magnetoelectric coupling are, respectively, one- and two-orders of magnitude larger than those same properties in the widely studied parent material BiFeO ₃ . This sought after combination of properties is found in a distinct tetragonal phase, which is different from rhombohedral and super-tetragonal variants of BiFeO ₃ , that emerges at x = 0.2 to 0.3 via combined chemical substitution and epitaxial strain. Structural and physical-property characterization, along with first-principles calculations, reveal a transition from monoclinic to tetragonal symmetry and suggest that short-range ordering of the titanium in the tetragonal phase results in ferrimagnetic spin ordering. This work demonstrates a unique single-phase multiferroic combining strong polarization, magnetization, and magnetoelectric coupling achieved through manipulation of the coupled chemical order and spin order; thereby addressing a major challenge in multiferroics research and providing a path toward practical room-temperature, efficient charge-to-spin and spin-to-charge conversion technologies.

Ferroelectricity at atomic-scale thickness would have important applications in next-generation electronics. Here, we report that a ferroelectric phase can be stabilized in titanium dioxide (TiO ₂ ) films, a commonly known dielectric that is widely used in semiconductor technologies, by reducing thickness to 2 ) and amorphous carbon films] demonstrate the feasibility of integration with a large variety of materials.

Magnons, the quanta of spin waves, have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. Here we demonstrate highly efficient magnon transport in a LaFeO₃/BiFeO₃/LaFeO₃ all-antiferromagnetic system, which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin–orbit-driven spin–charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, the non-volatility of the output voltage enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.