Sujoy Roy

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.

We demonstrate an experimental approach to determine magneto-optical effects that combines x-ray magnetic circular dichroism (XMCD) with x-ray interferometry, based on the concepts of Young's canonical double slit. By covering one of two slits with a magnetic thin film and employing XMCD, we show that it is possible to determine both the real and the imaginary parts of the complex refractive index by measuring the fringe shifts that occur due to a change in the sample magnetization. Our hybrid spectroscopic-interferometric methodology provides a means to probe changes in the magnetic refractive index in terms of the electron spin moment.

Ferroelectric materials exhibit a spontaneous electric polarization that can be reversed byan electric field ¹ , a property central to non-volatile memories ^2,3 , sensors ⁴ and actuators ^5,6 . In most conventional ferroelectric oxides, the polarization originates from a softening of a polar mode ⁷ . The amplitude of the mode along a given direction couples with the applied field along the same direction ⁸ . A long-standing challenge has been to achieve controlled switching of the in-plane polarization component using an out-of-plane electric field. We have discovered that a trilinear coupling between the in-plane and out-of-plane polarization, mediated via the octahedral tilts and rotations in the layered ferroelectric Bi ₄ Ti ₃ O ₁₂ naturally fulfills these requirements and enables trans-switching of the polarization state. In its monoclinic phase, this material hosts a large in-plane polarization (∼ 50 μC cm ^-2 ) driven by a proper ferroelectric instability, together with smaller out-of-plane polarization (∼ 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, analogous to the transconductance of modern CMOS electronics.

A nematic phase lacks translation order but has orientational order. Nematic phases have been discovered in a variety of systems, including liquid crystals, correlated materials, and superconductors. Here, we report on a magnetic nematic phase, where the basis components are composed of magnetic helices. We directly probed the order parameters associated with the magnetic helices using resonant soft x-ray scattering and find two distinct nematic phases with complex spatiotemporal signatures. Using x-ray correlation spectroscopy, we find that near the phase boundary between the two nematic phases, fluctuations coexist on multiple disparate timescales. Our micromagnetic simulations and density functional theory calculations show that the fluctuations occur concomitantly with a reorientation of the magnetic helices, indicating spontaneous symmetry breaking and the emergence of additional degrees of freedom. Our results provide a framework for characterizing exotic phases that can be extended to a broad class of physical systems.

Fluctuations of the order parameter near the phase transition boundary in complex oxides could be key to designing probabilistic bits and stochastic synapses. Direct probes of fluctuations how- ever are non-trivial due to the requirement of coherent spatio-temporal probes. Here, we report magnetic and lattice fluctuation dynamics in prototypical perovskite nickelate systems of NdNiO3 and SmNiO3 using X-ray photon correlation spectroscopy. For NdNiO3, our studies show strong coupling of structural and magnetic order parameters with slowdown of fluctuation timescales by one order of magnitude near Néel temperature irrespective of the epitaxial strain of the film. For SmNiO3, no such slowdown is observed emphasizing the distinct nature of structural fluctuations. Our measurements of diffraction wavevector dependence, indicate that short range structural fluctuations are surprisingly slow (3-5x slower) compared to long range fluctuations. Our results reveal novel insights into fluctuations enabled by coherent x-rays, expanding the unique capability of spatio-temporal probes to investigate quantum materials.

We demonstrate an experimental approach to determine magneto-optical effects which combines x-ray magnetic circular dichroism (XMCD) with x-ray interferometry, based on the concepts of Young's canonical double slit. By covering one of two slits with a magnetic thin film and employing XMCD, we show that it is possible to determine both the real and the imaginary parts of the complex refractive index by measuring the fringe shifts that occur due to a change in the sample magnetization. Our hybrid spectroscopic-interferometric methodology provides a means to probe changes in the magnetic refractive index in terms of the electron spin moment.

The Fe/Gd multilayer system hosts a number of magnetic phases, such as stripe, mixed stripe and skyrmion, skyrmion lattice, and isolated skyrmions for a wide range of temperature and magnetic field. We report different Hall transport signals in a Fe/Gd system through multimodal correlative resonant soft x-ray scattering (RSXS), Hall effect, magneto-optic Kerr effect, and transmission x-ray microscopy measurements. The simultaneous nature of the RSXS and Hall transport measurements allowed us to accurately connect various features in the transport data with the specific magnetic phases. We found that the topological Hall effect (THE) shows peaks with opposite signs, which we attribute to two different mechanisms. Our multimodal correlative study indicates that the sign reversal in THE occurs when the system transforms to and from a skyrmion lattice and low density isolated skyrmion phases. We propose that the skyrmion lattice contributes to the THE through a Berry phase induced emergent magnetic field mechanism in one case, and a skew scattering mechanism corresponding to the isolated low density skyrmion state.

Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We confirm the magnetic order within a single crystal of NiPS₃ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of β ~ 0.36, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principles density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations.

In addition to spin angular momentum, light can carry orbital angular momentum. The orbital angular momentum degree of freedom in the extreme ultraviolet and x-ray regimes enables fundamental studies of light–matter interactions and new methods to study materials. Advances in x-ray optics, as well as undulator radiation and high harmonic generation techniques, lead to the creation of beams with non-trivial phase structure, such as a helical phase structure, creating new possibilities for the use of extreme ultraviolet and x-ray photons with orbital angular momentum in probing complex electronic structures in matter. In this article, we review the generation and applications of orbital angular momentum beams in the x-ray and extreme ultraviolet regime. We discuss several recent works that exploit the orbital angular momentum degree of freedom and showcase the potential advantages of using these beams.

Presence of multiple competing periodicities may result in a system to go through states with modulated periodicities, an example of which is the self-similar staircase-like structure called the Devil's staircase. Herein we report on a novel staircase structure of domain periodicity in an amorphous and achiral Fe/Gd magnetic thin film wherein the reciprocal space wavevector \textbf{Q} due to the ordered stripe domains does not evolve continuously, rather exhibits a staircase structure. Resonant X-ray scattering experiments show jumps in the periodicity of the stripe domains as a function of an external magnetic field. When resolved in components, the step change along Q$_x$ was found to be an integral multiple of a minimum step height of 7 nm, which resembles closely to the exchange length of the system. Modeling the magnetic texture in the Fe/Gd thin film as an achiral spin arrangement, we have been able to reproduce the steps in the magnetization using a Landau-Lifshitz spin dynamics calculation. Our results indicate that anisotropy and not the dipolar interaction is the dominant cause for the staircase pattern, thereby revealing the effect of achiral magnetism.