R. RAMESH

Domain wall propagation in conventional ferroelectric materials under an electric field is often hindered by disorder and hence its velocity is orders-of-magnitude lower than the speed of sound. The recent discovery of quasi-long-ranged topological polar textures in oxide superlattices—such as polar vortices and skyrmions—offers an alternative platform to investigate domain wall motion in ferroelectrics, where atomic-scale disorder plays a minor role in nanotextures with characteristic size on the order of 10 nm. Here, we study the domain dynamics of polar textures in a PbTiO 3/SrTiO 3 superlattice using time-resolved electron diffraction and microscopy. Following photoexcitation, we observed a rapid suppression of the polar texture within 1 ps, where textures within one domain are preferentially melted. The selective melting leads to a fast expansion of the other domain, whose boundary propagates near the sound …

2D layered materials with broken inversion symmetry are being extensively pursued as  spin source layers to realize high‐efficiency magnetic switching. Such low‐symmetry layered systems are, however, scarce. In addition, most layered magnets with perpendicular magnetic anisotropy show a low Curie temperature. Here, the experimental observation of spin–orbit torque magnetization self‐switching at room temperature in a layered polar ferromagnetic metal, Fe2.5Co2.5GeTe2 is reported. The spin–orbit torque is generated from the broken inversion symmetry along the c‐axis of the crystal. These results provide a direct pathway toward applicable 2D spintronic devices.

Ultrafast stimuli can stabilize metastable states of matter inaccessible by equilibrium means. Establishing the spatiotemporal link between ultrafast excitation and metastability is crucial to understanding these phenomena. Here, we use single-shot optical-pump, X-ray-probe measurements to provide snapshots of the emergence of a persistent polar vortex supercrystal in a heterostructure that hosts a fine balance between built-in electrostatic and elastic frustrations by design. By perturbing this balance with photoinduced charges, a starting heterogenous mixture of polar phases disorders within a few picoseconds, resulting in a soup state composed of disordered ferroelectric and suppressed vortex orders. On the pico-to-nanosecond timescales, transient labyrinthine fluctuations form in this soup along with a recovering vortex order. On longer timescales, these fluctuations are progressively quenched by dynamical strain modulations, which drive the collective emergence of a single supercrystal phase. Our results, corroborated by dynamical phase-field modeling, reveal how ultrafast excitation of designer systems generates pathways for persistent metastability.

Superlattice films of layers of PbTiO 3 and SrTiO3 form polar vortices under the right lattice periodicity conditions. Low frequency and terahertz experiments observed both negative permittivity and interesting collective dynamics that could lead to other emergent behavior. Gigahertz complex permittivity data could provide a new insight into polar vortices dynamics and how emergent behavior develops with periodicity, but the measurements present challenges given the anisotropic nature of these complex thin films. Here, we tested a new metrology that uses coplanar waveguides designed to be sensitive to different components of the complex permittivity tensor. We tested 12 different 100 nm thick films with periodicities with and without a SrRuO 3 bottom electrode to understand how dispersion and tunability emerge with the formation of polar vortices. Taken together, these results provide new insights into the high …

Vector magnetometry is an essential tool in characterizing the distribution of currents and magnetization in a broad range of systems. Point defect sensors, like the nitrogen vacancy (NV) center in diamond, have demonstrated impressive sensitivity and spatial resolution for detecting these fields. Measuring the vector field at a single point in space using single defects, however, remains an outstanding challenge. We demonstrate that careful optimization of the static bias field can enable simultaneous measurement of multiple magnetic field components with enhanced sensitivity by leveraging the nonlinear Zeeman shift from transverse magnetic fields. This work quantifies the trade-off between the increased frequency shift from second-order Zeeman effects with decreasing contrast as off-axis field components increase, demonstrating the measurement of multiple components of the magnetic field from an exemplar antiferromagnet with a complex magnetic texture.

Antiferromagnets are promising platforms for transduction and transmission of quantum information via magnons—the quanta of spin waves—and they offer advantages over ferromagnets in regard to dissipation, speed of response and robustness to external fields. Recently, transduction was shown in a van der Waals antiferromagnet, where strong spin-exciton coupling enables readout of the amplitude and phase of coherent magnons by photons of visible light. This discovery shifts the focus of research to transmission, specifically to exploring the non-local interactions that enable magnon wave packets to propagate. Here we demonstrate that magnon propagation is mediated by long-range dipole–dipole interaction. This coupling is an inevitable consequence of fundamental electrodynamics and, as such, will likely mediate the propagation of spin at long wavelengths in the entire class of van der Waals magnets …

As CMOS technologies face challenges in dimensional and voltage scaling, the demand for novel logic devices has never been greater, with spin-based devices offering scaling potential, at the cost of significantly high switching energies. Alternatively, magnetoelectric materials are predicted to enable low-power magnetization control, a solution with limited device-level results. Here, we demonstrate voltage-based magnetization switching and reading in nanodevices at room temperature, enabled by exchange coupling between multiferroic BiFeO3 and ferromagnetic CoFe, for writing, and spin-to-charge current conversion between CoFe and Pt, for reading. We show that, upon the electrical switching of the BiFeO3, the magnetization of the CoFe can be reversed, giving rise to different voltage outputs. Through additional microscopy techniques, magnetization reversal is linked with the polarization state and …

A magnon is a collective excitation of the spin structure in a magnetic insulator and can transmit spin angular momentum with negligible dissipation. This quantum of a spin wave has always been manipulated through magnetic dipoles (that is, by breaking time-reversal symmetry). Here we report the experimental observation of chiral spin transport in multiferroic BiFeO3 and its control by reversing the ferroelectric polarization (that is, by breaking spatial inversion symmetry). The ferroelectrically controlled magnons show up to 18% modulation at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adjacent magnets, with a spin–torque efficiency comparable to the spin Hall effect in heavy metals. Utilizing such controllable magnon generation and transmission in BiFeO3, an all-oxide, energy-scalable logic is demonstrated composed of spin–orbit …