arXiv subjects

Ramamoorthy Ramesh

Publications and source records attributed to Ramamoorthy Ramesh.

At least 73 records · Page 4Linked to original sources

arXiv2021

Local Manipulation of Polar Skyrmions and Topological Phase Transitions

Topological phases such as polar skyrmions have been a fertile playground for ferroelectric oxide superlattices, with exotic physical phenomena such as negative capacitance. Herein, using phase-field simulations, we demonstrate the local control of the skyrmion phase with electric potential applied through a top electrode. Under a relatively small electric potential, the skyrmions underneath the electrode can be erased and recovered reversibly. A topologically protected transition from the symmetric to asymmetric skyrmion bubbles is observed at the edge of the electrode. While a topological transition to a labyrinthine domain requires a high applied potential, it can switch back to the skyrmion state with a relatively small electric potential. The topological transition from +1 to 0 occurs before the full destruction of the bubble state. It is shown that the shrinking and bursting of the skyrmions leads to a large reduction in the dielectric permittivity, the magnitude of which depends on the size of the electrode.

cond-mat.mtrl-sci↗

arXiv2021

A room temperature polar ferromagnetic metal

The advent of long-range magnetic order in non-centrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. This work reports a novel wurtzite-structure polar magnetic metal, identified as AA'-stacked (Fe0.5Co0.5)5-xGeTe2, which exhibits a Neel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies and ushers in a promising new framework for spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Electric Field Control of Chirality

Polar textures have attracted significant attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second harmonic generation based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric-field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of non-chiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field controllable order parameter.

cond-mat.mtrl-sci↗

arXiv2021

Unified Framework for Charge-Spin Interconversion in Spin-Orbit Materials

Materials with spin-orbit coupling are of great interest for various spintronics applications due to the efficient electrical generation and detection of spin-polarized electrons. Over the past decade, many materials have been studied, including topological insulators, transition metals, Kondo insulators, semimetals, semiconductors, and oxides; however, there is no unifying physical framework for understanding the physics and therefore designing a material system and devices with the desired properties. We present a model that binds together the experimental data observed on the wide variety of materials in a unified manner. We show that in a material with a given spin-momentum locking, the density of states plays a crucial role in determining the charge-spin interconversion efficiency, and a simple inverse relationship can be obtained. Remarkably, experimental data obtained over the last decade on many different materials closely follow such an inverse relationship. We further deduce two figure-of-merits of great current interest: the spin-orbit torque (SOT) efficiency (for the direct effect) and the inverse Rashba-Edelstein effect length (for the inverse effect), which statistically show good agreement with the existing experimental data on wide varieties of materials. Especially, we identify a scaling law for the SOT efficiency with respect to the carrier concentration in the sample, which agrees with existing data. Such an agreement is intriguing since our transport model includes only Fermi surface contributions and fundamentally different from the conventional views of the SOT efficiency that includes contributions from all the occupied states.

cond-mat.mes-hall↗

arXiv2021

Subterahertz collective dynamics of polar vortices

The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarization rather than spin have recently been realized in ferroelectric superlattices, promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics of such complex extended nanostructures which in turn underlies their functionalities. Using terahertz-field excitation and femtosecond x-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders of magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices. A previously unseen soft mode, hereafter referred to as a vortexon, emerges as transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond time scales. Its frequency is significantly reduced at a critical strain, indicating a condensation of structural dynamics. First-principles-based atomistic calculations and phase-field modeling reveal the microscopic atomic arrangements and frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens up opportunities for applications of electric-field driven data processing in topological structures with ultrahigh speed and density.

cond-mat.mes-hall↗

arXiv2020

Transferring Orbital Angular Momentum to an Electron Beam Reveals Toroidal and Chiral Order

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics(1-4). Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices(5). Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. To develop a mechanism that can control switching in polarization vortices, we utilize a high-energy electron beam and show that transverse currents are generated by polar order in the ballistic limit. We find that the presence of an electric toroidal moment in a ferro-rotational phase transfers a measurable torque and orbital angular momentum to the electron beam. Furthermore, we find that the complex polarization patterns, observed in these heterostructures, are microscopically chiral with a non-trivial axial component of the polarization. This chirality opens the door for the coupling of ferroelectric and optical properties.

cond-mat.mtrl-sci↗

arXiv2020

Antiferromagnetic Switching Driven by the Collective Dynamics of a Coexisting Spin Glass

The theory behind the electrical switching of antiferromagnets is premised on the existence of a well defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. In this study, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet Fe$_{1/3+δ}$NbS$_2$, which is rooted in the electrically-stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. The use of a spin glass' collective dynamics to electrically manipulate antiferromagnetic spin textures has never been applied before, opening the field of antiferromagnetic spintronics to many more material platforms with complex magnetic textures.

cond-mat.str-el↗

arXiv2020

Unexpected Giant Microwave Conductivity in a Nominally Silent BiFeO3 Domain Wall

Nanoelectronic devices based on ferroelectric domain walls (DWs), such as memories, transistors, and rectifiers, have been demonstrated in recent years. Practical high-speed electronics, on the other hand, usually demand operation frequencies in the giga-Hertz (GHz) regime, where the effect of dipolar oscillation is important. In this work, an unexpected giant GHz conductivity on the order of 103 S/m is observed in certain BiFeO3 DWs, which is about 100,000 times greater than the carrier-induced dc conductivity of the same walls. Surprisingly, the nominal configuration of the DWs precludes the ac conduction under an excitation electric field perpendicular to the surface. Theoretical analysis shows that the inclined DWs are stressed asymmetrically near the film surface, whereas the vertical walls in a control sample are not. The resultant imbalanced polarization profile can then couple to the out-of-plane microwave fields and induce power dissipation, which is confirmed by the phase-field modeling. Since the contributions from mobile-carrier conduction and bound-charge oscillation to the ac conductivity are equivalent in a microwave circuit, the research on local structural dynamics may open a new avenue to implement DW nano-devices for RF applications.

cond-mat.mtrl-sci↗

arXiv2019

Imaging uncompensated moments and exchange-biased emergent ferromagnetism in FeRh thin films

Uncompensated moments in antiferromagnets are responsible for exchange bias in antiferromagnet/ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magneto-thermal microscopy to image uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a 1st-order phase transition to a ferromagnetic state near 100~$^\circ$C. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (10-15~$^\circ$C) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk Néel order. This exchange coupling leads to the nucleation of unusual configurations in which different FM domains are pinned parallel, antiparallel, and perpendicular to the applied magnetic field before suddenly collapsing into a state uniformly parallel to the field.

cond-mat.mtrl-sci↗

arXiv2019

Tunable Giant Exchange Bias in an Intercalated Transition Metal Dichalcogenide

The interplay of symmetry and quenched disorder leads to some of the most fundamentally interesting and technologically important properties of correlated materials. It also poses the most vexing of theoretical challenges. Nowhere is this more apparent than in the study of spin glasses. A spin glass is characterized by an ergodic landscape of states - an innumerable number of possibilities that are only weakly distinguished energetically, if at all. We show in the material Fe$_x$NbS$_2$, this landscape of states can be biased by coexisitng antiferromagnetic order. This process leads to a phenomenon of broad technological importance: giant, tunable exchange bias. We observe exchange biases that exceed those of conventional materials by more than two orders of magnitude. This work illustrates a novel route to giant exchange bias by leveraging the interplay of frustration and disorder in exotic materials.

cond-mat.str-el↗

arXiv2018

A high-energy density antiferroelectric made by interfacial electrostatic engineering

Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.

cond-mat.mtrl-sci↗

arXiv2018

Discovery of Ordered Vortex Phase in Multiferroic Oxide Superlattices

Ferroics, characterized by a broken symmetry state with nonzero elastic, polar, or magnetic order parameters $\vec{u}$, are recognized platforms for staging and manipulating topologically-protected structures as well as for detecting unconventional topological phenomena. The unrealized possibility of producing ordered topological phases in magnetoelectric multiferroics, exhibiting coupled magnetic and polar order parameters, is anticipated to engender novel functionality and open avenues for manipulating topological features. Here, we report the discovery of an ordered $π_1$-$S_\infty$ vortex phase within single-phase magnetoelectric multiferroic BiFeO$_3$. The phase, characterized by positive topological charge and chiral staggering, is realized in coherent TbScO$_3$ and BiFeO$_3$ superlattices and established via the combination of direct- and Fourier-space analyses. Observed order-parameter morphologies are reproduced with a field model describing the local order-parameter stiffness and competing non-local dipole-dipole interactions. Anisotropies canting the order parameter towards $\left<100\right>$ suppress chiral staggering and produced a competing $π_1$-$C_{\infty v}$ vortex phase in which cores are centered.

cond-mat.mtrl-sci↗

arXiv2018

Voltage Control of Uni-directional Anisotropy in Ferromagnet-Multiferroic System

Demonstration of ultra-low energy switching mechanisms is an imperative for continued improvements in computing devices. Ferroelectric (FE) and multiferroic (MF) orders and their manipulation promises an ideal combination of state variables to reach atto-Joule range for logic and memory (i.e., ~ 30X lower switching energy than nanoelectronics). In BiFeO3 the coupling between the antiferromagnetic (AFM) and FE orders is robust at room temperature, scalable in voltage, stabilized by the FE order, and can be integrated into a fabrication process for a beyond-CMOS era. The presence of the AFM order and a canted magnetic moment in this system causes exchange interaction with a ferromagnet such as CoFe or LSMO. While previous work has shown that exchange coupling (uniaxial anisotropy) can be controlled with an electric field, several puzzling issues remain. Perhaps the most intriguing among them is that the BiFeO3-CoFe bilayer did not demonstrate any electrically controlled directional anisotropy, i.e., an exchange bias, which is a potential mechanism for 180o magnetic reversal and is independent of switching kinetics. However, what is needed/preferred for logic and memory is a magneto-electric mechanism that works analogous to an applied field, i.e a uni-directional anisotropy that is voltage modulated. Here, we present the evidence of electrical control of exchange bias of a laterally scaled spin valve that is exchange-coupled to BiFeO3 at room temperature. We show that the exchange bias in this bilayer is thermally robust, electrically controlled and reversible. We anticipate that magneto-electricity at such scaled dimensions provides a powerful pathway for computing beyond the modern nanoelectronics transistors by enabling a new class of non-volatile, ultra-low energy computing elements.

cond-mat.mes-hall↗

arXiv2018

Unexpected Termination Switching and Polarity Compensation in LaAlO3/SrTiO3 heterostructures

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically. An electronic asymetry has long been observed in the LaAlO3/SrTiO3 system. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first principles calcula- tions to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Differential voltage amplification from ferroelectric negative capacitance

It is well known that one needs an external source of energy to provide voltage amplification. Because of this, conventional circuit elements such as resistors, inductors or capacitors cannot provide amplification all by themselves. Here, we demonstrate that a ferroelectric can cause a differential amplification without needing such an external energy source. As the ferroelectric switches from one polarization state to the other, a transfer of energy takes place from the ferroelectric to the dielectric, determined by the ratio of their capacitances, which, in turn, leads to the differential amplification. {This amplification is very different in nature from conventional inductor-capacitor based circuits where an oscillatory amplification can be observed. The demonstration of differential voltage amplification from completely passive capacitor elements only, has fundamental ramifications for next generation electronics.

cond-mat.mes-hall↗

arXiv2017

Electrically Induced, Non-Volatile, Metal Insulator Transition in a Ferroelectric Gated MoS$_2$ Transistor

We demonstrate an electrically induced, non-volatile, metal-insulator phase transition in a MoS$_2$ transistor. A single crystalline, epitaxially grown, PbZr$_{0.2}$Ti$_{0.8}$O$_3$ (PZT) was placed in the gate of a field effect transistor made of thin film MoS$_2$. When a gate voltage is applied to this ferroelectric gated transistor, a clear transition from insulator to metal and vice versa is observed. Importantly, when the gate voltage is turned off, the remnant polarization in the ferroelectric can keep the MoS$_2$ in its original phase, thereby providing a non-volatile state. Thus a metallic or insulating phase can be written, erased or retained simply by applying a gate voltage to the transistor.

physics.app-ph↗

arXiv2017

Atomically flat single terminated oxide substrate surfaces

To achieve high quality epitaxial thin films and heterostructures of transition metal oxides with atomically controlled interfaces, one critical requirement is the use of atomically flat single terminated oxide substrates since the atomic arrangements and the reaction chemistry of the topmost surface layer of substrates determine the growth and consequent properties of the overlying films. Achieving the atomically flat and chemically single terminated surface state of commercially available substrates, however, requires judicious efforts because the surface of as-received substrates is of chemically mixed nature and also often polar. In this review, we summarize the surface treatment procedures to accomplish atomically flat surfaces with single terminating layer for various metal oxide substrates. We particularly focus on the substrates with lattice constant ranging from 4.00 to 3.70 angstrom, as the lattice constant of most perovskite materials falls into this range. For materials outside the range, one can utilize the substrates to induce compressive or tensile strain on the films and explore new states not available in bulk. The substrates covered in this review, which have been chosen with commercial availability and, most importantly, experimental practicality as a criterion, are KTaO3, REScO3 (RE = Rare-earth elements), SrTiO3, La0.18Sr0.82Al0.59Ta0.41O3 (LSAT), NdGaO3, LaAlO3, SrLaAlO4, and YAlO3. Analyzing all the established procedures, we conclude that atomically flat surfaces with selective A- or B-site single termination would be obtained for most commercially available oxide substrates. We further note that this topmost surface layer selectivity would provide an additional degree of freedom in searching for unforeseen emergent phenomena and functional applications in epitaxial oxide thin films and heterostructures with atomically controlled interfaces.

cond-mat.mtrl-sci↗

arXiv2017

Spin-Orbit Logic with Magnetoelectric Nodes: A Scalable Charge Mediated Nonvolatile Spintronic Logic

As nanoelectronics approaches the nanometer scale, a massive effort is underway to identify the next scalable logic technology beyond Complementary Metal Oxide Semiconductor (CMOS) computing. Such computing technology needs to improve switching energy & delay at reduced dimensions, allow improved interconnects and provide a complete logic/memory family. However, a viable beyond-CMOS logic technology has remained elusive. Here, we propose a scalable spintronic logic device which operates via spin-orbit transduction combined with magneto-electric switching. The proposed Magneto-Electric Spin Orbit (MESO) logic enables a new paradigm to continue scaling of logic performance to near thermodynamic limits for GHz logic (100 kT switching energy at 100 ps delay). The proposed MESO devices scale strongly and favorably with critical dimensions of the device, showing a cubic dependence of switching energy on size, (E_m proportional to W^3), and square dependence on voltage (E_m proportional to V^2). The excellent scaling is obtained thanks to the properties of the spin orbit effects (e.g. Inverse Spin Hall Effect (ISHE) and Inverse Rashba-Edelstein Effect (IREE)) and the dependence of capacitance on size. The operating voltages for these devices are predicted to be < 100 mV allowing a significant jump ahead of historic trends of scaling voltage with size and corresponding reduction of energy. Interconnect resistance is a critical obstacle for scaling beyond 10 nm dimensions. We project a less detrimental impact of interconnect resistance and show that MESO logic is amenable for highly resistive interconnects (100 uOhm.cm-1 mOhm.cm) which opens a possibility to use nano-metallic (width < bulk electron mean free path) or doped semiconducting wires (width<5 nm). A scalable, CMOS compatible, non-volatile logic family proposed here may enable the next multi-generational scaling of computing devices.

cond-mat.mes-hall↗