arXiv subjects

R. Ramesh

Publications and source records attributed to R. Ramesh.

At least 55 records · Page 3Linked to original sources

arXiv2015

Room-temperature antiferromagnetic memory resistor

The bistability of ordered spin states in ferromagnets (FMs) provides the magnetic memory functionality. Traditionally, the macroscopic moment of ordered spins in FMs is utilized to write information on magnetic media by a weak external magnetic field, and the FM stray field is used for reading. However, the latest generation of magnetic random access memories demonstrates a new efficient approach in which magnetic fields are replaced by electrical means for reading and writing. This concept may eventually leave the sensitivity of FMs to magnetic fields as a mere weakness for retention and the FM stray fields as a mere obstacle for high-density memory integration. In this paper we report a room-temperature bistable antiferromagnetic (AFM) memory which produces negligible stray fields and is inert in strong magnetic fields. We use a resistor made of an FeRh AFM whose transition to a FM order 100 degrees above room-temperature, allows us to magnetically set different collective directions of Fe moments. Upon cooling to room-temperature, the AFM order sets in with the direction the AFM moments pre-determined by the field and moment direction in the high temperature FM state. For electrical reading, we use an antiferromagnetic analogue of the anisotropic magnetoresistance (AMR). We report microscopic theory modeling which confirms that this archetypical spintronic effect discovered more than 150 years ago in FMs, can be equally present in AFMs. Our work demonstrates the feasibility to realize room-temperature spintronic memories with AFMs which greatly expands the magnetic materials base for these devices and offers properties which are unparalleled in FMs.

cond-mat.mtrl-sci↗

arXiv2015

Probing single magnon excitations in Sr$_2$IrO$_4$ using O $K$-edge resonant inelastic X-ray scattering

Resonant inelastic X-ray scattering (RIXS) at the $L$-edge of transition metal elements is now commonly used to probe single magnon excitations. Here we show that single magnon excitations can also be measured with RIXS at the $K$-edge of the surrounding ligand atoms when the center heavy metal elements have strong spin-orbit coupling. This is demonstrated with oxygen $K$-edge RIXS experiments on the perovskite Sr$_2$IrO$_4$, where low energy peaks from single magnon excitations were observed. This new application of RIXS has excellent potential to be applied to a wide range of magnetic systems based on heavy elements, for which the $L$-edge RIXS energy resolutions in the hard X-ray region is usually poor.

cond-mat.mtrl-sci↗

arXiv2014

Induced magnetization in La$_{0.7}$Sr$_{0.3}$MnO$_3$/BiFeO$_3$ superlattices

Using polarized neutron reflectometry (PNR), we observe an induced magnetization of 75$\pm$ 25 kA/m at 10 K in a La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO)/BiFeO$_3$ superlattice extending from the interface through several atomic layers of the BiFeO$_3$ (BFO). The induced magnetization in BFO is explained by density functional theory, where the size of bandgap of BFO plays an important role. Considering a classical exchange field between the LSMO and BFO layers, we further show that magnetization is expected to extend throughout the BFO, which provides a theoretical explanation for the results of the neutron scattering experiment.

cond-mat.str-el↗

arXiv2014

Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy

High-resolution X-ray photoemission electron microscopy (X-PEEM) is a well-established method for imaging ferroelectric domain structures. Here, we expand the scope of application of X-PEEM and demonstrate its capability for imaging and investigating domain walls in ferroelectrics with high-spatial resolution. Using ErMnO3 as test system, we show that ferroelectric domain walls can be visualized based on photo-induced charging effects and local variations in their electronic conductance can be mapped by analyzing the energy distribution of photoelectrons. Our results open the door for non-destructive, contract-free, and element-specific studies of the electronic and chemical structure at domain walls in ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2014

BiFeO3/La0.7Sr0.3MnO3 heterostructures deposited on Spark Plasma Sintered LaAlO3 Substrates

Multiferroic BiFeO3 (BFO) / La0.7Sr0.3MnO3 heterostructured thin films were grown by pulsed laser deposition on polished spark plasma sintered LaAlO3 (LAO) polycrystalline substrates. Both polycrystalline LAO substrates and BFO films were locally characterized using electron backscattering diffraction (EBSD), which confirmed the high-quality local epitaxial growth on each substrate grain. Piezoforce microscopy was used to image and switch the piezo-domains, and the results are consistent with the relative orientation of the ferroelectric variants with the surface normal. This high-throughput synthesis process opens the routes towards wide survey of electronic properties as a function of crystalline orientation in complex oxide thin film synthesis.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the Competition between Ferromagnetism and Antiferromagnetism in a Half-doped Manganite through Magnetoelectric Coupling

We investigate the possibility of controlling the magnetic phase transition of the heterointerface between a half-doped manganite La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and a multiferroic BiFeO$_{3}$ through magnetoelectric coupling. Using macroscopic magnetometry and element-selective x-ray magnetic circular dichroism at the Mn and Fe \emph{L}-edges, we discover that the ferroelectric polarization of BFO controls simultaneously the magnetization of BFO and LCMO. X-ray absorption spectra at oxygen \emph{K}-edge and linear dichroism at Mn \emph{L}-edge suggest that the interfacial coupling is mainly derived from the superexchange between Mn and Fe \emph{t}$_{2g}$ spins. The combination of x-ray absorption spectroscopy and mean field theory calculations reveals that the \emph{d}-electron modulation of Mn cations changes the magnetic coupling in LCMO, which controls the enhanced canted moments of interfacial BFO via the interfacial coupling. Our results demonstrate that the competition between ferromagnetic and antiferromagnetic instability can be modulated by an electric field at the heterointerface, providing another pathway for the electrical field control of magnetism.

cond-mat.str-el↗

arXiv2013

Symmetry-Tunable Environment in Functional Ferroic Heterostructures

We demonstrate a concept for post-growth symmetry control in oxide heterostructures. A functional oxide is sandwiched between two ferroelectric layers and inversion symmetry at the position of the oxide constituent is reversibly switched on or off by layer-selective electric-field poling. The functionality of this process is monitored by optical second harmonic generation. The generalization of our approach to other materials and symmetries is considered. We thus establish ferroic trilayer structures as device components in which symmetry-driven charge-, spin-, and strain-related functionalities can be activated and deactivated at will.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the electronic properties of J_eff=1/2 correlated semimetal in epitaxial perovskite SrIrO3

We investigated the electronic properties of epitaxially stabilized perovskite SrIrO3 and demonstrated the effective strain-control on its electronic structure. Comprehensive transport measurements showed that the strong spin-orbit coupling renders a novel semimetallic phase for the J_eff=1/2 electrons rather than an ordinary correlated metal, elucidating the nontrivial mechanism underlying the dimensionality-controlled metal-insulator transition in iridates. The electron-hole symmetry of this correlated semimetal was found to exhibit drastic variation when subject to bi-axial strain. Under compressive strain, substantial electron-hole asymmetry is observed in contrast to the tensile side, where the electron and hole effective masses are comparable, illustrating the susceptivity of the J_eff=1/2 to structural distortion. Tensile strain also shrinks the Fermi surface, indicative of an increasing degree of correlation which is consistent with optical measurements. These results pave a pathway to investigate and manipulate the electronic states in spin-orbit-coupled correlated oxides, and lay the foundation for constructing 5d transition metal heterostructures.

cond-mat.str-el↗

arXiv2013

Anisotropic magnetoresistance in antiferromagnetic semiconductor Sr2IrO4 epitaxial heterostructure

Lord Kelvin with his discovery of the anisotropic magnetoresistance (AMR) phenomenon in Ni and Fe was 70 years ahead of the formulation of relativistic quantum mechanics the effect stems from, and almost one and a half century ahead of spintronics whose first commercial applications relied on the AMR. Despite the long history and importance in magnetic sensing and memory technologies, the microscopic understanding of the AMR has struggled to go far beyond the basic notion of a relativistic magnetotransport phenomenon arising from combined effects on diffusing carriers of spin-orbit coupling and broken symmetry of a metallic ferromagnet. Our work demonstrates that even this seemingly generic notion of the AMR phenomenon needs revisiting as we observe the ohmic AMR effect in a nano-scale film of an antiferromagnetic (AFM) semiconductor Sr2IrO4 (SIO). Our work opens the recently proposed path for integrating semiconducting and spintronic technologies in AFMs. SIO is a particularly favorable material for exploring this path since its semiconducting nature is entangled with the AFM order and strong spin-orbit coupling. For the observation of the low-field Ohmic AMR in SIO we prepared an epitaxial heterostructure comprising a nano-scale SIO film on top of an epilayer of a FM metal La2/3Sr1/3MnO3 (LSMO). This allows the magnetic field control of the orientation of AFM spins in SIO via the exchange spring effect at the FM-AFM interface.

cond-mat.mtrl-sci↗

arXiv2013

Epitaxy-distorted spin-orbit Mott insulator in Sr2IrO4 thin films

High quality epitaxial thin films of Jeff=1/2 Mott insulator Sr2IrO4 with increasing in-plane tensile strain have been grown on top of SrTiO3(001) substrates. Increasing the in-plane tensile strain up to ~0.3% was observed to drop the c/a tetragonality by 1.2 %. X-ray absorption spectroscopy detected a strong reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the local electronic structure. While the most relaxed thin film shows a consistent dependence with previously reported single crystal bulk measurements, electrical transport reveals a charge gap reduction from 200 meV down to 50 meV for the thinnest and most epitaxy-distorted film. We argue that the reduced tetragonality plays a major role in the change of the electronic structure, which is reflected in the change of the transport properties. Our work opens the possibility for exploiting epitaxial strain as a tool for both structural and functional manipulation of spin-orbit Mott systems.

cond-mat.mtrl-sci↗

arXiv2012

Mutual induction of magnetic 3d and 4f order in multiferroic hexagonal ErMnO3

The complex interplay between the 3d and 4f moments in hexagonal ErMnO3 is investigated by magnetization, optical second harmonic generation, and neutron-diffraction measurements. We revise the phase diagram and provide a microscopic model for the emergent spin structures with a special focus on the intermediary phase transitions. Our measurements reveal that the 3d exchange between Mn^{3+} ions dominates the magnetic symmetry at 10 K < T < T_N with Mn^3+ order according to the Gamma_4 representation triggering 4f ordering according to the same representation on the Er^{3+}(4b) site. Below 10 K the magnetic order is governed by 4f exchange interactions of Er^{3+} ions on the 2a site. The magnetic Er^{3+}(2a) order according to the representation Gamma_2 induces a magnetic reorientation (Gamma_4 --> Gamma_2) at the Er^{3+}(4b) and the Mn^{3+} sites. Our findings highlight the fundamentally different roles the Mn^{3+}, R^{3+}(2a), and R^{3+}(4b) magnetism play in establishing the magnetic phase diagram of the hexagonal RMnO3 system.

cond-mat.str-el↗

arXiv2012

Observation of Coherent Helimagnons and Gilbert damping in an Itinerant Magnet

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe0.8Co0.2Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets. (*These authors contributed equally to this work)

cond-mat.str-el↗

arXiv2012

Unusual size effects on thermoelectricity in a strongly correlated oxide

We investigated size effects on thermoelectricity in thin films of a strongly correlated layered cobaltate. At room temperature, the thermopower is independent of thickness down to 6 nm. This unusual behavior is inconsistent with the Fuchs-Sondheimer theory, which is used to describe conventional metals and semiconductors, and is attributed to the strong electron correlations in this material. Although the resistivity increases, as expected, below a critical thickness of $\sim$ 30 nm. The temperature dependent thermopower is similar for different thicknesses but resistivity shows systematic changes with thickness. Our experiments highlight the differences in thermoelectric behavior of strongly correlated and uncorrelated systems when subjected to finite size effects. We use the atomic limit Hubbard model at the high temperature limit to explain our observations. These findings provide new insights on decoupling electrical conductivity and thermopower in correlated systems.

cond-mat.str-el↗

arXiv2011

Direct Observation of an Interface Dipole between Two Metallic Oxides Caused by Localized Oxygen Vacancies

Oxygen vacancies are increasingly recognized to play a role in phenomena observed at transition-metal oxide interfaces. Here we report a study of SrRuO3/La0.7Sr0.3MnO3 (SRO/LSMO) interfaces using a combination of quantitative aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density-functional calculations. Cation displacements are observed at the interface, indicative of a dipole-like electric field even though both materials are nominally metallic. The observed displacements are reproduced by theory if O vacancies are present in the near-interface LSMO layers. The results suggest that atomic-scale structural mapping can serve as a quantitative indicator of the presence of O vacancies at interfaces.

cond-mat.mtrl-sci↗

arXiv2011

Ferromagnetic Enhancement of CE-type Spin Ordering in (Pr,Ca)MnO$_3$

We present resonant soft X-ray scattering (RSXS) results from small band width manganites (Pr,Ca)MnO$_3$, which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance (CMR) effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic (FM-M) state and CE-type ordering.

cond-mat.str-el↗

arXiv2011

Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling

A planar slab of negative index material works as a superlens with sub-diffraction-limited imaging resolution, since propagating waves are focused and, moreover, evanescent waves are reconstructed in the image plane. Here, we demonstrate a superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime. The combination of near-field microscopy with a tunable free-electron laser allows us to address precisely the polariton modes, which are critical for super-resolution imaging. We spectrally study the lateral and vertical distributions of evanescent waves around the image plane of such a lens, and achieve imaging resolution of wavelength/14 at the superlensing wavelength. Interestingly, at certain distances between the probe and sample surface, we observe a maximum of these evanescent fields. Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy, and thermal sensors.

physics.optics↗

arXiv2010

Quantitative determination of anisotropic magnetoelectric coupling in BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructures

The transverse and longitudinal magnetoelectric susceptibilities (MES) were quantitatively determined for (001) heteroepitaxial BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructures. Both of these MES values were sharply enhanced at magnetic fields below 6 kOe and revealed asymmetric lineshapes with respect to the dc magnetic field, demonstrating the strain-induced magnetoelectric effect. The maximum transverse MES, which reached as high as $\sim$60 mV/cm Oe, was about five times larger than the longitudinal MES. This observation signifies that transverse magnetostriction of the CoFe$_{2}$O$_{4}$ nanopillars is enhanced more than the bulk value due to preferred magnetic domain alignment along the [001] direction coming from compressive, heteroepitaxial strain.

cond-mat.str-el↗

arXiv2010

Determination of the spin-flip time in ferromagnetic SrRuO3 from time-resolved Kerr measurements

We report time-resolved Kerr effect measurements of magnetization dynamics in ferromagnetic SrRuO3. We observe that the demagnetization time slows substantially at temperatures within 15K of the Curie temperature, which is ~ 150K. We analyze the data with a phenomenological model that relates the demagnetization time to the spin flip time. In agreement with our observations the model yields a demagnetization time that is inversely proportional to T-Tc. We also make a direct comparison of the spin flip rate and the Gilbert damping coefficient showing that their ratio very close to kBTc, indicating a common origin for these phenomena.

cond-mat.mtrl-sci↗