arXiv subjects

Padraic Shafer

Publications and source records attributed to Padraic Shafer.

34 records · Page 2Linked to original sources

arXiv2020

Emergent electric field control of phase transformation in oxide superlattices

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides (TMOs) provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized TMO that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up a new class of materials for voltage-controlled functionality.

cond-mat.mtrl-sci↗

arXiv2019

Coherent transfer of spin angular momentum by evanescent spin waves within antiferromagnetic NiO

Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz frequency within antiferromagnetic NiO at room temperature. Using element-specific and phase-resolved x-ray ferromagnetic resonance, we probe the injection and transmission of ac spin current, and demonstrate that insertion of a few nanometre thick epitaxial NiO(001) layer between a ferromagnet and non-magnet can even enhance the flow of spin current. Our results pave the way towards coherent control of the phase and amplitude of spin currents at the nanoscale, and enable the realization of spin-logic devices and spin current amplifiers that operate at GHz and THz frequencies.

cond-mat.mes-hall↗

arXiv2019

Damping enhancement in coherent ferrite/insulating-paramagnet bilayers

High-quality epitaxial ferrites, such as low-damping MgAl-ferrite (MAFO), are promising nanoscale building blocks for all-oxide heterostructures driven by pure spin current. However, the impact of oxide interfaces on spin dynamics in such heterostructures remains an open question. Here, we investigate the spin dynamics and chemical and magnetic depth profiles of 15-nm-thick MAFO coherently interfaced with an isostructural $\approx$1-8-nm-thick overlayer of paramagnetic CoCr$_2$O$_4$ (CCO) as an all-oxide model system. Compared to MAFO without an overlayer, effective Gilbert damping in MAFO/CCO is enhanced by a factor of $>$3, irrespective of the CCO overlayer thickness. We attribute this damping enhancement to spin scattering at the $\sim$1-nm-thick chemically disordered layer at the MAFO/CCO interface, rather than spin pumping or proximity-induced magnetism. Our results indicate that damping in ferrite-based heterostructures is strongly influenced by interfacial chemical disorder, even if the thickness of the disordered layer is a small fraction of the ferrite thickness.

cond-mat.mtrl-sci↗

arXiv2019

Unconventional crystal field splitting in non-centrosymmetric BaTiO$_3$ thin films

Understanding the crystal field splitting and orbital polarization in non-centrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking BaTiO$_3$ (BTO) as a representative material we investigate titanium crystal field splitting and orbital polarization in non-centrosymmetric TiO$_6$ octahedra with resonant X-ray linear dichroism at Ti $L_{2,3}$-edge. The high-quality BaTiO$_3$ thin films were deposited on DyScO$_3$ (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction (RHEED) and element specific X-ray absorption spectroscopy (XAS) were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal field splitting and orbital configuration ($d_{xz}$/$d_{yz}$ $<$ $d_{xy}$ $<$ $d_{3z^2-r^2}$ $<$ $d_{x^2-y^2}$ or $d_{xy}$ $<$ $d_{xz}$/$d_{yz}$ $<$ $d_{x^2-y^2}$ $<$ $d_{3z^2-r^2}$) according to Jahn-Teller effect, it is revealed that $d_{xz}$, $d_{yz}$, and $d_{xy}$ orbitals are nearly degenerate, whereas $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbitals are split with an energy gap $\sim$ 100 meV in the epitaxial BTO films. The unexpected degenerate states $d_{xz}$/$d_{yz}$/$d_{xy}$ are coupled to Ti-O displacements resulting from competition between polar and Jahn-Teller distortions in non-centrosymmetric TiO$_6$ octhedra of BTO films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.

cond-mat.str-el↗

arXiv2019

Evolution of charge order topology across a magnetic phase transition in cuprate superconductors

Charge order is now accepted as an integral constituent of cuprate high-temperature superconductors, one that is intimately related to other instabilities in the phase diagram including antiferromagnetism and superconductivity. Unlike nesting-induced Peierls-like density waves, the charge correlations in the CuO2 planes have been predicted to display a rich momentum space topology depending on the detailed fermiology of the system. However, to date charge order has only been observed along the high-symmetry Cu-O bond directions. Here, using resonant soft X-ray scattering, we investigate the evolution of the full momentum space topology of charge correlations in Ln2CuO4 (Ln=Nd, Pr) as a function of intrinsic electron doping. We report that, upon electron doping the parent Mott insulator, charge correlations first emerge in a hitherto-unobserved form, with full (Cinf) rotational symmetry in momentum-space. At higher doping levels, the orientation of charge correlations is sharply locked to the Cu-O bond high-symmetry directions, restoring a more conventional bidirectional charge order with enhanced correlation lengths. Through charge susceptibility calculations, we closely reproduce the drastic evolution in the topology of charge correlations across an antiferromagnetic quantum phase transition, highlighting the interplay between spin and charge degrees of freedom in electron-doped cuprates. Finally, using the established link between charge correlations and the underlying fermiology, we propose a revised phase diagram of Ln2CuO4 with a superconducting region extending toward the Mott limit.

cond-mat.str-el↗

arXiv2019

Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

cond-mat.mtrl-sci↗

arXiv2019

Ultrafast X-Ray Induced Changes of the Electronic and Magnetic Response of Solids Due to Valence Electron Redistribution

We report a novel mechanism, consisting of redistribution of valence electrons near the Fermi level, during interactions of intense femtosecond X-ray pulses with a Co/Pd multilayer. The changes in Co 3d valence shell occupation were directly revealed by fluence-dependent changes of the Co L$_3$ X-ray absorption and magnetic circular dichroism spectra near the excitation threshold. The valence shell redistribution arises from inelastic scattering of high energy Auger electrons and photoelectrons that lead to transient holes below and electrons above the Fermi level on the femtosecond time scale. The valence electron reshuffling effect scales with the energy deposited by X-rays and within 17 fs extends to valence states within 2 eV of the Fermi level. As a consequence the sample demagnetizes by more than twenty percent due to magnon generation.

cond-mat.mtrl-sci↗

arXiv2018

Inverted orbital polarization in strained correlated oxide films

Manipulating the orbital occupation of valence electrons via epitaxial strain in an effort to induce new functional properties requires considerations of how changes in the local bonding environment affect the band structure at the Fermi level. Using synchrotron radiation to measure the x-ray linear dichroism of epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that the orbital polarization of the Fe valence electrons is opposite from conventional understanding. Although the energetic ordering of the Fe 3d orbitals is confirmed by multiplet ligand field theory analysis to be consistent with previously reported strain-induced behavior, we find that the nominally higher energy orbital is more populated than the lower. We ascribe this inverted orbital polarization to an anisotropic bandwidth response to strain in a compound with nearly filled bands. These findings provide an important counterexample to the traditional understanding of strain-induced orbital polarization and reveal a new method to engineer otherwise unachievable orbital occupations in correlated oxides.

cond-mat.str-el↗

arXiv2018

Control of hidden ground-state order in NdNiO$_3$ superlattices

The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO$_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.

cond-mat.str-el↗

arXiv2018

Artificial two-dimensional polar metal at room temperature

Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO$_3$/SrTiO$_3$/LaTiO$_3$. A combination of atomic resolution scanning transmission electron microscopy with electron energy loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition

We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe electron valence undergoes no observable change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ~5-10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

cond-mat.mtrl-sci↗

arXiv2017

Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La$_{1-x}$Sr$_{x}$MnO$_{3}$)/(SrIrO$_{3}$) Superlattices

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La$_{1-x}$Sr$_{x}$MnO$_{3}$ and SrIrO$_{3}$, we find that all superlattices (0$\leq$x$\leq$1) exhibit ferromagnetism despite the fact that La$_{1-x}$Sr$_{x}$MnO$_{3}$ is antiferromagnetic for x$>$0.5. PMA as high as 4$\times$10$^6$ erg/cm$^3$ is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

cond-mat.str-el↗

arXiv2017

Role of polar compensation in interfacial ferromagnetism of LaNiO$_3$/CaMnO$_3$ superlattices

Polar compensation can play an important role in the determination of interfacial electronic and magnetic properties in oxide heterostructures. Using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk magnetometry, and transport measurements, we find that interfacial charge redistribution via polar compensation is essential for explaining the evolution of interfacial ferromagnetism in LaNiO$_3$/CaMnO$_3$ superlattices as a function of LaNiO$_3$ layer thickness. In insulating superlattices (4 unit cells or less of LaNiO$_3$), magnetism is dominated by Ni-Mn superexchange, while itinerant electron-based Mn-Mn double-exchange plays a role in thicker metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray scattering show that Ni-Mn superexchange contributes to the magnetization even in metallic superlattices. This Ni-Mn superexchange interaction can be explained in terms of polar compensation at the LaNiO$_3$-CaMnO$_3$ interface. These results highlight the different mechanisms responsible for interfacial ferromagnetism and the importance of understanding compensation due to polar mismatch at oxide-based interfaces when engineering magnetic properties.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic switching in granular FePt layers promoted by near-field laser enhancement

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle x-ray scattering at an x-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, one order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material, with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

cond-mat.mtrl-sci↗

arXiv2016

Observation of a Three-Dimensional Quasi-Long-Range Electronic Supermodulation in YBa2Cu3O7-x/La0.7Ca0.3MnO3 Heterostructures

Recent developments in high-temperature superconductivity highlight a generic tendency of the cuprates to develop competing electronic (charge) supermodulations. While coupled to the lattice and showing different characteristics in different materials, these supermodulations themselves are generally conceived to be quasi-two-dimensional, residing mainly in individual CuO2 planes, and poorly correlated along the c-axis. Here we observed with resonant elastic x-ray scattering a distinct type of electronic supermodulation in YBa2Cu3O7-x (YBCO) thin films grown epitaxially on La0.7Ca0.3MnO3 (LCMO). This supermodulation has a periodicity nearly commensurate with four lattice constants in-plane, eight out-of-plane, with long correlation lengths in three dimensions. It sets in far above the superconducting transition temperature and competes with superconductivity below this temperature for electronic states predominantly in the CuO2 plane. Our finding sheds new light on the nature of charge ordering in cuprates as well as a reported long-range proximity effect between superconductivity and ferromagnetism in YBCO/LCMO heterostructures.

cond-mat.str-el↗

arXiv2015

Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L$_{3,2}$-edges. This new ultrafast time-resolved capability is then applied to a fluence-dependent study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature. At the magnetic switching fuence, we corroborate the existence of a transient ferromagnetic-like state. The timescales of the dynamics, however, are longer than previously observed below the magnetization compensation temperature. Above and below the switching fluence range, we observe secondary demagnetization with about 5 ps timescales. This indicates that the spin thermalization takes longer than 5 ps.

cond-mat.mtrl-sci↗