Engineering topics

Garlow, Joseph A.

Publications and source records attributed to Garlow, Joseph A..

11 records

DOE OSTI2021

Signature of Many-Body Localization of Phonons in Strongly Disordered Superlattices

Many-body localization (MBL) has attracted significant research attention due to its immunity to thermalization, role in logarithmic entanglement entropy growth, and opportunities to reach exotic quantum orders. However, experimental realization of MBL in solid-state systems has remained challenging. Here we report evidence of a possible phonon MBL phase in disordered GaAs/AlAs superlattices. Through grazing-incidence inelastic X-ray scattering, we observe a strong deviation of the phonon population from equilibrium in disordered samples doped with ErAs nanodots at low temperature, signaling a departure from thermalization. This behavior occurs within finite phonon energy and wavevector windows, suggesting a localization-thermalization crossover. We support our observation by proposing a theoretical model for the effective phonon Hamiltonian in disordered superlattices, and showing that it can be mapped exactly to a disordered 1D Bose-Hubbard model with a known MBL phase. The large on-site repulsion qualitatively explains the experimental findings. Furthermore, our work provides mode-resolved experimental evidence of phonon localization, potentially extending the scope of MBL to disordered solid-state systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Unconventional domain-wall pairs and interacting Bloch lines in a Dzyaloshinskii-Moriya multilayer thin film

Bloch-lines (BLs) play a key role in determining the static and dynamic behavior of chiral domain walls and skyrmions in multilayer films with a significant Dzyaloshinskii-Moriya interaction (DMI). Here, using in-situ Lorentz phase microscopy, we reveal a new type of spin texture, termed a type-II domain wall pair (DWP), stabilized in DMI multilayer thin films with BLs. For a type-II DWP, the Bloch components of each complementary domain wall are parallel, implying opposite chirality. Additionally, we find that type-II DWPs preferentially form through bifurcation of a type-I DWP and require the formation of at least two BLs. We demonstrate a distinct phase jump associated with type-II DWPs, and further reveal the role they play in the formation of mixed-character skyrmions where the Bloch component that can be of either left- or right-handed chirality.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Bloch Chirality Induced by an Interlayer Dzyaloshinskii-Moriya Interaction in Ferromagnetic Multilayers

Chiral spin textures stabilized by the interfacial Dzyaloshinkii-Moriya interaction, such as skyrmions and homochiral domain walls, have been shown to exhibit qualities that make them attractive for their incorporation in a variety of spintronic devices. However, for thicker multilayer films, mixed textures occur in which an achiral Bloch component coexists with a chiral Néel component of the domain wall to reduce the demagnetization field at the film surface. We show that an interlayer Dzyaloshinkii-Moriya interaction can break the degeneracy between Bloch chiralities. We further find large population asymmetries and chiral branching in the Bloch component of the domain walls in well-ordered Co/Pd multilayers. This asymmetry is a result of the combined effect of the demagnetization field and an interlayer Dzyaloshinkii-Moriya interaction, and is strongly related to film thickness and structural ordering. This work paves the way toward the utilization of this effect toward controlling Bloch chirality in magnetic multilayers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr 2 Ge 2 Te 6

Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. In this work, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr 2 Ge 2 Te 6 . Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. In conclusion, our observation of topologically nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Deterministic Ferroelastic Domain Switching Using Ferroelectric Bilayers

Composition gradients, or dissimilar ferroelectric bilayers, demonstrate colossal electromechanical figures of merit attributed to the motion of ferroelastic domain walls. Yet, mechanistic understanding of polarization switching pathways that drive ferroelastic switching in these systems remains elusive. Here, the crucial roles of strain and electrostatic boundary conditions in ferroelectric bilayer systems are revealed, which underpin their ferroelastic switching dynamics. Using in situ electrical biasing in the transmission electron microscope (TEM), the motion of ferroelastic domain walls is investigated in a tetragonal (T) Pb(Zr,Ti)O 3 (PZT)/rhombohedral (R) PZT epitaxial bilayer system. Atomic resolution electron microscopy, in tandem with phase field simulations, indicates that ferroelastic switching is triggered by predominant nucleation at the triple domain junctions located at the interface between the T/R layers. Furthermore, this interfacial nucleation leads to systematic reversable reorientation of ferroelastic domain walls. Finally, deterministic ferroelastic domain switching, driven by the interfacial strain and electrostatic boundary conditions in the ferroelectric bilayer, provides a viable pathway toward novel design of miniaturized energy-efficient electromechanical devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Quantification of Mixed Bloch-Néel Topological Spin Textures Stabilized by the Dzyaloshinskii-Moriya Interaction in Co / Pd Multilayers

The three-dimensional structure of nanoscale topological spin textures stabilized by the Dzyaloshinskii-Moriya interaction is governed by the delicate competition between the exchange, demagnetization, and anisotropy energies. The quantification of such spin textures through direct experimental methods is crucial towards understanding the fundamental physics associated with their ordering, as well as their manipulation in spintronic devices. Here in this work, we extend the Lorentz transmission electron microscopy technique to quantify mixed Bloch-Néel chiral spin textures stabilized by the Dzyaloshinskii-Moriya interaction in Co / Pd multilayers. Analysis of the observed intensities under varied imaging conditions coupled to corroborative micromagnetic simulations yields vital parameters that dictate the stability and properties of the complex spin texture, namely, the degree of mixed Bloch-Néel character, the domain wall width, the strength of the Dzyaloshinskii-Moriya interaction, and the exchange stiffness. In conclusion, this approach provides the necessary framework for the application of quantitative Lorentz phase microscopy to a broad array of topological spin systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Dirac-electron-mediated magnetic proximity effect in topological insulator/magnetic insulator heterostructures

The possible realization of dissipationless chiral edge current in a topological insulator/magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. We report a polarized neutron reflectometry observation of Dirac-electron-mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi 0.2 Sb 0.8 ) 2 Te 3 /magnetic insulator EuS heterostructure. We are able to maximize the proximity-induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the Dirac point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity-induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator heterointerface for low-power spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Electron-beam-induced-current and active secondary-electron voltage-contrast with aberration-corrected electron probes

The ability to map out electrostatic potentials in materials is critical for the development and the design of nanoscale electronic and spintronic devices in modern industry. Electron holography has been an important tool for revealing electric and magnetic field distributions in microelectronics and magnetic-based memory devices, however, its utility is hindered by several practical constraints, such as charging artifacts and limitations in sensitivity and in field of view. In this article, we report electron-beam-induced-current (EBIC) and secondary-electron voltage-contrast (SE-VC) with an aberration-corrected electron probe in a transmission electron microscope (TEM), as complementary techniques to electron holography, to measure electric fields and surface potentials, respectively. These two techniques were applied to ferroelectric thin films, multiferroic nanowires, and single crystals. Electrostatic potential maps obtained by off-axis electron holography were compared with EBIC and SE-VC to show that these techniques can be used as a complementary approach to validate quantitative results obtained from electron holography analysis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Observation of stable Néel skyrmions in cobalt/palladium multilayers with Lorentz transmission electron microscopy

Néel skyrmions are of high interest due to their potential applications in a variety of spintronic devices, currently accessible in ultrathin heavy metal/ferromagnetic bilayers and multilayers with a strong Dzyaloshinskii–Moriya interaction. Here in this paper we report on the direct imaging of chiral spin structures including skyrmions in an exchange-coupled cobalt/palladium multilayer at room temperature with Lorentz transmission electron microscopy, a high-resolution technique previously suggested to exhibit no Néel skyrmion contrast. Phase retrieval methods allow us to map the internal spin structure of the skyrmion core, identifying a 25 nm central region of uniform magnetization followed by a larger region characterized by rotation from in- to out-of-plane. The formation and resolution of the internal spin structure of room temperature skyrmions without a stabilizing out-of-plane field in thick magnetic multilayers opens up a new set of tools and materials to study the physics and device applications associated with chiral ordering and skyrmions.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Coupling of bias-induced crystallographic shear planes with charged domain walls in ferroelectric oxide thin films

Polar discontinuity at interfaces plays deterministic roles in charge transport, magnetism, and even superconductivity of functional oxides. To date, most polar discontinuity problems have been explored in hetero-interfaces between two dissimilar materials. Here, we show that charged domain walls (CDWs) in epitaxial thin films of ferroelectric PbZr 0.2 Ti 0.8 O 3 are strongly coupled to polar interfaces through the formation of ½<101>{h0l} type crystallographic shear planes (CSPs). Using atomic resolution imaging and spectroscopy we illustrate that the CSPs consist of both conservative and nonconservative segments when coupled to the CDWs, where necessary compensating charges for stabilizing the CDWs are associated with vacancies at the CSPs. Lasly, the CDW/CSP coupling yields an atomically narrow domain walls, consisting of a single atomic layer of oxygen. This study shows that the CDW/CSP coupling is a fascinating venue to develop emergent material properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Large-Area Growth of Turbostratic Graphene on Ni(111) via Physical Vapor Deposition

Single-layer graphene has demonstrated remarkable electronic properties that are strongly influenced by interfacial bonding and break down for the lowest energy configuration of stacked graphene layers (AB Bernal). Multilayer graphene with relative rotations between carbon layers, known as turbostratic graphene, can effectively decouple the electronic states of adjacent layers, preserving properties similar to that of SLG. While the growth of AB Bernal graphene through chemical vapor deposition has been widely reported, we investigate the growth of turbostratic graphene on heteroepitaxial Ni(111) thin films utilizing physical vapor deposition. By varying the carbon deposition temperature between 800–1100°C, we report an increase in the graphene quality concomitant with a transition in the size of uniform thickness graphene, ranging from nanocrystallites to thousands of square microns. Combination Raman modes of as-grown graphene within the frequency range of 1650 cm ₋1 to 2300 cm ₋1 , along with features of the Raman 2D mode, were employed as signatures of turbostratic graphene. Bilayer and multilayer graphene were directly identified from areas that exhibited Raman characteristics of turbostratic graphene using high-resolution TEM imaging. Lastly, Raman maps of the pertinent modes reveal large regions of turbostratic graphene on Ni(111) thin films at a deposition temperature of 1100°C.

36 MATERIALS SCIENCE↗