James C. T. Lee

Oxide heterostructures have shown rich physics phenomena, particularly in the conjunction of exotic insulator–metal transition (IMT) at the interface between polar insulator LaAlO3 and non-polar insulator SrTiO3 (LaAlO3/SrTiO3). The polarization catastrophe model has suggested an electronic reconstruction, yielding to metallicity at both the interface and surface. Another scenario is the occurrence of surface oxygen vacancy at LaAlO3 (surface-Ov), which has predicted surface-to-interface charge transfer, yielding metallic interface but insulating surface. To clarify the origin of IMT, one should probe surface-Ov and the associated electronic structures at both the surface and the buried interface simultaneously. Here, using grazing-angle resonant soft x-ray scattering (GA-RSXS) supported with first-principles calculations, we reveal the co-existence of the surface-Ov state and the interface conducting state only in conducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-Ov state and the interface conducting state are absent for the insulating film. As a function of Ov density, while the surface-Ov state is responsible for the IMT, the spatial charge distribution is found responsible for a transition from two-dimensional-like to three-dimensional-like conductivity accompanied by spectral weight transfer, revealing the importance of electronic correlation. Our results show the importance of surface-Ov in determining interface properties and provide a new strategy in utilizing GA-RSXS to directly probe the surface and buried interface electronic properties in complex oxide heterostructures.

The detection and manipulation of antiferromagnetic domains and topological antiferromagnetic textures are of central interest to solid-state physics. A fundamental step is identifying tools to probe the mesoscopic texture of an antiferromagnetic order parameter. In this work, we demonstrate that Bragg coherent diffractive imaging can be extended to study the mesoscopic texture of an antiferromagnetic order parameter using resonant magnetic x-ray scattering. We study the onset of the antiferromagnet transition in PrNiO ₃ , focusing on a temperature regime in which the antiferromagnetic domains are dilute in the beam spot and the coherent diffraction pattern modulating the antiferromagnetic peak is greatly simplified. We demonstrate that it is possible to extract the arrangements and sizes of these domains from single diffraction patterns and show that the approach could be extended to a time-structured light source to study the motion of dilute domains or the motion of topological defects in an antiferromagnetic spin texture.

We study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations.

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

The dynamic response of dipole skyrmions in Fe/Gd multilayer films is investigated by ferromagnetic resonance measurements and compared to micromagnetic simulations. We detail thickness- and temperature-dependent studies of the observed modes as well as the effects of magnetic field history on the resonant spectra. Correlation between the modes and the magnetic phase maps constructed from real-space imaging and scattering patterns allows us to conclude that the resonant modes arise from local topological features such as dipole skyrmions but do not depend on the collective response of a close-packed lattice of these chiral textures. Using micromagnetic modeling, we are able to quantitatively reproduce our experimental observations which suggests the existence of localized spin-wave modes that are dependent on the helicity of the dipole skyrmion. We identify four localized spin-wave excitations for the skyrmions that are excited under either in-plane or out-of-plane rf fields. Lastly we show that dipole skyrmions and nonchiral bubble domains exhibit qualitatively different localized spin-wave modes.

The interesting physics and potential memory technologies resulting from topologically protected spin textures such as skyrmions have prompted efforts to discover new material systems that can host these kinds of magnetic structures. Here, we use the highly tunable magnetic properties of amorphous Fe/Gd multilayer films to explore the magnetic properties that lead to dipole-stabilized skyrmions and skyrmion lattices that form from the competition of dipolar field and exchange energy. Using both real space imaging and reciprocal space scattering techniques, we determined the range of material properties and magnetic fields where skyrmions form. Micromagnetic modeling closely matches our observation of small skyrmion features (∼50 to 70 nm) and suggests that these classes of skyrmions have a rich domain structure that is Bloch-like in the center of the film and more Néel-like towards each surface. Our results provide a pathway to engineer the formation and controllability of dipole skyrmion phases in a thin film geometry at different temperatures and magnetic fields.

"The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover, as a new type of magnetic domain structure with special topological structures, skyrmions feature outstanding magnetic and transport properties and may well have applications in data storage and other advanced spintronic devices, as readers will see in this book. Chapters address the relationships between physical properties of condensed matter, such as the AB effect, Berry phase effect, quantum Hall effect, and topological insulators. Overall, it provides a timely introduction to the fundamental aspects and possible applications of magnetic skyrmions to an interdisciplinary audience from condensed matter physics, chemistry, and materials science. "The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover, as a new type of magnetic domain structure with special topological structures, skyrmions feature outstanding magnetic and transport properties and may well have applications in data storage and other advanced spintronic devices, as readers will see in this book. Chapters address the relationships between physical properties of condensed matter, such as the AB effect, Berry phase effect, quantum Hall effect, and topological insulators. Overall, it provides a timely introduction to the fundamental aspects and possible applications of magnetic skyrmions to an interdisciplinary audience from condensed matter physics, chemistry, and materials science. "The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover, as a new type of magnetic domain structure with special topological structures, skyrmions feature outstanding magnetic and transport properties and may well have applications in data storage and other advanced spintronic devices, as readers will see in this book. Chapters address the relationships between physical properties of condensed matter, such as the AB effect, Berry phase effect, quantum Hall effect, and topological insulators. Overall, it provides a timely introduction to the fundamental aspects and possible applications of magnetic skyrmions to an interdisciplinary audience from condensed matter physics, chemistry, and materials science. "The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover, as a new type of magnetic domain structure with special topological structures, skyrmions feature outstanding magnetic and transport properties and may well have applications in data storage and other advanced spintronic devices, as readers will see in this book. Chapters address the relationships between physical properties of condensed matter, such as the AB effect, Berry phase effect, quantum Hall effect, and topological insulators. Overall, it provides a timely introduction to the fundamental aspects and possible applications of magnetic skyrmions to an interdisciplinary audience from condensed matter physics, chemistry, and materials science. "The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover,...

We show that properly engineered amorphous Fe-Gd alloy thin films with perpendicular magnetic anisotropy exhibit bound pairs of like-polarity, opposite helicity skyrmions at room temperature. Magnetic mirror symmetry planes present in the stripe phase, instead of chiral exchange, determine the internal skyrmion structure and the net achirality of the skyrmion phase. Our study shows that stripe domain engineering in amorphous alloy thin films may enable the creation of skyrmion phases with technologically desirable properties.

High spatial resolution magnetic x-ray spectromicroscopy at x-ray photon energies near the cobalt L₃ resonance was applied to probe an amorphous 50 nm thin SmCo₅ film prepared by off-axis pulsed laser deposition onto an x-ray transparent 200 nm thin Si₃N₄ membrane. Alternating gradient magnetometry shows a strong in-plane anisotropy and an only weak perpendicular magnetic anisotropy, which is confirmed by magnetic transmission soft x-ray microscopy images showing over a field of view of 10 μm a primarily stripe-like domain pattern but with local labyrinth-like domains. Soft x-ray ptychography in amplitude and phase contrast was used to identify and characterize local magnetic and structural features over a field of view of 1 μm with a spatial resolution of about 10 nm. There, the magnetic labyrinth domain patterns are accompanied by nanoscale structural inclusions that are primarily located in close proximity to the magnetic domain walls. Our analysis suggests that these inclusions are nanocrystalline Sm₂Co₁₇ phases with nominally in-plane magnetic anisotropy.