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Scanning tunneling microscopy was used to study the surfaces of 20-100 nm thick FeGe films grown by molecular beam epitaxy. An average surface lattice constant of ∼6.8 Å, in agreement with the bulk value, was observed via scanning tunneling microscopy, low energy electron diffraction, and reflection high energy electron diffraction. Each of the four possible chemical terminations in the FeGe films were identified by comparing atomic-resolution images, showing distinct contrast with simulations from density functional theory calculations. A detailed study of the atomic layering order and registry across step edges allows us to uniquely determine the grain orientation and chirality in these noncentrosymmetric films.

Skyrmion imaging and electrical detection via topological Hall (TH) effect are two primary techniques for probing magnetic skyrmions, which hold promise for next-generation magnetic storage. However, these two kinds of complementary techniques have rarely been employed to investigate the same samples. We report the observation of nanoscale skyrmions in SrIrO3/SrRuO3 (SIO/SRO) bilayers in a wide temperature range from 10 to 100 K. The SIO/SRO bilayers exhibit a remarkable TH effect, which is up to 200% larger than the anomalous Hall (AH) effect at 5 K, and zero-field TH effect at 90 K. Using variable-temperature, high-field magnetic force microscopy (MFM), we imaged skyrmions as small as 10 nm, which emerge in the same field ranges as the TH effect. These results reveal a rich space for skyrmion exploration and tunability in oxide heterostructures.

Ferrimagnetic insulators capped with a heavy metal are becoming an increasingly interesting materials system in spintronics due to their unique ability for electrical manipulation and detection of magnetic states and spin textures via spin-orbit torques. The ability to engineer magnetic anisotropy is a powerful tool for tuning the recently discovered phenomena in these bilayers such as electrical switching or the stabilization of topological magnetic textures. We observe large shifts in the magnetic anisotropy in ${\mathrm{Tm}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ and ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ thin films due to heavy-metal capping layers, which strongly depends on the orientation of the substrate and therefore the orientation of the epitaxial films. This work suggests large Rashba spin-orbit coupling at the metal/ferrimagnetic-insulator interface, which can be engineered in spintronic devices that utilize spin-orbit torques for electrical control of the magnetization in magnetic insulators.

Interfacial magnetic anisotropy in magnetic insulators has been largely unexplored. Recently, interface-induced skyrmions and electrical control of magnetization have been discovered in insulator-based heterostructures, which demand a thorough understanding of interfacial interactions in these materials. We observe a substantial, tunable interfacial magnetic anisotropy between Tm_{3}Fe_{5}O_{12} epitaxial thin films and fifteen nonmagnetic materials spanning a significant portion of the periodic table, which we attribute to Rashba spin-orbit coupling. Our results show a clear distinction between nonmagnetic capping layers from the d block and the p block. This work offers a new path for controlling magnetic phases in magnetic insulators for low-loss spintronic applications.

The quality of the tunnel barrier at the ferromagnet/graphene interface plays a pivotal role in graphene spin valves by circumventing the impedance mismatch problem, decreasing interfacial spin dephasing mechanisms and decreasing spin absorption back into the ferromagnet. It is thus crucial to integrate superior tunnel barriers to enhance spin transport and spin accumulation in graphene. Here, we employ a novel tunnel barrier, strontium oxide (SrO), onto graphene to realize high quality spin transport as evidenced by room-temperature spin relaxation times exceeding a nanosecond in graphene on silicon dioxide substrates. Furthermore, the smooth and pinhole-free SrO tunnel barrier grown by molecular beam epitaxy (MBE), which can withstand large charge injection current densities, allows us to experimentally realize large spin accumulation in graphene at room temperature. This work puts graphene on the path to achieve efficient manipulation of nanomagnet magnetization using spin currents in graphene for logic and memory applications.

Skyrmions are localized magnetic spin textures whose stability has been shown theoretically to depend on material parameters including bulk Dresselhaus spin orbit coupling (SOC), interfacial Rashba SOC, and magnetic anisotropy. Here, we establish the growth of a new class of artificial skyrmion materials, namely B20 superlattices, where these parameters could be systematically tuned. Specifically, we report the successful growth of B20 superlattices comprised of single crystal thin films of FeGe, MnGe, and CrGe on Si(111) substrates. Thin films and superlattices are grown by molecular beam epitaxy and are characterized through a combination of reflection high energy electron diffraction, x-ray diffraction, and cross-sectional scanning transmission electron microscopy (STEM). X-ray energy dispersive spectroscopy (XEDS) distinguishes layers by elemental mapping and indicates good interface quality with relatively low levels of intermixing in the [CrGe/MnGe/FeGe] superlattice. This demonstration of epitaxial, single-crystalline B20 superlattices is a significant advance toward tunable skyrmion systems for fundamental scientific studies and applications in magnetic storage and logic., Comment: 16 pages, 7 figures

Layered Zintl phase van der Waals (vdW) materials are of interest due to their strong spin-orbit coupling and potential for high mobility. Here, we report the successful growth of large area CaGe 2 films, as a model of layered Zintl phase materials, on atomically flat Ge(111) substrates by molecular beam epitaxy (MBE) using an alternating layer growth (ALG) protocol. Reflection high energy electron diffraction (RHEED) patterns of the Ge buffer layer and CaGe 2 indicate high quality two dimensional surfaces, which is further confirmed by atomic force microscopy (AFM), showing atomically flat and uniform CaGe 2 films. The appearance of Laue oscillations in X-ray diffraction (XRD) and Kiessig fringes in the X-ray reflectivity (XRR), which are absent in co-deposited CaGe 2 , confirms the uniformity of the CaGe 2 film and the smoothness of the interface. These results demonstrate a novel method of deposition of CaGe 2 that could be also applied to other layered Zintl phase vdW materials. Also, the high quality of the CaGe 2 film is promising for the exploration of novel properties of germanane.

Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. While magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm$_3$Fe$_5$O$_{12}$ bilayers, where the pristine Tm$_3$Fe$_5$O$_{12}$ epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm$_3$Fe$_5$O$_{12}$ is insulating, we demonstrate the detection of topological magnetic textures through a novel phenomenon: 'spin-Hall topological Hall effect' (SH-THE), where the interfacial spin-orbit torques allow spin-Hall-effect generated spins in Pt to experience the unique topology of the underlying skyrmions in Tm$_3$Fe$_5$O$_{12}$. This novel electrical detection phenomenon paves a new path for utilizing a large family of magnetic insulators in future skyrmion technologies.

We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by x-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of, 23 pages, 8 figures

We report experimental and theoretical evidence for the formation of chiral bobbers - an interfacial topological spin texture - in FeGe films grown by molecular beam epitaxy (MBE). After establishing the presence of skyrmions in FeGe/Si(111) thin film samples through Lorentz transmission electron microscopy and topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between film thickness and the slope of the susceptibility (d\c{hi}/dH). We present evidence for the evolution as a function of film thickness, L, from a skyrmion phase for L < LD/2 to a cone phase with chiral bobbers at the interface for L > LD/2, where LD ~ 70 nm is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction (DMI)., accepted to Phys. Rev. Materials

The magnetic proximity effect (MPE) is an important phenomenon for spin manipulation in spintronic devices, but to this date, researchers have been unable to demonstrate MPE using spinel ferrites, a versatile class of ferrimagnetic insulators with high Curie temperatures. In this study, magnetotransport measurements on platinum-cobalt ferrite bilayers demonstrate the presence of MPE, which results in induced ferromagnetic order in the platinum films. In contrast to earlier studies, the entire structure is grown by molecular beam epitaxy, which ensures the cleanest possible interfaces. These results enable the use of spinel ferrites for spin manipulation in future spin-logic devices.

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

We apply a low-field ($l100$ G) technique to measure the critical pair momentum ${p}_{c}$ in thin underdoped films of ${\mathrm{Y}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ reflecting a wide range of hole doping. We observe that ${p}_{c}\ensuremath{\propto}\ensuremath{\hbar}/\ensuremath{\xi}$ scales with ${T}_{c}$ and therefore superfluid density ${n}_{s}(T\ensuremath{\rightarrow}0)$ in our two-dimensional cuprate films. This relationship was famously predicted by a universal model of the cuprates with a doping-independent superconducting gap, but has not been observed by high field $(g{10}^{4}\phantom{\rule{0.28em}{0ex}}\mathrm{G})$ measurements of the coherence length $\ensuremath{\xi}$ because high-field cuprate phenomena obscure the simple scaling we observe at much lower fields.

We calculate the magnetic field above which vortices necessarily appear in a two-dimensional superconducting film in a nonuniform magnetic field applied near its center, as in two-coil measurements of superfluid density. Experiments suggest that free vortices appear when the Meissner screening current density is large enough that the free-energy barrier for separating tightly-bound vortex-antivortex (V-aV) pairs into free vortices is comparable to ${k}_{B}T$, not at the much lower thermodynamic critical field. Specifically, we calculate the applied magnetic field above which there exists no metastable state without vortices. There is a simple analogy with the appearance of phase-slip centers in a one-dimensional superconducting wire.

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy (MBE) and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 {\deg}C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy (AFM) shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films., Comment: 20 pages, 7 figures