To overcome the spatial resolution limit set by aperture-limited diffraction in traditional scanning transmission electron microscopy, microscopists have developed ptychography enabled by iterative phase retrieval algorithms and high-dynamic-range pixel array detectors. Current detector designs are limited by the data rate off chip, so a high-pixel-count detector has a proportionally lower frame rate than the few-segment detectors used for differential phase contrast (DPC) imaging. This slower acquisition speed leads to heightened vulnerability to scan noise, drift, and potential sample damage. This creates opportunities for repurposing fast segmented detectors for ptychography by trading a reduction in reciprocal space pixels for an increase in real space pixels. Here, we explore a strategy of oversampling in real space and instead apply detector pixel upsampling during the reconstruction process. We demonstrate the viability of achieving super-resolution ptychography on thin objects using only 2 x 2 detector pixels, surpassing the resolution of integrated DPC (iDPC) imaging. With optimization using simulated datasets and experiments on MoTe2/WSe2 bilayer moir & eacute; superlattices, we achieved super-resolution ptychography reconstructions under rapid acquisition conditions (37.5 pA, 1 mu s dwell time), yielding over 50% improvements in contrast and information limit compared to annular dark field and iDPC imaging on the same detectors.

Disordered iron germanium (FeGe) has recently garnered interest as a testbed for a variety of magnetic phenomena as well as for use in magnetic memory and logic applications. This is partially owing to its ability to host skyrmions and antiskyrmions—nanoscale whirlpools of magnetic moments that could serve as information carriers in spintronic devices. In particular, a tunable skyrmion–antiskyrmion system may be created through precise control of the defect landscape in B20-phase FeGe, motivating the development of methods to systematically tune disorder in this material and understand the ensuing structural properties. To this end, we investigate a route for modifying magnetic properties in FeGe. In particular, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au4+ ions, which creates a dispersion of amorphized regions that may preferentially host antiskyrmions at densities controlled by the irradiation fluence. To further tune the disorder landscape, we conduct a systematic electron diffraction study with in situ annealing, demonstrating the ability to recrystallize controllable fractions of the material at temperatures ranging from ∼150 to 250 °C. Finally, we describe the crystallization kinetics using the Johnson–Mehl–Avrami–Kolmogorov model, finding that the growth of crystalline grains is consistent with diffusion-controlled one-to-two dimensional growth with a decreasing nucleation rate.

Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic non-centrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals -- determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au$^{4+}$ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and anitskyrmions as storage bits and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.

Magnetic van der Waals heterostructures provide a unique platform to study magnetism and spintronics device concepts in the two-dimensional limit. Here, we report studies of exchange bias from the van der Waals antiferromagnet CrSBr acting on the van der Waals ferromagnet Fe3GeTe2 (FGT). The orientation of the exchange bias is along the in-plane easy axis of CrSBr, perpendicular to the out-of-plane anisotropy of the FGT, inducing a strongly tilted magnetic configuration in the FGT. Furthermore, the in-plane exchange bias provides sufficient symmetry breaking to allow deterministic spin-orbit torque switching of the FGT in CrSBr/FGT/Pt samples at zero applied magnetic field. A minimum thickness of the CrSBr greater than 10 nm is needed to provide a non-zero exchange bias at 30 K.

Cryo-EM is a powerful tool in structural biology, providing insights through techniques like single-particle analysis (SPA) and cryogenic electron tomography (cryo-ET). In thick specimens, challenges arise as an exponentially larger fraction of the transmitted electrons lose energy from inelastic scattering and can no longer be properly focused as a result of chromatic aberrations in the post-specimen optics. Rather than filtering out the inelastic scattering at the price of reducing potential signal, as is done in energy-filtered transmission electron microscopy (EFTEM), we show how a dose-efficient and unfiltered image can be rapidly obtained using tilt-corrected bright-field scanning-TEM (tcBF-STEM) data collected on a pixelated detector. Enhanced contrast and a 3-5x improvement in collection efficiency are observed for 2D images of intact bacterial cells and large organelles using tcBF-STEM compared to EFTEM for thicknesses beyond 500 nm. As a proof of concept for the technique’s performance in structural determination, we present an SPA map at a 7 Å nominal resolution for a highly symmetric virus-like particle (VLP) with 789 particles. These findings suggest applications for tcBF-STEM in cryo-EM of thicker cellular volumes where current approaches struggle. ### Competing Interest Statement The authors have declared no competing interest.

We present measurements of thermally generated transverse spin currents in the topological insulator Bi 2 Se 3 , thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi 2 Se 3 /CoFeB. We find that Bi 2 Se 3 does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is J s ∇ x T = (4.9 ± 0.9) × 10 6 ( ℏ 2 e ) A m − 2 K μ m − 1 , and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi 2 Se 3 is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi 2 Se 3 can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy.

Additional files for arXiv:2210.05636

Journal Article Achieving Super Resolution Ptychography with a Quadrant Detector Get access Xiyue S Zhang, Xiyue S Zhang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, United States Search for other works by this author on: Oxford Academic Google Scholar Zhen Chen, Zhen Chen School of Materials Science and Engineering, Tsinghua University, Beijing, China Search for other works by this author on: Oxford Academic Google Scholar Yu-Tsun Shao, Yu-Tsun Shao School of Applied and Engineering Physics, Cornell University, Ithaca, NY, United StatesMork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, United States Search for other works by this author on: Oxford Academic Google Scholar Yi Jiang, Yi Jiang Advanced Photon Source, Argonne National Laboratory, Lemont, IL, United States Search for other works by this author on: Oxford Academic Google Scholar Ariana Ray, Ariana Ray School of Applied and Engineering Physics, Cornell University, Ithaca, NY, United States Search for other works by this author on: Oxford Academic Google Scholar David A Muller David A Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, United StatesKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, United States Corresponding author: dm24@cornell.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 291–292, https://doi.org/10.1093/micmic/ozad067.134 Published: 22 July 2023

Heterostructures of two-dimensional (2D) van der Waals (vdW) magnets and topological insulators (TI) are of substantial interest as candidate materials for efficient spin-torque switching, quantum anomalous Hall effect, and chiral spin textures. However, since many of the vdW magnets have Curie temperatures below room temperature, we want to understand how materials can be modified to stabilize their magnetic ordering to higher temperatures. In this work, we utilize molecular beam epitaxy to systematically tune the Curie temperature ($T_C$) in thin film Fe$_3$GeTe$_2$/Bi$_2$Te$_3$ from bulk-like values ($\sim$220 K) to above room temperature by increasing the growth temperature from 300 $^\circ$C to 375 $^\circ$C. For samples grown at 375 $^\circ$C, cross-sectional scanning transmission electron microscopy (STEM) reveals the spontaneous formation of different Fe$_m$Ge$_n$Te$_2$ compositions (e.g. Fe$_5$Ge$_2$Te$_2$ and Fe$_7$Ge$_6$Te$_2$) as well as intercalation in the vdW gaps, which are possible origins of the enhanced Curie temperature. This observation paves the way for developing various Fe$_m$Ge$_n$Te$_2$/TI heterostructures with novel properties.

We demonstrate advantages of samples made by mechanical stacking of exfoliated van der Waals materials for controlling the topological surface state of a three-dimensional topological insulator (TI) via interaction with an adjacent magnet layer. We assemble bilayers with pristine interfaces using exfoliated flakes of the TI BiSbTeSe2 and the magnet Cr2Ge2Te6, thereby avoiding problems caused by interdiffusion that can affect interfaces made by top-down deposition methods. The samples exhibit an anomalous Hall effect (AHE) with abrupt hysteretic switching. For the first time in samples composed of a TI and a separate ferromagnetic layer, we demonstrate that the amplitude of the AHE can be tuned via gate voltage with a strong peak near the Dirac point. This is the signature expected for the AHE due to Berry curvature associated with an exchange gap induced by interaction between the topological surface state and an out-of-plane-oriented magnet.

Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe100−xTbx single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, ξDLj/t, with a record-high value of 0.036 ± 0.008 nm−1, and the damping-like torque efficiency ξDLj achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe100−xTbx layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm2. Surprisingly, for a given layer thickness, ξDLj shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices.

We report measurements demonstrating that when the Neel vector of the collinear antiferromagnet RuO2 is appropriately canted relative to the sample plane, the antiferromagnet generates a substantial out of plane damping-like torque. The measurements are in good accord with predictions that when an electric field, E is applied to the spin split band structure of RuO2 it can cause a strong transverse spin current even in the absence of spin-orbit coupling. This produces characteristic changes in all three components of the E induced torque vector as a function of the angle of E relative to the crystal axes, corresponding to a spin current with a well defined tilted spin orientation s approximately (but not exactly) parallel to the Neel vector, flowing perpendicular to both E and S. This angular dependence is the signature of an antiferromagnetic spin Hall effect with symmetries that are distinct from other mechanisms of spin-current generation reported in antiferromagnetic or ferromagnetic materials.

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. One such material is sputter-deposited B20 FeGe on Si, which has been actively investigated as the basis for low-power, high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal FeGe, probing the magnetism of sputtered B20 FeGe is more challenging because the sub-micron crystal grains add confounding contrast. We address the challenge of disentangling magnetic and grain contrast by applying 4-dimensional Lorentz scanning transmission electron microscopy using an electron microscope pixel array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic range detector becomes extremely important. Despite the small grain size in sputtered B20 FeGe on Si, using this approach we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

Journal Article Detection Limits for Imaging Chiral Magnetic Materials with 4-Dimensional Lorentz Scanning Transmission Electron Microscopy Get access Xiyue S Zhang, Xiyue S Zhang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Kayla X Nguyen, Kayla X Nguyen School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USADepartment of Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, IL, USA Search for other works by this author on: Oxford Academic Google Scholar Emrah Turgut, Emrah Turgut School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USATaiwan Semiconductor Manufacturing Company, San Jose, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Zhen Chen, Zhen Chen School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Celesta S Chang, Celesta S Chang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Yu-Tsun Shao, Yu-Tsun Shao School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Gregory D Fuchs, Gregory D Fuchs School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USAKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar David A Muller David A Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USAKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA Corresponding author: David.a.muller@cornell.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1698–1700, https://doi.org/10.1017/S1431927622006742 Published: 01 August 2022

In traditional scanning transmission electron microscopy (STEM) imaging, the spatial resolution limit is set by the diffraction limit from the probe-forming aperture. This limit can be overcome by ptychography, where we collect a 4-dimensional dataset consisting of a diffraction pattern (kx, ky) at each scan position (x, y) for phase reconstruction. With the development of iterative phase retrieval algorithms such as ePIE and high dynamical range pixel-array detectors that enable the acquisition of the full scattering distribution, images with usable information beyond the diffraction limit have been achieved [1]. One challenge for ptychography is the relatively slow acquisition speed of the 4D dataset – 0.1-1 ms/real space position compared to 0.1-10 μs for differential phase contrast (DPC) imaging with a quadrant i.e. (2x2) detector. The longer acquisition times lead to more noticeable scan noise, drift and damage. Here, we demonstrate that super-resolution ptychography is still possible with only 2 × 2 detector pixels when the real space sampling is sufficient and that the ptychography can significantly outperform the conventional DPC analysis. Smaller detectors such as segmented detectors are much faster thus we now open up the possibility of using such faster detectors for ptychography[2].

Magnetic skyrmions hold promise for spintronic devices. To explore the dynamical properties of skyrmions in devices, a nanoscale method to image spin textures in response to a stimulus is essential. Here, we apply a technique for operando electrical current pulsing of chiral magnetic devices in a Lorentz transmission electron microscope. In ferromagnetic multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI), we study the creation and annihilation of skyrmions localized by point-like pinning sites due to defects. Using a combination of experimental and micromagnetic techniques, we establish a thermal contribution for the creation and annihilation of skyrmions in our study. Our work reveals a mechanism for controlling skyrmion density, which enables an examination of skyrmion magnetic field stability as a function of density. We find that high-density skyrmion states are more stable than low-density states or isolated skyrmions resisting annihilation over a magnetic field range that increases monotonically with density.

Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fundamental studies of the spin currents that can arise from robust, intrinsic spin-orbit interaction (SOI) effects in more exotic systems including topological insulators, transition metal dichalcogenides with broken crystalline symmetry, Weyl and Dirac semimetals where gapless electronic excitations are protected by topology and symmetry. Here we experimentally study strong SOT from the topological semimetal IrO2 in (001) and (110) normal films, which exhibit distinctly different SHE strengths. Angle resolved photoemission spectroscopy studies have shown IrO2 exhibits Dirac nodal lines (DNL) in the band structure, which could enable a very high spin Hall conductivity (SHC). The (001) films exhibit exceptionally high damping like torque efficiency ranging from 0.45 at 293 K to 0.65 at 30 K which sets the lower bound of SHC that is ten times higher and of opposite sign than the theoretical prediction. We observe a substantial reduction of SHC in anisotropically strained (110) films, which suggests that the DNLs that are present in the (001) films and contribute to SHC, are disrupted and gapped due to the large anisotropic strain in (110) films, which in turn significantly lowers SHC. Very large value of SHC at room temperature of this Dirac semimetal could be very promising for the practical application.

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξDL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO2 thin films. IrO2 possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σSH. We find that the (001) films do exhibit exceptionally high ξDL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σSH to be 1.9 × 105 and 3.75 × 105 Ω-1 m-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξDL and σSH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σSH near the DNLs.

Strong damping-like spin-orbit torque (tau(DL)) has great potential for enabling ultrafast energy-efficient magnetic memories, oscillators, and logic. So far, the reported tau(DL)exerted on a thin-film magnet must result from an externally generated spin current or from an internal non-equilibrium spin polarization in non-centrosymmetric GaMnAs single crystals. Here, for the first time a very strong, unexpected tau(DL)is demonstrated from current flow within ferromagnetic single layers of chemically disordered, face-centered-cubic CoPt. It is established here that the novel tau(DL)is a bulk effect, with the strength per unit current density increasing monotonically with the CoPt thickness, and is insensitive to the presence or absence of spin sinks at the CoPt surfaces. This tau(DL)most likely arises from a net transverse spin polarization associated with a strong spin Hall effect, while there is no detectable long-range asymmetry in the material. These results broaden the scope of spin-orbitronics and provide a novel avenue for developing single-layer-based spin-torque memory, oscillator, and logic technologies.

Journal Article A Robust Basis for Grain Identification in Polycrystalline Thin Film Devices Using Cepstrum Transforms of 4D-STEM Diffraction Pattern Get access Xiyue Zhang, Xiyue Zhang Cornell University, Ithaca, New York, United States Search for other works by this author on: Oxford Academic Google Scholar Elliot Padgett, Elliot Padgett Cornell University, DC, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Lijun Zhu, Lijun Zhu Cornell University, Ithaca, New York, United States Search for other works by this author on: Oxford Academic Google Scholar Robert Buhrman, Robert Buhrman Cornell University, Ithaca, New York, United States Search for other works by this author on: Oxford Academic Google Scholar David Muller David Muller Cornell University, Ithaca, New York, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 1620–1622, https://doi.org/10.1017/S1431927620018759 Published: 01 August 2020