Arthur McCray

All Publications

• Accelerating iterative ptychography with an integrated neural network. Journal of microscopy McCray, A. R., Ribet, S. M., Varnavides, G., Ophus, C. 2025

  Abstract

  Electron ptychography is a powerful and versatile tool for high-resolution and dose-efficient imaging. Iterative reconstruction algorithms are powerful but also computationally expensive due to their relative complexity and the many hyperparameters that must be optimised. Gradient descent-based iterative ptychography is a popular method, but it may converge slowly when reconstructing low spatial frequencies. In this work, we present a method for accelerating a gradient descent-based iterative reconstruction algorithm by training a neural network (NN) that is applied in the reconstruction loop. The NN works in Fourier space and selectively boosts low spatial frequencies, thus enabling faster convergence in a manner similar to accelerated gradient descent algorithms. We discuss the difficulties that arise when incorporating a NN into an iterative reconstruction algorithm and show how they can be overcome with iterative training. We apply our method to simulated and experimental data of gold nanoparticles on amorphous carbon and show that we can significantly speed up ptychographic reconstruction of the nanoparticles.

  View details for DOI 10.1111/jmi.13407

  View details for PubMedID 40195648

• Direct observation of twisted stacking domains in the van der Waals magnet CrI₃ NATURE COMMUNICATIONS Jang, M., Lee, S., Cantos-Prieto, F., Kosic, I., Li, Y., McCray, A. C., Jung, M., Yoon, J., Boddapati, L., Deepak, F., Jeong, H., Phatak, C. M., Santos, E. G., Navarro-Moratalla, E., Kim, K. 2024; 15 (1): 5925

  Abstract

  Van der Waals (vdW) stacking is a powerful technique to achieve desired properties in condensed matter systems through layer-by-layer crystal engineering. A remarkable example is the control over the twist angle between artificially-stacked vdW crystals, enabling the realization of unconventional phenomena in moiré structures ranging from superconductivity to strongly correlated magnetism. Here, we report the appearance of unusual 120° twisted faults in vdW magnet CrI3 crystals. In exfoliated samples, we observe vertical twisted domains with a thickness below 10 nm. The size and distribution of twisted domains strongly depend on the sample preparation methods, with as-synthesized unexfoliated samples showing tenfold thicker domains than exfoliated samples. Cooling induces changes in the relative populations among different twisting domains, rather than the previously assumed structural phase transition to the rhombohedral stacking. The stacking disorder induced by sample fabrication processes may explain the unresolved thickness-dependent magnetic coupling observed in CrI3.

  View details for DOI 10.1038/s41467-024-50314-z

  View details for Web of Science ID 001272555000016

  View details for PubMedID 39009625

  View details for PubMedCentralID PMC11251270

• Topological Spin Textures in an Insulating van der Waals Ferromagnet. Advanced materials (Deerfield Beach, Fla.) Grebenchuk, S., McKeever, C., Grzeszczyk, M., Chen, Z., Šiškins, M., McCray, A. R., Li, Y., Petford-Long, A. K., Phatak, C. M., Ruihuan, D., Zheng, L., Novoselov, K. S., Santos, E. J., Koperski, M. 2024; 36 (24): e2311949

  Abstract

  Generation and control of topological spin textures constitutes one of the most exciting challenges of modern spintronics given their potential applications in information storage technologies. Of particular interest are magnetic insulators, which due to low damping, absence of Joule heating and reduced dissipation can provide energy-efficient spin-textures platform. Here, it is demonstrated that the interplay between sample thickness, external magnetic fields, and optical excitations can generate a prolific paramount of spin textures, and their coexistence in insulating CrBr3 van der Waals (vdW) ferromagnets. Using high-resolution magnetic force microscopy and large-scale micromagnetic simulation methods, the existence of a large region in T-B phase diagram is demonstrated where different stripe domains, skyrmion crystals, and magnetic domains exist and can be intrinsically selected or transformed to each-other via a phase-switch mechanism. Lorentz transmission electron microscopy unveils the mixed chirality of the magnetic textures that are of Bloch-type at given conditions but can be further manipulated into Néel-type or hybrid-type via thickness-engineering. The topological phase transformation between the different magnetic objects can be further inspected by standard photoluminescence optical probes resolved by circular polarization indicative of an existence of exciton-skyrmion coupling mechanism. The findings identify vdW magnetic insulators as a promising framework of materials for the manipulation and generation of highly ordered skyrmion lattices relevant for device integration at the atomic level.

  View details for DOI 10.1002/adma.202311949

  View details for PubMedID 38306214

• AI-enabled Lorentz microscopy for quantitative imaging of nanoscale magnetic spin textures NPJ COMPUTATIONAL MATERIALS McCray, A. C., Zhou, T., Kandel, S., Petford-Long, A., Cherukara, M. J., Phatak, C. 2024; 10 (1)

  View details for DOI 10.1038/s41524-024-01285-8

  View details for Web of Science ID 001232866000001

• Understanding the Effect of Curvature on the Magnetization Reversal of Three-Dimensional Nanohelices. Nano letters Fullerton, J., McCray, A. R., Petford-Long, A. K., Phatak, C. 2024; 24 (8): 2481-2487

  Abstract

  Comprehending the interaction between geometry and magnetism in three-dimensional (3D) nanostructures is important to understand the fundamental physics of domain wall (DW) formation and pinning. Here, we use focused-electron-beam-induced deposition to fabricate magnetic nanohelices with increasing helical curvature with height. Using electron tomography and Lorentz transmission electron microscopy, we reconstruct the 3D structure and magnetization of the nanohelices. The surface curvature, helical curvature, and torsion of the nanohelices are then quantified from the tomographic reconstructions. Furthermore, by using the experimental 3D reconstructions as inputs for micromagnetic simulations, we can reveal the influence of surface and helical curvature on the magnetic reversal mechanism. Hence, we can directly correlate the magnetic behavior of a 3D nanohelix to its experimental structure. These results demonstrate how the control of geometry in nanohelices can be utilized in the stabilization of DWs and control of the response of the nanostructure to applied magnetic fields.

  View details for DOI 10.1021/acs.nanolett.3c04172

  View details for PubMedID 38373326

• Raman Shifts in Two-Dimensional van der Waals Magnets Reveal Magnetic Texture Evolution. Nano letters Huang, Z., McCray, A. R., Li, Y., Morrow, D. J., Qian, E. K., Young Chung, D., Kanatzidis, M. G., Phatak, C., Ma, X. 2024; 24 (5): 1531-1538

  Abstract

  Two-dimensional (2D) van der Waals magnets comprise rich physics that can be exploited for spintronic applications. We investigate the interplay between spin-phonon coupling and spin textures in a 2D van der Waals magnet by combining magneto-Raman spectroscopy with cryogenic Lorentz transmission electron microscopy. We find that when stable skyrmion bubbles are formed in the 2D magnet, a field-dependent Raman shift can be observed, and this shift is absent for the 2D magnet prepared in its ferromagnetic state. Correlating these observations with numerical simulations that take into account field-dependent magnetic textures and spin--phonon coupling in the 2D magnet, we associate the Raman shift to field-induced modulations of the skyrmion bubbles and derive the existence of inhomogeneity in the skyrmion textures over the film thickness.

  View details for DOI 10.1021/acs.nanolett.3c03923

  View details for PubMedID 38286029

• Direct Observation of Magnetic Bubble Lattices and Magnetoelastic Effects in van der Waals Cr₂Ge₂Te₆ ADVANCED FUNCTIONAL MATERIALS McCray, A. C., Li, Y., Qian, E., Li, Y., Wang, W., Huang, Z., Ma, X., Liu, Y., Chung, D., Kanatzidis, M. G., Petford-Long, A. K., Phatak, C. 2023; 33 (26)

  View details for DOI 10.1002/adfm.202214203

  View details for Web of Science ID 000961599700001

• Thermal Hysteresis and Ordering Behavior of Magnetic Skyrmion Lattices. Nano letters McCray, A. R., Li, Y., Basnet, R., Pandey, K., Hu, J., Phelan, D. P., Ma, X., Petford-Long, A. K., Phatak, C. 2022; 22 (19): 7804-7810

  Abstract

  The physics of phase transitions in two-dimensional (2D) systems underpins research in diverse fields including statistical mechanics, nanomagnetism, and soft condensed matter. However, many aspects of 2D phase transitions are still not well understood, including the effects of interparticle potential, polydispersity, and particle shape. Magnetic skyrmions are chiral spin-structure quasi-particles that form two-dimensional lattices. Here, we show, by real-space imaging using in situ cryo-Lorentz transmission electron microscopy coupled with machine learning image analysis, the ordering behavior of Néel skyrmion lattices in van der Waals Fe3GeTe2. We demonstrate a distinct change in the skyrmion size distribution during field-cooling, which leads to a loss of lattice order and an evolution of the skyrmion liquid phase. Remarkably, the lattice order is restored during field heating and demonstrates a thermal hysteresis. This behavior is explained by the skyrmion energy landscape and demonstrates the potential to control the lattice order in 2D phase transitions.

  View details for DOI 10.1021/acs.nanolett.2c02275

  View details for PubMedID 36129969

• <i>In situ </i><i>o</i>bservation of the magnetization configuration and reversal in cylindrical nanowires APL MATERIALS Brajuskovic, V., McCray, A., Zhang, Y., Phatak, C. 2022; 10 (8)

  View details for DOI 10.1063/5.0097563

  View details for Web of Science ID 000843552200002

• Field-Dependent Magnetic Domain Behavior in van der Waals Fe₃GeTe₂ JOM Li, Y., Basnet, R., Pandey, K., Hu, J., Wang, W., Ma, X., McCray, A. C., Petford-Long, A. K., Phatak, C. 2022; 74 (6): 2310-2318

  View details for DOI 10.1007/s11837-022-05299-9

  View details for Web of Science ID 000792567600001

• Geometric control of emergent antiferromagnetic order in coupled artificial spin ices CELL REPORTS PHYSICAL SCIENCE Li, Y., Barrows, F., McCray, A. C., Cote, T., Friedman, D., Divan, R. S., Petford-Long, A. K., Phatak, C. 2022; 3 (4)

  View details for DOI 10.1016/j.xcrp.2022.100846

  View details for Web of Science ID 000795793000004

• Understanding Complex Magnetic Spin Textures with Simulation-Assisted Lorentz Transmission Electron Microscopy PHYSICAL REVIEW APPLIED McCray, A. C., Cote, T., Li, Y., Petford-Long, A. K., Phatak, C. 2021; 15 (4)

  View details for DOI 10.1103/PhysRevApplied.15.044025

  View details for Web of Science ID 000651421900001

• Orientational Disorder in Epitaxially Connected Quantum Dot Solids. ACS nano McCray, A. R., Savitzky, B. H., Whitham, K., Hanrath, T., Kourkoutis, L. F. 2019; 13 (10): 11460-11468

  Abstract

  Periodic arrays of strongly coupled colloidal quantum dots (QDs) may enable unprecedented control of electronic band structure through manipulation of QD size, shape, composition, spacing, and assembly geometry. This includes the possibilities of precisely engineered bandgaps and charge carrier mobilities, as well as remarkable behaviors such as metal-insulator transitions, massless carriers, and topological states. However, experimental realization of these theoretically predicted electronic structures is presently limited by structural disorder. Here, we use aberration-corrected scanning transmission electron microscopy to precisely quantify the orientational disorder of epitaxially connected QD films. In spite of coherent atomic connectivity between nearest neighbor QDs, we find misalignment persists with a standard deviation of 1.9°, resulting in significant bending strain localized to the adjoining necks. We observe and quantify a range of out-of-plane particle orientations over thousands of QDs and correlate the in-plane and out-of-plane misalignments, finding QDs misoriented out-of-plane display a statistically greater misalignment with respect to their in-plane neighbors as well. Using the bond orientational order metric ψ4, we characterize the 4-fold symmetry and introduce a quantification of the local superlattice (SL) orientation. This enables direct comparison between local orientational order in the SL and atomic lattice (AL). We find significantly larger variations in the SL orientation and a statistically robust but locally highly variable correlation between the orientations of the two differently scaled lattices. Distinct AL and SL behaviors are observed about a grain boundary, with a sharp boundary in the AL orientations, but a more smooth transition in the SL, facilitated by lattice deformation between the neighboring grains. Coupling between the AL and SL is a fundamental driver of film growth, and these results suggest nontrivial underlying mechanics, implying that simplified models of epitaxial attachment may be insufficient to understand QD growth and disorder when oriented attachment and superlattice growth occur in concert.

  View details for DOI 10.1021/acsnano.9b04951

  View details for PubMedID 31502825