Florian Kronast

Voltage control of magnetic states in multiferroic heterostructures represents a promising route toward voltage‐programmable magnetic devices. However, it remains challenging to achieve deterministic and non‐volatile electrical control of well‐defined magnetic states without the assistance of a magnetic field. Here we demonstrate repeatable and non‐volatile voltage‐driven interconversion between giant single‐domain and multidomain magnetic states in a strain‐coupled heterostructure comprising permalloy (Py) and 0.68Pb(Mg _1/3 Nb _2/3 )O _3– 0.32PbTiO ₃ (011) _pc (PMN‐PT) (pc = pseudocubic). Growth‐induced in‐plane uniaxial magnetic anisotropy yields a square hysteresis loop, indicating a remanent monodomain state that is consistent with 15 µm‐diameter XMCD‐PEEM images. Low‐voltage biasing of the PMN‐PT substrate repeatably induces a ∼90° in‐plane rotation of the magnetic easy axis and the concomitant formation of a multidomain state. At higher voltages, the easy axis rotates back to its original orientation, restoring the monodomain state. Cycling the ferroelectric substrate through minor loops near the coercive field yields non‐volatile and repeatable switching between the multidomain and monodomain states. These results provide a pathway for voltage‐programmable engineering of magnetic single‐domain states in multiferroic heterostructures.

We report the evolution of the antiferromagnetic domain structure of epitaxial La 0.45 Sr 0.55 MnO 3 (LSMO) ultrathin films with thickness in the range from 5–50 unit cells (uc), using x-ray photoemission electron microscopy (XPEEM). While the 5 uc thick LSMO shows no magnetic contrast down to ∼ 80 K , thicker films display a multidomain antiferromagnetic configuration with a nonmonotonic variation in the characteristic domain size from 0.3 µ m at 10 uc, to 3–5 µ m at 15–16 uc and 1 µ m for 50 uc. Post-growth annealing is found not to impact strongly the magnetic domain state. In contrast, for a 15 uc film grown on a substrate characterized by large atomic steps ( ∼ 1 µ m ) , we observe much larger antiferromagnetic domains and the presence of net magnetic moment in the form of stripes with alternating contrast, assigned to the signal from the top uncompensated spins of the A-type antiferromagnetic state of LSMO. From the combined antiferromagnetic domain structure and the net magnetic moment contrast, we determine the exact orientation of the Néel vector, including at domain walls. We describe the antiferromagnetic domain size distribution in terms of interface and bulk contributions to the density of defects that pin the antiferromagnetic domain walls and which determine the equilibrium domain configuration. Our results demonstrate the impact of thickness and defects on the antiferromagnetic domain size and constitute a stepping stone in controlling the antiferromagnetic domain state required for oxide antiferromagnetic device applications.

Crystal structure engineering is pivotal for tailoring material properties, particularly in atomically thin two-dimensional (2D) transition-metal chalcogenides that exhibit rich, crystal-phase-dependent electronic, optical, and magnetic properties. However, their crystal-phase-controlled synthesis remains a critical challenge that limits their practical applications. Here, we report the phase-controlled synthesis of atomically thin chromium telluride films, including CrTe₃, CrTe₂, Cr_1+δTe₂, and CrTe, on graphene/SiC substrates via precise tuning of molecular beam epitaxy parameters. The growth mechanism is further revealed. We have systematically characterized the different structures and electronic, chemical, and magnetic properties of these films using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, density functional theory calculations, X-ray photoelectron/absorption spectroscopy, and circular-polarized X-ray photoemission electron microscopy. Heterostructures of different chromium telluride phases are further fabricated, demonstrating band engineering enabled by phase control of chromium tellurides.

Recent predictions of orders of magnitude larger orbital current effects compared with spin currents have attracted considerable interest. However, orbital currents must first be converted into spin currents to interact with the static magnetization dominated by spin angular momentum in conventional magnets. By using a magnet dominated by orbital angular momentum (OAM), we demonstrate a 70-fold enhancement in orbital Hall magnetoresistance in cobalt II oxide/copper (CoO/Cu*), compared with spin Hall magnetoresistance in cobalt II oxide/platinum (CoO/Pt). This arises from interactions between dynamic OAM from surface-oxidized Cu* and static OAM in the antiferromagnetic insulator CoO. Our results show how by using OAM-dominated materials, we can harness the benefits of giant orbital currents that have not been possible using conventional spin-dominated magnets.

Recent advances in additive nanofabrication have enabled the realization of free‐standing three‐dimensional architectures with precisely controlled geometry and material composition at the nanoscale. Here, we investigate the magnetic properties of 3D PtC/ core–shell heterostructures with engineered curvature and shell thickness profiles. Using focused electron‐beam‐induced deposition in combination with temperature‐induced site‐selective chemical vapor deposition, we realize hollow magnetic nanoshells with different cross‐section, curvature and thickness profiles with outer dimensions down to 200 nm. Using X‐ray magnetic circular dichroism photoelectron emission microscopy (XMCD‐PEEM) and full‐scale micromagnetic simulations, we reveal distinct curvature‐ and thickness‐dependent magnetization reversal mechanisms with the formation of axially symmetric Néel domain walls (DWs) separating vortex domains with opposite circulations. We quantify the role of uncompensated magnetostatic charges in curved hollow geometries and show how curvature and thickness asymmetry shape the micromagnetic energy landscape, DW pinning and the corresponding stray‐field distributions.

The tremendous interest in the technology and underlying physics of all-optical switching of magnetization brings up the question of how fast the switching can occur and how high the frequency of writing the data with ultrafast laser pulses can be. To answer this question, we excited a GdFe ferrimagnetic alloy, the magnetization of which can be reversed by single laser pulses, a phenomenon known as toggle switching, by two pulses with a certain time delay in between. Using photoemission electron microscopy and Kerr microscopy for magnetic domain imaging, we explore the effects of varying fluences of the first and second pulse as well as the time delay between the two pulses. Our results show that when the fluence of the first pulse is adjusted just above the threshold of single-pulse switching, a second pulse with about 60 % of the fluence of the first pulse, arriving only 3 p s later, switches the magnetization back. This reswitching persists up to about 40 p s pulse separation. We interpret the latter as the time required for the sample to cool down and remagnetize after the first pulse. For shorter time delays below about 2 p s , no reswitching occurs. However, the effect of the two pulses adds up, enabling switching for fluences of both pulses below the threshold for single-pulse switching. Atomistic spin dynamics simulations are used to model the experimental data, successfully confirming our results.

Magnetic domain formation in two-dimensional materials offers insight into the fundamentals of magnetism and serves as a catalyst for the advancement of spintronics. In order to propel these developments, it is crucial to acquire an understanding of the evolution of magnetic ordering at the nanometer scale. In particular, two-dimensional magnetic insulators allow for the realization of atomically sharp magnetoresistive tunneling junctions with nonmagnetic electrodes, therefore lifting one of the major constraints for the realization of computing in memory based on magnetoresistive elements. In this study, we visualize magnetic ordering in monolayers of annite, a fully air-stable layered magnetic mica. Using a nanometer-scale scanning superconducting quantum interference device microscopy, we directly observe domain formation in this representative of two-dimensional magnetic phyllosilicates.

Water under 2D confinement exhibits unique structural and dynamic behaviors distinct from bulk water, including phase transitions and altered hydrogen-bonding networks, making it of great scientific interest. While confinement in 2D materials like graphene, mica, or hexagonal boron nitride has been reported, their lack of intrinsic hydrophilicity or metallic conductivity limits their suitability for probing the interplay between confined water and electronic transport. MXenes, a family of 2D transition metal carbides and nitrides, overcome these limitations by combining high metallic conductivity (~10⁴ S cm⁻¹) with hydrophilicity, offering a unique platform to investigate confined water dynamics and their influence on electronic properties. Here, we show that temperature and confinement drive structural transitions of water within MXene interlayers, including the formation of localized ice clusters, amorphous ice, and dynamic hydrogen-bonded networks. These transformations disrupt stacking order, inducing a reversible metal-to-semiconductor transition and conductivity hysteresis in MXene films. Upon heating to 340 K, the dissociation of ice clusters restores interlayer spacing and metallic behavior. Our findings experimentally establish MXenes as an exceptional platform for studying the phase change of confined water, offering new insights into how nanoscale water dynamics modulate electronic properties and enabling the design of advanced devices with tunable interlayer interactions.

The tremendous interest in the technology and underlying physics of all-optical switching of magnetization brings up the question of how fast the switching can occur and how high the frequency of writing the data with ultrafast laser pulses can be. To answer this question, we excited a GdFe ferrimagnetic alloy, the magnetization of which can be reversed by single laser pulses, a phenomenon known as toggle switching, by two pulses with a certain time delay in between. Using photoemission electron microscopy and Kerr microscopy for magnetic domain imaging, we explore the effects of varying fluences of the first and second pulse as well as the time delay between the two pulses. Our results show that when the fluence of the first pulse is adjusted just above the threshold of single-pulse switching, a second pulse with about 60% of the fluence of the first pulse, arriving only 3 ps later, switches the magnetization back. This reswitching persists up to about 40 ps pulse separation. We interpret the latter as the time required for the sample to cool down and remagnetize after the first pulse. For shorter time delays below about 2 ps, no re-switching occurs. However, the effect of the two pulses adds up, enabling switching for fluences of both pulses below the threshold for single-pulse switching. Atomistic spin dynamics simulations are used to model the experimental data, successfully confirming our results.

Artificial magnetic domains are induced into a soft-magnetic in-plane Co 60 Fe 20 B 19 Si 1 (CoFeBSi) alloy by interlayer coupling to a perpendicular-to-plane FePt underlayer. We present images documenting the thickness dependence of the size of magnetic domains inside the CoFeBSi ferromagnetic layer. The magnetic domain walls in FePt induce a continuous progression of domain walls into the CoFeBSi at the vicinity of the interface to FePt. With femtosecond laser pulses, the recurrent domain pattern in CoFeBSi has been investigated after reversing the magnetization and we observe a reorganization into the thickness-dependent multidomain state inside the CoFeBSi with strong domain-wall pinning at the interface.