Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii–Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here, we report that a cubic chiral insulator Co5TeO8 provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7 to 10 nm embedded in a complex phase diagram spanning 8 distinct phases. Capacitance anomalies throughout the phase diagram indicate ME coupling, pointing to the possibility of future E-field control of these textures. The temperature and field dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, ab initio calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co5TeO8 thus offers an experimental realization of sub-10-nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.

Electron magnetic circular dichroism (EMCD) in transmission electron microscopy (TEM) enables element-specific measurement of spin and orbital magnetic moments, analogous to x-ray magnetic circular dichroism (XMCD). While the EMCD technique offers unmatched spatial resolution, its quantitative accuracy remains under scrutiny, particularly in beam-splitter geometries with convergent probes. Here, we systematically evaluate the limits of quantitative EMCD analysis using the first-order magnetostructural transition in the functional phase-change material FeRh as a tunable magnetic reference. Unlike previous EMCD studies primarily focused on elemental ferromagnets such as Fe, we demonstrate its applicability to a correlated material exhibiting coupled structural and magnetic order. We demonstrate that the extracted orbital-to-spin moment ratio (mL/mS) remains within the same order of magnitude as XMCD benchmarks, despite a systematic reduction in absolute value, for TEM probes down to approximately 6 nm, thereby establishing the validity range for reliable quantification. For nanometer-sized probes with higher convergence angles, we observe an enhanced mL/mS, which we attribute to a combination of instrumental factors and sensitivity to nanoscale heterogeneity within the probed volume. Our results confirm that EMCD provides quantitative agreement with macroscale techniques under suitable conditions, while uniquely enabling spatially confined measurements of local magnetic moments in functional magnetic materials, and allowing the study of interfacial, defect-mediated, or phase-separated magnetism that is inaccessible to photon-based methods.

To date, controlling the steady-state electronic band structure in high-Tc cuprate superconductors has been achieved primarily through chemical doping or magnetic fields. Here, we present that ultrafast optical excitation can instead drive the electronic band structure of Bi2Sr2CaCu2O8+δ into a photostationary, long-lived excited state. At sufficiently high pump fluences, this state undergoes a Lifshitz transition of the Fermi surface, characterized by a change in topology from hole-like to electron-like. Time- and angle-resolved photoemission spectroscopy, supported by single-band tight-binding calculations, reveals that 1.6 eV photoexcitation induces band-structure evolutions closely analogous to those produced by chemical doping. These results point to an efficient photodoping mechanism involving cooperative effects, including charge transfer, renormalization of effective electronic correlations, and defect-assisted charge trapping. Our findings raise fundamental questions regarding thermalization processes occurring on timescales comparable to the laser repetition period in cuprates. More broadly, ultrafast optical control enables access to otherwise inaccessible regions of the phase diagram by tuning the pump fluence.

Controlling magnetic order on ultrafast timescales, driven by spintronic and recording applications, is one of the main directions of current research in magnetism. Despite major advances in understanding the temporal evolution of magnetic order upon its emergence or quenching, experimental demonstration of the local link between microstructure and dynamic nucleation is missing. Here, taking advantage of the high structural and magnetic resolution of in situ transmission electron microscopy, we observe that cumulative laser irradiation significantly alters the nucleation pathway of the first-order antiferromagnetic to ferromagnetic phase transition of FeRh thin films, causing the transition to switch from homogeneous to heterogeneous nucleation. This leads to a decrease of 20 K in transition temperature and the emergence of submicron magnetic vortices as preferential nucleation motifs. These vortices are pinned in the film by underlying dislocation networks. We observe that the dislocation networks are formed and rearranged upon repeated crossing of the phase transition using both femtosecond and picosecond laser pulses. Our results establish a direct link between defect formation and the microscopic morphology of the nucleated ferromagnetic phase, with broad implications for ultrafast stroboscopic experiments and defect-mediated phase transitions in functional materials.

Murunskite (K 2 FeCu 3 S 4 ) bridges the two known families of high‐temperature superconductors, cuprates and iron‐pnictides, structurally and electronically. Like these families, murunskite exhibits an antiferromagnetic (AF)‐like response with an ordered phase below 97 K. The magnetic iron atoms are randomly distributed over one‐quarter of the sites in two‐dimensional planes, while the remaining sites are occupied by non‐magnetic copper, evoking the notion of a high‐entropy magnetic alloy. This intriguing magnetic transition is studied by neutron, Mössbauer, and X‐ray photoelectron spectroscopy (XPS) measurements on single crystals. The AF order has a nearly commensurate quarterzone wave vector. In the paramagnetic state, Mössbauer spectroscopy identifies two iron sites, associated with Fe 3 + or Fe 2 + oxidation states as observed by XPS, which merge into a third site upon cooling, indicating an orbital transition. This cascade of local transitions transforms iron atoms from a fully orbitally and magnetically disordered state to a homogeneously ordered state in inverse space, while still being randomly distributed in real space. This finding challenges the traditional paradigm of magnetism in insulators, which relies on a direct connection between crystal structure and the location of magnetic moments.

Spatial and temporal light modulation is a well-established technology that enables dynamic shaping of the phase and amplitude of optical fields, significantly enhancing the resolution and sensitivity of imaging methods. Translating this capability to electron beams is highly desirable within the framework of a transmission electron microscope (TEM) to benefit from the nanometer spatial resolution of this instrument. In this work, we report on the experimental realization of a photonic-based free-electron modulator integrated into the column of two ultrafast TEMs for presample electron-beam shaping. Electron-photon interaction is employed to coherently modulate both the transverse and longitudinal components of the electron wave function (through lateral phase imprinting and temporal profiling, respectively), while leveraging dynamically controlled optical fields and tailored designs of the electron-laser-sample interaction geometry. Using energy- and momentum-resolved electron detection, we successfully reconstruct the shaped electron wave function at the TEM sample plane. These results demonstrate the ability to manipulate the electron wave function before probing the sample, paving the way for photonics-inspired imaging and spectroscopy techniques in ultrafast electron microscopy.

We demonstrate a novel shot-to-shot acquisition method for optical pump—keV electron energy probe in ultrafast scattering experiments. We integrate a phase-locked acquisition scheme at a repetition rate of 20 kHz in a conventional ultrafast electron diffraction setup. We proceed to a full characterization of the noise level in different configurations and for realistic scenarios. The shot-to-shot acquisition improves the signal-to-noise ratio by one order of magnitude and can be readily implemented in other high-repetition-rate electron diffraction and spectroscopy setups.

Frequency metrology lies at the heart of precision measurement. Optical frequency combs provide a coherent link uniting the microwave and optical domains in the electromagnetic spectrum, with profound implications in timekeeping, sensing and spectroscopy, fundamental physics tests, exoplanet search, and light detection and ranging. Here, we extend this frequency link to free electrons by coherent modulation of the electron phase by a continuous-wave laser locked to a fully stabilized optical frequency comb. Microwave frequency standards are transferred to the optical domain via the frequency comb, and are further imprinted in the electron spectrum by optically modulating the electron phase with a photonic chip-based microresonator. As a proof-of-concept demonstration, we apply this frequency link in the calibration of an electron spectrometer, and use the electron spectrum to measure the optical frequency. Our work bridges frequency domains differed by a factor of $\sim10^{13}$ and carried by different physical objects, establishes a spectroscopic connection between electromagnetic waves and free-electron matter waves, and has direct ramifications in ultrahigh-precision electron spectroscopy.

Scattering between charges and collective modes in materials governs phenomena such as electrical resistance, energy dissipation, and phase switching. Studying such scattering requires simultaneous access to ultrafast and momentum-resolved dynamics of single-particle and collective excitations, which remains an experimental challenge. Here, we present time- and momentum-resolved electron energy loss spectroscopy, and we apply it to graphite, demonstrating that large (Δ q ≃1.2 Å −1 ) photoexcited electron-hole pockets induce a renormalization of in-plane and bulk plasmons. This effect is explained by intra- and intervalley scattering processes mediated by E 2g and A ′ 1 phonon modes, which we directly observe via ultrafast electron diffraction and identify via ab initio calculations. Conversely, smaller electron-hole pockets (Δ q ≃0.7 Å −1 ) result in the renormalization of in-plane plasmons, which can only be partially explained by phonon-mediated scattering and thermal expansion. Our results highlight the importance of combining momentum- and time-resolved information to elucidate electronic scattering processes.

Recent advances in lasers and electron optics technology have allowed transmission electron microscopes to achieve high spatial and temporal resolution, making them capable of tracking atoms, charges and spin motions down to the attosecond and nanometre scales. This Primer discusses the most common and practical experimental implementation of time-resolved transmission electron microscopy and the stroboscopic mode for evaluating ultrafast reversible dynamics. An in-depth discussion of photo-induced near-field electron microscopy, a technique unique to laser-assisted electron microscopy, is also provided, covering its prospective applications in the study of coherent phenomena in quantum materials. The experimental strategies and limitations in investigating the structural dynamics of materials and nanostructures by imaging, diffraction and spectroscopy are also described in detail, with a direct comparison with more conventional and established techniques. We provide key information for new researchers who intend to use ultrafast transmission electron microscopy to address new challenges in specific materials science, condensed matter and nanophotonics. Laser-driven ultrafast tranmission electron microscopy (UTEM) approaches such as stroboscopic UTEM enable the study of ultrafast reversible processes at time resolutions at the femtosecond scale and beyond. This Primer focuses on stroboscopic UTEM, describing its experimental set-up and variants, and covers the various applications of this technique in condensed matter physics, including imaging structural dynamics, photo-induced near-field electron microscopy, attosecond-scale imaging, dark-field imaging and beyond.

Scattering between individual charges and collective modes in materials governs fundamental phenomena such as electrical resistance, energy dissipation, switching between different phases, and ordering. The study of such scattering requires a simultaneous access to the ultrafast momentum-resolved dynamics of single-particle and collective excitations, which remains as an experimental challenge. Here, we demonstrate time- and momentum-resolved electron energy-loss spectroscopy, and apply it to graphite showing that large (Δ q≃1.2 Å^-1) photoexcited electron-hole (e-h) pockets in the band structure induce a renormalization of the collective in-plane and bulk plasmons that can be described quantitatively by invoking intra- and inter-valley scattering processes mediated by E_2g and A_1' phonon modes, which we directly observe by ultrafast electron diffraction and identify via ab initio calculations. Conversely, the photoexcitation of smaller e-h pockets (Δ q≃0.7 Å^-1) close to the K point of graphite results in the renormalization of in-plane plasmons, which can only be partially explained by phonon-mediated scattering and thermal expansion. Our results show the importance of combining momentum- and time-resolved information to elucidate microscopic details associated with electronic scattering processes.

The time- and angle-resolved photoemission spectroscopy (trARPES) allows for direct mapping of the electronic band structure and its dynamic response on femtosecond timescales. Here, we present a new ARPES system, powered by a new fiber-based femtosecond light source in the vacuum ultraviolet range, accessing the complete first Brillouin zone for most materials. We present trARPES data on Au(111), polycrystalline Au, Bi2Se3, and TaTe2, demonstrating an energy resolution of 21 meV with a time resolution of <360 fs, at a high repetition rate of 1 MHz. The system is integrated with an extreme ultraviolet high harmonic generation beamline, enabling an excellent tunability of the time-bandwidth resolution.

Phase transitions occurring in nonequilibrium conditions can evolve through high-energy intermediate states inaccessible via equilibrium adiabatic conditions. Because of the subtle nature of such hidden phases, their direct observation is extremely challenging and requires simultaneous visualization of matter at subpicoseconds and subpicometer scales. Here, we show that a magnetite crystal in the vicinity of its metal-to-insulator transition evolves through different hidden states when controlled via energy-tuned ultrashort laser pulses. By directly monitoring magnetite’s crystal structure with ultrafast electron diffraction, we found that upon near-infrared (800 nm) excitation, the trimeron charge/orbital ordering pattern is destroyed in favor of a phase-separated state made of cubic-metallic and monoclinic-insulating regions. On the contrary, visible light (400 nm) activates a photodoping charge transfer process that further promotes the long-range order of the trimerons by stabilizing the charge density wave fluctuations, leading to the reinforcement of the monoclinic insulating phase. Our results demonstrate that magnetite’s structure can evolve through completely different metastable hidden phases that can be reached long after the initial excitation has relaxed, breaking ground for a protocol to control emergent properties of matter.

We establish a frequency link across the microwave, optical, and free-electron domains by a photonic chip-based electron phase modulator driven by a continuous-wave laser locked to a fully stabilized optical frequency comb.