arXiv subjects

Vojtěch Uhlíř

Publications and source records attributed to Vojtěch Uhlíř.

14 recordsLinked to original sources

arXiv2026

Vectorial Reconstruction of Magnetic Order Parameters using Electron Magnetic Linear Dichroism

Determining the orientation of compensated magnetic order with nanometer resolution remains a major challenge because conventional magnetic probes primarily detect net magnetization. Here, we establish electron magnetic linear dichroism as a quantitative probe of the spin axis in transmission electron microscopy. Explicit inclusion of vectorial core-level exchange splitting into mixed-dynamic-form-factor simulations accounting for dynamical diffraction enables the calculation of the momentum- and energy-resolved dichroic response for an arbitrary Néel-vector or magnetization orientation. The magnetic linear contribution can be separated from nonmagnetic anisotropy and exhibits a characteristic angular dependence that enables reconstruction of the spin axis from a finite set of momentum-resolved electron-energy-loss spectra. Simulations for the antiferromagnetic and ferromagnetic phases of cubic FeRh show that this sensitivity persists from parallel illumination to atomically confined probes and over a broad range of crystal orientations and thicknesses. Electron magnetic linear dichroism therefore provides a route toward nanoscale vector imaging of compensated magnetic order in antiferromagnets and related magnetic materials.

cond-mat.mes-hall↗

arXiv2026

Dislocation-Driven Nucleation Type Switching Across Repeated Ultrafast Magnetostructural Phase Transition

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 sub-micron 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 femtosecond and picosecond laser pulses. Our results establish a direct link between defect formation, nucleation energetics, and the microscopic morphology of the nucleated ferromagnetic phase, with broad implications for ultrafast stroboscopic experiments and defect-mediated phase transitions in functional materials.

cond-mat.mes-hall↗

arXiv2025

Sub-10 nm Quantification of Spin and Orbital Magnetic Moment Across the Metamagnetic Phase Transition in FeRh Using EMCD

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 ($m_\text{L}/m_\text{S}$) remains consistent with XMCD benchmarks for TEM probes down to approximately 6 nm, thereby establishing the validity range for reliable quantification. For nm-sized probes with higher convergence angles, we observe an enhanced $m_\text{L}/m_\text{S}$, 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.

cond-mat.mes-hall↗

arXiv2025

Ultrafast X-ray sonography reveals the spatial heterogeneity of the laser-induced magneto-structural phase transition in FeRh

Phase transitions are governed by both intrinsic and extrinsic heterogeneities, yet capturing their spatio-temporal dynamics remains a challenge. While ultrafast techniques track phase changes on femtosecond timescales, the spatial complexity and stochastic nature of the processes often remain hidden. Here, we present an experimental approach that combines well-established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and non-invasive probe. This ultrafast X-ray sonography can capture the spatio-temporal phase heterogeneity in great detail by resolving the phase-specific strain response. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh and identify the ferromagnetic phase to nucleate at the surface as narrow columnar domains of approximately $30\,\text{nm}$ diameter. Besides reconciling the diverse experimental results in the literature on FeRh, X-ray sonography offers a versatile platform for investigating a wide range of phase transitions accompanied by structural changes.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Manipulation of Spatially Confined Multiferroic Heuslers by Martensitic Microstructure Engineering

Magnetic shape-memory (MSM) Heuslers show a strong coupling between magnetic and structural characteristics, evidencing a correlation between magnetic, thermal, and mechanical properties through a magnetostructural martensitic transformation. This functional aspect makes MSM Heuslers promising for integration into smart micro/nanodevices, including sensors, energy harvesters, and actuators. Controlling the martensitic microstructure, which determines the magnetic characteristics, is among the key points for optimization of the magnetic functional properties of these materials at different length scales. Here, we report a strategy for manipulating the magnetic properties of spatially confined epitaxial Ni-Mn-Ga films grown on Cr(001)//MgO(001) by twinning configuration engineering in the low-temperature ferromagnetic (martensitic) phase. We show how the twinning configurations in the continuous films and the micropatterned structures can be switched from Y-type (showing negligible magnetic stray field) into X-type (presenting significant magnetic stray field) by a post-annealing process. Advanced characterization techniques enable us to analyze the atomic structure and the surface quality of the annealed samples and to disentangle the twin-switching phenomenon. The martensitic microstructure engineering reported in this study introduces a simple method for promoting the magnetic stray-field contribution at the surface of Ni-Mn-Ga epitaxial thin films and micropatterns initially showing a negligible magnetic stray field.

cond-mat.mtrl-sci↗

arXiv2025

Non-thermal electrons open the non-equilibrium pathway of the phase transition in FeRh

We use ultrafast x-ray diffraction to study non-equilibrium pathways of the phase transition in FeRh parametrized by the structural response. By increasing the pump-pulse duration beyond the electron-phonon coupling time, we suppress the electron-phonon non-equilibrium present upon femtosecond laser excitation but still photoexcite electrons to non-thermal states. Irrespective of the pump pulse duration, we find an optically induced nucleation of ferromagnetic domains on an $8\,\text{ps}$ timescale that starts as soon as the successively deposited energy surpasses the site-specific threshold energy. If in contrast, FeRh is only indirectly excited by diffusion of thermalized electrons from an opaque Pt cap layer, the ferromagnetic phase rises on a $50\,\text{ps}$ timescale. These findings unambiguously identify the photo-excitation of non-thermal electrons and not electron-phonon non-equilibria to enable the non-equilibrium pathway of the phase transition in FeRh.

cond-mat.mtrl-sci↗

arXiv2024

Accelerating the laser-induced phase transition in nanostructured FeRh via plasmonic absorption

By ultrafast x-ray diffraction we show that the laser-induced magnetostructural phase transition in FeRh nanoislands proceeds faster and more complete than in continuous films. We observe an intrinsic 8 ps timescale for nucleation of ferromagnetic (FM) domains in both types of samples. For the continuous film, the substrate-near regions, which are not directly exposed to light, are only slowly transformed to the FM state by domain wall motion following heat transport. In contrast, numerical modeling of the plasmonic absorption in the investigated nanostructure reveals a strong contribution near the FeRh/MgO interface. On average, the absorption is larger and more homogeneous in the nanoislands, enabling the phase transition throughout the entire volume at the intrinsic nucleation timescale.

cond-mat.mtrl-sci↗

arXiv2024

Speed limits of the laser-induced phase transition in FeRh

We use ultrafast x-ray diffraction (UXRD) and the polar time-resolved magneto-optical Kerr effect (tr-MOKE) to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of $8\,\text{ps}$ that is substantially slower than the speed of sound. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within $8\,\text{ps}$. For strong excitations we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in front- and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than $8\,\text{ps}$ after heating above the transition temperature.

cond-mat.mtrl-sci↗

arXiv2022

Preserving Metamagnetism in Self-Assembled FeRh Nanomagnets

We present self-assembly routes of metamagnetic FeRh nanoisland arrays using sputter deposition processing. Controlling the growth parameters and epitaxy allows tuning the size and shape of the FeRh nanoislands. While the phase transition between antiferromagnetic and ferromagnetic order is largely suppressed in nanoislands formed on oxide substrates via thermodynamic nucleation, we find that metamagnetism is largely preserved in nanomagnet arrays formed through solid-state dewetting. This behavior is strongly dependent on the resulting crystal faceting of the nanoislands, which is characteristic of each assembly route. Comparing the calculated surface energies for each magnetic phase of the nanoislands reveals that metamagnetism can be suppressed or allowed by specific geometrical configurations of the facets. Furthermore, we find that the spatial confinement leads to very pronounced supercooling and the absence of phase separation in the nanoislands. Finally, the supported nanomagnets are chemically etched away from the substrates to inspect the phase transition properties of self-standing nanoparticles. We demonstrate that solid-state dewetting is a feasible and scalable way to obtain supported and free-standing FeRh nanomagnets with preserved metamagnetism.

cond-mat.mtrl-sci↗

arXiv2021

Subpicosecond metamagnetic phase transition driven by non-equilibrium electron dynamics

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.

cond-mat.str-el↗

arXiv2020

Single-layer graphene on epitaxial FeRh thin films

Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(001) surface with unit vectors of graphene rotated by 30°, 15°, 11°, and 19° with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices.

cond-mat.mtrl-sci↗

arXiv2018

Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission

We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of the FeRh during the first-order phase transition identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi edge is a fingerprint of the magnetic status of the system that is independent of the long-range ferromagnetic alignment of the magnetic domains. We use this feature to follow the phase transition induced by a laser pulse in a pump-probe experiment and find that the magnetic transition occurs in less than 50 ps, and reaches its maximum in 100 ps.

cond-mat.str-el↗

arXiv2015

Stable room-temperature ferromagnetic phase at the FeRh(100) surface

Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at $\sim$400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1\%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We thus reveal that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that fully support the experimental findings.

cond-mat.mtrl-sci↗

arXiv2014

Critical phenomena of nano phase evolution in a first order transition

First order phase transitions occur discretely from one state to another, however they often display continuous behavior. To understand this nature, it is essential to probe how the emergent phase nucleates, interacts and evolves with the initial phase across the transition at microscopic scales. Here, the prototypical first-order magneto-structural transition in FeRh is used to investigate these phenomena. We find that the temperature evolution of the final phase exhibits critical behavior. Furthermore, a difference between the structure and magnetic transition temperatures reveals a novel intermediate phase created from the interface between the initial and nucleated final states. This emergent phase, characterized by its lack of spin order due to the competition between the antiferromagnetic and ferromagnetic interactions, leads to suppression of the dynamic aspect of the transition, generating a static mixed-phase-morphology. Understanding and controlling the transition process at this spatial scale is critical to optimizing functional device capabilities.

cond-mat.mes-hall↗