arXiv subjects

W. Widdra

Publications and source records attributed to W. Widdra.

7 recordsLinked to original sources

arXiv2026

Magneto-optical evidence for single-crystal-like magnetic switching of epitaxial antiferromagnetic LaFeO3 films

Strained epitaxial films of the antiferromagnetic orthoferrite LaFeO3 offer a promising platform for antiferromagnetic spintronics, yet their magnetic switching behavior and domain structure have remained largely unexplored due to the small magnitude of the weak ferromagnetic moment. Here, we demonstrate that longitudinal magneto-optical Kerr effect (MOKE) measurements provide a sensitive and direct probe of magnetic switching and domain processes in coherently strained LaFeO3 thin films grown on orthorhombic substrates. By employing DyScO3(110), GdScO3(110), and NdGaO3(110) substrates, we achieve straincontrolled, largely twin-free growth and identify the orientation of the orthorhombic c-axis through the presence or absence of a longitudinal MOKE signal. Compressively strained films exhibit large Kerr signals, rectangular hysteresis loops, and magnetic single-domain remanence over macroscopic areas. Tensile strain on orthorhombic substrates is associated with two competing structural effects on thin film orientation; in-plane magnetization has been identified in some films on GdScO3(110) by MOKE. Angle-dependent MOKE hysteresis follows the Kondorsky model, indicating domain-wall-controlled switching analogous to bulk single crystals. Kerr microscopy reveals abrupt domain nucleation and rapid domain-wall motion, with defects acting as pinning centers and governing the coercive field. Our results establish MOKE as an efficient optical tool for identifying orthorhombic orientation, probing magnetic switching of coupled weak magnetization and Neel vectors, and accessing domain dynamics in LaFeO3 films. This provides a foundation for strain-engineered orthoferrite thin films in antiferromagnetic spintronics and magnonics.

cond-mat.mtrl-sci↗

arXiv2025

Improved imaging of magnetic domains with a photoelectron emission microscope by utilizing symmetry and momentum selection

Imaging of magnetic domains with a photoelectron emission microscope operated with photon energies in the threshold regime often suffers from low contrast. In this work we show by symmetry considerations, photoemission calculations, and imaging experiments, how the contrast can be improved significantly. The key to both domain selectivity and sizable intensity asymmetries is, guided by symmetry considerations, selecting the momenta of the photoelectrons by a properly positioned contrast aperture. By comparing computational with experimental results for an Fe(001) surface we prove the feasibility of the approach.

cond-mat.mtrl-sci↗

arXiv2021

Electron pair emission from surfaces: Some general experimental considerations

We discuss some experimental facets of electron pair emission from surfaces using two different experimental approaches. In the first case the instrument consists of a pair of hemispherical analyzers which are operated with continuous primary beams of electrons or photons. The second instrument employs a pair of time-of-flight spectrometers which require a pulsed excitation source. A key experimental quantity is the ratio of `true' to `random' coincidences which can be determined in different ways. Regardless of the type of instrument the primary flux has to adopt a much smaller value than in single electron spectroscopy. We describe different approaches to obtain the relevant count rates, in particular the concept of operating with a delayed coincidence circuit. We also address the question on how to compare the two types of spectrometer in terms of their performance.

cond-mat.mtrl-sci↗

arXiv2020

Exchange-striction driven ultrafast nonthermal lattice dynamics in NiO

We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AF) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce a Q-resolved effective lattice temperature, and identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ~0.3 ps and persists for 25 ps. We associate this with transient changes to the AF exchange striction-induced lattice distortion, supported by the observation of a transient Q-asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order.

cond-mat.str-el↗

arXiv2015

Surface phonons of NiO(001) ultrathin films grown pseudomorphically on Ag(001)

For a ultrathin NiO(001) film of 4 monolayers (ML) thickness grown on Ag(001), the vibrational properties have been determined by high-resolution electron energy loss spectroscopy (HREELS). For the well-ordered pseudomorphically grown film, nine phonon modes have been identified and their dispersions have been revealed along the Gamma-X high-symmetry direction. The comparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO (001) phonon properties are already fully developed at 4 ML. Significant differences are found for the surface-localized phonon S6 which has an increased dispersion for the ultrathin film. The dipole-active Fuchs-Kliewer phonon-polariton exhibits a narrower lineshape than the mode found for a single-crystal surface, which might hint to a reduced antiferromagnetic coupling in the ultrathin film.

cond-mat.mtrl-sci↗

arXiv2015

Image potential states of ultrathin NiO films: A time-resolved two-photon photoemission study

Well-ordered ultrathin films of NiO have been prepared on an Ag(001) substrate using molecular beam epitaxy. With the help of angle-resolved two-photon photoemission (2PPE) a series of image potential states (IPS) for film thicknesses of 2--4 monolayers (ML) has been identified. By time-resolved 2PPE, the lifetimes of the first three IPS and their dependence on the oxide film thickness have been determined. While the lifetimes of the (n=1) IPS are all in the range of 27--42 fs, the values for the (n=2) IPS decrease from 85 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms of a coupling to the layer-dependent electronic structure of the NiO ultrathin films.

cond-mat.mtrl-sci↗

arXiv2002

Adsorbate-induced Surface Stiffening: Surface Lattice Dynamics of Ru(001)-(1x1)-O

The dynamical properties of the high-density Ru(001)-(1$\times$1)-O phase has been investigated by a combined high-resolution electron energy loss spectroscopy and density functional theory study. Due to a strong static outward relaxation of the first Ru layer a soft Rayleigh phonon mode is expected. However, a Rayleigh mode stiffening together with a new high energy in-plane phonon mode above the bulk bands is found which is related to a strong adsorbate-induced intralayer force constant stiffening which counteracts an interlayer softening. This structurally rather simple system with one surface atom per (1$\times$1) unit cell demonstrates the limited applicability of previously adopted models.

cond-mat.mtrl-sci↗