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The relationship between the magnetic interaction and photoinduced dynamics in antiferromagnetic perovskites is investigated in this study. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ thin films, commensurate spin ordering is accompanied by charge disproportionation, whereas SrFeO${}_{3}$ thin films show incommensurate helical antiferromagnetic spin ordering due to increased ferromagnetic coupling compared to La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$. To understand the photoinduced spin dynamics in these materials, we investigate the spin ordering through time-resolved resonant soft X-ray scattering. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$, ultrafast quenching of the magnetic ordering within 130 fs through a nonthermal process is observed, triggered by charge transfer between the Fe atoms. We compare this to the photoinduced dynamics of the helical magnetic ordering of SrFeO${}_{3}$. We find that the change in the magnetic coupling through optically induced charge transfer can offer an even more efficient channel for spin-order manipulation., 7 pages, 5 figures

Faraday discussions 237, 300 - 316 (2022). doi:10.1039/d2fd00005a, Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge-transfer insulator NiO to demonstrate that 1.5 eV sub-gap optical excitation leads to a renormalised NiO band-gap in combination with a significant reduction of the antiferromagnetic order. We employ element-specific X-ray reflectivity at the FLASH free-electron laser to demonstrate the reduction of the upper band-edge at the O 1s–2p core–valence resonance (K-edge) whereas the antiferromagnetic order is probed via X-ray magnetic linear dichroism (XMLD) at the Ni 2p–3d resonance (L$_2$-edge). Comparing the transient XMLD spectral line shape to ground-state measurements allows us to extract a spin temperature rise of 65 ± 5 K for time delays longer than 400 fs while at earlier times a non-equilibrium spin state is formed. We identify transient mid-gap states being formed during the first 200 fs accompanied by a band-gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band gap., Published by Soc., Cambridge [u.a.]

The authors measure the effects of uniaxial tensile-stress on a prototypical stripe-ordered cuprate, showing that a modest amount of stress creates large changes in the phase diagram.

We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump--probe resonant X-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering temperature of the NiMn layer allows to investigate effects induced by the magnetic order of the latter. The presence of antiferromagnetic order in NiMn speeds up the demagnetization of the ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layer., 20 pages, 6 figures

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L$_3$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state., 6 pages, 3 figures

Condensed Matter 4(3), 77 (2019). doi:10.3390/condmat4030077, In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. $\mathrm{La_{1.72}Sr_{0.28}NiO_{4}}$ is a prototypical strongly correlated perovskite showing spin stripes order. In this work we present the spatial distribution of the spin order inhomogeneity by applying micro X-ray diffraction to $\mathrm{La_{1.72}Sr_{0.28}NiO_{4}}$, mapping the spin-density-wave order below the 120 K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with negligible spin-density-wave order. The present work highlights the complex spatial nanoscale phase separation of spin stripes in nickelate perovskites and opens new perspectives of local spin order control by strain., Published by MDPI, Basel

Physical review letters 127(5), 057001 (2021). doi:10.1103/PhysRevLett.127.057001, We study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations., Published by APS, College Park, Md.

By comparing femtosecond laser pulse induced ferro- and antiferromagnetic dynamics in one and the same material - metallic dysprosium - we show both to behave fundamentally different. Antiferromagnetic order is considerably faster and much more efficiently manipulated by optical excitation than its ferromagnetic counterpart. We assign the fast and extremely efficient process in the antiferromagnet to an interatomic transfer of angular momentum within the spin system. Our findings do not only reveal this angular momentum transfer channel effective in antiferromagnets and other magnetic structures with non-parallel spin alignment, they also point out a possible route towards energy-efficient spin manipulation for magnetic devices., Comment: 3 figures, supplemental material included

Magnetic soft X-ray scattering at the Ho M5 resonance was characterized using thin films, thus reducing the effect of the strong photon absorption. Magnetic scattering factors are found to be as large as 200 r 0 at resonance, while the effective probing depth of the X-rays at the resonance maximum is of the order of 30 A at 2 θ = 25 ∘ . The huge enhancement of magnetic scattering allows one to study ultrathin Ho films, which is exploited to investigate the thickness dependence of the Neel temperature in these long-period antiferromagnets. The tunable X-ray penetration depth across the resonance can be used for magnetic depth profiling, e.g. to study the depth-dependent growth of antiferromagnetic domains across the ferromagnetic/antiferromagnetic phase transition in Dy metal films.

Systematic magnetic effects in the band structure of the heavy lanthanide metals Gd, Tb, Dy, and Ho were studied by angle-resolved photoemission from epitaxial films. The temperature-dependent exchange splitting exhibits a different behavior for delocalized and localized band states. For the delocalized Δ 2 bulk band states, a Stoner-like temperature dependence of the splitting is observed in ferromagnetic and helical antiferromagnetic phases. A more localized oxygen-induced surface state shows a temperature-independent contribution to the splitting. All observed magnetic splittings scale with the 4f spin moment.

We report very highly resolved photoemission spectra of ${\mathrm{NiS}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$ across the so-called metal-insulator transition as a function of temperature as well as composition. The present results convincingly demonstrate that the low temperature, antiferromagnetic phase is metallic, with a reduced density of states at ${E}_{F}$. This decrease is due possibly to the opening of gaps along specific directions in the Brillouin zone caused by the antiferromagnetic ordering.

The partially filled, magnetically-active d-like surface state on Tb(0001) was studied by photoemission and inverse photoemission at temperatures above as well as below the bulk Curie temperature of Tb metal. Upon cooling, the weight of the occupied surface state increases, while that of the empty surface state decreases. No temperature-dependent energy shifts of the surface states and, hence, no indication of a Stoner-like behavior of the exchange splitting were found. The results can be interpreted on the basis of a generalized theory of magnetism, which predicts a change of relative spectral weights of spin-majority and spin-minority subbands with temperature.

A collective order of spin and charge degrees of freedom into stripes has been predicted to be a possible ground state of hole-doped CuO(2) planes, which are the building blocks of high-temperature superconductors. In fact, stripe-like spin and charge order has been observed in various layered cuprate systems. For the prototypical high-temperature superconductor La(2-x)Sr(x)CuO(4), no charge-stripe signal has been found so far, but several indications for a proximity to their formation. Here we report the observation of a pronounced charge-stripe signal in the near surface region of 12-percent doped La(2-x)Sr(x)CuO(4). We conclude that this compound is sufficiently close to charge stripe formation that small perturbations or reduced dimensionality near the surface can stabilize this order. Our finding of different phases in the bulk and near the surface of La(2-x)Sr(x)CuO(4) should be relevant for the interpretation of data from surface-sensitive probes, which are widely used for La(2-x)Sr(x)CuO(4) and similar systems.

We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

We have performed a detailed high-resolution electron spectroscopic investigation of NiS$_2$ and related Se-substituted compounds NiS$_{2-x}$Se$_x$, which are known to be gapped insulators in the bulk at all temperatures. A large spectral weight at the Fermi energy of the room temperature spectrum, in conjunction with the extreme surface sensitivity of the experimental probe, however, suggests that the surface layer is metallic at 300 K. Interestingly, the evolution of the spectral function with decreasing temperature is characterized by a continuous depletion of the single-particle spectral weight at the Fermi energy and the development of a gap-like structure below a characteristic temperature, providing evidence for a metal-insulator crossover behavior at the surfaces of NiS$_2$ and of related compounds. These results provide a consistent description of the unusual transport properties observed in these systems., 12 pages, 3 figures

The thickness dependence of the helical antiferromagnetic ordering temperature ${T}_{N}$ was studied for thin Ho metal films by resonant magnetic soft x-ray and neutron diffraction. In contrast with the Curie temperature of ferromagnets, ${T}_{N}$ was found to decrease with film thickness $d$ according to $[{T}_{N}(\ensuremath{\infty})\ensuremath{-}{T}_{N}(d)]/{T}_{N}(d)\ensuremath{\propto}(d\ensuremath{-}{d}_{0}{)}^{\ensuremath{-}{\ensuremath{\lambda}}^{\ensuremath{'}}}$, where ${\ensuremath{\lambda}}^{\ensuremath{'}}$ is a phenomenological exponent and ${d}_{0}$ is of the order of the bulk magnetic period ${L}_{b}$. These observations are reproduced by mean-field calculations that suggest a linear relationship between ${d}_{0}$ and ${L}_{b}$ in long-period antiferromagnets.

We report on angle-resolved photoemission and inverse photoemission studies of Gd(0001) in the temperature range 60 to 400 K. It is found that with increasing temperature both occupied and unoccupied parts of the d-like surface state at \gG shift towards the Fermi level. This rules out a pure spin-mixing behavior with constant exchange splitting as claimed recently on the basis of spin-resolved PE data. Instead, the temperature variation of the surface-state exchange splitting is better described by a Stoner-like behavior, with an enhanced surface Curie temperature. Nevertheless, a small residual splitting above Tc of the order of 100 meV cannot be ruled out.

We report the observation and theoretical analysis of magnetic circular dichroism of low-spin Gd $4f$ photoemission (PE) lines in the Gd ${M}_{5}$ photoexcitation region. With circularly polarized x rays tuned to previously unresolved absorption lines of magnetized Gd metal, low-spin and high-spin $4f$ PE lines become nearly equally strong. The observations are in excellent agreement with a state-of-the-art atomic multiplet calculation proving that spin-flip processes in resonant PE from lanthanides occur predominantly during the photoexcitation step.