arXiv subjects

Sonia Francoual

Publications and source records attributed to Sonia Francoual.

18 recordsLinked to original sources

arXiv2026

Ultrasensitive strain modulation of terahertz magnons at a magnetic phase transition

Antiferromagnets typically host spin-wave (magnon) excitations in the terahertz (THz) regime, offering a promising platform for high-speed magnonic information technologies. Harnessing these excitations requires sensitive control of their spectral properties. Here we use resonant x-ray diffraction and Raman scattering to demonstrate uniaxial-strain control of the antiferromagnetic (AFM) ground state and THz magnon excitations in the layered Mott insulator Ca$_2$RuO$_4$. Although the states separated by the strain-induced phase transition differ only by the sign of the weak and partially frustrated interlayer interaction, their magnon energies differ by more than 10% (~ 0.3 THz). Our theoretical analysis explains this surprising observation by tracing the origin of both the sign reversal of the interlayer coupling and the magnon energy to the spin-orbital composition of the Ru valence electrons. The extreme strain sensitivity of the THz magnon energy near a magnetic phase transition opens up pathways towards a new generation of transition-edge magnonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Coexistence of charge density wave and field-tuned magnetic states in TmNiC$_2$

Exploring the relations between coexisting, cooperative, or competing types of ordering is a key to identify and harness the mechanisms governing the mutual interactions between them, and to utilize their combined properties. We have experimentally explored the response of the charge density wave (CDW) to various antiferromagnetic, metamagnetic, and field-aligned ferromagnetic states that constitute the magnetic phase diagram of TmNiC$_2$. The high resolution x-ray diffraction experiment employing synchrotron radiation at low temperature and high magnetic field, allowed to follow the superstructure satellite reflections, being a sensitive probe of CDW. This investigation not only reveals direct evidence that the charge density wave avoids even a partial suppression in the antiferromagnetic ground state but also proves that this state coexists, without any visible signatures of weakening, in the entire dome of the magnetically ordered phases, including the field-aligned ferromagnetic state. The calculations of the electronic and phonon structures support the experiment, revealing that the dominant contribution to the CDW transition stems from momentum-dependent electron-phonon coupling. We conclude that this mechanism prevents the CDW from vanishing, although the nesting conditions within the magnetically ordered phases deteriorate.

cond-mat.str-el↗

arXiv2025

Entropy-assisted, long-period stacking of honeycomb layers in an AlB2-type silicide

Configurational entropy can impact crystallization processes, tipping the scales between structures of nearly equal internal energy. Using alloyed single crystals of Gd2PdSi3 in the AlB2-type structure, we explore the formation of complex layer sequences made from alternating, two-dimensional triangular and honeycomb slabs. A four-period and an eight-period stacking sequence are found to be very close in internal energy, the latter being favored by entropy associated with covering the full configuration space of interlayer bonds. Possible consequences of polytype formation on magnetism in Gd2PdSi3 are discussed.

cond-mat.mtrl-sci↗

arXiv2024

Absence of bulk charge density wave order in the normal state of UTe$_2$

A spatially modulated superconducting state, known as pair density wave (PDW), is a tantalizing state of matter with unique properties. Recent scanning tunneling microscopy (STM) studies revealed that spin-triplet superconductor UTe$_2$ hosts an unprecedented spin-triplet, multi-component PDW whose three wavevectors are indistinguishable from a preceding charge-density wave (CDW) order that survives to temperatures well above the superconducting critical temperature, $T_{c}$. Whether the PDW is the mother or a subordinate order remains unsettled. Here, based on a systematic search for bulk charge order above $T_{c}$ using resonant elastic X-ray scattering (REXS), we show that the structure factor of charge order previously identified by STM is absent in the bulk within the sensitivity of REXS. Our results invite two scenarios: either the density-wave orders condense simultaneously at $T_{c}$ in the bulk, in which case PDW order is likely the mother phase, or the charge modulations are restricted to the surface.

cond-mat.str-el↗

arXiv2023

Observation of giant circular dichroism induced by electronic chirality

Chiral phases of matter, characterized by a definite handedness, abound in nature, ranging from the crystal structure of quartz to spiraling spin states in helical magnets. In $1T$-TiSe$_2$ a source of chirality has been proposed that stands apart from these classical examples as it arises from combined electronic charge and quantum orbital fluctuations. This may allow its chirality to be accessed and manipulated without imposing either structural or magnetic handedness. However, direct bulk evidence that broken inversion symmetry and chirality are intrinsic to TiSe$_2$ remains elusive. Here, employing resonant elastic scattering of x-rays, we reveal the presence of giant circular dichroism up to $\sim$ 40$\%$ at forbidden Bragg peaks that emerge at the charge and orbital ordering transition. The dichroism varies dramatically with incident energy and azimuthal angle. Comparison to calculated scattering intensities unambiguously traces its origin to bulk chiral electronic order in ${\mathrm{TiSe}}_2$ and establishes resonant elastic x-ray scattering as a sensitive probe to electronic chirality.

cond-mat.str-el↗

arXiv2023

Spectroscopic signatures and origin of a hidden order in Ba$_2$MgReO$_6$

Clarifying the underlying mechanisms that govern ordering transitions in condensed matter systems is crucial for comprehending emergent properties and phenomena. While transitions are often classified as electronically driven or lattice-driven, we present a departure from this conventional paradigm in the case of the double perovskite Ba$_2$MgReO$_6$. Leveraging resonant and non-resonant elastic x-ray scattering techniques, we unveil the simultaneous ordering of structural distortions and charge quadrupoles at a critical temperature of $T_\mathrm{q}$$\sim$33 K. Using a variety of complementary first-principles-based computational techniques, we demonstrate that while electronic interactions drive the ordering at $T_\mathrm{q}$, it is ultimately the lattice that dictates the specific ground state that emerges. Our findings highlight the crucial interplay between electronic and lattice degrees of freedom, providing a unified framework to understand and predict unconventional emergent phenomena in quantum materials.

cond-mat.str-el↗

arXiv2023

Resonant elastic X-ray scattering of antiferromagnetic superstructures in EuPtSi$_{3}$

We report resonant elastic X-ray scattering (REXS) of long-range magnetic order in EuPtSi$_{\text{3}}$, combining different scattering geometries with full linear polarization analysis to unambiguously identify magnetic scattering contributions. At low temperatures, EuPtSi$_{\text{3}}$ stabilizes type A antiferromagnetism featuring various long-wavelength modulations. For magnetic fields applied in the hard magnetic basal plane, well-defined regimes of cycloidal, conical, and fan-like superstructures may be distinguished that encompass a pocket of commensurate type A order without superstructure. For magnetic field applied along the easy axis, the phase diagram comprises the cycloidal and conical superstructures only. Highlighting the power of polarized REXS, our results reveal a combination of magnetic phases that suggest a highly unusual competition between antiferromagnetic exchange interactions with Dzyaloshinsky--Moriya spin--orbit coupling of similar strength.

cond-mat.str-el↗

arXiv2023

Coexistence of Multiple Stacking Charge Density Waves in Kagome Superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$

The recently discovered Kagome family ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) exhibits rich physical phenomena, including non-trivial topological electronic structure, giant anomalous Hall effect, charge density waves (CDW) and superconductivity. Notably, CDW in ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ is evidenced to intertwine with its superconductivity and topology, but its nature remains elusive. Here, we combine x-ray scattering experiments and density-functional theory calculations to investigate the CDWs in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and demonstrate the coexistence of 2 $\times$ 2 $\times$ 2 and 2 $\times$ 2 $\times$ 4 CDW stacking phases. Competition between these CDW phases is revealed by tracking the temperature evolution of CDW intensities, which also manifests in different transition temperatures during warming- and cooling measurements. We also identify a meta-stable quenched state of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ after fast-cooling process. Our study demonstrates the coexistence of competing CDW stacking in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, offering new insights in understanding the novel properties of this system.

cond-mat.supr-con↗

arXiv2022

Coexistence of local structural heterogeneities and long-range ferroelectricity in Pb-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

Environmentally benign (1-x)Ba(Ti$_{0.8}$Zr$_{0.2}$)O$_3$-x(Ba$_{0.7}$Ca$_{0.3}$)TiO$_3$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. \textbf{103}, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\sim$ 477 K. Temperature-dependent local structural investigations across the Zr K edge extended x-ray absorption fine structure spectroscopy reveal an anomalous collaboration between the ZrO$_{6}$ and TiO$_6$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below T$_{m}$, as well as their hierarchical arrangement, is observed by piezo-response force microscopy. Effects of the existence of the structural/polar heterogeneities below T$_{m}$ are observed also when polarizibilities of the poled and the unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits reduction of tetragonal distortion, which is consistent with earlier reports.

cond-mat.mtrl-sci↗

arXiv2021

Sublattice spin reversal and field induced $Fe^{3+}$ spin-canting across the magnetic compensation temperature in $Y_{1.5}Gd_{1.5}Fe_{5}O_{12}$ rare-earth iron garnet

In the present work $Fe^{3+}$ sublattice spin reversal and $Fe^{3+}$ spin-canting across the magnetic compensation temperature ($T_{Comp}$) are demonstrated in polycrystalline $Y_{1.5}Gd_{1.5}Fe_{5}O_{12}$ (YGdIG) by means of in-field $^{57}Fe$ M$\ddot{o}$ssbauer spectroscopy measurements. Corroborating in-field $^{57}Fe$ M$\ddot{o}$ssbauer measurements, both $Fe^{3+}$ & $Gd^{3+}$ sublattice spin reversal has also been manifested with x-ray magnetic circular dichroism (XMCD) measurement in hard x-ray region. Moreover from in-field $^{57}Fe$ M$\ddot{o}$ssbauer measurements, estimation and analysis of effective internal hyperfine field ($H_{eff}$), relative intensity of absorption lines in a sextet elucidated unambiguously the signatures of $Fe^{3+}$ spin reversal, their continuous transition and field induced spin-canting of $Fe^{3+}$ sublattices across $T_{Comp}$. Further, Fe K- (Gd $L_{3}$-) edge XMCD signal is observed to consist of additional spectral features, those are identified from $Gd^{3+}$ ($Fe^{3+}$) magnetic ordering, enabling us the extraction of both the sublattices ($Fe^{3+}$ & $Gd^{3+}$) information from a single edge analysis. The evolution of the magnetic moments as a function of temperature for both magnetic sublattices extracted either at the Fe K- or Gd $L_3$-edge agree quite well with values that are extracted from bulk magnetization data of YGdIG and YIG ($Y_{3}Fe_{5}O_{12}$). These measurements pave new avenues to investigate how the magnetic behavior of such complex system acts across the compensation point.

cond-mat.mtrl-sci↗

arXiv2021

Hidden Charge Order in an Iron Oxide Square-Lattice Compound

Since the discovery of charge disproportionation in the FeO$_2$ square-lattice compound Sr$_3$Fe$_2$O$_7$ by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained "hidden" to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO$_2$ planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty to detect them by conventional probes. We discuss implications of these findings for research on "hidden order" in other materials.

cond-mat.str-el↗

arXiv2021

Quantitative comparison of the magnetic proximity effect in Pt detected by XRMR and XMCD

X-ray resonant magnetic reflectivity (XRMR) allows for the simultaneous measurement of structural, optical and magnetooptic properties and depth profiles of a variety of thin film samples. However, a same-beamtime same-sample systematic quantitative comparison of the magnetic properties observed with XRMR and x-ray magnetic circular dichroism (XMCD) is still pending. Here, the XRMR results (Pt L$_{3}$ absorption edge) for the magnetic proximity effect in Pt deposited on the two different ferromagnetic materials Fe and Co$_{33}$Fe$_{67}$ are compared with quantitatively analyzed XMCD results. The obtained results are in very good quantitative agreement between the absorption-based (XMCD) and reflectivity-based (XRMR) techniques taking into account an ab initio calculated magnetooptic conversion factor for the XRMR analysis. Thus, it is shown that XRMR provides quantitative reliable spin depth profiles important for spintronic and spin caloritronic transport phenomena at this type of magnetic interfaces.

cond-mat.mtrl-sci↗

arXiv2020

Static magnetic proximity effects and spin Hall magnetoresistance in Pt/Y$_{3}$Fe$_{5}$O$_{12}$ and inverted Y$_{3}$Fe$_{5}$O$_{12}$/Pt bilayers

The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we compare thin film heterostructures with different order of the layer stacking and different interface properties. For standard Pt layers on Y$_{3}$Fe$_{5}$O$_{12}$ thin films, we do not detect any static magnetic polarization in Pt. These samples show an angle-dependent magnetoresistance behavior, which is consistent with the established spin Hall magnetoresistance. In contrast, for the inverted layer sequence, Y$_{3}$Fe$_{5}$O$_{12}$ thin films grown on Pt layers, Pt displays a finite induced magnetic moment comparable to that of all-metallic Pt/Fe bilayers. This magnetic moment is found to originate from finite intermixing at the Y$_{3}$Fe$_{5}$O$_{12}$/Pt interface. As a consequence, we found a complex angle-dependent magnetoresistance indicating a superposition of the spin Hall and the anisotropic magnetoresistance in these type of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution.

cond-mat.mtrl-sci↗

arXiv2020

Structurally assisted melting of excitonic correlations in 1T-TiSe2

The simultaneous condensation of electronic and structural degrees of freedom gives rise to new states of matter, including superconductivity and charge-density-wave formation. When exciting such a condensed system, it is commonly assumed that the ultrafast laser pulse disturbs primarily the electronic order, which in turn destabilizes the atomic structure. Contrary to this conception, we show here that structural destabilization of few atoms causes melting of the macroscopic ordered charge-density wave in 1T-TiSe2. Using ultrafast pump-probe non-resonant and resonant X-ray diffraction, we observe full suppression of the Se 4p orbital order and the atomic structure at excitation energies more than one order of magnitude below the suggested excitonic binding energy. Complete melting of the charge-density wave occurs 4-5 times faster than expected from a purely electronic charge-screening process, strongly suggesting a structurally assisted breakup of excitonic correlations. Our experimental data clarifies several questions on the intricate coupling between structural and electronic order in stabilizing the charge-density-wave in 1T-TiSe2. The results further show that electron-phonon-coupling can lead to different, energy dependent phase-transition pathways in condensed matter systems, opening new possibilities in the conception of non-equilibrium phenomena at the ultrafast scale.

cond-mat.str-el↗

arXiv2020

Advanced data analysis procedure for hard x-ray resonant magnetic reflectivity discussed for Pt thin film samples of various complexity

X-ray resonant magnetic reflectivity (XRMR) is a powerful method to determine the optical, structural and magnetic depth profiles of a variety of thin films. Here, we investigate samples of different complexity all measured at the Pt L$_3$ absorption edge to determine the optimal procedure for the analysis of the experimental XRMR curves, especially for nontrivial bi- and multilayer samples that include differently bonded Pt from layer to layer. The software tool ReMagX is used to fit these data and model the magnetooptic depth profiles based on a highly adaptable layer stack which is modified to be a more precise and physically consistent representation of the real multilayer system. Various fitting algorithms, iterative optimization approaches and a detailed analysis of the asymmetry ratio features as well as $χ^2$ (goodness of fit) landscapes are utilized to improve the agreement between measurements and simulations. We present a step-by-step analysis procedure tailored to the Pt thin film systems to take advantage of the excellent magnetic sensitivity and depth resolution of XRMR.

cond-mat.mtrl-sci↗

arXiv2019

Asymmetric modification of the magnetic proximity effect in Pt/Co/Pt trilayers by the insertion of a Ta buffer layer

The magnetic proximity effect in top and bottom Pt layers induced by Co in Ta/Pt/Co/Pt multilayers has been studied by interface sensitive, element specific x-ray resonant magnetic reflectivity. The asymmetry ratio for circularly polarized x-rays of left and right helicity has been measured at the Pt $L_3$ absorption edge (11567 eV) with an in-plane magnetic field ($\pm158$ mT) to verify its magnetic origin. The proximity-induced magnetic moment in the bottom Pt layer decreases with the thickness of the Ta buffer layer. Grazing incidence x-ray diffraction has been carried out to show that the Ta buffer layer induces the growth of Pt(011) rather than Pt(111) which in turn reduces the induced moment. A detailed density functional theory study shows that an adjacent Co layer induces more magnetic moment in Pt(111) than in Pt(011). The manipulation of the magnetism in Pt by the insertion of a Ta buffer layer provides a new way of controlling the magnetic proximity effect which is of huge importance in spin-transport experiments across similar kind of interfaces.

cond-mat.mtrl-sci↗

arXiv2018

Impact of magnetic moment and anisotropy of Co$_\textrm{1-x}$Fe$_\textrm{x}$ thin films on the magnetic proximity effect of Pt

We present a systematic study of the magnetic proximity effect in Pt, depending on the magnetic moment and anisotropy of adjacent metallic ferromagnets. Element-selective x-ray resonant magnetic reflectivity measurements at the Pt absorption edge (11565$\,$eV) are carried out to investigate the spin polarization of Pt in Pt/Co$_\textrm{1-x}$Fe$_\textrm{x}$ bilayers. We observe the largest magnetic moment of (0.72$\,\pm\,$0.03)$\, μ_\textrm{B}$ per spin polarized Pt atom in Pt/Co$_\textrm{33}$Fe$_\textrm{67}$, following the Slater-Pauling curve of magnetic moments in Co-Fe alloys. In general, a clear linear dependence is observed between the Pt moment and the moment of the adjacent ferromagnet. Further, we study the magnetic anisotropy of the magnetized Pt which clearly adopts the magnetic anisotropy of the ferromagnet below. This is depicted for Pt on Fe(001) and on Co$_\textrm{50}$Fe$_\textrm{50}$(001), which have a 45$^{\circ}$ relative rotation of the fourfold magnetocrystalline anisotropy.

cond-mat.mtrl-sci↗

arXiv2015

Static magnetic proximity effect in Pt/NiFe2O4 and Pt/Fe bilayers investigated by x-ray resonant magnetic reflectivity

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity was measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be $(0.6\pm0.1)\,μ_{B}$ for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of $0.02\,μ_{B}$ per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

cond-mat.mes-hall↗