arXiv subjects

L. S. I. Veiga

Publications and source records attributed to L. S. I. Veiga.

At least 19 recordsLinked to original sources

arXiv2025

Magnetostriction as the origin of the magnetodielectric effect in La2CoMnO6

The La2CoMnO6 (LCMO) perovskite has received a lot of attention due to its near room temperature magnetodielectric effect. Despite the recent efforts, the mechanism ruling the correlation between its magnetic and dielectric properties is not yet fully understood. In order to address this issue, we conducted a detailed investigation of the coupling between the structural, electronic and magnetic properties of a polycrystalline LCMO sample. Using magnetic field-dependent x-ray powder diffraction and measurements with a capacitive dilatometer, we show that applying an external magnetic field decreases the unit cell volume, thereby modifying the octahedral distortions. Experiments involving temperature and field-dependent x-ray absorption spectroscopy at the Co-L2,3 edges provide further evidence that the spin-orbit interaction of outermost Co 3d-orbital and the field-induced enhancement of covalence effects are the key contributors to the magnetostrictive effects. From a detailed analysis using multiplet and density functional theory calculations, we propose that the field-induced modulations of the orbital hybridization and the ligand-to-metal charge transfer are responsible for the changes in the dielectric response of LCMO, thus enabling a direct coupling between magnetic, elastic and dielectric properties in this material.

cond-mat.mtrl-sci↗

arXiv2024

Near-room-temperature ferrimagnetism and half-metallicity in disordered Ca1.5La0.5MnRuO6

The electron spin polarization on half-metallic double-perovskites is usually conditioned to the ordered rock-salt arrangement of the transition-metal ions along the lattice. In this work, we investigate a polycrystalline sample of the Ca1.5La0.5MnRuO6 compound by employing x-ray powder diffraction, high resolution transmission electron microscopy, x-ray absorption and magnetic circular dichroism at the Mn-L2,3 and Ru-M2,3 edges, magnetometry, electrical transport and first principles calculations in order to show that this is a fully disordered material exhibiting near room temperature ferrimagnetism and half-metallic conductivity, with significant intergrain tunneling magnetoresistance. These unprecedented results are compared to those of archetypical ordered double-perovskites, and discussed in terms of the Mn and Ru valences and their orbital hybridization.

cond-mat.mtrl-sci↗

arXiv2024

X-ray Magnetic Circular Dichroism in Altermagnetic $α$-MnTe

Altermagnetism is a recently identified magnetic symmetry class combining characteristics of conventional collinear ferromagnets and antiferromagnets, that were regarded as mutually exclusive, and enabling phenomena and functionalities unparalleled in either of the two traditional elementary magnetic classes. In this work we use symmetry and ab initio theory to explore X-ray magnetic circular dichroism (XMCD) in the altermagnetic class. Our results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy in altermagnets. As a representative material for our XMCD study we choose $α$-MnTe with the compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated both in theory and experiment. The predicted magnitude of XMCD lies well within the resolution of existing experimental techniques.

cond-mat.mtrl-sci↗

arXiv2023

Unusual magnetism of the axion-insulator candidate Eu$_5$In$_2$Sb$_6$

Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar noncollinear magnetic structure with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

cond-mat.str-el↗

arXiv2023

The Co$^{2+}$-Ir$^{5+}$ orbital hybridization in LaCaCoIrO$_6$ double perovskite

Here, we report on the structural, electronic, and magnetic properties of a polycrystalline sample of the LaCaCoIrO$_6$ double-perovskite investigated by means of synchrotron x-ray powder diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism at the Co and Ir $L_{2,3}$ edges, magnetometry, and electrical transport. Our results indicate a configuration of nearly Co$^{2+}$/Ir$^{5+}$ configuration for the transition-metal ions, with spin canting within the Co antiferromagnetic superstructure responsible for the ferromagnetic-like behavior observed below 100 K. The highly insulating character of LaCaCoIrO$_6$ and its positive magnetoresistance further suggest that this antiferromagnetic superexchange interaction occurs through an indirect hybridization between the Co $e_g$ orbitals

cond-mat.str-el↗

arXiv2023

Strain control of a bandwidth-driven spin reorientation in Ca$_{3}$Ru$_{2}$O$_{7}$

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca$_{3}$Ru$_{2}$O$_{7}$, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a $90^{\circ}$ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca$_{3}$Ru$_{2}$O$_{7}$, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO$_{6}$ octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.

cond-mat.str-el↗

arXiv2023

The 3d and 5d electronic structures and orbital hybridization in Ba- and Ca-doped La2CoIrO6 double perovskite

Here we present a detailed investigation of the Co and Ir local electronic structures in La1.5A0.5CoIrO6 (A = Ba, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir L2,3-edges and Co K-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for A = Ca below approximately 96 K, the competing magnetic phases and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the A = Ba, on the other hand, there is no evidence of long range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds is discussed in terms of the structural distortion that alters the strength of the Co - Ir couplings, with a relevant role played by the Co 3d eg - Ir 5d j = 1/2 hybridization.

cond-mat.mtrl-sci↗

arXiv2022

Robust Kagome Electronic Structure in Topological Quantum Magnets XMn6Sn6 (X = Dy, Tb, Gd, Y)

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab-initio calculation, we systematically investigate the electronic structure of XMn6Sn6 (X = Dy, Tb, Gd, Y) family compounds. We observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and 4f electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the XMn6Sn6 family, thus providing an ideal platform for the search and investigation on new emergent phenomena in magnetic topological materials.

cond-mat.str-el↗

arXiv2022

Probing electron-phonon interactions away from the Fermi level with resonant inelastic x-ray scattering

Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon (e-ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We performed carbon $K$-edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the $π^*$ and $σ^*$ electronic states. Our high-resolution data reveals detailed structure in the multi-phonon RIXS features that directly encodes the momentum dependence of the e-ph interaction strength. We develop a Green's-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the e-ph interaction through the Brillouin zone, being concentrated at the $Γ$ and $K$ points for the $π^*$ states, while being significant at all momenta for the $σ^*$ states. Our results advance the interpretation of phonon excitations in RIXS, and extend its applicability as a probe of e-ph interactions to a new range of out-of-equilibrium situations.

cond-mat.str-el↗

arXiv2021

Readout of a antiferromagnetic spintronics systems by strong exchange coupling of Mn2Au and Permalloy

In antiferromagnetic spintronics, the read-out of the staggered magnetization or Neel vector is the key obstacle to harnessing the ultra-fast dynamics and stability of antiferromagnets for novel devices. Here, we demonstrate strong exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits Neel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows us to benefit from the well-estabished read-out methods of ferromagnets, while the essential advantages of antiferromagnetic spintronics are retained. We show one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain pattern. Conversely, alignment of the Permalloy magnetization reorients the Mn2Au Neel vector, an effect, which can be restricted to large magnetic fields by tuning the ferromagnetic layer thickness. To understand the origin of the strong coupling, we carry out high resolution electron microscopy imaging and we find that our growth yields an interface with a well-defined morphology that leads to the strong exchange coupling.

cond-mat.mtrl-sci↗

arXiv2020

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

We show how complex modulated order can spontaneously emerge when magnetic interactions compete in a metal with polar lattice distortions. Combining neutron and resonant x-ray scattering with symmetry analysis, we reveal that the spin reorientation in Ca$_3$Ru$_2$O$_7$ is mediated by a magnetic cycloid whose eccentricity evolves smoothly but rapidly with temperature. We find the cycloid to be highly sensitive to magnetic fields, which appear to continuously generate higher harmonic modulations. Our results provide a unified picture of the rich magnetic phases of this correlated, multi-band polar metal.

cond-mat.str-el↗

arXiv2020

Correlated electron metal properties of the honeycomb ruthenate Na$_2$RuO$_3$

We report the synthesis and characterisation of polycrystalline Na$_2$RuO$_3$, a layered material in which the Ru$^{4+}$ ($4d^4$ configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na$_2$RuO$_3$ ($C2/c$ phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism ($χ_0 \sim 1.42(2)\times10^{-3}$ emu/mol Oe) with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility ($χ_0$) together with the Sommerfeld coeficient of $γ=11.7(2)$ mJ/Ru mol K$^2$ estimated from heat capacity measurements, gives an enhanced Wilson ratio of $R_W\approx8.9(1)$, suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behaviour, photoemission spectrocopy indicate a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near surface and interface effects, all other probes indicate that Na$_2$RuO$_3$ is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na$_2$RuO$_3$ is an antiferromagnetic insulator at low temperatures.

cond-mat.str-el↗

arXiv2020

Colossal magnetoresistance in a nonsymmorphic antiferromagnetic insulator

Here we investigate antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$, a nonsymmorphic Zintl phase. Our electrical transport data show that Eu$_{5}$In$_{2}$Sb$_{6}$ is remarkably insulating and exhibits an exceptionally large negative magnetoresistance, which is consistent with the presence of magnetic polarons. From {\it ab initio} calculations, the paramagnetic state of Eu$_{5}$In$_{2}$Sb$_{6}$ is a topologically nontrivial semimetal within the generalized gradient approximation (GGA), whereas an insulating state with trivial topological indices is obtained using a modified Becke-Johnson potential. Notably, GGA+U calculations suggest that the antiferromagnetic phase of Eu$_{5}$In$_{2}$Sb$_{6}$ may host an axion insulating state. Our results provide important feedback for theories of topological classification and highlight the potential of realizing clean magnetic narrow-gap semiconductors in Zintl materials.

cond-mat.str-el↗

arXiv2020

Unveiling charge density wave quantum phase transitions by x-ray diffraction

We investigate the thermal-driven charge density wave (CDW) transition of two cubic superconducting intermetallic systems Lu(Pt1-xPdx)2In and (Sr1-xCax)3Ir4Sn13 by means of x-ray diffraction technique. A detailed analysis of the CDW modulation superlattice peaks as function of temperature is performed for both systems as the CDW transition temperature T_CDW is suppressed to zero by an non-thermal control parameter. Our results indicate an interesting crossover of the classical thermal-driven CDW order parameter critical exponent from a three-dimensional universality class to a mean-field tendency, as T_CDW vanishes. Such behavior might be associated with presence of quantum fluctuations which influences the classical second-order phase transition, strongly suggesting the presence of a quantum critical point (QCP) at T_CDW = 0. This also provides experimental evidence that the effective dimensionality exceeds its upper critical dimension due to a quantum phase transition.

cond-mat.str-el↗

arXiv2020

Possible quantum fluctuations in the vicinity of the quantum critical point of $\mathbf{(Sr, Ca)_3Ir_4Sn_{13}}$ revealed by high-energy X-ray diffraction study

We explore the evolution of the structural phase transition of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$, a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature $T^*$ against pressure, which extrapolates to zero at a critical pressure of $\approx 1.79(4)$ GPa. The temperature evolution of the superlattice Bragg peak in $\rm{Ca_3Ir_4Sn_{13}}$ reveals a drastic decrease of the intensity and an increase of the linewidth when $T \rightarrow 0$ K and $p \rightarrow p_c$. Such anomaly is likely associated to the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$ thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors.

cond-mat.str-el↗

arXiv2020

Possible quantum paramagnetism in compressed Sr$_2$IrO$_4$

The effect of compression on the magnetic ground state of Sr$_2$IrO$_4$ is studied with x-ray resonant techniques in the diamond anvil cell. The weak interlayer exchange coupling between square-planar 2D IrO$_2$ layers is readily modified upon compression, with a crossover between magnetic structures around 7 GPa mimicking the effect of an applied magnetic field at ambient pressure. Higher pressures drive an order-disorder magnetic phase transition with no magnetic order detected above 17-20 GPa. The persistence of strong exchange interactions between $\mathrm{J_{eff}}=1/2$ magnetic moments within the insulating IrO$_2$ layers up to at least 35 GPa points to a highly frustrated magnetic state in compressed Sr$_2$IrO$_4$ opening the door for realization of novel quantum paramagnetic phases driven by extended $5d$ orbitals with entangled spin and orbital degrees of freedom.

cond-mat.str-el↗

arXiv2019

High-resolution resonant inelastic x-ray scattering study of the electron-phonon coupling in honeycomb $α$-Li$_2$IrO$_3$

The excitations in honeycomb $α$-Li$_2$IrO$_3$ have been investigated with high-resolution resonant inelastic x-ray scattering (RIXS) at the O K edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multi-phonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross-section, which has been proposed to describe EPC in transition metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of 3d TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding 5d based systems.

cond-mat.str-el↗

arXiv2019

Selective probing of magnetic order on Tb and Ir sites in stuffed Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$ using resonant X-ray scattering

We study the magnetic structure of the "stuffed" (Tb-rich) pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$, using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir $L_3$ and Tb $M_5$ edges, which provide selective sensitivity to Ir $5d$ and Tb $4f$ magnetic moments, respectively. At the Ir $L_3$ edge, we found the onset of long-range ${\bf k}={\bf 0}$ magnetic order below $T_{N}^\text{Ir}\sim$ 71 K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of $μ_{5d}^{\text{Ir}} \approx 0.34(3)\,μ_B$ at 5 K. At the Tb $M_5$ edge, long-range ${\bf k}={\bf 0}$ magnetic order appeared below $\sim40$ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the $M_5$ edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb$_{2}$Ir$_{2}$O$_{7}$, for which $T_{N}^{\text{Ir}} =130$ K and $μ_{5d}^{\text{Ir}}=0.56\,μ_B$, indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.

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