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DOE OSTI2022

Spin-orbit coupling induced demagnetization in Ni: Ab initio nonadiabatic molecular dynamics perspective

Spin-orbit coupling (SOC), which can induce spin flip during the relaxation of photoexcited charge carrier, plays a crucial role in spin dynamics. Here in this work, we have used time-domain ab initio nonadiabatic molecular dynamics (NAMD) method to study the SOC induced ultrafast demagnetization in Ni at 300K. The spin-diabatic representation using spin-polarized Kohn-Sham (KS) basis sets and spin-adiabatic representation using spinor basis sets have been applied, and both of them achieve demagnetization in Ni with a timescale around 100fs. The spin-diabatic representation suggests a picture that the electron-phonon coupling (EPC) provides direct energy relaxation channel among the same-spin states, while the SOC can induce spin flip. After photoexcitation, it is found the spin-minority electrons relax to the same-spin states rather than the opposite-spin states, since EPC is larger than SOC by one order of magnitude. By contrast, for the spin-majority electrons, spin flip occurs since there are no empty same-spin states as electron acceptor above the Fermi level. The different relaxation pathways for spin-majority and spin-minority electrons induce the demagnetization. The spin-adiabatic representation provides an Elliott-Yafet spin-phonon scattering picture. The SOC induced reduction of magnetic moment in Ni may induce magnon to drive further demagnetization. The ab initio NAMD simulation provides a critical angle to understand how the SOC and EPC affect demagnetization process in Ni.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Anomalous Hall effect in conical helimagnetic crystals

Spin-spiral texture can substantially change charge transport properties in helimagnets. Here we find the anomalous Hall effect (AHE) exhibiting the dramatic behavior with respect to chemical potential μ in conical magnetic structures. The direct conductivity demonstrates kinks, and the anomalous Hall current exhibits minima and maxima changing the sign. We analytically derive the expression for energy bands and eigenstates in the most general case. Because of the conical potential, the energy bands are split into two nonparabolic bands where the lower band can have one- or two-minima shapes in the k z direction ($\hat{z}$ is a direction of the spiral axis). We prove that the origin of the anomalous Hall effect is not topological and is due to the interplay between the asymmetry of energy bands in the x and z directions and spin restrictions in the phase space due to the conical potential. We also investigate the dependence of transport properties on cone half- angle θ, and find that the effects are most pronounced at θ=π/2 (a helical state). Electric current is calculated using the Boltzmann equation where the relaxation is caused by electron-acoustic phonon interaction. The transition probability is found to be a 2×2 matrix with nonvanishing off-diagonal elements indicating the strong interband transitions. The origin of interband transitions is because of the nature of the conical potential where conduction electron spins interact with localized magnetic moments. To verify the proposed theory, we calculate the temperature dependence of resistivity for MnSi crystals and find the discontinuity at the phase transition between conical and paramagnetic phases. The calculations are in the excellent agreement with the experimental data. In addition, we predict the discontinuity behavior for the anomalous Hall resistivity at the phase transition where the resistivity exhibits the abrupt change at T=T C , (a) to zero if the relativistic effects for the conduction electrons are small or (b) to a nonzero value if Rashba/Dresselhaus effects are taken into account.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Loop-current charge density wave driven by long-range Coulomb repulsion on the kagomé lattice

Recent experiments on vanadium-based nonmagnetic kagomé metals A⁢V 3 ⁢Sb 5 ⁢(A=K,Rb,Cs) revealed evidence for possible spontaneous time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered state. The long-sought-after quantum order of loop currents has been suggested as a candidate for the TRS breaking state. However, a microscopic model for the emergence of the loop-current CDW due to electronic correlations is still lacking. Here, in this work, we calculate the susceptibility of the real and imaginary bond orders on the kagomé lattice near van Hove filling, and reveal the importance of next-nearest-neighbor Coulomb repulsion V 2 in triggering the instability toward imaginary bond ordered CDW. The concrete effective single-orbital t–V 1 –V 2 model on the kagomé lattice is then studied, where t and V 1 are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram in the plane spanned by V 1 and V 2 at van Hove filling. The region dominated by V 1 is occupied by a 2⁢a 0 ×2⁢a 0 real CDW insulator with the inverse of Star-of-David (ISD) bond configuration. Increasing V 2 indeed drives a first-order transition from ISD to stabilized loop-current insulators that exhibit four possible current patterns of different topological properties, leading to orbital Chern insulators. We then extend these results away from van Hove filling and show that electron doping helps the stabilization of loop currents, and gives rise to doped orbital Chern insulators with emergent Chern Fermi pockets carrying large Berry curvature and orbital magnetic moment. Our findings provide a concrete model realization of the loop-current Chern metal at the mean-field level for the TRS breaking normal state of the kagomé superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Introduction of localized spin-state transitions in the optical absorption spectrum of Cr-doped GaN

Using a hybrid density functional, we study the electronic, magnetic, and neutral excitations of the paradigmatic Cr-doped GaN solid. Our results indicate that the ≈1% doped wurtzite GaN crystal retains the semiconducting nature of the host, associated with a magnetic moment of 3 μ B . Further, as a consequence of Cr doping, additional hybridized bands are formed within the intrinsic band gap of GaN, leading to a considerable band gap narrowing by 1.4 eV and low-energy optical transitions. Our results indicate dark Cr d-d transitions at very low energy (0.5–0.7 eV), followed by bright transitions in the visible energy range (1.8–2.2 eV). Characterization of the electron-hole pairs suggests that the latter originates from the internal transitions among hybridized Cr d –N p states. With the existence of these additional optical features within the visible energy window, one can expect an enhancement to the photoelectric conversion efficiency of GaN upon Cr doping, in addition to applications in spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Origin of magnetic ordering in half-Heusler RuMnGa

The half-Heusler alloy RuMnGa having valence electron count (VEC) 18 has recently been theoretically proposed to exhibit compensated ferrimagnetic (CFiM) character instead of the expected nonmagnetic ground state. On the other hand, a preliminary experimental study proposed ferromagnetic (FM) ordering. As no half-Heusler system with VEC 18 is known to exhibit magnetic ordering, we have investigated the details of crystal structure and magnetic properties of RuMnGa using a combination of experimental tools, viz., x-ray and neutron diffraction techniques, dc and ac susceptibility, isothermal magnetization, heat capacity, resistivity and neutron depolarisation measurements. Rietveld refinements of x-ray and neutron diffraction data suggest single phase nature of the compound with elemental composition RuMn 0.86 Ga 1.14 . We have shown that the system exhibits FM-type ordering owing to the inherent presence of this minor off-stoichiometry, showing very low magnetic moment. Further, the system also exhibits reentrant canonical spin-glass behavior, which is rarely observed in half-Heusler alloys. The temperature coefficient of resistivity changes its sign from negative to positive and further to negative as the temperature decreases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Assessment of exchange-correlation functionals on oxygen vacancy formation energies of metal oxides

Oxygen vacancies are a common defect in oxides and play vital roles in many technological applications such as oxygen separation, catalytic reactors, solid oxide fuel cells, and solar thermochemical water splitting. The oxygen vacancy formation energy is directly related to the oxide reduction enthalpy and the vacancy concentration and, therefore, is a key quantity underlying these applications. The complexity of measuring oxygen vacancy formation energies experimentally suggests the utility of calculations based on density functional theory (DFT). However, the calculated results are strongly dependent on the exchange-correlation functionals and other parameters such as the Hubbard U . In this work, we compare the performance of the meta-GGA strongly constrained and appropriately normed (SCAN) functional and the commonly used semilocal generalized gradient approximation (GGA) functionals with experimental values of structural parameters, band gaps, magnetic structures, and oxygen vacancy formation energies for six representative oxides, i.e., CaMnO 3 , SrMnO 3 , LaMnO 3 , YMnO 3 , LaFeO 3 , and CeO 2 . Further, our results show that SCAN usually has better agreement with the experimental lattice constants and band gaps and larger magnetic moments than the commonly used GGA functionals. Although SCAN overestimates the oxygen vacancy formation energies of transition metal oxides and therefore requires unusually large Hubbard U values to reproduce the experimental reduction enthalpies, it does predict the correct oxygen vacancy formation energy of CeO 2 , which challenges the commonly used GGA and hybrid functionals. Our results underscore the challenges that exist in describing these complex oxides by DFT and may shed light on developing more accurate exchange-correlation functionals for oxygen vacancy formation energy calculations.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet

Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet YTiO 3 , with spin-spin exchange disorder introduced via La substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functional theory calculations predict minimal changes in exchange constants as a result of average structural changes due to La substitution, in agreement with the data. The absence of significant changes in the spin-wave dispersion, the considerable lifetime effect, and the reduced ordered magnetic moment previously observed in the La-substituted system are qualitatively captured by an isotropic, nearest-neighbor, three-dimensional Furthermore, Heisenberg ferromagnet model with random antiferromagnetic exchange. We therefore establish Y 1-$x$ La $x$ TiO 3 as a model system for the study of antiferromagnetic spin-exchange disorder in a three-dimensional Heisenberg ferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Electron-phonon coupling in M⁢n 1−𝑥 ⁢F⁡e 𝑥⁢ Si

We present a study of the lattice dynamical properties of M⁢n 1−𝑥⁢ F⁡e 𝑥 ⁢Si with 0≤𝑥≤0.22. Employing time-of-flight neutron spectroscopy and inelastic x-ray scattering, we investigate the doping dependence of phonon energies, 𝐸 ph , and linewidths, Γ ph . We find anomalous softening and broadening of a phonon mode propagating along the direction. Ab initio lattice dynamical calculations link this softening to an enhanced electron-phonon coupling due to the doping-dependent changes of the Fermi surface. In conclusion, we discuss an interplay of increased electron-phonon coupling and reduced ordered magnetic moments in M⁢n 1−𝑥⁢ F⁡e 𝑥 ⁢Si.

Density functional theory↗

DOE OSTI2025

Mapping the three-dimensional fermiology of the triangular lattice magnet EuAg 4 Sb 2

In this paper, we report the temperature-field phase diagram as well as present a comprehensive study of the electronic structure and three-dimensional fermiology of the triangular-lattice magnet EuAg 4 ⁢Sb 2 , utilizing quantum oscillation measurements, angle-resolved photoemission spectroscopy, and first-principles calculations. The complex magnetic phase diagram of EuAg 4 ⁢Sb 2 highlights many transitions through nontrivial AFM states. Shubnikov-de Haas and de Haas-van Alphen oscillations were observed in the polarized ferromagnetic state of EuAg 4 ⁢Sb 2 , revealing three pairs of distinct spin-split frequency branches with small effective masses. A comparison of the angle-dependent oscillation data with first-principles calculations in the ferromagnetic state and angle-resolved photoemission spectra shows good agreement, identifying tubular hole pockets and hourglass-shaped hole pockets at the Brillouin zone center, as well as diamond-shaped electron pockets at the zone boundary. As the temperature increases, the frequency branches of the tiny hourglass pockets evolve into a more cylindrical shape, while the larger pockets remain unchanged. This highlights that variations in exchange splitting, driven by changes in the magnetic moment, primarily impact the small Fermi pockets without significantly altering the overall band structure. As a result, this is consistent with first-principles calculations, which show minimal changes near the Fermi level across ferromagnetic and simple antiferromagnetic states or under varying on-site Coulomb repulsion.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Noncollinear ground states of solids with a source-free exchange correlation functional

In this paper, we expand upon the source-free (SF) exchange correlation (XC) functional developed by Sangeeta Sharma and coworkers to plane-wave density functional theory (DFT) based on the projector augmented wave (PAW) method. This constraint is implemented by the current authors within the VASP source code, using a fast Poisson solver that capitalizes on the parallel three-dimensional fast Fourier transforms (FFTs) implemented in VASP. Using this modified XC functional, we explore the improved convergence behavior that results from applying this constraint to the GGA-PBE+U+J functional. In the process, we compare the noncollinear magnetic ground state computed by each functional and their SF counterpart for a select number of magnetic materials in order to provide a metric for comparing with experimentally determined magnetic orderings. We observe significantly improved agreement with experimentally measured magnetic ground-state structures after applying the source-free constraint. Furthermore, we explore the importance of considering probability current densities in spin-polarized systems, even under no applied field. We analyze the XC torque as well, in order to provide theoretical and computational analyses of the net XC magnetic torque induced by the source-free constraint. Along these lines, we highlight the importance of properly considering the real-space integral of the source-free local magnetic XC field. Our analyses on probability currents, net torque, and constant terms draw additional links to the rich body of previous research on spin-current density functional theory (SCDFT), and pave the way for future extensions and corrections to the SF corrected XC functional.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Magnetoelectric phase diagrams of multiferroicGdMn2O5

Electric and magnetic properties of multiferroic GdMn 2 O 5 in external magnetic fields were investigated to map out the magnetoelectric phases in this material. Due to strong magnetoelectric coupling, the dielectric permittivity is highly sensitive to phase boundaries in GdMn 2 O 5 , which allowed us to construct the field-temperature phase diagrams. Several phase transitions are observed which are strongly field dependent with respect to field orientation and strength. The phase diagram for a magnetic field along the crystallographic a axis corresponds well to a polarization step, as induced by 90° rotation of Gd magnetic moments. Our results support the model of two ferroelectric sublattices, Mn-Mn and Gd-Mn, with strong R -Mn (4f - 3d) interaction for the polarization in R Mn 2 O 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

First-principles investigation of structural and magnetic disorder in CuNiMnAl and CuNiMnSn Heusler alloys

Two quaternary Heusler alloys, equiatomic CuNiMnAl and CuNiMnSn, are studied using density functional theory to understand their tendency for atomic disorder on the lattice and the magnetic effects of disorder. Disordered structures with antisite defects of atoms of the same and different sublattices are considered, with the level of atomic disorder ranging from 3% to 25%. Formation energies and magnetic moments are calculated relative to the ordered ground state and combined with a simple thermodynamical model to estimate temperature effects. We predict the relative levels of disordering in the two equiatomic alloys with good correlation to experimental x-ray diffraction results. In conclusion, the effect of swaps involving Mn is also discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Hybrid excitations due to crystal field, spin-orbit coupling, and spin waves in LiFePO 4

Here, we report on the spin waves and crystal field excitations in single crystal LiFePO 4 by inelastic neutron scattering over a wide range of temperatures, below and above the antiferromagnetic transition of this system. In particular, we find extra excitations below T N = 50 K that are nearly dispersionless and are most intense around magnetic zone centers. Furthermore, we show that these excitations correspond to transitions between thermally occupied excited states of Fe 2 + due to splitting of the S = 2 levels that arise from the crystal field and spin-orbit interactions. These excitations are further amplified by the highly distorted nature of the oxygen octahedron surrounding the iron atoms. Above T N , magnetic fluctuations are observed up to at least 720 K, with an additional inelastic excitation around 4 meV, which we attribute to single-ion effects, as its intensity weakens slightly at 720 K compared to 100 K, which is consistent with the calculated cross sections using a single-ion model. This theoretical analysis, using the MF-RPA model, provides both detailed spectra of the Fe d shell and estimates of the average ordered magnetic moment and T N . By applying the MF-RPA model to a number of existing spin-wave results from other Li M PO 4 ( M = Mn , Co, and Ni), we are able to obtain reasonable predictions for the moment sizes and transition temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Pairing tendencies in a two-orbital Hubbard model in one dimension

The recent discovery of superconductivity under high pressure in the ladder compound BaFe 2 S 3 has opened a new field of research in iron-based superconductors with focus on quasi-one-dimensional geometries. In this publication, using the density matrix renormalization group technique, we study a two-orbital Hubbard model defined in one-dimensional chains. Our main result is the presence of hole binding tendencies at intermediate Hubbard U repulsion and robust Hund coupling J H / U = 0.25. Binding does not occur either in weak coupling or at very strong coupling. The pair-pair correlations that are dominant near half-filling, or of similar strength as the charge and spin correlation channels, involve hole-pair operators that are spin singlets, use nearest-neighbor sites, and employ different orbitals for each hole. As a result, the Hund coupling strength, presence of robust magnetic moments, and antiferromagnetic correlations among them are important for the binding tendencies found here.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

The existence of large instantaneous local magnetic moments in paramagnetic phases is a direct signature of Mott localization. In order to track the doping evolution of fluctuating local moments in iron-based superconductors, we jointly use two fast probes, x-ray emission and absorption spectroscopies. Exploring K- and Cr-hole-doped BaF e 2 A s 2 , we also find a systematic increase in the local moment with hole-doping, in contrast with inelastic neutron scattering measurements, which suggest an opposite trend. The results support the theoretical scenario in which a Mott insulating state that would be realized for half-filled conduction bands has an influence throughout the phase diagram of these iron pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Spin reorientation and magnetoelastic properties of ferromagnetic T b 1 - x N d x C o 2 systems with a morphotropic phase boundary

The spin reorientation (SR) and magnetoelastic properties of pseudobinary ferromagnetic Tb 1–x Nd x Co 2 (0 ≤ x ≤ 1.0) systems involving a morphotropic phase boundary (MPB) were studied by high-resolution synchrotron x-ray diffraction (XRD), magnetization, and magnetostriction measurements. The easy magnetization direction of the Laves phase lies along the < 111 > axis with x < 0.65, whereas it lies along the < 100 > axis for x > 0.65 below Curie temperature (TC). The temperature-dependent magnetization curves showed SR; this can be explained by a two-sublattice model. Based on the synchrotron (XRD) and magnetization measurements, the SR phase diagram for a MPB composition of Tb 0.35 Nd 0.65 Co 2 was obtained. Contrary to previously reported ferromagnetic systems involving MPB, the MPB composition of Tb 0.35 Nd 0.65 Co 2 exhibits a low saturation magnetization (MS), indicating a compensation of the Tb and Nd magnetic moments at MPB. The anisotropic magnetostriction (λS) first decreased until x = 0.8 and then continuously increased in the negative direction with further increase of Nd concentration. The decrease in magnetostriction can be attributed to the decrease of spontaneous magnetostriction λ 111 and increase of λ 100 with opposite sign to λ 111 . In conclusion, this paper indicates an anomalous type of MPB in the ferromagnetic Tb 1–x Nd x Co 2 system and provides an active way to design novel functional materials with exotic properties.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Effect of anti-site disorder on magnetism in La 2 NiMnO 6

La 2 NiMnO 6 has been reported to exhibit a paramagnetic to ferromagnetic transition with a transition temperature of ~ 260 K. However, most of its magnetic properties, such as the saturation magnetization and even the transition temperature, appear to vary considerably among different reports. This is possibly because the crystallographic structure as well as the extent of the anti-site disorder (ASD) at the Ni/Mn sites are strongly influenced by the choice of synthesis routes. There are diverse reports connecting the extent of ASD to the valencies of Ni and Mn ions, such as Ni 2+ -Mn 4+ and Ni 3+ -Mn 3+ , including suggestions of thermally induced valence transitions. Consequently, these reports arrive at very different conclusions on the mechanism behind the magnetic properties of La 2 NiMnO 6 . To address the correlation between ASD and valency, we have carried out a comparative study of two monoclinic La 2 NiMnO 6 polycrystals with different degrees of ASD. Using a combination of x-ray absorption spectroscopy, x-raymagnetic circular dichroism, andmagnetometry, we conclude that the valency of the transition metal ions, and the transition temperature, are insensitive to the extent of ASD. Furthermore, we find the magnetic moment decreases strongly with an increasing ASD. We attribute this effect to the introduction of antiferromagnetic interactions in the anti-site disordered regions.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

Here, isotope-selective 57 Fe nuclear resonant inelastic x-ray scattering (NRIXS) measurements and atomic-layer resolved density functional theory (DFT) calculations were used to investigate the effect of interfaces on the vibrational (phonon) density of states (VDOS) of (001)-oriented nanoscale Fe/Ag and Fe/Cr multilayers. The multilayers in the experiment contained isotopically enriched 57 Fe monolayers as probe layers located either at the Fe/Ag or Fe/Cr interfaces or in the center of the Fe films. This allows probing of the vibrational ynamics of Fe sites either at the buried interfaces or in the center of the Fe films. For Fe/Ag multilayers, distinct differences were observed experimentally between the Fe-partial VDOS at the interface and in the center. At the Fe/Ag interface, the high-energy longitudinal-acoustic (LA) phonon peak of Fe near ~35 meV is suppressed and slightly shifted to lower energy, and the low-energy part of the VDOS below ~20 meV is drastically enhanced, as compared to the Fe-specific VDOS in the center Fe layers or in bulk Fe. Similar phenomena are found to a less degree in the Fe/Cr multilayers. The measured Fe-partial VDOS was used to determine the Fe site-selective vibrational thermodynamic properties of the multilayers. Our theoretical findings for the layer-dependent VDOS of the multilayers are in qualitative agreement with the experimental results obtained by NRIXS. For Fe/Ag multilayers, which are characterized by a large atomic mass ratio, the experimental and theoretical results demonstrate phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces due to the energy mismatch between Ag and Fe LA phonons. These phenomena are reduced or suppressed in the Fe/Cr multilayers with their about equal atomic masses. Moreover, direction-projected Fe VDOS along the (nearly in-plane) incident x-ray beam was computed in order to address the intrinsic vibrational anisotropy of the Fe/Ag multilayer. We have also performed spin-resolved electronic band structure (DFT) calculations, predicting an enhanced magnetic moment (μ Fe = 2.8μ B ) of the interfacial Fe atoms and a high electron spin polarization (79%) at the Fermi energy for the Fe/Ag interface, as compared to the case of Fe center layers. This is a result of charge transfer from Fe to Ag at the interface. On the contrary, Cr tends to donate electrons to Fe, thus reducing the interfacial Fe moment (μ Fe = 1.9μ B ). This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗