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Mandrus, D.

Publications and source records attributed to Mandrus, D..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Antiferromagnetic Resonance and Terahertz Continuum in α - RuCl 3

We report measurements of optical absorption in the zigzag antiferromagnet α-RuCl 3 as a function of temperature T , magnetic field B , and photon energy ℏ ω in the range ~ 0.3 –8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at B = 0 that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of B , the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Terahertz dielectric analysis and spin-phonon coupling in multiferroic GeV 4 S 8

We present an investigation of the multiferroic lacunar spinel compound GeV 4 S 8 using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, T N =17K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the Néel temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform an analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Dimensionality Effects in FeGe 2 Nanowires: Enhanced Anisotropic Magnetization and Anomalous Electrical Transport

Here, we report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe 2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe 2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2017

Controlling the Topological Sector of Magnetic Solitons in Exfoliated Cr 1 / 3 NbS 2 Crystals

Here, we investigate manifestations of topological order in monoaxial helimagnet Cr 1/3 NbS 2 by performing transport measurements on ultrathin crystals. Upon sweeping the magnetic field perpendicularly to the helical axis, crystals thicker than one helix pitch (48 nm) but much thinner than the magnetic domain size (similar to 1 mu m) are found to exhibit sharp and hysteretic resistance jumps. We also show that these phenomena originate from transitions between topological sectors with a different number of magnetic solitons. This is confirmed by measurements on crystals thinner than 48 nm-in which the topological sector cannot change-that do not exhibit any jump or hysteresis. These results show the ability to deterministically control the topological sector of finite-size Cr 1/3 NbS 2 and to detect intersector transitions by transport measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structure and Dynamics Investigations of Sr/Ca-Doped LaPO 4 Proton Conductors

Proton conductors loom out of the pool of candidate materials with great potential to boost hydrogen alternatives to fossil-based resources for energy. Acceptor doped lanthanum orthophosphates are considered for solid oxide fuel cells (SOFCs) for their potential stability and conductivity at high temperature. By exploring the crystal and defect structure of x% Sr/Ca-doped LaPO 4 with different nominal Sr/Ca concentrations (x = 0 – 10) with Neutron powder diffraction (NPD) and X-ray powder diffraction (XRD), we confirm that Sr/Ca-doped LaPO 4 can exist as self-supported structures at high temperatures during solid oxide fuel cell operation. Thermal stability, surface topography, size distribution are also studied to better understand the proton conductivity for dry and wet compounds obtained at sintering temperatures ranging from 1200 to 1400 °C using a combination of scanning electron microscopy (SEM), Atomic Force Microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). In conclusion, the results suggest that Sr doped samples exhibit the highest proton conductivity of our samples and illustrate the impact of material design and versatile characterization schemes on the development of proton conductors with superior functionality.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Excitations and magnetization density distribution in the dilute ferromagnetic semiconductor Yb 14 MnSb 11

One rare example of a Kondo lattice compound with ferromagnetic dominated RKKY interactions is Yb$_{14}$MnSb$_{11}$. As a ferromagnetic semiconductor with $T_c \approx 53$~K, it is also a potential compound for exploration of spintronic devices. This material is furthermore one of the most efficient high temperature thermoelectrics. We describe measurements which answer remaining questions regarding the energy scales of the exchange interactions, the valence and the magnetization density distribution in this system. We also find that the system consists of RKKY exchange coupled Mn$^{2+}$ sites with nearest and next nearest exchange interactions dominating the magnetic spectrum with no significant magnetization density localized on other atomic sites. The extended spread of a negative magnetization around each of the Mn ions supports a Kondo screening cloud scenario for Yb$_{14}$MnSb$_{11}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Strong anisotropy within a Heisenberg model in the J eff = 1 2 insulating state of Sr 2 Ir 0.8 Ru 0.2 O 4

The dispersive magnetic excitations in Sr 2 IrO 4 have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with La 2 CuO 4 . In this paper, we investigate the inelastic spectra of Sr 2 Ir 0.8 Ru 0.2 O 4 with resonant inelastic x-ray scattering (RIXS) at the Ir L 3 edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to Sr 2 IrO 4 ; however, the disorder induced by the substitution of 20% Ir 4+ ions for Ru 4+ removes longer range exchange interactions. A large spin gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in Sr 2 Ir 0.8 Ru 0.2 O 4 with c-axis aligned moments compared to the basal plane moments in the parent. Finally, collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the J eff = 1/2 moments are diluted.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

μ SR and magnetometry study of superconducting 5% Pt-doped IrTe 2

In this paper, we present magnetometry and muon spin rotation ( SR) measurements of the superconducting dichalcogenide Ir 0.95 Pt 0.05 Te 2 . From both sets of measurements we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional s-wave model and yield fitting parameters consistent with a BCS weak coupling superconductor. Finally, we therefore see no evidence for exotic superconductivity in Ir 0.95 Pt 0.05 Te 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2016

Highly anisotropic exchange interactions of j eff = 1 2 iridium moments on the fcc lattice in La 2 B IrO 6 ( B = Mg , Zn )

Here we have performed inelastic neutron scattering (INS) experiments to investigate the magnetic excitations in the weakly distorted face-centered-cubic (fcc) iridate double perovskites La$_2$ZnIrO$_6$ and La$_2$MgIrO$_6$, which are characterized by A-type antiferromagnetic ground states. The powder inelastic neutron scattering data on these geometrically frustrated $j_{\rm eff}=1/2$ Mott insulators provide clear evidence for gapped spin wave excitations with very weak dispersion. The INS results and thermodynamic data on these materials can be reproduced by conventional Heisenberg-Ising models with significant uniaxial Ising anisotropy and sizeable second-neighbor ferromagnetic interactions. Such a uniaxial Ising exchange interaction is symmetry-forbidden on the ideal fcc lattice, so that it can only arise from the weak crystal distortions away from the ideal fcc limit. This may suggest that even weak distortions in $j_{\rm eff}=1/2$ Mott insulators might lead to strong exchange anisotropies. More tantalizingly, however, we find an alternative viable explanation of the INS results in terms of spin models with a dominant Kitaev interaction. In contrast to the uniaxial Ising exchange, the highly-directional Kitaev interaction is a type of exchange anisotropy which is symmetry-allowed even on the ideal fcc lattice. The Kitaev model has a magnon gap induced by quantum order-by-disorder, while weak anisotropies of the Kitaev couplings generated by the symmetry-lowering due to lattice distortions can pin the order and enhance the magnon gap. In conclusion, our findings highlight how even conventional magnetic orders in heavy transition metal oxides may be driven by highly-directional exchange interactions rooted in strong spin-orbit coupling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Structural and magnetic phase transitions in CeCu 6 - x T x ( T = Ag , Pd )

The structural and the magnetic properties of CeCu6-xAgx (0≤x≤0.85) and CeCu6-xPdx (0≤x≤0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of CeCu6-xAgx and CeCu6-xPdx as a function of Ag/Pd composition are reported. The end member, CeCu6, undergoes a structural phase transition from an orthorhombic (Pnma) to a monoclinic (P21/c) phase at 240 K. In CeCu6-xAgx, the structural phase transition temperature (Ts) decreases linearly with Ag concentration and extrapolates to zero at xS ≈0.1. The structural transition in CeCu6-xPdx remains unperturbed with Pd substitution within the range of our study. The lattice constant b slightly decreases with Ag/Pd doping, whereas a and c increase with an overall increase in the unit cell volume. Both systems, CeCu6-xAgx and CeCu6-xPdx, exhibit a magnetic quantum critical point (QCP), at x≈0.2 and x≈0.05, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (δ10 δ2), where δ1~0.62,δ2~0.25,x=0.125 for CeCu6-xPdx and δ1~0.64,δ2~0.3,x=0.3 for CeCu6-xAgx. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the c axis of the orthorhombic unit cell.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin-orbit driven magnetic insulating state with J eff =1/2 character in a 4d oxide

The unusual magnetic and electronic ground states of 5d iridates has been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here we present experimental and theoretical results on Sr 4 RhO 6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous J eff =1/2 Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy and find a magnetic insulating ground state with J eff =1/2 character.The unusual magnetic and electronic ground states of 5d iridates have been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here, we present experimental and theoretical results on Sr 4 RhO 6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous J eff =1/2 Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy, and find a magnetic insulating ground state with J eff =12 character.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Magnetic Correlations in the Quasi-Two-Dimensional Semiconducting Ferromagnet CrSiTe 3

Intrinsic, 2D ferromagnetic semiconductors are an important class of materials for overcoming dilute magnetic semiconductors’ limitations for spintronics. CrSiTe 3 is a particularly interesting material of this class, since it can likely be exfoliated to single layers, for which T c is predicted to increase dramatically. Establishing the nature of the bulk material’s magnetism is necessary for understanding the thin-film magnetic behavior and the material’s possible applications. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single crystalline CrSiTe 3 . We find a very small single ion anisotropy that favors magnetic ordering along the c-axis and that the measured spin waves fit well to a model in which the moments are only weakly coupled along that direction. Then, we find that both static and dynamic correlations persist within the ab-plane up to at least 300 K, which is strong evidence of the material's 2D characteristics that are relevant for future studies on thin film and monolayer samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A new apparatus design for high temperature (up to 950 °C) quasi-elastic neutron scattering in a controlled gaseous environment

A design for a sample cell system suitable for high temperature Quasi-Elastic Neutron Scattering (QENS) experiments is presented. The apparatus was developed at the Spallation Neutron Source in Oak Ridge National Lab where it is currently in use. The design provides a special sample cell environment under controlled humid or dry gas flow over a wide range of temperature up to 950 °C. Using such a cell, chemical, dynamical, and physical changes can be studied in situ under various operating conditions. While the cell combined with portable automated gas environment system is especially useful for in situ studies of microscopic dynamics under operational conditions that are similar to those of solid oxide fuel cells, it can additionally be used to study a wide variety of materials, such as high temperature proton conductors. The cell can also be used in many different neutron experiments when a suitable sample holder material is selected. The sample cell system has recently been used to reveal fast dynamic processes in quasi-elastic neutron scattering experiments, which standard probes (such as electrochemical impedance spectroscopy) could not detect. In this work, we outline the design of the sample cell system and present results demonstrating its abilities in high temperature QENS experiments.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

High antiferromagnetic transition temperature of a honeycomb compound SrRu 2 O 6

We study the high-temperature magnetic order in a quasi-two-dimensional honeycomb compound SrRu 2 O 6 by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu 2 O 6 crystallizes into the hexagonal lead antimonate (PbSb 2 O 6 , space group P31m) structure with layers of edge-sharing RuO6 octahedra separated by Sr 2+ ions. SrRu 2 O 6 is found to order at T N = 565 K with Ru moments coupled antiferromagnetically both in plane and out of plane. The magnetic moment is 1.30(2) μ B /Ru at room temperature and is along the crystallographic c axis in the G-type magnetic structure. We perform density functional calculations with constrained random-phase approximation (RPA) to obtain the electronic structure and effective intra- and interorbital interaction parameters. The projected density of states shows strong hybridization between Ru 4d and O 2p. By downfolding to the target t 2g bands we extract the effective magnetic Hamiltonian and perform Monte Carlo simulations to determine the transition temperature as a function of interand intraplane couplings. We find a weak interplane coupling, 3% of the strong intraplane coupling, permits three-dimensional magnetic order at the observed T N .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin-liquid ground state in the frustrated J 1 - J 2 zigzag chain system BaTb 2 O 4

We have investigated polycrystalline samples of the zigzag chain system BaTb 2 O 4 with magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation measurements. No magnetic transitions are observed in the bulk measurements, while neutron diffraction reveals low-temperature, short-range, intrachain magnetic correlations between Tb 3+ ions. Muon spin relaxation measurements indicate that these correlations are dynamic, as the technique detects no signatures of static magnetism down to 0.095 K. Altogether these findings provide strong evidence for a spin liquid ground state in BaTb 2 O 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic properties of the helimagnet Cr 1 / 3 NbS 2 observed by μ SR

We have performed muon spin rotation and relaxation (μSR) measurements on single crystals of the chiral helimagnet Cr 1/3 NbS 2 at zero to low magnetic field. The transition from the paramagnetic to helical magnetically ordered phase at zero field ismarked by the onset of a coherent oscillation of the zero-field muon spin polarization below a critical temperature T c . An enhancement of the muon spin precession frequency is observed below T ~ 50 K, where anomalous behavior has been observed in bulk transport measurements. The enhanced precession frequency indicates a low-temperature modification of the helical magnetic structure. A Landau free-energy analysis suggests that the low-temperature change in the magnetic structure is caused by a structural change. Finally, we also suggest a longer periodicity of helicity below T ~ 50 K, which can be verified by neutron-scattering experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Strong spin-lattice coupling in CrSiTe{sub 3}

CrSiTe{sub 3} has attracted recent interest as a candidate single-layer ferromagnetic semiconductor, but relatively little is known about the bulk properties of this material. Here, we report single-crystal X-ray diffraction, magnetic properties, thermal conductivity, vibrational, and optical spectroscopies and compare our findings with complementary electronic structure and lattice dynamics principles calculations. The high temperature paramagnetic phase is characterized by strong spin-lattice interactions that give rise to glassy behavior, negative thermal expansion, and an optical response that reveals that CrSiTe{sub 3} is an indirect gap semiconductor with indirect and direct band gaps at 0.4 and 1.2 eV, respectively. Measurements of the phonons across the 33 K ferromagnetic transition provide additional evidence for strong coupling between the magnetic and lattice degrees of freedom. The Si-Te stretching and Te displacement modes are sensitive to the magnetic ordering transition, a finding that we discuss in terms of the superexchange mechanism. Spin-lattice coupling constants are also extracted.

36 MATERIALS SCIENCE↗