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At least 847 records · Page 47
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DOE OSTI2015

CaMn 2 Sb 2 : Spin waves on a frustrated antiferromagnetic honeycomb lattice

We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn 2 Sb 2 :, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the H and L directions in reciprocal space, with a maximum excitation energy of ≈ 24 meV. These excitations are well described by spin waves in a Heisenberg model, including first and second neighbor exchange interactions, J 1 and J 2 , in the Mn plane and also an exchange interaction between planes. The determined ratio J 2 /J 1 ≈ 1/6 suggests that CaMn 2 Sb 2 : is the first example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the N´eel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveal a mean field ordering temperature ≈ 4 times larger than the reported N´eel temperature T N = 85 K, suggesting significant frustration arising from proximity to the tricritical point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Antiferromagnetism in EuCu 2 As 2 and EuCu 1.82 Sb 2 single crystals

Single crystals of EuCu 2 As 2 and EuCu 2 Sb 2 were grown from CuAs and CuSb self-flux, respectively. The crystallographic, magnetic, thermal, and electronic transport properties of the single crystals were investigated by room-temperature x-ray diffraction (XRD), magnetic susceptibility χ versus temperature T, isothermal magnetization M versus magnetic field H, specific heat C p (T), and electrical resistivity ρ(T) measurements. EuCu 2 As 2 crystallizes in the body-centered tetragonal ThCr 2 Si 2 -type structure (space group I4/mmm), whereas EuCu 2 Sb 2 crystallizes in the related primitive tetragonal CaBe 2 Ge 2 -type structure (space group P4/nmm). The energy-dispersive x-ray spectroscopy and XRD data for the EuCu 2 Sb 2 crystals showed the presence of vacancies on the Cu sites, yielding the actual composition EuCu 1.82 Sb 2 . The ρ(T) and C p (T) data reveal metallic character for both EuCu 2 As 2 and EuCu 1.82 Sb 2 . Antiferromagnetic (AFM) ordering is indicated from the χ(T),C p (T), and ρ(T) data for both EuCu 2 As 2 (T N = 17.5 K) and EuCu 1.82 Sb 2 (T N = 5.1 K). In EuCu 1.82 Sb 2 , the ordered-state χ(T) and M(H) data suggest either a collinear A-type AFM ordering of Eu +2 spins S = 7/2 or a planar noncollinear AFM structure, with the ordered moments oriented in the tetragonal ab plane in either case. This ordered-moment orientation for the A-type AFM is consistent with calculations with magnetic dipole interactions. As a result, the anisotropic χ(T) and isothermal M(H) data for EuCu 2 As 2 , also containing Eu +2 spins S = 7/2, strongly deviate from the predictions of molecular field theory for collinear AFM ordering and the AFM structure appears to be both noncollinear and noncoplanar.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Coexistence of antiferromagnetic and ferromagnetic spin correlations in SrCo 2 As 2 revealed by 59 Co and 75 As NMR

In nonsuperconducting, metallic paramagnetic SrCo 2 As 2 , inelastic neutron scattering measurements have revealed strong stripe-type q=(π,0) antiferromagnetic (AFM) spin correlations. Using nuclear magnetic resonance (NMR) measurements on 59 Co and 75 As nuclei, we demonstrate that stronger ferromagnetic (FM) spin correlations coexist in SrCo 2 As 2 . Our NMR data are consistent with density functional theory (DFT) calculations which show enhancements at both q=(π,0) and the in-plane FM q=0 wave vectors in static magnetic susceptibility χ(q). We suggest that the strong FM fluctuations prevent superconductivity in SrCo 2 As 2 , despite the presence of stripe-type AFM fluctuations. Since DFT calculations have consistently revealed similar enhancements of the χ(q) at both q=(π,0) and q=0 in the iron-based superconductors and parent compounds, our observation of FM correlations in SrCo 2 As 2 calls for detailed studies of FM correlations in the iron-based superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Field-induced spin density wave and spiral phases in a layered antiferromagnet

Here we determine the low-field ordered magnetic phases of the S=1 dimerized antiferromagnet Ba 3 Mn 2 O 8 using single crystal neutron diffraction. We find that for magnetic fields between μ 0 H=8.80 T and 10.56 T applied along the $1\bar{1}0$ direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ 0 H > 10.56 T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. Finally, the nature of these two transitions is fundamentally different: the low-field transition is a second order transition to a spin-density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Thermal conductivity in large-J two-dimensional antiferromagnets: Role of phonon scattering

Motivated by the recent heat transport experiments in 2D antiferromagnets, such as La 2 CuO 4 , where the exchange coupling J is larger than the Debye energy Θ D , we discuss different types of relaxation processes for magnon heat current with a particular focus on coupling to 3D phonons. In this work, we study thermal conductivity by these in-plane magnetic excitations using two distinct techniques, Boltzmann formalism within the relaxation-time approximation and memory-function approach. Within these approaches, a close consideration is given to the scattering of magnons by both acoustic and optical branches of phonons. A remarkable accord between the two methods with regards to the asymptotic behavior of the effective relaxation rates is demonstrated. Additional scattering mechanisms, due to grain boundaries, impurities, and finite correlation length in the paramagnetic phase, are discussed and included in the calculations of the thermal conductivity κ(T). Again, we demonstrate a close similarity of the results from the two techniques of calculating κ(T). Our complementary approach strongly suggests that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high temperature window of Θ D ≲T$\ll$J.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY 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

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

In this work, using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF 2 , show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF 2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF 2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF 2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. Lastly, the underlying mechanism of the uncompensated AF domain formation in Ni/FeF 2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Antiferromagnetic spin correlations and pseudogaplike behavior in Ca(Fe 1-x Co x ) 2 As 2 studied by 75 As nuclear magnetic resonance and anisotropic resistivity

We report 75 As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe 1–x Co x ) 2 As 2 (x=0.023, 0.028, 0.033, and 0.059) annealed at 350°C for 7 days. From the observation of a characteristic shape of 75 As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of x=0 (T N =170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in x=0.023 (T N =106 K) and x=0.028 (T N =53 K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (1/T 1 ), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature T* that was nearly independent of Co-substitution concentration, and it is attributed to a pseudogaplike behavior in the spin excitation spectra of these systems. The T* feature finds correlation with features in the temperature-dependent interplane resistivity, ρ c (T), but not with the in-plane resistivity ρ a (T). The temperature evolution of anisotropic stripe-type AFM spin fluctuations is tracked in the paramagnetic and pseudogap phases by the 1/T 1 data measured under magnetic fields parallel and perpendicular to the c axis. As a result, based on our NMR data, we have added a pseudogaplike phase to the magnetic and electronic phase diagram of Ca(Fe 1–x Co x ) 2 As 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electrodynamics of the antiferromagnetic phase in URu 2 Si 2

Here, we present data on the optical conductivity of URu 2-x (Fe,Os) x Si 2 . While the parent material URu 2 Si 2 enters the enigmatic hidden order (HO) phase below 17.5 K, an antiferromagnetic (AFM) phase is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as ω 2 . We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density wave gap.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr 2 IrO 4

Sr 2 IrO 4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at T N = 240 K. We report results of a comprehensive study of single-crystal Sr 2 Ir 1-x Tb x O 4 (0≤x≤0.03). This study found that mere 3% (x=0.03) tetravalent Tb 4+ (4f 7 ) substituting for Ir 4+ (rather than Sr 2+ ) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x = 0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund’s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. Furthermore, this work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic structure and crystal-field states of the pyrochlore antiferromagnet Nd 2 Zr 2 O 7

In this paper, we present synchrotron x-ray diffraction, neutron powder diffraction, and time-of-flight inelastic neutron scattering measurements on the rare earth pyrochlore oxide Nd 2 Zr 2 O 7 to study the ordered state magnetic structure and cystal-field states. The structural characterization by high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long-range all-in/all-out antiferromagnetic order below T N ≈0.4 K with propagation vector k = (0 0 0) and an ordered moment of 1.26(2) μ B /Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of 2.65μ B /Nd for the local <111> Ising ground state of Nd 3+ (J=9/2) suggesting that the ordering is partially suppressed by quantum fluctuations. The inelastic neutron scattering experiment further confirms the Ising anisotropic ground state of Nd 3+ and also reveals its dipolar-octupolar character which possibly induces the quantum fluctuation. Lastly, the crystal-field level scheme and ground state wave function have been determined.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Monoclinic crystal structure of α - RuCl 3 and the zigzag antiferromagnetic ground state

We have proposed the layered honeycomb magnet α - RuCl 3 as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j eff = 1/2 Ru 3 + magnetic moments. We report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We also report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the j eff = 1/2 picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below T N ≈ 13 K. Our analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Antiferromagnetism and hidden order in isoelectronic doping of URu 2 Si 2

In this paper, we present muon spin rotation (mu SR) and susceptibility measurements on single crystals of isoelectronically doped URu 2-x T x Si 2 (T = Fe, Os) for doping levels up to 50%. Zero field (ZF) μ SR measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment-hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping.

36 MATERIALS SCIENCE↗
DOE OSTI2016

XY antiferromagnetic ground state in the effective S = 1 2 pyrochlore Yb 2 Ge 2 O 7

We report neutron scattering and muon spin relaxation measurements (μSR) on the pyrochlore antiferromagnet Yb 2 Ge 2 O 7 . Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=$\frac{7}{2}$ Yb 3+ ion in this environment. The crystal electric field ground state doublet in Yb 2 Ge 2 O 7 corresponds primarily to m J =± $\frac{1}{2}$ with local XY anisotropy, consistent with an S eff = 12 description for the Yb moments. $μ$SR measurements reveal the presence of an ordering transition at T N =0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the $k$=(000)$Γ_5$ ground state with an ordered magnetic moment of 0.3(1)$μ_B$ per Yb ion. We compare and contrast this phenomenology with the low-temperature behavior of Yb 2 Ti 2 O 7 and Er 2 Ti 2 O 7 , the prototypical S eff = $\frac{1}{2}$ XY pyrochlore magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Simultaneous metal-insulator and antiferromagnetic transitions in orthorhombic perovskite iridate Sr 0.94 Ir 0.78 O 2.68 single crystals

The orthorhombic perovskite SrIrO 3 is a semimetal, an intriguing exception in iridates where the strong spin-orbit interaction coupled with electron correlations tends to impose an insulating state. Here, we report results of our investigation of bulk single-crystal Sr 0.94 Ir 0.78 O 2.68 or Ir-deficient, orthorhombic perovskite SrIrO 3 . It retains the same crystal structure as stoichiometric SrIrO 3 but exhibits a sharp, simultaneous antiferromagnetic (AFM) and metal-insulator (MI) transition occurring in the basal-plane resistivity at 185 K. Above it, the basal-plane resistivity features an extended regime of almost linear temperature dependence up to 800 K but the strong electronic anisotropy renders an insulating behavior in the out-of-plane resistivity. The Hall resistivity undergoes an abrupt sign change and grows below 40 K, which along with the Sommerfeld constant of 20 mJ/mol K 2 suggests a multiband effect. All results including our first-principles calculations underscore a delicacy of the paramagnetic, metallic state in SrIrO 3 that is in close proximity to an AFM insulating state. The contrasting ground states in isostructural Sr 0.94 Ir 0.78 O 2.68 and SrIrO 3 illustrate a critical role of lattice distortions and Ir deficiency in rebalancing the ground state in the iridates. Finally, the concurrent AFM and MI transitions reveal a direct correlation between the magnetic transition and formation of an activation gap in the iridate, which is conspicuously absent in Sr 2 IrO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

Here, the surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a (LaFeO 3 ) 8 /(SrFeO 3 ) 1 superlattice film capped with four layers of LaFeO 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from Fe 3+ in the bulk to Fe 2+ at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Strong magnetic correlations to 900 K in single crystals of the trigonal antiferromagnetic insulators SrMn 2 As 2 and CaMn 2 As 2

Crystallographic, electronic transport, thermal, and magnetic properties are reported for SrMn 2 As 2 and CaMn 2 As 2 single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl 2 Si 2 -type structure (space group $P\bar{3}$ m1), in agreement with the literature. Electrical resistivity ρ versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn 2 As 2 and 61 meV for CaMn 2 As 2 . In a local-moment picture, the Mn +2 3d 5 ions are expected to have high-spin S=5/2 with spectroscopic splitting factor g≈2. Magnetic susceptibility χ and heat capacity Cp measurements versus T reveal antiferromagnetic (AFM) transitions at T N =120(2) K and 62(3) K for SrMn 2 As 2 and CaMn 2 As 2 , respectively. The anisotropic χ(T≤T N ) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. Finally, the χ(T) data for both compounds and the Cp(T) for SrMn 2 As 2 show strong dynamic short-range AFM correlations from T N up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Spin-Josephson effects in exchange coupled antiferromagnetic insulators

The spin superfluid analogy can be extended to include Josephson-like oscillations of the spin current. In a system of two antiferromagnetic insulators (AFMIs) separated by a thin metallic spacer, a threshold spin chemical potential established perpendicular to the direction of the N&#233el vector field drives terahertz oscillations of the spin current. This spin current also has a nonlinear, time-averaged component which provides a “smoking gun” signature of spin superfluidity. The time-averaged spin current can be detected via the inverse spin Hall effect in a metallic spacer with large spin-orbit coupling. The physics illustrated here with AFMIs also applies to easy-plane ferromagnetic insulators. We suggest these findings may provide a new approach for experimental verification of spin superfluidity and realization of a terahertz spin oscillator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗