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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Spin transport through the metallic antiferromagnet FeMn

Here, we investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni 80 Fe 20 /FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. By using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents, respectively. Furthermore, the latter can extend to relatively large distances (approximate to 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr 2 Ir 1 - x Ru x O 4

Here, we study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr 2 Ir 1–x RuxO 4 . The maximum energy of the magnetic excitation remains robust up to x = 0.77, with a gap opening at low dopings and increasing to over 150 meV at x = 0.77. At these higher Ru concentrations, the dispersive magnetic excitations in Sr 2 IrO 4 are rendered essentially momentum independent. Up to a Ru concentration of x = 0.77, both experiments and first-principles calculations show the Ir J eff = 1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Pressure effects in the itinerant antiferromagnetic metal TiAu

Here, we report the pressure dependence of the Néel temperature T N up to P ≈ 27 GPa for the recently discovered itinerant antiferromagnet (IAFM) TiAu. The T N (P) phase boundary exhibits unconventional behavior in which the Néel temperature is enhanced from T N ≈ 33 K at ambient pressure to a maximum of T N ≈ 35 K occurring at P ≈ 5.5 GPa. Upon a further increase in pressure, T N is monotonically suppressed to ~22 K at P ≈ 27 GPa. We also find a crossover in the temperature dependence of the electrical resistivity ρ in the antiferromagnetic (AFM) phase that is coincident with the peak in T N (P), such that the temperature dependence of ρ = ρ 0 + A n T n changes from n≈3 during the enhancement of T N to n ≈ 2 during the suppression of T N . Based on an extrapolation of the T N (P) data to a possible pressure-induced quantum critical point, we estimate the critical pressure to be P c ≈ 45 GPa.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Influence of classical anisotropy fields on the properties of Heisenberg antiferromagnets within unified molecular field theory

Here, a comprehensive study of the influence of classical anisotropy fields on the magnetic properties of Heisenberg antiferromagnets within unified molecular field theory versus temperature T , magnetic field H , and anisotropy field parameter h A1 is presented for systems comprised of identical crystallographically-equivalent local moments. The anisotropy field for collinear z-axis antiferromagnetic (AFM) ordering is constructed so that it is aligned in the direction of each ordered and/or field-induced thermal-average moment with a magnitude proportional to the moment, whereas that for XY anisotropy is defined to be in the direction of the projection of the moment onto the xy plane, again with a magnitude proportional to the moment. Properties studied include the zero-field Néel temperature T N , ordered moment, heat capacity, and anisotropic magnetic susceptibility of the AFM phase versus T with moments aligned either along the z axis or in the xy plane. Also determined are the high-field magnetization perpendicular to the axis or plane of collinear or planar noncollinear AFM ordering, the high-field magnetization along the z axis of a collinear z -axis AFM, spin-flop (SF), and paramagnetic (PM) phases, and the free energies of these phases versus T , H , and h A1 . Phase diagrams at T =0 in the H z – h A1 plane and at T > 0 in the H z – T plane are constructed for spins S =1/2. For h A1 =0, the SF phase is stable at low field and the PM phase at high field with no AFM phase present. As h A1 increases, the phase diagram contains the AFM, SF, and PM phases. Further increases in h A1 lead to the disappearance of the SF phase and the appearance of a tricritical point on the AFM-PM transition curve. Furthermore, applications of the theory to extract h A1 from experimental low-field magnetic susceptibility data and high-field magnetization versus field isotherms for single crystals of AFMs are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Effects of interaction strength, doping, and frustration on the antiferromagnetic phase of the two-dimensional Hubbard model

Recent quantum-gas microscopy of ultracold atoms and scanning tunneling microscopy of the cuprates reveal new detailed information about doped Mott antiferromagnets, which can be compared with calculations. Using cellular dynamical mean-field theory, we map out the antiferromagnetic (AF) phase of the two-dimensional Hubbard model as a function of interaction strength U, hole doping δ, and temperature T . The Néel phase boundary is nonmonotonic as a function of U and δ. Frustration induced by second-neighbor hopping reduces Néel order more effectively at small U. The doped AF is stabilized at large U by kinetic energy and at small U by potential energy. The transition between the AF insulator and the doped metallic AF is continuous. At large U, we find in-gap states similar to those observed in scanning tunneling microscopy. Finally, we predict that, contrary to the Hubbard bands, these states are only slightly spin polarized.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Sequential structural and antiferromagnetic transitions in BaFe 2 Se 3 under pressure

We present the discovery of superconductivity in the two-leg ladder compound BaFe 2 S 3 has established the 123-type iron chalcogenides as a novel and interesting subgroup of the iron-based superconductor family. However, in this 123 series, BaFe 2 Se 3 is an exceptional member, with a magnetic order and crystalline structure different from all others. Recently, an exciting experiment reported the emergence of superconductivity in BaFe 2 Se 3 at high pressure [J. Ying et al., Phys. Rev. B 95, 241109(R) (2017)]. In this paper, we report a first-principles study of BaFe 2 Se 3 . Our analysis unveils a variety of qualitative differences between BaFe 2 S 3 and BaFe 2 Se 3 , including in the latter an unexpected chain of transitions with increasing pressure. First, by gradually reducing the tilting angle of iron ladders, the crystalline structure smoothly transforms from Pnma to Cmcm at ~6 GPa. Second, the system becomes metallic at 10.4 GPa. Third, its unique ambient-pressure Block antiferromagnetic ground state is replaced by the more common stripe (so-called CX-type) antiferromagnetic order at ~12 GPa, the same magnetic state as the 123-S ladder. This transition is found at a pressure very similar to the experimental superconducting transition. Finally, all magnetic moments vanish at 30 GPa. This reported theoretical diagram of the complete phase evolution is important because of the technical challenges to capture many physical properties in high-pressure experiments. The information obtained in our calculations suggests different characteristics for superconductivity in BaFe 2 Se 3 and BaFe 2 S 3 : in 123-S pairing occurs when magnetic moments vanish, while in 123-Se the transition region from Block- to CX-type magnetism appears to catalyze superconductivity. Lastly, an additional superconducting dome above ~30 GPa is expected to occur.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Anisotropic antiferromagnetic order in the spin-orbit coupled trigonal-lattice Ca 2 Sr 2 IrO 6

Here, we used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca 2 Sr 2 IrO 6 . The crystal structure is determined to be R ¯3 with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below T N = 13.1 K. The magnetic wave vector is identified as (0,0.5,1) with ferromagnetic coupling along the a axis and antiferromagnetic correlation along the b axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34° toward the c axis. The ordered moment is 0.66(3) μ B /Ir, larger than other iridates where iridium ions form corner- or edge-sharing IrO 6 octahedral networks. The tilting angle is reduced to ≈19° when a magnetic field of 4.9 T is applied along the c axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap ~0.5 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Gradual enhancement of stripe-type antiferromagnetism in the spin-ladder material BaFe 2 S 3 under pressure

We report the recent discovery of pressure-induced superconductivity in BaFe 2 S 3 and BaFe 2 Se 3 has generated considerable interest in the 123-type iron chalcogenides. The structure of these materials features iron atoms arranged in quasi-one-dimensional ladders with either stripe-type or block-type antiferromagnetism at ambient pressure, constituting a new parent state for iron-based superconductivity in need of detailed study. Here, we report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFe 2 S 3 under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published results. In contrast to our experimental observations, the calculations predict a reduction of the ordered moment with pressure. In conclusion, we suggest that the robustness of the stripe-type antiferromagnetism is due to strong electron correlations not fully considered in the calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Anomalous connection between antiferromagnetic and superconducting phases in the pressurized noncentrosymmetric heavy-fermion compound CeRhG e 3

Unconventional superconductivity frequently emerges as the transition temperature of a magnetic phase, typically antiferromagnetic (AFM), is suppressed continuously toward zero temperature. Here in this paper, we report contrary behavior in pressurized CeRhGe 3 , a noncentrosymmetric heavy-fermion compound. We find that its pressure-tuned AFM transition temperature (T N ) appears to avoid a continuous decrease to zero temperature by terminating abruptly above a dome of pressure-induced superconductivity. Near 21.5 GPa, evidence for T N suddenly vanishes, the electrical resistance becomes linear in temperature, and the superconducting transition reaches a maximum. Analysis of high-pressure resistance and x-ray-absorption spectroscopy measurements suggest that the anomalous connection between antiferromagnetic and superconducting phases in pressurized CeRhGe 3 is associated with proximity to a critical valence instability.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin-Chirality-Driven Ferroelectricity on a Perfect Triangular Lattice Antiferromagnet

Magnetic field (B) variation of the electrical polarization P c ( ∥c) of the perfect triangular lattice antiferromagnet RbFe(MoO 4 ) 2 is examined up to the saturation point of the magnetization for B⊥c. P c is observed only in phases for which chirality is predicted in the in-plane magnetic structures. No strong anomaly is observed in P c at the field at which the spin modulation along the c axis, and hence the spin helicity, exhibits a discontinuity to the commensurate state. These results indicate that the ferroelectricity in this compound originates predominantly from the spin chirality, the explanation of which would require a new mechanism for magnetoferroelectricity. Lastly, the obtained field-temperature phase diagrams of ferroelectricity well agree with those theoretically predicted for the spin chirality of a Heisenberg spin triangular lattice antiferromagnet.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Remarkably robust and correlated coherence and antiferromagnetism in (Ce 1-x La x )Cu 2 Ge 2

We present magnetic susceptibility, resistivity, specific heat, and thermoelectric power measurements on (Ce 1-x La x )Cu 2 Ge 2 single crystals (0 ≤ x ≤ 1). With La-substitution, the antiferromagnetic temperature T N is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of our measurements for x > 0.8. Surprisingly, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below T coh up to ~0.9 of La, indicating a small percolation limit ~9% of Ce. T coh as a function of magnetic field was found to have different behavior for x < 0.9 and x > 0.9. Remarkably, (T coh ) 2 at H = 0 was found to be linearly proportional to T N . In conclusion, the jump in the magnetic specific heat δC m at T N as a function of T K /T N for (Ce 1-x La x )Cu 2 Ge 2 follows the theoretical prediction based on the molecular field calculation for the S = 1/2 resonant level model.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Spin Nernst effect of magnons in collinear antiferromagnets

In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the $\Gamma$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. As a result, possible material candidates are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Magnon Spin Nernst Effect in Antiferromagnets

We predict that a temperature gradient can induce a magnon-mediated spin Hall response in an antiferromagnet with nontrivial magnon Berry curvature. We develop a linear response theory which gives a general condition for a Hall current to be well defined, even when the thermal Hall response is forbidden by symmetry. We apply our theory to a honeycomb lattice antiferromagnet and discuss a role of magnon edge states in a finite geometry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Two-Dimensional J eff = 1 / 2 Antiferromagnetic Insulator Unraveled from Interlayer Exchange Coupling in Artificial Perovskite Iridate Superlattices

We report an experimental investigation of the two-dimensional J eff =1/2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO 3 ) 1 , (SrTiO 3 ) m ] (m=1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m = 2 and m = 3 samples than the m = 1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO 3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Néel temperature from 150 K for m = 1 to 40 K for m = 2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO 3 layer number m, for maintaining canting-induced weak ferromagnetism. In conclusion, the nearly unaltered transition temperature between the m = 2 and the m = 3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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 OSTI2018

From Ising Resonant Fluctuations to Static Uniaxial Order in Antiferromagnetic and Weakly Superconducting CeCo ( In 1 - x Hg x ) 5 ( x = 0.01 )

CeCo (In 0.990 Hg 0.010 ) 5 is a charge doped variant of the d-wave CoCoIn 5 superconductor with coexistent antiferromagnetic and superconducting transitions occurring at T N = 3.4 and T c = 1.4 K, respectively. We use neutron diffraction and spectroscopy to show that the magnetic resonant fluctuations present in the parent superconducting phase are replaced by collinear c -axis magnetic order with three-dimensional Ising critical fluctuations. No low-energy transverse spin fluctuations are observable in this doping-induced antiferromagnetic phase and the dynamic resonant spectral weight predominately shifts to the elastic channel. Static (τ > 0.2 ns) collinear Ising order is proximate to superconductivity in CeCoIn 5 and is stabilized through hole doping with Hg.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Topological Transitions Induced by Antiferromagnetism in a Thin-Film Topological Insulator

Magnetism in topological insulators (TIs) opens a topologically nontrivial exchange band gap, providing an exciting platform for manipulating the topological order through an external magnetic field. Here, we show that the surface of an antiferromagnetic thin film can magnetize the top and the bottom TI surface states through interfacial couplings. During the magnetization reversal, intermediate spin configurations are ascribed from unsynchronized magnetic switchings. This unsynchronized switching develops antisymmetric magnetoresistance spikes during magnetization reversals, which might originate from a series of topological transitions. With the high Néel ordering temperature provided by the antiferromagnetic layers, the signature of the induced topological transition persists up to ~$90 K$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality in CaK ( Fe 1 - x Ni x ) 4 As 4 Revealed by NMR

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe 1-x Ni x ) 4 As 4 exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our 75 As nuclear magnetic resonance studies on CaK(Fe 1-x Ni x ) 4 As 4 (0≤ x ≤ 0.049) revealed that CaKFe 4 As 4 is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe 1-x Ni x ) 4 As 4 highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe 4 As 4 an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗