Gregory John MacDougall

Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2.

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below T_{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k_{B}T_{C}. This is in good agreement with the canonical value of 5 k_{B}T_{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Static and dynamic magnetism in underdoped superconductor BaFe1.92Co0.08As2.

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through TC. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below TC additional intensity appears at an energy of approximately 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.

Static magnetic order and superfluid density of RFeAs(O,F) (R=La,Nd,Ce) and LaFePO studied by muon spin relaxation: Unusual similarities with the behavior of cuprate superconductors

J. P. Carlo, Y. J. Uemura, ∗ T. Goko, 2, 3 G. J. MacDougall, J. A. Rodriguez, W. Yu, G. M. Luke, Pengcheng Dai, N. Shannon, S. Miyasaka, S. Suzuki, S. Tajima, G. F. Chen, W. Z. Hu, J. L. Luo, and N. L. Wang Department of Physics, Columbia University, New York, New York 10027, USA Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3, Canada Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA H H Wills Physics Laboratory, University of Bristol, BS8 1TL Bristol, United Kingdom Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, Peoples Republic of China (Dated: May 14, 2008)

Magnetic order and ice rules in the multiferroic spinel FeV 2 O 4

We present a neutron-diffraction study of FeV2O4, which is rare in exhibiting spin and orbital degrees of freedom on both cation sublattices of the spinel structure. Our data confirm the existence of three structural phase transitions previously identified with x-ray powder diffraction and reveal that the lower two transitions are associated with sequential collinear and canted ferrimagnetic transitions involving both cation sites. Through consideration of local crystal and spin symmetry, we further conclude that Fe2+ cations are ferro-orbitally ordered below 135 K and V3+ orbitals order at 60 K, in accordance with predictions for vanadium spinels with large trigonal distortions and strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium spins at low temperature obey ice rules more commonly associated with the frustrated rare-earth pyrochlore systems.

Strain dependence of transition temperatures and structural symmetry of BiFeO3 within the tetragonal-like structure

The influence of strain-imposed in-plane lattice symmetry on the structural and magnetic properties of tetragonal-like BiFeO3 is investigated by x-ray and elastic neutron scattering. We find that an increase in the in-plane distortion results in an increase of the Neel temperature from 313 ± 5 K to 324 ± 3 K for films grown on YAlO3 and LaAlO3, respectively. The change in magnetic ordering temperature is reproduced in three-dimensional Heisenberg Monte-Carlo simulations. These results show that strain cannot be treated as a single scalar number or simply as a direct consequence of the lattice mismatch between the film material and the substrate.

Muon spin relaxation studies of magnetic-field-induced effects in high-Tc superconductors

Muon spin relaxation measurements in high transverse magnetic fields [FORMULA: SEE TEXT] revealed strong field-induced quasistatic magnetism in the underdoped and Eu-doped (La,Sr)2CuO4 and La1.875Ba0.125CuO4, existing well above Tc and TN. The susceptibility counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards T approximately 25 K. No field-induced magnetism was detected in overdoped La1.81Sr0.19CuO4, optimally doped Bi2212, and Zn-doped YBa2Cu3O7.

Kinetically inhibited order in a diamond-lattice antiferromagnet.

Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here, we present a comprehensive single-crystal neutron scattering study of CoAl2O4, a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Néel ordering. Below the temperature T∗ = 6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T∗ had previously been associated with the onset of glassy behavior. Our new results suggest instead that T∗ signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing first-order phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials.

Muon spin rotation measurements of heterogeneous field response in overdoped La2-xSrxCuO4

Transverse-field muon spin rotation measurements of overdoped La{sub 2-x}Sr{sub x}CuO{sub 4} reveal a large broadening of the local magnetic field distribution in response to applied field, persisting to high temperatures. The field response is approximately Curie-Weiss-like in temperature and is largest for the highest doping investigated. Such behavior is contrary to the canonical Fermi-liquid picture commonly associated with the overdoped cuprates and implies extensive heterogeneity in this region of the phase diagram. A possible explanation for the result lies in regions of staggered magnetization about dopant cations, analogous to what is argued to exist in underdoped systems.

Structural transition and orbital glass physics in near-itinerant CoV2O4

In this study, the ferrimagnetic spinel CoV2O4 has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron di raction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak (Δa/a ~ 10–4), first order structural phase transition at T* = 90 K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak 1.25meV spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below T*.