The recent observations of superconductivity at temperatures up to 55K in compounds containing layers of iron arsenide [1,2,3,4] have revealed a new class of high temperature superconductors that show striking similarities to the more familiar cuprates. In both series of compounds, the onset of superconductivity is associated with the suppression of magnetic order by doping holes and/or electrons into the band [5] leading to theories in which magnetic fluctuations are either responsible for or strongly coupled to the superconducting order parameter [6]. In the cuprates, theories of magnetic pairing have been invoked to explain the observation of a resonant magnetic excitation that scales in energy with the superconducting energy gap and is suppressed above the superconducting transition temperature, Tc. Such resonant excitations have been shown by inelastic neutron scattering to be a universal feature of the cuprate superconductors [7], and have even been observed in heavy fermion superconductors with much lower transition temperatures [8,9,10]. In this paper, we show neutron scattering evidence of a resonant excitation in Ba0.6K0.4Fe2As2, which is a superconductor below 38 K [4], at the momentum transfer associated with magnetic order in the undoped compound, BaFe2As2, and at an energy transfer that is consistent with scaling in other strongly correlated electron superconductors. As in the cuprates, the peak disappears at Tc providing the first experimental confirmation of a strong coupling of the magnetic fluctuation spectrum to the superconducting order parameter in the new iron arsenide superconductors.Unconventional superconductivity has been the subject of considerable theoretical and experimental interest since the discovery of superconductivity in CeCu 2 Si 2 and other heavy fermion compounds [11], an interest that was only intensified by the discovery of cuprate superconductors with transition temperatures in excess of 100 K [6]. Although significant progress has been made, the origin of unconventional superconductivity is still not understood. The observation of a magnetic resonance in the spin excitation spectrum which appears concurrently with the onset of superconductivity in both the high T c cuprates [12,13,14,15,16] and the heavy fermion superconductors [8,9,10] offers the tantalizing possibility of a unifying theme for unconventional superconductivity that spans a diverse range of superconducting materials. Recently, a new family of superconductors containing layers of Fe 2 As 2 has been discovered with T c s in excess of 50 K stimulating considerable experimental and theoretical activity [1,2,3]. Although there is mounting evidence that the superconductivity in this new family is also unconventional [17], there is as yet no consensus concerning the mechanism giving rise to superconductivity or even the superconducting pairing symmetry. In this letter, we describe neutron scattering data that confirm for the first time the existence of a resonant spin excitation below T c in the iron arsenide ma...

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors 1-3 , a central question has been whether magnetism in these materials arises from weakly correlated electrons 4,5 , as in the case of spin density wave in pure chromium 6 , requires strong electron correlations 7 , or can even be described in terms of localized electrons 8,9 such as the AF insulating state of copper oxides 10 . Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe 1.9 Ni 0.1 As 2 (T c = 20 K), which allows us to obtain the size of the fluctuating magnetic moment m 2 , and its energy distribution 11,12 . We find that superconducting BaFe 1.9 Ni 0.1 As 2 and AF BaFe 2 As 2 (ref. 13) both have fluctuating magnetic moments m 2 ≈ 3.2 µ 2 B per Fe(Ni), which are similar to those found in the AF insulating copper oxides 14,15 . The common theme in both classes of high-temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high-temperature superconductivity 16 .In the undoped state, iron pnictides such as BaFe 2 As 2 form a metallic low-temperature orthorhombic phase with the antiferromagnetic (AF) structure as shown in Fig. 1a (ref. 17). Inelastic neutron scattering measurements have mapped out spin waves throughout the Brillouin zone in the AF orthorhombic and paramagnetic tetragonal phases 13 . On Co-and Ni-doping to induce optimal superconductivity via electron doping, the orthorhombic structural distortion and static AF order in BaFe 2 As 2 are suppressed and the system becomes tetragonal and paramagnetic at all temperatures 18 . In previous inelastic neutron scattering experiments on optimally electron-doped Ba(Fe, Co, Ni) 2 As 2 superconductors 11,12,[19][20][21][22] , spin excitations up to ∼120 meV were observed. However, the lack of spin excitation data at higher energies in absolute units precluded a comparison with spin waves in undoped BaFe 2 As 2 . Only the absolute intensity measurements in the entire Brillouin zone can reveal the effect of electron doping on the overall spin excitation spectra and allow a direct comparison with the results in the AF insulating copper oxides 14,15 . For the experiments, we chose to study well-characterized electron-doped BaFe 1.9 Ni 0.1 As 2 (refs 20,22) because large single crystals were available 23 and their properties are similar to Co-doped BaFe 2 As 2 (refs 11,12,19,21,24).By comparing spin excitations in BaFe 1.9 Ni 0.1 As 2 and BaFe 2 As 2 throughout the Brillouin zone, we were able to probe how electron doping and superconductivity affect the overall spin

The origin of the superconducting state in the recently discovered Fe-based materials 1-3 is the subject of intense scrutiny. Neutron scattering 4-7 and NMR (ref. 8) measurements have already demonstrated a strong correlation between magnetism and superconductivity. A central unanswered question concerns the nature of the normal-state spin fluctuations that may be responsible for the pairing. Here we present inelastic neutron scattering measurements from large single crystals of superconducting and non-superconducting Fe 1+y Te 1−x Se x . These measurements indicate a spin fluctuation spectrum dominated by two-dimensional incommensurate excitations extending to energies greater than 250 meV. Most importantly, the spin excitations in Fe 1+y Te 1−x Se x have four-fold symmetry about the (1, 0) wavevector (square-lattice (π,π) point). Moreover, the excitations are described by the identical wavevector and can be characterized by the same model as the normal-state spin excitations in the high-T C cuprates 9-11 . These results demonstrate commonality between the magnetism in these classes of materials, which perhaps extends to a common origin for superconductivity.

The frustrated pyrochlore magnet Yb2Ti2O7 has the remarkable property that it orders magnetically, but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe in addition to dispersive magnons also a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low and high field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set we re-evaluate the spin Hamiltonian finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields. The lattice of corner-shared tetrahedra realized in cubic A 2 B 2 O 7 pyrochlores and AB 2 O 4 spinels, is a canonical lattice to explore correlated magnetism in the presence of strong geometric frustration effects. In the strongly spin-orbit coupled rare earth pyrochlores, experiment has uncovered materials offering a tremendously rich spectrum of magnetic behavior. Notable examples include classical spin ice physics as in the rare-earth titanates (Ho/Dy) 2 Ti 2 O 7 where Ising antiferromagnetism leads to an emergent classical electrostatics at low temperatures [1] and "order-by-disorder" in XY antiferromagnets where thermal and quantum fluctuations lift a large frustration-induced degeneracy resulting in unconventional magnetic order as in Er 2 Ti 2 O 7 [2-5]. Currently, much of the interest in this field concentrates on a handful of materials that seem to fall outside a semiclassical understanding of these systems. The pyrochlore Yb 2 Ti 2 O 7 [6-25], where Kramers Yb 3+ ions behave as effective spin 1/2 moments, is quite unique in its behavior: in high applied magnetic fields dispersive magnons were observed [12], which are apparently replaced by a broad continuum of scattering at zero field [16] despite the presence of ferromagnetic order. This exotic behavior is not yet understood. To make progress one would like to know i) how the broad scattering continuum in zero field originates from quantum fluctuations, whether those fluctuations are also present at high field and, if so, how they manifest themselves, ii) how the sharp magnons "disappear" over a wide range of the Brillouin zone as the field is lowered. Here we experimentally answer those questions by studying the behavior in a magnetic field applied along the cubic [001] direction, which has not been explored in detail before and which, we will show, allows for a transparent interpretation of the phase diagram and evolution of the spectrum in a magnetic field. The experiment also allows us to re-visit the parametrization of the magnetic exchange, which is a critical ingredien...

We report neutron scattering measurements of cooperative spin excitations in antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide superconductor. The data extend up to ∼ 100 meV and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.PACS numbers: 74.25. Ha, 74.70.Dd, 75.30.Ds, 78.70.Nx One of the greatest challenges presented by the recently discovered iron pnictide superconductors 1 is to identify the electron pairing interaction which permits the formation of a superconducting condensate. In conventional superconductors this interaction is provided by the exchange of a phonon. For the iron pnictides, however, theoretical calculations 2,3 indicate that the electron-phonon coupling is too weak to account for the observed high critical temperatures. Attention has therefore turned to other types of bosonic excitations which could mediate the pairing interaction.One such candidate is spin fluctuations 4 . In common with the layered cuprates, superconductivity in the pnictides is found in close proximity to parent phases which exhibit long-range antiferromagnetic order 5,6 . However, unlike the cuprates, whose magnetic properties are governed by strong superexchange interactions between localized spin-1 2 moments in a single Cu 3d x 2 −y 2 orbital, magnetism in the pnictides is more itinerant in character and derives from multiple d orbitals. It may also involve a degree of frustration. In magnetically ordered materials the dominant magnetic excitations are coherent spin waves. Wavevector-resolved measurements of the spinwave spectrum by inelastic neutron scattering provide information on the fundamental magnetic interactions and can also reveal effects due to itinerancy and frustration. Such studies on the magnetically ordered parent phases of unconventional superconductors like the cuprates and iron pnictides are important to establish the characteristic energy scales of the spin fluctuations and also to provide a reference against which changes associated with superconductivity can be identified.Here we present neutron scattering data on the collective spin excitations in antiferromagnetic BaFe 2 As 2 . We find that the spin excitation spectrum has a very steep dispersion within the FeAs layers with a bandwidth in the region of 0.17 eV, not much less than that in the cuprates. Such a high characteristic energy suggests that spin fluctuations are a serious candidate to mediate high temperature superconductivity in the iron pnictides.The parent phase BaFe 2 As 2 becomes superconduct- ing on doping with holes 7 or on application of pressure 8 . At T s = 140 K, BaFe 2 As 2 undergoes a structural transition from tetragonal to orthorhombic and simultaneously develops three-dimensional long-range antiferromagnetic order 9,10...

With the discovery of molecular complexes exhibiting slow relaxation of the magnetization and magnetic hysteresis at low temperature, research activity in the field of molecular magnetism based on coordination compounds has experienced spectacular growth. [1] These nanomagnets, called single-molecule magnets (SMMs), [1][2][3] straddle the quantum/ classical interface showing quantum effects, such as quantum tunneling of the magnetization and quantum phase interference, and have potential applications in molecular spintronics, ultra-high density magnetic information storage, and quantum computing at the molecular level. [3] The motivation of much of this research activity has been provided by the prospect of integrating SMMs into nanosized devices. The origin of the SMM behavior is the existence of an energy barrier that prevents reversal of the molecular magnetization, [1] although the currently observed energy barriers are (relatively) low and therefore SMMs act as magnets only at very low temperature. To increase the height of the energy barrier and therefore to improve the SMM properties, systems with large spin-ground states and/or with large magnetic anisotropy are required. The early examples of SMMs were clusters of transition metal ions, [2] but recently mixed 3d/4f metal aggregates, [4] low-nuclearity 4f metal complexes, [5] and even mononuclear complexes (called single-ion magnets, SIMs) of lanthanide, [6] actinide, [7] and transition-metal ions [8] have been reported to exhibit slow relaxation of the magnetization.It should be noted that for integer-spin systems with D < 0 fast quantum tunneling of the magnetization (QTM) through the mixing of AE Ms levels may suppress the observation of slow magnetic relaxation through a thermally activated mechanism. QTM is promoted by transverse zero-field splitting (E), hyperfine interactions, and/or dipolar interactions. [1] The application of a small direct current (dc) field, stabilizing the negative Ms levels with regard to the positive ones, may remove the degeneracy of the AE Ms levels on either side of the energy barrier, tilting the system out of resonance and, on occasion, enabling the thermally activated mechanism. For non-integer spin systems with D < 0, the mixing of the degenerate ground state AE Ms levels through transverse anisotropy (E) is forbidden, thus favoring observation of the thermally activated relaxation process. [9] This situation, together with the fact that mononuclear species can exhibit larger anisotropies than their multinuclear counterparts (the

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Using inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO 3 single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriyalike term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO 3 provides a fundamental ingredient for understanding of the novel magnetic properties of BiFeO 3 .