Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic A1−xA ′ x MnO3 manganites (where A and A ′ are rare-and alkaline-earth ions), which when combined with previous work elucidate the systematics of the interactions as a function of carrier concentration x, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange (J4/J1) that controls the zone boundary magnon softening changes systematically with x, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.Determining the evolution of the elementary magnetic excitations in A 1−x A ′ x MnO 3 (where A and A ′ are rareand alkaline-earth ions respectively) is the first step in understanding the magnetic interactions in these doped perovskite manganites. According to the conventional double-exchange (DE) mechanism [1], the motion of charge carriers in the metallic state of A 1−x A ′ x MnO 3 establishes a ferromagnetic (FM) interaction between spins on adjacent Mn 3+ and Mn 4+ sites. In the strong Hundcoupling limit, spin-wave excitations of a DE ferromagnet below the Curie temperature T C can be described by a Heisenberg Hamiltonian with only the nearest neighbor exchange coupling [2]. At the long wavelength (small wavevector q), spin-wave stiffness D measures the average kinetic energy of charge carriers and therefore should increase with increasing x [2, 3]. While spin dynamics of some manganites initially studied appeared to follow these predictions [4,5], later measurements revealed anomalous zone boundary magnon softening deviating from the nearest neighbor Heisenberg Hamiltonian for other materials with x ∼ 0.3 [6,7,8,9,10]. Three classes of models have been proposed to explain the origin of such deviations. The first is based on the DE mechanism, considering the effect of the on-site Coulomb repulsion [3] or the conducting electron band (e g ) filling dependence of the DE and superexchange interactions [11]. The second suggests that magnon-phonon coupling [8,12] or the effects of disorder on the spin excitations of DE systems [13] is the origin for the zone boundary magnon softening. Finally, quantum fluctuations of the planar (x 2 −y 2 )-type orbital associated with the A-type antiferromagnetic (AF) ordering may induce magnon softening as the precursor of such AF order [14]. Although all these models appear to be reasonable in explaining the zone boundary magnon softening near x = 0.3, the lack of complete spinwave dispersion data for A 1−x A ′ x MnO 3 with x < 0.3 and x > 0.4 means that one cannot test the doping dependence of different mechanisms and, therefore, the origin of the magnon softening is still unsettled.Very recently, Endoh et al.[15] measured spinwave excitations in the FM phase of Sm 0.55 Sr 0.45 MnO 3 (SSMO45) and found that the dispersion can be described phenomenologically by the Heisenberg model with the nearest ne...

A neutron scattering study of heavily hole-overdoped superconducting KFe2As2 revealed a well-defined low-energy incommensurate spin fluctuation at [π(1 ± 2 δ),0] with δ = 0.16. The incommensurate structure differs from the previously observed commensurate peaks in electron-doped AFe2As2 (A = Ba, Ca, or Sr) at low energies. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)2As2 at high energies. A band structure calculation suggests interband scattering between bands around the Γ and X points as an origin of this incommensurate peak. The perpendicular direction of the peak splitting can be understood within the framework of multiorbital band structure. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped samples, which can be responsible for the appearance of superconductivity in the heavily hole-doped samples.

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe 2 As 2 single crystals. With the field along the c axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the c axis, and thus d-wave or anisotropic s-wave pairing. However, the strong temperature dependence of the signal at T T c shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the c axis.

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ErNi 2 B 2 C is the first material in which a microscopic coexistence between weak ferromagnetism ͑WFM͒ and superconductivity ͑SC͒ was confirmed by neutron diffraction technique. In the present paper, we report the detailed WFM structure and its ordering process. In the WFM phase, weak but clear peaks appear at (n/20 0 0) with nϭeven. The detailed analysis revealed that one from 20 Er magnetic moments contributes to the WFM ordering and such spins form ferromagnetic sheets that appear with a periodicity of 10a (ϳ35 Å).