Recent neutron scattering measurements on the La2−xSrxCuO4 system have revealed a drastic change of the incommensurate static spin correlations from diagonal in the insulating region to parallel in the superconducting region. We report the doping dependence of the ordered magnetic moment for the hole concentration region 0.03 ≤ x ≤ 0.12, focusing on the relationship between the static magnetism and the superconductivity. The elastic magnetic crosssection decreases monotonically with increasing x for 0.03 ≤ x ≤ 0.07. We find that the ordered magnetic moment µ varies from ∼ 0.18 µB/Cu (x = 0.03) to ∼ 0.06 µB/Cu (x = 0.07). No significant anomaly is observed at the insulator-superconductor boundary (x ∼ 0.055). The elastic magnetic cross section is enhanced in the vicinity of x = 0.12 where resolution limited width peaks are observed in neutron scattering measurements and where the apparent magnetic and superconducting transitions coincide.74.72. Dn, 75.30.Fv, 75.50.Ee
Inelastic neutron scattering with high wave-vector resolution has characterized the propagation of transverse spin wave modes near the antiferromagnetic zone center in the metastable domain state of a random field Ising magnet. A well-defined, long wavelength excitation is observed despite the absence of long-range magnetic order. Direct comparisons with the spin wave dispersion in the long-range ordered antiferromagnetic state reveal no measurable effects from the domain structure. This result recalls analogous behavior in thermally disordered anisotropic spin chains but contrasts sharply with that of the phonon modes in relaxor ferroelectrics.PACS numbers: 75.30. Ds, 75.50.Lk Disorder in a condensed matter system can strongly influence mode propagation through the medium, providing a unique perspective on its microscopic behavior. To gain a more comprehensive understanding of such effects from disorder, we have performed a neutron scattering study of the long wavelength spin waves of the frozen domain state of a random field magnet. Despite the wealth of attention directed at the random field problem [1], little attention has been directed toward the spin wave dispersion in these systems. Our study has been motivated in part by experiments on the phonons in several relaxor ferroelectrics including Pb(Zn 1/3 Nb 2/3 )O 3 and Pb(Mg 1/3 Nb 2/3 )O 3 that have demonstrated interesting anomalies associated with the disorder in these systems [2,3,4,5]. Specifically, at wave vectors near 0.2 A −1 , the dispersion of the transverse optic branch appears to drop precipitously into the acoustic branch. This phenomenon has been identified with a sharply wavevector dependent overdamping associated with the presence of polar nanoregions in the relaxors [2]. To test the generality of such behavior for systems with disorder, we have performed a search for analogous effects in the spin waves of the domain state of the random field magnet. In marked contrast to the phonons in the relaxors, we observe well-defined long wavelength excitations transverse to the ordering axis, despite the absence of long range order, and find no measurable effect on the spin waves from the domain structure. We identify the persistence of these modes as an apparently generic feature of disordered magnetic systems with anisotropy, as first realized in thermally disordered spin chains [6,7,8].The experiments were performed on a diluted antiferromagnet, Mn x Zn 1−x F 2 with x = 0.5, cooled in a magnetic field parallel to the Ising axis. The physics of the diluted Ising antiferromagnet in a uniform field can be mapped directly on to that of a random field Ising magnet [9], providing a method for experimental realization of the random field system. MnF 2 has a tetragonal rutiletype structure with lattice constants a = 4.87Å and c = 3.31Å. The spin interaction is primarily Heisenberglike, and a weak dipolar interaction between the Mn moments accounts for the small Ising anisotropy that aligns the Mn spins along the c axis (i.e., the tetragonal axis). Dil...
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