PHYSICAL REVIEW B VOLUME 39, NUMBER 4 1 FEBRUARY 1989 Static and dynamic spin fluctuations in superconducting La2 Sr Cu04 R. J. Birgeneau Physics Department, Brookhaven National Laboratory, Upton, New York 11973 and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Y. Endoh and K. Kakurai Department of Physics, Tohoku University, Sendai 980, Japan Y. Hidaka and T. Murakami Tokai, Ibaraki 319-11, Japan M. A. Kastner and T. R. Thurston Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 G. Shirane Physics Department, Brookhaven National Laboratory, Upton, New York 11973 K. Yamada Physics Department, Brookhaven National Laboratory, Upton, New York 11973 and Department of Physics, Tohoku University, Sendai 980, Japan (Received 31 August 1988) %'e report a neutron scattering study of the spin fluctuations in samples of Lai. 89Sro. i~Cu04 which are — 80% superconducting with T, =10 K. The structure factor S(Q) refiects three- dimensional modulated spin correlations with an in-plane correlation length of order 18 ~6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasielastic ( I tsE l & 0.5 meV) at low temperatures. No significant differences are ob- served in the normal and superconducting states. A variety of experiments has indicated that the Cu02 lamellar superconducting materials exhibit novel but com- plicated magnetic effects. ' Recent neutron experi- ments in La2 — „Sr„Cu04 with 0.02 ~ x ~ 0. 14 show that as x increases the Cu + moment is preserved but the basic antifer rom agnetic state becomes progressively shorter ranged. ' These experiments, however, are not definitive vis-a-vis the nature of the magnetism in the superconduct- ing state since the samples studied there exhibit a Meiss- ner fraction of at best 15%. Recently, however, two of us (Y. H. and T.M. ) have made significant progress in the growth of single-crystal La2 — „Sr„Cu04 of high crystal- line perfection and with a large Meissner fraction ( — 809o). In this note we report a neutron scattering study of samples of La2 — „Sr Cu04 with T, =10 K and an 80% Meissner fraction. The high quality of the samples has al- lowed a much more thorough study of the spin correla- tions than was possible previously. We find a number of new results. First, the static factor S(Q) exhibits a com- plicated three-dimensional incommensurate structure with a characteristic two-dimensional (2D) correlation length of order 18+'6 A. The low-energy (~&E~ &0. 5 meV) part of S(Q) exhibits pronounced three dimen-- sional correlations at all temperatures (5 to 350 K). The response function evolves with temperature from being predominantly inelastic ( ~ AE ~ &0. 5 meV) at high tem- peratures to mainly quasielastic ( ~ hE ~ & 0.5 meV) at low temperatures, T ~ 50 K. The integrated intensity is, however, preserved; further it is close to that observed un- der identical spectrometer conditions for pure La2Cu04. The experiments were carried out on the H7 and H4M triple-axis spectrometers at the Brookhaven High Flux Beam Reactor. As will be discussed below, the experi- ments proved to be rather difficult and, thus, required a novel approach to data collection. Specifically, it was discovered early in the experiments that there was a strik- ing thermal evolution in the distribution in energy of the scattering so that it was essential to separate the quasi- elastic ( ~ IsE ~ & 0.5 meV) and integrated inelastic ( ~ AE ~ & 0.5 meV) contributions to S(Q). Data were ob- tained primarily using neutrons with energy E; =14. 7 meV and collimations 40'-40'-40'-80'; the monochromator and analyzer were pyrolytic graphite (PG) (002) and the incoming beam passed through two PG filters in order to eliminate completely any A, /2 neutrons. The spectrometer was set up in the triple-axis mode and all scans were car- ried out twice, first detecting neutrons scattered oA' the PG analyzer so that ~ AF. ~ & 0.5 meV and second detect- ing neutrons passing straight through the analyzer. The eff'ective reAectivity of the analyzer was measured to be 78% so that by subtracting 29% of the first scan from the second scan, one obtained precisely in the latter scan the intensity integrated over all energies with ~IsE l &0. 5 ~9 2868 1989 The American Physical Society