Structure and Spin Dynamics of La0.85_{0.85}Sr0.15_{0.15}MnO3_3

Neutron scattering has been used to study the structure and spin dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0, 0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$ dependence at low T, and the damping at small q follows $q^4T^{2}$. An anomalously strong quasielastic component, however, develops at small wave vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the $e_g$ electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript

The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe

Stripes Induced by Orbital Ordering in Layered Manganites

Spin-charge-orbital ordered structures in doped layered manganites are investigated using an orbital-degenerate double-exchange model tightly coupled to Jahn-Teller distortions. In the ferromagnetic phase, unexpected diagonal stripes at $x$=$1/m$ ($m$=integer) are observed, as in recent experiments. These stripes are induced by the orbital degree of freedom, which forms a staggered pattern in the background. A $\pi$-shift in the orbital order across stripes is identified, analogous to the $\pi$-shift in spin order across stripes in cuprates. At $x$=1/4 and 1/3, another non-magnetic phase with diagonal static charge stripes is stabilized at intermediate values of the $t_{\rm 2g}$-spins exchange coupling.Comment: reordering of figure

Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites

In order to study the magnetic excitation behaviors in colossal magnetoresistance manganites, a magnon-phonon interacting system is investigated. Sudden broadening of magnon linewidth is obtained when a magnon branch crosses over an optical phonon branch. Onset of the broadening is approximately determined by the magnon density of states. Anomalous magnon damping at the brillouine zone boundary observed in low Curie temperature manganites is explained.Comment: 4 pages incl. 4 figs. New e-mail: [email protected]

Neutron and X-ray evidence of charge melting in ferromagnetic layered colossal magnetoresistance manganites

Recent x-ray and neutron scattering studies have revealed static diffuse scattering due to polarons in the paramagnetic phase of the colossal magnetoresistive manganites La2-2xSr1+2xMn2O7, with x = 0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combined ferromagnetic and metal-insulator transition in the x = 0.40 system because of the sudden charge delocalization, while persisting at low temperature in the antiferromagnetic x = 0.44 system. The "melting" of the polaron ordering as we cool through T-C occurs with the collapse of the polaron scattering itself in the x = 0.40 system. This short-range polaron order is characterized by an ordering wave vector q = (0.3,0,1) that is almost independent of x for x greater than or equal to 0.38, and is consistent with a model of disordered stripes. (C) 2001 American Institute of Physics