Structure and Spin Dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$

TL;DR

This study uses neutron scattering to analyze the structure and spin dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$, revealing a ferromagnetic phase, spin-canted phase onset, lattice distortions, and complex damping behaviors linked to electron itinerancy.

Contribution

It provides detailed insights into the magnetic excitations, phase transitions, and damping mechanisms in La$_{0.85}$Sr$_{0.15}$MnO$_3$, highlighting the role of electron itinerancy in spin dynamics.

Findings

Identification of ferromagnetic order below T_C=235 K.

Observation of a spin-canted phase below T_{CA}=205 K.

Detection of strong damping and anomalous quasielastic scattering at high temperatures.

Abstract

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.