Spin waves throughout the Brillouin zone and magnetic exchange coupling in ferromagnetic metallic manganites La$_{1-x}$Ca$_{x}$MnO$_3$ ($x=0.25,0.30$)

TL;DR

This study maps spin wave excitations across the entire Brillouin zone in ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ and compares experimental results with theoretical models to understand magnetic exchange interactions.

Contribution

It provides comprehensive experimental data on spin wave dispersions in La$_{1-x}$Ca$_x$MnO$_3$ and evaluates the applicability of a Heisenberg model with multiple exchange interactions.

Findings

Spin wave excitations are similar at long wavelengths for both compounds.

Significant softening of excitations occurs near the Brillouin zone boundary in La$_{0.7}$Ca$_{0.3}$MnO$_3$.

A Heisenberg model with nearest and fourth neighbor interactions describes the data well.

Abstract

Using time-of-flight and triple-axis inelastic neutron spectroscopy, we determine spin wave excitations throughout the Brillouin zone for ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in their low temperature metallic states. While spin wave excitations in the long wavelength limit (spin stiffness $D$) have similar values for both compounds, the excitations near the Brillouin zone boundary of La$_{0.7}$Ca$_{0.3}$MnO$_3$ are considerable softened in all symmetry directions compared to that of La$_{0.75}$Ca$_{0.25}$MnO$_3$. A Heisenberg model with the nearest neighbor and the fourth neighbor exchange interactions can describe the overall dispersion curves fairly well. We compare the data with various theoretical models describing the spin excitations of ferromagnetic manganites.