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

摘要

Using time-of-flight and triple-axis inelastic neutron spectroscopy, wedetermine spin wave excitations throughout the Brillouin zone for ferromagneticmanganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in their low temperaturemetallic states. While spin wave excitations in the long wavelength limit (spinstiffness $D$) have similar values for both compounds, the excitations near theBrillouin zone boundary of La$_{0.7}$Ca$_{0.3}$MnO$_3$ are considerablesoftened in all symmetry directions compared to that ofLa$_{0.75}$Ca$_{0.25}$MnO$_3$. A Heisenberg model with the nearest neighbor andthe fourth neighbor exchange interactions can describe the overall dispersioncurves fairly well. We compare the data with various theoretical modelsdescribing the spin excitations of ferromagnetic manganites.