Spin waves throughout the Brillouin zone and magnetic exchange coupling in the ferromagnetic metallic manganites La_(1-x)Ca_xMnO_3 (x = 0.25, 0.30)

摘要

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_xMnO_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 considerably 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.