Origin of Spin Wave Gap in Metallic Ferromagnet La1-χAχMnO3

The spin wave spectrum of the double-exchange model with anisotropic Hund coupling is investigated using the spin wave approximation and Green's function approach based on Furukawa's work [J. Phys. Soc. Jpn. 65 (1996) 1174]. It is found that there exists a spin gap, which agrees with the observations in the inelastic neutron scattering experiments for La_0.7Sr_0.3MnO₃ and La_0.7Pb_0.3MnO₃. The spin gap is directly related to the anisotropy of Hund coupling. The ratio of the xy and z Hund coupling strengths is estimated from experimental data and the possible origin of the spin gap from the Jahn-Teller effect is pointed out.