SPIN DYNAMICS IN LA1-XSRXMNO3

The spin dynamics of the La1-xSrxMnO3 system have been studied from th e lightly (x < 0.1) to the heavily doped (x > 0.17) limit. In the low- doping limit we show that the spin waves exhibit a finite-energy gap a nd a strongly anisotropic dispersion relation, which can be described by a strong in-plane ferromagnetic coupling and a weak inter-plane ant iferromagnetic coupling. As the dopant concentration increases, the sp in-wave energy gap and the inter-plane coupling both decrease consider ably with x. For x = 0.11 a second magnetic-phase transition occurs at T = 122 K evidenced by the appearance of new magnetic peaks at (0, 0, 1/2) and of a new gap in the spin-wave spectrum at finite q = (0, 0, 1/4). In the metallic ferromagnetic state (x > 0.17), the system behav es like an isotropic ferromagnet at low T, with a quadratic long-wavel ength dispersion relation E=E-0+Dq(2) and no significant anisotropy ga p (E-0 < 0.04 meV). The spin-wave stiffness constant D increases stron gly with x, conserving a large D-T=0/kT(c) ratio, indicative of an iti nerant soft ferromagnet. In this limit, the x-dependence of the stiffn ess constant is found to be in good agreement with the double-exchange model with an effective bandwidth of about 0.3 eV and a Hund's coupli ng constant 4J(H) similar to 4.7eV. (C) 1998 Elsevier Science B.V. All rights reserved.