Spin dynamics of lightly doped La1−xSrxMnO3 (abstract)

Abstract

Neutron scattering has been used to study the spin dynamics of the lightly doped La1−xSrxMnO3, with x between 0 and 18. We show that as the dopant concentration increases from 0 to 0.09, the magnetic long-range order progressively changes from a slightly to a strongly canted antiferromagnetic order, whereas the transition temperature is continuously reduced from about 140 K to about 120 K. The spin waves show a strongly anisotropic dispersion relation that can be described by a strong ferromagnetic in-plane coupling Jab and a weak antiferromagnetic out-of-plane coupling Jc. The spin wave dispersion exhibits a finite gap that continuously decreases with the dopant concentration from about 2.5 meV for x=0 to about 1.5 meV for x=0.09. Furthermore, while the ferromagnetic in-plane coupling is slightly enhanced from SJab∼1.8 meV for the undoped sample to SJab∼2.0 meV for 9% doping, the out-of-plane antiferromagnetic coupling is strongly reduced from SJc∼−1.2 meV to SJc∼−0.4 meV. In the same concentration range, the system remains orthorhombic with a continuously decreasing lattice distortion, which could indicate that part of the reduction in the out-of-plane antiferromagnetic coupling can be ascribed to steric effects, whereas the enhancement of the ferromagnetic coupling could be solely associated with electronic effects. Preliminary studies of the spin-wave linewidths in the two systems x=0 and x=0.09 show a significant increase in the damping of the in-plane ferromagnetic spin waves. These results clearly show that La1−xSrxMnO3 evolves into a two-dimensional ferromagnetic system near the concentration x=0.1.