SPIN-WAVES IN THE ANTIFERROMAGNET PEROVSKITE LAMNO3 - A NEUTRON-SCATTERING STUDY

As part of a general work on doped manganese perovskites, we have carr ied out detailed neutron-scattering experiments on powder and single c rystals of the othorhombic phase of undoped LaMnO3. The temperature de pendence of the sublattice magnetization has been determined in the an tiferromagnetic phase (T-N = 139.5 K), and the critical exponent is be ta = 0.28, well below that corresponding to a pure three-dimensional H eisenberg antiferromagnet. We have measured the dispersion of the spin waves propagating in the highest symmetry directions solving the prob lems related to twinning. The whole spin wave spectrum is well account ed for with a Heisenberg Hamiltonian and a single ion anisotropy term responsible for the easy magnetization direction (b axis). This term i nduces a gap of 2.7 meV at low temperature in the spin wave dispersion curve. An important result is that the ferromagnetic exchange integra l (J(1) approximate to 0.83 meV), coupling the spins within the ferrom agnetic basal plane (a,b), is larger by a factor 1.4 than the antiferr omagnetic exchange integral (J(2) approximate to -0.58 meV) coupling s pins belonging to adjacent MnO2 planes along c.