Spin waves in the antiferromagnet perovskite LaMn${\mathrm{O}}_3$: A neutron-scattering study

As part of a general work on doped manganese perovskites, we have carried out detailed neutron-scattering experiments on powder and single crystals of the othorhombic phase of undoped LaMn${\mathrm{O}}_3$. The temperature dependence of the sublattice magnetization has been determined in the antiferromagnetic phase ($T_N=139.5\mathrm{}$ K), and the critical exponent is $\beta =0.28\mathrm{}$, well below that corresponding to a pure three-dimensional Heisenberg antiferromagnet. We have measured the dispersion of the spin waves propagating in the highest symmetry directions solving the problems related to twinning. The whole spin wave spectrum is well accounted for with a Heisenberg Hamiltonian and a single ion anisotropy term responsible for the easy magnetization direction (b axis). This term induces a gap of 2.7 meV at low temperature in the spin wave dispersion curve. An important result is that the ferromagnetic exchange integral ($J_1\approx 0.83$ meV), coupling the spins within the ferromagnetic basal plane (a,b), is larger by a factor 1.4 than the antiferromagnetic exchange integral ($J_2\approx -0.58\mathrm{}$ meV) coupling spins belonging to adjacent Mn${\mathrm{O}}_2$ planes along c.