R-dependence of spin exchange interactions in RMnO3 (R = rare-earth ions)

Magnon excitations have been investigated for PrMnO3 showing an A-type antiferromagnetic ordering and TbMnO3 forming a long-period spin ordering by inelastic neutron scattering. Combined with a result of LaMnO3 [K. Hirota et al.: J. Phys. Soc. Jpn. 65 (1996) 3736], the systematic data revealed that the ferromagnetic exchange interaction between nearest-neighbor (NN) Mn sites in the ab plane markedly decreases in RMnO3 with the ionic radius of R. In addition, finite next-nearest-neighbor (NNN) coupling is evident in TbMnO3. In contrast, the exchange interaction along the c axis shows a much weaker R dependence consistent with the Mn-O-Mn bond angle. These observations confirm a scenario that explains the evolution of spin structures in RMnO3 in terms of the competition between the NN and NNN interactions.