<b>R</b>-Dependence of Spin Exchange Interactions in<b>R</b>MnO₃(<b>R</b>= Rare-Earth Ions)

Abstract

Magnon excitations have been investigated for PrMnO 3 showing an A -type antiferromagnetic ordering and TbMnO 3 forming a long-period spin ordering by inelastic neutron scattering. Combined with a result of LaMnO 3 [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 a b plane markedly decreases in R MnO 3 with the ionic radius of R . In addition, finite next-nearest-neighbor (NNN) coupling is evident in TbMnO 3 . 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 R MnO 3 in terms of the competition between the NN and NNN interactions [T. Kimura et al. : Phys. Rev. B 68 (2003) 060403(R)].