Quantum Effect on the Ground State of the Triple-Perovskite Ba3MNb2O9 (M = Co, Ni, and Mn) with Triangular-Lattice

As the simplest example of geometrical frustration, the two-dimensional triangular lattice antiferromagnet exhibits the mismatch between the lattice geometry and spin-exchange interaction, which has been the subject of intensive studies due to its exotic quantum phenomena. In this work, we performed detailed studies of the magnetic structures and spin wave excitations by neutron powder diffraction and inelastic neutron scattering measurements on the triple-perovskite oxides Ba₃MNb₂O₉ (M = Co, Ni, and Mn) with triangular-lattice geometry. The interplay between the frustrated interaction and easy-plane/axis anisotropy gives rise to two magnetic phase transition temperatures for Ba₃CoNb₂O₉ (Ba₃MnNb₂O₉) and only one for Ba₃NiNb₂O₉. The linear spin-wave theory +1/S calculations indicate that both spatial dimensionality and the spin size have a significant impact on the strength of quantum fluctuations, which lead to their different magnetic ground states and exotic physical properties. Moreover, the effects of the thermal fluctuations are presented for Ba₃NiNb₂O₉.