Existence of Co³⁺ Low-Spin State in TbBaCo₂O_5.5

NMR studies have been carried out on a single-crystal sample of TbBaCo2O5.5 with an oxygen-deficient perovskite structure. We have also carried out neutron diffraction studies and added new results to our previously published data. By analyzing these data, the magnetic structures and the spin states of the system, which have been the subjects of strong controversy, are argued in the ferromagnetic and antiferromagnetic phases. We show that among the various magnetic structures that have been proposed, only our noncollinear structures can reproduce all the experimental results accumulated up to now.