Magnetic and Structural Studies on Two-Dimensional Antiferromagnets (MCl)LaNb2O7 (M = Mn, Co, Cr)

We report magnetic and structural studies on the two-dimensional antiferromagnets (MCl)LaNb2O7 (M = Mn, Cr, Co), prepared by topochemical reactions of a layered perovskite RbLaNb2O7. Electron diffraction of these oxyhalides revealed a superstructure with a root 2a x root 2a cell for M = Mn and Co, and a 2a x 2a cell for M = Cr, indicating that the MCl networks are distorted from an ideal square lattice. Neutron diffraction experiments showed that M = Mn and Co exhibit a (pi 0 pi) antiferromagnetic order as observed for the S = 1/2 counterparts. (CoCl)LaNb2O7 with a strong spin anisotropy shows an antiferro to weak-ferromagnetic transition at low field, followed by novel two-step metamagnetic transitions likely associated with a 1/2 plateau for 27-54 T. Possible spin structures under magnetic field are discussed in terms of an Ising-type model. By contrast, (CrCl)LaNb2O7 exhibits a (pi pi pi) order, which is the first observation among related oxyhalides, and a spin-flop transition at 12 T due to a weak spin anisotropy. These results suggest that a slight difference in the MCl structure and spin anisotropy provides a crucial influence on the magnetic properties.