Collinear order in frustrated quantum antiferromagnet on square lattice (CuBr)LaNb2O7

Magnetic susceptibility, heat capacity, high-field magnetization and neutron diffraction measurements have been performed on a two-dimensional S = 1/2 square-lattice system (CuBr)LaNb2O7, prepared by a topotactic ion-exchange reaction of a nonmagnetic double-layered perovskite RbLaNb2O7. (CuBr)LaNb2O7 exhibits a second-order magnetic transition at 32 K, in marked contrast to a spin-singlet nature for its Cl-based counterpart (CuCl)LaNb2O7, despite nearly identical structural parameters. The magnetic structure is a novel collinear antiferromagnetic (CAF) ordering characterized by a modulation vector q = (pi, 0, pi) with a reduced moment of 0.6 mu(B). Mixed ferromagnetic nearest-neighbor (J(1)) and anti ferromagnetic second-nearest-neighbor (J(2)) interactions are of comparable strength (J(1/)k(B) = -35.6 K and J(2)/k(B) = 41.3 K), placing the system in a more frustrated region of the CAF phase than ever reported.