Collinear Order in Frustrated Quantum Antiferromagnet on Square Lattice (CuBr)LaNb₂O₇

Abstract

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)LaNb 2 O 7 , prepared by a topotactic ion-exchange reaction of a nonmagnetic double-layered perovskite RbLaNb 2 O 7 . (CuBr)LaNb 2 O 7 exhibits a second-order magnetic transition at 32 K, in marked contrast to a spin-singlet nature for its Cl-based counterpart (CuCl)LaNb 2 O 7 , despite nearly identical structural parameters. The magnetic structure is a novel collinear antiferromagnetic (CAF) ordering characterized by a modulation vector q = (π, 0, π) with a reduced moment of 0.6µ B . Mixed ferromagnetic nearest-neighbor ( J 1 ) and antiferromagnetic 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.