J₁-J₂ Square-Lattice Heisenberg Antiferromagnets with 4d¹ spins: AMoOPO₄Cl (A = K, Rb)

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Magnetic properties of AMoOPO₄Cl (A = K, Rb) with Mo⁵⁺ ions in the 4d¹ electronic configuration are investigated by magnetization, heat capacity and NMR measurements on single crystals, combined with powder neutron diffraction experiments. The magnetization measurements reveal that they are good model compounds for the spin-1/2 J₁-J₂ square lattice magnet with the first and second nearest-neighbor interactions. Magnetic transitions are observed at around 6 and 8 K in the K and Rb compounds, respectively. In contrast to the normal Néel-type antiferromagnetic order, the NMR and neutron diffraction experiments find a columnar antiferromagnetic order for each compound, which is stabilized by a dominant antiferromagnetic J₂. Both compounds realize the unusual case of two interpenetrating J₂ square lattices weakly coupled to each other by J₁.