Case study of bilayered spin-1/2 square lattice compound VO(HCOO)₂ · (H₂O)

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline VO(HCOO)₂ · (H₂O) by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron-spin-resonance measurements. The compound crystallizes in an orthorhombic structure with space group Pcca. The crystal lattice features distorted VO₆ octahedra connected via HCOO linkers (formate anions), forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high-field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish the quasi-two-dimensional nature of the compound with nearest-neighbor interaction J₁/k_B ≃ 11.7 K and next-nearest-neighbor interaction J₂/k ≃ 0.02 K. A Néel antiferromagnetic ordering sets in at T_N ≃ 1.1 K. The ratio θ_CW/T_N ≃ 10.9 reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of T_N with magnetic field, consistent with the structural data.