Superposition of ferromagnetic and antiferromagnetic spin chains in the quantum magnet BaAg₂Cu[VO₄]₂

Based on density functional theory band-structure calculations, quantum Monte Carlo simulations, and highfield magnetization measurements, we address the microscopic magnetic model of BaAg₂Cu[VO₄]₂ that was recently proposed as a spin- 1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of J ⁽¹⁾ _a ~ −19 K and J ⁽²⁾ _a ~ 9.5 K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic a direction, and slightly differ in the mutual arrangement of the magnetic CuO₄ plaquettes and nonmagnetic VO₄ tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds.