Quantum magnetism of ferromagnetic spin dimers in α-KVOPO₄

Magnetism of the spin- 1/2 α-KVOPO₄ is studied by thermodynamic measurements, ³¹P nuclear magnetic resonance, neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of θ_CW ≃ 15.9 K and the saturation field of μ₀H_s ≃ 11.3 T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below T_N ≃ 2.7 K in zero field. Magnetic structure with the propagation vector k = (0, 1/2, 0) and the ordered magnetic moment of 0.58 μ_B at 1.5 K exposes a nontrivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value (1 μ_B) indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the VO₆ octahedra.