Magnetic properties of LiCu3O3 : A quasi-two-dimensional antiferromagnet on a depleted square lattice

LiCu₃O₃ is a novel 2D S=1/2 antiferromagnet with randomly depleted square lattice. The crystal structure contains two types of square planes with different Cu²⁺→ Li⁺ substitution rates (20% and 40%). ⁷Li NMR and magnetization measurements performed on single crystals of LiCu₃O₃ revealed the occurrence of magnetic order at Tc1=123 K and the change of the magnetic state at Tc2≈30 K. The high temperature transition can be attributed to establishment of magnetic order in planes with higher concentration of magnetic ions and the low temperature transition -- to the magnetic ordering in planes with lower concentration of magnetic ions. Broad continuous NMR spectra below Tc1 reflect a continuous distribution of values or directions of magnetic moments in LiCu₃O₃ typical for spiral, spin-modulated magnetic structures or structures with frozen disorder. Magnetization measurements revealed a spin-flop transition which indicates weak uniaxial anisotropy of the spin structure. Relatively small value of magnetic susceptibility at all orientations of applied magnetic field shows that magnetic structure is rigid, since the estimated value of saturation field derived from differential susceptibility measured at μ₀ H=5 T is μ₀Hₛₐₜ ≈200 T.