Magnetic and thermodynamic properties of Li2VOSiO4:: A two-dimensional S=1/2 frustrated antiferromagnet on a square lattice -: art. no. 024409

NMR, muon spin rotation (mu SR), magnetization, and specific-heat measurements in Li2VOSiO4 powders and single crystals are reported. Specific-heat and magnetization measurements evidence that Li2VOSiO4 is a frustrated two-dimensional S=1/2 Heisenberg antiferromagnet on a square lattice with a superexchange coupling J(1), along the sides of the square, almost equal to J(2), the one along the diagonal (J(2)/J(1) = 1.1 +/- 0.1 with J(2) + J(1) = 8.2+/-1 K). At T-c similar or equal to 2.8 K a phase transition to a low-temperature collinear order is observed. T-c and the sublattice magnetization, derived from NMR and mu SR, were found practically independent on the magnetic field intensity up to 9 T. The critical exponent of the sublattice magnetization was estimated beta similar or equal to0.235, nearly coincident with the one predicted for a two-dimensional XY system on a finite size. The different magnetic properties found above and below T-c are associated with the modifications in the spin Hamiltonian arising from a structural distortion occurring just above T-c.