Two-step antiferromagnetic transition and moderate triangular frustration in Li2Co(WO4)(2)

We present a detailed investigation of the magnetic properties of the spin-3/2 system Li2Co(WO4)(2) by means of magnetic susceptibility and specific heat. Our experimental results show that in Li2Co(WO4)(2) short-range antiferromagnetic (AFM) correlations appear near chi(max) similar to 11 K and two successive long-range AFM phase transitions are observed at T-N1 similar to 9 and T-N2 similar to 7 K. The frustration factor vertical bar Theta vertical bar/T-N1 similar to 3 indicates that the system is moderately frustrated, which is identifiable by the broken triangular symmetry within both ab and bc planes for the triclinic crystal structure. The magnetic isotherm at temperatures below TN2 shows a field-induced spin-flop transition, and a complete H-T phase diagram for the two-step AFM system is mapped. Ab initio band-structure calculations suggest that the strongest exchange coupling does not correspond to the shortest Co-Co distance along the a axis but rather along the diagonal direction through a Co-O-W-O-Co supersuperexchange path within the bc plane.