Two-step antiferromagnetic transition and moderate triangular frustration in Li2 Co(WO4)2

We present a detailed investigation of the magnetic properties of the spin-32 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 χmax∼11K and two successive long-range AFM phase transitions are observed at TN1∼9 and TN2∼7K. The frustration factor |Θ|/TN1∼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. © 2014 American Physical Society