Incommensurate-commensurate magnetic phase transition in double tungstate Li2Co(WO4)(2)

Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li2Co(WO4)(2) samples. Under zero magnetic field, two successive magnetic transitions at T (N1) similar to 9.4 K and T (N2) similar to 7.4 K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li2Co(WO4)(2) enters an incommensurate magnetic state with a temperature dependent k between T (N1) and T (N2). The magnetic propagation vector locks-in to a commensurate value k = (1/2, 1/4, 1/4) below T (N2). The antiferromagnetic structure is refined at 1.7 K with Co2+ magnetic moment 2.8(1) mu (B), consistent with our first-principles calculations.