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 Li_2Co(WO_4)_2 samples. Under zero magnetic field, two successive magnetic transitions at T_\rm N1\sim 9.4 K and T_\rm N2\sim 7.4 K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li_2Co(WO_4)_2 enters an incommensurate magnetic state with a temperature dependent \bm k between T_\rm N1 and T_\rm N2. The magnetic propagation vector locks-in to a commensurate value \bm k = (1/2, 1/4, 1/4) below T_\rm N2. The antiferromagnetic structure is refined at 1.7 K with Co^2+ magnetic moment 2.8(1) \mu_\rm B, consistent with our first-principles calculations.