Spin glass behaviour in ferromagnetic La2CoMnO6 perovskite manganite

La2CoMnO6 perovskite manganite was synthesized by solid-state reaction. Rietveld refinement of powder x-ray diffraction data indicated that this compound crystallized with an orthorhombic structure containing distorted (Co/Mn)O-6 octahedra (Pnma; a=5.5 Angstrom; b=7.2 Angstrom; c=5.4 Angstrom. The dc magnetization and real part (x') of the ac susceptibility were measured at dc fields up to 1 T and an ac field of 0.1 Oe at frequencies of 21, 217, 600 and 2000 Hz over a wide range of temperatures from 300 K down to 4.2 K. A ferromagnetic transition was observed at T-c = 220 K. A spin glass ground state was also determined from the shift of a peak at 220 K in the real part of the ac susceptibility with frequency and slow spin relaxation for T < T-c as indicated by the time dependence of the remanent magnetization. The spin glass state arises from a ferromagnetic state and might be caused by competition between the ferromagnetic and antiferromagnetic interaction.