Cluster Spin-glass-like Dynamics in Mn-rich La 2 Ni 0.5 Mn 1.5 O 6 Ferromagnetic Insulator

We report on the physical properties of Mn-rich, nonstoichiometric La2Ni0.5Mn1.5O6ferromagnetic insulator, prepared by sol-gel method. The single-phase orthorhombic Pbnm structure for the compound was confirmed by x-ray diffraction measurements. Dc magnetization measurements revealed a high saturation magnetization of ∽5.95 μB/f.u. at 5 K, and a ferromagnetic to paramagnetic transition at ∽162 K. Ac magnetic susceptibility measurements confirmed a broad frequency-dependent anomaly at lower temperatures indicating the presence of spin-glass type magnetic interactions. The ac susceptibility data have been discussed within the framework of the critical slowing down model and Vogel-Fulcher law, and confirmed the cluster spin-glass dynamics with a relaxation time of the order of 10-5s. The valence of Ni and Mn ions was verified from the x-ray absorption near edge structure spectroscopy. The origin of cluster spin-glass state was discussed in terms of several possible magnetic exchange interaction pathways among Ni and Mn ions.