Role of Co/Mn Interaction in Developing Griffiths Phase with Reducing Particle Size in La₂CoMnO₆

A comprehensive study on the transition from the non-Griffiths phase (NGP) to the Griffiths phase (GP) is reported in double-perovskite La₂CoMnO₆ with the reduction of particle sizes. The bulk compound exhibits two ferromagnetic (FM) transitions, a prominent one below around T_C ∼ 155 K and a weak one below T_C1 ∼ 85 K. However, the materials in the nanocrystalline form show only one FM transition below T_C ∼ 220 K. Interestingly, short-range FM interactions consisting of small FM clusters have been observed above T_C for all of the systems. However, the nature of intercluster interactions of the preformed FM clusters in the PM matrix differs in bulk and nanomaterials. The antiferromagnetic (AFM) and FM intercluster interactions, respectively, result in the evolution of NGP in bulk and GP in nanomaterials. A detailed magnetization and transport study along with the theoretical analysis reveal that the existent AFM intercluster interaction and the enhanced antisite disorder (ASD) restrict the formation of GP in the bulk material. In contrast, due to the FM intercluster interaction and, as a consequence of the pronounced grain boundary as well as surface pressure effects in core-shell nanostructured materials, ASDs are reduced and GP emerges in those systems.