Anti-site Disorder Driven Unusual Magnetic Properties in Sm\( _{2} \)NiMnO\( _{6} \) Double Perovskite

Cation ordered double perovskite RE \(_{2}\) $_2^{}$ NiMnO \(_{6}\) $_6^{}$ (RE: rare-earth) is commonly believed to show two distinct magnetic phase transitions, one at T = T \( _{C} \) $_C^{}$ because of Ni-O-Mn ferromagnetic (FM) super exchange interaction and another at T = T \( _{d} \) $_d^{}$ due to the polarization of RE spins opposite with respect to Ni-Mn network. In the present work, the structural, electronic, and magnetic properties of anti-site disordered Sm \( _{2} \) $_2^{}$ NiMnO \( _{6} \) $_6^{}$ (SNMO) have been investigated. It is observed that the presence of intrinsic B-site disorder results in an additional antiferromagnetic (AFM) coupling, mediated via Ni-O-Ni and Mn-O-Mn local bond pairs. As a consequence, the magnetic behavior of SNMO comprises of co-existing FM-AFM phases. Field-dependent inverted cusp-like trend in temperature-dependent magnetization, thermal irreversibility in cooling-warming paths of susceptibility, two-step reversibility in magnetic isotherm, and non-monotonicity in the thermal evolution of coercivity and remanence indicate the presence of competing magnetic phases over a wide range of temperature values (T \( _{d} < \) $_d^{}<$ T < T \( _{C} \) $_C^{}$ ).