Temperature dependent magnetic, dielectric and Raman studies of partially disordered La₂NiMnO₆

Abstract

We report a detailed magnetic, dielectric and Raman studies on partially disordered and biphasic double perovskite La₂NiMnO₆. DC and AC magnetic susceptibility measurements show two magnetic anomalies at T_C1~270 K and TC2~240 K, which may indicate the ferromagnetic ordering of the monoclinic and rhombohedral phases, respectively. A broad peak at a lower temperature (Tsg~70 K) is also observed indicating a spin-glass transition due to partial anti-site disorder of Ni2+ and Mn4+ ions. Unlike the pure monoclinic phase, the biphasic compound exhibits a broad but a clear dielectric anomaly around 270 K which is a signature of magneto-dielectric effect. Temperature-dependent Raman studies between the temperature range 12–300 K in a wide spectral range from 220 cm⁻¹ to 1530 cm⁻¹ reveal a strong renormalization of the first as well as second-order Raman modes associated with the (Ni/Mn)O6 octahedra near T_C1 implying a strong spin–phonon coupling. In addition, an anomaly is seen in the vicinity of spin-glass transition temperature in the temperature dependence of the frequency of the anti-symmetric stretching vibration of the octahedra.