  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 TC1 ~ 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 K to 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 TC1
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.
