Evidence of a new magnetic anomaly below the ferromagnetic Curie temperature in highly ordered La₂CoMnO₆

Bulk double perovskite La₂CoMnO₆ (LCMO) is an insulating ferromagnet with Curie temperature (T_C) of 225 K. The monoclinic P2₁ /n phase of LCMO was synthesized using sol-gel technique. Temperature dependence of magnetization showed two distinct transitions at 226 and 218 K corresponding to Mn⁴⁺–O–Co²⁺ ferromagnetic (FM) superexchange (T_C ~226 K) and Co²⁺–O–Co²⁺ antiferromagnetic (AFM) interactions, respectively. A relatively high value of T_C and µ_eff ~ 7.51 µ_B confirm the highly ordered state of Co and Mn ions. Interestingly, AC measurements indicate the presence of two additional transitions: a frequency dependent peak at 135 K which signifies a spin-glass-like state and an anomaly at ~40 K due to low-temperature Mn⁴⁺–O–Mn⁴⁺ AFM interactions. A careful examination of the temperature and field dependence of magnetic entropy change (ΔS_M), calculated via the magnetocaloric effect, supports the above findings. Specifically, our results indicate the presence of a low-field AFM phase (218 K) just below the ferromagnetic transition (226 K) that is suppressed at a critical field, which has not been previously reported.