Competing magnetic interactions and emergent phase diagrams in double perovskite Y₂Ni_xCo_1−xMnO₆

We present a comparative study of double perovskites Y₂Ni_xCo_1−xMnO₆ for x = 1, 0.5, and 0. The polycrystalline samples of Y₂Ni_xCo_1−xMnO₆ with space group P2₁/n were synthesized via sol-gel technique. X-ray photoelectron spectroscopy (XPS) confirms the presence of majority Ni²⁺/Mn⁴⁺ in Y₂NiMnO₆ (YNMO) and a mixed valence state Ni^2+/3+/Co^2+/3+/Mn^4+/3+ in co-doped Y₂Ni_0.5Co_0.5MnO₆ (YNCMO). The temperature dependent magnetization results suggest homogeneous substitution of Ni ions with Co ions with minimal contribution of 3 + cationic magnetic interactions in YNCMO. The superexchange ferromagnetic (FM) interaction has a major contribution to the magnetism of YNMO. The strength of antiferromagnetic (AFM) coupling resulting from the antiphase boundaries in Y₂Ni_xCo_1−xMnO₆ increases with the increase in the Co concentration. The AC susceptibility (AC-χ) study further validates the presence of higher density of antiphase boundaries, which is reflected by the dynamics of domain walls in YNCMO. The isothermal magnetic entropy change (ΔS_M) as functions of temperature and magnetic field is exploited to assess the stabilization of different magnetic phases. The findings of ΔS_M (T,µ₀H) lead to the proposed new magnetic phase diagrams for Y₂Ni_xCo_1−xMnO₆ for x = 1 and 0.5, in comparison with the previously established phase diagram of Y₂CoMnO₆.