Magnetic, dielectric and transport characteristics of Ln2CoMnO6 (Ln=Nd and Sm) double perovskite ceramics

• Ferromagnetic Curie point generally decreases with decreasing 〈Co–O–Mn〉 bond angle.
• Only one relaxor-like dielectric peak with strong frequency dispersion is observed.
• Both magnetic and dielectric properties have similar variation tendency.
• Transport behaviors conform to three-dimensional variable-range hopping mechanism.

Magnetic and dielectric properties of Nd2CoMnO6 and Sm2CoMnO6 double perovskite ceramics have been investigated by comparing with those of La2CoMnO6 and Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics. Differing from the single magnetic state of Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics, both Nd2CoMnO6 and Sm2CoMnO6 are multiphase magnetic state with the monoclinic symmetry (space group P21/n). Ferromagnetic Curie point TC generally decreases with decreasing 〈Ni/Co–O–Mn〉 bond angles which depends on lanthanide ionic radius (RLn). This result indicates that the 〈Ni/Co–O–Mn〉 bond angle plays an important role in magnetic properties of Ln2MMnO6 (Ln=La, Nd and Sm, M=Ni and Co) ceramics. Meanwhile, only one relaxor-like dielectric peak with strong frequency dispersion is observed in Nd2CoMnO6 and Sm2CoMnO6 ceramics, which is similar with that of La2CoMnO6. The dielectric constant ε′ also decreases with decreasing RLn. Both magnetic and dielectric properties of Ln2CoMnO6 (Ln=La, Nd and Sm) ceramics have similar variation tendency with decreasing RLn, and have been correlated with cos2〈Co–O–Mn〉, demonstrating some coupling between magnetic and dielectric response. In addition, transport behaviors of the present ceramics conform to the three-dimensional variable-range hopping mechanism, which may be attributed to the phase separation nature.