Theoretical and experimental investigation of the structural and magnetic properties of La2NiMnO6

•La2NiMnO6nanoparticles with 36 nm were prepared.•Citric acid, glutaraldehyde and chitosan were used in the synthesis method.•The crystal structure is monoclinic with P21/n space group.•Ferromagnetic phase in T < 150 K and paramagnetic phase in ambient conditions.•DFT calculations applied to investigate the structural, electronic, and magnetic.