Canted ferrimagnetism in the distorted double perovskite La₃Mn₂NbO₉

The synthesis, structural characterization, and magnetic properties of La₃Mn₂NbO₉ double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P2₁/n. The Mn²⁺, Mn³⁺ and Nb⁵⁺ cations are distributed at the 2d and 2c Wyckoff sites, with a high cation order degree in the latter, mainly occupied by Mn²⁺/Mn³⁺. The Mn³⁺ presence was established from X-ray Emission Spectroscopy and X-ray Photoelectron Spectroscopy. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a= 5.6063(3) Å, b= 5.8155(3) Å, c= 7.9877(2) Å y β= 89.89(1)°. Magnetization measurements indicate the presence of ferromagnetic and antiferromagnetic interactions with T_C ∼19 K and T_N ∼11 K, respectively, with a magnetic cell of symmetry Γ₁(A_g). The analysis of the neutron data suggests a canted ferrimagnetic spin structure where an antiferromagnetic unit-cell in a and c axis, and ferromagnetic in b axis with a propagation vector k= (0 ½ 0) was used. The effective magnetic moment obtained experimentally is μ_eff= 5.50 μ_B/mol Mn^2+/3+, slightly lower than the theoretical one for spin only (5.83 μ_B).