Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni2+ and Mn4+ cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn4+ and Ni2+ moments. This spin alignment can be rationalized taking into account the Goodenough–Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni2+ and Mn4+ (Ni2+–O–Mn4+ angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.