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 <i>P</i>2₁/<i>n</i> 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 (<i>T</i>_CM) decreases abruptly as the size of the rare earth decreases, since <i>T</i>_CM 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 <i>T</i>_CM