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

RNiMnO (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 P2/n space group, with an almost complete order between Ni and Mn 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 LaNiMnO, with a ferromagnetic structure involving Mn and Ni moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (T) decreases abruptly as the size of the rare earth decreases, since T is mainly influenced by the superexchange interaction between Ni and Mn (Ni-O-Mn angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below T.We acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through the projects MAT2013-41099-R and CSD2007-00045 and funding of the Universidad Complutense de Madrid and Banco Santander through the project UCM2014-971703. J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Spanish Ministry of Economy and Competitiveness