TEMPERATURE EVOLUTION OF THE MAGNETIC-STRUCTURES IN R2BANIO5 OXIDES

The magnetic structures of R2BaNiO5 (R = Nd, Tb, Dy, Ho, Er, Tm, Yb an d Y) have been characterized by means of neutron powder diffraction fr om 1.5 K to room temperature, Three dimensional antiferromagnetic orde ring is established for both R and Ni sublattices at relatively high t emperatures. The propagation vector of the magnetic structures of the whole family is k = [1/2 0 1/2]. At 1.5 K the directions of the magnet ic moments differ because of the different anisotropy of the rare eart h ions, The strong magnetic moment reduction of Ni2+ cations is probab ly due to the combination of low-dimensional quantum effects and coval ency. For the compounds with the biggest rare earth ions, the Ni momen ts rotate from nearly parallel to the a-axis toward the c-axis with de creasing temperature. A smooth re-orientation, governed by the magneti c anisotropy of R3+, seems to occur below and close to T-N.