Temperature evolution of the magnetic structures in R2BaNiO5 oxides

The magnetic structures of R2BaNiO5 (R = Nd, Tb, Dy, Ho, Er, Tm, Yb and Y) have been characterized by means of neutron powder diffraction from 1.5 K to room temperature, Three dimensional antiferromagnetic ordering is established for both R and Ni sublattices at relatively high temperatures. 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 magnetic moments differ because of the different anisotropy of the rare earth ions, The strong magnetic moment reduction of Ni2+ cations is probably due to the combination of low-dimensional quantum effects and covalency. For the compounds with the biggest rare earth ions, the Ni moments rotate from nearly parallel to the a-axis toward the c-axis with decreasing temperature. A smooth re-orientation, governed by the magnetic anisotropy of R3+, seems to occur below and close to T-N.