Magnetic structures of R 5Ni2In4 and R 11Ni4In9 (R = Tb and Ho): strong hierarchy in the temperature dependence of the magnetic ordering in the multiple rare-earth sublattices

The magnetic properties and magnetic structures of the R5Ni2In4 and the microfibrous R11Ni4In9 compounds with R  =  Tb and Ho have been examined using magnetization, heat capacity, and neutron diffraction data. Rare earth atoms occupy three and five symmetrically inequivalent rare earth sites in R 5Ni2In4 and R 11Ni4In9 compounds, respectively. As a result of the intra- and inter-magnetic sublattice interactions, the magnetic exchange interactions are different for various rare earth sites; this leads to a cascade of magnetic transitions with a strong hierarchy in the temperature dependence of the magnetic orderings. A transition at T C  =  125 K in Tb5Ni2In4 [κ 1  =  (0, 0, 0)] leads to a ferro/ferrimagnetic order where the magnetic ordering in one of the three R-sublattices leads to the ordering of another one; the third sublattice stays non-magnetic. New magnetic Bragg peaks appearing below T N  =  20 K can be indexed with the incommensurate magnetic propagation vector κ 2  =  (0, 0.636, ½); at T N  =  20 K a cycloidal spin order, which acts mostly upon the third R-sublattice, occurs. Ho5Ni2In4 establishes first antiferromagnetism [κ  =  (0, 0, 0)] at T N  =  31 K on two R-sublattices; then the system becomes ferro/ferrimagnetic at T C  =  25 K with the third sublattice ordering as well. Tb11Ni4In9 has three magnetic transitions at T C  =  135 K, T N1  =  35 K and at T N2  =  20 K; they are respectively coupled to the appearance of different propagation vectors [κ 1  =  (0, 0, 0), κ 2  =  (0, 0, ½), κ 3  =  (0, 1, ½)], which themselves are operating differently on the five different R-sublattices. Two sublattices remain mostly ferromagnetic down to lowest temperature while the three others are predominantly coupled antiferromagnetically. In Ho11Ni4In9 a purely antiferromagnetic order, described by four different magnetic propagation vectors [κ 1  =  (0, 0.62, 0), κ 2  =  (0, 1, 0), κ 3  =  (0, 0, ½), κ 4  =  (0, 1, ½)], succeedingly includes all five different sublattices on cooling through transitions at T N1  =  22 K, T N2  =  12 K, T N3  =  8 K and T N4  =  7 K. The strength of the magnetic interactions of the different sublattices can be linked to structural details for both R5Ni2In4 and R11Ni4In9 compounds.