Magnetic structures of R5Ni2In4 and R11Ni4In9 (R = Tb and Ho): strong hierarchy in the temperature dependence of the magnetic ordering in the multiple rare-earth sublattices

Abstract

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 R5Ni2In4 and R11Ni4In9 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 TC = 125 K in Tb5Ni2In4 [κ1 = (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 TN = 20 K can be indexed with the incommensurate magnetic propagation vector κ2 = (0, 0.636, ½); at TN = 20 K a cycloidal spin order, which acts mostly upon the third R-sublattice, occurs. Ho5Ni2In4 establishes first antiferromagnetism [κ = (0, 0, 0)] at TN = 31 K on two R-sublattices; then the system becomes ferro/ferrimagnetic at TC = 25 K with the third sublattice ordering as well. Tb11Ni4In9 has three magnetic transitions at TC = 135 K, TN1 = 35 K and at TN2 = 20 K; they are respectively coupled to the appearance of different propagation vectors [κ1 = (0, 0, 0), κ2 = (0, 0, ½), κ3 = (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 [κ1 = (0, 0.62, 0), κ2 = (0, 1, 0), κ3 = (0, 0, ½), κ4 = (0, 1, ½)], succeedingly includes all five different sublattices on cooling through transitions at TN1 = 22 K, TN2 = 12 K, TN3 = 8 K and TN4 = 7 K. The strength of the magnetic interactions of the different sublattices can be linked to structural details for both R5Ni2In4 and R11Ni4In9 compounds.