Magnetic order in the frustrated Ising-like chain compound Sr$_3$NiIrO$_6$

Oxides of the family A$_3$MM'O$_6$ (A = alkaline-earth metal, M, M'= transition metal) attracted a lot of attention because of their unconventional magnetic properties due to the interplay between low dimensionality, magnetic frustration and magnetocrystalline anisotropy. In these compounds, the M and M' ions form chains which are distributed on a triangular lattice. We studied the 5d-based system Sr$_3$NiIrO$_6$, which is in the strong spin-orbit coupling regime, by single crystal magnetization measurements and neutron powder diffraction. The magnetization revealed a large easy-axis of anisotropy confining the Ni$^{2+}$ and Ir$^{4+}$ magnetic moments along the chains. Besides, the zero-field-cooled and field-cooled measurements show that there are two characteristic temperatures: T$_1$ = 75 K and T$_2$ = 17 K. The first one is associated with the appearance of a magnetic order with a propagation vector k = (0, 0, 1). At T$_2$, the susceptibility reaches a maximum followed by a sudden drop. The magnetic structure was determined from neutron powder diffraction only up to a global phase. However, symmetry arguments allowed determining the exact nature of the magnetic ground state below T$_2$, thus clarifying the universal magnetic properties of this family of compounds.