  The dynamical properties of free and bound domain-wall excitations in
Ising-chain materials have recently become the focus of intense research
interest. New materials and spectrometers have made it possible to control the
environment of coupled Ising chains by both effective internal and applied
external fields, which can be both longitudinal and transverse, and thus to
demonstrate how the resulting magnetic phase transitions and the nature of the
associated excited states obey fundamental symmetry properties. In RbCoCl$_3$,
the weakly coupled Ising chains form a triangular lattice whose frustrated
geometry and magnetic ordering transitions at low temperature open new
possibilities for the Ising-chain environment. We have investigated the
structure and magnetism in RbCoCl$_3$ by high-resolution x-ray diffraction and
neutron scattering measurements on powder and single crystal samples between
1.5 K and 300 K. Upon cooling, the Co$^{2+}$ spins develop one-dimensional
antiferromagnetic correlations along the chain axis ($c$-axis) below 90 K.
Below the first N\'eel temperature, $T_{N1}$ = 28 K, a partial 3D magnetic
order sets in, with propagation vector ${\vec k}_1$ = (1/3,1/3,1), the moments
aligned along the $c$-axis and every third chain uncorrelated from its
neighbours. Only below a second magnetic phase transition at $T_{N2}$ = 13 K
does the system achieve a fully ordered state, with two additional propagation
vectors: ${\vec k}_2$ = (0,0,1) establishes a "honeycomb" $c$-axis order, in
which 1/3 of the chains are subject to a strong effective mean field due to
their neighbours whereas 2/3 experience no net field, while ${\vec k}_3$ =
(1/2,0,1) governs a small, staggered in-plane ordered moment. We conclude that
RbCoCl$_3$ is an excellent material to study the physics of Ising chains in a
wide variety of temperature-controlled environments.
