  We have measured extremely large coercive magnetic fields of up to 55~T in
Sr$_3$NiIrO$_6$, with a switched magnetic moment $\approx 0.8~\mu_{\rm B}$ per
formula unit. As far as we are aware, this is the largest coercive field
observed thus far. This extraordinarily hard magnetism has a completely
different origin from that found in conventional ferromagnets. Instead, it is
due to the evolution of a frustrated antiferromagnetic state in the presence of
strong magnetocrystalline anisotropy due to the overlap of spatially-extended
Ir$^{4+}$ 5$d$ orbitals with oxygen 2$p$ and Ni$^{2+}$ 3$d$ orbitals. This work
highlights the unusual physics that can result from combining the extended $5d$
orbitals in Ir$^{4+}$ with the frustrated behaviour of triangular lattice
antiferromagnets.
