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 ex
traordinarily 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.
