Magnetic-Octupole Order in Neptunium Dioxide?

The phase transition occurring at 25 K in $\mathrm{Np}{\mathrm{O}}_2$, discovered almost 50 years ago, is the most long-lasting mystery in the physics of actinide compounds. Theories based on magnetic or electric-quadrupole order lead inevitably to fundamental, qualitative inconsistencies with observations. We show that the phenomenology of $\mathrm{Np}{\mathrm{O}}_2$ can be understood if the order parameter is assumed to be a magnetic octupole of ${\Gamma }_2$ ( $\mathrm{xyz}$) symmetry. $\mathrm{Np}{\mathrm{O}}_2$ is the first compound for which indications of an octupolar phase transition have been found.