Magnetic-octupole order in neptunium dioxide?

The phase transition occurring at 25 K in NpO2, discovered almost 50 years ago, is the most longlasting 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 th e phenomenology of NpO2 can be understood if the order parameter is assumed to be a magnetic octupole of Gamma(2) (xyz) symmetry. NpO2 is the first co mpound For which indications of an octupolar phase transition have been fou nd.