Field-induced magnetic charge in a cubic Laves compound UAl2

Magnetic diffraction of polarized neutrons by the cubic Laves compound UAl2 in a magnetic field has unveiled weak Bragg spots that are nominally forbidden. On the one hand, they can be viewed as magnetic analogues of the basis-forbidden (2, 2, 2) reflection in diamond-type structures that has been painstakingly and frequently investigated over almost a century. Alternatively, the pattern of weak intensities can be assigned to Dirac multipoles imbedded in field-induced magnetic charge. To this end, a published diffraction pattern is successfully confronted with intensities calculated from the appropriate magnetic space-group (Imm′a′) that includes Dirac dipoles (anapoles) to describe the basis-forbidden magnetic reflections (Ho + Ko + Lo = 4n + 2), and conventional (axial) dipole and octupole multipoles to describe basis-allowed magnetic reflections.