  Neutron scattering is a powerful tool to study magnetic structures and
dynamics, benefiting from a precisely established theoretical framework. The
neutron dipole moment interacts with electrons in materials via their magnetic
field, which can have spin and orbital origins. Yet in most experimentally
studied cases the individual degrees of freedom are well described within the
dipole approximation, sometimes accompanied by further terms of a multipolar
expansion that usually act as minor corrections to the dipole form factor. Here
we report a unique example of neutrons diffracted mainly by magnetic octupoles.
This unusual situation arises in a quantum spin ice where the electronic
wavefunction becomes essentially octupolar under the effect of correlations.
The discovery of such a new type of quantum spin liquid that comes with a
specific experimental signature in neutron scattering is remarkable, because
these topical states of matter are notoriously difficult to detect.
