Nuclear order in copper: New type of antiferromagnetism in an ideal fcc system

A new ordering vector k=(2\ensuremath{\pi}/a)(0, 2/3, 2/3) for fcc antiferromagnets has been found by neutron-diffraction experiments at nanokelvin temperatures in the nuclear-spin system of a $^{65}\mathrm{Cu}$ single crystal. The corresponding reflection together with the previously observed (100) Bragg peak show the presence of three antiferromagnetic phases, separated by two first-order phase transitions, as the external magnetic field is varied between zero and ${\mathit{B}}_{\mathit{c}}$=0.25 mT. The kinetics of the transitions was observed to be on the order of seconds.