Observation of Nuclear Antiferromagnetic Order in Copper by Neutron Diffraction at Nanokelvin Temperatures

Long-range nuclear antiferromagnetic order in the nuclear-spin system of a ⁶⁵Cu single crystal has been observed by neutron diffraction. By use of a linear position-sensitive detector, the time evolution of the antiferromagnetic (100) Bragg peak was followed during the warmup of the nuclear-spin system. The peak intensity was found to depend strongly on the external magnetic field between $B=0$ and the critical field Bc=0.25 mT.