Long-range nuclear antiferromagnetic order in the nuclear-spin system of a ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ 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 $B=0\mathrm{}$ and the critical field $B_c=0.25\mathrm{}$ mT.

• Received 16 March 1988

DOI:https://doi.org/10.1103/PhysRevLett.60.2418