Neutron studies of nuclear magnetism at ultralow temperature

Nuclear magnetic order in copper and silver has been investigated by neutron diffraction. Antiferromagnetic order is observed in these simple, diamagnetic metals at temperatures below 50 nK and 560 pK, respectively. Both crystallize in the FCC-symmetry which is fully frustrated for nearest-neighbour interactions. Silver is a good example for a frustrated, spin 1/2 Heisenberg system. The antiferromagnetic structure is of type I with a very simple phase diagram. In copper the exchange interaction is similar to silver, but there the dipolar interaction has comparable strength. This leads to a very complex phase diagram in applied fields. The low-and the high-field structures are of type I, while in intermediate fields the unconventional ordering vector q = 2 pi(1,1/3,1/3) is observed. Strong hysteresis effects indicate first-order phase boundaries in copper. (C) 1998 Elsevier Science B.V. All rights reserved.