NEUTRON STUDIES OF NUCLEAR MAGNETISM AT ULTRALOW TEMPERATURE

Nuclear magnetic order in copper and silver has been investigated by n eutron diffraction. Antiferromagnetic order is observed in these simpl e, diamagnetic metals at temperatures below 50 nK and 560 pK, respecti vely. Both crystallize in the FCC-symmetry which is fully frustrated f or nearest-neighbour interactions. Silver is a good example for a frus trated, spin 1/2 Heisenberg system. The antiferromagnetic structure is of type I with a very simple phase diagram. In copper the exchange in teraction is similar to silver, but there the dipolar interaction has comparable strength. This leads to a very complex phase diagram in app lied fields. The low-and the high-field structures are of type I, whil e 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.