Theoretical studies of nuclear ordering

SELECTED RESEARCH INTERESTS

Theoretical studies of nuclear ordering

In the LTL, theoretical work on nuclear magnetic ordering has closely followed the progress in the experimental front. Extensive calculations have been made for copper, silver, and rhodium to understand the collective properties of their nuclear spins. These studies are also relevant outside the field of nuclear magnetism since the spin assemblies in metals are good realizations of several widely investigated model systems.

Motivated by the richness of the measured phase diagram of nuclear spins in copper, most of our work has concentrated on determining the antiferromagnetically ordered spin structures in this metal. The observed interplay of two types of antiferromagnetic ordering, which are characterized by the (1 0 0) and (0 2/3 2/3) Bragg reflections, was successfully explained in terms of a model based on mean-field theory. It was found that these two antiferromagnetic orders can coexist in a single magnetic domain. The model predicted which of the 12 symmetrically equivalent (0 2/3 2/3) Bragg reflections are stable for different directions of the external magnetic field. The theoretical results were vindicated by subsequent neutron-diffraction experiments.

The major puzzle that remained after the neutron diffraction measurements on copper was the spin structure in the B = 0.17 - 0.25 mT antiferromagnetic region when the field is aligned along the [111] crystalline axis. For other high-symmetry field alignments, the (1 0 0) order was observed. Theoretical calculations showed that the experimentally so far undiscovered phase should display a Bragg reflection of the form (h k l) in which the indices are all positive and different from each other.

Our recent studies have analyzed spin configurations in fcc antiferromagnets with simple (Type-I) order. These spin configurations are stabilized by subtle fluctuation effects, which have either thermal or quantum origin. Earlier theoretical approaches yielded different answers to the problem but a coherent picture is now emerging. Nuclear spins in silver are expected to form a Type-I antiferromagnet below the observed transition temperature T_N = 560 pK. With increasing field, the ground state should transform from a structure with a single Type-I modulation to a state with three Type-I modulations. At negative nuclear spin temperatures a ferromagnetic domain structure is predicted, consistent with measurements. Theoretical results for silver can soon be tested by neutron-diffraction experiments in Berlin.