Nuclear magnetic ordering in metals with strong hyperfine coupling and electronic magnetic ordering in heavy fermion compounds. Final report

We have observed nuclear ferromagnetic ordering in the Van Vleck paramagnet PrNi5 (Tc=0.4 mK) and, for the first time, in the non-hyperfine enhanced nuclear magnet, AuIn2 (Tc=35 μK), under the condition of thermal equilibrium using integral methods, e.g. nuclear ac susceptibility and nuclear heat capacity. In the described project, we have also observed for the first time a spontaneous nuclear antiferromagnetic ordering in the hyperfine enhanced nuclear spin system PrS (TN=90 μK) as well as electronic antiferromagnetism in the heavy fermion compound CeCu6 (TN=2.5 mK). However, the microscopic spin structures of these materials are still unknown so far. For this reason, we have built a nuclear demagnetization refrigerator for neutron scattering experiments at ultralow temperatures. First measurements of n-scattering have been performed on CeCu6 yielding a n-reflection depending on temperature near TN. However, the most important result in our project was achieved at the investigation of the interplay between nuclear magnetism and superconductivity in AuIn2 and Al. For AuIn2, we observed a reduction of the superconducting critical field caused by the nuclear magnetic ordering and, particularly, a coexistence of (nuclear) ferromagnetism and (type-I) superconductivity, unseen so far. For future investigations of the coexistance, n-scattering could be the most informative experimental technique because of the possibility of neutrons to penetrate into superconducting materials. (orig.)