Abstract. The interplay of magnetism and superconductivity has intrigued physicists since the late fifties. The advance of ultralow temperature technology has recently allowed to extend these studies to nuclear magnetism.
We have measured the influence of nuclear magnetism on superconducting critical fields B_s.c of Sn, AuIn₂, Al, and AuAl₂ at ultralow temperatures, 17 μK ⋜ T ⋜ 1 K. The materials have been chosen in respect to their very different hyperfine coupling expressed by the Korringa constants K (Sn, AuIn₂ , Al, AuAl₂) = (0.05, 0.11, 1.8, 18) Ksec. Both, for the strongly and for the weakly coupled nuclear paramagnets, Sn, AuIn₂, and Al a reduction of B_s.c is observed being proportional to the nuclear magnetization . However, for AuAl₂ with its extremely weak hyperfine coupling between Al nuclear moments and conduction electrons, almost no influence of nuclear magnetization on B_s.c = 1.2 mT is observed within our experimental resolution of 0.1 μT.
In addition, we had observed formerly a spontaneous nuclear ferromagnetic transition of the strongly interacting In nuclei in AuIn₂ at T_m.c = 37 μK. Besides the above mentioned reduction of B_s.c in the nuclear paramagnetic state, we observe a sudden reduction of B_s.c by 40% below T_m.c leading then to the first observation of coexistence of (nuclear) ferromagnetism and superconductivity.

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