Bulg. J. Phys. vol.27 no.1 (2000), pp. 020-036

How Nuclear Magnetism Impairs Superconductivity

T. Herrmannsdorfer 1, F. Pobell2
1Physikalisches lnstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
2Forschungszentrum Rossendorf, D-01314 Dresden, Germany
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 Bs.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 Bs.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 Bs.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 Tm.c = 37 μK. Besides the above mentioned reduction of Bs.c in the nuclear paramagnetic state, we observe a sudden reduction of Bs.c by 40% below Tm.c leading then to the first observation of coexistence of (nuclear) ferromagnetism and superconductivity.

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