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We report on a new type of magnetic superconductor investigated by susceptibility measurements of AuIn 2 at 25 mK # T # 207 mK and 0.01 mT # B # 2.00 mT. These experiments have been performed to study the interplay between nuclear ferromagnetism and type-I superconductivity. We observe a decrease of the critical field B s and a broadening of the superconducting transition of the type-I superconductor AuIn 2 ͓T s ͑B 0͒ 207 mK͔ caused by the coexisting nuclear ferromagnetic state starting at T c 37 mK. This is the first study on the interplay of nuclear magnetism and superconductivity. [S0031-9007(97)02378-8] PACS numbers: 74.25.Ha, 75.40.Gb Studies on the interplay between magnetism and superconductivity during the past decades have led to an understanding of many effects associated with the interaction of these two phenomena. Most of these studies were concerned with the coexistence of antiferromagnetism and superconductivity, whereas the coexistence of long-range ferromagnetism and superconductivity has never been observed [1,2]. The destruction of superconductivity at a critical temperature T s2 due to the onset of electronic ferromagnetism below the temperature T s1 , where superconductivity appears, was discovered in ErRh 4 B 4 [3] and HoMo 6 S 8 [4]. For these materials it was found that oscillatory magnetic order can coexist with superconductivity in a very narrow temperature range just above T s2 [1,2]. For HoMo 6 Se 8 it was observed that superconductivity may coexist with an oscillatory magnetic state with ferromagnetic tendency down to lowest temperatures [5]. However, it was not possible until now to investigate the interplay of nuclear magnetic ordering and superconductivity, because until recently spontaneous nuclear magnetic ordering in metals had been reported only for the nonsuperconducting metals Ag, Cu [6], PrCu 6 , PrNi 5 , and Pr 12x Y x Ni 5 [7]. Recently, we have reported on the observation of nuclear ferromagnetic ordering of In nuclei in AuIn 2 at T c 35 mK [8]. This compound is a type-I superconductor with T s ͑B 0͒ 207 mK and B s ͑T 0͒ 1.45 mT [8,9]. The nuclear magnetic ordering is caused by dominantly ferromagnetic indirect exchange interaction of RKKY type between the nuclear magnetic moments m͑In͒ 5.50 m n . The good thermal coupling of these nuclear magnetic moments to the conduction electrons (Korringa constant k 0.11 Ksec) enabled measurements of nuclear specific heat, nuclear ac susceptibility, and nuclear magnetic resonance in thermal equilibrium to T nuc T el Ӎ 30 mK [8]. All these measurements were performed in static fields B $ 2.00 mT to suppress the superconducting state of AuIn 2 .In the present work we investigate the field range B # 2.00 mT to study the interplay between nuclear ferromagnetism and superconductivity in AuIn 2 . Our data show for the first time the coexistence of nuclear ferromagnetism and superconductivity, and, more generally, we find that AuIn 2 is the first type-I superconductor where coexistence with magnetic ordering occurs. We observe a strong de...

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Nuclear magnetic resonance, ac susceptibility, and nuclear heat capacity of Pt samples with different concentrations of magnetic impurities x in the ppm range have been studied at magnetic fields (0 ± 0.05) mT ⩽ B⩽248 mT and at temperatures 0.3 μK ⩽ T⩽100 mK. Spin-lattice and spin-spin relaxation times of 195Pt strongly depend on the impurity content x, and the nuclear heat capacity data show enhanced values which scale with x at low polarization of the nuclear spin system. We present a model to describe the heat capacity data with a spatially varying internal field caused by the magnetic impurities. There is no indication for a nuclear magnetic ordering of 195Pt to a nuclear temperature of Tn = 0.3 μK and an electronic temperature of Tel = 1.5 μK.

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