dc magnetization measurements on CeCoIn 5 reveal a first-order phase transition at H c2 for both Hʈa and c axes in the isothermal magnetization M (H) below 0.7 K, indicating a strong Pauli paramagnetic suppression in the even-parity pairing. M (T) in the normal state above H c2 exhibits non-Fermi-liquid behavior down to 150 mK, implying the existence of antiferromagnetic fluctuations behind the unconventional superconductivity. We observed an unusual peak effect for Hʈc in fields 5-30 kOe below 150 mK(ϭ0.06T c ), whose anomalous temperature dependence cannot be simply explained by ordinary mechanisms.Since in 1979, 1 HF superconductivity has been attracting interest in the field of strongly correlated electron systems. Recent experimental and theoretical progress indicates that most of the HF superconductors are likely to be of an unconventional type. Until recently, to our best knowledge, CeCu 2 Si 2 was the only Ce-based HF superconductor at ambient pressure. The superconductivity in CeCu 2 Si 2 , however, is rather difficult to understand because of the complicated magnetic phase diagram. 2 Quite recently, two tetragonal Ce-based HF compounds have been discovered by Petrovic et al. to become superconducting at ambient pressure: CeXIn 5 ͓XϭIr ͑Ref. 3͒ and Co ͑Ref. 4͔͒. To our best knowledge, CeCoIn 5 has the highest T c (ϭ2.3 K) among the HF superconductors known at present. The specific heat, 4 thermal conductivity, 6 and NMR relaxation rate 8 of CeCoIn 5 show power-law temperature dependencies below T c , suggesting an unconventional superconductivity with anisotropic energy gap. Very recently, the NMR Knight-shift measurement 7,8 has revealed even-parity pairing in the superconducting state, and the angle-dependent thermal-conductivity measurement 9 has identified that the gap symmetry is k x 2 Ϫk y 2 , pointing to the fact that the pairing interaction is mediated by magnetic fluctuations. An interesting observation with respect to this point is the non-Fermiliquid ͑NFL͒ behavior in the specific heat divided by temperature C/T, showing a remarkable upturn on cooling when the superconductivity is suppressed by magnetic field. 4,5 To our best knowledge the origin of the NFL behavior has not been clarified yet.One of the features of the HF superconductors is that the orbital limiting field is relatively high despite low T c , because of the small Fermi velocity of the carriers. In addition, the HF superconductors possess quite large normal-state paramagnetic susceptibility at sufficiently low temperature, reflecting the high density of states. These facts lead to an interesting situation in which the paramagnetic energy near H c2 becomes a significant fraction of the superconducting condensation energy. 10,11 It was theoretically pointed out that a second-order transition at H c2 changes into a first-order one below ϳ0.56 T c for the singlet pairing, provided that the normal-state spin susceptibility is large enough. 12,13 Subsequent theoretical studies predicted that in the case of a clean limit (lӷ 0 ) a firs...
