Evidence of Ferromagnetic Spin Fluctuations in the single crystalline MgCNi$_{3}$ superconductor

Despite strong expectation of the ferromagnetic ground state or the strong ferromagnetic fluctuation in MgCNi$_{3}$, the real ground state shows only superconductivity without any evidence of magnetism. At least the ferromagnetic spin fluctuations should be appeared in this strong ferromagnetic Ni based compound, but no clear observation of them is absent. Lack of the single crystalline and use of the poly-crystalline MgCNi$_{3}$ had hindered to obtain the intrinsic signal from the spin fluctuations due to the strong scattering of the defects or grain boundaries. To clarify this issue, we prepared single crystalline MgCNi$_{3}$ and measured the resistivity as a function of temperature and field. Important role of the spin fluctuations as the pair breaker in the MgCNi$_{3}$ single crystals was confirmed in these measurements while comparing the prediction from the modified Eliashberg equation including the spin fluctuations by O. V. Dolgov \textit{et. al.} The unusual normal-state resistivity and upward curvature of $H_{c2}(T)$ near $T_{c}$ also supported the existence of the additional scattering due to the spin fluctuation scattering in addition to the typical electron-phonon scattering in BCS theory.