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Ferromagnetic spin fluctuations in itinerant-electron magnetic compounds RCo9Si4 (R = Y, La) were investigated by using magnetization and 59Co-NMR measurements. YCo9Si4 is an itinerant ferromagnet, while LaCo9Si4 is a nearly ferromagnetic metal. In YCo9Si4, the field dependence of the magnetization is well described by Takahashi’s spin-fluctuation theory for weak itinerant ferromagnets. In both YCo9Si4 and LaCo9Si4, the magnetic susceptibility, the Knight Shift and 1/T1T show a similar temperature dependence, which suggests that the ferromagnetic spin fluctuation is dominant in these compounds. The difference in the magnetic ground state of these compounds is possibly attributed to the fact that the energy scale of the ferromagnetic spin fluctuation in LaCo9Si4 is larger than that in YCo9Si4. [ABSTRACT FROM AUTHOR]