Weak itinerant ferromagnetism in YCo9Si4 below about 25 K is studied by means of magnetisation, specific heat, and resistivity measurements. Single crystal X-ray Rietveld refinements at room temperature reveal a fully ordered distribution of Y, Co and Si atoms within the tetragonal space group I4/mcm isostructural with LaCo9Si4. The latter exhibits itinerant electron metamagnetism with an induced moment of about 1 mu_B/f.u. above 6 T, whereas YCo9Si4 exhibits a spontaneous magnetisation M0~12 Am^2/kg at 2 K which corresponds to an ordered moment of about 1.6 mu_B/f.u. indicating weak itinerant ferromagnetism.
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