Glass formation phenomena and magnetic properties in the temperature range 5 - 600K for Nd50Fe40Si10 melt-spun ribbons with thicknesses up to 150 mum were investigated. Fully amorphous structures have been found for Nd50Fe40Si10 ribbons thinner than 80 mum, while the ribbons with thicknesses between 80 and 110 mum are partially crystallised. The dependence of the coercivity on both the ribbons' thickness and the temperature is ascribed to the existence of two magnetic phases. The differences obtained between zero-field-cooled (M-ZFC) and field-cooled (M-FC) magnetisation curves and the maximum that appears on the M-ZFC curves prove the coexistence of two types of magnetic order, spin-glass like behaviour on short distances and ferromagnetic order on long distances. The disappearance of the amorphous phase from Nd50Fe40Si10 ribbons leads to the drastic decrease of the coercivity force and magnetisation. Submitting the molten alloy to different annealing times before ejection changes the glass-forming ability and the magnetic properties of the amorphous ribbons because of microstructural changes.