We have determined by polarized neutron diffraction the spin density of a ferromagnetic (Tc=0.28K) tempo derivative: the 4(p-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl, where the nearest and next nearest neighbours N-O sites construct a two-dimensional network parallel to the ((a) over right arrow, (b) over right arrow) plane. Most of the spin density lies on the N and O atoms of the NO radical and is equally shared between these two atoms. As for the pure tempone, the two carbon atoms neighbouring the nitrogen of the N-O group in the 6-member ring, carry significant spin populations of different signs: -0.074(12) for atom C3 and +0.063(17) for atom C4. This is not consistent with the coupling mechanism proposed previously which implied that the populations of both C3 and C4 should be negative. A tentative explanation for the coupling mechanisms, taking into account the spin density distribution, is discussed.