  Defect-induced ferromagnetism has triggered a lot of investigations and
controversies. The major issue is that the induced ferromagnetic signal is so
weak that it can sufficiently be accounted for by trace contamination. To
resolve this issue, we studied the variation of the magnetic properties of SiC
after neutron irradiation with fluence covering four orders of magnitude. A
large paramagnetic component has been induced and scales up with defect
concentration, which can be well accounted for by uncoupled divacancies.
However, the ferromagnetic contribution is still weak and only appears in the
low fluence range of neutrons or after annealing treatments. First-principles
calculations hint towards a mutually exclusive role of the concentration of
defects: Defects favor spin polarization at the expense of magnetic
interaction. Combining both experimental and first-principles calculation
results, the defect-induced ferromagnetism can be understood as a local effect
which cannot be scaled up with the volume. Therefore, our investigation answers
the long-standing question why the defect-induced ferromagnetic signal is weak.
