The magnetic susceptibility is negative for:

As we know the relation between the magnetic permeability and susceptibility of material is, ${\mu }_r=1+{\chi }_m$ $\dots \left(\mathrm{i}\right)$

$\because$ For diamagnetic substances, ${\mu }_r<1$

So, according to Eq. (i), the magnetic susceptibility $\left({\chi }_m\right)$ of diamagnetic substance will be negative. While in case of para and ferromagnetic substances subsceptibility is positive.