The magnetic susceptibility is negative for:

As we know the relation between the magnetic permeability and susceptibility of material is, ${\mu}_{r} = 1 + {χ}_{m} \ldots ( i )$

$∵$ For diamagnetic substances, ${\mu}_{r} < 1$

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