What is Magnetic susceptibility?

The magnetic susceptibility $\chi$ of a material is the ratio of its magnetisation to the applied field strength; the molar susceptibility $\chi_{\mathrm m}$ is the susceptibility per mole. Paramagnetic substances have $\chi > 0$, diamagnetic ones a small $\chi < 0$. Chemists often use the cgs value of $\chi_{\mathrm m}$, in $\mathrm{cm}^{3}\,\mathrm{mol}^{-1}$, equal to the SI value in $\mathrm{m}^{3}\,\mathrm{mol}^{-1}$ divided by $4\pi\times10^{-6}$.