Logical Answer:

Susceptibility comes from the word “susceptible” means the easily affected. As I have discussed earlier that lines of force are expelled or repelled from diamagnetic materials, when these materials are placed in external magnetic field. It means diamagnetic materials are not effected by the field. Therefore the susceptibility of diamagnetic materials is negative.

Analytical Reason:

As we have discussed in the earlier articles, that relative permeability of diamagnetic materials is lesser than 1 and  also there is a relation between relative permeability (μ_r)and susceptibility (χ):

μ_r = 1 + χ

where is μ_r

or χ = μ_r – 1

and μ_r is less than 1

therefore, χ is negative

Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0. In case of any question, you can put your question in the comment section.