Logical Answer:

Susceptibility comes from the word “susceptible” means the easily affected. As I have discussed earlier that lines of force are weakly attracted towards paramagnetic materials, when these materials are placed in external magnetic field. It means paramagnetic materials are weakly effected by the field. Therefore the susceptibility of diamagnetic materials is positive but small.

Analytical Reason:

As we have discussed in the earlier articles, that relative permeability of diamagnetic materials is greater 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 greater than 1

therefore, χ is positive

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.