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.