Relative magnetic permeability of diamagnetic materials is always less than unity that is
μr < 1
When a specimen of diamagnetic material is placed in a magnetising field, the lines of magnetic force prefer not to pass through the specimen. This is as if a diamagnetic material expels magnetic field lines. Therefore the field lines within the sample is decreased from B to a smaller value.
therefore B/H < 1
As μ = B/H
Therefore μ < 1 for diamagnetic material
Conceptual explanation: Do you know that permeability word comes from the word “permission”? As it is the natural property of diamagnetic material to repel the lines of force (due to structureof diamagnetic material). Now, if they repel the lines of force or in other words they do not permit the magnetic lines of force to pass through it. Therefore, the μ < 1 for diamagnetic materials
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0. In case of any doubt, you can put your question in the comment section.