When a ferromagnetic material is magnetised, its length either expands or contracts in the direction of the magnetisation. This effect is called magnetostriction or Joule effect.
In other words, magnetostriction refers to the changes in the dimensions of a ferromagnetic material when it is subjected to magnetising field.
Magnetostriction occurs due to the rotation of domains on applying the magnetic field. This rotation of domains produces a strain in the material resulting in its compression or expansion.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0