In an atom, electron revolves around the nucleus in a closed orbit. Since electron is a charged particle, so its orbit around the nucleus is equivalent to a current loop. When electron revolves in anticlockwise direction, the current is clockwise. Thus, upper face of the electron loop acts as south pole and lower face acts as north pole. Thus an atom behaves like a magnetic dipole.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0