Magnetic field due the orbital motion of electron:
A point charge is  is moving in a classical orbit with a velocity . At a given instant, the field it creates at the nucleus, is

 is directed from the nucleus towards the charge .
Hence the magnetic field due to the orbital motion of the electron is

The interaction energy between the nuclear magnetic moment and the orbital motion of electron
Using special case of Wigner-Eckart theorem, ( Lande formula)



So we can substitute,  and we get