Q:

How is magnetic field created in an atom according to quantum model

How is magnetic field created in an atom according to quantum model? I mean, we are taught about the magnetic field by bohr's model which assumes that electron revolves around the nucleus in a circular path. But in reality it doesn't do so. So the field will keep changing its orientation. And it would be hard to align for them in external magnetic field if they are ferromagnetic or paramagnetic.

A:

Observables like quantised energy levels and quantised angular momentum of an atom are obtained by finding eigensolutions of the Schrödinger Equation (here for the Hydrogen atom).
Separation into three parts allows to obtain the Colatitude and Azimuthal equations which allows to calculate the quantised angular momentum of the hydrogen atom, giving rise to the electron's orbital magnetic moment.
The electron itself also has an intrinsic so-called spin magnetic moment which can only take on two values.
The net magnetic moment of an atom is the vector sum of its orbital and spin magnetic moments.

