Motion of a magnetic moment placed in a uniform magnetic field (classical) In the book, it is written that a magnetic moment placed in uniform magnetic field processes about the direction of the field. Can someone explain why?
 A: An atom can have a magnetic dipole moment and an angular momentum. These can be thought of as be caused by the orbital motion of an unpaired electron acting like a loop of current.  If you put a loop of current in a uniform magnetic field, it will experience a torque which attempts to align the area vector with the field.  (Consider the magnetic forces on opposite sides of the loop.)  This torque vector then deflects the angular momentum vector in a direction which leads to precession. (Look up gyroscopic motion.)  If the dipole is not associated with angular momentum, it will not precess.
A: What do we have:

*

*An electron with its magnetic dipole.

*Under the influence of an external magnetic field, the electron is deflected in all cases where the field orientation is not parallel to the direction of motion.

*The electron emits photons tangential to the radius of the deflection.

The last two points indicate that the recoil from the photon emission is the cause of the deflection.
At the time of Lorentz, it was not known that the path of an electron is not a circular path but a spiral path and that the deflection is accompanied by radiation. Therefore, the precession of the electron was postulated in analogy to the gyroscope, which R.W.Bird mentioned in his answer. Since this analogy, we talk about spin, although any physicist will tell you that there is nothing rotating at all.
As long as the electron's direction of motion is not parallel to the external field, a kind of precession actually takes place. The crux of the matter is the following.
As long as the dipole is not aligned with the external magnetic field, the field exerts a dipole moment on the electron and the electron begins a rotational motion. This, an acceleration, leads to photon emission. This in turn tilts the electron back to its unaligned state. BTW, the more accurate description of the trajectory of the electron through this is a spiral path of mandarin slices.
In each section of the movement (each mandarin slice), a photon is emitted and each time the electron goes through the cycle of aligning with the magnetic field and tilting away due to the photon emission.
