# How does spin flip take place?

An electron in a constant magnetic field, say $B_z~ \hat{z}$ starts precessing about $\hat{z}$-direction. One can flip the spin by providing a time-varying field $B_x \sin(\omega t)~ \hat{x}$ in the $\hat{x}$-direction. The logic often given is that the linearly polarized field can be treated as a sum of the left and right circular field. The field component with is rotating in the same sense and same frequency as that of precession (Larmor frequency) is responsible for the spin flip. Could someone explain the actual mechanism behind the spin-flip in this context?

• If you hang a spinning gyroscope from a string, with its axis perpendicular to the string, you can watch it precess. Take note of the frequency and direction of its precession. Now swing the top end of the string in a small circle at about that same frequency, first in the same direction as the precession and then in the opposite direction to the precession. The gyroscope will orient itself either up of down depending on the direction of your circle. This is a consequence of classical mechanics. Pretty much the same happens when the spin of an electron is flipped by an applied RF field. – S. McGrew Apr 14 '18 at 13:21
• Thanks, @S. McGrew. It seems a good analogy. Could you suggest some reference where I can find this gyroscope-flip phenomenon? – W. Voltera Apr 14 '18 at 16:58
• I've posted an answer, mostly consisting of my last comment, but including links to a youtube video lecture about gyroscope precession, along with my own explanation. – S. McGrew Apr 14 '18 at 20:55