I am reading this article: The Nature of the Electron by Don Lincoln in The Physics Teacher, Volume 54 (2016), pg. 203, and I ran across the part where he talks about measuring the magnetic moment of the electron by placing it in a magnetic field.
A magnetic moment in an external magnetic field experiences a torque, causing it (the magnetic moment) to precess around the direction of the external field. That makes sense, except, the magnetic moment of the electron is proportional to its spin. So aren't we effectively performing a measurement of the spin of an electron when we place it in a magnetic field to measure its magnetic moment? Shouldn't it have to then choose to be either aligned or anti-aligned with the external magnetic field, just like when we measure the spin along some arbitrary axis, we find either spin up or spin down? If so, then it seems that it shouldn't precess, because the torque would be zero.