Therefore, B ends up in the lower energy state and A in the upper state.
It is convenient to define a rotating frame of reference which rotates about the Z axis at the Larmor frequency.
If the net magnetization is placed along the -Z axis, it will gradually return to its equilibrium position along the +Z axis at a rate governed by T 1.
In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance.
For example, a net magnetization vector along the Y' axis will end up along the -Y' axis when acted upon by a 180 o pulse of B 1 along the X' axis.
The frequency ν depends on the gyromagnetic ratio, γ of the particle.