Random direction of nuclear spins outside a magnetic field.

(A) Random direction of nuclear spins outside a magnetic field.  When spins are placed in a large magnetic field (oriented here along the z axis), they begin to (B) precess about that direction and (C) slowly align themselves until (D) they are all aligned parallel to the applied field.  A radio frequency pulse coherently rotates these spins perpendicular to the applied field so that they (E) precess coherently, producing a detectable magnetic field.  Over time (F) the spins realign themselves with the magnetic field until they are again in the state shown in Figure D.