 Circularly rotating magnetic drive

For positive gyromagnetic ratio in static field $B\hat{z}$, transverse components $B_1\cos (\omega t+\phi )$ and $-B_1\sin (\omega t+\phi )$ rotate with the spin precession. Their rotating-frame Hamiltonian operator is time independent, with transverse Rabi angular frequency $\Omega =\gamma B_1$. A linearly polarized field of peak amplitude $B_{\mathrm{peak}}$ contains two oppositely rotating components and gives resonant $\Omega =|\gamma |B_{\mathrm{peak}}/2$ in the rotating-wave approximation. Negative gyromagnetic ratio requires opposite rotation sense.