We study theoretically the spectrum $\mathbf{F}$(s) of spin-dependent transition rates within dipolar $D$ and exchange $J$ coupled pairs of two spins with $S=\frac{1}{2}$ undergoing Rabi oscillations due to a coherent magnetic-resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange $J$ is stronger than the driving field ${\Omega }_R$ (in the frequency units), the frequency components of the Rabi oscillation do not depend on $J$, rather they are determined by the relation between the ${\Omega }_R$ and $D$. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of ${\Omega }_R/D$ and $\delta /D$, where $\delta$ is detuning of the driving ac field from the Larmor frequency. When ${\Omega }_R\gg D$, the two lower frequencies approach $s={\Omega }_R$, while the upper line approaches $s=2{\Omega }_R$. Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different ${\Omega }_R/D$ and $\delta /D$. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with ${\Omega }_R$: its frequency decreases with ${\Omega }_R$ at ${\Omega }_R<D$. Upon further increase of ${\Omega }_R$, this frequency then passes through a minimum and, eventually, approaches $s={\Omega }_R$. Nonmonotonic behavior of the frequencies is accompanied by nonmonotonic behavior of the respective oscillation intensity.

• Received 22 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.165205