Cross Polarization Imaging with Magnetic Resonance Force Microscopy

Cross Polarization (CP) is a standard method in Nuclear Magnetic Resonance Spectroscopy (NMR) for signal enhancement of nuclei with a low gyromagnetic ratio and was recently applied in MRFM [1]. We demonstrate two techniques based on CP. In the first we perform frequency-sweept CP to enhance the polarization of low-$\gamma$ S-spins. In the second method the S spins are used as a polarization sink for the high-$\gamma$ I-spins coupled to them. The I-spins can be completely depolarized by adding phase-jumps to the frequency-sweep of the S-spin channel, allowing that their presence is detected indirectly via the high-$\gamma$, often more abundant I-spins with improved SNR. 1D images with $\mu$m-resolution of a $KPF_{6}$ - $CaF_{2}$ sample are presented for both techniques. In the example the depolarization scheme allowed for an order of magnitude signal-to-noise ratio enhancement over direct detection. [1] Q. Lin et al., Phys. Rev. Lett. 2006, 96, 137604.