  Zero- and ultralow-field nuclear magnetic resonance (ZULF NMR) has
experienced rapid development and provides an excellent tool for diverse
research fields ranging from materials science, quantum information processing
to fundamental physics. The detection of ZULF NMR signals in samples with
natural abundance remains a challenging endeavor, due to the limited
sensitivity of NMR detectors and thermal polarization. In this work, we
demonstrate a femtotesla potassium spin-exchange relaxation-free (SERF)
magnetometer designed for ZULF NMR detection. A potassium vapor cell with high
buffer gas pressure and high atomic number density is used in the magnetometer.
With absorption spectroscopy and SERF effect, the key parameters of the vapor
cell are characterized and applied to optimize the magnetometer sensitivity. To
combine our SERF magnetometer and ZULF NMR detection, a custom-made vacuum
chamber is employed to keep NMR sample close to the magnetometer cell and
protect the sample from undesired heating effects. Gradiometric measurement is
performed to greatly reduce the magnetic noise. With the phase calibration
applied, the gradiometric measurement achieves 7-fold enhancement in
magnetic-field sensitivity compared to the single channel and has a magnetic
noise floor of 1.2 fT/Hz$^{1/2}$. Our SERF magnetometer exhibits high
sensitivity and is promising to realize ZULF NMR detection of samples with
natural abundance.
