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.
