A functional magnetic resonance imaging (fMRI) method that focuses on neural magnetic fields has great potential to detect neural activities more directly than the conventional method. Because this fMRI method does not depend on blood-oxygenation-level-dependent contrast, improved temporal and spatial resolutions can be expected. Among various approaches of this fMRI method, the one that uses a spin-lock imaging sequence has attracted wide attention because of the possibility to detect small oscillating magnetic fields. To understand the mechanism of this approach, we visualized magnetization behavior during the spin-lock module with externally applied oscillating magnetic fields. A fast-and-simple method with matrix operations was used to solve a time-dependent Bloch equation. In addition, we investigated the influence of the duration of the spin-lock pulse in the spin-lock module, which interacts with the external oscillating magnetic fields, on magnetic resonance signals. Furthermore, to detect minute magnetic fields in the order of sub-nT, we carried out phantom studies on the practical use of this method as an fMRI approach. A single-loop coil generating oscillating magnetic fields was placed inside a saline-filled phantom. Time-dependent performance of magnetization during the spin-lock module was thus visually demonstrated to aid understanding of the mechanism of the fMRI method with the spin-lock imaging sequence. In addition to this visualization, we found that the decrease in magnetization depends on the duration of the spin-lock pulse. Longer durations are appropriate for detecting minute sub-nT magnetic fields such as neural magnetic fields. Furthermore, we were able to detect magnetic fields of approximately 200 pT by choosing a spin-lock pulse of long duration and increasing the number of MR image acquisitions. Our results provide useful information for the understanding of the mechanism of direct detection of oscillating neural magnetic fields using MRI with a spin-lock imaging sequence. In addition, we propose an improved selection scheme for the duration of the spin-lock pulse and the feasibility of detecting oscillating magnetic fields of 200 pT considering practical application of fMRI.