Magneto-Photonic Effect of Fe₃O₄@SiO₂ Nanorods for Visualizing the Direction of Magnetic Fields with High Spatiotemporal Resolution

In this work, we demonstrate the visualization of the complex magnetic fields by utilizing the magneto-photonic effect of Fe₃O₄@SiO₂ nanorod suspension with one-to-one correspondence between the visible colors and magnetic field directions. The selected anisotropic nanorods possess appropriate saturated magnetization and high electrostatic repulsion, which is magnetically direction-responsive but strength-insensitive, accurately detecting the field direction while eliminating the influence from intensity. The combined experiment-simulation study validates the accuracy of the simulation, allowing us to further determine the intensity distribution of the magnetic field. The packed photonic device's high spatial (∼20 μm) and temporal (∼1 ms) resolutions were confirmed by time-resolved ultrasmall-angle X-ray scattering (USXAS) tests, as well as observations using an optical microscope and a high-speed camera. Our work provides a new technique for visualizing magnetic fields and opens an avenue toward further studying and utilizing complex magnetic fields for various purposes.