Differential optical imaging of magnetization in sub-micron magnetic structures

Magneto-optical techniques can enable convenient in situ mapping of magnetization, but become difficult for sub-micron structures due to edge artifacts. We present a magneto-optical Kerr effect (MOKE) microscopy method that achieves imaging of sub-micron-scale structures through frequency- and phase-multiplexed detection. Two orthogonal optical choppers encode the M_x, M_y, and M_z components of magnetization in separate frequency and phase channels, removing the need for sequential measurements. To enhance spatial resolution and suppress edge artifacts, a 3D-printed coil stage tuned to resonance produces an oscillating magnetic field of ±6 mT at 10 kHz, synchronized with dual lock-in detection. We have demonstrated the technique on permalloy films patterned into squares and nanoislands with widths down to 100 nm. This differential imaging approach produces magnetization scans that agree with simulations, whereas conventional MOKE images are completely dominated by edge artifacts. We show how these measurements can reveal the orientation of magnetization in magnetic nanoislands, permitting optical imaging of artificial spin ice structures.