1：Experimental Design and Method Selection:

The holder integrates three stacked electromagnets with 180-μm gap length to deflect electrons back in parallel to the optical axis. A specimen prepared by FIB was positioned in the gap by using an AFM cantilever.

2：Sample Selection and Data Sources:

A polycrystalline Nd-Fe-B sintered magnet (with coercivity of 1.1 T) was in-situ observed.

3：1 T) was in-situ observed. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: The holder consists of three miniaturized electromagnets with 300 × 300-μm pole area and 180-μm pole gap. A 40-μm-wide cantilever commercially available for atomic force microscopy (AFM) was used to position the specimen.

4：Experimental Procedures and Operational Workflow:

The specimen was in a thermally demagnetized state at the beginning of the observation. A positive field up to 0.2 T was applied until the specimen reached magnetic saturation, and then the field was reduced to zero and turned negative.

5：2 T was applied until the specimen reached magnetic saturation, and then the field was reduced to zero and turned negative. Data Analysis Methods:

5. Data Analysis Methods: Magnetic-field magnitude was determined on the basis of experimentally measured electron-deflection angles and induction-field profiles along the electron-beam path calculated by finite element electromagnetic simulation.