Off-axis electron holographic mapping of magnetic domains in Nd ₂Fe₁₄B

Off-axis electron holography employing a field-free Lorentz lens has been used to provide direct imaging of magnetic induction in Nd₂Fe ₁₄B with nanometer-scale resolution and high signal-to-noise. Using this technique, reconstructed phase images have been used to measure domain wall widths for 90° and 180° walls in a sintered sample were measured to be no greater than 7 and 9 nm, respectively. Induction maps show an unusual array of 90° domains in a hot-press sample, with singularities at the intersections of the domains. In situ thermal annealing of the hot-pressed sample resulted in magnetization rotation accompanied by domain wall movement and evidence for pinning at a structural defect. Heating of the sample above 400°C resulted in the accumulation of small particles in thin regions and decrease in magnetic contrast.