Although it might not be obvious, a material like lead has multiple magnetic fields, one inside and another on its surface. Until recently, researchers could confidently measure only surface fields. Now, in a recent Nature Communications paper, materials scientists in Germany describe a way to create detailed maps of the direction and strength of magnetic fields inside bulky materials like those used in electric engines and high-efficiency transformers.
The new method uses neutrons, neutrally charged subatomic particles that spin in the opposite direction of a magnetic field’s alignment. Because neutrons are so small and chargeless, they easily penetrate many materials. The team took advantage of these properties, scanning a sample of lead with neutrons that all had the same spin. When they hit a magnetic field inside the sample, their spin changed detectably, letting researchers plot the information.
The image shows the magnetic field in and around the lead sample (outlined in white). Warmer colors denote stronger sections of the magnetic field.
[This story originally appeared in print as "Inner Magnetism."]