# Synopsis: Magnetic Fields Measured with Negative Muons

A magnetic-field measurement technique that uses negative muons, rather than the usual positive muons, has probed hydrogen diffusion in a hydrogen storage material.

A technique called muon spin rotation and relaxation ($𝜇\text{SR}$) can measure a material’s internal magnetic field by implanting muons in the material and observing their decay products. Typically, $𝜇\text{SR}$ works with positively charged muons, but a new study has demonstrated magnetic field measurements with negatively charged muons. Using the negative muon beam at the Japan Proton Accelerator Research Complex (J-PARC), the researchers observed the magnetic field produced by hydrogen ions inside a hydrogen storage material called magnesium hydride. The experiments demonstrate that negative-muon-based $𝜇\text{SR}$ could complement positive-muon-based $𝜇\text{SR}$, especially for materials containing light elements like hydrogen.

In $𝜇\text{SR}$, a beam of spin-polarized muons is shot into a target material, where some of the muons come to rest. The exact resting location depends on the muon charge: positive muons settle in the crystal lattice (mimicking atoms), whereas negative muons fall into orbits around atoms (mimicking electrons). When one of these embedded muons eventually decays, measurements of its decay products can reveal information about the internal magnetic field around the muon’s location.

Positive muons are usually better suited for magnetic-field measurements, as their spin polarization remains higher than negative muons during insertion. Nevertheless, negative muons could prove useful, as Jun Sugiyama, from Toyota Central Research & Development Laboratories in Japan, and colleagues found in their study of magnesium hydride. The flow of hydrogen ions within the material should cause fluctuations in its internal magnetic fields, but positive muons move around too much in magnesium hydride to give a clear reading. By using negative muons, which are captured (and held fixed) by magnesium atoms, the team succeeded in observing the expected field fluctuations.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

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