A New Theory of Muon Spin Relaxation
Antimuons implanted in a crystal precess in response to magnetic fields generated by nearby ions, then they decay into positrons—all within a few microseconds. Each positron shoots off in the direction that the antimuon’s spin was pointing when the antimuon decayed. Since the 1970s, this process, known as muon spin relaxation ( SR), has been used to infer crystals’ atomic-scale magnetic structures from the positrons’ spatial distributions. The inference is straightforward when the local magnetic fields are quasistatic or randomly fluctuating, but in many interesting cases they retain memory of previous configurations—the fields have temporal correlations. Now Elvis Arguelles and Osamu Sugino of the University of Tokyo have devised a theory that accounts for such memory [1].
Temporal correlations that affect SR arise in several ways. When ions jiggle, they tend to induce magnetic fluctuations that are correlated with each other. The antimuons themselves can also induce fluctuations when they thermalize during implantation. To accommodate these and other sources of memory, Arguelles and Sugino posited that an antimuon’s spin is coupled to a local magnetic environment with two parts: a quasistatic background field associated with the material’s average ionic structure and a fluctuating field produced by time-dependent changes in the local ion density. Using a mathematical framework developed in the 1960s, the pair went on to derive an equation for the antimuon’s spin that captures the time-dependent fluctuations in two ways: random magnetic-field fluctuations that remain correlated over time and a history-dependent torque from the surrounding environment. To evaluate their theory, Arguelles and Sugino applied it to model the industrially important cathode material lithium cobalt oxide. The thermally activated hopping of lithium ions within the cathode material induces correlated fluctuations that the theory successfully reproduced.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- E. F. Arguelles and O. Sugino, “Open quantum system theory of muon spin relaxation in materials,” Phys. Rev. B 114, 034313 (2026).