Someone call @keithw. He put a tremendous amount of brainpower into getting the Unicode side of this puzzle "right" (or, as "right" as it is possible to be) when he wrote mosh. I'm sure he and Hashimoto could have a grand old (but zero-width non-joining) conversation!
AIUI, there is a technical criterion for an ambient EM field to imbue circuits within it with broken time-reversal symmetry.
One example of a system that meets the criterion is a ferromagnet. Another is this altermagnet.
One example of a system that doesn’t meet the criterion is a diamagnet. Another is the anti-ferromagnet.
Roughly speaking, some systems are microscopically “asymmetric enough” to be useful in a certain way, and others are “too symmetrical.”
Ferromagnets have a downside that altermagnets avoid: their microscopic fields don’t average out to zero over macroscopic distances.
I think, but honestly don’t really understand, that the goal is to cause the material to treat currents of spin-up and spin-down charge carriers (think electrons or holes) dissimilarly. Constructing materials that distinguish between charge carriers of differing spin is a step towards spintronics. Again, I don’t know why that’s important, but it is what it is.
https://arxiv.org/abs/2405.02389