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.
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.