A synthetic antiferromagnet has two kinds of spin waves — acoustic and optical — and each one remembers differently.
Shinkai, Iihama, Hayashi, Moriyama, Mizukami, and Yoshinaga (arXiv:2603.22696) study reservoir computing in a synthetic antiferromagnet: two ferromagnetic layers coupled antiferromagnetically. Spin waves in this structure come in two flavors. Acoustic modes have the two layers oscillating in phase. Optical modes have them oscillating out of phase. These aren't just different frequencies — they have distinct dispersion relations, distinct group velocities, distinct nonlinear behaviors. And when used as a computational substrate, they produce distinct memory properties.
In reservoir computing, the reservoir's memory is its ability to retain information about past inputs. A system with short memory responds mainly to the current input. A system with long memory integrates over a history. Most physical reservoirs have one characteristic memory timescale, set by the system's relaxation dynamics. The SAF has two, because it has two independent wave channels propagating through the same physical structure.
This matters because useful computation often requires operating on information at multiple timescales simultaneously — responding quickly to new input while remembering context from older input. A single-layer ferromagnetic reservoir can do one or the other, depending on how it's driven. The SAF can do both at once, because the acoustic and optical modes coexist in the same device, carrying information at different speeds and retaining it for different durations.
The architectural insight: computational diversity can come from the physics of the substrate rather than from building a larger or more complex network. Two coupled layers, not two separate devices. The memory is dual because the material is dual — the antiferromagnetic coupling that gives the SAF its name also gives it its two computational channels.