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A translator works in two worlds at once: to an outside observer, they are silent and unobtrusive, but inside their mind, a flurry of activity unfolds along clear lines of meaning. Altermagnets, a recently discovered class of magnetic materials, are much the same. In ordinary space, their magnetization is fully compensated, as in antiferromagnets: the tiny magnetic arrows of neighboring atoms point in opposite directions and cancel each other out. From the outside, the material appears nonmagnetic. But in the momentum space of electrons (also called reciprocal space), the electronic levels split by spin, as if in a strong ferromagnet. This gives altermagnets their unique properties.

For a long time, this internal contradiction could not be reliably detected. Researchers directed ultrashort laser pulses at a MnTe crystal and measured how they reflected. In response, two signals emerged at once. The first is the Kerr effect, familiar from ferromagnetic disks: the plane of light polarization rotates by a tiny angle that depends on magnetization. The second is the Voigt effect, characteristic of antiferromagnets: what changes is not the rotation but the very shape of the light spot: its ellipticity. Both effects appeared simultaneously but decayed at different rates — the Kerr effect vanished faster.

The difference in signal lifetimes is reminiscent of two echoes in the mountains: one fades quickly, the other lingers longer. The fast echo, the Kerr effect, is tied to the instantaneous response of the electronic structure. The slow echo, the Voigt effect, arises from the more sluggish rearrangement of magnetic order. Such a combination makes altermagnets a promising platform for ultrafast optical electronics, where information is controlled not by currents but by flashes of light. And the translator, silent on the outside, continues their dual work — now in full view of researchers.