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"Thanks to the high precision and sensitivity of our measurements, we could detect the characteristic alternating splitting of the energy levels corresponding to opposite spin states and thus demonstrate that manganese telluride is neither a conventional antiferromagnet nor a conventional ferromagnet but belongs to the new altermagnetic branch of magnetic materials," says Juraj Krempasky, beamline scientist in the Beamline Optics Group at PSI and first author of the study.

The topological insulator is a ferromagnet -- a type of magnet whose electrons spin the same way -- while FeTe is an antiferromagnet, whose electrons spin in alternating directions.

According to the researchers, adding in the ferromagnet has been particularly challenging.

"We focused on a cobalt-tin-sulfur-based ferromagnet because its topological electronic state is suited to control the polarity of the anomalous Nernst effect according to our previous theoretical study," stated Fujiwara.

Crucially, the cavity vacuum fluctuations alone are sufficient to change the material's magnetic order from a zigzag antiferromagnet into a ferromagnet.