In the grand symphony of magnetism, a duo long reigned: boisterous ferromagnetism and silent antiferromagnetism, where spin-soloists cancel each other out. But recently, a third voice joined the ensemble — altermagnetism. Imagine a conductor making the orchestra play soundlessly, yet each instrument carries its own part: an altermagnet separates electrons by spin without creating magnetization. The trouble is that in the flat world of two-dimensional materials, such a talent was almost unheard of — until chemists stepped in, wielding the tool of surface functionalization.

Numerical simulations show: a single surface hydrogenation, oxidation, or fluorination breaks inversion and mirror symmetries, transforming mundane spin degeneracy into exquisite d-wave splitting. In the hydrogenated iron-selenide monolayer (Fe₂Se₂H), pairs of Weyl point-antipodes with opposite Chern numbers are born, and spin-orbit coupling opens a topological gap. Thus begins the subtle dance of quantum fields: the wave-particle duality of electrons acquires precise spin polarization: they are not just particles, but waves with perfectly tuned orientation.

Transport calculations in structures like Au/Fe₂Se₂H/barrier/Fe₂Se₂H/Au revealed giant tunneling magnetoresistance—1870%. That’s almost 20 times higher than modern commercial magnetic tunnel junctions. When the Néel vectors in the two layers are parallel, the spin filters are aligned, and current flows freely; when antiparallel, the channel is blocked. Like a lock obedient to a chemical key, the device controls the flow of spin-polarized electrons. The Pauli principle, forbidding two electrons from occupying the same state, here turns into selective transmission: one channel is a high-speed freeway, the other a solid wall.

Looking ahead, chemical engineering can be extended to other antiferromagnetic monolayers. Moreover, combining with FeSe’s innate superconductivity opens a path to hybrid devices where altermagnetic and superconducting correlations intertwine in novel quantum states. This discovery is already changing the rules of the game: instead of passively searching for altermagnets, scientists gain an active method to construct them, and the record magnetoresistance promises spintronic memory and logic with previously unattainable density and speed.