Ruthenium oxide has long been regarded as a textbook altermagnet — a material where magnetic moments of neighboring atoms point in opposite directions but do not fully cancel due to the crystal's special symmetry. New measurements show that inside the crystal, there is no magnetism at all. All the magnetic action is confined to the surface.

Using spin-resolved photoemission, scientists observed a narrow surface state with identical spin polarization at opposite momenta. Such a pattern is not seen in altermagnets. Calculations confirmed that magnetism appears only on the fully oxygen-terminated surface. There, charge flows from ruthenium to oxygen, and the neighboring ruthenium sublattices acquire moments of +0.48 and −0.04 Bohr magnetons. This is ferrimagnetic order, like two magnets of different strengths pointing toward each other.

The discovery reconciles years of debate: some experiments saw magnetism in RuO2, others did not. It turns out that everything depended on how surface-sensitive the method was. Now it is clear: RuO2 is not an altermagnet but a material with surface ferrimagnetism. This matters for spintronics, which relies on surface and interface effects.