Tuning Surface-Electron Spins on Fe₃ O₄ (111) through Chemisorption of Carbon Monoxide

The adsorption-induced flip of electron spin at interfaces is an important but poorly understood phenomenon for magnetic devices, sensors, and heterogeneous catalysis, due to the difficulties in determining the surface spins at atomic resolution. We present an evolutionary magnetic order searching method that allows efficient identification of the ground state spin configuration of magnetic bulk and surfaces. Using this approach, we have discovered for the first time a set of adsorption-induced near-degenerate surface magnetic states on the Fe₃ O₄ (111) surface. Molecular adsorption of CO causes a destabilization of the magnetic states of the clean surface leading to a set of near-degenerate surface magnetic states at medium coverage, which causes an abrupt increase of the magnetic entropy on the surface. The empty 2π* orbital of CO, which could accommodate the back donation of the spin density in the Fe d orbitals, plays a key role for the CO adsorption-induced spin transition.