Direct observation of spin-polarized surface state on Cr(001) and its exchange coupling with SP-STM tip

Spin-polarized scanning tunneling microscopy (SP-STM) is capable of investigating magnetic properties of nanostructures and surface states, offering insights into magnetic textures and magnetic interactions at the atomic scale. In this work, via SP-STM, we studied the electronic and magnetic properties of an atomically flat Cr(001) surface. The commensurate spin density wave (C-SDW) domain of Cr(001) exhibit in-plane ferromagnetism, which induces exchange splitting of surface states and generates spin-polarized surface states. Using quasiparticle interference measurement (QPI), we clearly observed the energy dispersion of these spin-polarized surface states. Interestingly, when the spin of tip is polarized in parallel/antiparallel to the spin of the surface, we found significant energy shift (up/down) of surface states band. This shift is much larger than the Zeeman splitting of applied magnetic field, suggesting that it is caused by intrinsic magnetic coupling between tip and surface states, which act as the two electrodes of the magnetic tunneling junction (MTJ) . Meanwhile, at the C-SDW anti-phase domain wall with vanishing spin, we observed a weakened surface state dispersion, which likely due to the disappearance of exchange splitting. Our observation indicates there exists a ferromagnetic coupling between the spin-polarized tip and sample surface, even at relatively large tunneling barrier. This provides new understandings on the inter-layer coupling of a MTJ at atomic scale.