Origin of the spin polarization of magnetic scanning tunneling microscopy tips

Using first-principles calculations, we demonstrate that the vacuum spin polarization of commonly used Fe-coated scanning tunneling microscopy (STM) tips is positive at the Fermi energy-opposite to that of Fe surfaces-and is often lower than expected from magnetic thin films. We consider single Fe atoms and pyramids of five Fe atoms on Fe (001) and (110) surfaces as models of STM tips. While the spin polarization of the local density of states (LDOS) at the apex atom of all considered tips is negative close to the Fermi energy and dominated by minority d electrons, the spin polarization of the vacuum LDOS, crucial for the tunneling current, is positive and controlled by majority states of sp character. These states derive from the atomic 4s and 4p orbitals and provide a large spillout of charge density into the vacuum. If we replace the Fe apex atom by a Cr, Mn, or Co atom, the vacuum spin polarization remains positive at the Fermi energy, and it is much enhanced for Cr or Mn in the favorable antiferromagnetic spin alignment with respect to the Fe tip body. At energies above the Fermi level, the spin polarization can change sign due to the contribution from antibonding minority d states. Single Mn and Fe atoms on a nonmagnetic tip provided, for example, by a Cu (001) surface display a similar vacuum LDOS with a small positive spin polarization in good agreement with recent experimental findings. For Cr-coated tips, we observe that the spin polarization can display a change in sign very close to the Fermi energy which can complicate the interpretation of the measured asymmetry in spin-polarized tunneling spectroscopy.