Vacuum tunneling of spin-polarized electrons detected by scanning tunneling microscopy

Abstract

We show that the scanning tunneling microscope (STM), operated in ultrahigh vacuum, can be made sensitive to the electron spin and therefore to magnetic structures down to the atomic scale. The experiments were performed with a ferromagnetic CrO2 tip providing electrons of high spin polarization. As sample, we used a Cr(001) single crystal which offers an ideal magnetic test structure. The experimental results obtained with the ferromagnetic CrO2 tip characteristically differ from those obtained with a nonmagnetic tungsten tip. We give an expression, relating the local electron spin polarization of the tunneling junction with quantities directly measurable with the STM. We also discuss possible contributions from magnetic dipole and exchange forces as well as the feasibility of magnetic exchange force microscopy.