Ferromagnetic materials have a spin-dependent density of states, i.e. the density of states of spin-down and spin-up electrons differ. This difference results in an observable magnetization. The summed density over all occupied states having the larger number of states is referred to as the majority density of states and the other correspondingly as the minority density of states. In general, the majority electrons, i.e. the electrons of the majority density of states, have a spin-up character. If the tip and sample are made of a magnetic material, the tunnel current is dependent on the density of states and the spin-dependent tunnel current. The tunneling current spectroscopy with a magnetic tip is therefore referred to as spin-polarized scanning tunneling spectroscopy (sp-STS).
Three-dimensional representations of topographic STM images (500x500nm²) with superimposed magnetic contrast. (a) 0.5 ML Fe (a) and (b) 1.5 ML Fe to Mo (110).