Structural and magnetic properties of Fe epitaxially grown on W(001) surfaces have been investigated by non-contact Atomic Force Microscopy (nc-AFM) and Magnetic Force Microscopy (MFM) under ultra high vacuum conditions and in external magnetic fields at 8 K.
Iron deposited on W(001) grows pseudomorphically up to the fourth monolayer [1]. If an amount of θ ~ 1.3 ML of iron is deposited on the tungsten substrate, elevated substrate temperatures lead to step flow growth, resulting in second layer islands forming on a closed wetting layer. During growth, adsorbates resting on the W(001) substrate tend to move to the wetting layer. Whereas the second layer iron islands and the second layer iron film growing along the step edges of the underlying tungsten substrate are found to be relatively clean, atomic scale nc-AFM images reveal numerous defects and contaminations on the first layer.
When imaging the surface, an electrostatic contrast with bias dependent apparent step heights is obtained by nc-AFM, which is due to the different work functions of first and second iron monolayer. The work function difference can be determined utilizing frequency shift vs. bias voltage curves. Kelvin Probe Force Microscopy (KPFM) allows to map the work function and reveals the real topography.
Although iron is the prototypical ferromagnet, the pseudomorphic growth of iron on W(001) leads to a c(2x2) antiferromagnetic arrangement of the first monolayer [2]. However, MFM images indicate, that the ferromagnetic alignment is recovered within the second layer islands, which exhibit an in-plane contrast. The data are in agreement with spin-polarized scanning tunnelling microscopy studies, where a magnetization along the <110> direction has been found [1,2]. The first monolayer below the ferromagnetic second layer might be either antiferromagnetic (like the whole wetting layer), or ferromagnetic as the iron layer above. Field-dependent MFM measurements were performed to address this open issue.
[1] K. v. Bergmann et al., Phys. Rev. B 70, 174455 (2004).
[2] A. Kubetzka et al., Phys. Rev. Lett. 94, 087204 (2005). |