R. Wiesendanger et al., WIGNER GLASS ON THE MAGNETITE (001) SURFACE OBSERVED BY SCANNING-TUNNELING-MICROSCOPY WITH A FERROMAGNETIC TIP, Journal of vacuum science & technology. B, Microelectronics and nanometer structures processing, measurement and phenomena, 12(3), 1994, pp. 2118-2121
A clean and well-ordered (001) surface of a slightly nonstoichiometric
natural single crystal of magnetite has been prepared under ultrahigh
vacuum conditions by in situ thermal annealing. The surface was caref
ully characterized by low energy electron diffraction, Auger electron
spectroscopy, and x-ray photoelectron spectroscopy prior to our scanni
ng tunneling microscope investigations. Atomic resolution was achieved
by using either tungsten or in situ prepared ferromagnetic iron tips.
In contrast to the results obtained with the tungsten tips, the ferro
magnetic probe tips revealed an atomic-scale contrast between the diff
erent magnetic iron ions of magnetite on the octahedrally coordinated
B sites which form static arrays of pairs with short-range order. The
clean magnetite (001) surface appears to be in a Wigner glass state ev
en at room temperature with electron pairs localized on adjacent ions
as the basic unit. The degree of order is likely to be limited by the
slight nonstoichiometry of the magnetite sample as deduced from temper
ature-dependent measurements of the electrical resistivity.