Paper SS1-TuM4
Termination and Identification of Surface Species on the Magnetite (111) Surface Studied by Scanning Tunneling Microscopy

Tuesday, November 10, 2009, 9:00 am, Room M

Besides its magnetic properties, magnetite exhibits an interesting first order phase transition called the Verwey transition (T_V~120 K). Although there have been many atomistic investigation using scanning tunneling microscopy (STM) at room temperature (RT), the most stable structure of Fe₃O₄(111) has not been yet in complete agreement. By employing STM and scanning tunneling spectroscopy (STS) at 77 K, we have studied the Fe₃O₄(111) surface using a naturally grown single crystal. STM images show a hexagonal lattice with an approximately 0.6 nm periodicity, no remarkable difference from RT observations. Based on the comparison of STS with a theoretical calculations, this surface can be assigned to the Fe_tet termination, where the layer of Fe cations at the tetrahedral site terminates the crystal, supporting results of several early studies. In addition to these findings, we found two distinct features on the as-prepared surface. These features are considered to be the same species as those seen in the RT STM study [1] from the site and the bias dependence of their appearances. We were able to obtain direct evidence of their origin being water by intentionally adsorbing water both at 77 K and at RT, in agreement with previous predictions [1]. For instance, by injecting tunneling electrons into as-dosed intact water molecules at 77 K, we could induce reactions to one of the originally observed species. In contrast to RT experiment, however, their diffusion requires much higher energy, which may be explained by the inefficient electron-phonon (e.g., OH stretching mode) coupling due to the smaller number of electronic states available near the Fermi level below the Verwey transition temperature.

[1] Sh. Shaikhutdinov and W. Weiss, J. Mol. Catal. A: Chem. 158, 129-133 (2000)