Correlation Between Nanoscale Structural, Electronic, and Magnetic Properties of Thin Films by Scanning-Probe Microscopy
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MRS BULLETIN/AUGUST 1997
copy (STS) establishes the correlation between atomic structure and local electronic properties,2'3 while spin-polarized scanning tunneling spectroscopy (SPSTS) relates local electronic and magnetic properties.45 Additionally the correlation
Topographic Structure
Electronic Structure
Spin-Polarized STM/STS
Magnetic Structure
Figure 1. Experimental approach to study the correlation between structural, electronic, and magnetic properties of thin films based on scanning-probe microscopies and spectroscopies.
between topographical and magnetic structure can be addressed by magnetic force microscopy (MFM)6'7 though the spatial resolution is limited to about 1050 nm. Since magnetic thin films are usually sensitive to oxidation, their preparation and characterization have to be performed in situ under ultrahigh vacuum (UHV) conditions in order to achieve reproducible experimental conditions. While STS is now routinely applied in UHV, it has only been recently that the first MFM studies under UHV conditions have been reported.8 In the following, representative examples of the application of STS, SPSTS, and MFM in ultrathin Fe, Gd, and Co films will be presented. Scanning-TunnelingSpectroscopy Studies of Ultrathin Fe(110) and Gd(0001) Films on W(110) The growth behavior of Fe on W(llO) has recently been studied in detail by scanning tunneling microscopy (STM).9 Although there is a large lattice misfit of 9.4%, Fe grows pseudomorphically on W(llO) in the first monolayer (ML). At room temperature the initial stage of growth is characterized by the formation of two-dimensional (2D) islands that do not coalesce up to a coverage of 0.6 ML. Figure 2a shows an STM topograph of a 0.5-ML Fe film grown at room temperature on a stepped W(llO) substrate. The 2D iron islands are of monolayer height and have a lateral extent of about 5 nm. Some small amount of Fe tends to decorate the monoatomic steps of the W(llO) substrate. This however is difficult to see in constant-current STM images because the difference in the apparent topographic height between Fe and W almost vanishes. Therefore we have applied local tunneling spectroscopy to look for chemical-specific fingerprints.1011 Figure 2b shows spectra as obtained above the bare W(llO) substrate (A) or above the 2D Fe islands (B). While the ill dfi((i)-characteristic (where 7 is the tunneling current and U is the applied sample voltage) measured above the Fe islands exhibits a pronounced peak centered at U = +0.2 V corresponding to an empty rf-state,12 the spectrum measured above the bare W(llO) substrate shows no significant feature in this bias-voltage regime. Based on the pronounced difference between the tunneling spectra measured above Fe and W, a chemicalspecific imaging of this ultrathin film system can be achieved by spatially resolved measurements of the differential tunneling conductivity dI/dU(x,y) for
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Nanoscale Structural, Electronic, and Magnetic Properties of Thin Films
iipr
f
jif.
jieW A
1.0 -1.0 -0.5 0.0 0.5 1.0 1.5
Figure 2. (a)
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