SPLEEM of Magnetic Surfaces and Layered Structures
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help of an extraction electro-optical system, which is combined with an inefficient polarization detector. SPLEEM, on
6.0 ML
11.0
ML
the other hand, relies on globally imaging the reflected intensity distribution in a "parallel" surface image, which results from illuminating the surface with polarized electrons. Although these two principal approaches for magneticsurface imaging have been recognized for a number of years,6 SPLEEM has only recently47"11 become practical with the advent of new sources for polarized electrons with reasonably high degrees of polarization (activated GaAs and strained layer epitaxial GaAs photocathodes). Although the two approaches to magnetic-surface imaging, SEMPA and SPLEEM, are very different instrumentally, they are related in principle. As shown by theoretical symmetry considerations, the polarization P' of the (lowenergy) secondary electrons created by the scattering of an unpolarized, primary electron beam by a magnetic surface is equivalent to the asymmetry A, which describes the difference in reflection intensity of a polarized, primary electron beam (of low energy) impinging on a magnetic target material. Recognizing this relationship between P' and A and considering the inefficiency of detectors for polarized electrons in SEMPA, electron microscopy of magnetic surfaces using polarized, primary electrons has been discussed in the past.6 The main advantage of this microscopy approach is the detection of reflected electron intensities instead of polarization, which is much more efficient. However, such proposals were limited to (electrostatic) low-energy scanning-microscopy techniques of insufficient resolution capability, to name only the most obvious shortcomings. With the advent of LEEM and higher efficiency sources for polarized electrons,13 high-speed global-imaging magneticsurface microscopy has now become a viable alternative to SEMPA. The image contrast in LEEM is mainly due to diffraction in the crystal lattice of the sample or is caused by geometrical interference effects on the sample surface.1 The magnetic image contrast in SPLEEM is proportional to the asymmetry A for the reflection of up- or downpolarized primary electrons of very low energy: A = (Jup - /d()wn)/(/up + Jdnwn) x
Figure 1. Four selected growth stages of Co on W(110) at a 790-K substrate temperature showing morphological and magnetic image contrast. The corresponding Co thicknesses in monolayers are (a) 4.2 ML, (b) 6.0 ML, (c) 9.0 ML, and (d) 11.0 ML.
where P is the polarization of the primary electron beam. The magnitude of the magnetic contrast depends not only on the local magnetization M in the sample surface but also on the band
MRS BULLETIN/OCTOBER 1995
SPLEEM of Magnetic Surfaces and Layered Structures
t = 195 sec
t = 220 sec (1 ML)
Figure 2. In situ determination of exact monolayer coverage for Co on W(110) at 650 K.
structure of ferromagnetic materials. For a given crystallographic direction, the allowed states differ for up- and downspin incident electrons as a function of en
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