Motivation

This chapter describes the motivations for undertaking the theoretical and experimental study, of polarized electron reflection from a $Co/Cu(001)$ structure, presented in this thesis. The material in this chapter will be repeated in section 2.8, and much of it may seem more lucid there, after an introduction to the theoretical principles surrounding polarized electron reflection studies; it is reproduced here because some understanding of the motives for the study is likely to be helpful in reading that theoretical introduction.

Colleagues at the Cavendish Laboratory had earlier performed a series of measurements on the system formed by growing a $0.5\,\mathrm{ML}$ $Mn/6\,\mathrm{ML}$ $Co/Cu(001)$ multi-layer [14,15,16]. Low-energy electron diffraction (LEED) revealed, through the existence of $(\frac{1}{2}\frac{1}{2})$ order spots, a $c(2\times{}2)$ (chessboard-like) superstructure in the plane of the film.

Three possible explanations were advanced for the $c(2\times{}2)$ superstructure [14,15,16]:

• Inter-diffusion of copper atoms through the cobalt layer may have led to the formation of a $MnCu$ surface alloy [17,18]. However, these experiments were performed at room temperature, which is too cold to permit significant inter-diffusion through $6ML$ $Co$ [19]. This suggestion was therefore ruled out.
• The manganese may have had an internal $c(2\times{}2)$ anti-ferromagnetic configuration. However, the measured $(\frac{1}{2}\frac{1}{2})$ spot intensity was $\sim 10$ times that predicted for magnetic diffraction of this type by Tamura et al. [20]. This phenomenon is therefore incapable of providing a complete explanation of the observed superstructure.
• An ordered $MnCo$ surface alloy may have been formed. Given the evidence against the other two models, the observed diffraction pattern was attributed to $MnCo$ alloy formation.

Magneto-optical Kerr effect (MOKE) measurements were taken by colleagues at the Cavendish Laboratory as part of the same project [14,15,16]. The $[110]$ direction Kerr signal was seen to increase with the addition of manganese, in the regime where the alloy exists. The immediate reaction to this is that the system's magnetic moment is increasing, i.e. that the manganese atoms that are being added are ferro-magnetically aligned, both with each other and with the cobalt. However, the simultaneous drop in $[100]$ Kerr signal did not paint the same picture, and it may be that a change in magneto-optical response, rather than a genuine addition of magnetic moments, is responsible for the increased $[110]$ signal. The possibilities are in particular need of investigation, given that a series [21,22,23] of numerical calculations, using different approximations, have produced widely differing predictions about the magnetic properties of this system.

The polarization of a reflected electron beam from the sample surface provides an alternative to MOKE, for measuring the magnetization, through the difference, for the two electron spins, in density of states at a particular energy created by the exchange splitting, and as such provides an experiment which is interesting by virtue of having an unpredictable result. The experiments presented in this thesis, using a $Co/Cu(001)$ structure, are intended as an intermediate stage on the way to similar measurements on $MnCo/Co/Cu(001)$, although they could equally well lead to similar measurements on other multi-layer structures of more direct industrial relevance, such as metal/ferromagnet or semiconductor/ferromagnet spin valves. This possibility may help to address the conclusions, concerning the motivation for experimental research projects, of statistical research the author has presented elsewhere [10].

In addition, the measurements for $Co/Cu(001)$, in themselves, allow a trial run of estimating the parameters in the new theory of this chapter, and comparing a version of that theory, in which no Weiss field affects the probing electrons, against another version, in which an adjustable Weiss field affects the probing electrons. This task is undertaken in section 5.3.