Association

56 3. Experimental details
does not fulfill the above-mentioned rule. We remark moreover, that the MCD amplitude
depends on the direction of the k e− vector, for this we refer to literature. 145 Here, we focus on
the geometry used in this thesis: the electron wave vector k e− is in 10 ◦ off-normal emission,
and the axial vector of the light polarization q is either parallel or antiparallel to the in-plane
sample magnetization M. In this setup, every symmetry operation (i. e. rotation of the sample
by the n-axis or mirroring by planes normal to the surface) which changes M also alters k e− .
Thus, the rule of Feder allows one to observe the MCD effect in photoemission.
In a photoemission spectrum including MCD, the intensity of a peak changes when the polarization
of the incoming light is changed from left-circularly polarized light (LCP) to rightcircularly
polarized light (RCP), or as an alternative the magnetization of the sample is reversed.
The MCD asymmetry is defined as
MCD ≡ I(σ+ , M) − I(σ − , M)
I(σ + , M)+I(σ − , M) = I(σ+ , +M) − I(σ + , −M)
I(σ + , +M)+I(σ + , −M) . (3.21)
The MCD photoemission experiment is conducted with hard X-rays at the HAXPES beamline
P09 at the high brilliance storage ring PETRA III (see Ch. 3.4.3). In practice, a constant helicity
of the incoming light (e. g. σ + ) is used, and to obtain the MCD asymmetry in eq. (3.21),
the magnetization of the thin film is altered to be either parallel or antiparallel with respect
to the k vector of the incoming light (see Figs. 3.20, and 3.16 on p. 51). This offers a high experimental
consistency: neither a modification of the X-ray beam (by changing the diamond
phase retarder) nor a rotation of the sample position (by 180 ◦ ) is needed in order to reverse
the MCD effect.
Figure 3.20.: Geometrical setup for the HAXPES measurement of the magnetic circular dichroism effect
(MCD).