H G
P
iP
fM
Figure 3.9: Reflection geometry with neutron polarization perpendicular to the sample sur- face. In this geometry the experiment is directly sensitive to a non-collinear arrangements of magnetic moments. The interaction of the polarized neutron beam with a non-collinear structure of magnetic moments is depicted leading to spin flip.
of the domains. When the size of the domains is smaller than the coherence length, the neutrons probe the average magnetization vector of the domain distribution within the coherence length. In addition, the lateral fluctuation of the magnetization vector leads to off-specular scattering.
3.3
X-ray reflectometry and diffraction
We performed X-ray reflectivity and X-ray diffraction measurements to characterize the chemical depth profile and the out-of-plane texture of the prepared samples. Both types of experiments are performed on a Siemens D5000 X-ray diffractometer (Institut E21, Technische Universit¨at M¨unchen).
The instrument runs with a copper source providing a X-ray beam with a wavelength λ = 0.154 nm (CuKα). Originally, it is designed as a dedicated powder diffractometer but additionally, the instrument has been upgraded to perform high quality reflectivity measurements. Therefore, it has been equipped with commercially available compo- nents: a sample table including tilting facility, a high precision knife-edge collimator, an automatic attenuator and a secondary monochromator (figure 3.10).
The beam is collimated by a slit system, which can be varied in order to run the experiments with a proper beam divergence. One slit is introduced in the path of the incident beam close to the sample and two slits are placed in the reflected beam. The knife edge collimator, which acts as an additional slit, allows to narrow the beam divergence for the requirements of reflectometry experiments. The zero position of the knife edge is adjusted to the center of the straight X-ray beam, thus, it serves also as a reference for the alignment of the sample surface to the center of the beam. A monochromator (graphite) is installed in the reflected beam path, in front of the de- tector (scintillation counter), in order to filter out the Kβ-line of the Cu source as well
= ? D @ C B A > > >
Figure 3.10: X-ray diffractometer Siemens D5000: standard θ/2θ-goniometer, upgraded for reflectivity measurements on thin films and multilayers. a) source (Cu), b) slits, c) knife edge collimator, d) sample table, e) automatic attenuator, f ) secondary monochromator (graphite), g) detector (scintillation counter) and h) sample.
as fluorescence that might originate from the sample. Especially fluorescence from Fe in FeCoV layers is suppressed reducing the background in the experiments. The auto- matic attenuator allows reducing the beam intensity if it would cause saturation of the detector resulting in wrong counting rates. Especially for reflectometry, the automated attenuator enables to measure the range of total reflection with the attenuator in the beam and to continue the range of low reflectivity without attenuator, avoiding manual operation and interruption of the experiment.
The diffractometer is interfaced to a PC that controls the sample alignment and the experiments. The software is able to run various types of scans, which can even be combined to a sequence and executed one after the other. A complete set of exper- iments can be performed on one sample, fully automated. Data visualization during experiments and the processing afterwards as well is provided by adequate software tools including data treatment like merging, background correction etc..
For diffraction experiments, the slit width of 0.6 mm was chosen at every position resulting in a relaxed beam divergence of 0.18◦. A well collimated beam is needed for reflectivity measurements. Therefore, an arrangement of slits was installed with widths of 0.1 mm in the incident beam and 0.2 and 0.05 mm in the reflected beam. In addition the knife edge collimator was inserted into the path of the beam, almost touching the surface of the sample. The gap between the film surface and the knife edge was set to 0.005 mm resulting in a divergence of ≈ 0.008◦. Table 3.3 summarizes the setups for beam collimation conducting either reflectometry or diffraction.
3.3. X-RAY REFLECTOMETRY AND DIFFRACTION 43
Reflectometry Diffraction
Width slit 1 [mm] 0.1 0.6
Width slit 2 [mm] 0.2 0.6
Width slit 3 [mm] 0.05 0.6
Position of knife edge [mm] 0.005 ± 0.002 out
Resolution ∆θ [◦] ≈ 0.008 0.18
Table 3.3: Configurations of the slits and the knife edge collimator for reflectometry and diffraction. The numbering of the slits follows the X-ray propagation with slit 1 in the incident beam and slit 2 and 3 in the reflected beam. The knife edge position is its distance to the sample surface.
Prior to the measurement the sample had to be aligned. Therefore, the knife edge was brought into its zero position representing the center of the beam. Then, the sample was moved from below towards the knife edge until it just did not touch. This step was in combination with the tilting facility, which enabled to adjust the sample surface parallel to the knife edge. After that, the knife edge was slightly lifted to create a narrow gap. For the final step of the sample alignment, a rocking scan at grazing incidence and a fixed detector angle was performed and the absolute sample angle was obtained from the center of the specular peak. The alignment of the sample was done using the beam collimation for reflectometry.
Specular reflectivity was typically measured from 0 to 3◦ of the incident angle in steps of 0.005◦. The typical time for counting was 10 s per data point. The regime of total reflection was scanned with the attenuator in the beam until the intensity drops below 1000 counts/s. The rest of the scan was performed without beam attenuator. A small overlap of the two ranges was included, which enabled to merge the two parts of the reflectivity profile. In addition to the specular scan, a longitudinal scan was performed with a constant offset of the sample angle of 0.03◦ from the θ/2θ geometry. This scan measured the diffuse intensity next to the specular rod including the background at the instrument. The intensity of the longitudinal scan was subtracted from the intensity of the specular scan providing the pure specular signal.
Off-specular reflectivity was measured on selected samples of the trilayer series. There- fore, a sequence of rocking scans was performed at various detector angles. The limits for the rocking scans were on either side the horizon of the sample surface with respect to the incident and the exit beam. This range increased with increasing 2θ angles. The detector angle was varied between 0.86◦ ≤ 2θ ≤ 4◦ in order to limit the total time for the mapping of one sample. The step widths were 0.01◦ for θ and 0.02◦ for 2θ. The measuring time per data point was 10 s. A small C code was developed to link all rocking scans and arrange the data in columns of Qx, Qz and intensity.
Diffraction scans were performed in a θ/2θ mode where the scattering vector remained perpendicular to the sample surface. Thus, the experiments probed the crystal struc- ture of the layers along their growth direction, i.e the vertical texture. The scans typically covered a range of 33◦ ≤ 2θ ≤ 68◦. The 2θ step size was 0.05◦ and at each data point the intensity was accumulated for 60 s. The background was determined
from the level of counts between the Bragg peaks and subtracted from the measured intensity.