An advanced magnetic reflectometer
A new experimental setup dedicated to the measurement of soft-x-ray magnetic absorption spectroscopy and soft-x-ray
resonant magnetic reflectometry (soft-XRMR) is presented. XRMR is the combination of standard x-ray reflectometry with x-ray magnetic circular dichroism which provides chemical and magnetic depth profiles of layered thin-film samples. This new diffractometer is optimized for a broad variety of sample systems. Therefore a balanced design focusing on high magnetic fields, low temperatures, and full freedom of rotation has been realized in UHV.
The specular reflection of x-rays from a multilayered thin-film sample is measured in a so-called θ–2θ geometry, where the sample is rotated by an angle θ and the detector, respectively, by the double angle 2θ.
Such a measurement requires the detector and the sample to be freely rotatable and their axis to be aligned properly. Beside this general setup the experiment is intended for XMCD measurements.
The cryosample variable temperature insert (VTI) is intended to provide a large temperature range for measurements while allowing azimuthal sample rotation and measurement of the sample electron yield via the total drain current.
A broad variety of sensors and detectors is included in the experiment in order to be able to measure not only the
specular reflection but also absorption and fluorescence from the sample.
To be able to measure magnetic reflectivity curves, element selective hystereses, and dichroic x–ray absorption
spectra, it is necessary to apply an external magnetic field to the sample. This field should be fast switching and freely scalable. Two field coils with yoke setup are used to achieve this.
The key features and improvements of the system can be summarized as follows:
- Simultaneous detection of TEY, FY, reflection, diffuse background and I0
- Maximum applicable magnetic field 260 ± 0.5 mT (optimal 650 mT with 10 mm gap)
- Vacuum < 5⋅10−10 mbar in the main chamber, <5⋅10−8 mbar in loadlock
- temperature range: 38–480 K (<20 K planned)
- θ–2θ range: 0° – 65° (resolution < 0.03°) corresponding to qZ = 0 – 0.64 ± 4⋅10−4 Å (at 700 eV)
- 360° azimuthally sample rotation (resolution < 0.5°)
- S/N (asymmetries): IRefl < 1‰, TEY < 1‰, FY < 4‰
- Saving of alignment time
- Fast loadlock and sample transfer (<1h)
- 10x reduction of measuring time
- 50x reduction of sample transfer time
First measurements at the UE56/2-PGM1 beamline at BESSY II proved that the design is able to provide absorption (TEY and FY) and reflectivity spectra of highest quality. Detailed element selective hystereses of deeply buried layers can be measured in reflection. For the future it is planned to include a large-area high-resolution charge coupled device (CCD) on the 2θ–axis. This will allow measuring the specular and diffuse scattering and thus investigate in-plane magnetic effects such as domains. The CCD will capture more than 200 frames/s allowing reasonable integration times and a high dynamic range. The low–temperature range will be extended below 20 K by using an alternative sample holder without azimuthal rotation but better thermal coupling.
CAD drawing showing the whole experimental setup for soft X-ray resonant reflectivity
Schematic drawing of the experimental setup. The scattering geometry corresponds to the geometry of the longitudinal Kerr effect. More details on the individual components are given in the text. Note that the experiment is connected to the beamline on the left side.
Magnetic field as measured with the hall probe attached to the pole shoe. The solid circles show the standard configuration with 30mm gap while the square symbols show the field for a gap of 10mm. The inset shows the low field region of the 30mm gap configuration.
Photography of the detector mounted in the chamber. (1) is the opening of the detector which is mounted behind; (2) is the motor driven aperture slit and (3) is an additional diode of the same type which acts as background reference.
CAD drawing of the main chamber showing all relevant components: the main body of the cryo-VTI (1) and the attached sample holder (4), the detector diode mounting (2), the inner part of the magnet yoke (3), and the two UHV ball bearings [(5) and (6)], carrying magnet and detector, respectively.