The steady and reliable operation of the apparatus with continuous sources has been proven by perfoming high spatial resolution experiments in “imaging mode”. With an accurately focused 1.5 keV electron beam, the sample area has been scanned in steps of 100x100 µm measuring the secondary electron yield with the Mott detector.
Figure 1 “Imaging mode” of Ultraspin, from a test Fe-p(1x1)O thin film grown on MgO. a. map of the secondary electron yield. b. Map of the spin polarization. The quantization axis for this measurement is parallel to the x axis (where magnetic field is applied by an electromagnet), but two more figures can be obtained quasi-simultaneously for the other directions. In this case, with a 100 nm Fe film, magnetization is fully in plane, and along other directions no contrast is observed.
At each pixel, the film magnetization in the horizontal plane was reversed. Total signal from the four channels (Figure 1 a.) corresponds to a secondary electron emissivity map (in a way analogous to a
Secondary Electron Micrography, or SEM image). The signal arising from the difference of the image acquired for each magnetization (Figure 1 b.) gives a map of the spin polarization (similar to Secondary Electron Micrograph with Polarization Analysis, i.e. a SEMPA image).
Figure 2Comparison of a measurement with a continuous excitation source (statistical electron gun) and one with a pulsed one (microsecond-gated electron gun). The performance of the detector in pulsed mode is uncompromised. The error bars on the pulsed acquisition points are slightly larger due to the much lower average current resulting from gating of electron gun.
We successfully upgraded the acquisition electronics of our electron spin polarization detector to operate with sharp pulses of photoemitted electrons, maximizing the efficiency of the set-up (Figure 2).
Ultraspin has now been transferred into the Fermi Experimental Hall (Figure 3), and is undergoing a second development phase to become the endstation of a new laser facility parallel to Fermi Free Electron Laser, the Joint Sprint (CNR) and T-Rex Laboratory (Elettra).
Figure 3Phases of Ultraspin endstation moving to SPRINT hutch. The endstation is easily moveable and after the second step of development it will sit on a single frame with an extremely compact profile. It will also be possible to move it to Fermi beamlines in a single step.