Abstract
Spin-resolved photoemission is one of the most direct ways of measuring the magnetization of a ferromagnet. If all valence band electrons contribute, the measured average spin polarization is proportional to the magnetization. This is even the case if electronic excitations are present, and thus is of particular interest for studying the response of the magnetization to a pump laser pulse. Here, we demonstrate the feasibility of ultrafast spin-resolved photoemission using free electron laser (FEL) radiation and investigate the effect of space charge on the detected spin polarization. The sample is exposed to the radiation of the FEL FLASH in Hamburg. Surprisingly, the measured spin polarization depends on the fluence of the FEL radiation: a higher FEL fluence reduces the measured spin polarization. Space-charge simulations can explain this effect. These findings have consequences for future spin-polarized photoemission experiments using pulsed photon sources.
Highlights
Photoemission spectroscopy allows for direct access to the band structure of a solid [1, 2]
The high peak intensity of the free electron laser (FEL) is challenging for all photoemission experiments [16]: the immediate release of electrons from the sample causes a cloud of electrons in front of the sample
If the space charge reduces the amount of these low energy but highly polarized electrons, the polarization reduction observed in the FEL experiments could be explained solely by a space-charge effect
Summary
Photoemission spectroscopy allows for direct access to the band structure of a solid [1, 2]. Photoelectron spectroscopy can be extended by a spin detector in order to resolve the spin-split band structure of magnetic samples [5,6,7]. FELs are challenging sources, especially in the case of photoemission experiments [16]: in classical photoemission experiments the usual flux of the source is rarely higher than 1014 photons per second (e.g. a synchrotron source) At these conditions, the number of generated photoelectrons at any time is low enough that they hardly interact with each other. The average spin polarization of the photoelectrons is expected to remain independent of space-charge effects This conjecture is experimentally tested at the PG2 beamline [17, 18] of the free electron laser in Hamburg (FLASH). An iron film on tungsten (110) serves as the magnetic sample
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