Observation of spin voltage and accumulation by spin-resolved femtosecond photoelectron spectroscopy

The generation of spin current pulses by laser-driven demagnetization links the field of ultrafast magnetism to spintronics. So far, this spin transport and its cause could only be observed indirectly. We demonstrate that femtosecond spin injection can be observed on the femtosecond time scale by spin and time resolved photoemission experiments. We study thin, epitaxial iron films which are excited by a 800 nm pump laser beam. Photoemission by a higher harmonic generation source (photon energy: 21 eV) in combination with an electron spin polarimeter is used to measure the chemical potentials of the minority- and majority electrons. This way, we observe the spin voltage, which acts as the driving force for the spin current. If we deposit a thin gold film onto the iron sample and excite the iron film through the transparent substrate, we can study spin injection and accumulation. The spin polarization in Au rises on the femtosecond time scale and decays within < 1 ps. The decay time depends on the Au film thickness. This thickness dependence can be described by a "spin capacitance", which is similar to the capacitance in charge-based electronics.