Local and Nonlocal Relaxation Dynamics of Hot Electrons in Au/Fe/MgO(001) Thin Films
Photoexcited hot charge carriers exhibit excess energies that could be harnessed for a wide range of applications, such as in photovoltaic cells for the conversion of sunlight into energy. The energy of the charge carriers also leads to scattering mechanisms in the constituents or at the interfaces of heterostructures, which compete with their use in application devices. Therefore, the spatial, temporal, and energetic distribution of hot charge carriers is important for their efficient extraction and utilization. This work investigates the ultrafast local and nonlocal relaxation dynamics of hot electrons in epitaxially grown Au/Fe/MgO(001) thin films. Femtosecond time-resolved two-photon photoemission spectroscopy (tr-2PPE) was used to study the energy-dependent ultrafast electron dynamics in Au and Fe buried media/interfaces. Experiments were performed with two different geometries: (i) pump-probe at the Au surface (FP) and (ii) pumping Fe layer and probing at the Au surface (BP). The propagation of electrons from a ferromagnetic metal (FM) to a noble metal (NM) through the FM/NM interface is of particular interest in this work. As a first step for investigating the energy-dependent hot electron relaxation dynamics in Au/Fe/MgO(001) thin films, femtosecond time-resolved 2PPE intensities as a function of Au thickness dAu and intermediate state energy E − EF are explored. It is found that optical excitation of the Fe layer leads to a temporal shift in the intensity built-up of the transient 2PPE yields. The thicker the Au layer, the more pronounced the temporal shift in the intensity built-up, which is attributed to transport effects. This nonlocal effect was not observed in the tr-2PPE measurements with the Au-side pump geometry. Analysis of the interplay among local and nonlocal relaxation dynamics led to the conclusion that hot electrons propagate from the Fe layer to the Au layer surface in a superdiffusive manner. It is also shown in this work that the Fe-side pump geometry can separate the local scattering rates in the constituents of the Au/Fe heterostructure, which can not be inferred from the Au-side pump data analysis. However, tr-2PPE experiments with the Au-side pumping approach reveal the first image potential state IPS (n = 1) at the Au(001) surface. Another spectral feature is observed by the Fe-side pumping approach, which is attributed to the excitation of d-band minority states in Fe, and, thus, demonstrates the sensitivity of the Fe-side pump geometry to the hot electron relaxation dynamics in buried media of metallic heterostructures. This work establishes Fe-side pumping in femtosecond two-photon photoemission spectroscopy and demonstrates a tool for analyzing hot electron relaxation and transport dynamics in FM/NM heterostructures energy-resolved in the time domain.