Ultrafast charge and spin dynamics at solid interfaces : Investigated with femtosecond time-resolved second harmonic spectroscopy

This work investigates the ultrafast charge and spin dynamics on heterogeneous solid interfaces via femtosecond time-resolved second harmonic spectroscopy. With epitaxial Co/Cu(001) films and iron porphyrin molecules adsorbed on Cu(001) two model interface systems for ferro- and paramagnetically ordered overlayers have been studied, respecti-vely. The charge and spin transfer across heterogeneous interfaces are currently areas of interest as being fundamental steps for future spintronic devices. From a technological point of view, one would like to have spin injection with minimal spin dissipation. Investigation of these effects down to fundamental picture in nanometer length scales and femtosecond timescales are of great importance.

In the first part, the present work addresses the length scale of laser induced spin current via systematic thickness dependent studies of epitaxial Co films on a Cu(001) substrate. The spatially inhomogeneous magnetization dynamics is analyzed by time-resolved magnetization-induced second harmonic generation (MSHG), which probes the spin dynamics at vacuum/Co and Co/Cu(001) interfaces. The transient magnetization in-depth profile depends on the Co film thickness. For the Co films less than 3 nm, the vacuum/Co interface is more strongly demagnetized than Co/Cu interface, which is explained by spin transfer with an inelastic mean free paths (MFP) of about 3 nm. While for film thickness larger than 3 nm, a sign change of transient magnetization gradient occurs, which reflects that only spins near Co/Cu interface region can escape into the Cu substrate.

Elementary processes and related timescales at Co/Cu(001) interfaces are further investigated through a combined effort of time-resolved MSHG experiments and  ab initio time-dependent density functional theory (TDDFT). Based on the agreement between experiment and theory on ultrathin Co/Cu(001) films, spin dependent charge transfer from Co films to Cu substrate has been identified. A spin back transfer can occur due to a resonant optical transition in the Co/Cu(001) interface layers. The spin transfer processes govern the dynamics in the first 35 fs after laser excitation. As a local process competing to spin transfer, spin angular momentum dissipates through spin-orbit coupling on the 100 fs timescale, and eventually flows into the crystal lattice, which serves as a sink for angular momentum.

In addition, photoinduced charge transfer dynamics on in situ prepared monolayer Fe-porphyrins (FeOEP) on Cu(001), which serve as a prototype molecule-metal interface,  have been studied by time-resolved second harmonic generation (SHG) spectroscopy. Through wavelength and polarization dependent SHG studies, it was found that a SHG enhancement occurs at 2.2 eV photon energy, which can be assigned to interface-assisted charge transfer resonance. In order to verify the resonant excitation, pump-probe SHG experiments are performed at on- and off-resonant photon energies. Distinctive pump effect and SHG relaxation time of 244 ± 22 fs has been observed at 2.2 eV at the FeOEP/Cu(001) interface, which is slower compared to bare Cu(001) with 168 ± 17 fs. Since the electron lifetime in the interface molecular state are expected to be longer than for Cu(001), slower relaxation dynamics observed at 2.2 eV for FeOEP/Cu(001) indicates resonant charge transfer excitation from metal to unoccupied molecular state.

These results give an insight into the fundamental physical interactions at the interfaces, involving charge/spin transfer and dissipation that give rise to many exciting phenomena, which offer a clear potential for future device technology.

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