O. Andreyev

Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission

A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt-CuPc interface. We report a spin injection efficiency of 85-90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10-30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss < or = 200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.

Space charge effects in photoemission with a low repetition, high intensity femtosecond laser source

In this paper, we present experimental results on the effect of space charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distributions of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a N dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where...

Spin-Flip Processes and Ultrafast Magnetization Dynamics in Co: Unifying the Microscopic and Macroscopic View of Femtosecond Magnetism

The femtosecond magnetization dynamics of a thin cobalt film excited with ultrashort laser pulses has been studied using two complementary pump-probe techniques, namely, spin-, energy-, and time-resolved photoemission and the time-resolved magneto-optical Kerr effect. Combining the two methods, it is possible to identify the microscopic electron spin-flip mechanisms responsible for the ultrafast macroscopic magnetization dynamics of the cobalt film. In particular, we show that electron-magnon excitation does not affect the overall magnetization even though it is an efficient spin-flip channel on the sub-200 fs time scale. Instead, we find experimental evidence for the relevance of Elliott-Yafet-type spin-flip processes for the ultrafast demagnetization taking place on a time scale of 300 fs.

Energy-Resolved Electron-Spin Dynamics at Surfaces of p-Doped GaAs

Electron-spin relaxation at different surfaces of p-doped GaAs is investigated by means of spin, time, and energy-resolved two-photon photoemission. These results are contrasted with bulk results obtained by timeresolved Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus spin-flip mechanism. Due to the reduced hole density in the band bending region at the 100 surface the spin-relaxation time increases over two orders of magnitude towards lower energies. At the flat-band 011 surface a constant spin relaxation time in agreement with our measurements and calculations for bulk GaAs is obtained.

Relaxation of Hot Electrons in Solids of Reduced Dimensions

The dynamics of optically excited electrons in solids of reduced dimensions is studied by means of time-resolved two-photon photoemission. In ultrathin films, the lifetime measurements show a clear change by the transition from bulk to layer dimensions smaller than the mean free path of the excited electrons. In silver nano-particles, the electron dynamics does not only depend on the reduced dimension of the systems but rather how the light is coupled into the system (on or off the Mie plasmon resonance). The later effect can be understood in terms of the increased radiation damping in nanoparticles.

Spin Injection and Spin Dynamics at CuPC/GaAs(100) Interface

Submitted for the MAR07 Meeting of The American Physical Society Spin Injection and Spin Dynamics at CuPc/GaAs (100) Interface HUANJUN DING, YONGLI GAO, Department of Physics and Astronomy, University of Rochester, MARINA SANCHEZ-ALBANEDA, MIRKO CINCHETTI, JANPETER WÜSTENBERG, OLEKSIY ANDREYEV, MICHAEL BAUER, MARTIN AESCHLIMANN, Department of Physics, University of Kaiserslautern — Spin injection from GaAs (100) to organic semiconductor copper phthalocyanine (CuPc) has been investigated experimentally with spin-resolved two-photon photoemission (SR-2PPE) spectroscopy. The spin-polarized electrons are originally generated in GaAs through optical pumping with femtosecond time resolution and injected into CuPc film. We observed an enhancement in spin polarization at the interface after initial CuPc deposition. This demonstrates that interface spin scattering is insignificant, which is similar to our previous results of spin injection at CuPc/Co interface. The spin polarization dropped when the CuPc film became thick, an effect attributed to bulk attenuation in CuPc. The lifetime of the unoccupied orbits in CuPc was also studied with red-blue excitation of photon energy of 1.6 eV and 3.2 eV, respectively. There was a strong asymmetry in the time-resolved spectra, and an unexpected long lifetime for the low intermediate state was observed. A simple explanation of this phenomenon will be discussed. Huanjun Ding Department of Physics and Astronomy, University of Rochester Date submitted: 27 Dec 2006 Electronic form version 1.4