Sebastian Schäfer

Laser-induced charge separation in CdSe nanowires.

A combination of electrostatic force microscopy and optical microscopy was used to investigate the charge state of individual CdSe nanowires upon local illumination with a focused laser beam. The nanowires were found to be positively charged at the excitation spot and negatively charged at the distant end(s). For high laser powers, the amount of accumulated charges increases logarithmically with the laser power. These effects are described by a diffusion-based model where the results are in good agreement with the experimentally observed effects. On the basis of this model the charge imbalance along the nanowire should establish in the course of nanoseconds. The net charge separation within homogeneous nanowires upon local illumination is of importance for several electronic devices.

Interfacial perpendicular magnetic anisotropy and damping parameter in ultra thin Co2FeAl films

B2-ordered Co2FeAl films were synthesized using an ion beam deposition tool. A high degree of chemical ordering ∼81.2% with a low damping parameter (α) less than 0.004 was obtained in a 50 nm thick film via rapid thermal annealing at 600 °C. The perpendicular magnetic anisotropy (PMA) was optimized in ultra thin Co2FeAl films annealed at 350 °C without an external magnetic field. The reduced thickness and annealing temperature to achieve PMA introduced extrinsic factors thus increasing α significantly. However, the observed damping of Co2FeAl films was still lower than that of Co60Fe20B20 films prepared at the same thickness and annealing temperature.

Spectroscopic investigation of electrochemically charged individual (6,5) single-walled carbon nanotubes.

Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift.

Microstructural and ferromagnetic resonance properties of epitaxial nickel ferrite films grown by chemical vapor deposition

Microstructural and ferromagnetic resonance properties of epitaxial nickel ferrite (NiFe2O4) films grown by direct liquid injection chemical vapor deposition are reported. While high-quality epitaxial growth of NiFe2O4 films on (100)-oriented MgAl2O4 substrate is confirmed by high resolution transmission electron microscopy, bright field (diffraction contrast) TEM studies reveal the presence of dislocations and also dark diffused contrast areas, which originate from antiphase domains. Angle and frequency-dependent ferromagnetic resonance (FMR) experiments are conducted to determine the magnetic anisotropy and the magnetic relaxation. A low out-of-plane FMR linewidth of ∼160 Oe has been observed at a frequency of 10 GHz.

Charge separation in CdSe/CdTe hetero-nanowires measured by electrostatic force microscopy

The charge state of free standing axial type-II CdSe/CdTe hetero-nanowires is monitored via electrostatic force microscopy. The CdSe and the CdTe segment which are identified by Raman spectroscopy are found to be negatively and positively charged, respectively. The charge state is monitored without and with local illumination. We found that the magnitude of opposite charging in the respective nanowire segment is increasing with illumination power, which is attributed to a charge separation of the photogenerated electron-hole pairs across the CdSe/CdTe interface.

Frequency-selective control of ferromagnetic resonance linewidth in magnetic multilayers

We report on a frequency-specific linewidth broadening of the ferromagnetic resonance (FMR) mode of a NiFe free layer within a magnetic multilayer stack. The FMR studies reveal a significant broadening of the FMR linewidth of the free layer at frequencies where this resonance is degenerate with FMR modes stemming from other layers within the multilayer stack. By pinning part of the magnetic multilayer to an antiferromagnet, we tailor a ferromagnetic linewidth behavior that is anisotropic for a specific frequency.

Magnetic damping and spin polarization of highly ordered B2 Co2FeAl thin films

Epitaxial Co2FeAl films were synthesized using the Biased Target Ion Beam Deposition technique. Post annealing yielded Co2FeAl films with an improved B2 chemical ordering. Both the magnetization and the Gilbert damping parameter were reduced with increased B2 ordering. A low damping parameter, ∼0.002, was attained in B2 ordered Co2FeAl films without the presence of the L21 Heusler phase, which suggests that the B2 structure is sufficient for providing low damping in Co2FeAl. The spin polarization was ∼53% and was insensitive to the chemical ordering.