R. Vollmer

Spin-wave excitation in ultrathin Co and Fe films on Cu(001) by spin-polarized electron energy loss spectroscopy (invited)

We demonstrate, that spin-polarized electron energy loss spectroscopy is a unique tool to determine spin-wave dispersion curves of ultrathin films and surfaces over the whole Brillouin zone. For ultrathin Co films on Cu(001) the excitation of a spin wave appears as a well defined peak in the loss spectrum. 3 ML Fe films on 1 ML Co/Cu(001) films show spin-wave excitation peaks as well though much broader compared to the Co case. The spin-wave energy at the surface zone boundary of about 140 meV for the Fe film is nearly a factor of 2 smaller than that of the Co films.

Interface mode for NaCl overlayers on the Ge(001) surface

Abstract The structure and dynamical behaviour of NaCl overlayers on the Ge(001) surface have been investigated with helium atom scattering (HAS). For coverages of 2 and 3 layers of NaCl on the Ge(001) surface, a low energy mode at about 2 meV has been zone boundary and its disappears with increasing NaCl coverage. Morover, the experimental data shown ab avoided crossing between the Rayleigh wave of NaCl and this low energy mode. A slab calculation within the framework of a simple force constant model provides an interpretation of the experimental features and indicates that the low energy mode is an interafact mode.

Nonlinear magneto-optical Kerr measurements on Fe(110)

For the clean Fe(110) surface Kerr angles close to 90° are observed in the frequency doubled light and for the oxygen covered surface asymmetries up to 100%. In the wavelength range from 740 to 840 nm a sizable amount of the second harmonic light is possibly generated by the bulk quadrupole like nonlinear polarization especially for p-polarized incident light. For s-polarized incident light a strong surface contribution is found.

Second harmonic generation from the Cu(001) surface

We report on measurements of the second harmonic (SH) yield from a Cu(001) surface in the wavelength range between 740 nm and 840 nm which is below the one photon excitation threshold of d-electrons. The SH-yield varies strongly with the azimuthal angle of the Cu crystal indicating a significant contribution of bulk anisotropic SH generation.

Nonlinear and linear Kerr studies of Co/Cu multilayers

Magnetization induced optical second harmonic generation (MSHG) in combination with the magneto-optical Kerr effect (MOKE) was used to study thin Co films and Cu/Co multilayers of thicknesses between 1 and 20 monolayers (ML) grown on a Cu(001) substrate. From the different dependence of MSHG and MOKE on Co film thickness we get direct information about the interface sensitivity of MSHG. Modifying the vacuum/Co interface by gas adsorption yields the relative contributions of the vacuum/Co and the Co/Cu(001)-substrate interfaces. Both MSHG and MOKE data can be accurately described by a multiple reflection model.

Strong coupling of Rayleigh phonons to charge density waves in 1T-TaS2

Abstract The dispersion curves of charge density wave (CDW)-induced phonons have been measured in 1T-TaS 2 by inelastic He atom scattering along both crystallographic and CDW symmetry directions. Evidence is given for a strong coupling of the CDW to Rayleigh waves, i.e. to quasi-transverse acoustic modes. A pseudocharge model analysis of lattice dynamics accounts for the CDW-induced splitting of the Raman-active modes originated from the folding of transverse acoustic modes.

High wave vector spin waves in ultrathin Co-films investigated by spin-polarized electron energy loss spectroscopy

The capabilities of spin polarized electron energy loss spectroscopy are demonstrated with results obtained on ultrathin Co-films. Co is investigated in two crystalline phases, the fcc- and the hcp-phase. In both cases, the spin-wave signals for wave-vector transfers up to the surface Brillouin zone for film thickness of only a few atomic layers are measured. In both systems, a pronounced dispersion of the spin waves is found. The measured data is in good agreement with the calculated dispersions of surface spin waves within a nearest neighbor Heisenberg model. From the fits of the calculated to the measured dispersions, a value for the product of the exchange coupling constant and the magnetic moment (JS) of about 15 meV for both crystalline phases is obtained. Due to the similar values of JS, the spin-wave energies are similar for wave-vector transfers much smaller than the surface Brillouin zone boundary. For high wave-vector transfers, the spin-wave energies differ significantly reflecting the larger surface Brillouin zone for the hcp Co-lattice caused by the different arrangement of atoms at the surface.