M.M.J. Bischoff

Use of voltage pulses to detect spin-polarized tunneling

The present letter describes a method to make a spin-polarized scanning tunneling microscopy tip by applying voltage pulses between a W tip and a magnetic sample. This spin-polarized tip has the similar characteristics as an Fe-coated W tip, which was confirmed by observations of antiferromagnetically coupled ferromagnetic Mn(001) layers (>3 ML) grown on an Fe(001) whisker at 370 K. Furthermore, we demonstrate that these voltage pulses can vary the tip magnetization direction.

STM-induced photon emission spectroscopy of granular gold surfaces in air

Abstract Light emission has been detected under ambient conditions in the tip–sample region of a scanning tunneling microscope (STM) consisting of an etched gold tip and a granular gold film. The photon yield as a function of surface geometry (photon mapping) has been studied. By means of STM, it was possible to measure photon emission spectra locally. We have studied the effect of grain size and applied bias voltage on the spectrum. We found that the peak position in the photon emission spectrum shifts to a shorter wavelength when increasing the bias voltage and shifts to a longer wavelength when tunneling to larger grains. These effects can be understood in a simple model which considers tunneling electrons exciting localized surface plasmons which decay by emitting photons.

Influence of Impurities on Localized Transition Metal Surface States

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

Origin of Magnetic Contrast in Spin-Polarized Scanning Tunneling Spectroscopy: Experiments on Ultra-Thin Mn Films

Normalized differential tunneling conductivities obtained with Fe-coated W tips show a spin-polarized peak around +0.8 V on ultrathin bct Mn films grown on Fe(001)-whiskers. This spin-polarized peak results in a clear magnetic contrast in spectroscopic images. Our normalization removes the influence of the tunneling probability and makes the spectroscopic curves most reliable for a derivation of the spin-resolved sample density of states (DOS) at positive voltages. From this analysis we conclude that the magnetic contrast in our spectroscopic maps is caused by a highly polarized DOS. Furthermore, a tip polarization of about 15% is found.

Data Evaluation for Spin‐Polarized Scanning Tunneling Spectroscopy Measurements

Spin‐polarized scanning tunneling spectroscopy measurements have been done on ultrathin bct Mn films grown on Fe(001)‐whiskers. Emphasis is placed on the relationship between the feedback setpoint chosen for the tunneling spectroscopy measurements and the normalization procedures for the data, and the values obtained for the spin polarization of the surface electronic structure.

Influence of a spin-polarized tip on the Fe(001) surface

A detailed study of ab initio calculations for an iron parallel electrode and a tip–surface system, using the localized spherical wave method and the supercell-slab approach is presented. The calculations show that within the medium inter-electrode distance range (about 2.9 to 5.7 A) the interaction between two parallel electrodes has a strong influence on the local surface electronic structure, while the magnetic moment is almost constant. A tip induces changes of the specific surface states near the Fermi level. However, the spin-polarization of the tip shows no significant influence on the surface states.

Study of Spin-Polarized Scanning Tunneling Microscopy / Spectroscopy on Ultrathin Magnetic Films

Spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) opens up new real atomic scale magnetism. Interlayer exchange coupling of ultrathin Fe(001) films grown on the antiferromagnetically stacked bct-Mn(001) layers have been studied. The ordered c(2x2)-MnAu(001) surface alloy films were studied and found a high magnetic moment for Mn spins (>4 Bohr magneton), an antiferromagnetic coupling between layers, and a ferromagnetic coupling in the same layer.

Spin-polarized scanning tunneling spectroscopy study of epitaxial iron(001) layers on bct-manganese(001)

In this work, the correlation between layer spacing and magnetic coupling of successive Fe layers on the bct-Mn by means of spin-polarized scanning microscopy and spectroscopy (SP-STM/STS) is studied. In ultra-high-vacuum, Fe(001) epitaxial layers are grown at room temperature (RT) on bct-Mn(001) and are deposited on a single crystalline Fe(001)-whisker. These layers show a bct structure with the same in-plane lattice constant as the substrate and the bct-Fe. With ferromagnetic Fe-coated W tips, the magnetic structures of the first to the eighth Fe layer at RT are also studied. Since the substrate Mn layers couple antiferromagnetically, the first Fe layers grown on the odd and even Mn layers show magnetic contrast. Surprisingly, the contrast is also observed between the second and first Fe layers. This means that the first Fe layer also couples antiferromagnetically with the second layer. Similar SP-STS measurements for Fe films with other thicknesses are done and results show that the first to fourth Fe layers on bct-Mn have non-collinear helical magnetic coupling. The inter-layer exchange coupling constant J/sub k/ turns out to be strongly correlated to the step height or interlayer distance d, i.e., J/sub k/<0 for d>0.155 nm and J/sub k/>0 for d<0.155 nm. Also, it is shown from SP-STS results that J/sub k2/<0 for 00 for k>5.