R. Hoffmann

Conductance of gold nanojunctions thinned by electromigration

We investigate nanocontact formation by thermally assisted electromigration of gold nanowires. An automatic cycling process allows us to follow a line of constant power dissipated at the nanocontact up to resistances corresponding to 10–20 conductance quanta. The contacts are thinned in a controlled way by voltage-induced heating. In the ballistic regime, oscillations of the conductance histograms show oscillations typical for atomic discreteness.

The effective quality factor at low temperatures in dynamic force microscopes with Fabry–Pérot interferometer detection

The oscillation of a cantilever in ultrahigh vacuum dynamic force microscopy is sometimes measured with the help of a Fabry–Perot interferometer. We show that the photoinduced forces present in such an interferometer can artificially increase or decrease the effective quality factor of the cantilever. We examine this effect on the basis of a PtIr-coated rectangular silicon cantilever at temperatures between 7.7 and 30 K. By measuring resonance curves we demonstrate that the magnitude of the effective quality factor changes with cantilever-fiber distance, laser power, and temperature.

Mechanical manifestations of rare atomic jumps in dynamic force microscopy.

The resonance frequency and the excitation amplitude of a silicon cantilever have been measured as a function of distance to a cleaved KBr(001) surface with a low-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify two regimes of tip-sample distances. Above a site-dependent critical tip-sample distance reproducible data with low noise and no interaction-induced energy dissipation are measured. In this regime reproducible SFM images can be recorded. At closer tip-sample distances, above two distinct atomic sites, the frequency values jump between two limiting curves on a timescale of tens of milliseconds. Furthermore, additional energy dissipation occurs wherever jumps are observed. We attribute both phenomena to rarely occurring changes in the tip apex configuration which are affected by short-range interactions with the sample. Their respective magnitudes are related to each other. A specific candidate two-level system is also proposed.

Spin-polarized current versus stray field in a perpendicularly magnetized superconducting spin switch

In superconducting Co∕Pt–Nb–Co∕Pt multilayers with perpendicular magnetic anisotropy, the superconducting transition temperature Tc is considerably higher in the parallel (P) orientation of the two Co∕Pt magnetizations compared to the antiparallel (AP) orientation. Low temperature magnetic-force microscopy reveals that the individual ferromagnetic layers are uniformly magnetized in both configurations and do not exhibit a magnetic stray field arising from a multidomain state. We also demonstrate that the difference ΔTc=TcP−TcAP between both Tc values is enhanced at higher transport currents due to the increasing imbalance of spin-polarized charge carriers in the superconducting Nb layer. Hence, the spin switch can be controlled by the transport current in future superconducting devices.

Site-specific force-vector field studies of KBr(001) by atomic force microscopy.

The spatial orientation and magnitude of forces acting between the tip atoms of an atomic force microscope tip and the surface atoms of an atomically clean surface can be determined by force field measurements. We compare two force-vector fields obtained along and between the ionic lattice sites of a KBr(001) surface with atomistic simulations for two differently configured tips. This careful analysis allows us to identify the K(+)-termination of the tip apex as well as the polarity of the KBr lattice.

STM-induced surface aggregates on metals and oxidized silicon

We have observed an aggregation of carbon or carbon derivatives on platinum and natively oxidized silicon surfaces during STM measurements in ultra-high vacuum on solvent-cleaned samples previously structured by e-beam lithography. We imaged the aggregated layer with scanning tunneling microscopy (STM) as well as scanning electron microscopy (SEM). The amount of the aggregated material increases with the number of STM scans and with the tunneling voltage. Film thicknesses of up to 10 nm with five successive STM measurements of the same area have been obtained.

Temperature-dependent scanning tunneling spectroscopy on the Si(557)-Au surface

Room-temperature and low-temperature (77 K) scanning tunneling spectroscopy (STS) and voltage-dependent scanning tunneling microscopy (STM) data are used to study the local electronic properties of the quasi-one-dimensional Si(557)-Au surface in real space. A gapped local electron density of states near the Gamma-point is observed at different positions of the surface, i.e., at protrusions arising from Si adatoms and step-edge atoms. Within the gap region, two distinct peaks are observed on the chain of localized protrusions attributed to Si adatoms. The energy gap widens on both types of protrusions after cooling from room temperature to T = 77 K. The temperature dependence of the local electronic properties can therefore not be attributed to a Peierls transition occurring for the step edge only. We suggest that more attention should be paid to finite-size effects on the one-dimensional segments.

High resolution quantitative magnetic force microscopy

Summary form only given. The bit size in magnetic recording has reached the resolution limit of presently available magnetic force microscopes. The next generation of magnetic force microscopes are expected to measure the magnetic field quantitatively, obtain a lateral resolution below 10 nm, and achieve a complete separation between topographical and magnetic information.

Applications of Tip Calibration in Magnetic Force Microscopy (MFM)

The investigation of magnetic recording device is an important application of MFM. The MFM has become a standard tool for imaging the recorded domains, imaging the stray field of the write head and characterizing the response of the magnetic readout sensor. However there are two problems of the standard MFM technique that limit its usefulness: the lacking re- producibility of measurements made with different tips, and the difficulty to relate the measured signal to a physical quantity. In a recent paper, we have introduced a calibration method that can solve these problems [1]. Here, calibrated MFM tips are used to reconstruct the sample stray field and magnetization from the MFM measurement. Furthermore, it is shown that the tip stray field can be determined from the calibration, which may be useful for measuring the local response of magnetic read heads.