K. Mukasa

Atomic resolved imaging of cleaved NiO(100) surfaces by NC-AFM

We have observed the cleaved NiO(100) surface by means of the UHV non-contact atomic force microscopy (NC-AFM). Atomically resolved NC-AFM images of NiO(100)-cleaved surface has successfully obtained with atomically distinguishable defects at room temperature. The distance of the observed periodic protrusions is about 4.2 A and the periodicity corresponds to that of sublattice of NiO single crystal.

Possibility of Observing Spin-Polarized Tunneling Current Using Scanning Tunneling Microscope with Optically Pumped GaAs

A scanning tunneling microscope (STM) experiment demonstrating the spin-polarized tunneling effect was performed. A GaAs thin-film sample pumped by circularly polarized light, and a ferromagnetic polycrystalline Ni tip are used. The tunneling current is perturbed by modulating the power and polarization of the pumping light. The perturbation arises due to three dominant effects: the thermal expansion of the tip or sample, the variation in excited carrier concentration in GaAs and the spin-polarized tunneling effect. The spin-polarized effect can be distinguished by observing the current dependence on the bias. Well-adjusted pumping optics allows the separate detection of the spin-polarized signal, which depends upon the circular polarization of the light and the tip magnetization. The present experiment indicates the feasibility of a spin-polarized STM with a non-magnetic tip.

Exchange Interaction between Magnetic Moments of Ferromagnetic Sample and Tip: Possibility of Atomic-Resolution Images of Exchange Interactions using Exchange Force Microscopy

The exchange interaction and force acting between magnetic moments of a ferromagnetic sample and tip are calculated using the one-dimensional electron-gas model. It is suggested from an estimate of their magnitudes that an atomic-resolution image of exchange interaction of the transition-metal sample and tip can be obtained with the use of atomic force microscopy (AFM).

Anisotropic Factor of Electrical Conductivity in p-Bi2Te3 Crystals

Temperature variation of electrical conductivity in two crystallographic directions (σ∥ and σ⟂) of p-Bi2Te3 crystals has been measured. The anisotropic factor, σ∥/σ⟂, shows an exponential form involving an activation energy. This can be interpreted as an effect of defects between the layers on the electronic density of states.

Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.

Spin-polarized STM and its family

Abstract Since STM was invented in 1982 it has proved to be the most powerful tool for studying local electronic surface structure and has also led to a number of related scanning tunneling microscopy methods to be developed. One of them is spin-polarized STM (SP-STM) which will image the local magnetic characteristics of the material surfaces. In 1992 we succeeded in detecting the tunneling current, which depends on the tip magnetization and the circular polarization of the light-pumping GaAs sample. These preliminary experiments and their results indicate the feasibility of SP-STM with a non-magnetic tip. Furthermore we are investigating exchange force microscopy which will play an important role in the evaluation of insulating surface magnetism. This paper outlines both investigations together with recent progress in these areas. We also report on the experimental results of SP-STM in a vacuum.

SPIN-POLARIZED TUNNELING BY SPIN-POLARIZED SCANNING TUNNELING MICROSCOPY

A spin-polarized scanning tunneling microscope (STM) using a GaAs tip has been studied to observe the magnetization with nanoscale resolution for a sample magnetized in the plane of the film. A STM image of Si(111) 7×7 surface reconstruction using the GaAs tip was observed clearly under ultra-high-vacuum conditions. We found that the cleaved GaAs tip has a capability of high atomic resolution. Next, circularly polarized light irradiated the GaAs tip, and the I–V characteristics were measured for magnetic Ni80Fe20 and nonmagnetic Au samples. Changes in the tunneling current, which is estimated to be approximately 7%, were observed only for the NiFe sample under the circularly polarized light irradiation. It suggests that the change in the tunneling current is due to the spin-polarized tunneling from the GaAs tip to the NiFe.

Scanning Tunneling Spectroscopy of c(2*2) Reconstructed Fe Thin-Film Surfaces.

Fe thin films with flat surfaces are obtained on a MgO(001) substrate at a growth temperature of 550 K. The surfaces with atomically flat and wide terraces exhibit a c(2×2) reconstructed structure. To evaluate the effect of impurity atoms at the surface on the surface structures, scanning tunneling spectroscopy (STS), reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS) studies were performed. The differential conductivity (dI/dV) spectrum of the c(2×2) Fe(001) thin-film surfaces indicates an intense peak at the sample bias voltage of 0.4 V. Since there is no clear evidence of impurity adatoms forming such a surface structure, we expect that the topmost atoms are Fe, and that the observed peak originates from surface states.

Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.

Tunneling Conductance of Al–Al Oxide–p‐Bi2Te3 Junctions

We measured tunneling conductance of Al-Al oxide-p-Bi 2 Te 3 tunnel junctions. A small increase of the tunneling conductance was observed in the energy gap region. The intensity of the increase of tunneling conductance decreased by annealing the p-Bi 2 Te 3 films. Therefore, it is considered that this increase in tunneling conductance is closely related to energy states caused by defects in p-Bi 2 Te 3 films. For the first time, energy states caused by defects are revealed by tunneling experiments in p-Bi 2 Te 3 films.