Hirofumi Oka

Spin-Dependent Quantum Interference Within a Single Magnetic Nanostructure

Wave-Particle Duality The dual-wave nature of particles is nowhere more evident than in a confined space, where standing waves are formed with wavelengths that depend on particle energy. This so-called quantum interference has been observed in nanostructures using surface probes such as scanning tunneling microscopy. Now, Oka et al. (p. 843) use the spin-polarized version of this technique to study spin-dependent quantum interference on a triangular nanoscale cobalt island deposited on a copper surface. They observe the modulation of the magnetization, with the pattern depending on the energy of the interfering electrons. The experimental results are in good agreement with simulations, which indicate that the magnetization at a given energy and position largely depends on which of two electron spin states present dominates. Magnetization modulation is observed on a cobalt nanoisland using spin-polarized scanning tunneling microscopy. Quantum interference is a coherent quantum phenomenon that takes place in confined geometries. Using spin-polarized scanning tunneling microscopy, we found that quantum interference of electrons causes spatial modulation of spin polarization within a single magnetic nanostructure. We observed changes in both the sign and magnitude of the spin polarization on a subnanometer scale. A comparison of our experimental results with ab initio calculations shows that at a given energy, the modulation of the spin polarization can be ascribed to the difference between the spatially modulated local density of states of the majority spin and the nonmodulated minority spin contribution.

Characterization of tips for spin-polarized scanning tunneling microscopy

We propose a conclusive characterization of the magnetic configuration of tips for spin-polarized scanning tunneling microscopy studies. We show that both careful tip preparation and characterization by tunneling spectroscopy need to be augmented by in-field measurements to ensure a reliable analysis of a magnetic contrast in spin-polarized scanning tunneling microscopy studies.

Magnetic Response and Spin Polarization of Bulk Cr Tips for In-Field Spin-Polarized Scanning Tunneling Microscopy

The magnetic properties of bulk Cr tips have been investigated by spin-polarized scanning tunneling spectroscopy (SP-STS). To extract the properties of the Cr tips, we performed low-temperature SP-STS measurements on a well-known model system: nanometric Co islands on Cu(111). Our experiments indicate the existence of uncompensated magnetic moments at the apex of the Cr tips, which rotate in the direction of the applied vertical magnetic field and become aligned with it at approximately 2 T. We extracted a tip spin polarization of 45% at the Fermi energy. We showed that the tip spin polarization can change with a modification of the tip apex.

Atomic structure and spin polarization at the apex of tips used in spin-polarized scanning tunneling microscopy

To analyze spin-polarized scanning tunneling microscopy (STM) studies quantitatively, we evaluated the atomic structure and spin polarization at the apex of Cr/W and Fe/W tips using field ion microscopy (FIM) and field-emitted electron polarimetry, respectively. The patchwork-patterned H2-FIM images of the Cr/W tip indicated partially developed Cr planes, and the spin polarization at the surface was 10 ? 3% at room temperature. H2-FIM images of the Fe/W tip indicate the crystalline order of Fe layers on the W tip, and its spin polarization was 41 ? 2%. These first results allow us to quantify the spin polarization in spin-dependent STM measurements.

The impact of structural relaxation on spin polarization and magnetization reversal of individual nano structures studied by spin-polarized scanning tunneling microscopy

The application of low temperature spin-polarized scanning tunneling microscopy and spectroscopy in magnetic fields for the quantitative characterization of spin polarization, magnetization reversal and magnetic anisotropy of individual nano structures is reviewed. We find that structural relaxation, spin polarization and magnetic anisotropy vary on the nm scale near the border of a bilayer Co island on Cu(1 1 1). This relaxation is lifted by perimetric decoration with Fe. We discuss the role of spatial variations of the spin-dependent electronic properties within and at the edge of a single nano structure for its magnetic properties.

Temperature dependence of the superconducting proximity effect quantified by scanning tunneling spectroscopy

Here, we present the first systematic study on the temperature dependence of the extension of the superconducting proximity effect in a 1–2 atomic layer thin metallic film, surrounding a superconducting Pb island. Scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal the spatial variation of the local density of state on the film from 0.38 up to 1.8 K. In this temperature range the superconductivity of the island is almost unaffected and shows a constant gap of a 1.20 ± 0.03 meV. Using a superconducting Nb-tip a constant value of the proximity length of 17 ± 3 nm at 0.38 and 1.8 K is found. In contrast, experiments with a normal conductive W-tip indicate an apparent decrease of the proximity length with increasing temperature. This result is ascribed to the thermal broadening of the occupation of states of the tip, and it does not reflect an intrinsic temperature dependence of the proximity length. Our tunneling spectroscopy experiments shed fresh light on the fundamental issue of the temperature dependence of the proximity effect for atomic monolayers, where the intrinsic temperature dependence of the proximity effect is comparably weak.

New insights into nanomagnetism: spin-polarized scanning tunneling microscopy and spectroscopy studies

We perform low-temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy measurements in magnetic fields to gain new insights into nanomagnetism. We use the magnetic field to change and control magnetizations of a sample and a magnetic tip, and measure the magnetic hysteresis loops of individual Co nano-islands on Cu(111). We also exploit the high spatial resolution of SP-STM in magnetic fields to measure maps of the differential conductance within a single Co nano-island. In connection with ab initio calculations, we find that the spin polarization is not homogeneous but spatially modulated within the nano-island. We ascribe the spatial variation of the spin polarization to spin-dependent electron confinement within the Co nano-island.

Switching Fields of Individual Co Nanoislands

We explore the magnetization reversal process of individual Co nanoislands grown on Cu(111) by low temperature spin-polarized scanning tunneling microscopy (spin-STM) and spectroscopy (spin-STS). We measure hysteresis loops of the differential conductance of single Co islands in magnetic fields of up to 4 T. From such hysteresis loops we extract the magnetic switching field of single Co islands as a function of island size. Tentatively we analyze the size dependence of the switching field using the venerable model of thermally assisted coherent magnetization reversal. We present evidence for the failure of that model to explain our experimental results. We propose that the magnetization reversal process within individual Co nanoislands on Cu(111) is a non-coherent process.

Carbon-Induced Superstructure on Cr(001) Thin-Film Surfaces

We have successfully fabricated an interesting carbon-induced superstructure on Cr(001) thin film surfaces deposited on MgO(001) substrates with a c(2×2)-C/Fe(001) seed layer. Reflection high energy electron diffraction shows that atomic arrangements of the C atoms on the Cr(001) thin film surfaces strongly depend on the film thickness of the Cr(001) thin films. The C atoms form reconstructions with three- and two-fold periodicities within the Cr directions in the thickness range between 0.6 and 2.4 nm, and above 2.4 nm, respectively. Scanning tunneling microscopy and low energy electron diffraction reveal that a surface atomic arrangement of 2-nm-thick Cr(001) thin film surfaces is a c(3√2×√2)R45°-C reconstructed structure.