G. Tarrach

Topographic and Magnetic-Sensitive Scanning Tunneling Microscope Study of Magnetite

The topographic and magnetic surface structure of a natural single crystal of magnetite (Fe304), a common mineral, has been studied from the submicrometer scale down to the atomic scale with a scanning tunneling microscope having nonmagnetic tungsten as well as ferromagnetic iron probe tips. Several different (001) crystal planes were imaged to atomic resolution with both kinds of tips. A selective imaging of the octahedrally coordinated Fe B-sites in the Fe-O planes, and even a selective imaging of the different magnetic ions Fe2+ and Fe3+, has been achieved, demonstrating for the first time that magnetic imaging can be realized at the atomic level.

Progress towards spin‐polarized scanning tunneling microscopy

Solutions to the main problems in operating a spin‐polarized scanning tunneling microscope are discussed. Preliminary experimental results obtained in the course of implementing these solutions are reported. Atomic resolution on Si(111) and Si(100) is achieved with a scanning tunneling microscope (STM) using chromium and iron tips. Fabrication of antiferromagnetic tips of Cr, MnNi, and MnPt is described. A technique of preparation of clean (100) surfaces of Fe3O4 (magnetite) is given. Low‐energy electron diffraction patterns were obtained on Fe3O4 for the first time. The first STM experimental results obtained on magnetite in air and in ultrahigh vacuum are reported. Atomic resolution is obtained on Fe3O4 (100) with an ultrahigh vacuum scanning tunneling microscope using iron and tungsten tips. This is the first successful observation of atomic resolution on a ferromagnetic sample using a ferromagnetic tip.

Local structure of the Si(100) surface studied by scanning tunneling microscopy

Abstract The local structure of the Si(100)2 × 1 surface has been studied on extended 2 × 1 reconstructed terraces without vacancies as well as on small rotational domains separated by monoatomic steps. On defect-free terraces, only symmetric dimers were found whereas the presence of monoatomic steps seems to stabilize buckled dimers or zig-zag structures either by the electric field distribution or by strain fields associated with these steps. The smallest 2 × 1 reconstructed domains observed have a side length of less than 60 A, indicating a very short range driving force for the Si(100)2 × 1 reconstruction.

Magnetic imaging at the atomic level

Magnetic contrast at the atomic level has been observed for the first time in scanning tunneling microscopy experiments on a magnetite (Fe3O4(001)) surface using in-situ prepared ferromagnetic Fe tips. A periodic corrugation with a 12 Å periodicity is clearly observed along the rows of FeB-sites which corresponds to the repeat period of Fe2+ and Fe3+ along these rows. This periodicity is not observed by using non-magneticW tips although the rows of FeB-sites can be resolved as well. The magnetic contrast observed with Fe tips is attributed to the different spin configurations of the magnetic ions Fe2+ and Fe3+ in Fe3O4.

Evidence for Selective Imaging of Different Magnetic Ions on the Atomic Scale by Using a Scanning Tunnelling Microscope with a Ferromagnetic Probe Tip

Atomic resolution has been obtained on a magnetite (Fe3O4(001)) surface using a scanning tunnelling microscope with nonmagnetic tungsten and ferromagnetic iron tips. Selective imaging of Fe2 + and Fe3 + on B-sites is achieved by using the ferromagnetic Fe probe tip, reflecting the different spin configurations of the two magnetic ions. Local order in the distribution of Fe2 + and Fe3 + on the (001) surface is found at room temperature, well above the bulk order-disorder (Verwey) transition.

Recent advances in scanning tunneling microscopy involving magnetic probes and samples

We report on recent developments in the field of STM performed with magnetic probes and samples. The choice of appropriate magnetic sensors and their in situ preparation will be described. We further focus on the information obtained in spin-polarized scanning tunneling microscopy (SPSTM) as well as on possible modes of operation for simultaneous acquisition of topographic and magnetic data. The prospects for SPSTM and related magnetic sensitive SXM techniques will be discussed.

Scanning Tunnelling Microscopy Study of Si(111) 7×7 in the Presence of Multiple-Step Edges

We have used scanning tunnelling microscopy to investigate the Si(111) 7×7 reconstruction on narrow terraces separated by multiple-step edges as high as several nanometers. Atomic resolution with a corrugation of up to 2.5 A could be achieved on both sides of these multiple step edges indicating a remarkable stability of the scanning tip. Besides the alignment of the step direction, we have studied defects in the vicinity of the step edges. Such defects occur preferentially on the lower terrace adjacent to a step edge, which can be explained by the growth behaviour of the 7×7 phase upon cooling.

Scanning tunneling microscopy study of the degree of dimer asymmetry on the Si(001)-(2 × 1) surface

Abstract The degree of dimer asymmetry on the Si(001)-(2 × 1) surface as seen in high resolution scanning tunneling microscopy (STM) images has been studied in detail. It is found that the degree of dimer asymmetry can change drastically from one terrace to another. The degree of dimer asymmetry has also been studied as a function of tip material using non-magnetic W and ferromagnetic Fe tips.

Recent advances in spin-polarized scanning tunneling microscopy

Abstract We present recent advances in the field of spin-polarized scanning tunneling microscopy (SPSTM) since our first report on the successful observation of vacuum tunneling of spin-polarized electrons between a half-metallic ferromagnetic CrO2 sensor tip and an antiferromagnetic Cr(001) sample. We mainly focus on the in-situ preparation and testing of a variety of ferromagnetic and antiferromagnetic tips as well as on novel promising test samples such as Fe3O4(001) and Si(001). Atomic resolution in STM images obtained with a ferromagnetic tip and a ferromagnetic sample has been observed for the first time.

Scanning tunneling microscopy on laser- and thermal-annealed Si(111): transitions from 7 × 7 reconstructed to disordered surface structures

Abstract Transitions from ordered to disordered surface structures have been examined by scanning tunneling microscopy (STM) on laser- and thermal-annealed Si(111). STM is shown to be a unique technique to study surface disorder on the atomic scale. Statistical information about the observed surface structures is obtained by using the autocorrelation function.