Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO₂ tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO₂ tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.     

Magnon excitation with spin-polarized scanning tunneling microscopy

Using spin-polarized scanning tunneling microscopy, the local excitation of magnons in Fe and Co has been studied. A large cross section for magnon excitation was found for bulk Fe samples while for thin Co films on Cu(111) the cross section linearly scales with film thickness. Recording inelastic tunneling spectra with Fe coated W tips in a magnetic field, the magnonic nature of the excitation was proven. Magnon excitation could be detected without the use of a separating insulating layer opening up the possibility to directly study magnons in magnetic nanostructures via spin-polarized currents.     

Photon emission with the scanning tunneling microscope

By placing a photon detector near the tip-sample region of a scanning tunneling microscope, we have measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV. Such spectra contain electronic-structure information comparable to inverse photoemission spectroscopy, but with high lateral/spatial resolution. The implications of this new observation are discussed.     

自旋极化扫描隧道显微镜的研究进展

自旋极化扫描隧道显微镜（spin-polarized scanning tunneling microscope,SP-STM）将扫描隧道显微镜（scanning tunneling microscope,STM）的实空间分辨率和对自旋敏感的磁成像技术结合起来,已经成为人们研究纳米磁性物理的最有效工具之一.文章介绍了SP-STM的工作原理及其在低维磁性物理领域的应用和最新进展,如对磁性薄膜、磁性纳米岛、磁性原子及不共线结构的研究等,并对我国在这一领域的研究现状和发展前景进行简要评述.     

自旋极化扫描隧道显微镜的研究进展

自旋极化扫描隧道显微镜(spin-polarized scanning tunneling microscope,SP-STM)将扫描隧道显微镜(scanning tunneling microscope,STM)的实空间分辨率和对自旋敏感的磁成像技术结合起来,已经成为人们研究纳米磁性物理的最有效工具之一.文章介绍了SP-STM的工作原理及其在低维磁性物理领域的应用和最新进展,如对磁性薄膜、磁性纳米岛、磁性原子及不共线结构的研究等,并对我国在这一领域的研究现状和发展前景进行简要评述.     

Recent advances in spin-polarized scanning tunneling microscopy

Considerable progress in the field of spin-polarized scanning tunneling microscopy (SP-STM) has been achieved recently by gaining a high degree of control with regard to properties of the tunneling tip. It is found that by choosing the appropriate material for the magnetic thin film coating of the tip sensitivity to either the samples in-plane or perpendicular magnetization component can be achieved. Using SP-STM in external magnetic fields, domains and domain walls of two atomic layers thick Fe on a W(110) substrate are studied. A residual domain of enhanced stability against remagnetization is observed. Furthermore, a new imaging mechanism is identified which allows the use of even non-magnetic tungsten tips to observe contrast between magnetic domains and domain walls. The effect exploited is a modification of the electronic band structure which is induced by spin–orbit coupling. PACS 75.50.Ak; 75.75.+a; 68.37.Ef     

Fluctuation dominated Josephson tunneling with a scanning tunneling microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k Omega. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltage. In the presence of microwave fields ( f = 15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.     

Joule heating and spin-transfer torque investigated on the atomic scale using a spin-polarized scanning tunneling microscope

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.     

Observation of spin-polarized surface states on ultrathin bct Mn(001) films by spin-polarized scanning tunneling spectroscopy

We report the observation of a magnetic contrast of up to 20% in the scanning tunneling spectroscopy dI/dV maps obtained with Fe-coated tips on Mn(001) layers grown on an Fe(001) whisker at 370 K. These nanometer resolution microscopy results show that the layers couple antiferromagnetically. By normalizing the dI/dV curves by tunneling probability functions, we found a spin-dependent peak on the body-centered-tetragonal (bct) Mn(001) surface at +0.8 V, whose high spin polarization gives rise to the dI/dV map contrast. Band structure calculations allow one to identify the +0.8 V peak as due to two spin-polarized d(z(2)) surface states.     

Complex magnetism in ultra-thin films: atomic-scale spin structures and resolution by the spin-polarized scanning tunneling microscope

In this paper we present a density functional theory investigation of complex magnetic structures in ultra-thin films. The focus is on magnetically frustrated antiferromagnetic Cr and Mn monolayers deposited on a triangular lattice provided by a Ag (111) substrate. This involves non-collinear magnetic structures, which we treat by first-principles calculations on the basis of the vector spin-density formulation of the density functional theory. We find for Cr/Ag (111) a coplanar non-collinear periodic 120° Néel structure, for Mn/Ag (111) a row-wise antiferromagnetic structure, and for Fe/Ag (111) a ferromagnetic structure as magnetic ground states. The spin-polarized scanning tunneling microscope (SP–STM) operated in the constant-current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. We discuss a recent application of this operation mode of the SP–STM on Mn/W (110), which led to the first observation of a two-dimensional antiferromagnet on a non-magnetic metal. The future potential of this approach is demonstrated by calculating SP–STM images for different magnetic structures of Cr/Ag (111). The results show that the predicted non-collinear magnetic ground state structure can clearly be discriminated from competing magnetic structures. A general discussion of the application of different operation modes of the SP–STM is presented on the basis of the model of Tersoff and Hamann. Received: 07 May 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Resolving noncollinear magnetism by spin-polarized scanning tunneling microscopy

We present a density functional theory (DFT) investigation of magnetically frustrated Mn monolayers deposited on the triangular lattice of the Cu(1 1 1) surface. Noncollinear magnetic structures are treated on the basis of the vector spin-density formulation of the DFT. The spin-polarized scanning tunneling microscope operated in the constant-current mode is proposed as a powerful tool to investigate these complex magnetic structures.     

电子自旋共振扫描隧道显微镜

单电子自旋极有可能发展成为未来信息学的基础。以电子自旋为核心的新型单分子或单原子器件将最终成为基本信息单元,基于单电子的自旋态将有可能构筑未来量子计算机的量子比特。但是,如何实现对单个电子自旋及其相干态和纠缠态的测量和控制,目前仍然是一个很大的挑战。作为调控单个电子自旋的重要实验手段,电子自旋共振扫描隧道显微镜的发展一直备受关注。文章简要介绍了电子自旋共振扫描隧道显微镜的基本概念,阐述了其发展历史和最新进展,归纳了机理探索的研究成果,论述了该设备研发面临的挑战与对策,并对未来的发展和应用做了展望。     

Photovoltaic characterization of WSe2 with the scanning tunneling microscope

In the present study we use the Scanning Tunneling Microscope (STM) as an instrument to investigate the photovoltaic properties of semiconducting materials. The surfaces of the layered semiconductor WSe₂ were optically illuminated during the tunneling process. The resulting photo-induced tunneling current (PITC) was measured as a function of the wavelength. Microscopic information on the energy dependent generation and recombination of the photo-electrons in the vicinity of the tunneling tip was obtained by this method without the necessity of covering the surface with a conducting electrode. The analysis of the wavelength dependence of the PITC points at the existence of excitonic excitations. Compared to the spectral response of conventional photosensitive heterodiodes our PITC spectra exhibit a more structured response and an increase of the signal in the UV region. Finally, PITC studies on differently treated WSe₂ surfaces show an improved photoactivity for WSe₂ crystals treated with NaI/I₂ solutions.     

Observation of subcellular nanostructure of single neurons with an illumination mode photon scanning tunneling microscope

We report about the observation of microtubules lying underneath the cell membrane of neural process in neurons with a resolution as high as that of an electron microscope by an illumination mode photon scanning tunneling microscope. Nanoapertures used in our observations were fabricated by means of selective chemical etching and metal coating of an optical fiber. The narrowest observed tube has got an average diameter of 26 nm. Comparing this with its nominal value of 25 nm, the difference which is considered as a measure of resolution (δ) is 1 nm implying a resolution comparable to that of an electron microscope in imaging dielectric specimens. This was possible due to the presence of a boundary between the glass and the metal coating and also due to the use of an aperture of almost the same size as that of the microtubule that enhances the detection.     

Determining the spin polarization of surfaces by spin-polarized scanning tunneling spectroscopy

During the last few years, spin-polarized scanning tunneling microscopy and spectroscopy has been developed as a reliable tool to image surface magnetic domain structures of bulk materials as well as thin films and nanostructured systems. In principle, this technique also allows for the determination of the energy-resolved spin polarization of the sample P_S(E) with nanometer resolution, information which might play a crucial role in understanding systems like, for example, non-magnetic adatoms on magnetic surfaces. A main problem in quantifying P_S(E), however, arises from the fact that, in contrast to planar junctions, the tip–sample distance generally varies with the magnitude and direction of the surface magnetization, since the distance is controlled indirectly by the tunneling current that is itself spin-polarized. We employ a simple model of the tunneling process to investigate this issue and show that a normalization of the dI/dU spectra with the total conductance I/U is insufficient to correct for their distance dependence. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-5311, E-mail: kubetzka@physnet.uni-hamburg.de