Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal spin structure of magnetic vortex cores. Using tips coated with a layer of antiferromagnetic chromium, we obtained images of the curling in-plane magnetization around and of the out-of-plane magnetization inside the core region. The experimental data are compared with micromagnetic simulations. The results confirm theoretical predictions that the size and the shape of the vortex core as well as its magnetic field dependence are governed by only two material parameters, the exchange stiffness and the saturation magnetization that determines the stray field energy.

This paper investigates the origin of the threshold switching effect in niobium oxide based filamentary switching cells.

We have observed variable negative differential resistance (NDR) in scanning tunneling spectroscopy measurements of a double layer of C 60 molecules on a metallic surface.Minimum to maximum current ratios in the NDR region are tuned by changing the tunneling barrier width. The multi-layer geometry is critical, as NDR is not observed when tunneling into a C 60 monolayer. Using a simple model we show that the observed NDR behavior is explained by voltage-dependent changes in the tunneling barrier height. 2Negative differential resistance (NDR) is a crucial property of several important electronic components [1,2]. Originally observed in highly doped tunneling diodes [3], NDR has been seen in a variety of systems and caused by several different mechanisms [4,5,6,7,8]. Here we present a scanning tunneling spectroscopy (STS) study showing the appearance of NDR in the tunneling signature of thin molecular C 60 films deposited on Au(111). NDR is completely absent for tunneling into a single C 60 monolayer, but emerges when tunneling into second and higher layers of C 60 . In previous STS studies of molecular systems NDR has been commonly attributed to the convolution of energetically localized tip states with the molecular density of states [7]. The NDR observed in our study is inconsistent with this interpretation, but instead stems from the voltage dependence of the tunneling barrier height [4]. We further find that the relative decrease in current, induced by the NDR, increases with increasing tunneling barrier width, allowing for tunability of the NDR behavior. This behavior is explained by using a simple tunneling model. Our experiments were conducted using a homebuilt ultrahigh vacuum (UHV) STM with a PtIr tip. The single-crystal Au(111) substrate was cleaned in UHV and dosed with C 60 using a calibrated Knudsen cell evaporator before being cooled to 7K in the STM stage. dI/dV spectra and images were measured through lock-in detection of the ac tunneling current driven by a 451Hz, 10mV (rms) signal added to the junction bias under open-loop conditions (bias voltage here is defined as the sample potential referenced to the tip). All data were acquired at 7K.Figure 1(a) shows the topographic structure of a single layer of C 60 (monolayer), a second layer of C 60 (bilayer), and a third layer of C 60 (trilayer). Each layer is well ordered 3 and has a topographic structure consistent with previous measurements performed on similar monolayer and layered C 60 systems [9,10]. The step height of each C 60 layer is ~8.0Å.Step edges in the underlying Au(111) substrate lead to 2Å steps that run through the C 60 layers. dI/dV spectra performed on the C 60 monolayer and bilayer are shown in Fig. 1 (b).These spectra exhibit several common features: a shoulder in the filled density of states (V<0) and two peaks in the empty density of states (V>0) that arise from tunneling into the C 60 highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital LUMO, and LUMO+1, respectively [11]. In the monolayer (bilayer) s...

We present a low-temperature scanning tunneling microscopy (STM) study of K(x)C60 monolayers on Au(111) for 3 < or = x < or = 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C60 molecules. The Jahn-Teller effect, a charge-induced mechanical deformation of molecular structure, is directly visualized in the K4C60 monolayer at the single-molecule level. These results, along with theoretical analyses, provide strong evidence that the transition from metal to insulator in K(x)C60 monolayers is caused by the Jahn-Teller effect.

We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the He3 pot of an entirely UHV compatible He3 refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the energy resolution is about 100 μeV.

The ability to tune competing interactions in the fullerides arises from advances in our ability to grow well-controlled heterogeneous molecular films. Here we describe measurements on potassium doped C 60 (K x C 60 ) ultra-thin films having variable thickness from one to three layers (layer index i = 1, 2, and 3) for three specific doping concentrations (x = 3, 4, and 5). Fig. 1a displays a scanning tunneling microscope (STM) topograph of a representative K x C 60 multilayer on Au(111), where the color scale highlights the plateau structure. Narrow slivers of C 60 -free voids containing only K atoms 3 (brown) exist between continuous patches of K x C 60 . Islands of second (blue) and third layer (red) K x C 60 can be seen residing on top of the first K x C 60 layer (green). The average layer thickness is ~9.9 Å, greater than the 8 Å spacing found in undoped C 60 films 11 .We begin by describing our results for a multilayer of the x = 3 metallic system.Layer-dependent electronic structure in K 3 C 60 can be seen in Fig. 2a, which shows spatially-averaged dI/dV spectra measured at three different layer levels. Within each layer the spectrum is highly uniform with no sign of spatial inhomogeneity such as that found in the surface of bulk fullerides 12 . The first layer dI/dV displays a wide peak at the Fermi energy (E F ), reflecting the large electronic density of states (DOS) of a metallic LUMO-derived band (LUMO = Lowest Unoccupied Molecular Level). In contrast, the second layer spectrum shows a sharp dip at E F , indicating the emergence of an energy gap that tends to split the band into two halves. A similar gap-like feature persists in the third layer. The width of the gap-like feature (measured between adjacent local maxima) is ~ 0.2 eV, a much larger value than the superconducting gap 2∆ sc ~ 6 meV found in bulk K 3 C 60 13.The spatial arrangement of C 60 molecules also changes dramatically with layer index. The first layer of K 3 C 60 (Fig. 2c) exhibits a complex 3 3 × superstructure of bright molecules having different orientation from their dimmed nearest neighbors 8 . In the second layer ( Fig. 2d), however, C 60 molecules form a very simple hexagonal lattice (lattice constant a ~10.5 Å) with long-range orientational ordering. The tri-star-like topography of each molecule suggests that C 60 in the second layer is oriented with a hexagon pointing up 14 . The third layer topograph is the same as the second layer. 4The insulating x = 4 multilayer system displays a similar trend. Fig. 3a shows dI/dV spectra measured on a K 4 C 60 plateau structure where the number of layers is varied from i = 1 to 3. First layer spectra (i = 1) exhibit an insulating energy gap ∆ ~ 0.2 eV that is induced by molecular Jahn-Teller (JT) distortion 8 . As the layer index increases from i = 1 to 3, the energy gap opens continuously (by layer 3 the gap has well-defined edges and a flat bottom). The gap amplitudes observed here are estimated to be ∆ ~ 0.6 eV and 0.8 eV for layer 2 and 3 respectively. As seen in the metallic x = 3 ...

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K atom donates a constant amount of charge (approximately 0.6 electron charge) to the C60 host, thereby enabling its molecular electronic structure to be precisely and reversibly tuned.

We have studied the thickness dependent domain configuration of single-crystal nanoscale Fe islands on W(110) by spin-polarized scanning tunneling microscopy. The experimental results are compared with micromagnetic calculations. For very thin islands, the uniaxial surface anisotropy of Fe/W(110) leads to a single domain state. With increasing island thickness, the magnetostatic energy becomes increasingly important resulting in different flux closure configurations.