We simulate the electronic system of ejected electrons arising when a tip, positioned few 10 nm away from a surface, is operated in the field emission regime. We find that, by repeated quantum reflections (“quantum skipping”), electrons produced at the nanoscale primary site are able to reach the macroscopic environment surrounding the tip-surface region. We observe the hallmark of quantum skipping in an energy filtered experiment that detects the spin of the ejected electrons.

In scanning field emission microscopy (SFEM), a tip (the source) is approached to few (or a few tens of) nanometres distance from a surface (the collector) and biased to field-emit electrons. In a previous study (Zanin et al. 2016 Proc. R. Soc. A 472, 20160475. (doi:10.1098/rspa.2016.0475)), the field-emitted current was found to change by approximately 1% at a monatomic surface step (approx. 200 pm thick). Here we prepare surface domains of adjacent different materials that, in some instances, have a topographic contrast smaller than 15 pm. Nevertheless, we observe a contrast in the field-emitted current as high as 10%. This non-topographic collector material dependence is a yet unexplored degree of freedom calling for a new understanding of the quantum mechanical tunnelling barrier at the source site that takes into account the properties of the material at the collector site.

Scanning Field Emission Microscopy with Polarization Analysis was recently introduced to detect the spin polarization of electrons excited in the field emission regime of Scanning Tunnelling Microscopy. In this work, a miniature electron energy analyzer, called Bessel Box, is implemented into the Scanning Field Emission Microscope with Polarization Analysis setup. It is used to filter electrons according to their energy before they reach the spin detector. The Bessel Box allows, e.g., the spin polarization of elastically scattered electrons to be compared with the spin polarization obtained with the full energy spectrum. We use this technology to measure the local in-plane polarization signal as a function of the magnetic field B at room temperature for 10 monolayers Fe deposited on top of a W(011)-single crystal surface through a half mask (half of the surface is covered with Fe, the other half is uncovered). The spin polarization at the Fe-W crossing drops sharply from 9 % above Fe to 0 % above W(011) only if the elastically scattered electrons are selected for spin analysis. The mechanism of signal generation in Scanning Field Emission Microscope with Polarization Analysis including the formation of cascade of inelastically scattered electrons is discussed as an explanation for the different spin polarization profiles observed with and without Bessel Box (energy filtered).

In this study, we use Scanning Field Emission Microscopy (SFEM) combined with a miniature electron energy analyzer known as a Bessel box to measure electron energy spectra emitted from a sample. Previous studies using SFEM have revealed that the work function (ϕ) of the material under study has a significant role to play in the formation of the signal intensity. Hence, in order to understand the role of ϕ in greater detail, a sample of W(110) (ϕ = 5.25 eV) and a sample of Cs deposited on W(110) (ϕ ≈ 1.7 eV) were investigated. STM images show that the Cs covered surface has a speckled appearance indicating small Cs islands. The electron energy loss spectra obtained (which are the first using the Bessel box in SFEM) show differing structure in the elastic peak region. Monte Carlo (MC) simulations including quantum mechanical "bouncing" have been carried out. The results are consistent with MC simulations of the electrons escaping from the tip-sample junction.

Scanning Tunneling Microscopy is performed in the conventional (tunneling) and in the field-emission regime. Images of W(110)-surfaces with and without some carbon content are taken in the constant current mode, in which the tip-target vertical distance displaces to compensate for the changes of the tunneling, respectively, field emission current. In the field emission regime, we observe tip-target displacements that are not related to the topographic contrast.

In the Fowler-Nordheim regime of Scanning Tunneling Microscopy (STM) the tip-target distance is few nanometers to few tens of nanometers. In this situation the tunneling between tip and target is completely suppressed. Instead, electrons can be field-emitted from the tip and their impact with the target might excite electrons off the surface. Under certain circumstances, the excited electrons escape the tip-target junction and build a new electronic system, absent in the tunneling regime of STM. A recent experiment discovered that this electronic system is spin polarized at nanoscale tip-target distances. Here we provide a comprehensive review of all strategies that have been adopted to perform the spin polarized experiments in the Fowler-Nordheim regime of STM. We also present new data that complement the proof of spin polarization and specifically underline the magnetic imaging potential of this new technology, which we call Scanning Field Emission Microscopy with Polarization Analysis (SFEMPA).

When a scanning tunneling microscope is operated at tip-target distances ranging from few nanometers to few tens of nanometers (Fowler-Nordheim or field emission regime), a new electronic system appears, consisting of electrons that escape the tip-target junction. If the target is ferromagnetic, this electronic system is spin polarized. Here, we use these spin polarized electrons to image magnetic domains in thin films. As two components of the spin polarization vector are detected simultaneously, the imaging of the local magnetization has vectorial character. The tip is nonmagnetic, i.e., the magnetic state of the target is not perturbed by the act of probing. We expect this spin polarized technology, which scales down scanning electron microscopy with polarization analysis by bringing the source of primary electrons in close proximity to the target, to find its main applications in the imaging of noncollinear, weakly stable spin excitations.

the design of the low-temperature scanning probe microscope, which works in field emission regime with spin polarization analysis, is proposed. A performance at temperature of 77 K has been achieved. The first result of STM imaging with atomic resolution is demonstrated.

With the aim of improving detection and analysis of energy filtered electrons in the Scanning Field-Emission Microscope (SFEM) and of the spin polarised electrons in the SFEM with Polarisation Analysis (SFEMPA) tests are performed on a miniature electron detection unit employing a Bessel Box energy analyser. Even in conventional electron microscopes, the detection of low-energy electrons (with kinetic energies of the order of 100eV or lower) is inherently difficult due to the presence of electrostatic (and magnetic) fields in proximity of the beam-target interaction region, inhibiting the escape of these electrons and complicating the interpretation of their detected signal. The reduced dimensions of such a compact energy analyser - with a length of 1&1/2 channeltrons - consent its employment close to the sample surface, thus minimising the aforementioned fields effects. Experimental results demonstrating the capability of this analyser to collect electron spectra are discussed.

Secondary electrons emitted from a scanning field-emission microscope are spin analyzed with a Mott detector. Spin polarization up to 15% is observed with a lateral resolution of less than 5 nm, with a potential resolution of even less than 1 nm. In this paper the proof of principle is conducted by comparing this method with a well-established method of spin mapping and with reference samples examined by these two microscopes.

Nature Communications 7: Article number: 13611 (2016); Published 5 December 2016; Updated 17 January 2017 The original version of this Article contained a typographical error in the spelling of the author S.A. Cannas, which was incorrectly given as S. Cannas. This has now been corrected in both the PDF and HTML versions of the Article.

The current flowing through a Mott spin junction depends on the relative spin orientation of the two ferromagnetic layers comprising the "source" and "drain" sides of the junction. The resulting current asymmetry is detected as giant or tunnelling magnetoresistance depending on whether the two ferromagnets are separated by a metal or an insulator. Based on the fundamental principles of reciprocity for spin-dependent electron scattering, one can envisage a one-magnet-only spin junction in which the source is non-magnetic, and the spin information is encoded by the spin polarisation of the electrons that have crossed or are backscattered from the drain magnetic layer. The practical significance of using an unpolarised source is that the state of the magnetic layer can be modified without affecting the process of probing it. Whether this reciprocity is realised in the actual junctions is not yet known. Here, we demonstrate a nano-sized, one-magnet-only Mott spin junction by measuring the finite spin polarisation of the backscattered electrons. Based on this finding, we conclude that since the junction acts as a spin filter, the magnetic layer must experience a spin transfer that could become detectable in view of the high current densities achievable in this technology.

Nature Communications 7: Article number: 13611 (2016); Published 5 December 2016; Updated 17 January 2017 The original version of this Article contained a typographical error in the spelling of the author S.A. Cannas, which was incorrectly given as S. Cannas. This has now been corrected in both the PDF and HTML versions of the Article.

Nature Communications 7: Article number: 13611 (2016); Published 5 December 2016; Updated 17 January 2017 The original version of this Article contained a typographical error in the spelling of the author S.A. Cannas, which was incorrectly given as S. Cannas. This has now been corrected in both the PDF and HTML versions of the Article.

We perform scanning tunnelling microscopy (STM) in a regime where primary electrons are field-emitted from the tip and excite secondary electrons out of the target-the scanning field-emission microscopy regime (SFM). In the SFM mode, a secondary-electron contrast as high as 30% is observed when imaging a monoatomic step between a clean W(110)-and an Fe-covered W(110)-terrace. This is a figure of contrast comparable to STM. The apparent width of the monoatomic step attains the 1 nm mark, i.e. it is only marginally worse than the corresponding width observed in STM. The origin of the unexpected strong contrast in SFM is the material dependence of the secondary-electron yield and not the dependence of the transported current on the tip-target distance, typical of STM: accordingly, we expect that a technology combining STM and SFM will highlight complementary aspects of a surface while simultaneously making electrons, selected with nanometre spatial precision, available to a macroscopic environment for further processing.

The paramagnetic-to-ferromagnetic phase transition is classified as a critical phenomenon due to the power-law behaviour shown by thermodynamic observables when the Curie point is approached. Here we report the observation of such a behaviour over extraordinarily many decades of suitable scaling variables in ultrathin Fe films, for certain ranges of temperature T and applied field B. This despite the fact that the underlying critical point is practically unreachable because protected by a phase with a modulated domain structure, induced by the dipole-dipole interaction. The modulated structure has a well-defined spatial period and is realized in a portion of the (T, B) plane that extends above the putative critical temperature, where thermodynamic quantities do not display any singularity. Our results imply that scaling behaviour of macroscopic observables is compatible with an avoided critical point.

The recently developed technique Scanning Tunneling Microscopy in the Field Emission regime (STM FE) is based on the Russell Young's topografiner technology [1]. The set-up is a no contacting device consisting of a sharp tip approached vertically to a conducting surface at variable distances and biased with a small voltage with respect to the surface (Figure 1). The system builds a junction across which electrons can be transferred from the tip apex to the surface by direct quantum mechanical tunneling if the distance is in the sub nanometer region. In this case the surface topography can be imaged with atomic spatial resolution using the STM technique. In the STM FE mode, the distance between tip and collector is increased and the current is dominated by electrons emitted from the tip into the vacuum region via electric field assisted tunneling. In the experiment, secondary electrons carrying chemical and magnetic information are produced via diverse mechanisms on the surface of the sample and form a cloud in the vicinity around the interaction region in a macroscopic environment. Some of them are attracted by an electric potential to the entrance of a lens system designed for focusing the electrons into a counter and a Mott detector. The imaging of the sample with a resolution in the nanoscale depends on the amount of secondary electrons that effectively reach the detectors. We use COMSOL Multiphysics® to optimize the electrostatic and geometric parameters by calculating the trajectories of the secondary electrons from the sample to the final position on the detectors. The set of results achieved on the physical characterization of the instrument refers to a situation where a nanoscale quantum process, comprising field emission, secondary electron production and electron transport in the presence of strongest electric fields is shown to couple efficiently to a macroscopic environment. An accurate modeling of this multiscale process requires the solution of the Laplace and movement equations in two separate subsystems with different space scales. In the first component, the system consists of a tungsten tip in front of a silicon surface in a subspace of some hundreds of nanometers [2]. In the second component, the geometry is generated by importing a simplified version of the CAD-construction files of the instrument (in a scale of centimeters) (Figure 2). In both cases the electrostatic problem is solved using the AC/DC Module and the trajectories of the electrons are calculated with the Particle Tracing Module. For coupling the two simulations, in analogy to the model "Syngas Combustion in a Round-Jet Burner" (Model Library), the final positions and velocities on the walls of the first Component are used as initial values in the second Component (Figure 3). Most of the calculations are performed on a cluster. First results shown that only a few electrons can escape from the interaction region if their energies are about 70 eV. From the escaped electrons, a 10% can reach the Mott detector (Figure 4).

The possibility of resolving magnetic-textures in real space at atomic scale may trigger novel fundamental and applicative perspectives. We report on energy-resolved surface imaging with a new technique called Near Field-Emission SEM, which confirms the feasibility of electron spectroscopy and magnetic-domain mapping with nanometer spatial resolution.