Charge Transport inside TiO2 Memristors Prepared via FEBID.

We fabricated memristive devices using focused electron beam-induced deposition (FEBID) as a direct-writing technique employing a Pt/TiO2/Pt sandwich layer device configuration. Pinching in the measured current-voltage characteristics (i-v), the characteristic fingerprint of memristive behavior was clearly observed. The temperature dependence was measured for both high and low resistive states in the range from 290 K down to about 2 K, showing a stretched exponential behavior characteristic of Mott-type variable-range hopping. From this observation, a valence change mechanism of the charge transport inside the TiO2 layer can be deduced.

Granular Hall Sensors for Scanning Probe Microscopy.

Scanning Hall probe microscopy is attractive for minimally invasive characterization of magnetic thin films and nanostructures by measurement of the emanating magnetic stray field. Established sensor probes operating at room temperature employ highly miniaturized spin-valve elements or semimetals, such as Bi. As the sensor layer structures are fabricated by patterning of planar thin films, their adaption to custom-made sensor probe geometries is highly challenging or impossible. Here we show how nanogranular ferromagnetic Hall devices fabricated by the direct-write method of focused electron beam induced deposition (FEBID) can be tailor-made for any given probe geometry. Furthermore, we demonstrate how the magnetic stray field sensitivity can be optimized in situ directly after direct-write nanofabrication of the sensor element. First proof-of-principle results on the use of this novel scanning Hall sensor are shown.

Spin-wave spectroscopy of individual ferromagnetic nanodisks.

The increasing demand for nanoscale magnetic devices requires development of 3D magnetic nanostructures. In this regard, focused electron beam induced deposition (FEBID) is a technique of choice for direct-writing of complex nano-architectures with applications in nanomagnetism, magnon spintronics, and superconducting electronics. However, intrinsic properties of nanomagnets are often poorly known and can hardly be assessed by local optical probe techniques. Here, an original spatially resolved approach is demonstrated for spin-wave spectroscopy of individual circular magnetic elements with sample volumes down to about 10-3 µm3. The key component of the setup is a coplanar waveguide whose microsized central part is placed over a movable substrate with well-separated CoFe-FEBID nanodisks which exhibit standing spin-wave resonances. The circular symmetry of the disks allows for the deduction of the saturation magnetization and the exchange stiffness of the material using an analytical theory. A good correspondence between the results of analytical calculations and micromagnetic simulations is revealed, indicating a validity of the used analytical model going beyond the initial thin-disk approximation used in the theoretical derivation. The presented approach is especially valuable for the characterization of direct-write magnetic elements opening new horizons for 3D nanomagnetism and magnonics.

Crystalline Niobium Carbide Superconducting Nanowires Prepared by Focused Ion Beam Direct Writing.

Superconducting planar nanostructures are widely used in applications, e.g., for highly sensitive magnetometers and in basic research, e.g., to study finite size effects or vortex dynamics. In contrast, 3D superconducting nanostructures, despite their potential in quantum information processing and nanoelectronics, have been addressed only in a few pioneering experiments. This is due to the complexity of fabricating 3D nanostructures by conventional techniques such as electron-beam lithography and to the scarce number of superconducting materials available for direct-writing techniques, which enable the growth of 3D free-standing nanostructures. Here, we present a comparative study of planar nanowires and free-standing 3D nanowires fabricated by focused electron- and ion (Ga+)-beam induced deposition (FEBID and FIBID) using the precursor Nb(NMe2)3(N- t-Bu). FEBID nanowires contain about 67 atomic percent C, 22 atomic percent N, and 11 atomic percent Nb, while FIBID samples are composed of 43 atomic percent C, 13 atomic percent N, 15.5 atomic percent Ga, and 28.5 atomic percent Nb. Transmission electron microscopy shows that FEBID samples are amorphous, while FIBID samples exhibit a fcc NbC polycrystalline structure, with grains about 15-20 nm in diameter. Electrical transport measurements show that FEBID nanowires are highly resistive following a variable-range-hopping behavior. In contradistinction, FIBID planar nanowires become superconducting at Tc ≈ 5 K. In addition, the critical temperature of free-standing 3D nanowires is as high as Tc ≈ 11 K, which is close to the value of bulk NbC. In conclusion, FIBID-NbC is a promising material for the fabrication of superconducting nanowire single-photon detectors (SNSPD) and for the development of 3D superconductivity with applications in quantum information processing and nanoelectronics.

Spin-Wave Phase Inverter upon a Single Nanodefect.

Local modification of magnetic properties of nanoelements is a key to design future-generation magnonic devices in which information is carried and processed via spin waves. One of the biggest challenges here is to fabricate simple and miniature phase-controlling elements with broad tunability. Here, we successfully realize such spin-wave phase shifters upon a single nanogroove milled by a focused ion beam in a Co-Fe microsized magnonic waveguide. By varying the groove depth and the in-plane bias magnetic field, we continuously tune the spin-wave phase and experimentally evidence a complete phase inversion. The microscopic mechanism of the phase shift is based on the combined action of the nanogroove as a geometrical defect and the lower spin-wave group velocity in the waveguide under the groove where the magnetization is reduced due to the incorporation of Ga ions during the ion-beam milling. The proposed phase shifter can easily be on-chip integrated with spin-wave logic gates and other magnonic devices. Our findings are crucial for designing nanomagnonic circuits and for the development of spin-wave nano-optics.

Tunable magnetism on the lateral mesoscale by post-processing of Co/Pt heterostructures.

Controlling magnetic properties on the nanometer-scale is essential for basic research in micro-magnetism and spin-dependent transport, as well as for various applications such as magnetic recording, imaging and sensing. This has been accomplished to a very high degree by means of layered heterostructures in the vertical dimension. Here we present a complementary approach that allows for a controlled tuning of the magnetic properties of Co/Pt heterostructures on the lateral mesoscale. By means of in situ post-processing of Pt- and Co-based nano-stripes prepared by focused electron beam induced deposition (FEBID) we are able to locally tune their coercive field and remanent magnetization. Whereas single Co-FEBID nano-stripes show no hysteresis, we find hard-magnetic behavior for post-processed Co/Pt nano-stripes with coercive fields up to 850 Oe. We attribute the observed effects to the locally controlled formation of the CoPt L10 phase, whose presence has been revealed by transmission electron microscopy.

Towards an optimum design for electrostatic phase plates.

Charging of physical phase plates is a problem that has prevented their routine use in transmission electron microscopy of weak-phase objects. In theory, electrostatic phase plates are superior to thin-film phase plates since they do not attenuate the scattered electron beam and allow freely adjustable phase shifts. Electrostatic phase plates consist of multiple layers of conductive and insulating materials, and are thus more prone to charging than thin-film phase plates, which typically consist of only one single layer of amorphous material. We have addressed the origins of charging of Boersch phase plates and show how it can be reduced. In particular, we have performed simulations and experiments to analyze the influence of the insulating Si3N4 layers and surface charges on electrostatic charging. To optimize the performance of electrostatic phase plates, it would be desirable to fabricate electrostatic phase plates, which (i) impart a homogeneous phase shift to the unscattered electrons, (ii) have a low cut-on frequency, (iii) expose as little material to the intense unscattered beam as possible, and (iv) can be additionally polished by a focused ion-beam instrument to eliminate carbon contamination accumulated during exposure to the unscattered electron beam (Walter et al., 2012, Ultramicroscopy, 116, 62-72). We propose a new type of electrostatic phase plate that meets the above requirements and would be superior to a Boersch phase plate. It consists of three free-standing coaxial rods converging in the center of an aperture (3-fold coaxial phase plate). Simulations and preliminary experiments with modified Boersch phase plates indicate that the fabrication of a 3-fold coaxial phase plate is feasible.

Catalytic purification of directly written nanostructured Pt microelectrodes.

In the majority of cases, nanostructures prepared by focused electron beam induced deposition employing an organometallic precursor contain predominantly carbon-based ligand dissociation products. This is unfortunate with regard to using this high-resolution direct-write approach for the preparation of nanostructures for various fields, such as mesoscopic physics, micromagnetism, metaoptical phenomena in the visible spectral range, or others. Following early attempts of postprocessing Pt-based structures prepared by focused electron beam induced deposition at several hundred degrees Celsius in a reactive gas atmosphere, recent work has focused on developing in situ purification processes by using a stationary O2 flux in combination with electron irradiation to oxidize the carbonaceous component of the deposits. Here we show that this purification process is driven by the catalytic activity of Pt and in fact does not rely on the parallel electron irradiation process to function, if the O2 exposure is done in a pulsed fashion. We suggest a multistep cleaning mechanism which results in pure, nanoporous Pt. By suitably chosen beam parameters, high-resolution Pt dot and line structures with dimensions below 10 nm can thus be conveniently obtained. In temperature-dependent resistance measurements, we find the typical metallic behavior of Pt. In low-temperature magnetoresistance measurements, we see clear evidence for weak antilocalization effects and deduce a dephasing length of 234 nm at 1.2 K. We consider this to be a promising starting point for developing this approach into a versatile preparation technique for Pt-based mesoscopic structures, in particular since the purification process can be run in parallel on different deposits. We furthermore anticipate that our results will spur further research on purification approaches for nanostructures prepared by focused electron beam induced deposition containing a catalytically active metal species such as Pd-, Fe-, or Co-based deposits.

Focused electron beam induced deposition: A perspective.

BACKGROUND: Focused electron beam induced deposition (FEBID) is a direct-writing technique with nanometer resolution, which has received strongly increasing attention within the last decade. In FEBID a precursor previously adsorbed on a substrate surface is dissociated in the focus of an electron beam. After 20 years of continuous development FEBID has reached a stage at which this technique is now particularly attractive for several areas in both, basic and applied research. The present topical review addresses selected examples that highlight this development in the areas of charge-transport regimes in nanogranular metals close to an insulator-to-metal transition, the use of these materials for strain- and magnetic-field sensing, and the prospect of extending FEBID to multicomponent systems, such as binary alloys and intermetallic compounds with cooperative ground states. RESULTS: After a brief introduction to the technique, recent work concerning FEBID of Pt-Si alloys and (hard-magnetic) Co-Pt intermetallic compounds on the nanometer scale is reviewed. The growth process in the presence of two precursors, whose flux is independently controlled, is analyzed within a continuum model of FEBID that employs rate equations. Predictions are made for the tunability of the composition of the Co-Pt system by simply changing the dwell time of the electron beam during the writing process. The charge-transport regimes of nanogranular metals are reviewed next with a focus on recent theoretical advancements in the field. As a case study the transport properties of Pt-C nanogranular FEBID structures are discussed. It is shown that by means of a post-growth electron-irradiation treatment the electronic intergrain-coupling strength can be continuously tuned over a wide range. This provides unique access to the transport properties of this material close to the insulator-to-metal transition. In the last part of the review, recent developments in mechanical strain-sensing and the detection of small, inhomogeneous magnetic fields by employing nanogranular FEBID structures are highlighted. CONCLUSION: FEBID has now reached a state of maturity that allows a shift of the focus towards the development of new application fields, be it in basic research or applied. This is shown for selected examples in the present review. At the same time, when seen from a broader perspective, FEBID still has to live up to the original idea of providing a tool for electron-controlled chemistry on the nanometer scale. This has to be understood in the sense that, by providing a suitable environment during the FEBID process, the outcome of the electron-induced reactions can be steered in a controlled way towards yielding the desired composition of the products. The development of a FEBID-specialized surface chemistry is mostly still in its infancy. Next to application development, it is this aspect that will likely be a guiding light for the future development of the field of focused electron beam induced deposition.

Universal conductance correction in a tunable strongly coupled nanogranular metal.

We present temperature-dependent conductivity data obtained on a sample set of nanogranular Pt-C with finely tuned intergrain tunnel coupling strength g. For samples in the strong-coupling regime g > g(C), characterized by a finite conductivity for T→0, we find a logarithmic behavior at elevated temperatures and a crossover to a √T behavior at low temperatures over a wide range of coupling strengths g(C) ≈ 0.25 < g ≤ 3. The experimental observation for g > 1 is in very good agreement with recent theoretical findings on ordered granular metals in three spatial dimensions. The results indicate a validity of the predicted universal conductivity behavior that goes beyond the immediate range of the approach used in the theoretical derivation.

Binary Pt-Si nanostructures prepared by focused electron-beam-induced deposition.

Binary systems of Pt-Si are prepared by electron-beam-induced deposition using the two precursors, trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPt(Me)(3)) and neopentasilane (Si(SiH(3))(4)), simultaneously. By varying the relative flux of the two precursors during deposition, we are able to study composites containing platinum and silicon in different ratios by means of energy-dispersive X-ray spectroscopy, atomic force microscopy, electrical transport measurements, and transmission electron microscopy. The results show strong evidence for the formation of a binary, metastable Pt(2)Si(3) phase, leading to a maximum in the conductivity for a Si/Pt ratio of 3:2.

A tunable strain sensor using nanogranular metals.

This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)(3)]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals.