Near-Field Scanning Microwave Microscope at 30mK
Scanning techniques such as atomic force microscopy (AFM) or scanning tunnelling microscopy (STM) offer a wide range of material investigation possibilities. Depending on the environmental conditions and the required scanning resolution scanning probe microscopes are hard to design and setup and not seldom homemade devices, adapted to their special proposes and requirements. A new low temperature near-field scanning microwave microscope working at 30 mK, designed to meet emerging needs of the quantum technologies sector, was now developed by the group of Sebastian de Graaf at NPL (National Physical Laboratory, Teddington, UK) in collaboration with the group of Prof. Sergey Kubatkin (Chalmers University of Technology, SE). It combines microwave characterization up to 6 GHz with STM or AFM techniques. The environment of a dilution cryostat makes special demands on the stability and stiffness of the used components. The group used a set of ANPx100 and ANPz100 nanopositioners (former versions of the ANP101 positioners) to align the sample with the tip in x, y, and z direction and a small ANPz51 positioner for the RF waveguide positioning. First verifications of the instrument showed the capability to image dielectric contrast down to the single microwave photon regime.
S. Geaney et al; Scientific Report 9, 12539 (2019)
Discovery of Intermediate State in the Metal-Insulator Transition
A closer look at the state configuration of a first-order metal-insulator transition illuminates the coexistence of the metallic and insulating phases at ultra fast timescales. Based on their former work  the group of Jian Shen at the Fudan University (Shanghai, CN) witnessed a third long-living intermediate state emerging at the photoinduced first-order metal-insulator transition of La0.325Pr0.3Ca0.375MnO3, building a bridge between the other two states .
The attoAFM used in dual pass mode delivered not only the topography of the sample but also the magnetic distribution on the nm scale. Combined with Magneto Optical Kerr Effect (MOKE) measurements the authors identified the third state as a mixture of the ferromagnetic metallic and the charged-ordered insulating states. These findings open the door to create two completely different characteristic length of phase separation in first order metal-insulator transitions. In a next step it will be interesting to see if the results can be adapted to other first-order metal-insulator transitions in different condensed matter systems.
Ultimate Thermal Stability and Ultra-Low Drift
In order to characterize both the low-frequency drift of the atomic force miroscope unit of the attoCSFM with respect to the sample, a carbon nanotube (CNT) was imaged (a). By scanning the same line across the CNT (green line in overview image) 500 subsequent times within 42 minutes (b), a line-to-line position jitter below 1 nm and a long-term drift of less than 3 nm were observed (c), demonstrating the outstanding thermal and mechanical stability of the attoCSFM. After several hours of thermalization, drifts below 1 nm/h can be achieved (d). The part of drift due to scanner creep is constantly monitored interferometrically and can therefore be corrected.
(attocube application labs, 2012; sample courtesy of A. Hartschuh, LMU Munich, Germany)
This measurement was realized with the Combined Atomic Force & Confocal Microscope.
A new way to modulate exciton-complex emissions of TMDs
A new type of atomically layered transition-metal dichalcogenides (TMD) was developed by Mr. Taishen Li and co-workers from the University of Science and Technology of China (Hefei, China): a triangular inkslab-like WSe2 homojunction with a monolayer in the inner surrounded by a multilayer frame.Optical and scanning photocurrent microscopy (SPCM) measurements performed with the attoCFM I, cooled by a closed cycle attoDRY1000 to cryogenic temperatures, shows a clear redshift of the photoluminescence peaks from the center to the edge region of the inner monolayer, reflecting a high charge density gradient. In addition, a significant rectifying behavior and photovoltaic response across the homojunction is observed. All in all, the results lead to efficient modulation of the exciton-complex emissions of TMDs.
T. Li, et al., ACS Nano 12 (5), pp 4959–4967 (2018)
Measurements of field-driven transformation of a domain pattern
The group of Erik Folven at the Norwegian University of Science and Technology (Trondheim, Norway) used an attoAFM I for MFM measurements with a closed cycle attoDRY1000 to demonstrate how topological defects may be invoked to understand magnetic domain state transitions. The atomically sharp and magnetized tip of the microscope is scanned across the thin film surface to pick up the out-of plane stray fields from the sample and thus is sensitive primarily to spin textures such as domain walls and defects. The MFM measurements taken at 5K help to understand and describe micromagnetic domain state transitions and to assess their stability in remanence. This insight may open for new approaches to control the switching properties of micro- and nanomagnets.
S. D. Sloetjes, et al., Appl. Phys. Lett. 112, 042401 (2018)
Quantized Conduction on Domain Walls of a Magnetic Topological Insulator
In a paper published in Science, researchers from the University of Tokyo and RIKEN (Japan) have studied “Quantized conduction on domain walls of a magnetic topological insulator” using an attoAFM/MFM in a 3He-cryostat down to 500 mK. In their paper, Yasuda et al. designed and fabricated magnetic domains in the quantum anomalous Hall state, and proved the existence of the chiral one-dimensional edge conduction along the domain walls through transport measurements.
This discovery would permit fully electrical control of the mobile domain walls and chiral edge states, which may lead to the realization of low-power-consumption spintronic, memory and quantum information processing devices in the future.
Yasuda et al., Science 358, 1311–1314 (2017)
Local Conductivity Mapping and PFM on BFO Thin Film
In this application, the versatility of the cryogenic attoAFM I was demonstrated on an ultra-thin film of BFO. A simple box writing and reading measurements was performed. During the writing phase, a DC voltage of -10 V was applied to write a box. For the reading, a 5 Vpp AC excitation at ≈42 kHz on top of a -2 V DC voltage was used. Combining both AC and DC voltage at the same time allows for a simultaneous measurement of PFM (right image) and local conductivity (left image).
(attocube application labs, 2014; sample courtesy of N. Domingo, ICN Barcelona, Spain)
Low Temperature Piezoresponse Force Microscopy on BiFeO
Piezoresponse Force Microscopy (PFM) is a standard tool at room temperature to investigate new materials, especially multiferroics. However, in many cases the scientifically interesting phases only exist at low temperatures or high magnetic fields, what demands the extension of this technique to extreme conditions. In collaboration with our customers, we adapted our attoAFM based on the general purpose ASC500 for PFM measurements. In the measurements shown on the left, we investigated BiFeO3 a well know room temperature multiferroic. The figure shows piezoresponse amplitude after a box in the box writing at 160 K on the sample.
(attocube application labs, 2013; Sample courtesy of Neus Domingo & Gustau Catalan, CIN2 Barcelona, Centre d’Investigació en Nanociència i Nanotecnologia, Bellaterra, Spain)
Internal magnetic fields in natural sands
In this application, samples of natural sand were investigated with a cryogenic sSQUID as described above. The measurements indicate large internal magnetic field variations over tens of microns with up to 2 mT, as well as variations in excess of 50 µT over smaller ranges. These findings clearly show that unaccounted internal fields can significantly alter NMR data in unknown samples.
This measurement was realized with the Scanning SQUID Platform.
Piezo-Response Force Measurements on Ferroic Oxide Films
The renaissance of multiferroics in which at least two ferroic or antiferroic orders coexist, is motivated by fundamental aspects as well as by their possible applications in the field of spintronics. Magnetoelectric coupling allows for instance the reversal of the ferroelectric polarization by a magnetic field or the control of the magnetic order by an electric field. Most of the ferromagnetic-ferroelectric compounds exhibit both orders at low temperature.
In the measurements presented here PFM data have been taken on a layered heterostructure (150 nm BiFeO3-Mn on top of 35 nm of SrRuO3 on a SrTiO3 (001) substrate) recorded at 82 K with a standard attoAFM I. Two squares have been written, a 1 x 1 µm2 and a smaller, rotated one with ±15 V tip voltage. It can be noted that the amplitude goes to zero in the domain walls and that the outside area shows natural domains.
(Images and data courtesy of K. Bouzehouane and S. Fusil, Unité Mixte de Physique CNRS/Thales, Paris, France)
Dynamic Visualization of Nanoscale Vortex Motion using attoSTM in an attoLiquid3000
Matias Timmermans and co-workers invented an innovative technique removing the lack of temporal resolution in STM imaging. They used an attoSTM in an attoLIQUID3000 3He cryostat to measure and study vortex motion in 2H-NbSe2 on a much shorter time scale. By applying a small AC magnetic field they induced a periodic movement of the vortices. The external perturbation results in a distinct smearing of the vortex in the images. Instead of collecting several consecutive images, the tunnelling current is recorded at each point over three cycles of the excitation. The exceptional thermal and spatial stability of the attoSTM in the attoLiquid3000 allows further analysis of the time dependence of this signal at each point. Using an additional lock-in technique more details and understanding of the vortex motion is revealed. By mapping the first and a second harmonic of the tunnelling signal (see upper figures), they were able to visualize changes of the vortex lattice when the vortex density is increased by increasing the DC magnetic field.
In a next step, they used the AC excitation as a time reference to track the motion of individual vortices in time. This results in time resolved snapshots of the vortex motion, which allows them to construct a movie frame by frame. This visualization procedure is unprecedented and promises a much better understanding of the dynamical behaviour of the superconducting condensate (see lower figures). Contrary to the expectation the vortex does not move in a line but follows a circular motion, due to a potential created by atoms and/or vortices.
Low Temperature Surface Piezoelectricity in SrTiO3 using Piezo-Response Force Microscopy
SrTiO3 is one of the most investigated materials from the ferroelectric perovskite titanates family due to the variety of physical phenomena ranging from incipient ferroelectricity to superconductivity. Nowadays, considerable interest to SrTiO3 is conditioned by the observations of additional anomalies in the quantum paraelectric regime of SrTiO3, which could be described in terms of a coherent quantum state occurring below T≈37 K. It is supposed that these anomalies are related to the existence of large polarization clusters. Visualizing the dynamic of ferroelectric nanoscale structure at low temperatures may shed light on the mechanisms of the T≈37 K anomaly.
Imaging fractional incompressible stripes in integer quantum Hall effect
Nicola Paradiso, Stefan Heun, and co-workers measured the fractional quantum Hall effect in quantum point contacts using an attoAFM III in an attoLIQUID3000 at very low temperature (300 mK) and high magnetic field (≈8 T). They demonstrated that fractional features were unambiguously observed in every integer quantum Hall constriction. These ground-breaking Scanning Gate Microscopy experiments pave the way to a better understanding of the role of fractional phases in the field of coherent quantum transport!
Scanning Hall Probe Microscopy at 300 mK with ANP positioners
The magnetic properties of superconducting and ferromagnetic materials at ultra-low temperatures represent some of the most interesting contemporary problems in condensed matter physics. These properties are typically investigated using a magnetic force microscope or a scanning Hall probe microscope (SHPM). In this note, we report on a self-built SHPM capable of working at temperatures as low as 300 mK and magnetic fields of up to 10 T, while still having sub-micron lateral spatial resolution.
Piezo-Controlled Exfoliation of Graphene
In the group of Prof. Gosh at the IIS in Bangalore, researcher Kinikar and his coworkers managed to measure the conductance of narrow stripes of graphene during their exfoliation. A metal tip is crashed into a graphite HOPG crystal using an attocube positioner for vacuum application, namely the ANPz101, and slowly retracted via a piezo tube. Conductance is measured from the tip through the HOPG crystal. The setup situated inside a SEM is shown in picture 1. The graphite piece sticking on the tip will thereby be torn to a single layer of graphene. Mechanically torn graphene has highly crystalline edges, leading to quantized conductance. This is due to one-dimensional channels forming at the edges each with a conductance of 2e2/h (graph 2). A similar setup was used in a cryostat for high magnetic field measurements.
Kinikar: “The attocubes have been with us for over a decade, and they still work perfectly!”
This measurement was realized with the ANPz101/HV - linear z-nanopositioner.
A Kinikar et al; Nature Nanotechnology 12, 564-568 (2017).
HF-SPM using attocube nano-positioners in magnetic fields above 30 T
In an outstanding setup, Benjamin Bryant and Lisa Rossi (High Field Magnet Laboratory, Radboud University, Nijmegen, NL), together with the SPM group of Alex Khajetoorians (Radboud University), designed a high field scanning probe microscope (HF-SPM) for operation at cryogenic temperatures and in extreme magnetic fields up to 38 T. The high magnetic field is provided using a water-cooled Bitter magnet: noise from the cooling water creates a highly challenging vibration environment for SPM. An ANPz30 nanopositioner controls the coarse approach of an atomic force microscope cantilever to a scanned sample. The attocube positioner provides for a modular design that makes it easy not only to change the components if needed but also allows the flexibility to employ different cantilever or sample holders. Due to the compactness and rigid design of the positioner the sensitivity to vibrational noise is reduced, which is critical for SPM in the extreme environment of the Bitter magnet.
This measurement was realized with the ANPz30/LT - linear z-nanopositioner.
Conductive-Tip AFM Measurements on Ruthenium
In this application, atomic steps on Ruthenium were investigated using conductive-tip AFM. Atomic steps as well as spiral dislocations can be identified on the molecular beam epitaxy-grown sample. The contrast in this measurement is highly enhanced due to a difference in conductance between edges and flat plateaus. Such high contrast was not observed in the accompanying topographic image. A voltage of +10 mV was applied to the standard Pt-coated AFM tip, while the sample was grounded via a current amplifier with gain 106 V/A. The measurement was performed at room temperature in a 20 mbar He atmosphere.
(Sample and measurement courtesy of V. Da Costa, J.-F. Dayen, B. Doudin, IPCMS-DMONS,CNRS/University of Strasbourg, France)
Domain Imaging in BaFeO
The 15 µm x 15 µm sized image shows a sample of BaFeO recorded with an attoSHPM, recorded at 4.2 K. The SHPM sensor was kept in a constant height of about 200 nm. The color scale spans 106 mT (dark to bright), while the S/N ratio of this measurement yields an exceptional 50 000:1. Note that SHPM records absolute field strength as opposed to MFM techniques, that record only field gradients.
(attocube application labs, 2011; sample courtesy of R. Kramer, Institut Néel, CNRS, Grenoble)
Vortex Imaging via Scanning Hall Probe Microscopy
SHPM measurements on a degraded Bi2Sr2CaCu2O8+x substrate have been performed demonstrating strong surface pinning effects at 4.2 K and 2.5 Gauss external magnetic field. The figure shows the vortex distribution measured in constant height of approx. 100 nm above the surface.
(attocube applications labs, 2011; sample courtesy of A. Erb, TU Munich, Germany)
Scanning Gate Microscopy at 300 mK
In this measurement, an attoAFM III was operated inside an attoLIQUID3000 cryostat at 300 mK in scanning gate microscopy mode (SGM) - investigating the trajectory and interaction of edge channels of a split-gate quantum point contact (QPC) device in the Quantum Hall (QH) regime. By scanning the SGM tip over the surface of the QPC at constant height and by simultaneously measuring and plotting the source-drain current, conductance maps were obtained. The image to the left is an example of such a conductance map depicting the characteristic branched-flow of electrons at zero magnetic field, which in turn shows electron interference fringes and the actual electron path (T = 400 mK, 2DEG density n2D = 3.37 x 1011 cm-2).
(Data and images were generously provided by S. Heun et al., NEST, CNR-INFM and Scuola Normale Superiore, Pisa, Italy.)
KPFM of Au-on-Pt Pattern
The kelvin probe force microscopy (KPFM) measurements shown here were performed on a test sample consisting of a Au layer on a Pt substrate in dual pass mode at cryogenic temperatures of 4K. The KPFM image was recorded during the second line with a lift height of about 50 nm. The color scale spans approximately 130 mV, and the image size is 11.9 µm x 11.9 µm.We found a KPFM contrast of approx. 35 mV, and a KPFM resolution (noise level) of approx. 2.6 mV.
(attocube application labs 2014)
attoAFM/CFM in Toploading Insert
The presented data was taken with a mk-compatible version of the attoAFM/CFM mounted on a toploading insert of a Leiden Cryogenics closed-cycle dilution refrigerator. The sample temperature was 60 mK during an AFM scan with a speed of 400nm/s. The images nicely demonstrates that the delicate microscope works very well even under these extreme conditions.
This measurement was realized with the attoAFM/CFM.
Tuning Fork based AFM measurements at cryogenic temperatures
The attoAFM III is a tuning fork based setup for highly precise low temperature measurements. The non-optical design faciliates e.g. measurements on light-sensitive samples using conductive STM-type tips.
The distance feedback is done by detecting the tuning fork vibration using a Phase-Locked Loop (PLL) together with a feedback loop. The PLL tracks the resonance of the tuning fork, whereas the feedback loop keeps the z-distance in such way that the frequency shift (vs. the free oscillation) remains at a certain level.
The presented data was measured using uncapped, stacked InAs Quantum Dots in a GaAs matrix with an attoAFM III inside an attoLIQUID.
Scanning Tunneling Spectroscopy and Vortex Imaging on NbSe2 with attoAFM III / STM I at 315 mK
Scanning Tunneling Spectroscopy (STS) is a useful tool to characterize material properties, especially on superconductors at ultra low temperatures. In a series of experiments STS measurements as well as vortex imaging on NbSe2 have been performed at a temperature of only 315 mK. The tests show excellent stability of the combined attoAFM/STM microscope setup as well as the possibility to apply stable voltages in the micro-Volt range.