Probing Quantum Phase Transition inside Superconducting Dome
Unconventional superconductors (UCS) stand in permanent focus of physicists hoping to harness high-temperature superconductivity, and thus paving the path towards more affordable and sustainable energy usage in future. Elucidating the interplay between antiferromagnetic quantum phase transition (QPT) and superconducting state is crucial for understanding the UCS.
Experimentally this interplay is usually probed from the normal-state side. The group of Ruslan Prozorov (Ames Lab, USA) probed it from the superconducting side via measurements of the London penetration depth λ in a class of iron-pnictides by performing NV magnetometry with an attoAFM/CFM in an attoLIQUID1000 cryostat. Their results reveal a peak in λ coinciding with QPT, which unexpectedly speaks in favor of an universal QPT in iron-pnictides, irrespective of the disorder level.
K.R. Joshi et al., New J. Phys. 22, 053037 (2020)
Nanoscale Imaging and Control of Domain-Wall Hopping with an NV Center Microscope
Domain walls in magnetic wires may prove useful for future spintronic devices, and hence their nanoscale characterization is an important steps towards useful applications. As demonstrated by the group of Vincent Jaques in Science, their NV center microscope based on the attoAFM/CFM allows to image domain walls in a 1 nm thick ferromagnetic nanowire with high resolution as well as jumps between pinning sites of individual domain walls. At the same time, they showed that the domain walls can be moved along the wire by inducing jumps via local heating due to a high local laser power. Since the domain walls are pinned by nearest pinning site, this allows to probe and image the pinning landscape of the sample quite efficiently.
(Images courtesy of V. Jacques, University of Montpellier, FR)
Tetienne et al ., Science 344, 1366(2014)
Optical Magnetometer Reveals Lack of Conventional Meissner Effect in Iron-based Superconductors
Scientists at Ames Laboratory (US), Ruslan Prozorov and Naufer Nusran, together with graduate student Kamal Joshi succeeded to visualize spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. The experimental setup is based on the Attocube AFM/CFM, a low-temperature AFM combined with a confocal microscope. Attocube’s high NA low temperature microscope objective is used to optically access the nitrogen-vacancy centers in diamond, which are essentially atomic-scale magnetic sensors.
N. M. Nusran et al., New J. Phys 20, 043010 (2018)
NV-Center Based Nanomagnetometry
Given its premier mechanical and thermal stability, the attoAFM/CFM is the ideal platform for nanoscale magnetic imaging employing an AFM tip with a diamond nanocrystal that contains a single nitrogen-vacancy (NV) center -. Local magnetic fields are subsequently evaluated by measuring the Zeeman shifts of the NV defect spin sublevels. In the particular case of NV-center magnetometry, an external microwave field is emitted and tuned in frequency such that local spin resonance occurs. This condition can subsequently be detected by a decrease in photoluminescence intensity of the NV-center, referred to as ODMR (optically detected magnetic resonance). Using a Lock-in and feedback loop technique allows to maintain spin resonance while rastering the sample, allowing to record a local magnetic field map with nanometer resolution.
In this example, magnetic imaging of a hard disk sample with random bit orientation was performed in the group of V. Jacques at LPQM, ENS-Cachan, France. 
Example 1 (a,b): Quantitative imaging using ODMR based method with NV-center scanned at d1 = 250 nm above the sample. (a) Schematic of the measurement. (b) Quantitative magnetic field distribution recorded with the lockin technique (13 nm pixel size, 110 ms acquisition time per pixel). The inset shows a line-cut taken along the dashed white line in the image. 
Example 2 (c,d): All-optical method with NV center closer to the sample surface. (c) Schematic of the measurement. (d) All optical photoluminescence image (no microwave field applied) recorded with the NV-scanning probe magnetometer in tapping mode (8 nm pixel size, 20 ms acquisition time per pixel). Comparisons with simulations indicates that the tip surface distance is roughly d2 = 30 nm. Fine white dotted lines are plotted along the direction of the hard disk tracks as a guide for the eye. 
Related publications based on the attoAFM/CFM (2012-2016)
 A. Dréau et al., Phys. Rev. Lett. 113, 137601 (2014)
 A. Dréau et al., Phys. Rev. Lett. 110, 060502 (2013)
 L. Rondin et al., Nature communications 4, 2279 (2013)
 J.-P. Tetienne et al., Phys. Rev. B 87, 235436 (2013)
 J.-P. Tetienne et al., New J. Phys. 14, 103033 (2012)
 A. Dréau et al., Phys. Rev. B 85, 134107 (2012)
 L. Rondin et al., Appl. Phys. Lett. 100, 153118 (2012)
Quantitative Nanoscale Vortex-Imaging of Superconductors
Understanding the microscopic mechanisms of superconductivity could be greatly facilitated by non-invasive tools that allow for quantitative imaging with nanometric resolution over a large range of temperatures and high magnetic fields. Based on the attoAFM/CFM, the group of Patrick Maletinsky (Univ. of Basel) reports on cryogenic measurements using NV center magnetometry. Their technique allows to extract quantitative data on the local magnetic field of individual superconducting vortices in YBCO with high sensitivity and spatial resolution. By determining the local London penetration depth, they find that the so called Pearl-vortex model explains the data much better and allows for fitting of additional parameters than the standard monopole model. Their experiments constitute an impressive example for how far the practical use of the NV center based magnetometry tools has already evolved.
(Images courtesy of P. Maletinsky, University of Basel, CH)
L. Thiel et al., Nature Nanotechnology 11, 677-681 (2016).
Vortex Barriers in Iron Pnictides
Iron-pnictide high-temperature superconductors are widely studied, but many open questions still remain. Using an attoAFM I for magnetic force microscopy, the group of O. Auslaender has studied twin boundaries and their interaction with vortices over a range of magnetic fields and temperatures. They find that stripes parallel to the twin boundaries repel vortices, effectively hindering vortex motion, and hence potentially affecting the critical current in such materials.
(Data courtesy of O. Auslaender, Technion, Israel)
A. Yagil et al., Phys. Rev. B 94, 064510 (2016)
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)
Material composition and strain analysis of single semiconductor quantum dots
E.A. Chekhovich and his colleagues at the University of Sheffield developed an elegant modification of an optical technique to analyze material composition in semiconductor quantum dots. This new approach now allows analysis of strained structures, which was extremely difficult before.
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.