Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits.

Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.

Strong In-Plane Magnetization and Spin Polarization in (Co0.15Fe0.85)5GeTe2/Graphene van der Waals Heterostructure Spin-Valve at Room Temperature.

Van der Waals (vdW) magnets are promising, because of their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, so far, most of the vdW magnet-based spintronic devices have been limited to cryogenic temperatures with magnetic anisotropies favoring out-of-plane or canted orientation of the magnetization. Here, we report beyond room-temperature lateral spin-valve devices with strong in-plane magnetization and spin polarization of the vdW ferromagnet (Co0.15Fe0.85)5GeTe2 (CFGT) in heterostructures with graphene. Density functional theory (DFT) calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Magnetization measurements reveal the above room-temperature ferromagnetism in CFGT and clear remanence at room temperature. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices, such as efficient spin injection and detection. Further analysis of spin transport and Hanle spin precession measurements reveals a strong in-plane magnetization with negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus revealing its potential application in spintronic technologies.

A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe5 GeTe2 /Graphene Heterostructure.

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe5 GeTe2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe5 GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe5 GeTe2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5 GeTe2 along with the presence of negative spin polarization at the Fe5 GeTe2 /graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics.

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Nanopatterning of Weak Links in Superconducting Oxide Interfaces.

The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30-100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces.

Magnetism and ion diffusion in honeycomb layered oxide [Formula: see text].

In the quest for developing novel and efficient batteries, a great interest has been raised for sustainable K-based honeycomb layer oxide materials, both for their application in energy devices as well as for their fundamental material properties. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K[Formula: see text]) dynamics in materials using e.g. NMR and related techniques has so far been very challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation ([Formula: see text]SR) technique ideal for probing ions dynamics in these types of energy materials. Here we present a study of the low-temperature magnetic properties as well as K[Formula: see text] dynamics in honeycomb layered oxide material [Formula: see text] using mainly the [Formula: see text]SR technique. Our low-temperature [Formula: see text]SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at [Formula: see text] K. Further [Formula: see text]SR studies performed at higher temperatures reveal that potassium ions (K[Formula: see text]) become mobile above 200 K and the activation energy for the diffusion process is obtained as [Formula: see text] meV. This is the first time that K[Formula: see text] dynamics in potassium-based battery materials has been measured using [Formula: see text]SR. Assisted by high-resolution neutron diffraction, the temperature dependence of the K-ion self diffusion constant is also extracted. Finally our results also reveal that K-ion diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for potentially improving ion diffusion as well as K-ion battery device performance using nano-structuring and surface coatings of the particles.

Detection of interictal epileptiform discharges: A comparison of on-scalp MEG and conventional MEG measurements.

OBJECTIVE: Conventional MEG provides an unsurpassed ability to, non-invasively, detect epileptic activity. However, highly resolved information on small neuronal populations required in epilepsy diagnostics is lost and can be detected only intracranially. Next-generation on-scalp magnetencephalography (MEG) sensors aim to retrieve information unavailable to conventional non-invasive brain imaging techniques. To evaluate the benefits of on-scalp MEG in epilepsy, we performed the first-ever such measurement on an epilepsy patient. METHODS: Conducted as a benchmarking study focusing on interictal epileptiform discharge (IED) detectability, an on-scalp high-temperature superconducting quantum interference device magnetometer (high-Tc SQUID) system was compared to a conventional, low-temperature SQUID system. Co-registration of electroencephalopraphy (EEG) was performed. A novel machine learning-based IED-detection algorithm was developed to aid identification of on-scalp MEG unique IEDs. RESULTS: Conventional MEG contained 24 IEDs. On-scalp MEG revealed 47 IEDs (16 co-registered by EEG, 31 unique to the on-scalp MEG recording). CONCLUSION: Our results indicate that on-scalp MEG might capture IEDs not seen by other non-invasive modalities. SIGNIFICANCE: On-scalp MEG has the potential of improving non-invasive epilepsy evaluation.

A 7-Channel High-[Formula: see text] SQUID-Based On-Scalp MEG System.

OBJECTIVE: To present the technical design and demonstrate the feasibility of a multi-channel on-scalp magnetoencephalography (MEG) system based on high critical temperature (high-[Formula: see text]) superconducting quantum interference devices (SQUIDs). METHODS: We built a liquid nitrogen-cooled cryostat that houses seven YBCO SQUID magnetometers arranged in a dense, head-aligned array with minimal distance to the room-temperature environment for all sensors. We characterize the performance of this 7-channel system in terms of on-scalp MEG utilization and present recordings of spontaneous and evoked brain activity. RESULTS: The center-to-center spacing between adjacent SQUIDs is 12.0 and 13.4 mm and all SQUIDs are in the range of 1-3 mm of the head surface. The cryostat reaches a base temperature of  âˆ¼ 70 K and stays cold for 16 h with a single 0.9 L filling. The white noise levels of the magnetometers is 50-130 fT/Hz1/2 at 10 Hz and they show low sensor-to-sensor feedback flux crosstalk ( 0.6%). We demonstrate evoked fields from auditory stimuli and single-shot sensitivity to alpha modulation from the visual cortex. CONCLUSION: All seven channels in the system sensitively sample neuromagnetic fields with mm-scale scalp standoff distances. The hold time of the cryostat furthermore is sufficient for a day of recordings. As such, our multi-channel high-[Formula: see text] SQUID-based system meets the demands of on-scalp MEG. SIGNIFICANCE: The system presented here marks the first high-[Formula: see text] SQUID-based on-scalp MEG system with more than two channels. It enables us to further explore the benefits of on-scalp MEG in future recordings.

Grazing-angle reflectivity setup for the low-temperature infrared spectroscopy of two-dimensional systems.

A new optical setup is described that allows the reflectivity at grazing incidence to be measured, including ultrathin films and two-dimensional electron systems (2DES) down to liquid-helium temperatures, by exploiting the Berreman effect and the high brilliance of infrared synchrotron radiation. This apparatus is well adapted to detect the absorption of a 2DES of nanometric thickness, namely that which forms spontaneously at the interface between a thin film of LaAlO3 and its SrTiO3 substrate, and to determine its Drude parameters.

Characterization of Binding of Magnetic Nanoparticles to Rolling Circle Amplification Products by Turn-On Magnetic Assay.

The specific binding of oligonucleotide-tagged 100 nm magnetic nanoparticles (MNPs) to rolling circle products (RCPs) is investigated using our newly developed differential homogenous magnetic assay (DHMA). The DHMA measures ac magnetic susceptibility from a test and a control samples simultaneously and eliminates magnetic background signal. Therefore, the DHMA can reveal details of binding kinetics of magnetic nanoparticles at very low concentrations of RCPs. From the analysis of the imaginary part of the DHMA signal, we find that smaller MNPs in the particle ensemble bind first to the RCPs. When the RCP concentration increases, we observe the formation of agglomerates, which leads to lower number of MNPs per RCP at higher concentrations of RCPs. The results thus indicate that a full frequency range of ac susceptibility observation is necessary to detect low concentrations of target RCPs and a long amplification time is not required as it does not significantly increase the number of MNPs per RCP. The findings are critical for understanding the underlying microscopic binding process for improving the assay performance. They furthermore suggest DHMA is a powerful technique for dynamically characterizing the binding interactions between MNPs and biomolecules in fluid volumes.

Homogeneous Differential Magnetic Assay.

Assays are widely used for detection of various targets, including pathogens, drugs, and toxins. Homogeneous assays are promising for the realization of point-of-care diagnostics as they do not require separation, immobilization, or washing steps. For low concentrations of target molecules, the speed and sensitivity of homogeneous assays have hitherto been limited by slow binding kinetics, time-consuming amplification steps, and the presence of a high background signal. Here, we present a homogeneous differential magnetic assay that utilizes a differential magnetic readout that eliminates previous limitations of homogeneous assays. The assay uses a gradiometer sensor configuration combined with precise microfluidic sample handling. This enables simultaneous differential measurement of a positive test sample containing a synthesized Vibrio cholerae target and a negative control sample, which reduces the background signal and increases the readout speed. Very low concentrations of targets down to femtomolar levels are thus detectable without any additional amplification of the number of targets. Our homogeneous differential magnetic assay method opens new possibilities for rapid and highly sensitive diagnostics at the point of care.

Grooved Dayem Nanobridges as Building Blocks of High-Performance YBa2Cu3O7-δ SQUID Magnetometers.

We present noise measurements performed on a YBa2Cu3O7-δ nanoscale weak-link-based magnetometer consisting of a superconducting quantum interference device (SQUID) galvanically coupled to a 3.5 × 3.5 mm2 pick-up loop, reaching white flux noise levels and magnetic noise levels as low as [Formula: see text] and 100 fT/[Formula: see text] at T = 77 K, respectively. The low noise is achieved by introducing grooved Dayem bridges (GDBs), a new concept of a weak link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.

Development of a Sensitive Induction-Based Magnetic Nanoparticle Biodetection Method.

We developed a novel biodetection method for influenza virus based on AC magnetic susceptibility measurement techniques (the DynoMag induction technique) together with functionalized multi-core magnetic nanoparticles. The sample consisting of an incubated mixture of magnetic nanoparticles and rolling circle amplified DNA coils is injected into a tube by a peristaltic pump. The sample is moved as a plug to the two well-balanced detection coils and the dynamic magnetic moment in each position is read over a range of excitation frequencies. The time for making a complete frequency sweep over the relaxation peak is about 5 minutes (10 Hz⁻10 kHz with 20 data points). The obtained standard deviation of the magnetic signal at the relaxation frequency (around 100 Hz) is equal to about 10-5 (volume susceptibility SI units), which is in the same range obtained with the DynoMag system. The limit of detection with this method is found to be in the range of 1 pM.

Author Correction: Induced unconventional superconductivity on the surface states of Bi2Te3 topological insulator.

The original version of this Article contained an error in Fig. 6b. In the top scattering process, while the positioning of both arrows was correct, the colours were switched: the first arrow was red and the second arrow was blue, rather than the correct order of blue then red.

Author Correction: Induced unconventional superconductivity on the surface states of Bi2Te3 topological insulator.

The original version of this Article omitted the following from the Acknowledgements:"This work was partly supported by the Research Council of Norway through its Centres of Excellence funding scheme, project number 262633, QuSpin."This has now been corrected in both the PDF and HTML versions of the article.

Volume-amplified magnetic bioassay integrated with microfluidic sample handling and high-Tc SQUID magnetic readout.

A bioassay based on a high-Tc superconducting quantum interference device (SQUID) reading out functionalized magnetic nanoparticles (fMNPs) in a prototype microfluidic platform is presented. The target molecule recognition is based on volume amplification using padlock-probe-ligation followed by rolling circle amplification (RCA). The MNPs are functionalized with single-stranded oligonucleotides, which give a specific binding of the MNPs to the large RCA coil product, resulting in a large change in the amplitude of the imaginary part of the ac magnetic susceptibility. The RCA products from amplification of synthetic Vibrio cholera target DNA were investigated using our SQUID ac susceptibility system in microfluidic channel with an equivalent sample volume of 3 µl. From extrapolation of the linear dependence of the SQUID signal versus concentration of the RCA coils, it is found that the projected limit of detection for our system is about 1.0 × 105 RCA coils (0.2 × 10-18 mol), which is equivalent to 66 fM in the 3 µl sample volume. This ultra-high magnetic sensitivity and integration with microfluidic sample handling are critical steps towards magnetic bioassays for rapid detection of DNA and RNA targets at the point of care.

Induced unconventional superconductivity on the surface states of Bi2Te3 topological insulator.

Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral p x + ip y wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi2Te3 topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral p x + ip y component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and π coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi2Te3 flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

Towards a quantum resistance standard based on epitaxial graphene.

The quantum Hall effect allows the international standard for resistance to be defined in terms of the electron charge and Planck's constant alone. The effect comprises the quantization of the Hall resistance in two-dimensional electron systems in rational fractions of R(K) = h/e(2) = 25,812.807557(18) Omega, the resistance quantum. Despite 30 years of research into the quantum Hall effect, the level of precision necessary for metrology--a few parts per billion--has been achieved only in silicon and iii-v heterostructure devices. Graphene should, in principle, be an ideal material for a quantum resistance standard, because it is inherently two-dimensional and its discrete electron energy levels in a magnetic field (the Landau levels) are widely spaced. However, the precisions demonstrated so far have been lower than one part per million. Here, we report a quantum Hall resistance quantization accuracy of three parts per billion in monolayer epitaxial graphene at 300 mK, four orders of magnitude better than previously reported. Moreover, by demonstrating the structural integrity and uniformity of graphene over hundreds of micrometres, as well as reproducible mobility and carrier concentrations across a half-centimetre wafer, these results boost the prospects of using epitaxial graphene in applications beyond quantum metrology.