| 1. |
- Panchal, V., et al.
(författare)
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Magnetic Scanning Probe Calibration Using Graphene Hall Sensor
- 2013
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Ingår i: IEEE Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 49:7, s. 3520-3523
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic force microscopy (MFM) offers a unique insight into the nanoscopic scale domain structures of magnetic materials. However, MFM is generally regarded as a qualitative technique and, therefore, requires meticulous calibration of the magnetic scanning probe stray field (B-probe) for quantitative measurements. We present a straightforward calibration of B-probe using scanning gate microscopy on epitaxial graphene Hall sensor in conjunction with Kelvin probe force microscopy feedback loop to eliminate sample-probe parasitic electric field interactions. Using this technique, we determined B-probe similar to 70 mT and similar to 76 mT for probes with nominal magnetic moment similar to 1 x 10(-13) and > 3 x 10(-13) emu, respectively, at a probe-sample distance of 20 nm.
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| 2. |
- Felton, S., et al.
(författare)
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Micromagnetic studies of Fe/Co ellipses with competing anisotropy contributions
- 2004
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 272-276:SUPPL. 1, s. e1297-
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Tidskriftsartikel (refereegranskat)abstract
- The effects of competing magneto-crystalline and shape anisotropies on magnetization reversal were studied in situ in arrays of sub-micron Fe/Co ellipses of compositions Fe2/Co6 and Fe8/Co3 with magnetic force microscopy (MFM). A simple model assigning magnetization values to the different types of domain structures observed in the MFM images was used to estimate the field dependence of the total magnetization of a sample. The agreement with macroscopic magnetization measurements is discussed.
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| 3. |
- Hanson, Maj, et al.
(författare)
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Effects of size and interactions on the magnetic behaviour of elliptical (001)Fe nanoparticles
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - 0304-8853 .- 1873-4766.
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACT: Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.Article · Sep 2007 · Journal of Magnetism and Magnetic Materials
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| 4. |
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| 5. |
- Hanson, Maj, 1939, et al.
(författare)
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Influence of post-annealing on the properties of Fe50Pt50 film and submicron size particles
- 2004
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 272-276:SUPPL. 1, s. e1297-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of a 48 nm thick Fe50Pt50 film and submicron size elements made of the film were investigated. The initial film was grown by DC magnetron sputtering and post-annealed during 30 min at 300°C. Arrays of circular dots with diameters d = 200, 300 and 550 nm and a reference sample (diameter 1.7mm) were made of the film by electron lithography and Ar ion milling. After structural analysis by X-ray diffraction (XRD) and characterization by magnetization measurements and magnetic force microscopy, all samples were further annealed during 30min at 600°C. The XRD scans show that this led to an improvement of the crystalline quality in all samples, to a degree depending on the size of the dots. Both the structural and magnetic measurements imply that after the second heat treatment all samples have the easy magnetocrystalline direction (c-axis) mainly in the plane of the film, while the initial structures were characterized by a random distribution of c-axes.
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| 6. |
- Hanson, Maj, 1939, et al.
(författare)
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Magnetic domain structures in submicron-size particles of epitaxial Fe (001) films: shape anisotropy and thickness dependence
- 2002
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 66, s. 144419-1
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Tidskriftsartikel (refereegranskat)abstract
- In this work the influence of shape anisotropy on the magnetic hysteresis and zero-field magnetic domain state of submicron-size particles of Fe was investigated. Arrays of particles having circular (diameter dc=550 or 200 nm), rectangular (900 nm by 300 nm), or elliptical (450 nm by 150 nm) shape were prepared by electron lithography and ion-beam milling of epitaxial Fe (001) films of thicknesses t=50, 30, 15, and 10 nm. The samples were characterized by magnetization measurements and magnetic-force microscopy (MFM). All films have cubic anisotropy, for t=50, 30, and 15 nm with the same anisotropy constant as bulk Fe: K1=(4.30.1)104 J m-3. For t=10 nm the effective anisotropy is increased, K1=5.7104 J m-3, due to surface effects. The effects of the interplay between the magnotocrystalline and shape anisotropies are observed as the lateral extension of the films is decreased. The circular particles with dc=550 nm have closed magnetic domain structures with a fourfold symmetry, compatible with the magnetocrystalline anisotropy, for all thicknesses. In the rectangular particles a gradual change is observed as the thickness decreases. For t=50 nm a diamond structure comprising three closed-domain substructures is formed in the demagnetized state. The rectangles with t=30 and 15 nm are multidomains with the number of closed substructures decreasing to 2 and 2 or 1, respectively. The thickness dependence of the domain structure and an accompanying change of character in the MFM contrast are explained by an increasing amount of Neel component in the domain walls with a width that increases with decreasing film thickness. The rectangles with t=10 nm are quasisingle domains, forming a flower state. The small circular particles (dc=200 nm) and the elliptical ones, both with t=10 nm, are considered to be stable single domains in zero field. Judging from the hysteresis curves, magnetization reversal does not occur by coherent rotation in any of the particles
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| 7. |
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| 8. |
- Kazakova, O, et al.
(författare)
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Epitaxial graphene on SiC(000(1)over-bar): functional electrical microscopy studies and effect of atmosphere
- 2013
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Ingår i: Nanotechnology. - : Institute of Physics: Hybrid Open Access. - 0957-4484 .- 1361-6528. ; 24:21
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Tidskriftsartikel (refereegranskat)abstract
- Surface potential distribution, V-CPD, and evolution of atmospheric adsorbates on few and multiple layers (FLG and MLG) of graphene grown on SiC(000 (1) over bar) substrate have been investigated by electrostatic and Kelvin force microscopy techniques at T = 20-120 degrees C. The change of the surface potential distribution, Delta V-CPD, between FLG and MLG is shown to be temperature dependent. The enhanced Delta V-CPD value at 120 degrees C is associated with desorption of adsorbates at high temperatures and the corresponding change of the carrier balance. The nature of the adsorbates and their evolution with temperature are considered to be related to the process of adsorption and desorption of the atmospheric water on MLG domains. We demonstrate that both the nano- and microscale wettability of the material are strongly dependent on the number of graphene layers.
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| 9. |
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| 10. |
- Kazakova, O., et al.
(författare)
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Influence of interfaces on the magnetic properties of submicron elements
- 2004
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 272-276:III, s. 1640-1641
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the influence of interfaces on the properties of patterned elliptical elements made of epitaxial (BCC) Fe2/Co6 multilayers and polycrystalline Fe25Co75 alloy films, both containing 75 at% Co. Magnetic force microscopy shows that the formation of stable single domains in zero field is more likely in ellipses made of the alloy than of the multilayers. Magnetization measurements demonstrate considerably different behaviour of elements made of homogeneous and layered films, which imply that the magnetization reversal occurs through the formation of quasi-single domain states in the Fe25/Co75 ellipses and mainly by gradual rotation and switching in the FeCo elements.
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| 11. |
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| 12. |
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| 13. |
- Kazakova, O., et al.
(författare)
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Submicron size particles of Fe/Co multilayers: Effects of tunable anisotropy
- 2002
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Ingår i: INTERMAG Europe 2002 - IEEE International Magnetics Conference. - 0780373650 ; , s. Article number 1000796-
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Konferensbidrag (refereegranskat)abstract
- Particle arrays prepared by electron beam lithography and ion beam milling of magnetic films can be designed to yield a wide range of magnetic properties. The shape and size of the particles as well as the distance between them may be varied independently of each other. Furthermore, by choosing multilayers as starting materials the intrinsic magnetic properties, e.g. the crystalline anisotropy may be tuned. In the present work we have extended our earlier studies of fcc Co and Fe submicron structures, with an investigation of arrays of elliptical particles (150 nm x 450 nm x 20 nm) prepared of Fe/Co multilayers. In these the crystalline anisotropy varies between those of Co and Fe. The zero-field state of the magnetic particles, the strength of the dipolar interactions, as well as the processes of magnetization reversal vary significantly with the composition of the starting film. An example is shown in the magnetic force microscopy (MFM) images of 20 nm thick particles of elliptical shape made of pure Co with 100% single domains (See figure la), Fe2ML/Co6ML with 75% single domains and pure Fe with no single domains. The images represent the zero-field state after demagnetization. We conclude that the critical size at which the particles will form a stable single domain state in zero field varies in a way that can be explained by the magnetocrystalline anisotropy decreasing when going from Co to Fe/Co to Fe.
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| 14. |
- Ludwig, Frank, et al.
(författare)
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Magnetic, Structural, and Particle Size Analysis of Single- and Multi-Core Magnetic Nanoparticles
- 2014
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Ingår i: IEEE Transactions on Magnetics. - 0018-9464 .- 1941-0069. ; 50:11
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Tidskriftsartikel (refereegranskat)abstract
- We have measured and analyzed three different commercial magnetic nanoparticle systems, both multi-core and single-core in nature, with the particle (core) size ranging from 20 to 100 nm. Complementary analysis methods and same characterization techniques were carried out in different labs and the results are compared with each other. The presented results primarily focus on determining the particle size-both the hydrodynamic size and the individual magnetic core size-as well as magnetic and structural properties. The used analysis methods include transmission electron microscopy, static and dynamic magnetization measurements, and Mossbauer spectroscopy. We show that particle (hydrodynamic and core) size parameters can be determined from different analysis techniques and the individual analysis results agree reasonably well. However, in order to compare size parameters precisely determined from different methods and models, it is crucial to establish standardized analysis methods and models to extract reliable parameters from the data.
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| 15. |
- Melios, C., et al.
(författare)
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Detection of Ultralow Concentration NO2 in Complex Environment Using Epitaxial Graphene Sensors
- 2018
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Ingår i: ACS Sensors. - : American Chemical Society (ACS). - 2379-3694. ; 3:9, s. 1666-1674
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate proof-of-concept graphene sensors for environmental monitoring of ultralow concentration NO2 in complex environments. Robust detection in a wide range of NO2 concentrations, 10-154 ppb, was achieved, highlighting the great potential for graphene-based NO2 sensors, with applications in environmental pollution monitoring, portable monitors, automotive and mobile sensors for a global real-time monitoring network. The measurements were performed in a complex environment, combining NO2/synthetic air/water vapor, traces of other contaminants, and variable temperature in an attempt to fully replicate the environmental conditions of a working sensor. It is shown that the performance of the graphene-based sensor can be affected by coadsorption of NO2 and water on the surface at low temperatures (≤70 °C). However, the sensitivity to NO2 increases significantly when the sensor operates at 150 °C and the cross-selectivity to water, sulfur dioxide, and carbon monoxide is minimized. Additionally, it is demonstrated that single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene.
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| 16. |
- Melios, C., et al.
(författare)
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Tuning epitaxial graphene sensitivity to water by hydrogen intercalation
- 2017
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 9:10, s. 3440-3448
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Tidskriftsartikel (refereegranskat)abstract
- The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.
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| 17. |
- Panchal, V., et al.
(författare)
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Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies
- 2016
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC (0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N-2, O-2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O-2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O-2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O-2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.
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| 18. |
- Panchal, V., et al.
(författare)
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Local electric field screening in bi-layer graphene devices
- 2014
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Ingår i: Frontiers of Physics. - : Frontiers Media SA. - 2296-424X. ; 2, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- © 2014 Panchal, Giusca, Lartsev, Yakimova and Kazakova. We present experimental studies of both local and macroscopic electrical effects in uniform single- (1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.
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| 19. |
- Panchal, V, et al.
(författare)
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Small epitaxial graphene devices for magnetosensing applications
- 2012
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 111:7, s. 07E509-
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Tidskriftsartikel (refereegranskat)abstract
- Hall sensors with the width range from 0.5 to 20.0 mu m have been fabricated out of a monolayer graphene epitaxially grown on SiC. The sensors have been studied at room temperature using transport and noise spectrum measurements. The minimum detectable field of a typical 10-mu m graphene sensor is approximate to 2.5 mu T/root Hz, making them comparable with state of the art semiconductor devices of the same size and carrier concentration and superior to devices made of CVD graphene. Relatively high resistance significantly restricts performance of the smallest 500-nm devices. Carrier mobility is strongly size dependent, signifying importance of both intrinsic and extrinsic factors in the optimization of the device performance.
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| 20. |
- Panchal, V., et al.
(författare)
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Surface potential variations in epitaxial graphene devices investigated by Electrostatic Force Spectroscopy
- 2012
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Ingår i: Proceedings of the IEEE Conference on Nanotechnology. - : IEEE. - 1944-9399 .- 1944-9380. - 9781467321983
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Konferensbidrag (refereegranskat)abstract
- Electrostatic Force Spectroscopy and Scanning Kelvin Probe Microscopy techniques are used to study the performance of side-gated Hall devices made of epitaxial graphene on 4H-SiC(0001). Electrostatic Force Spectroscopy is a novel method which allows quantitative surface potential measurements with high spatial resolution. Using these techniques, we calibrate work function of the metal coated tip and define the work functions for single and double-layer graphene. We also show that the use of moderate strength electrical fields in the side-gate geometry does not notably change the performance of the device.
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| 21. |
- Panchal, V., et al.
(författare)
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Visualisation of edge effects in side-gated graphene nanodevices
- 2014
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of similar to 60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene.
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| 22. |
- Schier, P., et al.
(författare)
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European Research on Magnetic Nanoparticles for Biomedical Applications : Standardisation Aspects
- 2020
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Ingår i: 21st Polish Conference on Biocybernetics and Biomedical Engineering, PCBBE 2019. - Cham : Springer Verlag. - 9783030298845 ; , s. 316-326
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Konferensbidrag (refereegranskat)abstract
- Magnetic nanoparticles have many applications in biomedicine and other technical areas. Despite their huge economic impact, there are no standardised procedures available to measure their basic magnetic properties. The International Organization for Standardization is working on a series of documents on the definition of characteristics of magnetic nanomaterials. We review previous and ongoing European research projects on characteristics of magnetic nanoparticles and present results of an online survey among European researchers.
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| 23. |
- Tzalenchuk, A.Y., et al.
(författare)
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Graphene and the universality of the quantum Hall effect
- 2013
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Ingår i: Proceedings of the International School of Physics "Enrico Fermi". - 1879-8195 .- 0074-784X. - 9781614993254 ; 185, s. 323-350
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Konferensbidrag (refereegranskat)abstract
- The quantum Hall effect allows the standard for resistance to be defined in terms of the elementary charge and Planck's constant alone. The effect comprises the quantization of the Hall resistance in two-dimensional electron systems in rational fractions of RK=h/e2=25812.8074434(84) Ω (Mohr P. J. et al., Rev. Mod. Phys., 84 (2012) 1527), 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. In this lecture we show that graphene - a single layer of carbon atoms - beats these well-established semiconductor materials as the system of choice for the realisation of the quantum resistance standard. Here we shall briefly describe graphene technology, discuss the structure and electronic properties of graphene, including the unconventional quantum Hall effect and then present in detail the route, which led to the most precise quantum Hall resistance universality test ever performed. © Società Italiana di Fisica.
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| 24. |
- Tzalenchuk, A.Y., et al.
(författare)
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Towards a quantum resistance standard based on epitaxial graphene
- 2010
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Ingår i: Nature Nanotechnology. - 1748-3387 .- 1748-3395. ; 5:3, s. 186-189
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Tidskriftsartikel (refereegranskat)abstract
- The quantum Hall effect(1) 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) = 25 812.807 557(18) Omega, the resistance quantum(2). 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(3-5). Graphene should, in principle, be an ideal material for a quantum resistance standard(6), because it is inherently two-dimensional and its discrete electron energy levels in a magnetic field (the Landau levels(7)) are widely spaced. However, the precisions demonstrated so far have been lower than one part per million(8). 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.
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| 25. |
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