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- Araujo, Carlos Moyses, et al.
(författare)
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Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites
- 2014
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 4, s. 4686-
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Tidskriftsartikel (refereegranskat)abstract
- We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.
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| 5. |
- Glans, Per-Anders
(författare)
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Electronic structure of some SiC and Be surfaces
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon carbide (SiC) is a wide-bandgap semiconductor whose properties make it a good candidate for high-power, high-voltage, high-temperature and highfrequency devices. Beryllium (Be) is a metal with some unusual electronic properties.In this thesis synchrotron radiation and photoemission have been utilized as tools to extract information from both the core levels and valence band states of different SiC and Be surfaces.Several earlier SiC investigations have dealt with the silicon face and not so many with the carbon face. In paper I the carbon faced 6H-SiC(0001̅ ) surface is studied. The C 1s and Si 2p core levels are characterized for different photonenergies and after annealing to different temperatures. A 3 x 3 reconstruction developed after annealing to 950° C. After heating to 1200° C, spectra showed evidence of surface graphitization.In paper II, an investigation of the SiO2 /SiC-interface using high energy photoemission spectroscopy is reported. A low defect density in the interface is crucial for the manufacturing of power MOSFETs. One limiting factor for the formation of a high quality oxide is thought to be a carbon containing by-product at the interface. In this investigation, no carbon could be detected at the interface, but instead a 3 Å layer of graphite like carbon was found on top of the oxide.Paper III and the first part of paper IV deals with the surface state on the √3 x √3 reconstructed surfaces of 6H-SiC(0001) and 3C-SiC(111) respectively. In paper III the sensitivity of the surface state to oxygen and Na exposures was investigated and the symmetry and dispersion of the surface state was determined in both paper III and IV.Theoreticians have proposed a Mott-Hubbard model to explain the semiconducting behavior of this reconstruction, but this would also give a surface state with the periodicity of the √3 x √3 Surface Brillouin Zone. This periodicity has not been observed for either the 6H-SiC(0001) or the 3C-SiC(111) crystals.In the second part of paper IV, an investigation similar to the one in paper I has been done, but on the 3C-SiC(111) and 3C-SiC(1̅1̅1̅ ) surfaces. Similar results as for the 6H and 4H polytypes were found for both the Si-face and the C-face.The mean values of the surface core level shifts (SCLS) were lower for 3C than for 4H and 6H, but when taking the error bars into account the shifts cannot be said to be different. For both the 6H-SiC(0001̅ ) and the 3C-SiC(1̅1̅1̅ ) the C 1s spectra for the 3 x 3 reconstruction indicate the possibility of a quite complex atomic structure. Paper V and VI deal with two different Be surfaces. Paper V reports a fourthlayer core level shift on Be(0001), which is unmatched by any other metal. In paper VI experimental evidence for a model for the assignment of the SCLSs on Be(101̅ 0) earlier proposed by theory is reported. The most intuitive way is to assign the largest shift with the first layer, the second largest shift with the second layer and so forth. This was also supported by the intensity relations of the components. Other groups have later proposed that the component with the largest shift is from the second layer, the second largest shift is from the first layer and that the third largest shift is from the third layer. In paper VI the intensity ratios calculated using a multiple scattering method show that the component with the largest shift is connected to the second layer, the second largest shift to layers three and four and the smallest shift to the first layer.
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| 6. |
- Kapilashrami, Mukes, et al.
(författare)
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Understanding the magnetic interaction between intrinsic defects and impurity ions in room-temperature ferromagnetic Mg1-xFexO thin films
- 2016
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 28:15
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the nature and characteristics of the intrinsic defects and impurities in the dielectric barrier separating the ferromagnetic electrodes in a magnetic tunneling junction is of great importance for understanding the often observed 'barrier-breakdown' therein. In this connection, we present herein systematic experimental (SQUID and synchrotron-radiation-based x-ray absorption spectroscopy) and computational studies on the electronic and magnetic properties of Mg1-xFexO thin films. Our studies reveal: (i) defect aggregates comprised of basic and trimer units (Fe impurity coupled to 1 or 2 Mg vacancies) and (ii) existence of two competing magnetic orders, defect- and dopant-induced, with spin densities aligning anti-parallel if the trimer is present in the oxide matrix. These findings open up new avenues for designing tunneling barriers with high endurance and tunneling effect upon tuning the concentration/distribution of the two magnetic orders.
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| 8. |
- Li, Xin, et al.
(författare)
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Effects of domain size on x-ray absorption spectra of boron nitride doped graphenes
- 2016
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 109:8
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Tidskriftsartikel (refereegranskat)abstract
- Doping is an efficient way to open the zero band gap of graphene. The control of the dopant domain size allows us to tailor the electronic structure and the properties of the graphene. We have studied the electronic structure of boron nitride doped graphenes with different domain sizes by simulating their near-edge X-ray absorption fine structure (NEXAFS) spectra at the N K-edge. Six different doping configurations (five quantum dot type and one phase-separated zigzag-edged type) were chosen, and N K-edge NEXAFS spectra were calculated with large truncated cluster models by using the density functional theory with hybrid functional and the equivalent core hole approximation. The opening of the band gap as a function of the domain size is revealed. We found that nitrogens in the dopant boundary contribute a weaker, red-shifted pi* peak in the spectra as compared to those in the dopant domain center. The shift is related to the fact that these interfacial nitrogens dominate the lowest conduction band of the system. Upon increasing the domain size, the ratio of interfacial atom decreases, which leads to a blue shift of the pi* peak in the total NEXAFS spectra. The spectral evolution agrees well with experiments measured at different BN-dopant concentrations and approaches to that of a pristine h-BN sheet.
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| 9. |
- Zhang, Hui, et al.
(författare)
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Comprehensive electronic structure characterization of pristine and nitrogen/phosphorus doped carbon nanocages
- 2016
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 103, s. 480-487
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structures of carbon nanocages (CNCs) and nitrogen/phosphorus doped carbon nanocages (N-CNCs/P-CNCs) have been studied by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant X-ray emission spectroscopy (RXES). The doping configurations for N/P dopants are identified from the experiments. The results have shown that there are three major doping configurations for nitrogen but only one doping configuration for phosphorus. The nitrogen doping reveals the complex coexistence of graphite-like, pyridine-like and pyrrole-like configurations that are proved by density functional theory (DFT) simulations, while the phosphorus doping presents only the "graphite-like" configuration. The different configuration profiles result in less atomic structure ordering of N-CNCs than that of P-CNCs. XAS spectra obtained from both surface and bulk sensitive detection suggest different chemical environments between the interior and shell for all types of nanocages. The electronic structure modifications show significant difference between nitrogen and phosphorus doping from the DOS calculations. (C) 2016 Elsevier Ltd. All rights reserved.
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