Browsing by Subject "Magnetic moments"

Now showing 1 - 10 of 10

Open Access
The effect of gadolinium doping on the structural, magnetic and photoluminescence properties of electrospun bismuth ferrite nanofibers
(Elsevier Ltd, 2015) George Philip G.; Senthamizhan, A.; Srinivasan Natarajan, T.; Chandrasekaran G.; Annal Therese H.
Gadolinium (Gd) doped Bismuth ferrite (BFO) nanofibers (Bi1-xGdxFeO3 (x=0.0, 0.05, 0.10, 0.15 and 0.20)) were synthesized via electrospinning. Scanning Electron Microscope (SEM) analysis showed that the diameter of the nanofibers ranged from 150 to 250 nm. X-Ray Diffraction (XRD) analysis revealed a structural phase transition with varying 'x', the compositions with x≤0.10 have crystal structures with space group R3c, while the compositions with x > 0.10 have crystal structures with space group Pnma. Vibrating Sample Magnetometer (VSM) analysis exhibited the weak ferromagnetic nature of the BFO nanofibers. However an increase in the saturated magnetic moment with increase in Gd dopant concentration was observed. The Photoluminescence (PL) spectra of the Bi:1-x :x nanofibers show enhanced Near Band Emission (NBE) intensity at x=0.10 due to the passivation of oxygen vacancies by Gd doping. © 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Open Access
Effects of silicon and germanium adsorbed on graphene
(A I P Publishing LLC, 2010) Aktürk, E.; Ataca, C.; Çıracı, Salim
Based on the first-principles plane wave calculations, we studied the adsorption of Si and Geon graphene. We found that these atoms are bound to graphene at the bridge site with a significant binding energy, while many other atoms are bound at the hollow site above the center of hexagon. It is remarkable that these adatoms may induce important changes in the electronic structure of graphene even at low coverage. Semimetallic graphene becomes metallized and attains a magnetic moment. The combination of adatom orbitals with those of ππ- and π∗π∗-states of bare graphene is found responsible for these effects.
Open Access
Electronic and magnetic properties of zinc blende half-metal superlattices
(A I P Publishing LLC, 2004) Fong, C. Y.; Qian, M. C.; Pask, J. E.; Yang, L. H.; Dag, S.
Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0mB and 14.0mB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.
Open Access
Energetics and Electronic Structures of Individual Atoms Adsorbed on Carbon Nanotubes
(American Chemical Society, 2004) Durgun, Engin; Dag, S.; Çıracı, Salim; Gülseren, O.
The adsorption of individual atoms on the semiconducting and metallic single-walled carbon nanotubes (SWNT) has been investigated by using the first principles pseudopotential plane wave method within density functional theory. The stable adsorption geometries and binding energies have been determined for a large number of foreign atoms ranging from alkali and simple metals to the transition metals and group IV elements. We have found that the character of the bonding and associated physical properties strongly depends on the type of adsorbed atoms, in particular, on their valence electron structure. Our results indicate that the properties of SWNTs can be modified by the adsorbed foreign atoms. Although the atoms of good conducting metals, such as Zn, Cu, Ag, and Au, form very weak bonds, transition-metal atoms such as Ti, Sc, Nb, and Ta and group IV elements C and Si are adsorbed with a relatively high binding energy. Owing to the curvature effect, these binding energies are larger than the binding energies of the same atoms on the graphite surface. We have showed that the adatom carbon can form strong and directional bonds between two SWNTs. These connects can be used to produce nanotube networks or grids. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as ID nanomagnets or nanoconductors, conducting connects, and so forth.
Open Access
Half-metallic silicon nanowires: First-principles calculations
(American Physical Society, 2007) Durgun, Engin; Çakır, D.; Akman, N.; Çıracı, Salim
From first-principles calculations, we predict that specific transition metal (TM) atom-adsorbed silicon nanowires have a half-metallic ground state. They are insulators for one spin direction, but show metallic properties for the opposite spin direction. At high coverage of TM atoms, ferromagnetic silicon nanowires become metallic for both spin directions with high magnetic moment and may have also significant spin polarization at the Fermi level. The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Present results are not only of scientific interest, but also can initiate new research on spintronic applications of silicon nanowires. © 2007 The American Physical Society.
Open Access
Highly monodisperse low-magnetization magnetite nanocubes as simultaneous T1–T2 MRI contrast agents
(Royal Society of Chemistry, 2015) Sharma, V. K.; Alipour, A.; Soran-Erdem, Z.; Aykut, Z. G.; Demir, Hilmi Volkan
We report the first study of highly monodisperse and crystalline iron oxide nanocubes with sub-nm controlled size distribution (9.7 ± 0.5 nm in size) that achieve simultaneous contrast enhancement in both T1- and T2-weighted magnetic resonance imaging (MRI). Here, we confirmed the magnetite structure of iron oxide nanocubes by X-ray diffraction (XRD), selected area electron diffraction (SAED) pattern, optical absorption and Fourier transformed infrared (FT-IR) spectra. These magnetite nanocubes exhibit superparamagnetic and paramagnetic behavior simultaneously by virtue of their finely controlled shape and size. The magnetic measurements reveal that the magnetic moment values are favorably much lower because of the small size and cubic shape of the nanoparticles, which results in an enhanced spin canting effect. As a proof-of-concept demonstration, we showed their potential as dual contrast agents for both T1- and T2-weighted MRI via phantom studies, in vivo imaging and relaxivity measurements. Therefore, these low-magnetization magnetite nanocubes, while being non-toxic and bio-compatible, hold great promise as excellent dual-mode T1 and T2 contrast agents for MRI. © 2014 The Royal Society of Chemistry.
Open Access
Interaction of adatoms and molecules with single-layer arsenene phases
(American Chemical Society, 2016-06) Ersan, F.; Aktürk, E.; Çıracı, Salim
Recent studies have shown that arsenic can form single-layer phases in buckled honeycomb as well as symmetric washboard structures, named as arsenene. These structures are stable even in freestanding form and are nonmagnetic semiconductors in the energy range which is suitable for various electronic applications. In this study we investigated the adsorption of selected adatoms (H, Li, B, C, N, O, Al, Si, P, Cl, Ti, Ga, Ge, As, Se, and Sb) and physisorption of molecules (H2, O2, and H2O) to these two arsene phases. Since the interaction of these adspecies with arsenene are studied using large supercells, the coupling between adspecies is minimized, and hence our results can be interpreted to mimic the effects of isolated adatom or physisorbed molecule. It is found that the adatoms form strong chemisorption bonds and hence modify the atomic structure and physical properties locally. Some of the adatoms give rise to significant local reconstruction of the atomic structure. Electronic states of some adatoms become spin polarized and attain net magnetic moments; they may even display half-metallic character at high coverage. A majority of adsorbed atoms give rise to localized states in the fundamental band gap. We showed that the interactions between H2, O2, and H2O molecules and single-layer arsenene are rather weak and do not cause any significant changes in the physical properties of these molecules, as well as those of arsenene phases. However, some of these molecules can be dissociated at the edges of the flakes of arsenene structures; their constituents are adsorbed to the edge atoms and cause local reconstructions.
Open Access
Magnetization of graphane by dehydrogenation
(AIP Publishing, 2009) Şahin, H.; Ataca, C.; Çıracı, Salim
Using first principles calculations, we show that each hydrogen vacancy created at graphane surface results in a local unpaired spin. For domains of hydrogen vacancies the situation is, however, complex and depends on the size and geometry of domains, as well as whether the domains are single or double sided. In single-sided domains, hydrogen atoms at the other side are relocated to pair the spins of adjacent carbon atoms by forming ππ-bonds. Owing to the different characters of exchange coupling in different ranges and interplay between unpaired spin and the binding geometry of hydrogen, vacancy domains can attain sizable net magnetic moments.
Open Access
Oscillatory exchange coupling in magnetic molecules
(IOP Publishing, 2007) Sevincli, H.; Senger, R. T.; Durgun, Engin; Çıracı, Salim
Recently, first-principles calculations based on the spin-dependent density functional theory (DFT) have revealed that the magnetic ground state of a finite linear carbon chain capped by two transition metal (TM) atoms alternates between ferromagnetic and antiferromagnetic configurations depending on the number of carbon atoms. The character of indirect exchange coupling in this nanoscale, quasi-zero-dimensional system is different from those analogous extended structures consisting of magnetic layers separated by a non-magnetic spacer (or magnetic impurities in a non-magnetic host material) and a formulation based on an atomic picture is needed. We present a tight-binding model which provides a theoretical framework to the underlying mechanism of the exchange coupling in molecular structures. The model calculations are capable of reproducing the essential features of the DFT results for the indirect exchange coupling and the atomic magnetic moments in the TM-Cn-TM structures as functions of the number of carbon atoms. In nanostructures consisting of a few atoms the concepts of extended wavefunctions and the band theory lose their validity, and hence the oscillatory exchange coupling turns out to be a consequence of quantum interference effects due to the spin-dependent onsite and hopping energies. © IOP Publishing Ltd.
Open Access
Spintronic properties of zigzag-edged triangular graphene flakes
(AIP Publishing LLC, 2010) Şahin, H.; Senger, R. T.; Çıracı, Salim
We investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for specific flakes. Geometry and size dependent transport properties of graphene flakes may be crucial for spintronic nanodevice applications.