A model for describing structural pointlike defects in nanoscaled ferromagnetic materials is presented. Its details are explicitly developed whenever interacting with a vortex-like state comprised in a thin nanodisk. Among others, our model yields results for the vortex equilibrium position under the influence of several defects along with an external magnetic field in good qualitative agreement with experiments. We also discuss how such defects may affect the vortex motion, like its gyrotropic oscillation and dynamical polarization reversal.
We theoretically study the effect of a material defect (material void) on switching errors associated with magneto-elastic switching of magnetization in elliptical magnetostrictive nanomagnets having in-plane magnetic anisotropy. We find that the error probability increases significantly in the presence of the defect, indicating that magneto-elastic switching is particularly vulnerable to material imperfections. Curiously, there is a critical stress value that gives the lowest error probability in both defect-free and defective nanomagnets. The critical stress is much higher in defective nanom…
We report here the results of micromagnetic simulations of square artificial spin ice (ASI) systems with defects. The defects are introduced by misaligning of a nanomagnet at the vertex. In these defective systems, we are able to stabilize emergent monopole-like state by applying a small external field. We observe a systematic change of dipolar energies of the systems with varying misalignment angle. The fields at which the emergent monopoles are created vary linearly with the dipolar energies of the systems. Our results clearly show that the magnetization reversal of the ASI systems is intric…
We discuss elementary topological defects in soft magnetic nanoparticles in the thin-film geometry. In the limit dominated by magnetostatic forces the low-energy defects are vortices (winding number n = +1), cross ties (n = -1), and edge defects with n = -1/2. We obtain topological constraints on the possible composition of domain walls. The simplest domain wall in this regime is composed of two -1/2 edge defects and a vortex, in accordance with observations and numerics.
Nanomagnets with small shape anisotropy energy barriers on the order of the thermal energy have unstable magnetization that fluctuates randomly in time. They have recently emerged as promising hardware platforms for stochastic computing and machine learning because the random magnetization states can be harnessed for probabilistic bits. Here, we have studied how the statistics of the magnetization fluctuations (e.g. the power spectral density) is affected by (i) moderate variations in the barrier height of the nanomagnet and (ii) the presence of structural defects, in order to assess how robus…
Much attention has been focused on the design of low barrier nanomagnets (LBM), whose magnetizations vary randomly in time owing to thermal noise, for use in binary stochastic neurons (BSN) which are hardware accelerators for machine learning. The performance of BSNs depend on two important parameters: the correlation time associated with the random magnetization dynamics in a LBM, and the spin-polarized pinning current which stabilizes the magnetization of a LBM in a chosen direction within a chosen time. Here, we show that common fabrication defects in LBMs make these two parameters unpredic…
In this theoretical investigation we address the effect of defects on thermal excitations in square-lattice dipolar arrays. The geometry of the nanomagnets, adopted from recent experiments [A. Farhan \textit{et al.}, Nature Phys.\ \textbf{9} (2013) 375], allows for thermal activation at room temperature. It turns out that excitations can efficiently be pinned at defects. Furthermore, ferromagnetic strings of nanomagnets that connect a pair of defects can be produced; their lengths are closely related to the distance of the defects and the topology of the magnetic ground-state configuration. We…
We have theoretically studied how resonant spin wave modes in an elliptical nanomagnet are affected by fabrication defects, such as small local thickness variations. Our results indicate that defects of this nature, which can easily result from the fabrication process, or are sometimes deliberately introduced during the fabrication process, will significantly alter the frequencies, magnetic field dependence of the frequencies, and the power and phase profiles of the resonant spin wave modes. They can also spawn new resonant modes and quench existing ones. All this has important ramifications f…
Magneto-elastic (straintronic) switching of bistable magnetostrictive nanomagnets is an extremely energy-efficient switching methodology for (magnetic) binary switches that has recently attracted widespread attention because of its potential application in ultra-low-power digital computing hardware. Unfortunately, this modality of switching is also error very prone at room temperature. Theoretical studies of switching error probability of magneto-elastic switches have predicted probabilities ranging from 10E-8-10E-3 at room temperature for ideal, defect-free nanomagnets, but experiments with r…
We discuss the structure of the so-called "vortex" domain walls in soft magnetic nanoparticles. A wall of this kind is a composite object consisting of three elementary topological defects: two edge defects with winding numbers -1/2 and a vortex with a winding number +1 between them. We provide a qualitative model accounting for the energetics of such a domain wall.
Magnetodynamical properties of nanomagnets are affected by the demagnetizing fields created by the same nanoelements. In addition, magnetocrystalline anisotropy produces an effective field that also contributes to the spin dynamics. In this article we show how the dimensions of magnetic elements can be used to balance crystalline and shape anisotropies, and that this can be used to tailor the magnetodynamic properties. We study ferromagnetic ellipses patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50 x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonan…
We investigate spin dynamics of artificial spin ice (ASI) where topological defects confine magnon modes in Ni$_{81}$Fe$_{19}$ nanomagnets arranged on an interconnected kagome lattice. Brillouin light scattering microscopy performed on magnetically disordered states exhibit a series of magnon resonances which depend on topological defect configurations detected by magnetic force microscopy. Nanomagnets on a Dirac string and between a monopole-antimonopole pair show pronounced modifications in magnon frequencies both in experiments and simulations. Our work is key for the creation and annihilat…
Nanomagnets form the building blocks for a gamut of miniaturized energy-efficient devices including data storage, memory, wave-based computing, sensors and biomedical devices. They also offer a span of exotic phenomena and stern challenges. The progress in the magnetization dynamics of single nanomagnets and one- and two-dimensional arrays of nanostructures in the form of dots, antidots, nanoparticles, binary and bicomponent structures and patterned multilayers have been presented. Progress in unconventional and new structures like artificial spin ice and three-dimensional nanomagnets and spin…
Defect energy formation, lattice distortions and electronic structure of cubic In2O3 with Sn, Ga and O impurities were theoretically investigated using density functional theory. Different types of point defects, consisting of 1 to 4 atoms of Sn, Ga and O in both substitutional and interstitial (structural vacancy) positions, were examined. It was demonstrated, that formation of substitutional Ga and Sn defects are spontaneous, while formation of interstitial defects requires an activation energy. The donor-like behavior of interstitial Ga defects with splitting of conduction band into two sub…
Using density functional theory and non-equilibrium Greens function technique, we performed theoretical investigations on the structural and transport properties of zigzag silicene nanoribbons with Stone-Wales defect. The calculated formation energy is significantly lower than that of graphene and silicene, which implies the high stability of such defect in SiNRs. Negative differential resistance can be observed within certain bias voltage range in both perfect and SW defected SiNRs. In order to elucidate the mechanism the NDR behavior,the transmission spectra and molecular projected self-cons…
In this work, we have systematically studied structural, electronic and magnetic properties of atomic scale defects in 2D transition metal dichalcogenides MX$_2$, (M = Mo and W; X = S, Se and Te) by density functional theory. Various types of defects, e.g., X vacancy, X interstitial, M vacancy, M interstitial, MX and XX double vacancies have been considered. It has been found that the X interstitial has the lowest formation energy ($\sim$ 1 eV) for all the systems in the X--rich condition whereas for M--rich condition, X vacancy has the lowest formation energy except for \ce{MTe2} systems. Bot…
We study the band dispersion of graphene with randomly distributed structural defects using two complementary methods, exact diagonalization of the tight-binding Hamiltonian and implementing a self-consistent T matrix approximation. We identify three distinct types of impurities resulting in qualitatively different spectra in the vicinity of the Dirac point. First, resonant impurities, such as vacancies or 585 defects, lead to stretching of the spectrum at the Dirac point with a finite density of localized states. This type of spectrum has been observed in epitaxial graphene by photoemission s…
The influence of intrinsic defects of 1T-TaS2 on charge density waves (CDW) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to the single CDW site, with single exception which effectively behaves as a dopant, leading to band bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved by lo…
Direct observation of vortex states in an antiferromagnetic layer have been recently reported [Wu, et al, Nature Phys. 7, 303 (2011)]. In contrast to their analogues in ferromagnetic systems, namely in nanomagnets, the vortex core of antiferromagnets are not expected (and have not been observed) to present gyrotropic or any other remarkable dynamics, even when external fields are applied. Using simulated annealing and spin dynamics techniques we have been able to describe a number of properties of such a vortex state. Besides of being in agreement with reported results, our results also indica…
Sn$_{0.97-y}$Co$_{0.03}$Ni$_{y}$O$_{2}$ (0 $\leq y \leq$ 0.04) nanocrystals, with average crystallite size in the range of 7.3 nm ($y$=0.00) to 5.6 nm ($y$=0.04), have been synthesized using pH-controlled chemical co-precipitation technique. The non-stoichiometric Sn related defects and the O related stoichiometric Frenkel defects arising in the nanocrystals because of co-doping have been identified and their effect on the structural and optical properties of the nanocrystals have been extensively studied. It has been observed, using XPS that on increasing the Ni co-doping concentration ($y$),…
The potential energy surface (PES) of interlayer interaction of twisted bilayer graphene with vacancies in one of the layers is investigated via density functional theory (DFT) calculations with van der Waals corrections. These calculations give a non-negligible magnitude of PES corrugation of 28 meV per vacancy and barriers for relative sliding of the layers of 7 - 8 meV per vacancy for the moiré pattern with coprime indices (2,1) (twist angle 21.8$^\circ$). At the same time, using the semiempirical potential fitted to the DFT results, we confirm that twisted bilayer graphene without defects …
The electrical and chemical properties of carbon nanotubes vary significantly with different chirality and diameter, making the experimental determination of these structural properties important. Here, we show that the vibrational density of states (VDOS) contains information on the structure of carbon nanotubes, particularly at low frequencies. We show that the diameter and chirality of the nanotubes can be determined from the characteristic low frequency $L$ and $L'$ modes in the VDOS. For zigzag nanotubes, the $L$ peak splits into two peaks giving rise to another low energy $L"$ peak. The …
An unusual variety of impact-related diamond from the Popigai impact structure - yakutites - is characterized by complementary methods including optical microscopy, X-ray diffraction, radiography and tomography, infra-red, Raman and luminescence spectroscopy providing structural information at widely different scales. It is shown that relatively large graphite aggregates may be transformed to diamond with preservation of many morphological features. Spectroscopic and X-ray diffraction data indicate that the yakutite matrix represents bulk nanocrystalline diamond. For the first time, features o…
We propose an extensive report on the simulation of electronic transport in 2D graphene in presence of structural defects. Amongst the large variety of such defects in sp$^2$ carbon-based materials, we focus on the Stone-Wales defect and on two divacancy-type reconstructed defects. First, based on ab initio calculations, a tight-binding model is derived to describe the electronic structure of these defects. Then, semiclassical transport properties including the elastic mean free paths, mobilities and conductivities are computed using an order-N real-space Kubo-Greenwood method. A plateau of mi…
Quantum transport properties of disordered graphene with structural defects (Stone-Wales and divacancies) are investigated using a realistic π-π* tight-binding model elaborated from ab initio calculations. Mean free paths and semiclassical conductivities are then computed as a function of the nature and density of defects (using an order-N real-space Kubo-Greenwood method). By increasing of the defect density, the decay of the semiclassical conductivities is predicted to saturate to a minimum value of 4e^2/πh over a large range (plateau) of carrier density (> 0.5 10^{14}cm^{-2}). Additionally,…
Recently experiments showed that nodal structural defects are readily formed in the synthesis of single-walled carbon nanotubes (SWNTs) and consequently, SWNTs are likely to deviate from well-defined seamless tubular structures. Here, using graphene-helix growth model, we describe structural details of feasible nodal defects in SWNTs and investigate how mechanical and electronic properties of SWNTs would change in the presence of them using computational methods. Surprisingly atomistic simulations of SWNTs with nodal defects show excellent agreement with previous structural, tensile, and ball-…
We discuss memory effects in the conductance of hopping insulators due to slow rearrangements of structural defects leading to formation of polarons close to the electron hopping states. An abrupt change in the gate voltage and corresponding shift of the chemical potential change populations of the hopping sites, which then slowly relax due to rearrangements of structural defects. As a result, the density of hopping states becomes time dependent on a scale relevant to rearrangement of the structural defects leading to the excess time dependent conductivity.
Stone-Wales (SW) defects are favorably existed in graphenelike materials with honeycomb lattice structure and potentially employed to change the electronic properties in band engineering. In this paper, we investigate structural and electronic properties of SW defects in bulk silicene and its nanoribbons as a function of their concentration using the methods of periodic boundary conditions with first-principles calculations. We first calculate the formation energy, structural properties, and electronic band structures of SW defects in bulk silicene, with dependence on the concentration of SW d…
We report on an alternative route based on nanomechanical folding induced by AFM tip to obtain weakly interacting multi-layer graphene (wi-MLG) from a chemical vapor deposition (CVD) grown single-layer graphene (SLG). The tip first cuts, then pushes and folds graphene during zigzag movements. The pushed graphene has been analyzed using various Raman microscopy plots: $A_D /A_G \times E_L{}^4$ vs $Γ_G$, $ω_{2D}$ vs $Γ_{2D}$, $Γ_{2D}$ vs $Γ_G$, $ω_{2D+/-}$ vs $Γ_{2D+/-}$, and $A_{2D-}/A_{2D+}$ vs $A_{2D}/A_G$. We show that the SLG in plane properties are maintained under the folding process and …
Laser writing with ultrashort pulses provides a potential route for the manufacture of three-dimensional wires, waveguides and defects within diamond. We present a transmission electron microscopy (TEM) study of the intrinsic structure of the laser modifications and reveal a complex distribution of defects. Electron energy loss spectroscopy (EELS) indicates that the majority of the irradiated region remains as $sp^3$ bonded diamond. Electrically-conductive paths are attributed to the formation of multiple nano-scale, $sp^2$-bonded graphitic wires and a network of strain-relieving micro-cracks.
We study the properties of localized vibrational modes associated with structural defects in a sheet of graphene. For the example of the Stone-Wales defects, one- and two-atom vacancies, many-atom linear vacancies, and adatoms in a honeycomb lattice, we demonstrate that the local defect modes are characterized by stable oscillations with the frequencies lying outside the linear frequency bands of an ideal graphene. In the frequency spectral density of thermal oscillations, such localized defect modes lead to the additional peaks from the right side of the frequency band of the ideal sheet of g…
We use laser-cooled ion Coulomb crystals in the well-controlled environment of a harmonic radiofrequency ion trap to investigate phase transitions and defect formation. Topological defects in ion Coulomb crystals (kinks) have been recently proposed for studies of nonlinear physics with solitons and as carriers of quantum information. Defects form when a symmetry breaking phase transition is crossed nonadiabatically. For a second order phase transition, the Kibble-Zurek mechanism predicts that the formation of these defects follows a power law scaling in the rate of the transition. We demonstra…
The chapter combines analytical (statistical-thermodynamic and kinetic) with numerical (Kubo-Greenwood-formalism-based) approaches used to ascertain an influence of the configurations of point (impurities, vacancies) and line (grain boundaries, atomic steps) defects on the charge transport in graphene. Possible substitutional and interstitial graphene-based superstructures are predicted and described. The arrangements of dopants over sites or interstices related with interatomic-interaction energies governing the configurations of impurities. Depending on whether the interatomic interactions a…
We provide a thorough study of a carbon divacancy, a fundamental but almost unexplored point defect in graphene. Low temperature scanning tunneling microscopy (STM) imaging of irradiated graphene on different substrates enabled us to identify a common two-fold symmetry point defect. Our first principles calculations reveal that the structure of this type of defect accommodates two adjacent missing atoms in a rearranged atomic network formed by two pentagons and one octagon, with no dangling bonds. Scanning tunneling spectroscopy (STS) measurements on divacancies generated in nearly ideal graph…
Thermoelectric power factors of defective scandium nitride nanostructures from first principles
IT4Innovations, VŠB – Technical University of Ostrava（俄斯特拉发理工大学 IT4 创新中心）；Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University（查理大学数学与物理学院凝聚态物理系）
Ruthenium dioxide (RuO$_2$) is an important metal widely used in nanoelectronic devices. It plays indispensable roles in the applications as catalyst and supercapacitors. A good understanding of the origin of the flicker or 1/$f$ noise in RuO$_2$ will advance the design and efficiency of these applications. We demonstrate in a series of sputtered RuO$_2$ polycrystalline films that the 1/$f$ noise originates from fluctuating oxygen vacancies which act as dynamical structural defects, i.e., moving scattering centers. Reducing the number of oxygen vacancies by adjusting thermal annealing conditio…
