arXiv subjects

Biplab Sanyal

Publications and source records attributed to Biplab Sanyal.

At least 73 records · Page 4Linked to original sources

arXiv2014

Fe clusters (Fe$_n$, n=1-6) chemisorbed on vacancy defects in graphene: Stability, spin-dipole moment and magnetic anisotropy

In this work, we have studied the chemical and magnetic interactions of Fe$_n$ ; n=1-6 clusters with vacancy defects (monovacancy to correlated vacancies with six missing C atoms) in a graphene sheet by ab-initio density functional calculations combined with Hubbard U corrections for correlated Fe- d electrons. It is found that the vacancy formation energies are lowered in the presence of Fe, indicating an easier destruction of the graphene sheet. Due to strong chemical interactions between Fe clusters and vacancies, a complex distribution of magnetic moments appear on the distorted Fe clusters which results in reduced averaged magnetic moments compared to the free clusters. In addition to that, we have calculated spin-dipole moments and magnetic anisotropy energies. The calculated spin-dipole moments arising from anisotropic spin density distributions, vary between positive and negative values, yielding increased or decreased effective moments. Depending on the cluster geometry, the easy axis of magnetization of the Fe clusters shows in-plane or out-of-plane behavior.

cond-mat.mtrl-sci↗

arXiv2014

A real-space study of random extended defects in solids : application to disordered Stone-Wales defects in graphene

We propose here a first-principles, parameter free, real space method for the study of disordered extended defects in solids. We shall illustrate the power of the technique with an application to graphene sheets with randomly placed Stone-Wales defects and shall examine the signature of such random defects on the density of states as a function of their concentration. The technique is general enough to be applied to a whole class of systems with lattice translational symmetry broken not only locally but by extended defects and defect clusters. The real space approach will allow us to distinguish signatures of specific defects and defect clusters.

cond-mat.mtrl-sci↗

arXiv2013

Designing Fe Nanostructures at Graphene/h-BN Interfaces

Tailor-made magnetic nanostructures offer a variety of functionalities useful for technological applications. In this work, we explore the possibilities of realizing Fe nanostructures at the interfaces of 2D graphene and h-BN by ab initio density functional calculations. With the aid of ab initio Born-Oppenheimer molecular dynamics simulations and diffusion barriers calculated by nudged elastic band method, we find that (i) diffusion barriers of Fe on BN are much smaller than those on graphene, (ii) the Fe adatoms form clusters within a short time interval (~2.1 ps) and (iii) Fe clusters diffuse easily across the C-N interface but become immobile at the C-B interface. The calculated magnetic exchange coupling between Fe clusters at C-B interfaces varies non-monotonically as a function of the width of BN separating the graphene parts. One may envisage design of magnetic nanostructures at the C-B interface of 2D graphene/h-BN hybrids to realize interesting applications related to spintronics.

cond-mat.mtrl-sci↗

arXiv2013

Exchange parameters and adiabatic magnon energies from spin-spiral calculations

We present a method of extracting the exchange parameters of the classical Heisenberg model from first-principles calculations of spin-spiral total energies based on density functional theory. The exchange parameters of the transition-metal monoxides MnO and NiO are calculated and used to estimate magnetic properties such as transition temperatures and magnon energies. Furthermore we show how to relate the magnon energies directly to differences in spin-spiral total energies for systems containing an arbitrary number of magnetic sublattices. This provides a comparison between magnon energies using a finite number of exchange parameters and the infinite limit.

cond-mat.mtrl-sci↗

arXiv2013

A first principles study of the stability and mobility of defects in titanium carbide

We have performed density functional calculations of the formation energies of substitutional transition metal (TM) defects, C vacancies, and C interstitial defects in TiC. In addition we have evaluated the migration energy barriers for C atoms in the presence of TM impurities. We find that the solubility of TM impurities taken from the 3d TM series is low and only Sc and V impurities can be dissolved into TiC at equilibrium. In addition, we find that the migration energy barriers of C in TiC are greatly affected by the presence of TM impurities: The migration barriers are generally lower in the presence of impurities compared to pure TiC and show a clear dependence on the atomic size of the TM impurities. We propose that the mobility of C in TiC will be the highest in the presence of TM impurities from the middle of the 3d TM series.

cond-mat.mtrl-sci↗

arXiv2012

A microscopic model for ultrafast remagnetization dynamics

In this letter, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above Stoner-Curie temperature by a strong laser fluence. Combining first principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation and a three temperature model, we show the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macro spin moment that evolves non-monotonically.

cond-mat.mtrl-sci↗

arXiv2012

Manipulation and detection of spin state of Iron-Porphyrin by dedicated chemisorption on magnetic substrates

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this letter, we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via a dedicated chemisorption on magnetic substrates of different species and orientations. It is shown that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S=2) from a low spin state (S=1) is achieved. A ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Finally, we illustrate the possibility of detecting a change in the molecular spin state by x-ray magnetic circular dichroism, Raman spectroscopy and spin-polarized scanning tunneling microscopy.

cond-mat.mtrl-sci↗

arXiv2012

Functionalization of edge reconstructed graphene nanoribbons by H and Fe: A density functional study

In this paper, we have studied functionalization of 5-7 edge-reconstructed graphene nanoribbons by ab initio density functional calculations. Our studies show that hydrogenation at the reconstructed edges is favorable in contrast to the case of unreconstructed 6-6 zigzag edges, in agreement with previous theoretical results. Thermodynamical calculations reveal the relative stability of single and dihydrogenated edges under different temperatures and chemical potential of hydrogen gas. From phonon calculations, we find that the lowest optical phonon modes are hardened due to 5-7 edge reconstruction compared to the 6-6 unreconstructed hydrogenated edges. Finally, edge functionalization by Fe atoms reveals a dimerized Fe chain structure along the edges. The magnetic exchange coupling across the edges varies between ferromagnetic and antiferromagnetic ones with the variation of the width of the nanoribbons.

cond-mat.mes-hall↗

arXiv2012

A new first principles approach to calculate phonon spectra of disordered alloys

The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this letter, a new first-principles approach based on special quasi random structures and itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni$_{0.5}$Pt$_{0.5}$ alloy. The agreement between our results with the experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic impurities in graphane with dehydrogenated channels

We have investigated the electronic and magnetic response of a single Fe atom and a pair of interacting Fe atoms placed in patterned dehydrogenated channels in graphane within the framework of density functional theory. We have considered two channels: "armchair" and "zigzag" channels. Fully relaxed calculations have been carried out for three different channel widths. Our calculations reveal that the response to the magnetic impurities is very different for these two channels. We have also shown that one can stabilize magnetic impurities (Fe in the present case) along the channels of bare carbon atoms, giving rise to a magnetic insulator or a spin gapless semiconductor. Our calculations with spin-orbit coupling shows a large in-plane magnetic anisotropy energy for the case of the armchair channel. The magnetic exchange coupling between two Fe atoms placed in the semiconducting channel with an armchair edge is very weakly ferromagnetic whereas a fairly strong ferromagnetic coupling is observed for reasonable separations between Fe atoms in the zigzag-edged metallic channel with the coupling mediated by the bare carbon atoms. The possibility of realizing an ultrathin device with interesting magnetic properties is discussed.

cond-mat.mes-hall↗

arXiv2012

A first principles study of magnetism in Pd$_{3}$Fe under pressure

Recent experiments on Pd$_{3}$Fe intermetallics [Phys. Rev. Lett. 102, 237202 (2009)] have revealed that the system behaves like a classical invar alloy under high pressure. The experimental pressure-volume relation suggests an anomalous volume collapse and a substantial increase in bulk modulus around the pressure where invar behavior is observed. With the help of first-principles density functional theory based calculations, we have explored various magnetic phases (ferromagnetic, fully and partially disordered local moment, spin spiral) in order to understand the effect of pressure on magnetism. Our calculations reveal that the system does not undergo a transition from a ferromagnetic to a spin-disordered state as was thought to be the possible mechanism to explain the invar behavior of this system. We rather suggest that the anomaly in the system could possibly be due to the transition from a collinear state to non-collinear magnetic states upon the application of pressure.

cond-mat.mtrl-sci↗

arXiv2012

Graphene as a reversible spin manipulator of molecular magnets

One of the primary objectives in molecular nano-spintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this letter, we demonstrate by first principles density functional calculations, as well as second order perturbation thoery, that a strain induced change of the spin state, from S=1 $\to$ S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in a sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

cond-mat.mtrl-sci↗

arXiv2012

Ferrimagnetism, Antiferromagnetism and Magnetic Frustration in La$_{2-x}$Sr$_{x}$CuRuO$_{6}$ (0 <= x <= 1)

We studied structural and magnetic properties of a series of insulating double perovskite compounds, La$_{2-x}$Sr$_{x}$CuRuO$_{6}$ (0 $\le$ x $\le$ 1), representing doping via \textit{A}-site substitution. The end members La$_{2}$CuRuO$_{6}$ and LaSrCuRuO$_{6}$ form in monoclinic structure while the intermediate Sr doped compounds stabilise in triclinic structure. The Cu and Ru ions sit on alternate B-sites of the perovskite lattice with $\sim$15% anti-site defects in the undoped sample while the Sr doped samples show a tendency to higher ordering at B-sites. The undoped (x = 0) compound shows a ferrimagnetic-like behaviour at low temperatures. In surprising contrast to the usual expectation of an enhancement of ferromagnetic interaction on doping, an antiferromagnetic-like ground state is realized for all doped samples (x $>$ 0). Heat capacity measurements indicate the absence of any long range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest neighbour Ru -- O -- Cu 180$^\circ$ superexchange interaction and the next nearest neighbour Ru -- O -- O -- Ru 90$^\circ$ superexchange interaction as well as by the formation of anti-site defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for $x$ = 0 and $x$ = 1 support this interpretation.

cond-mat.str-el↗

arXiv2012

Effect of disorder on the electronic properties of graphene: a theoretical approach

In order to manipulate the properties of graphene, its very important to understand the electronic structure in presence of disorder. We investigate, within a tight-binding description, the effects of disorder in the on-site (diagonal disorder) term in the Hamiltonian as well as in the hopping integral (off-diagonal disorder) on the electronic dispersion and density of states by augmented space recursion method. Extrinsic off-diagonal disorder is shown to have dramatic effects on the two-dimensional Dirac-cone, including asymmetries in the band structures as well as the presence of discontinuous bands in certain limits. Disorder-induced broadening, related to the scattering length (or life-time) of electrons, is modified significantly with the increasing strength of disorder. We propose that our theory is suitable to study the effects of disorder in other 2D materials, e.g., a boron nitride monolayer.

cond-mat.mes-hall↗

arXiv2011

Unusual Structure and Magnetism in MnO Nanoclusters

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.

cond-mat.mes-hall↗

arXiv2011

A theoretical analysis of inertia-like switching in magnets: applications to a synthetic antiferromagnet

The magnetization dynamics of a synthetic antiferromagnet subject to a short magnetic field pulse, has been studied by using a combination of first-principles and atomistic spin dynamics simulations. We observe switching phenomena on the time scale of tens of picoseconds, and inertia-like behavior in the magnetization dynamics. We explain the latter in terms of a dynamic redistribution of magnetic energy from the applied field pulse to other possible energy terms, such as the exchange interaction and the magnetic anisotropy, without invoking concepts such as inertia of an antiferromagnetic vector. We also demonstrate that such dynamics can also be observed in a ferromagnetic material where the incident field pulse pumps energy to the magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2011

A route towards finding large magnetic anisotropy in nano-composites: application to a W$_{1-x}$Re$_x$/Fe multilayer

We suggest here a novel nano-laminate, 5[Fe]/2[W$_x$Re$_{1-x}$] (x=0.6-0.8), with enhanced magnetic hardness in combination with a large saturation moment. The calculated magnetic anisotropy of this material reaches values of 5.3-7.0 MJ/m$^3$, depending on alloying conditions. We also propose a recipe in how to identify other novel magnetic materials, such as nano-laminates and multilayers, with large magnetic anisotropy in combination with a high saturation moment.

cond-mat.mtrl-sci↗

arXiv2011

Carbon release by selective alloying of transition metal carbides

We have performed first principles density functional theory calculations on TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to Zn. The theory is accompanied with experimental investigations, both as regards materials synthesis as well as characterization. Our results show that by dissolving a metal with a weak ability to form carbides, the stability of the alloy is lowered and a driving force for the release of carbon from the carbide is created. During thin film growth of a metal carbide this effect will favor the formation of a nanocomposite with carbide grains in a carbon matrix. The choice of alloying elements as well as their concentrations will affect the relative amount of carbon in the carbide and in the carbon matrix. This can be used to design the structure of nanocomposites and their physical and chemical properties. One example of applications is as low-friction coatings. Of the materials studied, we suggest the late 3d transition metals as the most promising elements for this phenomenon, at least when alloying with TiC.

cond-mat.mtrl-sci↗