Fe Atomic Chains Research Articles

Electronic and Magnetic Properties of Building Blocks of Mn and Fe Atomic Chains on Nb(110).

We present results for the electronic and magnetic structure of Mn and Fe clusters on Nb(110) surface, focusing on building blocks of atomic chains as possible realizations of topological superconductivity. The magnetic ground states of the atomic dimers and most of the monatomic chains are determined by the nearest-neighbor isotropic interaction. To gain physical insight, the dependence on the crystallographic direction as well as on the atomic coordination number is analyzed via an orbital decomposition of this isotropic interaction based on the spin-cluster expansion and the difference in the local density of states between ferromagnetic and antiferromagnetic configurations. A spin-spiral ground state is obtained for Fe chains along the [] direction as a consequence of the frustration of the isotropic interactions. Here, a flat spin-spiral dispersion relation is identified, which can stabilize spin spirals with various wave vectors together with the magnetic anisotropy. This may lead to the observation of spin spirals of different wave vectors and chiralities in longer chains instead of a unique ground state.

Coupling of Yu-Shiba-Rusinov states in one-dimensional chains of Fe atoms on Nb(110)

The hybridization of Yu-Shiba-Rusinov states in dimers of magnetic impurities leads to molecular-like bonding and antibonding modes. In many-impurity systems, the interaction gives rise to YSR bands and can even result in the formation of a topologically non-trivial superconducting state, characterized by Majorana fermions at the edges of the system. To obtain a more detailed understanding of these interactions, we investigate the coupling of YSR states in short one-dimensional Fe chains on clean Nb(110). We observe a splitting of the single-atom YSR peaks into multiple states with even or odd spatial symmetry and identify a peculiar dependence of the even and odd states' energy position on the chain length.

Plasmon Excitations in Spin-Polarized Iron Atomic Chains: A Time-Dependent Density Functional Theory Study

By using the time-dependent density functional theory calculations, we study the plasmon excitations in Fe atomic chains with different numbers of atom and different Fe-Fe separations, which are now possible to fabricate experimentally. Because of the Fe valence 3d orbitals and the spin polarization, the excitations along the chain (longitudinal (L) mode) and perpendicular to the chain (transverse (T) modes) are found to be largely different from those in noble metal and alkali metal atomic chains. First, the strength of dipole response becomes weaker due to the more localized Fe 3d states; Second, the L mode is mixed with some multipole contribution; Third, the end mode of the T modes is absent.

Polarization of the Majorana quasiparticles in the Rashba chain

We demonstrate that the selective equal–spin Andreev reflection (SESAR) spectroscopy can be used in STM experiments to distinguish the zero–energy Majorana quasiparticles from the ordinary fermionic states of the Rashba chain. Such technique, designed for probing the p–wave superconductivity, could be applied to the intersite pairing of equal–spin electrons in the chain of magnetic Fe atoms deposited on the superconducting Pb substrate. Our calculations of the effective pairing amplitude for individual spin components imply the magnetically polarized Andreev conductance, which can be used to ‘filter’ the Majorana quasiparticles from the ordinary in–gap states, although the pure spin current (i.e., perfect polarization) is impossible.

Magnetic properties of a single iron atomic chain encapsulated in armchair carbon nanotubes: A Monte Carlo study

The magnetic properties have been investigated of FeCuxC1−x for a Fe atom chain wrapped in armchair (N,N) carbon nanotubes (N=4,6,8,10,12) diluted by Cu2+ ions using Monte Carlo simulations. The thermal total magnetization and magnetic susceptibility are found. The reduced transition temperatures of iron and carbon have been calculated for different N and the exchange interactions. The total magnetization is obtained for different exchange interactions and crystal field. The Magnetic hysteresis cycles are obtained for different N, the reduced temperatures and exchange interactions. The multiple magnetic hysteresis is found. This system shows it can be used as magnetic nanostructure possessing potential current and future applications in permanent magnetism, magnetic recording and spintronics.

High-resolution studies of the Majorana atomic chain platform

Ordered assemblies of magnetic atoms on the surface of conventional superconductors can be used to engineer topological superconducting phases and realize Majorana fermion quasiparticles (MQPs) in a condensed matter setting. Recent experiments have shown that chains of Fe atoms on Pb generically have the required electronic characteristics to form a 1D topological superconductor and have revealed spatially resolved signatures of localized MQPs at the ends of such chains. Here we report higher resolution measurements of the same atomic chain system performed using a dilution refrigerator scanning tunneling microscope (STM). With significantly better energy resolution than previous studies, we show that the zero bias peak (ZBP) in Fe chains has no detectable splitting from hybridization with other states. The measurements also reveal that the ZBP exhibits a distinctive 'double eye' spatial pattern on nanometer length scales. Theoretically we show that this is a general consequence of STM measurements of MQPs with substantial spectral weight in the superconducting substrate, a conclusion further supported by measurements of Pb overlayers deposited on top of the Fe chains. Finally, we report experiments performed with superconducting tips in search of the particle-hole symmetric MQP signature expected in such measurements.

Encapsulating “armchair” carbon nanotubes with “zigzag” chains of Fe atoms

Ab initio calculations of structural, electron, and magnetic properties of “armchair” carbon nanotubes (NT) encapsulated by a “zigzag” chain of Fe atoms Fe2@(n,n)m (m = 1, 2; n = 4, 5, 6, 7, 8, 9), are performed within the framework of the density functional theory. It is shown that optimizing the structure along the NT axis can significantly impact the binding energy of the NT and the Fe atom chain. It follows from the calculations that Fe2@(5,5) is the most stable of all the investigated encapsulated nanotubes. A two-fold decrease in the concentration of Fe in an encapsulated NT converts the system from exothermic to endothermic (Fe2@(5,5)m) and vice versa (Fe2@(6,6)m)). For large radii of an encapsulated NT (>4.13 Å) the binding energy of the NT and the Fe atom chain goes to zero, and the magnetic moments of the Fe atoms and the deviation of the Fe atoms from the NT axis go toward analogous values of the free “zigzag” Fe atom chain.

Classical and quantum aspects of spin interaction in3dchains on aCu3N-Cu(110) molecular network

We present a study of the magnetic states and exchange coupling in transition-metal Mn, Fe, and Co atomic chains deposited on a self-corrugated $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$-Cu(110) molecular network by means of first-principles calculations based on the density functional theory. The various adsorption sites on a bumping area of a self-corrugated $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$ layer are investigated where the atomic chains are formed at the initial stage of nanowire growth. We demonstrate, by calculating the ground-state magnetic configurations, that the exchange coupling, magnetic order, and anisotropies in atomic chains depend sensitively on their chemical composition and adsorption sites on the $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$ network. We find that the exchange interactions in atomic chains could lead to ferromagnetic or antiferromagnetic coupling of atomic spins depending on the position of the chain on the surface. The classical spin dynamics is investigated by means of the kinetic Monte Carlo method based on transition-state theory. Moreover we evaluate the Heisenberg--Dirac--Van Vleck quantum spin Hamiltonian for calculations of the magnetic susceptibility, in order to demonstrate the existence of quantum entanglement in the antiferromagnetic atomic chains at low temperatures.

Graphene nanoribbons production from flat carbon nanotubes

Graphene nanoribbons are of great interest for pure and applied sciences due to their unique properties which depend on the nanoribbon edges, as, for example, energy gap and antiferromagnetic coupling. Nevertheless, the synthesis of nanoribbons with well-defined edges remains a challenge. To collaborate with this subject, here we propose a new route for the production of graphene nanoribbons from flat carbon nanotubes filled with a one-dimensional chain of Fe atoms by first principles calculations based on density functional theory. Our results show that Fe-filled flat carbon nanotubes are energetically more stable than non flattened geometries. Also we find that by hydrogenation or oxygenation of the most curved region of the Fe-filled flat armchair carbon nanotube, it occurred a spontaneous production of zigzag graphene nanoribbons which have metallic or semiconducting behavior depending on the edge and size of the graphene nanoribbon. Such findings can be used to create a new method of synthesis of regular-edge carbon nanoribbons.

Visualizing Majorana fermions in a chain of magnetic atoms on a superconductor

A chain of magnetic atoms on the surface of a superconductor provides a versatile platform for realizing a one-dimensional topological superconductivity phase with edge-bounded Majorana fermions zero modes. This platform lends itself to spatial resolved measurements with scanning tunneling microscope (STM) that enables direct visualization of the presence of a localized Majorana zero mode. Experiments on self-assembled chains of Fe atoms on the surface of Pb show that such a system can be experimentally fabricated and studied using various high-resolution STM measurement techniques. Spatial and energy resolved STM experiments provide strong evidence for Majorana bound states that emerge due to the combination of Fe’s ferromagnetism and spin–orbit coupling of the superconducting Pb substrate. These studies provide a roadmap for optimizing topological superconductivity in this one-dimensional platform and its extension to realize chiral two-dimensional superconductors.

Topological matter. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor.

Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

Structural, electronic and magnetic properties of chiral nanotubes filled with a linear chain of iron

Density-functional calculations of the electronic structure of (n,n/2) chiral carbon nanotubes filled with a linear chain of Fe atoms were conducted for the first time. It was found that upon encapsulating a chain of Fe atoms, the initially semiconducting nanotubes Fe5@(4,2), Fe5@(6,3) and Fe5@(8,4) became metallic. In the case of the Fe5@(8,4) structure, only the iron chain was conductive. All the considered nanotubes maintained large energy of the magnetic anisotropy, which is characteristic for a free linear chain of iron atoms. The magnetic moment on an iron atom was found to vary nonmonotonically with increasing the chiral index n; for the Fe5@(6,3) nanotube, the magnetic moment reached that of a free linear chain of iron atoms.

Spin-polarized currents generated by magnetic Fe atomic chains

Fe-based devices are widely used in spintronics because of high spin-polarization and magnetism. In this work, freestanding Fe atomic chains, the thinnest wires, were used to generate spin-polarized currents due to the spin-polarized energy bands. By ab initio calculations, the zigzag structure was found to be more stable than the wide-angle zigzag structure and had a higher ratio of spin-up and spin-down currents. By our theoretical prediction, Fe atomic chains have a sufficiently long thermal lifetime only at T ≦̸ 150 K, while C atomic chains are very stable even at T = 1000 K. This means that the spintronic devices based on Fe chains could work only at low temperatures. A system constructed by a short Fe chain sandwiched between two graphene electrodes could be used as a spin-polarized current generator, while a C chain could not be used in this way. The present work may be instructive and meaningful to further practical applications based on recent technical developments on the preparation of metal atomic chains (Proc. Natl. Acad. Sci. USA 107 9055 (2010)).

Effect of alloying of magnetic and non-magnetic low reactivity atoms into atomic chain

We have theoretically studied the possible formation of evenly mixed Pt–X, Au–X, Pd–X bimetallic atomic chains (ACs) with X = Co, Fe, and Ni. The results show that Pt–Fe ACs are the most energetically favorable. First principles calculations revealed that the energy of alloy formation is dependent on the d-band filling of the magnetic component (Fe, Co, and Ni). Thus, we found that formation of stable evenly mixed ACs with Ni atoms as the magnetic component is not possible. Moreover, we found that the alloying energy is dependent on the geometry of the AC. We found that the energy of alloy formation remains unchanged in linear ACs and drastically decreases by ∼1 eV under chain contraction, accompanied by a transition of the AC from a linear to a zigzag configuration. Our electronic structure calculations revealed the emergence of a ferromagnetic transition under stretching of the AC in all the bimetallic chains with Fe atoms as the magnetic component (Pt–Fe, Pd–Fe, and Au–Fe) from the ferromagnetic to the antiferromagnetic state.

Half-metal behaviour mediated by self-doping of topological line defect combining with adsorption of 3d transition-metal atomic chains in graphene

We present a systematic study of 3d transition-metal (TM) atomic chains adsorbed on a line defect consisting of octagonal and pentagonal carbon rings embedded in a perfect graphene sheet. The results suggest that Sc, Ti, V, Cr and Mn atoms can be spontaneously adsorbed at octagonal sites and form an atomic chain along the line defect in graphene, while Fe, Co and Ni atomic chains are adsorbed at pentagonal sites. Moreover, the Sc, Ti, V, Cr, Mn and Fe chains embedded in line defect of graphene structures show half-metallic characters. These results propose a new route to explore the potential applications of the system in spintronic devices.

Ab initio study of iron nanowires encapsulated inside silicon nitride nanotubes

In the present paper, the iron nanowires (containing single Fe atomic chain and Fen nanowire (n=5, 9 and 13)) encapsulated in (8,8) silicon nitride nanotubes (SiNNTs) have been investigated systematically using the first-principles within GGA. For the pristine (8,8) SiNNT, a ferromagnetic ground state is more favorable, and the semiconducting character is observed. After single Fe atom chain encapsulated inside (8,8) SiNNT, two possible configurations are determined depending on the distance from the wire to the tubewall. Furthermore, these two configurations keep high spin-dependent transport and thus can be used in spintronics devices. As for the Fen nanowires encapsulated in (8,8) SiNNTs (Fen@(8,8)), the spin-dependent transport are badly disturbed, but the stabilities of metal wires are reinforced in Fen@(8,8) systems. In particular, an enhanced ferromagnetism is observed after the Fe13 nanowire encapsulated into the (8,8) SiNNT. The results suggest that the Fen@(8,8) systems can be used in the magnetic storage industries.

Significance of nutation in magnetization dynamics of nanostructures

The dynamics of magnetic moments in nanostructures is closely linked to that of gyroscopes. The Landau-Lifshitz-Gilbert equation describes precession and relaxation but does not include nutation. Both precession and relaxation have been observed in experiments, in contrast to nutation. The extension of the atomistic Landau-Lifshitz-Gilbert equation by a nutation term allows us to study the significance of nutation in magnetization dynamics of nanostructures: for a single magnetic moment, a chain of Fe atoms, and Co islands on Cu(111). We find that nutation is significant at low-coordination sites and on the time scale of about 100 fs; its observation challenges strongly today's experimental techniques.

Electronic and magnetic properties of Fe, Co and Ni atomic chains encapsulated in BN nanotube bundle

The electronic and magnetic properties of the (6, 6) BN nanotube bundle in the presence of the Fe, Co and Ni atomic chains have been studied by the first principles calculations in the framework of the density functional theory (DFT). Results show that the our systems have ferromagnetic properties. All TM@(6, 6) BNNTs bundle systems are formed endothermically. The spin polarization and the magnetic moment of the system are depend on the position and type of the transition metal atomic chains. The Ni atomic chain has maximum spin polarization and maximum stability.

First principle study of the electronic and magnetic properties of a single iron atomic chain encapsulated in boron nitrite nanotubes

The electronic and magnetic properties for a single Fe atom chain wrapped in armchair ( n , n ) boron nitride nanotubes (BNNTs) ( 4 ≤ n ≤ 6 ) are investigated through the density functional theory. By increasing the nanotube diameter, the magnetic moments, total magnetic moments and spin polarization of Fe @ ( n , n ) systems are increased. We have calculated the majority and minority density of states (DOS) of armchair Fe @ ( 6 , 6 ) BNNT. Our results show that the magnetic moment of the system come mostly from the Fe atom chain. The magnetic moment on an Fe atom, the total magnetic moment and spin polarization decrease by increasing the axial separation of the Fe atom chain for the Fe @ ( 6 , 6 ) system. The Fe @ ( 6 , 6 ) BNNT can be used in the magnetic nanodevices because of higher magnetic moment and spin polarization.

Magnetic and electronic properties of a single iron atomic chain encapsulated in carbon nanotubes: A first-principles study

Under the generalized gradient approximation (GGA), the magnetic and electronic properties have been investigated for a Fe atom chain wrapped in armchair ( n, n) carbon nanotubes (CNTs) (2 ≤ n ≤ 6)) by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. After simply moving the Fe atom chain parallel to tube axis to make Fe atom locates on the perpendicular of the tube wall through the center of a hexagon by carbon–carbon bonds, all Fe@( n, n) systems including the narrow Fe@(2,2) and Fe@(3,3) systems exhibit metallic character and the Fe atom chain maintains its magnetic moment. Total density of states (DOS) and projected densities of states (PDOS) analyses show that the spin polarization and the magnetic moment of Fe@( n, n) systems come mostly from the Fe atom chain. And with increasing n and thus tube diameter, the difference between the minority spin and the majority spin at the Fermi level increases for the PDOS onto Fe atom and thus for the DOS of Fe@( n, n) systems. This trend is also indicated quantitatively by the magnetic moment on Fe atom and spin polarization for Fe@( n, n) systems. The higher magnetic moment and spin polarization of the Fe@(6,6) system show it can be used as magnetic nanostructure possessing potential current and future applications in permanent magnetism, magnetic recording, and spintronics.