An {\it ab initio} study of the magnetic and electronic properties of Fe, Co, and Ni nanowires on Cu(001) surface

Magnetism at the nanoscale has been a very active research area in the past decades, because of its novel fundamental physics and exciting potential applications. We have recently performed an {\it ab intio} study of the structural, electronic and magnetic properties of all 3$d$ transition metal (TM) freestanding atomic chains and found that Fe and Ni nanowires have a giant magnetic anisotropy energy (MAE), indicating that these nanowires would have applications in high density magnetic data storages. In this paper, we perform density functional calculations for the Fe, Co and Ni linear atomic chains on Cu(001) surface within the generalized gradient approximation, in order to investigate how the substrates would affect the magnetic properties of the nanowires. We find that Fe, Co and Ni linear chains on Cu(001) surface still have a stable or metastable ferromagnetic state. When spin-orbit coupling (SOC) is included, the spin magnetic moments remain almost unchanged, due to the weakness of SOC in 3$d$ TM chains, whilst significant orbital magnetic moments appear and also are direction-dependent. Finally, we find that the MAE for Fe, and Co remains large, i.e., being not much affected by the presence of Cu substrate.Comment: 4 pages, 2 figure

Magnetism in reduced dimensions

We propose a short overview of a few selected issues of magnetism in reduced dimensions, which are the most relevant to set the background for more specialized contributions to the present Special Issue. Magnetic anisotropy in reduced dimensions is discussed, on a theoretical basis, then with experimental reports and views from surface to single-atom anisotropy. Then conventional magnetization states are reviewed, including macrospins, single domains, multidomains, and domain walls in stripes. Dipolar coupling is examined for lateral interactions in arrays, and for interlayer interactions in films and dots. Finally thermally-assisted magnetization reversal and superparamagnetism are presented. For each topic we sought a balance between well established knowledge and recent developments.Comment: 13 pages. Part of a Special Issue of the C. R. Physique devoted to spinelectronics (2005

Metallic, magnetic and molecular nanocontacts

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained

STRUCTURES, PROPERTIES AND FUNCTIONALITIES OF MAGNETIC DOMAIN WALLS IN THIN FILMS, NANOWIRES AND ATOMIC CHAINS: MICROMAGNETIC AND AB INITIO STUDIES

Structures, properties and functionalities of magnetic domain walls in thin film, nanowires and atomic chains are studied by micromagnetic simulations and ab initio calculations in this dissertation. For magnetic domain walls in thin films, we computationally investigated the dynamics of one-dimensional domain wall line in ultrathin ferromagnetic film, and the exponent α = 1.24 ± 0.05 is obtained in the creep regime near depinning force, indicating the washboard potential model is supported by our simulations. Furthermore, the roughness, creep, depinning and flow of domain wall line with commonly existed substructures driven by magnetic field are also studied. Our simulation results demonstrate that substructures will decrease the roughness exponent ζ, increase the critical depinning force, and reduce the effective creep energy barrier. Current induced domain-wall substructure motion is also studied, which is found quite different from current induced domain wall motion. For magnetic domain walls in nanowires, field and current induced domain wall motion is studied, and some relevant spintronic devices are proposed based on micromagnetic simulations. Novel nanometer transverse-domain-wall-based logic elements, 360° domain wall generator and shift register are proposed. When spinpolarized current is applied, the critical current for domain wall depinning can be substantially reduced and conveniently tuned by controlling domain wall number in the pile-up at pinning site, in analogy to dislocation pile-up responsible for Hall-Petch effect in mechanical strength. Furthermore, threshold currents for domain wall depinning and transportation through circular geometry in planar nanowire induced by spin transfer torques and spin-orbit torques are theoretically calculated. In addition, magnetic vortex racetrack memory which combines both conceptions of magnetic vortex domain walls and racetrack is also proposed using micromagnetic simulations. For magnetic domain walls in Ni atomic chains, a truly magnetic domain wall structure and the single domain switching process are investigated by both ab initio studies and spin dynamics simulations. Spin moment softening effect caused by the hybridization effect between two spin channels is considered. The atomic domain wall as narrow as 4 atom-distance with slight spin moment softening effect indicates a relatively evident ballistic magnetoresistance effect, and the large EB indicates the strong stability of single domain state

On-surface synthesis of sandwich-molecular wires

In this thesis, the on-surface synthesis of sandwich-molecular wires — one-dimensional chains of metal atoms and cyclic molecules in an alternating sequence — is investigated. The use of a low-symmetry substrate for the global and uniaxial alignment of these wires is introduced, making them accessible to spatially averaging techniques. Further, the effects of different types of metal atoms and molecules on the organometallic synthesis and the resulting compounds are investigated as well as their influence on the electronic and magnetic properties. First, the synthesis of a single-crystal sheet of graphene (Gr) on the two-fold symmetric substrate Ir(110) is demonstrated, which is achieved by thermal decomposition of C2H4 at 1500 K. The structure of the Gr sheet is investigated using scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED). While the bare Ir(110) substrate is strongly reconstructed, the adsorbed Gr layer is found to suppress this reconstruction and large flat terraces are observed. The two-fold symmetry of the substrate is imprinted onto the moiré of Gr with Ir(110), resulting in a clear wave pattern of nm wavelength. A strong stripe-like modulation of the electronic properties and binding energies is observed. Complementary angleresolved photoemission spectroscopy (ARPES) measurements and ab initio calculations show, that the Gr is strongly bound to the substrate and the characteristic Dirac cone in the electronic band structure is absent. This anisotropic pattern is demonstrated to enable uniaxial alignment of sandwich-molecular wires and templated adsorption of aromatic molecules. Furthermore, Gr/Ir(110) allows the on-surface synthesis of transition-metal dichalcogenide layers and the growth of epitaxial layers on unreconstructed Ir(110) under the Gr sheet. Second, the introduced two-fold symmetric Gr/Ir(110) is used for the in-depth characterization of uniaxially aligned sandwich-molecular wires consisting of the lanthanide europium (Eu) and the eight-membered carbon ring cyclooctatetraene (Cot). Using STM and LEED, the alignment effect along the [001] direction of the Ir substrate is found to persist up to several multilayers of the organometallic film. The electronic band structure of the one-dimensional wires is investigated with ARPES. A flat band 1.85 eV below the Fermi energy is found, while no π-derived bands could be observed. Using complementary density-functional theory (DFT) calculations, X-ray photoelectron spectroscopy and by exchanging the Eu within the wires by the alkaline-metal barium (Ba), this flat band could be attributed to the localized Eu 4f states. By fitting the relative position of the 4f-derived band with respect to the lower-lying σ states in the DFT calculations to the ARPES measurements, the Hubbard U of the organometallic system is derived. X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) are utilized to probe the magnetic behavior of the wire film at low temperatures. An electronic configuration of 4f7 is found with a resulting magnetic spin moment of 7 µB and an easy axis magnetization along the wires. Third, the on-surface synthesis of organometallic compounds containing the rare-earth metal thulium (Tm) and Cot is reported, which are characterized using STM, XAS/XMCD and thermal desorption spectroscopy. On undoped Gr/Ir(111), a disperse phase of TmCot monomers is observed for low coverages, which for high coverages coexists with an additional island phase. Complementary DFT calculations find that the monomers bind to the substrate through charge transfer with a Tm electronic configuration of 4f12. This configuration is confirmed using XMCD measurements and an out-of plane easy axis anisotropy of the resulting magnetic moments is observed. Intriguingly, the chemical reaction pathway during the on-surface synthesis can be changed through the suppression of charge transfer into Gr by n-doping of the substrate. As a result, islands of parallelly aligned sandwich-molecular wires are formed. It is found, that the average wire length can be controlled by changing the Tm/Cot flux ratio during the organometallic synthesis, going from small wire fragments to wires exceeding 100 formula units. Finally, the on-surface synthesis of metal–Cot sandwich-molecular wires is studied for metal atoms which are electronically similar to Eu and effects of modifications to the Cot ring as well as the growth on a metal oxide substrate are investigated using STM. Similar to Eu, also Ba and the rare-earth metal ytterbium (Yb) form sandwich-molecular wires on undoped Gr/Ir(111). In both cases, islands of interlocking and parallelly aligned wires of high crystalline quality are formed. Although the growth mechanism is the same as for Eu, differences in the morphology of the resulting islands are observed. The growth of sandwich-molecular wires consisting of rareearth metals and tetramethyl-Cot was studied. In the case of Eu islands of parallel wires are found, though at a higher growth temperature compared to the synthesis using Cot. The wire islands show a strong height modulation which is explained by the additional methyl groups. Furthermore, tetramethyl-Cot leads to a weaker interaction with the substrate. Lastly, polar Eu oxide on Ir(111) was used as substrate for the synthesis of Eu–Cot wires. Although the resulting wire islands are less ordered compared to the growth on Gr/Ir(111), they were still found to be phase pure. In the scientific appendix, the intercalation of Ba and Yb under Gr/Ir(111) is presented, both forming a (√3×√3)R30° intercalation layer (for Ba with respect to Gr, and for Yb with respect to Ir). The mass spectrum of the Cot molecule and the thermal desorption spectrum of a multilayer of Cot adsorbed to Gr/Ir(111) are shown, together with STM data of an adsorbed monolayer film of Cot at low temperatures. The mass spectra of tetramethyl-Cot and Dibenzo-Cot are presented, together with thermal desorption spectra of multilayers adsorbed on Gr/Ir(111) for both molecules. Finally, the STM data of an adsorbed monolayer of Dibenzo-Cot molecules on Gr/Ir(111) are presented

Quantum properties of atomic-sized conductors

Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to single atom upon further stretching. For some metals it is even possible to form a chain of individual atoms in this fashion. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. The properties of the contact are then dominantly determined by the nature of this atom. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics. Properties investigated include multiple Andreev reflection, shot noise, conductance quantization, conductance fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum effects show up in the mechanical properties of the contacts, as seen in the force and cohesion energy of the nanowires. We review this reseach, which has been performed mainly during the past decade, and we discuss the results in the context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have been compressed. A higher-resolution version can be found at: http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB zip PostScript