Magnetic properties of Co–Si alloy clusters

We have investigated the electronic structure and the magnetic properties of Co–Si alloy clusters using ab initio spin-polarized density functional calculations. The possible CoSi₂, CoSi, and Co₂Si phase clusters with oblique hexagon prism, icosahedron, and cuboctahedron structures are introduced. The CoSi phase cluster with icosahedron structure has the largest binding energy and amount of charge transfer. We found that HOMO-LUMO gap, magnetic moment, and spin polarization for the Co–Si alloy clusters with icosahedron structure increase with Co concentration. The Si atoms in the CoSi phase with icosahedron structure have negative magnetic moment.     

Needle-like domain structure in Co films deposited on Mo (1 1 0)

Magnetization reversal processes and domain structures have been studied in Mo(1 1 0)/Co(0 0 0 1)/Au(1 1 1) structures grown by molecular beam epitaxy on monocrystalline (11–20) sapphire substrates. Wedge-shaped samples with different Co thickness gradients relative to the Mo [0 0 1] direction were fabricated. Observation of the domain structure was performed at room temperature using Kerr microscopy in a Co thickness range varying from 5 to 50 nm, where the magnetization is oriented in the plane of the sample. A Co thickness-dependent coercivity field was determined through analysis of the domain wall position during the reversal process. A preferential orientation of magnetic domain walls was found, with the domains being needle-like. The orientation, as well as the size of the needles, depends on the Co thickness and the orientation of the magnetic field applied in the sample plane.     

Coverage-dependent spin reorientation transition temperature of the Fe double-layer on W(1 1 0) observed by scanning tunneling microscopy

The magnetic domain structure of the Fe double-layer on W(1 1 0) is investigated using a variable-temperature scanning tunneling microscope. At low temperature the well-known periodic magnetic stripe domain structure is identified via the observation of domain walls. This is done with a non-spin-polarized tip by taking advantage of a spin–orbit coupling effect. At higher temperature a reorientation to an in-plane easy axis is observed. The spin reorientation temperature is found to be coverage-dependent and it is determined for samples with a coverage of 1.5–2.2 atomic layers of Fe on W(1 1 0).     

Magnetic structures of exchange bias Ni/FeF2(1 1 0) interface

Electronic and magnetic structures of ferromagnetic (FM)/antiferromagnetic (AFM), Ni/FeF₂(1 1 0), with a compensated AFM interface are investigated by using the full-potential linearized augmented plane-wave method. We find that magnetic structures at the AFM interface are perturbed by a contact with the FM material, where the superexchange interaction through the interface F excites and induces a small net moment at the AFM interface. These results predicted may play an important role for explaining the exchange bias in this system, and rule out the exchange bias mechanisms with the spin-flop coupling and the magnetic moment reorientation.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Interaction of Ag and O on W(1 1 0) studied by scanning tunneling microscopy

The structural changes of Ag films on W(1 1 0) upon coadsorption of oxygen have been studied by scanning tunneling microscopy. The exposure of one monolayer Ag to oxygen leads to a phase separation into an Ag bilayer and patches of O-covered W(1 1 0). The effective Ag island thickness increases linearly with oxygen exposure. For Ag submonolayer-islands the onset of the bilayer formation is delayed, the induction period increases with the available free W area. We conclude that the steps of the transport process are (1) dissociation of oxygen on W and on the Ag islands, (2) site exchange of atomic oxygen with Ag atoms predominantly at the island edges - while on W(1 1 0) the oxygen is immobile, (3) diffusion of the displaced Ag atoms to the island edges where they are incorporated into the monolayer and (4) initiation of Ag bilayer formation, once the W(1 1 0) is saturated with O. This indicates an unexpected activity of the Ag monolayer on W(1 1 0) towards oxygen dissociation. In case of a reversed deposition sequence, where submonolayer quantities of Ag are adsorbed on an oxygen-precovered W(1 1 0) surface, growth of Ag clusters is observed. The distribution of cluster size and cluster height depends critically on the spatial order within the predeposited oxygen overlayer - it is obvious that the oxygen overlayer on the W surface acts as a structured template for preferential Ag nucleation.     

Structural and magnetic properties of evaporated Fe thin films on Si(1 1 1), Si(1 0 0) and glass substrates

We present experimental results on the structural and magnetic properties of series of Fe thin films evaporated onto Si(1 1 1), Si(1 0 0) and glass substrates. The Fe thickness, t, ranges from 6 to110 nm. X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used to study the structure and surface morphology of these films. The magnetic properties were investigated by means of the Brillouin light scattering (BLS) and magnetic force microscopy (MFM) techniques. The Fe films grow with (1 1 0) texture; as t increases, this (1 1 0) texture becomes weaker for Fe/Si, while for Fe/glass, the texture changes from (1 1 0) to (2 1 1). Grains are larger in Fe/Si than in Fe/glass. The effective magnetization, 4πM_eff, inferred from BLS was found to be lower than the 4πM_S bulk value. Stress induced anisotropy might be in part responsible for this difference. MFM images reveal stripe domain structure for the 110 nm thick Fe/Si(1 0 0) only.     

Low-frequency noise and inelastic tunneling spectroscopy in Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) epitaxial magnetic tunnel junctions

We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm². The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm²) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure.     

Magnetic properties and giant magnetoresistance in electrodeposited Co–Ag granular films

Small cobalt particles embedded in a silver matrix have been prepared using the electrodeposition technique. The size of the clusters is controlled by the deposition potential and the Co growth time. Structural, magnetic and magneto-transport properties of Co–Ag samples have been investigated as a function of the Co concentration between 2 and 40 at% cobalt. Superparamagnetic behavior is evidenced for the low contents of cobalt while long-range magnetic order appears at higher Co concentrations. The particles size has been determined from magnetic properties and from the X-ray diffraction technique, and varies between 3.5 and 9 nm. Magnetoresistance passes through a maximum as a function of the cobalt concentration. A maximum of ∼4% GMR is obtained at room temperature while GMR reaches a value of 14% at 10 K.     

Spin-spiral and domain wall structures in monolayer Fe/W(1 1 0)

The stability of spin-spiral and domain wall structures in an Fe monolayer on a W(1 1 0) substrate is theoretically investigated. By analyzing the exchange parameters obtained from first principles total energy calculations, we find that a competition between the nearest-neighbor ferromagnetic and long-distant antiferromagnetic exchange interactions leads to a stabilization of the spin-spiral structures. When the strong magnetocrystalline anisotropy (MCA) arising from the Fe/W(1 1 0) interface is introduced, however, the formation of the spin-spiral structures is suppressed and the ground state appears to be the ferromagnetic state—as observed in experiments. In addition, the strong MCA is found to play a key role in determining the domain wall structures.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

High-resolution X-ray diffraction studies of combinatorial epitaxial Ge (0 0 1) thin-films on Ge (0 0 1) substrates

We report high-resolution X-ray diffraction studies of combinatorial epitaxial Ge (0 0 1) thin-films with varying doping concentrations of Co and Mn grown on Ge (0 0 1) substrates. The crystalline structure of the epitaxial thin-film has been determined using crystal-truncation rod (CTR) measurements and fitting analysis. By analyzing the fine interference fringes in the CTR intensity profile, strain sensitivity of ∼0.003% has been achieved. Using this method, the evolution of interfacial structures has been quantified as a function of doping concentration.     

Influences of the interfacial state between Co and a Si substrate on the magnetic properties of Co/Si(1 1 1) films

Morphology and magnetic properties of Co/Si(1 1 1) interfaces have been investigated using scanning tunneling microscope and surface magneto-optic Kerr effect techniques. As deposited at room temperature for Co/Si(1 1 1), defects have been observed with shapes of dark patches and bright islands on the surface with different Co coverage. The defect formation causes a rough interface. For subsequently deposited Co layers, the interfacial state between Co and the Si substrate results in the appearance of both the longitudinal and polar Kerr loops. After annealing treatments, interdiffusion of Co atoms and Si(1 1 1) substrate occurs as revealed by Auger electron spectroscopy. Scanning tunneling microscope images show the formation of Si clusters with average diameter of 10 nm at high temperatures. The disappearance of ferromagnetism of the films occurs due to the structural and compositional changes.     

Ab initio study of the fcc-WC(1 0 0) surface and its interaction with cobalt monolayers

The WC(1 0 0) surface has been studied by using ab initio methods of the density functional theory and pseudopotentials. Calculations have shown that surface and undersurface atoms move from their bulk positions. Namely, carbon atoms moved outward, while tungsten atoms moved inward. Five geometric cases for Co/WC(1 0 0) system were compared: (A) Co atoms are above C atoms; (B) Co atoms are above W atoms; (C) Co atoms are in the four-fold sites above WC pairs; (D and E) Co atoms are above the W-W-C and C-C-W three-fold sites, respectively - and the (A) case has been found to be energetically preferable. In all cases, Co layers have been found to be ferromagnetic. The densities of states for the bulk fcc-WC, the WC(1 0 0) surface, and the WC/Co system were compared.     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.