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.     

Anti-ferromagnetic contrast in NiO (0 0 1) studied with threshold photoemission electron microscopy

 A detailed study is presented of the potential of threshold photoemission electron microscopy (PEEM) for the imaging of anti-ferromagnetic (AF) domains of NiO (0 0 1). Characteristic patterns with large asymmetry have been observed experimentally. Upon heating the sample to temperatures significantly above the Néel temperature, the patterns clearly remain visible and a magnetic origin can therefore be excluded. The patterns probably originate in polishing damage, with the bright areas corresponding to areas with a high oxygen deficiency (with enhanced electron emission). After sputter cleaning the sample, thereby removing the dominant patterns, no significant asymmetries in electron emission remain. Obviously, for our samples the AF asymmetries as measured with threshold PEEM are less than the detection limits of our setup of 0.5%. The conclusion is supported by model calculations, which show that the asymmetry should have a distinct angular dependence and which give an estimate of the maximum asymmetry of below 1%.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

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.     

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).     

Theoretical study on the temperature-induced structural transition of the Si(1 1 3) surface

The temperature-induced structural transition of the Si(1 1 3) surface is investigated by ab initio calculations. In this study, it is found that the room-temperature phase and the high-temperature phase have the 3 × 2 interstitial structure and the 3 × 1 interstitial structure, respectively. The existence of the 3 × 2 and 3 × 1 interstitial structures is supported by the analysis of scanning tunneling microscopy (STM) images and the calculation of surface core level shifts using final state pseudopotential theory. The theoretical STM images of interstitial structures are in good agreement with the STM images suggested by experiments. The analysis of STM images provides an insight into the characteristics of domain boundaries observed frequently in experiments. It is also found that the domain boundary can be formed by local 3 × 1 interstitial structures on the 3 × 2 interstitial surface. The theoretical analysis of the surface core level shifts reveals that the surface core levels in experiment originate from the interstitial structures. The lowest values in the surface core level shifts are found to be associated with the 2p core level shifts of the interstitial atoms.     

Robust half-metallicity at the zincblende CrTe(0 0 1) surfaces and its interface with ZnTe(0 0 1)

All electron full potential calculations based on spin density functional theory are performed to study cubic zincblende (ZB) and hexagonal NiAs structures of bulk CrTe, free (0 0 1) surfaces of ZB CrTe, and interface of ZB CrTe with ZnTe(0 0 1). The ferromagnetic NiAs structure is reported to be about 0.26 eV more stable than the ferromagnetic ZB phase while ZB CrTe is found to be a half-metallic ferromagnet with a half-metallic gap of about 2.90 eV. Thermodynamic stability of CrTe(0 0 1) surfaces are studied in the framework of ab-initio thermodynamic. The obtained phase diagram evidences more stability of the Te terminated surface compared with the Cr termination. We discuss that both Te and Cr ideal terminations of CrTe(0 0 1) retain bulk-like half-metallic property but with a reduced half-metallic gap compared with bulk value. The structural, electronic, magnetic, and band alignment properties of the ZB CrTe/ZnTe(0 0 1) interface are computed and a rather large minority valence band offset of about 1.09 eV is observed in this heterojunction.     

The micromagnetic ground states in epitaxial Fe (0 0 1) microstructures

The magnetic structure of planar epitaxial microstructures Fe (0 0 1) fabricated as rectangles and crosses of 200 nm to 20 μm sizes was studied using magnetic force microscopy and micromagnetic calculations. Most characteristic micromagnetic ground states were examined. The effect of microstructure sizes and orientation, with respect to the crystallographic axes, on the magnetic structure was considered. It was found that the micromagnetic ground state is a sequence of vortices or a diamond structure and also the Landau structure when the long microstructure axis is parallel to the easy axis of magnetization (EAM). When the microstructure sizes become smaller than 0.5 μm, micromagnetic states convert to a quasisingle-domain state. In microstructures oriented along the hard axis of magnetization in the film plane, micromagnetic ground states display either a concertina structure or a sequence of vortices and antivortices. In cross-shaped microstructures, the magnetic structure of the arms of the cross was found to be similar to that observed in rectangular microstructures of the same size and orientation relative to the crystallographic axes. However, the magnetic structure of the central part of the cross is not a superposition of the magnetic structures extended from the arms. It is determined by the EAM direction.     

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.     

Theoretical study of oxygen adsorption at the Fe(1 1 0) and (1 0 0) surfaces

Electronic, magnetic and structural properties of atomic oxygen adsorbed in on-surface and subsurface sites at the two most densely packed iron surfaces are investigated using density functional theory combined with a thermodynamics formalism. Oxygen coverages varying from a quarter to two monolayers (MLs) are considered. At a 1/4 ML coverage, the most stable on-surface adsorption sites are the twofold long bridge sites on the (1 1 0), and the fourfold-hollow sites on the (1 0 0) surface. The presence of on-surface oxygen atoms enhances the magnetic moments of the atoms of the two topmost Fe layers. Detailed results on the surface magnetic properties, due to O incorporation, are presented as well. Subsurface adsorption is found unfavored. The most stable subsurface O, in tetrahedral positions at the (1 0 0) and octahedral ones at the (1 1 0) surface, are characterized by substantially lower binding than that in the on-surface sites. Subsurface oxygen increases the interplanar distance between the uppermost Fe layers. The preadsorbed oxygen overlayer enhances binding of subsurface O atoms, particularly for tetrahedral sites beneath the (1 1 0) surface.     

Nanostructure formation aided by self-organised Bi nanolines on Si(0 0 1)

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(0 0 1) surface. It is suggested that hydrogen-passivated single-domain Si(0 0 1) produced by the formation of defect-free, hundreds of nm long, and 1.2-1.5 nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge-Si dimer structures on the Si(0 0 1) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Half-metallicity characteristic at zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1)

Electronic and magnetic properties of the zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1) semiconductors are studied within the framework of the density-functional theory using the FPLAPW+lo approach. We found that the Cr-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for the Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that at phase transition has occurred. Also the half-metallicity character is preserved at both CrSb/GaSb and CrSb/InAs interfaces. The conduction band minimum (CBM) of CrSb in the minority spin case lies about 0.63 eV above that of InAs, suggesting that the majority spin can be injected into InAs without being flipped to the conduction bands of the minority spin. On the other hand the CrSb/GaSb interface has a greater valence band offset (VBO) compared with the CrSb/InAs interface and the minority electrons have lower contribution in the injected currents and hence more efficient spin injection into the GaSb semiconductor. Thus the CrSb/GaSb and CrSb/InAs heterojunctions can be useful in the field of spintronics.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

Simulation of STM images of p(3 × 2)/Na/Ge(0 0 1) surface

Adsorption of Na on the Ge(0 0 1) surface is known to be a cause of surface reconstruction. It is expected to find one Na atom per unit cell of the reconstructed surface, however, the precise atomic configuration of this system is still a matter of controversy. Consequently, the aim of our present theoretical study is to examine the atomic structure of stable p(3 × 2)/Na/Ge(0 0 1) surfaces with and without the possible change of the number of Ge atoms in the surface layer (so-called mass transport). Structural and electronic properties of the considered system are investigated using the local-orbital density functional method. Our considerations are completed by a simulation of STM images of the structures following from molecular dynamics calculations.     

Relaxations in the 1 × 5 reconstruction of Pt(1 1 0)

Pt(1 1 0) is one of the most closely investigated metal surface structures because it displays a variety of “missing-row” reconstructions, which are only marginally stable. The ground state is usually found to have 1 × 2 translational symmetry, but a 1 × 3 form has also been seen. Between 1 × 2 and 1 × 3, a series of disordered structures has been recorded, which shows a slight preference for 1 × 5 periodicity. Under the preparation conditions used in this study, a stable 1 × 5 structure was found for Pt(1 1 0). Investigation by surface X-ray diffraction has led to a complete three-dimensional structure, which closely resembles an alternation of 1 × 2 and 1 × 3 unit cells. Pt(1 1 0) shows an interesting example of two “homometric” structures that are indistinguishable by diffraction, but are distinguishable by virtue of their subsurface relaxation pattern.     

Stabilization of the ferromagnetic configuration through alloying for monolayers of V, Cr, Mn on Pt(0 0 1) substrate

The magnetism is investigated for pseudomorphic monolayers of (V, Cr and Mn) and their substitutional c(2 × 2) alloyed structures adsorbed on Pt(0 0 1) semi-infinite surfaces using the semiempirical tight-binding method. Vanadium pseudomorphic monolayer exhibits a c(2 × 2) magnetic ordering, whereas Cr and Mn overlayers exhibit both c(2 ×  2) and p(1 × 1) magnetic structures. The substitutional alloyed overlayers exhibit ferrimagnetic structures that leads to appreciable net surface magnetization.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Reconstructions 3 × 3 and √3 × √3 on SiC(0 0 0 1) studied using RHEED

The 3 × 3 and √3 × √3 reconstructions on 6H-SiC(0 0 0 1) surface were obtained via depositing thin silicon layer and annealing it in ultrahigh vacuum (without Si flux). Rocking curves of reflection high energy electron diffraction (RHEED) were measured for integer and fractional order beams. They were fitted with results of many-beam calculation on the basis of dynamical theory of RHEED to determine structural parameters. For √3 × √3 superstructure, it was found that the occupancy of adatom states is 0.45 (incomplete coverage). In the sequence of Si-C double layers ABCACB, the lattice is terminated with the layer A. For 3 × 3 superstructure, the rocking curves support the model with twisted tetra-cluster. The best-fit twist is as half of that predicted in ab initio calculations; it is due to limited source of Si atoms to build up the superstructure. Larger twist correlates with higher occupancy of corner sites and with slower cooling rate of the sample after annealing.     

Electronic structures of the suboxide films formed on TiC(1 0 0) and ZrC(1 0 0) surfaces: Density functional theory studies

Density functional theory (DFT) calculations have been performed to elucidate the electronic structures of the TiO-like film on TiC(1 0 0) and the ZrO-like film on ZrC(1 0 0), which are assumed to be monolayers of suboxide films with (1 × 1) periodicity with respect to the substrate (1 0 0) surfaces. It was revealed that the electronic structures of both films were characterized by the existence of a band around 6 eV and a band around the Fermi level. The former and latter bands were mostly composed of O 2p and metal d orbitals, respectively, indicating the substantial ionic nature of the film. The calculated DOS well reproduced the previously obtained photoelectron spectra. From the inspection of the optimized structures, it was found that the both suboxide films have rippled structures; the metal and oxygen atoms are displaced vertically downward and upward, respectively, maintaining the (1 × 1) structures.