Surface Spin Density Research Articles

Density and T1 of Surface and Bulk Spins in Diamond in High Magnetic Field Gradients

We report on surface and bulk spin density measurements of diamond, using ultra-sensitive magnetic force microscopy with magnetic field gradients up to 0.5 T/$\mu$m. At temperatures between 25 and 800 mK, we measure the shifts in the resonance frequency and quality factor of a cantilever with a micromagnet attached to it. A recently developed theoretical analysis allows us to extract a surface spin density of 0.072 spins/nm$^2$ and a bulk spin density of 0.4 ppm from this data. In addition, we find an increase of the $T_1$ time of the surface spins in high magnetic field gradients due to the suppression of spin diffusion. Our technique is applicable to a variety of samples other than diamond, and could be of interest for several research fields where surface, interface or impurity bulk spin densities are an important factor.

Electronic properties of one-dimensional nanostructures of the Bi2Se3 topological insulator

We theoretically study the electronic structure and spin properties of one-dimensional nanostructures of the prototypical bulk topological insulator Bi$_2$Se$_3$. Realistic models of experimentally observed Bi$_2$Se$_3$ nanowires and nanoribbons are considered using the tight-binding method. At low energies, the band structures are composed of a series of evenly spaced degenerate sub-bands resulting from circumferential confinement of the topological surface states. The direct band gaps due to the non-trivial $\pi$ Berry phase show a clear dependence on the circumference. The spin-momentum locking of the topological surface states results in a pronounced 2$\pi$ spin rotation around the circumference with the degree of spin polarization dependent on the the momentum along the nanostructure. Overall, the band structures and spin textures are more complicated for nanoribbons, which expose two distinct facets. The effects of reduced dimensionality are rationalized with the help of a simple model that considers circumferential quantization of the topological surface states. Furthermore, the surface spin density induced by electric current along the nanostructure shows a pronounced oscillatory dependence on the charge-carrier energy, which can be exploited in spintronics applications.

Current-induced spin polarization at the surface of metallic films: A theorem and anab initiocalculation

The broken inversion symmetry at the surface of a metallic film (or, more generally, at the interface between a metallic film and a different metallic or insulating material) greatly amplifies the influence of the spin-orbit interaction on the surface properties. The best known manifestation of this effect is the momentum-dependent splitting of the surface state energies (Rashba effect). Here we show that the same interaction also generates a spin-polarization of the bulk states when an electric current is driven through the bulk of the film. For a semi-infinite jellium model, which is representative of metals with a closed Fermi surface, we prove as a theorem that, regardless of the shape of the confinement potential, the induced surface spin density at each surface is given by ${\bf S} =-\gamma \hbar {\bf \hat z}\times {\bf j}$, where ${\bf j}$ is the particle current density in the bulk, ${\bf \hat z}$ the unit vector normal to the surface, and $\gamma=\frac{\hbar}{4mc^2}$ contains only fundamental constants. For a general metallic solid $\gamma$ becomes a material-specific parameter that controls the strength of the interfacial spin-orbit coupling. Our theorem, combined with an {\it ab initio} calculation of the spin polarization of the current-carrying film, enables a determination of $\gamma$, which should be useful in modeling the spin-dependent scattering of quasiparticles at the interface.

A statistical correlation investigation for the role of surface spins to the spin relaxation of nitrogen vacancy centers

We investigated the influence of spins on surface of nanodiamonds (NDs) to the longitudinal relaxation time (T1) and transverse relaxation time (T2) of nitrogen vacancy (NV) centers in ND. A spherical model of the NDs was suggested to account for the experimental results of T1 and T2, and the density of surface spins was roughly estimated based on the statistical analysis of experimental results of 72 NDs containing a single NV center. For NDs studied here, the T1 of NV center inside is highly dependent to the surface spins of the NDs. However, for the T2 of NV center, intrinsic contributions must be much pronounced than that by surface spins. In other words, T1 of an NV center in NDs is more sensitive to the change of the surface spin density than T2.

First-principles study of oxygen adsorption on Fe(1 1 0) surface

We investigate three superstructures c(2×2), c(3×1) and c(1×1) of oxygen atom adsorption on Fe(110) surface by using first-principles density functional theory. The former two superstructures have been found by experiments at low oxygen exposures. The oxygen adsorption results in some electron transfer from surface Fe atom to O atom due to the strong electronegativity of oxygen atom, which also causes a positive work function change. For all three superstructures, the calculated surface spin density at Fermi level is positive, unlike that on a clean Fe(110) surface. The adsorbate–substrate hybridization is mainly from the interaction between O 2p and Fe 3d orbitals, and O 2p state exhibits obvious exchange spin–split induced by the Fe–O magnetic interaction. The adsorbate O 2p bands of c(2×2) and c(3×1) superstructures exhibit a weaker dispersion, whereas those bands of c(1×1) structure show larger dispersion due to the O–O interaction. Through the band structure calculation, we also calculate the exchange splitting energy of O 2px orbital for c(3×1) structure in k space and compare them with experimental results.

Noncollinear Surface Spin Density by Surface Reconstruction in the Alloy NiMn

At the (001) surface of the alloy Ni(50)Mn(50), a noncollinear spin density is observed in real space by spin-polarized scanning tunneling microscopy. The spin density of individual atoms also varies in both size and direction as a function of bias voltage, indicating a noncollinearity in the energy domain. The noncollinearity is driven by a surface reconstruction which breaks the otherwise high surface symmetry. First-principles electronic-structure calculations support the experimental observations and evidence the interplay of reconstruction and spin-orbit coupling.

Metastable Defects in Tritiated Amorphous Silicon

AbstractThe appearance of optically or electrically induced defects in hydrogenated amorphous silicon (a-Si:H), especially those that contribute to the Staebler-Wronski effect, has been the topic of numerous studies, yet the mechanism of defect creation and annealing is far from clarified. We have been observing the growth of defects caused by tritium decay in tritiated a Si-H instead of inducing defects optically. Tritium decays to 3He, emitting a beta particle (average energy of 5.7 keV) and an antineutrino. This reaction has a half âlife of 12.5 years. In these 7 at.% tritium-doped a-Si:H samples each beta decay will create a defect by converting a bonded tritium to an interstitial helium, leaving behind a silicon dangling bond. We use ESR (electron spin resonance) and PDS( photothermal deflection spectroscopy) to track the defects. First we annealed these samples, and then we used ESR to determine the initial defect density around 1016 to 1017 /cm3 , which is mostly a surface spin density. After that we have kept the samples in liquid nitrogen for almost two years. During the two years we have used ESR to track the defect densities of the samples. The defect density increases without saturation to a value of 3x1019/cm3 after two years, a number smaller than one would expect if each tritium decay were to create a silicon dangling bond (2x1020/cm3). This result suggests that there might be either an annealing process that remains at liquid nitrogen temperature, or tritium decay in clustered phase not producing a dangling bond due to bond reconstruction and emission of the hydrogen previously paired to Si-bonded tritium atom. After storage in liquid nitrogen for two years, we have annealed the samples. We have stepwise annealed one sample at temperatures up to 200°C, where all of the defects from beta decay are annealed out, and reconstructed the annealing energy distribution. The second sample, which was grown at 150°C, has been isothermally annealing at 300 K for several months. The defects remain well above their saturation value at 300 K, and the shape of decay suggests some interaction between the defects.

Electronic Structure and Magnetism of Antiferromagnetic Oxide Surface-First-principles Calculation-

Recent first-principles studies on antiferromagnetic oxide surfaces of MnO(001) and NiO(001) are reviewed. By structural optimization for the surface relaxation, small surface contraction and rampling relaxation where the surface transition-metal ions are displaced outwards are obtained. Surface electronic structures of the both oxides are still insulating and the spin magnetic moment at the transition-metal site is almost unchanged from its bulk value. The most interesting feature is the existence of finite spin magnetic moment at the surface oxygen site despite of no moment at oxygen in bulk by symmetry. The surface spin density distribution may be possibly observed by using exchange force microscopy technique recently proposed.

Electron spin resonance in mechanically fractured silicon carbide crystals

It is well known that the surface which is formed during the mechanical fracture of crystals of silicon, diamond, silicon carbide, and several other materials manifests paramagnetic properties [ 1]. The ESR signal which is observed is usually ascribed to "broken bonds" which are localized in the surface layer. The scatter in the data for the surface spin density of the centers (2- 1012-2 �9 1013 cm -2 for ehippings [2, 3] and 3- 1014 cm -2 after crushing, polishing, grinding, etc., of silicon crystals [4, 5]) provides grounds for suggesting that there is a relationship between the formation of surface states and the conditions under which the crystals are fractured. The problem of fracture is closely associated with that of comminution, which is one of the most extensively used processes in powder technology, and also with the problem of the mechanical activation of disperse media. The lack of a single opinion concerning the nature of mechanical activation stimulated us to carry out a detailed investigation of the properties of powders as a function of the conditions under which they were prepared. In the present work, we have studied the properties of powders prepared by crushing silicon carbide in air in a mortar, with a ball mill, and with a disintegrator (UDA-2 with six in-line, fingerlike rotors at maximum impact rates of 100, 200, and 270 m/see) at room temperature. Crystalline powders of technical grade ~-SiC with an average particle size of N200 t~m served as the initial samples. The ESR spectra were measured on an RE-1301 radiospectromete r at room temperature. At 77~ an intense ESR signal was manifested by the samples which arose from impurity nitrogen atoms and made it difficult to observe the lines from the

Spin-Dependent Absorption of Electrons in a Ferromagnetic Metal

It is found that the current collected by a ferromagnet placed in an electron beam depends on the orientation of the incident electron spin. At certain energies, only electrons with spins parallel or antiparallel to the net surface spin density cause a net target current. The spin dependence is caused by the influence of the exchange interaction on the elastic scattering. Inelastic scattering measurements show that the spin dependence of the production of secondary electrons is small.

Surface spin-density wave in the Cr-Re alloy

A phenomenological theory of surface spin-density waves is applied to the Cr-Re alloy. It is found that surface spin-density waves may exist on the surface of this alloy at temperatures up to 0.9K above the bulk transitions temperature TN.

The effects of bulk doping on the ESR signal of clean Si surfaces

Identical ESR signals were observed on ultrahigh vacuum crushed Si single crystals having B or P covering the entire doping range. The g-value, linewidth and line shape of the ESR signal were unaffected by the bulk doping. Maximum surface spin density of ∼2 × 10 14 cm −2 was observed for samples with bulk doping corresponding to neutral surface condition. The surface spin density decreased slightly as the bulk Fermi level moved towards the band edges. Oxygen exposure had no effect on the g-value, but the linewidth was reduced by ∼15 per cent. The resonance line shape was converted by oxygen from a very broad one to an almost perfect Lorentzian line. The surface spin density of oxygen-covered surfaces was almost independent of the bulk doping and equal roughly to the maximum spin density for clean surfaces. The results therefore show that (1) the resonance centers are localized on the surface (2) there is one-to-one correspondence between the resonance centers and surface states (3) the resonance centers and hence the surface states form two discrete levels or bands ≲ 10 −4 eV in width. Analysis of the ESR, surface states and Leed Measurements indicates that the rows of s-type unpaired electrons of Haneman's model are responsible for the ESR signal as well as both the donor and acceptor surface states.