Surface Magnetic Moment Research Articles

Electronic and magnetic structure of the Cr(001) surface

Density functional theory (DFT) calculations are carried out to study the electronic and magnetic structure of the (001) surface of chromium. Our aim is to identify and characterize the most prominent electronic surface states and make the connection with the main experimental results. We show that a low dispersive minority spin surface state at the center of the surface Brillouin zone plays a crucial role. This surface state of Δ1 symmetry at 0.58 eV above the Fermi level exhibits a predominantly dz2 as well as pz orbital character. Local density of states (LDOS) analysis in the vacuum above the surface shows that the sharp feature originating from this surface state persists far away above the surface because of the slow decay rate of the pz wavefunction. Finally, by artificially lowering the surface magnetic moment on the outermost surface layer we find excellent agreement with experiments for . In addition, we propose that some extra spin polarized scanning tunneling spectroscopy (SP-STS) experiments should be made at smaller tip–surface distances to reveal additional features originating from the majority spin dz2 surface state.

Ab-initio calculation of C and CO adsorption on the Co (110) surface

The adsorption energies, structural, electrical and magnetic properties of adsorption of C and CO on the fcc Co (110) surface have been investigated using density functional theory. The preferential adsorption site for the fcc Co (110) has been calculated. For the case of C adsorption, the preferential adsorption site is the long-bridge for both the 0.5 and 1.0 monolayers (ML) coverages. Whilst for the CO case, the preferential adsorption sites are at the atop and short-bridge site for 0.5 and 1.0 ML coverages respectively. Structurally, the first two layers of the bare Co (110) surface expand whereas the second and third layers contract. Upon adsorption of either C or CO, however, the degree of expansion and compression reduces. Magnetically, the adsorbates were found to couple ferrimagnetically to the surface and suppress the magnetic moment of the Co layers beneath them. The C adsorbate has a much stronger suppression effect as compared to the CO adsorbate. At 0.5 ML coverage, the C adatom suppresses up to 47% of the magnetic moment in the surface layer compared to a clean Co (110), whereas the CO adsorbate only suppresses up to 16%. For the 1.0 ML coverage case, both the C and CO adsorbates suppress almost equivalently well at 68% and 63% respectively. We also report on a correlation between the amount of charge transfer and the degree of suppression of the surface magnetic moment. Finally, we observe that the electronic charge is shared in the [001] direction for the C adsorbate and in the [11¯0] direction for the CO adsorbate. This anisotropy of surface bonding, in conjunction with the ligand field theory, explains the mechanism behind the spin reorientation transition that occurs uniquely on the CO/Co(110) system.

Magnetic configuration, electronic structure, and stability of the low-index surfaces ofη-Mn3N2: A first-principles study

The low-index (001), (010), and (110) surfaces of $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ are investigated using the ab initio generalized gradient (GGA) density functional method. For the bulk crystal, the GGA approach is found to predict correctly its electronic structure and the antiferromagnetic ordering along the [001] direction, with Mn magnetic moments close to 3${\ensuremath{\mu}}_{B}$. The more stable surfaces are the (001) terminated in a MnN plane and the (010), both observed experimentally in $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ thin films grown on MgO (001) crystals. While in the (001) the surface plane consists of Mn atoms ferromagnetically coupled through N, in the (010) surface rows of Mn antiferromagnetically aligned alternate along the [001] direction. Furthermore, our results suggest that, due to its relatively small surface energy, the (110) surface with a singular one-dimensional ribbonlike 8.46 \AA{} width ferromagnetic structure could also be stabilized. Despite their differences, the magnetic configuration of all the surfaces is dictated by the cleavage of the layerwise antiferromagnetic structure of bulk Mn${}_{3}$N${}_{2}$. The broken symmetry of the surface induces atomic corrugations, which however do not alter drastically either the spin configuration or the electronic structure. Mn-N bond states tend to be close to the bottom of the valence bands, and although an enhancement of the surface magnetic moments are obtained, they remain close to the bulk values. All the surfaces are metallic as bulk $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ crystals.

Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures

We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.

Atomic arrangement and magnetism of iron silicide on Fe(1 0 0) surface

We investigated surface magnetic structure in the initial stage of Si deposition on an Fe(100) surface by spin-polarized ion scattering spectroscopy (SP-ISS). We found silicidation at the Si/Fe interface after Si deposition followed by annealing at 823K. The silicidation occurs by the incorporation of silicon into the Fe substrate via the substitutional site of bcc Fe. After annealing, the incorporated Si atoms are distributed in surface layers several nanometers thick. The SP-ISS analysis revealed that the average magnetic moment of Fe in the silicide surface layer is about 70 % of that of Fe in the Fe(100) surface layer, whereas that of Si is almost zero. These surface magnetic moments are discussed in terms of the local magnetic environment. It is likely that the outermost surface of the silicide layer has an atomic arrangement similar to that of Fe3Si(100) with surface termination by the Fe–Si plane.

Increased surface spin stability in γ-Fe2O3 nanoparticles with a Cu shell

γ-Fe2O3 nanoparticles were coated with a Cu shell in situ during synthesis. An interfacial monolayer of CuO in the Cu-coated γ-Fe2O3 nanoparticles was discovered that stabilized the disordered surface spins of γ-Fe2O3 nanoparticles. Element-specific x-ray absorption spectroscopy at the L-edges for Cu and Fe indicated the magnetic moment of the Cu in the shell interacted with the γ-Fe2O3 nanoparticle’s surface magnetic moments. This exchange interaction between the Fe and Cu at the interface permitted an overall Cu moment in CuO (an antiferromagnet typically) that altered the γ-Fe2O3 nanomagnetism. Increasing the Cu shell thickness also increased the total Fe magnetism of the nanoparticles.

Field dependence of the interfacial Cu in Cu-coated γ-Fe2O3 nanoparticles

Evidence of elemental copper magnetism in Cu-coated γ-Fe2O3 nanoparticles was revealed by element specific x-ray absorption spectroscopy and magnetic circular dichroism (XMCD). An interfacial layer of magnetic CuO was discovered that interacted with the Fe3+ surface magnetic moments. This unexpected exchange interaction canted the moments of the CuO, so that a non-zero net magnetic moment was measured. XMCD measurements as a function of field on the copper coated γ-Fe2O3 nanoparticles indicated that the CuO magnetic moment was aligned with the octahedral Fe3+ and its magnetization was modified by the changing applied field. With increasing copper thickness, a stronger field dependence of the Fe3+ and Cu2+ magnetization was observed.

Tuning the surface magnetism of γ-Fe2O3 nanoparticles with a Cu shell

An interfacial monolayer of CuO in Cu-coated γ-Fe2O3 nanoparticles enables significantly decreased intrinsic surface spin disorder compared to bare γ-Fe2O3 nanoparticles. Element specific x-ray absorption spectroscopy at the L-edges for Cu and Fe indicates that the magnetic moment of the CuO in the shell interacts with the γ-Fe2O3 nanoparticle’s surface magnetic moments. This exchange interaction cants the moments of the CuO resulting in a non-zero Cu moment, altering the γ-Fe2O3 nanomagnetism.

Surface properties of 3d transition metals

Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.

Spin-glass magnetism of surface rich Au cluster film

The magnetic response of two-dimensional uncapped Au islands tens of nanometers in size was measured using a superconducting quantum interference device magnetometer. The magnetization vs. field was found to exhibit a clear ferromagnetic-like hysteresis loop and a large positive remanent magnetization was acquired after momentary application of a strong magnetic field. The Au magnetic response is assumed to be due to the surface effects in the nanosized islands and the large fraction of canted surface magnetic moments which interaction and mutual orientation differ from the bulk diamagnetic order.

Ab initio prediction of magnetically dead layers in freestanding γ-Ce(111)

It is well known that the surface of nonmagnetic α-Ce is magnetically ordered, i.e., γ-like. One then might conjecture, in agreement with previous theoretical predictions, that the γ-Ce may also exhibit at its surfaces even more strongly enhanced γ-like magnetic ordering. Nonetheless, our result shows that the (111)-surfaces of magnetic γ-Ce are neither spin nor orbitally polarized, i.e., α-like. Therefore, we predict, in contrast to the nonmagnetic α-phase which tends to produce magnetically ordered γ-like thin layers at its free surfaces, the magnetic γ-phase has a tendency to form α-like dead layers. This study, which explains the suppressed (promoted) surface magnetic moments of γ-Ce (α-Ce), shows that how nanoscale can reverse physical properties by going from bulk to the surface in isostructural α- and γ-phases of cerium. We predict using our freestanding surface results that a typical unreactive and nondiffusive substrate can dramatically influence the magnetic surface of cerium thin films in contrast to most of the uncorrelated thin films and strongly correlated transition metals. Our result implies that magnetic surface moments of α-Ce(111) can be suddenly disappeared by increasing lattice mismatch at the interface of a typical unreactive and nondiffusive substrate with cerium overlayers.

Neutron powder diffraction and magnetic studies of mesoporous Co 3O 4

Samples of mesoporous Co 3O 4, created by using mesoporous silicas KIT-6 and SBA-16 as hard templates to control the growth of Co 3O 4 have been investigated with SQUID magnetometry and neutron powder diffraction, to reveal the effects of high surface area on the magnetic and electronic properties. DC magnetic susceptibility measurements show lower Néel ordering temperatures and lower magnetic moments than in a “bulk” reference. A lower second transition temperature is also observed in the mesoporous samples, associated with the freezing of the surface (shell) magnetic moments. Measurements taken with increasing applied field at constant temperature show the materials to be antiferromagnetic as expected. Complementary parametric neutron powder diffraction studies show similar trends between the two mesoporous samples when looking at their Néel temperatures, and verify long range order within the samples.

Controlled Synthesis and Morphology-Dependent Electromagnetic Properties of Hierarchical Cobalt Assemblies

Hierarchical cobalt assemblies such as spheres, flowers with dendritic petals, and flowers with sharp petals are successfully synthesized via a facile liquid-phase reduction method by simply adjusting the reaction conditions. The morphology evolution process and transformation mechanism from spheres to dendrites and finally to flowers have been systematically investigated. It is determined that coercivity Hc depends more on sample size than on shape anisotropy, while saturation magnetization Ms is greatly affected by pinned surface magnetic moment. Even at a thinner thickness, as-synthesized cobalt samples exhibit stronger microwave absorbing ability compared with reported cobalt in the same frequency band. Especially, the cobalt flowers with dendritic petals exhibit the strongest absorption in middle frequency because incident wave and reflected wave are totally canceled at matching thickness. The architectural design of material morphologies is critical for improving properties toward future application.

Comparing magnetotransport and surface magnetic properties of half-metallic CrO2 films grown by low pressure and atmospheric pressure chemical vapor deposition

CrO2 films prepared by low pressure chemical vapor deposition (LPCVD) using Cr(CO)6 precursor have been investigated and compared with epitaxial half metallic CrO2 films prepared at atmospheric pressure (APCVD) using CrO3 precursor for their magnetotransport and surface magnetic properties. LPCVD films showed higher resistivity than APCVD epitaxial (100) CrO2 films prepared on (100) TiO2 substrates. Magnetoresistance of LPCVD films is comparable to that of APCVD films. X-ray magnetic circular dichroism suggests a reduced surface magnetic moment for LPCVD films. This reduced magnetic moment is attributed to antiferromagnetic alignment of the uncompensated Cr spins in the Cr2O3 surface layer.

Origin of ferromagnetism in molybdenum dioxide fromab initiocalculations

We have performed spin-polarized calculations of the unexpected ferromagnetism in ultrathin films of molybdenum dioxide $({\text{MoO}}_{2})$ within the framework of density-functional theory. It is found that the ideal bulk ${\text{MoO}}_{2}$ is metallic and nonmagnetic. Bulk ${\text{MoO}}_{2}$ with Mo vacancy, O vacancy, Mo interstitial, or O interstitial remains to be nonmagnetic. Using slab calculation, we observed ferromagnetism in both oxygen-rich and -poor ${\text{MoO}}_{2}$ (100) surfaces with average surface magnetic moment 1.53 and $0.69{\ensuremath{\mu}}_{\text{B}}$ per surface Mo atom, respectively. The partial density of states of surface Mo atom at the Fermi level $({E}_{\text{F}})$ is much larger than that of the Mo atom in the center of the slab and in bulk ${\text{MoO}}_{2}$, which indicates that ferromagnetism in surface (100) is due to Stoner instability. Enrichment of oxygen at the surface is found to be more favorable for ferromagnetism in ${\text{MoO}}_{2}$ (100). The $2p$ states of surface oxygen atoms are significantly hybridized with the $4d$ states of Mo atoms and are appreciably spin polarized.

The influence of hydrogen on the electronic and magnetic structures of TM(0 0 1) (TM=Fe, Co, Ni, and Cu) surfaces and interfaces: Ab-initio calculations

We present ab-initio investigation of the electronic and magnetic structure of TM(001) surfaces and TM/Cu(001) systems (TM=Fe, Co, Ni, Cu) with and without hydrogen adsorbed layer. The adsorption energy of hydrogen atom is found to be energetically more stable above the surface layer of Ni(001) surface than other TM(001) surfaces. The adsorption energies of hydrogen on TM/Cu(001) systems are larger than those on TM(001) surfaces. The relaxed geometries show that hydrogen has a strong influence on the interlayer distance. Furthermore, a marked reduction of Fe, Co, and Ni surface magnetic moments to 2.54, 1.41 and 0.25μB, respectively, is obtained due to the presence of hydrogen.

Geometric properties of magnetized black hole event horizons and ergosurfaces

In this paper we focus in the geometric properties of the magnetized Kerr-Newman metric. Three applications are considered. First, the event horizon surface area is calculated and from there we derive the first law of thermodynamics for magnetized black holes. We have obtained analytical expressions for the surface gravity, angular velocity, electric potential, and magnetic moment at the magnetized Kerr-Newman black hole event horizon. An approximate expression for the surface area of the magnetized black hole ergosurface was also obtained. Second, we study the magnetized Kerr-Newman black hole's circumferences. We found that for small values of the angular momentum (| | < 0.1) the event horizon has a prolate spheroid shape. Increasing the value of the angular momentum will change the event horizon shape from a prolate ellipsoid to an oblate spheroid. For small values of the angular momentum and charge the ergosurface shape is an oblate spheroid. Increasing these two parameters will change the ergosurface shape from a oblate spheroid to a prolate spheroid. Third, analytical expressions for the magnetized Kerr-Newman event horizon and ergosurface Gaussian curvatures were obtained although not explicitly shown. Instead a graphical analysis was carried out to visualize regions where Gaussian curvatures take negative or positive values. We found that the Gaussian curvature at the event horizon poles has negative values and do not satisfy Pelavas condition. Therefore, these regions can not be embedded in E3. However, the magnetized Kerr-Newman ergosurface can be embedded in E3 regardless the negative Gaussian curvature values in some regions of the ergosurface.

Stereo atomscope and diffraction spectroscopy—Atomic site specific property analysis

Direct three-dimensional atomic structure analysis by stereo atomscope, valence-band angular momentum analysis by circularly polarized photoelectron diffraction, and atomic-layer-resolved magnetic structure analysis by diffraction spectroscopy are reviewed. The circular dichroism of photoelectron forward focusing peak rotation around the incident-light axis reflects the orbital angular momentum of excited core level and is inversely proportional to the distance between the emitter and scatterer atoms. This is the basis for the stereo photograph of the atomic arrangements. Furthermore, these rotations are also found in the case of the valence band photoelectrons. The rotation corresponds to the orbital angular momentum quantum number of valence band. Simultaneously, the photoelectron structure factor, that corresponds to the interference of photoelectron waves from atomic orbitals within a unit cell, was observed. The origin of the dual behavior that appeared in the observation of local angular momentum from a delocalized valence band is discussed. Lastly, local electronic and magnetic structure information has been obtained by diffraction spectroscopy. X-ray absorption and magnetic circular dichroism spectra from different atomic layers of the Ni thin film were disentangled by use of Auger electron diffraction. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, ( n × 1 ) ( n = 2 , 3 ) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p ( n × 1 ) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.

Atomic-Layer Resolved Magnetic and Electronic Structure Analysis of Ni Thin Film on a Cu(001) Surface by Diffraction Spectroscopy

Up until now there has been no direct method for detecting the electronic and magnetic structure of each atomic layer at the surface, which is an essential analysis technique for nanotechnology. For this purpose, we have developed a new method, diffraction spectroscopy, based on the photon energy dependence of the angular distribution of Auger electron emission. We have applied this method to analyze the magnetic structure of a Ni ultrathin film on a Cu(001) surface around the spin reorientation transition. Atomic-layer resolved x-ray absorption and magnetic circular dichroism spectra were obtained. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.