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Spin-polarised neutron reflection is used to determine the magnetisation of ultrathin epitaxial fcc Fe films sandwiched by Cu (001). A 3.0ML (monolayer) film had a measured average magnetic dipole moment per atom of 1.9±0.6 Bohr magnetons/atom which is the highest value yet observed for an fcc Fe film on Cu and is consistent with the high spin-moment phase predicted theoretically for bulk fcc iron. This state only appears to be stable for a narrow range of film thicknesses. Raising the film growth temperature leads to a considerable decrease in the observed moment.

Nanocrystalline nickel, zinc and manganese ferrites synthesized by reverse micelle synthesis technique were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction techniques, and the magnetic behavior studied by Superconducting Quantum Interference Device (SQUID). The three ferrites exhibit blocking temperatures of 16, 20, and 35 K, respectively. The difference in the blocking temperatures was attributed to the magnetocrystalline anisotropy and L–S coupling. The saturation magnetization of the three ferrites at 300 K was 25, 8, and 6 emu/g, respectively and at 2 K was 35, 17, and 15 K, respectively. The lower saturation magnetization in the case of nickel and manganese ferrite compared to their bulk counterparts is attributed to a core-shell like particle morphology. The increase in the saturation magnetization in the case of zinc ferrite nanoparticles is attributed to the change in the cation distribution from a normal spinel to a mixed spinel structure in the nanocrystalline form.

The deposition of a monolayer of Fe on Cu(100) at 300 K is well known to yield a surface that is far from a perfect monolayer of Fe. Both interdiffusion and agglomeration of Fe occur. In the present work, it is found that if the Cu(100) surface is saturated with F atoms prior to the deposition of Fe, most of the interdiffusion and agglomeration of the Fe is suppressed. The surface can then be cooled to 100 K, and the F removed by electron stimulated desorption. This procedure leaves the Fe atoms at the surface frozen in place. The Fe monolayer is not structurally perfect (the LEED spots are broadened), but it appears to be much improved over any other known technique for producing Fe monolayers on Cu(100). A monolayer of high-surface-free-energy metal on low-surface-free-energy substrate may be viewed as a fundamental building block for atomic engineering of novel superlattices. The work reported here is a promising new approach to achieving that end.

The structural and microscopic analyses of Si//spl alpha/-Fe/sub 2/O/sub 3//Co(NiFe)/Cu/Co(NiFe) bottom GMR spin valves with Co or NiFe free and pinned layers were performed from the point of view of material characteristics. The input sputtering power, substrate bias, and sputtering power source (RF, or DC magnetron) were used as experimental variables. It was confirmed that the crystal structure of /spl alpha/-Fe/sub 2/O/sub 3/ related to the antiferromagnetic crystalline anisotropy, the surface roughness associated with coupling field (bias field) between the free and the pinned layers through the nonmagnetic Cu layer, and the interdiffusion were crucial factors in determining the GMR performance of Si//spl alpha/-Fe/sub 2/O/sub 3//Co(NiFe)/Cu/Co(NiFe). The increase of Ni-Cu intermixing region due to the Cu interdiffusion was shown to be a much more dominant factor as compared to the interfacial surface roughness in reduction of GMR ratio in /spl alpha/-Fe/sub 2/O/sub 3/ bottom spin valve.

The structural and microscopic analyses of Si/α-Fe 2 O 3 ,/Co(NiFe)/Cu/Co(NiFe) bottom GMR spin valves with Co or NiFe free and pinned layers were performed from the point of view of material characteristics. The input sputtering power, substrate bias, and sputtering power source (RF, or DC magnetron) were used as experimental variables. It was confirmed that the crystal structure of α-Fe 2 O 3 related to the anti-ferromagnetic crystalline anisotropy, the surface roughness associated with coupling field(bias field) between the free and the pinned layers through the nonmagnetic Cu layer, and the inter-diffusion were crucial factors in determining the GMR performance of Si/α-Fe 2 O 3 ,/Co(NiFe)/Cu/Co(NiFe). The increase of Ni-Cu intermixing region due to the Cu inter-diffusion was shown to be a much more dominant factor as compared to the interfacial surface roughness in reduction of GMR ratio in α-Fe 2 O 3 bottom spin valve.

We present results of magnetoresistance measurements on a 50 nm NiO/2.7 nm Co/2.2 nm Cu/3.1 nm Co/2.5 nm Au spin valve performed in vacuum at temperatures up to 400/spl deg/C. Values of /spl Delta/R/R, R, and coercivities were found to be reversible up to 200/spl deg/C. Irreversible changes in transport properties observed at and above 250/spl deg/C were concurrent with changes in the interlayer coupling. A striking result is that the coercivity of the NiO-coupled Co film remains large for temperatures at and above the Neel temperature of NiO. Evidence is presented for the existence of 'coercivity transfer' between the Co layers.

R. D. McMichael, M. D. Stiles, P. J. Chen, and W. F. Egelhoff Jr. Citation: Journal of Applied Physics 83, 7037 (1998); doi: 10.1063/1.367725 View online: http://dx.doi.org/10.1063/1.367725 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/83/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spatial control of spin-wave modes in Ni80Fe20 antidot lattices by embedded Co nanodisks Appl. Phys. Lett. 99, 202502 (2011); 10.1063/1.3662841 Exchange bias of NiO/NiFe: Linewidth broadening and anomalous spin-wave damping J. Appl. Phys. 93, 7723 (2003); 10.1063/1.1557964 Ferromagnetic resonance mode interactions in periodically perturbed films J. Appl. Phys. 91, 8647 (2002); 10.1063/1.1456382 Thermally assisted reversal of exchange biasing in NiO and FeMn based systems Appl. Phys. Lett. 72, 492 (1998); 10.1063/1.120795 Microwave permeability of ferromagnetic thin films with stripe domain structure J. Appl. Phys. 81, 4057 (1997); 10.1063/1.364877

The X-ray photoelectron spectrum of molecular N2 adsorbed in a c(2 × 2) structure on Ni(100) shows two n1s peaks in the fully-screened N1s region around 400 eV binding energy and a shake-up structure around 405 eV. The assignment of the two fully-screened peaks to the two inequivalent N atoms is established on the basis of the angular variation in the peak intensities. This assignment provides important support for a Born-Haber cycle analysis of these peaks.

X-ray photoelectron spectra of thin (up to 100 A) Ag films deposited on Al(100) under ultrahigh vacuum conditions indicate that the Ag electronic structure does not converge on bulk Ag character until a thickness of around 40 A or about 20 monolayers. This is surprising since theoretical calculations suggest convergence around 3 or 4 monolayers if the growth mode is layer by layer. This suggests the film is highly granular (at least, below about 20 monolayers). Strikingly similar data for Cu and Au films together with known properties of Au suggest that all three metals grow initially with a morphology that may roughly be described as tightly packed overlapping clusters whose diameter approximately equals the average film thickness.

Carbon monoxide oxidation over the (111)iridium surface was found to obey a Langmuir-Hinshelwood reaction rate expression indicative of a surface reaction between a chemisorbed carbon monoxide molecule and a chemisorbed oxygen atom. An extremely small value of the preexponential factor of the rate coefficient (approximately 10−11 cm2/sec) was determined, while the activation energy to the surface reaction was found to be 10.5 kcal/mole. The small preexponential factor may well be due to the presence of large, independent islands of each adsorbed reactant on the surface. Since the reaction would be restricted to the region of the boundaries of the islands, this would result in an apparently small preexponential factor of the rate coefficient. The primary difference between (111)iridium and various surfaces of platinum and palladium insofar as the reaction between adsorbed oxygen and adsorbed carbon monoxide is concerned is that only at elevated temperatures does the iridium surface exhibit the reaction rates displayed by platinum and palladium at room temperature. Moreover, extensive oxidation of the (111)iridium surface (e.g., 2 × 10−7 Torr oxygen for 10 min at 700 °K) was found to induce, under certain conditions, marked changes in the oxidative properties of the surface. These changes are related to the formation of a nonstoichiometric oxide in the near-surface region although the oxygen atoms in this oxide do not participate in the carbon monoxide oxidation reaction.

The chemisorption of CO on Ir(111) has been investigated both by XPS and UPS. It has been demonstrated that XPS is a useful technique for the determination of isosteric heats adsorption. The shape of the chemisorbed CO 1π—5σ peaks in the UPS data for Ir(111) is different from that previously published for Ir(100). This may well be indicative of an influence of the geometric structure of the surface upon the electronic structure of the chemisorption bond.

This review presents an overview of the theory and of various successful approaches to the interpretation of core-level binding-energy shifts observed in photoelectron spectroscopy. The review concentrates specifically on shifts since most of the chemical and physical insights provided by core levels are derived not from the core-level binding energies themselves but from shifts they exhibit. The theoretical background is presented at a level readily accessible to the general reader. Particular attention is paid to relative merits of the two basically different conceptual frameworks for interpreting core-level binding-energy shifts, the initial-state-final-state approach and the equivalent-core approach.

Angle resolved X-ray photoelectron (XPS) studies of clean Ni(100) and of exitaxial Cu and Co films on Ni(100) have been interpreted with the aid of single scattering cluster calculations. It is found that for atoms in the top few atomic layers, photoelectron forward scattering by overlying atoms in the lattice causes XPS peak intensities to be enhanced at angles corresponding to nearest neighbor and next-nearest neighbor internuclear axes. Angle resolved XPS should thus be an excellent approach for gaining structural information on, for example, epitaxial overlayers or surface reconstructions.

X-ray and ultraviolet photoelectron spectroscopy have been used to clarify the mechanism of the CO oxidation reaction by NO on Ir(111). It was found that at room temperature, NO and CO chemisorb on Ir(111) but do not react. At elevated temperatures, adsorbed NO decomposes, and the adsorbed atomic oxygen reacts with CO to form CO2 which is desorbed rapidly. The adsorbed atomic nitrogen thermally desorbs as N2. In the absence of CO, NO decomposes on Ir(111) at elevated temperatures until oxygen accumulates to such an extent that the catalytic activity of the surface is reduced. Furthermore, the photoelectron spectra establish the significant point that the unpaired spin density of the 2π molecular orbital of NO is not quenched by adsorption on Ir(111).

The epitaxial growth of ultrathin films of pseudomorphic fcc-Fe on Cu(100) has been studied with several structure sensitive techniques. It has been found that the growth is not via a Frank-van der Merwe (layer-by-layer) mechanism, as proposed by several others, but exhibits Fe agglomeration, and at or slightly above 300 K, Cu surface segregation. This manner of growth is shown to be consistent with the generally recognized parameters which affect thin film growth. Several adsorbed gas species are found to affect strongly the growth process and the resulting film morphology. The conditions for growth of the best quality Fe films, sandwiches, and superlattices on Cu(100) are presented and are contrasted with the conditions which have been used in previous studies.

The present work consists of a study by X-ray photoelectron spectroscopy (XPS) of the adsorption of NO and its interaction with CO on the Ir(III) surface. In addition to a determination of the details of the reduction of NO per se, this type of investigation is of considerable value in elucidating the reversible oxidation-reduction of the Ir surface during the course of the chemical reaction. The properties of Group VIII transition metal catalysts in their working state depend critically on the effective oxidation state of the surface.

Ultraviolet photoelectron spectra show that even at the threshold of detectability, Pd, Ni, and Cu atoms deposited on the basal plane of cleaved graphite nucleate into clusters sufficiently large to exhibit valence band and core binding energies characteristics of bulk metals. However at very low coverage of these metals on amorphous carbon, the spectra show that the valence d -bands have decreased widths and larger binding energies than the bulk metals. Core binding energies show a similar increase at very low coverage. These effects occur because at low coverage on amorphous carbon these metals are present as isolated adatoms.

Ultraviolet and x-ray photoelectron spectroscopies, UPS and XPS, have been used to study the adsorption states of NO on the Ir(111) and Ir(110) surfaces. Two states of approximately equal population are found on Ir(111), whereas one state predominates on Ir(110). The predominant state on Ir(110) decomposes thermally at a significantly higher temperature than the two states on Ir(111). Steady-state NO decomposition studies demonstrate that Ir is a better decomposition catalyst than a reduction catalyst, and the presence of CO is required to prevent poisoning by a surface oxide. The formation of a surface oxide of low chemical reactivity begins at a significantly lower temperature with NO as the oxidizing agent than with O/sub 2/. 6 figures.

The comparison of the photoelectron spectra of chemisorbed layers taken with glancing photon incidence and those with normal photon incidence permits a qualitative determination of the degree of delocalisation of the various orbitals at the surface. For glancing photon incidence, both localised and delocalised orbitals can be photoionised. However, for normally incident photons, those orbitals localised around particular atomic sites have a much higher photoionisation cross section than those orbitals extensively delocalised in the plane of the surface. Examples of this behaviour in the spectra of hydrogen and carbon monoxide on W(100) are presented.

Using X-ray photoelectron spectroscopy (XPS) to monitor the concentration of adsorbed oxygen on Ir(111), it was found that at temperatures well below the onset of thermal desorption, the adsorbed oxygen disappears from the surface. A pressure of approximately 10−7 Torrof O2 over the crystal surface ensures that the adsorbed oxygen is not removed via reaction with background CO. The implication is that oxygen is diffusing into the bulk of the crystal. Support for this idea comes from the observation of a small, broad peak in the X-ray photoelectron spectrum of the “clean” surface at the same binding energy as the oxygen signal and the difficulty in removing the oxygen which contributes to this peak. Evidently, this peak derives from oxygen dissolved in the Ir lattice. The saturation coverage of oxygen is estimated to be 7.9 × 1014atoms/cm2 by comparison with XPS data for a CO saturated surface. This coverage is consistent with the interpretation that the (2×2) oxygen LEED pattern arises from three independent domains of (2×1) oxygen superstructures rotated 120° with respect to each other.

The adsorption of oxygen in Ir(111) has been investigated under ultrahigh vacuum conditions. Thermal desorption spectra of O 2 indicate a coverage-dependent activation energy for desorption of E d ≅ (65−10 Θ ) kcal/mole. The adsorption proceeds via a highly mobile precursor state giving a constant value for the sticking probability until saturation coverage is nearly reached. The work function change at saturation coverage of oxygen is Δφ = 0.56 ± 0.04 eV. The induction period necessary for the formation of a (2 × 2) LEED pattern indicates that the activation energy for diffusion of the oxygen adatoms on Ir(111) is in the range 16–19 kcal/mole Upon heating the crystal to moderate temperatures (e.g., ∼700K), the surface oxygen is partially converted into an unreactive oxide which is present in the near surface region.

The adsorption of CO on Ir(111) has been found to be a non-activated process with an adsorption probability of approximately unity, independent of coverage up to a coverage to a (√3 × √3)R30° LEED structure, i.e., one third of a monolayer based on the number of Ir substrate atoms. Furthermore, CO has been found to adsorb non-dissociatively on Ir(111) at temperatures up to 533 K. An analysis of the photoemission cross sections for the orbitals of adsorbed CO indicates that, upon adsorption, the 5σ orbital of CO loses the appreciable 2s character it possesses in gaseous CO. This interpretation suggests a method for determining the nature of the chemisorption bond.

Oscillations in the intensity of the specular beam in reflection high energy electron diffraction (RHEED) have been observed during the epitaxial growth of Cu on Cu(100) and face-centered cubic (fcc) Fe on fcc Fe. These oscillations are not observed in the initial stages of growth of Fe on Cu(100). Thus it is inferred that the homo-epitaxial growth of both metals is by a Frank-van der Merwe (layer-by-layer) mechanism but that the hetero-epitaxial growth of fcc Fe on Cu(100) is accompanied by strong agglomeration of the first Fe monolayer followed by gradual burying of the Cu substrate. This burying is complete after approximately 5 ML at 255 K. In addition to facilitating the determination of the active growth mechanism, these experimental results allow for a simple, effective means for calibrating the incident metal flux from the MBE vapor sources.

In a study of the interaction of methanol and formaldehyde with an atomically clean W(100) surface at 300 K, ultra-violet photoelectron spectroscopy (U has been used to identify chemisorbed species and to investigate the nature of the chemical ical bonding in adsorbed layers of mixed composition. Tempe programmed thermal desorption has been used to assist in the interpretation of the data. Methanol and formaldehyde, when exposed to a clean W(100) surf decompose to give initially a mixture of adsorbed CO and H. After further exposure, increasing CO coverage causes loss of H. This is followed by the ad of molecular species producing photoelectron spectra and desorption products which are dependent on the coverage of pre-adsorbed CO. The nature of the molecular species, in relation to gas phase methanol and formaldehyde, is discussed.