We present a microscopic investigation of the temperature dependence of stripe domains in perpendicularly magnetized Ni films on Cu(001) using photoelectron emission microscopy in combination with x-ray magnetic circular dichroism (XMCD) in the resonant absorption of soft x rays. When the temperature approaches the Curie temperature of the system, the average width of the observed stripe domains is reduced along with the XMCD contrast. In addition, the domains become mobile. A quantitative analysis of the temperature-dependent motion of the domains yields an exponential behavior of the domain mobility with temperature, pointing toward thermally activated processes.

The rf accelerating structures of the Compact Linear Collider (CLIC) require a material capable of sustaining high electric field with a low breakdown rate and low induced damage. Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN in order to test candidate materials and surface preparations, and have a better understanding of the breakdown mechanism under ultrahigh vacuum in a simple setup. Conditioning speeds and breakdown fields of several metals and alloys have been measured. The average breakdown field after conditioning ranges from 100 MV/m for Al to 850 MV/m for stainless steel, and is around 170 MV/m for Cu which is the present base-line material for CLIC structures. The results indicate clearly that the breakdown field is limited by the cathode. The presence of a thin cuprous oxide film at the surface of copper electrodes significantly increases the breakdown field. On the other hand, the conditioning speed of Mo is improved by removing oxides at the surface with a vacuum heat treatment, typically at 875 degrees C for 2 hours. Surface finishing treatments of Cu samples only affect the very first breakdowns. More generally, surface treatments have an effect on the conditioning process itself, but not on the average breakdown field reached after the conditioning phase. In analogy to rf, the breakdown probability has been measured in dc with Cu and Mo electrodes. The dc data show similar behavior as rf as a function of the applied electric field.

Motivated by the need of novel materials for the CLIC accelerating structures to resist mechanical fatigue, the copper based metals Copper Zirconium C15000 (CuZr) and GlidCop Al-15 C15715 have been investigated by DC breakdown measurements, and compared with commercially pure Oxygen-free Copper C10100 (Cu-OFE). In all three cases the saturated breakdown fields (Esat) are similar, despite significant differences in their tensile strengths. In addition, the choice of mechanical surface preparation techniques influences the final breakdown characteristics. For both CuZr and GlidCop immediate conditioning takes place when the surfaces are prepared by milling. For electro discharge machined (EDM) surfaces, however, several breakdown events are needed to obtain saturation. Specifically, for EDM treated CuZr and GlidCop, ~50 and ~200 breakdown events are required to reach Esat.

A comprehensive investigation of the effect of Ar, ${\mathrm{H}}_{2}$, air, and CO on the saturated breakdown field (${E}_{\mathrm{sat}}$) of molybdenum, tungsten, and copper has been performed at intensive breakdown conditioning. A significant decrease in ${E}_{\mathrm{sat}}$ is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of $\ensuremath{\sim}{10}^{\ensuremath{-}5}\text{ }\mathrm{mbar}$, the decrease in ${E}_{\mathrm{sat}}$ is found to be up to 50% and $\ensuremath{\sim}30%$ compared to UHV conditions for molybdenum and tungsten, respectively. In addition, a $\ensuremath{\sim}30%$ decrease is found when molybdenum is conditioned with a $\ensuremath{\sim}{10}^{\ensuremath{-}5}\text{ }\mathrm{mbar}$ CO pressure. Surface analysis measurements and breakdown conditioning in ${\mathrm{O}}_{2}$ ambience imply that the origin of the decrease in ${E}_{\mathrm{sat}}$ is closely linked to oxide formation on the electrode surfaces. Ex situ heat treatment of molybdenum is shown to improve the breakdown characteristics drastically.

The selection of candidate materials for the accelerating cavities of the Compact Linear Collider (CLIC) is carried out in parallel with high power RF testing. The maximum DC breakdown field of copper, copper alloys, refractory metals, aluminium and titanium have been measured with a dedicated setup. Higher maximum fields are obtained for refractory metals and for titanium, which exhibits, however, important damages after conditioning. Fatigue behaviour of copper alloys has been studied for surface and bulk by pulsed laser irradiation and ultrasonic excitation, respectively. The selected copper alloys show consistently higher fatigue resistance than copper in both experiments. In order to obtain the best local properties in the device a possible solution is a bi-metallic assembly. Junctions of molybdenum and copper-zirconium UNS C15000 alloy, achieved by HIP (Hot Isostatic Pressing) diffusion bonding or explosion bonding were evaluated for their mechanical strength. The reliability of the results obtained with both techniques should be improved. Testing in DC and radiofrequency (RF) is continued in order to select materials for a bi-metal exhibiting superior properties with respect to the combination C15000-Mo.

The need for high accelerating gradients for the future 30 GHz multi-TeV e + e - Compact Linear Collider (CLIC) at CERN has triggered a comprehensive study of DC breakdown fields of metals in UHV. The study shows that molybdenum (Mo), tungsten (W), titanium (Ti) and TiVAl reach high breakdown fields, and are thus good candidates for the iris material of CLIC structures. A significant decrease in the saturated breakdown field (Esat) is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of 10 -5 mbar, the decrease in Esat is found to be 50% and 30% for molybdenum and tungsten, respectively. In addition, a 30% decrease is found when molybdenum is conditioned with a CO pressure of ~1·10 -5 mbar. Surface analysis measurements and breakdown conditioning in O2 ambience imply that the origin of the decrease in Esat is closely linked to oxide formation on the cathode surface. ‘Ex-situ’ treatments by ion bombardment of molybdenum effectively reduce the oxide layers, and improve the breakdown characteristics of the metal drastically.

The need for high accelerating gradients for the future 30 GHz multi-TeV e + e - Compact Linear Collider (CLIC) at CERN has triggered a comprehensive study of DC breakdown fields of metals in UHV. The study shows that molybdenum (Mo), tungsten (W), titanium (Ti) and TiVAl reach high breakdown fields, and are thus good candidates for the iris material of CLIC structures. A significant decrease in the saturated breakdown field (Esat) is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of 10 -5 mbar, the decrease in Esat is found to be 50% and 30% for molybdenum and tungsten, respectively. In addition, a 30% decrease is found when molybdenum is conditioned with a CO pressure of ~1·10 -5 mbar. Surface analysis measurements and breakdown conditioning in O2 ambience imply that the origin of the decrease in Esat is closely linked to oxide formation on the cathode surface. 'Ex-situ' treatments by ion bombardment of molybdenum effectively reduce the oxide layers, and improve the breakdown characteristics of the metal drastically.

The CLIC study is investigating a number of different materials at different frequencies in order to find ways to increase achievable accelerating gradient and to understand what are the important parameters for high-gradient operation. So far a series of rf tests have been made with a set of identical-geometry 30 GHz and X-band structures in copper, tungsten and molybdenum. A new test of a 30 GHz copper accelerating structure has been completed in CTF3 with pulse lengths up to 70 ns. The new results are presented and compared to the previous structures to determine dependencies of quantities such accelerating gradient, material, frequency, pulse length, conditioning rate, breakdown rate and surface damage.

The CLIC study is actively investigating a number of different materials in an effort to find ways to increase achievable accelerating gradient. So far a series of rf tests have been made with a set of identical-geometry structures: a W-iris 30 GHz structure, a Mo-iris 30 GHz structure (with pulses as long as 16 ns) and a scaled Moiris X-band structure. A second Mo-iris 30 GHz structure of the same geometry has now been tested in CTF3 with pulse lengths up to 350 ns. The structure was conditioned to a gradient of 140 MV/m with a 70 ns pulse length and a breakdown rate slope of 13 MV/m per decade has been measured.

This document aims at giving the status at mid-2006 of some studies and tests on the CLIC high-gradient accelerating structures, by gathering together the content of two papers which have been published in the 2006 European Particle Accelerator Conference, in Edinburgh. This summary concerns the study carried out on the fatigue data for copper alloys in the particular stress pattern present in RF cavities on the one side, and on the RF tests made at 11.4 and 30 GHz with identical-geometry structures of different materials on the other side. Both efforts tend to find ways to reach the high accelerating gradients desirable for the future multi-TeV e+eCompact Linear Collider (CLIC). EU contract number RII3-CT-2003-506395 CARE/ELAN Document-2006-017

Exchange coupling between a magnetized thin film cobalt on mica substrate and adsorbed manganese(III)-tetraphenyl-porphyrin chloride (MnTPPCl) molecules is studied by X-ray magnetic circular dichroism. In the regime of a submonolayer coverage of MnTPPCl a clear circular dichroism is observed at the Mn LIII,II-edge, which is reverted with opposite grazing incidence with respect to the spin moments in the cobalt film. This result shows that the two species are ferromagnetically coupled to each other. Further evidence for the role of exchange interaction as the primary cause of the induced magnetism in MnTPPCl is observed on hysteresis loops of Co and MnTPPCl.

Chemically tunable molecular magnets exhibit new and attractive properties, particularly in the field of spintronics. In this work, the magnetic interaction between a magnetized thin film cobalt substrate and adsorbed manganese(III)–tetraphenylporphyrin chloride (MnTPPCl) molecules has been studied by X-ray magnetic circular dichroism (XMCD). For MnTPPCl submonolayer coverages circular dichroism is observed at the Mn LIII,II-edges. From temperature dependent studies and element-specific hysteresis curves, it is concluded that a net magnetization is induced on the complexed Mn in the adsorbed molecule. To our knowledge, this is the first clear evidence that exchange coupling between a large organic adsorbate and a ferromagnetic substrate are observable by XMCD.

Adsorption of carbon monoxide on La-Rh(l 0 0) surface alloys has been studied by photoelectron spectroscopy. low-energy electron diffraction (LEED) and temperature programmed desorption (TPD). The surface alloys were formed by depositing overlayers of La in the monolayer regime on a Rh(1 00) substrate, and by subsequent annealing to about 1350 K. Carbon monoxide (CO) was dosed to saturation at various temperatures, The LEED pattern of the dissociated CO overlayer shows a c(4 x 4) structure with respect to the Rh(1 0 0) substrate. The LEED structure for the submonolayer alloy takes on a ring structure superposed on a (1 x 1) structure, At higher temperatures an apparent (10 x 10) superstructure is superposed on the ring structure.The TPD results shows that four different peaks are present in the desorption traces, From the core level spectroscopic data it is argued that the two first peaks are a combination of direct CO desorption, CO dissociation and association e CO desorption. The remaining two peaks are assumed to be related to associative desorption from atomic carbon and oxygen in a combination of different bonding states and different adsorbate interactions.Rh 3d core level spectra shows that the two peaks related to the surface atoms are shifted towards the bulk peak for CO on the alloy. The La 5p core level spectra shows the splitting of the core level into two peaks for CO and dissociated CO on the alloy. The 4sigma and 5sigma/1pi molecular levels of CO on the alloy are observed in the valence band spectra, with similar binding energies as for CO on a disordered La layer. In particular, a stronger rehybridisation of the CO 4sigma state with the electronic states of the surface alloy as compared to CO on clean Rh(1 0 0). is argued to be responsible for dissociation of CO on the alloy at low temperatures. (C) 2001 Published by Elsevier Science B.V.

Acetylene chemisorption and dissociation on the Co(I 120) surface has been studied using high-resolution core level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). The adsorbed acetylene molecules are found to dissociate at about 200 K, which is significantly lower than the dissociation onset reported for the system C2H2/Co(0 0 0 1). NEXAFS measurements show that acetylene hybridises strongly with the Co(1 1 (2) over bar 0) surface, forming antibonding states below the ionisation limit. which are not present in the gas-phase. In the temperature region from 200 to 300 K a dehydrogenated fragment, possibly of the form C2H or C-2, is found to co-exist with molecular acetylene. Further heating to 450 K leads to decomposition of this fragment to graphitic carbon, while an ordered (5 x 2) carbon overlayer starts to form at the expensive of molecular acetylene. At 570 K this ordered overlayer is fully developed. By combining results from photoemission spectroscopy measurements, LEED and STM, a hard sphere model for the carbon overlayer relative to the Co substrate is proposed. Above similar to600 K, a substantial decrease in the amount of ordered carbon atoms is seen, leaving mainly graphitic carbon on the Co(1 1 (2) over bar 0) surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Acetylene chemisorption and dissociation on the Co(112̄0) surface has been studied using high-resolution core level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). The adsorbed acetylene molecules are found to dissociate at about 200 K, which is significantly lower than the dissociation onset reported for the system C2H2/Co(0 0 0 1). NEXAFS measurements show that acetylene hybridises strongly with the Co(112̄0) surface, forming antibonding states below the ionisation limit, which are not present in the gas-phase. In the temperature region from 200 to 300 K a dehydrogenated fragment, possibly of the form C2H or C2, is found to co-exist with molecular acetylene. Further heating to 450 K leads to decomposition of this fragment to graphitic carbon, while an ordered (5×2) carbon overlayer starts to form at the expensive of molecular acetylene. At 570 K this ordered overlayer is fully developed. By combining results from photoemission spectroscopy measurements, LEED and STM, a hard sphere model for the carbon overlayer relative to the Co substrate is proposed. Above ∼600 K, a substantial decrease in the amount of ordered carbon atoms is seen, leaving mainly graphitic carbon on the Co(112̄0) surface.

The chemisorption of acetylene on the Co(0001) single crystal surface has been studied using high resolution core level photoemission, X-ray absorption spectroscopy and low energy electron diffraction. At low temperatures (less than or equal to 300 K) acetylene forms a strong chemisorption bond to the substrate, creating a state of hybridisation close to sp(3). The molecules are confirmed to be oriented with the C-C axis parallel to the surface. The vibrational splitting in the photoemission spectra due to excitation of the C-H stretch vibration is determined to be (389 +/- 8) meV, which is similar to6% lower then the reported C-H stretch vibration frequency for gaseous acetylene. The S-factor of the C-H-vibrational mode is determined to be (0.17 +/- 0.02), considerably higher than those reported for C2H2 adsorbed on other substrates. (C) 2002 Published by Elsevier Science B.V.

In the present work surface alloy formation when La on Rh(100) is annealed to 1350 K is investigated by photoelectron spectroscopy, low energy electron diffraction (LEED), and temperature programmed desorption (TPD). When a sub-monolayer of La deposited on Rh(100) is annealed, a LEED pattern with streaked fractional order spots appears. By increasing the La coverage a modified c(2×2) structure is formed. In this case split fractional order spots indicative of anti-phase domain effects are observed in LEED. Analyses of the photoemission spectra of the Rh3d5/2 and La5p core levels show that an ordering takes place in going from the as-deposited to the annealed system, which results in a Rh terminated surface. TPD experiments of CO adsorbed on the annealed La/Rh(100) system show CO desorption peaks at significantly lower temperature than CO on the as-deposited La/Rh(100), but still higher than the desorption peaks found for CO on Rh(100). From these results it is concluded that a true, ordered surface alloy is formed with new electronic states.