A. Gaupp

Soft-x-ray polarimeter with multilayer optics: complete analysis of the polarization state of light

The design of a versatile high-precision eight-axis ultrahigh-vacuum-compatible polarimeter is presented. This multipurpose instrument can be used as a self-calibrating polarization detector for linearly and circularly polarized UV and soft-x-ray light. It can also be used for the characterization of reflection or transmission properties (reflectometer) or polarizing and phase-retarding properties (ellipsometer) of any optical element. The polarization properties of Mo/Si, Cr/C, Cr/Sc, and Ni/Ti multilayers used in this polarimeter as polarizers in transmission and as analyzers in reflection have been investigated theoretically and experimentally. In the soft-x-ray range, close to the p edges of Sc, Ti, and Cr, resonantly enhanced phase retardation of the transmission polarizers of as much as 18 degrees has been measured. With these newly developed optical elements the complete polarization analysis of soft-x-ray synchrotron radiation can be extended to the water-window range from 300 to 600 eV.

Faraday rotation spectra at shallow core levels: 3p edges of Fe, Co, and Ni

We present magneto-optical ( MO) Faraday spectra measured at the 3(p) edges of Fe, Co, and Ni. A polarization analysis of the final state of the transmitted radiation is employed to determine the F ...

Broadband Mo∕Si multilayer transmission phase retarders for the extreme ultraviolet

Experimental results on aperiodic broadband transmission molybdenum/silicon multilayer phase retarders for the extreme ultraviolet range are presented. The broadband phase retarders were designed using a numerical method and made using direct current magnetron sputtering on silicon nitride membrane. The polarization properties of these aperiodic transmission phase retarders have been investigated using the soft x-ray polarimeter at BESSY-II. The measured phase shift was about 42° in the wavelength range of 13.8–15.5nm, and the corresponding s-component transmission (Ts) decreased from 6% to 2% with increasing wavelength.

Complete polarization analysis of extreme ultraviolet radiation with a broadband phase retarder and analyzer

The polarization state of the BESSY UE56/1-PGM beamline radiation in the broad wavelength range of 12.7–15.5nm was measured using a molybdenum/silicon transmission phase retarder and a reflection analyzer with aperiodic multilayer interference structures, which can broaden the spectral response of these optical elements. The characteristics of the circular polarized undulator radiation, as well as the polarization properties of the two polarizing elements, were determined by a complete polarization analysis. Furthermore, the polarization of the radiation as a function of the undulator shift setting was also measured at the wavelength of 13.1nm by use of the broadband phase retarder-analyzer pair.

Multilayer transmission phase shifters for the carbon K edge and the water window

A multilayer transmission filter with 100 periods Cr/C has been produced in order to achieve a significant phase retardation for photon energies just below the carbon K edge. The phase retardation of this device was characterized and despite being smaller than expected the filter was suitable for the polarization characterization of a soft x‐ray beamline with a primary standard. Neither the filter nor the beamline were found to change their characteristics in the course of seven months. Considering the state of the art of multilayer coatings additional model calculations show that these transmission filters can be applied also at 400 eV photon energy with an upper limit at 500 eV.

Extreme ultraviolet broadband Mo/Y multilayer analyzers

Broadband extreme ultraviolet molybdenum/yttrium aperiodic multilayer analyzers were designed for polarization experiments in 8.5–11.7nm wavelength range. The multilayer analyzers were made using direct current magnetron sputtering and characterized using the soft x-ray polarimeter at BESSY-II facility. Measured s reflectivities at the Brewster angle are 5.5% for a multilayer designed for 8.5–10.1nm wavelength range and 6.1% for one designed for 9.1–11.7nm. The multilayers also exhibit high polarization degree up to 98.79%. In addition, the multilayer was also measured over 38°–52° angular range at the fixed wavelength of 10.2nm and the mean s reflectivity is 6.2%.

Soft X-ray magneto-optical constants at the Fe 2p edge determined by Bragg scattering and Faraday effect

Bragg scattering on a Fe/C multilayer in L-MOKE geometry, using circularly polarised undulator radiation, was exploited to determine the optical constants of ferromagnetic Fe across the Fe 2p absorption edges. An alternative data set was obtained from Faraday experiments on identical samples measured in transmission. Both methods lead to a direct determination of both the refractive and absorptive part of the optical constant.

X-ray magnetic circular dichroism in reflection geometry: A tool for investigating surface magnetism in thin films

Surface and interface magnetisms in oxide thin films and heterostructures have been a recurrent topic during the past years due to their relevance in the implementation of magnetoelectronic devices. Magneto-optical techniques, such as x-ray magnetic circular dichroism, turn out to be a very efficient tool to study surface magnetism due to their sensitivity to magnetic and chemical variations across the sample depth. Nevertheless, the application of the sum rules for the determination of the spin magnetic moment might lead to uncertainties as large as 40%. To overcome this problem we present an alternative approach consisting in using x-ray magnetic circular dichroism in reflection geometry. Data analysis by using a computer code based on a 4×4 matrix formalism indicates that surface and interface roughnessas are of major relevance for a proper description of the experimental data and a correct interpretation of the results. By using such an approach, we discuss the presence of a narrow surface region with...

Surface degradation of magnetic properties in manganite thin films proved with magneto-optical techniques in reflection geometry

Magneto-optical techniques in reflection geometry turn out to be a very efficient tool to study the surface magnetism due to their sensitivity to magnetic and chemical variations across the sample depth. The existence of a surface layer of about one to five unit cells with strongly depressed magnetic properties, when compared with the rest of the film, in La2∕3Ca1∕3MnO3 thin films is shown. These results strongly support previous theoretical predictions as well as recent findings showing the insulating nature of the topmost layers in these materials.

Mn valence instability in La 2 ∕ 3 Ca 1 ∕ 3 Mn O 3 thin films

A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at 1000 C in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the Mn2+ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the Mn2+ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.