J.-P. Kappler

DEIMOS: A beamline dedicated to dichroism measurements in the 350–2500 eV energy range

The DEIMOS (Dichroism Experimental Installation for Magneto-Optical Spectroscopy) beamline was part of the second phase of the beamline development at French Synchrotron SOLEIL (Source Optimisée de Lumière à Energie Intermédiaire du LURE) and opened to users in March 2011. It delivers polarized soft x-rays to perform x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray linear dichroism in the energy range 350-2500 eV. The beamline has been optimized for stability and reproducibility in terms of photon flux and photon energy. The main end-station consists in a cryo-magnet with 2 split coils providing a 7 T magnetic field along the beam or 2 T perpendicular to the beam with a controllable temperature on the sample from 370 K down to 1.5 K.

Study of 5d magnetism in rare-earth-transition-metal (Fe, Co) intermetallic compounds by magnetic circular X-ray dichroism

Abstract Magnetic circular X-ray dichroism (MCXD) experiments have been performed at the L 2,3 of rare-earth (RE) elements in several Re-transition-metal (Fe, Co) compounds. Experiments at the L 2,3 edges give, in principle, direct information on the 5d magnetic properties of the RE atom which results from the hybridization between the 3d and the 5d electrons. We show in this paper that the use of MCXD experiments at the L 2,3 absorption edges seems to be really complicated when localized 4f electrons are present in the ground state. This unexpected situation is clearly demonstrated by comparing the results obtained on a cerium mixed-valent compound (CeFe 2 ), in which the 4f electrons are highly hybridized with the conduction electrons, and those obtained on a normal trivalent cerium (4f 1 ) compound (CeRu 2 Ge 2 ). Our results suggest that the 4f electrons play an essential role in the description of MCXD at the edges of rare earths.

Use of an asymmetric wiggler to study the magnetic circular x‐ray dichroism of Ni, Ho, and Tb

The asymmetric wiggler, inserted since the end of 1990 on the Super‐Aco storage ring in Orsay, has been built to produce a high rate and high flux of circularly polarized soft x rays in the 100–3000 eV range. This insertion device is particularly well suited to measure x‐ray absorption spectra corresponding to dipole transitions towards electronic orbitals that are directly responsible for magnetism (3d states of first row transition metals, 4f states of rare earths). In this article we report the first magnetic circular x‐ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.

Experimental evidence of pressure-induced magnetic phase transition in Fe72Pt28 Invar alloy

X-ray magnetic circular dichroism (XMCD) measurements have been performed under high pressure and used to study the disordered Fe72Pt28 Invar alloy in conjunction with x-ray diffraction. The high spin to low spin state transition is observed by both techniques and is compared with previous work. Furthermore, XMCD gives the first evidence of the complete disappearance of the magnetic moment at high pressure in this alloy. It also proves the existence of an hysteresis cycle due to the pressure-dependent magnetic moment.

Magnetic circular X-ray dichroism at high magnetic field and low temperature in ferrimagnetic HoCo2 and paramagnetic Ho2O3

Abstract This letter presents Magnetic Circular X-Ray Dichroism at the Ho-M5 edge in the ferrimagnetic HoCo2 and for the first time in a paramagnetic compound: the Ho2O3 oxide. The latter experiment is possible because of an experimental set-up allowing to work under high magnetic field (up to 7T) and at low temperature. the experimental spectra, isotropic absorption and dichroism, are compared to calculated ones using a fully local multiplet interaction model. The intensity of the Magnetic Circular X-Ray Dichroism being proportional to the average magnetic moment on the probed atom, its value is deduced from our measurements. Finally, we show that the shape of the Magnetic Circular X-Ray Dichroism is independent of the applied magnetic field, its intensity following the behaviour of the magnetic moment.

Magnetic Circular Dichroism at High Magnetic Field and Low Temperature

The asymmetric wiggler inserted since the end of 1990 on the Super-ACO storage ring in Orsay, has been built to produce high rate and high flux of circularly polarized soft X-rays in the 400-2000 eV range. This insertion device is particularly well suited to measure x-ray absorption spectra corresponding to dipole transitions towards electronic orbitals which are directly responsible for the magnetism (3d states of first row transition metals, 4f states of rare-earths). We report the first magnetic circular x-ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.

Versatile variable temperature insert at the DEIMOS beamline for in situ electrical transport measurements

The design and the first experiments are described of a versatile cryogenic insert used for its electrical transport capabilities. The insert is designed for the cryomagnet installed on the DEIMOS beamline at the SOLEIL synchrotron dedicated to magnetic characterizations through X-ray absorption spectroscopy (XAS) measurements. This development was spurred by the multifunctional properties of novel materials such as multiferroics, in which, for example, the magnetic and electrical orders are intertwined and may be probed using XAS. The insert thus enables XAS to in situ probe this interplay. The implementation of redundant wiring and careful shielding also enables studies on operating electronic devices. Measurements on magnetic tunnel junctions illustrate the potential of the equipment toward XAS studies of in operando electronic devices.

Evidence of Multielectron Excitations in Magnetic Circular X-ray Dichroism of Rare Earth Compounds

This letter reports the first experimental evidence of multielectron excitations in magnetic circular x-ray dichroism (MCXD) spectra, measured at the LI,II,III-edges of Rare Earths (RE) in several RE-Transition Metal (Fe, Co) compounds. These multielectron features involve transitions from the 2p and 4d levels and exhibit a very strong magnetic enhancement compared to normal MCXD.

Direct observation of a highly spin-polarized organic spinterface at room temperature

Toward the design of large-scale electronic circuits that are entirely spintronics-driven, organic semiconductors have been identified as a promising medium to transport information using the electron spin. This requires a ferromagnetic metal-organic interface that is highly spin-polarized at and beyond room temperature, but this key building block is still lacking. We show how the interface between Co and phthalocyanine molecules constitutes a promising candidate. In fact, spin-polarized direct and inverse photoemission experiments reveal a high degree of spin polarization at room temperature at this interface.