X-ray absorption and dichroism of transition metals and their compounds

Circularly polarized soft x-rays have been used with an imaging photoelectron microscope to record images of magnetic domains at a spatial resolution of 1 micrometer. The magnetic contrast, which can be remarkably large, arises from the fact that the x-ray absorption cross section at inner-shell absorption edges of aligned magnetic atoms depends on the relative orientation of the photon spin and the local magnetization direction. The technique is element-specific, and, because of the long mean free paths of the x-rays and secondary electrons, it can record images of buried magnetic layers.

Element-Specific Magnetic Microscopy with Circularly Polarized X-rays

Saturation effects are determined in x-ray magnetic circular dichroism spectra, acquired by electron yield techniques. It is shown that sum-rule extraction of the number of d holes, orbital moment, and spin moment are affected for Fe, Co, and Ni. In particular, errors in the extracted orbital moment values due to saturation effects can be in excess of 100% and even yield the wrong sign for films as thin as 50 \AA{}. They are significant even for film thicknesses of a few monolayers. Errors for the derived values for the number of d holes and the spin moment are considerably smaller but may be of the order 10--20 %. Correction factors are given for quantities obtained from sum rule analysis of electron yield data of Fe, Co, and Ni as a function of film thickness and x-ray incidence angle.

Electron-yield saturation effects in L-edge x-ray magnetic circular dichroism spectra of Fe, Co, and Ni

A theoretical description of magneto-optical effects in metallic transition metal systems on the basis of an itinerant description for the underlying electronic structure is presented and applications to the magneto-optical Kerr effect and the circular dichroism in x-ray absorption are discussed Simple arguments based on symmetry considerations as well as electronic excitation schemes are given to show that magneto-optical effects are caused by the interplay of magnetic ordering and spin - orbit coupling The various band structure techniques developed to deal with this situation are reviewed with an emphasize on fully relativistic methods based on the Dirac equation The theoretical framework to calculate magneto-optical spectra on the basis of the underlying band structure is outlined in some detail In addition, a number of sum rules derived by various authors are presented that allow one to estimate spin and orbital magnetic moments of an absorbing atom from the magnetic x-ray dichroism spectra For the magneto-optical Kerr effect, as well as the magnetic x-ray dichroism, application of the theoretical formalism is presented for a great variety of systems, ie pure elements, compounds, alloys and multilayer systems For both kinds of spectroscopies it is demonstrated that model calculations which allow one to manipulate the exchange splitting and the spin - orbit coupling strength give valuable information for a better understanding of the rather complex spectra Concerning the sum rules, this technique together with the direct calculation of the dichroism spectra allow for a stringent test of the various assumptions on which these rules are based

https://iopscience.iop.org/article/10.1088/0034-4885/59/12/003/pdf

Magneto-optical effects in transition metal systems

Synchrotron radiation (SR) is having a very large impact on interdisciplinary science and has been tremendously successful with the arrival of third generation synchrotron x-ray sources. But the revolution in x-ray science is still gaining momentum. Even though new storage rings are currently under construction, even more advanced rings are under design (PETRA III and the ultra high energy x-ray source) and the uses of linacs (energy recovery linac, x-ray free electron laser) can take us further into the future, to provide the unique synchrotron light that is so highly prized for today's studies in science in such fields as materials science, physics, chemistry and biology, for example. All these machines are highly reliant upon the consequences of Einstein's special theory of relativity. The consequences of relativity account for the small opening angle of synchrotron radiation in the forward direction and the increasing mass an electron gains as it is accelerated to high energy. These are familiar results to every synchrotron scientist. In this paper we outline not only the origins of SR but discuss how Einstein's strong character and his intuition and excellence have not only marked the physics of the 20th century but provide the foundation for continuing accelerator developments into the 21st century.

/pdf/review-of-third-and-next-generation-synchrotron-light-kuhktl7uox.pdf

Review of third and next generation synchrotron light sources

The spin-dependent absorption of circularly polarized x-rays is studied at theL-edges of ferromagnetic Gd and Tb metal. At theL1-edge a spin-dependent part of the absorption coefficient of 10−3–10−2 is observed. Strong resonance absorption known as “white line” occurs at theL2- andL3-absorption onset. Correlated with it one finds large spin-dependent absorption effects with amplitudes of a few percent. The spin-dependent absorption spectra reflect the profiles of the spin densities of the states populated in the absorption process. Thep-states show spin densities correlated with the first two flat bands above the Fermi level. The spin density of thed-like states is concentrated in the energy range of the white line. In Gd a splitting of ∼(0.5–0.6) eV of the unoccupied 5d spin up and spin down bands is indicated for both spin-orbit partners. In Tb a large dependence of the 5d spin density on the spin-orbit configuration is observed. The experimental results on the spin densities in Gd are compared with band structure calculations for the ferromagnetic ground-state. The theoretical and experimental spin density profiles agree well for thep-states but not for thed-states. The discrepancy concerning thed-states may be attributed to core-hole polarization effects in the absorption process.

Spin-dependent photoabsorption at theL-edges of ferromagnetic Gd and Tb metal

Large spin-dependent absorption effects have been observed with circularly polarized photons of energies between 10.8 keV and 13.8 keV at theL
2,3-edges of Os, Ir, Pt, and Au impurities (3 at.%) in iron. The largest value of the spin-dependent absorption coefficient (µ
 c
 /µ
 o
 ∼+0.22) is found at the PtL
2-edge. From the energy dependence of the spin-dependent absorption at theL
2- andL
3-edges the spin-density profiles of thed
3/2-andd
5/2-projected states populated in the absorption process are derived. Thed
5/2-spin densities deduced from theL
3 spin-dependent absorption spectra agree well with spin-polarized band-structure calculations for the ferromagnetic ground state. A comparison of thed
3/2- andd
5/2-spin densities shows a strong contribution of the spin-orbit effects to the exchange splitting in Os, whereas only small spin-orbit effects are indicated for Ir, Pt and Au impurities.

Strong spin-dependent absorption at theL 2,-edges of 5d-impurities in iron

Spin‐dependent absorption spectroscopy with circularly polarized synchrotron radiation is presented as a new method to study magnetism in magnetic solids. In the near‐edge absorption region, the spin‐dependent absorption coefficient probes the spin density of unoccupied states of selected angular momentum at the absorber atom. The spin‐dependent absorption effects as function of temperature contain information on the local magnetic coupling. A large spin‐dependent part of the absorption of ∼+0.22 at the Pt L2 edges in PtFe is observed in transmission and fluorescence mode.

Spin‐dependent K‐ and L‐absorption measurements (invited)

Spin-dependent absorption spectroscopy as a method to study the spin density of unoccupied states is demonstrated in case of the Fe K-edge and Pt L 2,3 -edges in the ferromagnetic alloy Fe 80 Pt 20 . Large relative spin-dependent parts of the absorption coefficient μ c /μ 0 ∼+23% at the Pt L 2 - and μ c /μ 0 ∼−12% at the Pt L 3 -edges have been found. The μ c /μ 0 -spectra are interpreted in terms of spin density profiles of p- and d-projected states near the Fermi level populated in the absorption process and are compared with results of spin-resolved band-structure calculations.

W. Wilhelm

Papers

Spin-dependent photoabsorption at theL-edges of ferromagnetic Gd and Tb metal

Strong spin-dependent absorption at theL 2,-edges of 5d-impurities in iron

Spin‐dependent K‐ and L‐absorption measurements (invited)

Spin-dependent absorption at the K- and L2,3-edges in ferromagnetic Fe80Pt20 alloy