Tripositive europium attracts wide interest in diverse fields such as phosphors, sensors, and time-gated bioimaging agents based upon its optical emission spectra. Some inaccurate descriptions of these spectra are being amplified throughout the literature. In this tutorial review the background of electronic states, energy levels and transition intensities is provided as a pre-requisite for clarifying these misconceptions. The topics discussed encompass the electric dipole nature of intraconfigurational 4f electronic transitions, the use of europium as site symmetry and centrosymmetry probe, as well as an indicator of nephelauxetic effects. The frequently mis-used term Stokes shift is also clarified and alternative terms are given for the situations where it is incorrectly applied.

Some misconceptions concerning the electronic spectra of tri-positive europium and cerium

Electronic Energy Level Structure

Epitaxial Gd films were prepared in ultrahigh vacuum (UHV) on W(110) substrates. Cleanliness, growth mode, and surface crystallography were analyzed using Auger electron spectroscopy (AES) and low‐energy electron diffraction (LEED). Stranski–Krastanov growth is established by AES for a substrate temperature Ts =450 °C. The LEED patterns correspond to the hcp(0001) surface. For nominal thicknesses in the range 1≲d≲50 A a typical double diffraction interference pattern occurs. For d≳70 A both the satellite pattern in LEED and the W signal in AES vanish, indicating coherent film surfaces for this thickness range. The films can be remanently magnetized as shown by in situ measurements of the magneto‐optical Kerr effect and by spin‐polarized low‐energy electron diffraction (SPLEED).

Preparation of remanently ferromagnetic Gd(0001)

A fully relativistic first-principles electronic structure calculation method is presented for magnetic materials. The method is based on the local spin moment density concept for relativistic Hamiltonians. In order to obtain manageable Kohn-Sham-Dirac equations including magnetic fields, the orbital contribution to the four-current density is omitted. The starting point is Takeda's relativistic generalisation of the augmented spherical wave method for non-magnetic crystals (RASW). In its basic form, the proposed method for magnetic crystals is only slightly more involved as RASW, and still takes all relativistic and spin polarisation effects into account, from first principles (including the Delta l=2 coupling). The treatment of relativistic and spin polarisation effects can be called 'on equal footing'. In both relevant limits the method is exact (within the mentioned framework). Furthermore, a more elaborate scheme is suggested, which is a systematic improvement of the basic scheme. A comparison is made with other recently published methods. Finally, results of self-consistent calculations for ferromagnetic Ni and Gd, performed with the basic scheme, are compared with previous calculations and experimental data from the literature. For Ni, the results are in good agreement both with previous calculations and with experiment. For Gd interesting new results have been obtained concerning the spectroscopic splitting factor g. The influence of the choice for an explicit exchange and correlation functional is studied as well as the influence of the coupling between l and l+2 levels.

http://iopscience.iop.org/article/10.1088/0953-8984/1/44/009/pdf

Spin-polarised relativistic electronic structure calculations

On the basis of an effective Dirac-Hamiltonian, which describes the interaction of a quasiparticle or quasihole with a semi-infinite crystalline ferromagnet well below the Curie temperature, a one-step-model Green function theory of energy-, angle- and spin-resolved ultraviolet photoemission and bremsstrahlung is formulated. The elastic photocurrent spin density matrix is evaluated in a manner analogous to a Schrodinger-equation-based layer-KKR method (due to Pendry). Since the authors spin-dependent formalism includes both spin-orbit coupling and magnetic exchange, its applicability extends from ferromagnets a fortiori to non-magnetic materials, in particular those with strong spin-orbital coupling.

https://iopscience.iop.org/article/10.1088/0022-3719/18/5/016/pdf

Relativistic theory of photoemission and bremsstrahlung from ferromagnets

A fully relativistic multiple scattering theory for low-energy electron diffraction (LEED) from ferromagnets has been extended to allow for arbitrary magnetisation orientation and to take account of thermal lattice vibrations. Time-reversal and spatial symmetries were used to derive general results and to reduce the computational labour. Numerical results are presented for the ferromagnetic surfaces Ni(001), Fe(110) and Gd(0001). The exact spin-orbit and exchange-induced scattering asymmetries are found to be excellently approximated by spin-orbit-only and exchange-only asymmetries as well as by experimentally more convenient asymmetries in the cases of Ni and Fe, and more moderately so in the case of Gd. A new type of scattering asymmetry, which is brought about by an interplay between spin-orbit and exchange interaction already for unpolarised primary electrons, is predicted to reach sizeable values (up to about 40% for Gd) and to respond very sensitively to assumed changes in the magnetisation of the top layer. This suggests, firstly, the feasibility of studying surface ferromagnetism by unpolarised LEED and, secondly, an enhanced accuracy and reliability when using polarised electrons.

http://iopscience.iop.org/article/10.1088/0022-3719/17/30/018/pdf

B Ackermann

Papers

Relativistic diffraction theory: Study of 3d and 4f surface ferromagnetism by polarised and unpolarised electrons