Investigation of the electronic and magnetic structure of materials with high spin polarization by means of experimental and theoretical methods
Zusammenfassung der Projektergebnisse
This project examined the electronic and magnetic structure of half-metallic ferromagnets by using experimental and theoretical methods. The main experimental emphasis was focused on the occupied states of Heusler compounds that were spectroscopically examined. The calculations were focused on ground state properties. • The theoretical part of the project used ab initio calculations to predict new compounds, and to explain the properties of the compounds produced and investigated in the other projects of the research unit. Beyond ground state properties, calculations of Curie temperatures as well as phonon and magnon excitations in Heusler compounds were performed. • The main experimental task was the spectroscopic investigation of the bulk electronic structure by means of synchrotron radiation based hard X-ray photoemission (HAXPES). This method overcomes the high surface sensitivity of regular photoemission spectroscopy and is truly bulk sensitive. The method was applied to bulk materials as well as thin films and complete tunneling junctions. The calculation of the electronic and magnetic structure using Local Spin Density Approximation (LSDA) was successfully used to predict and to understand the properties and experiments on materials with high spin polarisation. This success was based on the permanent collaboration between experiment and theory. First theoretical effort was concentrated on explaining the electronic and magnetic structure of the substitutional compound Co2Cr0.6Fe0.4Al and its related parents. The electronic structures of new materials were calculated in parallel with experiments on these compounds and their preparation by P 1. Co2FeSi and Co2Mn1-xFexSi are examples of these new compounds. Besides this work on compounds that were parallely investigated in experiments, a comprehensive study of other possible Co2 and/or Mn based Heusler compounds was performed using the base of experimental data that was available. Some of the new compounds were synthesised by P 1 to demonstrate that they exist in the L21 structure. As a result of analyzing the calculations, new possible substitutional compounds, for example, Co2FeAl1-xSix, were suggested and successfully used in TMR devices. Based on the electronic structure, we calculated electronic transport and vibrational properties of several Heusler compounds. Especially for Co2 based Heusler compounds the linear dependence of the Curie temperature on the magnetic moments was explained. Based on these findings, computer codes for frozen magnons (spiral spin density waves) were developed. Photoemission data were up to now analyzed only on density of states, matrix elements, and mean free path arguments. The importance of the atomic multiplet splitting was demonstrated comparing LDA-based calculations for the x-ray absorption L-edges of Mn atom in Rh2MnGe with the experimental spectrum. The importance of the dynamical corrections was shown for the low-energy spectral properties of halfmetallic ferromagnets. Studies of locally-correlated materials demonstrate that the LDA+DMFT scheme can be used for the computation of valence-band spectra at high energies. At the same time the account of correlations gives the consistent description for the ground-state properties. It was shown that in combination with the CPA, LDA+DMFT calculation retains the correct variation of the Fermi energy, spin-polarization and the magnetic moments through the whole range of Co2FexMn1-xSi compounds. The correct FOR 559 - 16 electron structure delivered by LDA+DMFT explains the corresponding evolution from the hole-dominated to electron-like charge transport when going through the above mentioned series. Photoemission data for Co2Cr0.6Fe0.4Al revealed in addition to the effect of disorder, that there may be an influence of electron-electron correlation in Heusler compounds. Hard x-ray photoelectron spectroscopy (HAXPES) results showed a strong deviation of the structure of the valence band from the structure that is expected from the calculated density of states. High resolution HAXPES measurements of the valence band of polycrystalline Co2Mn1-xFexSi (x = 0, 0.5, 1), excited by photons having an energy of approximately 8 keV, was used to show that the high energy spectra indicate the bulk electronic structure better than low energy XPS spectra. High resolution measurements of the valence band close to the Fermi energy indicate the existence of a gap in the minority states for all three alloys. HAXPES from the valence band of buried Heusler thin films (Co2MnSi and Co2FeAl0.5Si0.5) excited by photons of about 6keV energy was carried out. The measurements were performed on thin films covered by MgO and SiOx with different thickness from 1nm to 20nm of the insulating layer and additional AlOx or Ru protective layers. It was shown that the insulating layer does not affect the high energy spectra of the Heusler compound close to the Fermi energy. The high resolution measurements of the valence band close to the Fermi energy indicated a very large electron mean free path of the electrons through the insulating layer. The spectra of the buried thin films agreed well with previous measurements from bulk samples. The valence band spectra of the two different Heusler compounds exhibit clear differences in the low lying s bands as well as close to the Fermi energy. The valence band spectra have been used to estimate the mean free path of the electrons through the MgO layer to be 17 nm at kinetic energies of about 6 keV. The electron inelastic mean free path was experimentally determined for Co2MnSi in a wide energy range. As expected for materials with unfilled d-shells, its value is slightly smaller than the calculated one. The buried Co2MnSi thin film resembles the valence band of the bulk sample that confirms its promise as an electrode for spintronic devices. The electronic structure of the buried thin films at 20K does not differ from the one measured at RT. Accounting for the bulk sensitivity of HAXPES, this illustrates that the electronic structure of the thin Co2MnSi film itself does not depend on the temperature for T ≤300 K. This fact rules out the possibility that the temperature dependence of the TMR is related to the changes in the bulk electronic structure of the electrodes. The observed temperature dependence of the TMR has to be directly related to the properties of the Co2MnSi—MgO interface.
