Zusammenfassung
Half-metallic ferromagnets, such as the Heusler compounds with formula
X(2)YZ, are expected to show an integer value for the spin magnetic
moment. In contrast to experiments, calculations give noninteger values
in certain cases where the compounds are based on X=Co. In order to
explain deviations of the magnetic moment calculated for such compounds,
the dependence of the electronic structure on the lattice parameter was
studied theoretically. In the local density approximation (LDA), the
minimum total energy of Co2FeSi is found for the experimental lattice
parameter, but the calculated magnetic moment is approximately 12% too
low. In addition, half-metallic ferromagnetism and a magnetic moment
equal to the experimental value of 6 mu(B) are found only after
increasing the lattice parameter by more than 6%. To overcome these
discrepancies, the LDA+U scheme was used to respect on-site electron
correlation in the calculations. For Co2FeSi, these calculations showed
that an effective Coulomb exchange interaction U-eff=U-J in the range of
approximately 2-5 eV leads to half-metallic ferromagnetism and to the
measured integer magnetic moment at the measured lattice parameter.
Finally, it is shown for Co2MnSi that correlation may also serve to
destroy the half-metallic behavior if the correlation becomes too strong
(above 2 eV for Co2MnSi and above 5 eV for Co2FeSi). These findings
indicate that on-site correlation may play an important role in the
description of Heusler compounds with localized moments.
