| We use the Gutzwiller variational theory to investigate the paramagnetic and antiferromagnetic phases of LaOFeAs. Our analysis of multi-band Hubbard models shows that neither Hartree--Fock mean-field theories nor effective spin models describe the pnictides adequately. In contrast to Hartree--Fock theory, the Gutzwiller theory predicts that antiferromagnetic order requires substantial values of the local Hund's-rule exchange interaction. For a five-band model, the iron eg-electrons substantially contribute to the ordered moment, and they strongly polarize the t2g-electrons so that the antiferromagnetic phase has a large moment. A proper description of the magnetic order in LaOFeAs requires the inclusion of the arsenic 4p bands. We shall present first results for an appropriate 3d-4p eight-band model. |
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