Ferromagnetic transition in ordered double perovskites and related alloys

We study the electronic and magnetic properties of ordered perovskites Sr<Subscript> 2</Subscript>FeMO<Subscript>6</Subscript> (where M is a transition metal), among which some compounds, like Sr<Subscript>2</Subscript>FeMoO<Subscript>6</Subscript> are half-metallic with high Curie temperature while Sr<Subscript>2</Subscript>FeWO<Subscript>6</Subscript> is an antiferromagnetic insulator. Using a double exchange type model with an interaction between localized spins and conduction electrons together with a tight-binding Hamiltonian and the renormalized perturbation expansion method, we study the behavior of T<Subscript>c</Subscript> as a function of the number of conduction electrons and also as a function of the Fe-M energy difference. We also consider the Sr<Subscript>2</Subscript>FeMo<Subscript>x</Subscript>W<Subscript>1-x</Sub script>O<Subscript>6</Subscript> compounds which present a magnetic and metal-insulator transition as a function of doping. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005