Strong ferromangnetism and weak antiferroamgnetism in double perovskites: Sr₂Fe{/it M}O₆ ({/it M}=Mo, W and Re)

Double perovskites Sr$_2$FeMO$_6$ (M=Mo and Re) exhibit significant colossal magnetoresistance even at room temperature due to the high Curie Temperature (419K and 401K). However, such a high Curie Temperature is puzzling, given the large separation between magnetic elements (Fe). Moreover, with M=W, the electronic and magnetic properties suddenly change to insulating and antiferromagnetic with the N{\'e}el temperature of only 16$\sim$37 K. Based on detailed electronic structure calculations, a new mechanism is proposed which stabilizes the strong ferromagnetic state for M=Mo and Re and is passivated for M=W.