A new approach to increase the Curie temperature of Fe-Mo double perovskites

Sr2FeMoO6 and related double perovskites are nowadays intensely investigated due to their potential in the field of spintronics. It has been previously shown that the Curie temperature (TC) of double perovskites can be increased by injecting carriers in the conduction band. We report here on an alternative approach to reinforce the magnetic interaction, and thus raise TC. It can be suspected that the introduction of Fe excess in the Fe-Mo sub-lattice, which would lead into the appearance of nearest neighbour Fe-O-Fe antiferromagnetic spin coupling, could reinforce the next-near neighbour Fe-O-Fe-O-Fe ferromagnetic ordering and thus raise the Curie temperature. The plausibility of this mechanism was checked, in the first place, by means of Monte Carlo simulations. Afterwards, Nd2x Ca2-2x Fe1+x Mo1-x O6 series was prepared and fully characterized, being found that the Curie temperature rises as much as (delta)TC ~75 K when the Fe content is increased. We argue that this is a genuine magnetic exchange effect, not related neither to steric distortions nor band filling.