Large intragranular low field magnetoresistance in polycrystalline S₂Fe_1-xAl_xMoO₆ compounds

Polycrystalline powders of the family of compounds Sr₂Fe_1-xAl_xMoO₆ (x = 0, 0.05, 0.10, 0.15, 0.30) were prepared according to the standard solid-state reaction . XRD patterns and SEM images show that the structure and the grain size of these compounds were not changed and no impurity was observed by doping with Al in Sr₂ FeMoO₆. In the cationic ordered segregation, a large ferrimagnetic patch can be separated into some smaller ones by the Al doping. So the chain of the ferrimagnetic exchange path will be shortened and the strength of exchange will be weakened. Therefore, their moments become more sensitive to external field. Furthermore, when the Al ion replaces the Fe ion in the disordered segregation, the antiferromagnetic coupling becomes weaker. As a result, the low-field magnetoresistance is enhanced. But the smaller ferrimagnetic segregates become more sensitive to thermal agitation at high temperatures. The weakness of the Al-doped SFMO lies in its stronger temperature dependence of the saturated moments and the low-field magnetoresistance.