Influence of nonmagnetic Al ions on magnetoresistance of double-perovskite Sr ₂Fe _1-xAl _xMoO ₆ (0≤x≤0.30)

The structural, magnetic, and magnetoresistance properties of the double-perovskite series Sr2 Fe1-x Alx MoO6 (0≤x≤0.30) were systematically investigated in order to clarify the influence of nonmagnetic Al ions on the magnetoresistance. The structural refinements of these samples show that the degree of cationic order increases gradually from 88.5% for x=0 to 92% for x=0.30 without any change in the crystal structure. The magnetization measurements reveal that the substitution of nonmagnetic Al ion for Fe ion enhances the magnetic moment per Fe ion significantly. In addition, the magnetic-field dependence of magnetization and magnetoresistance of these Sr2 Fe1-x Alx MoO6 samples were all fitted excellently by taking into account the contributions from ferromagnetic-coupled Fe-O-Mo region and nonferromagnetic- coupled regions. The fitting results indicate that the low-field magnetoresistance can be greatly enhanced due to the separation of the cationic-ordered Fe-O-Mo regions by the paramagnetic Mo-O-Al-O-Mo chains introduced through Al doping. Furthermore, doping nonmagnetic Al ions also suppress the formation of antiferromagnetic Fe-O-Fe antiphase boundaries, and then lead to the improvement of cation ordering and the reduction of magnetoresistance under high field.