Influence of doping Al on the magnetoresistance of polycrystalline Sr2FeMoO6

In this paper, polycrystalline Sr2Fe1-xAlxMoO6 (x = 0.0, 0.05, 0.10, 0.15, 0.30) samples were fabricated by traditional solid-state reaction. X-ray diffraction and scan electron microscopy show that all samples have single double perovskite structure and no obvious change was found in the grain size and grain boundary after doping. The magnetoresistance of Sr2FeMoO6 was greatly enhanced by doping Al as x increased from zero to 0.30, but it showed a maximum at x = 0.15 under high field over 0.5 T. These phenomena were found to originate from the change of intragranular spin-dependent scattering in Sr2FeMoO6 after doping Al. The role of the replacement of Fe with Al is twofold. Firstly, it broke the ferrimagnetic chain of O-Fe-O-Mo-O into smaller chains that become more sensitive to the external magnetic field. So the low-field magnetoresistance was enhanced remarkably with the increase of Al. Secondly, it suppressed the antiferromagnetic exchange in the antiphase boundary arising from the anti-site defects. So the spin in the antiphase boundary appeared to align more easily to the external magnetic field and the high-field magnetoresistance was decreased gradually with increasing the doping concentration. Therefore, the magnetoresistance of Sr2FeMoO6 reaches a threshold saturated value at x = 0.15 at high field. However, doping Al also make the magnetoresistance of SFMO show stronger temperature dependence.