Antisite defects versus grain boundary competition in the tunneling magnetoresistance of the Sr₂FeMoO₆ double perovskite

We present magnetic and transport measurements on the Sr ₂FeMoO₆ double perovskite, and study the tunneling magnetoresistance (TMR) response when the amount of Fe/Mo disorder (antisite defects) and the grain boundary (GB) properties are changed. We prepared two samples with different amounts of disorder (saturation magnetization M _S ≈ 3.0 and 1.6μ_B, respectively) and performed several GB oxidation treatments. While M_S remained unchanged, the resistivity (ρ) was increased by more than five orders of magnitude after oxidation and the TMR showed an appreciable increase, regardless of the amount of Fe/Mo disorder. At low ρ values, the more disordered sample (M _S = 1.6μ_B) shows a diminished TMR due to a reduced electron spin polarization. On the contrary, when the GBs dominate in the transport properties the effect of the disorder is washed away and both samples exhibit the same improved TMR values.