The many facets of tunneling magnetoresistance in \(\hbox {Sr}_2\hbox {FeMoO}_6\)

The nature and shape of the magnetoresistance versus magnetic field responses of polycrystalline \(\hbox {Sr}_2\hbox {FeMoO}_6\) has been a matter of intense debate. Other than the conventional intergranular tunneling magnetoresistance with nonmagnetic grain boundaries as tunnel barriers, intragranular contributions due to the presence of magnetic anti-phase boundaries as barriers and magnetically frustrated grain surface barriers giving rise to spin-valve-type magnetoresistance also turn out to be important. We find that each of these descriptions are partially true, while it is the physical state of the sample resulting from different sample synthesis and treatment conditions that defines the relative contributions of different mechanisms. It is also shown that a purely conventional intergranular TMR response can be realized only when small grain, well ordered pellets are annealed at or above 1500 \(^{\circ }\hbox {C}\). Low temperature annealing of already highly ordered samples lead to formation of \(\hbox {SrMoO}_4\) patches which further modify the magnetoresistance of this material. The dissolution of this phase takes place only after annealing above 1100 \(^{\circ }\hbox {C}\) which leads to the enhancement in the moment and ordering of the sample.