Modulations of interlayer exchange coupling through ultrathin MgO-based magnetic tunnel junctions: First-principles study

Ultrathin MgO-based magnetic tunnel junction (MTJ) features high electron/heat current density, presenting important applications in spintronics. Here, we report a first-principles study of the interlayer exchange coupling (IEC) through ultrathin MgO-based MTJs. We investigate the effects of different modulations on the IEC, including temperature, different interfacial disorders, and the type and thickness of the ferromagnetic (FM) materials. It is found that the interfacial disorders, such as oxygen vacancies, boron and carbon impurities, can significantly influence the magnitude and sign of the IEC. The presence of interfacial disorders enhances the anti-FM coupling contribution and reduces the FM coupling contribution to the total IEC, and can thus change the total IEC from FM to Anti-FM in the ultrathin MTJ. We also find that FM materials have important effects on IEC: the IEC with CoFe alloy exhibits much weaker dependence on the interfacial disorders and temperature than that with the Fe. Our first-principles results provide a good explanation for the serious inconsistency between previous experimental measurements. Moreover, by studying the junction structure Vacuum/FM1/MgO/FM2 (FM1, FM2=Fe, CoFe), we find that the ultrathin FM1 layers can dramatically enhance the FM IEC and the IEC enhancement significantly depends on the combination of FM1-FM2. We show that the enhanced FM IEC with ultrathin FM1 can be sustained with a considerable amount of surface roughness in FM1 and interfacial disorder.