Structure, Magnetism And Magnetoresistance Of Bilayers And Organic Spin Valves Based On Fe₄N

Fe₄N has potential application value in spintronic devices because of its high saturation magnetization, high Curie temperature and high spin polarization of-100%. Exchange bias?EB? structure is an essential part in magnetic tunnel junctions and spin valves for achieving the low-power consumption in a practical application. However, there are no reports on the EB in the Fe₄ N system. Besides, due to low cost price and good flexibility, organic spin valve?OSV? has brighter future in spintronic devices. Therefore, combined Fe₄ N with organic semiconductor, magnetoresistance effect of the OSVs based on Fe₄ N was studied.In this work, the EB and anisotropic magnetoresistance?AMR? of epitaxial Fe₄N/CoN bilayers fabricated by facing-target reactive sputtering are studied in detail. Fe₄N/Alq₃/Co OSVs are fabricated by facing-target sputtering and thermal evaporation. The magnetoresistance?MR? effects of OSVs are investigated in depth.The EB sign reverses from negative to positive with increasing temperature in the Fe₄N/CoN epitaxial bilayers. This novel phenomenon is independent on cooling magnetic field or training effect. Positive EB can be attributed to the antiferromagnetic interfacial coupling and frustrated interfacial spin structures. This transition of the sign appears in all the bilayers with different Fe₄ N and CoN?tCoN? thicknesses, and not only appears at low temperatures. At tCoN=10 and 12 nm, it has a high transition temperature of 200 K. For the bilayer with 4-nm Fe₄ N, the interfacial magnetization reversal has a complex trend. The unsmooth hysteresis loops can be ascribed to the disordered reversal of the ferromagnetic?FM? magnetization and the combination of many complex magnetic structures.The AMR effect of Fe₄N/CoN bilayers also reflects the EB phenomenon. Phase shift and rectangular-like AMR appear at low temperatures, which can be related to the interfacial exchange coupling. The presence of EB field makes the magnetization lag behind external field, so phase shift happens. With the increase of Fe₄ N thickness, rectangular-like AMR becomes more apparent. The rectangular-like AMR may come from the competition of various contributions including EB-induced unidirectional anisotropy, the intrinsic AMR of Fe₄ N and higher order anisotropy induced by interfacial spin scattering.Fe₄N/Alq₃/Co OSVs with different Alq₃ thicknesses?t? exhibit an inverse MR effect, which is attributed to opposite effective spin polarization at two FM/Alq₃ interfaces. Asymmetric hysteresis loops at low temperatures are consistent with the asymmetric MR loops, which is due to the complicate magnetic structures at Alq₃/Co interface. The interfacial diffusion between ferromagnetic layer and organic layer is weak due to the advantages of facing-target sputtering. MR response at t=5 nm suggests the thin ill-defined layer. Below 120 nm, MR increases with the increased t. The reason of enhanced MR is the weakened impact of ill-defined layer. The reduced MR at t=260 nm comes from the decline of spin polarization.