Influence of the growth technique on the coupling and magnetoresistance of Co/Ru sandwiches

Abstract

A comparative study of the structure and magnetotransport properties of Co/Ru sandwiches grown by evaporation in ultrahigh vacuum (UHV) and by plasma sputtering is reported. The crystalline structure of both types of samples has been studied by x-ray diffraction and transmission electron microscopy and the interfacial morphology by nuclear magnetic resonance. The comparison shows that the main structural difference comes from the crystalline quality: the evaporated samples are single crystals whereas the sputtered samples are polycrystalline and not textured. This difference in structure has no significant effect on the magnetoresistance, which is small for both growth techniques and is attributed to the strong interfacial mixing observed in both cases. The crystalline quality has a stronger effect on the exchange coupling. Indeed, its strength is more than a factor of 2 larger in the UHV samples than in the sputtered ones. Moreover, we observe that the shape of the room-temperature coupling oscillations obtained with the UHV samples is in good agreement with ab initio calculations. This indicates that the coupling is very homogeneous in those samples. Surprisingly the agreement is still good at low Ru thicknesses where ferromagnetic bridges are usually observed. This has been studied in more detail by temperature-dependent magnetometry. It shows that the absence of ferromagnetic bridges is due to the interfacial magnetization reduction (a consequence of the interfacial mixing), which prevents, at room temperature only, the appearance of ferromagnetic bridges.