High thermal durability of Ru-based synthetic antiferromagnet by interfacial engineering with Re insertion

Synthetic antiferromagnets (SAFs), composed of Ru spacer with a Re insertion layer, reveal superior thermal stability up to 450 °C annealing, making the back-end of line process a wider manufacturing window and tolerance to integrate the perpendicular magnetic tunneling junctions (P-MTJs) into CMOS process. The coupling strength decays significantly for SAFs with single Ru spacer after annealing above 400 °C. Due to the characteristics of refractory metals, Re can behave as a diffusion barrier during annealing. Furthermore, the Re spacer can still keep reasonable RKKY coupling strength. Therefore, the SAFs with Ru/Re composite spacers exhibit higher RKKY coupling strength than Ru spacers after 450 °C annealing. In addition, we discovered the different enhancements for the upper and lower interfacial Re insertion, which was attributed to the varied defect formation at interfaces. The stacking fault was formed at the upper Ru/Co interface in as-deposited state. When Re was inserted at the upper interface, the diffusion between Co and Ru was significantly suppressed and the stacking fault can be eliminated during annealing, leading to enhanced interlayer coupling. Through the interfacial engineering, we may have more degrees of freedom to tune the SAF performance and thus enhance process compatibility of P-MTJ to the CMOS process.

Magnetic properties of SAFs. We first prepared the samples with a sandwich structure, Re/Ru/Re, in which two Re layers were fixed at 0.18 nm and Ru was varied. The Re layer is about one monolayer inserted between Ru and Co. The sample with a single Ru spacer is also made for comparison. The hysteresis loops were measured by using VSM and the interlayer coupling strength J ex was determined by the following formula 9,15,16 J ex = M s tH ex www.nature.com/scientificreports/ M s is saturation magnetization of the ferromagnetic layer and t is thickness of ferromagnetic layers. H ex , the exchange field strength induced by antiferromagnetic coupling, is the loop shift along the x-axis (field-axis) of M-H loop. The J ex value is plotted as a function of total spacer thickness in Fig. S1 of Supplementary Information. Compared to the sample with a single Ru spacer, the sample with composite spacer reveals similar dependence of interlayer coupling on the spacer thickness, which has a maximum J ex value around 0.79 nm (Ru 2nd RKKY anti-parallel coupling peak). This result indicates that the fixed spacer thickness at 0.79 nm for the composite spacer may still provide the highest coupling strength. Since RKKY strength strongly depends on the spacer thickness, we believe that it is proper to fix the composite spacer at the same thickness for fair comparison with a single Ru spacer. Therefore, for our experimental design, we made all the samples composed of various composite spacers with the same total thickness (0.79 nm). On the other hand, the J ex generated by the Re/Ru/Re spacer was smaller than that by a single Ru spacer because Re provides less coupling strength. Therefore, the bilayer structure with only one side Re is a possible way to reach a balance between the J ex value and thermal durability.
The sample with a composite spacer, Ru 0.61 nm/Re 0.18 nm, was made to compare with one consisting of a single Ru (0.79 nm) spacer and their hysteresis loops before and after 450 °C annealing are shown in Fig. 2. In the as-deposited state, the largest H ex 6.5 kOe for the sample with a single Ru spacer can be achieved, shown in Fig. 2a, which corresponds to the J ex of 1.02 erg/cm 2 , comparable to the reported value at 2nd RKKY peak 10 . The interlayer coupling J ex is slightly decreased to 0.93 erg/cm 2 for the as-deposited sample with the Ru/Re composite spacer, shown in Fig. 2a. After annealing at 450 °C for 1 h, J ex degrades to 0.35 erg/cm 2 for the sample with a single Ru spacer (Fig. 2b), but the J ex value of the sample with a composite spacer is only reduced to 0.57 erg/cm 2 . With the less reduction on J ex , the composite spacer shows more robust thermal durability and keeps relatively good J ex after 450 °C annealing.
Effects of Re insertion on RKKY coupling strength J ex . To further explore effects of Re insertion, we prepared the SAF samples with various composite spacers. We made two types of insertion, Ru/Re and Re/ Ru, that is, insertion of Re in the upper or lower interface, respectively. In addition, we varied the Ru thickness from 0.18 to 0.61 nm but kept the total spacer thickness about the same (0.79 nm), locating at the region of 2nd AF coupling peak of single Ru. Figure 3a displays the dependence of J ex on Re thickness for the as-deposited and annealed samples. In both spacers, J ex has highest value at Re 0.18 nm (Ru 0.61 nm) and remains similar strength at Re 0.29 nm (Ru 0.50 nm). J ex decreases with further increasing Re thickness, which may result from the smaller RKKY coupling strength of Re. On the other hand, the upper Re insertion (Ru/Re spacer) exhibits higher J ex than the lower Re insertion (Re/Ru spacer). Since J ex strength strongly depends on the Ru (0002) texture grown on (111)-oriented [Co/Pt] n multilayers 17 , we speculate that the thin Re insertion on the top of [Co/Pt] n (the lower insertion case, Re/Ru) may slightly deteriorate the subsequently deposited Ru crystallinity, resulting in a lower J ex . After 450 °C annealing, J ex of all samples drops obviously. To verify the crystallinity of Ru and Re, we prepared samples of Si/SiO 2 //Ta 5/Pt 2/Co 0.6/Re (or Ru) 5 nm for XRD measurements, as shown in Fig. S2 of Supplementary Information. The sample with Re shows quite weak signal unlike strong textured Ru, which is consistent with the previous report that Re was not easy to be crystallized when the film thickness is too thin 18 . Therefore, we suggest that very thin Re in our layer structure may grow in relatively poor crystallization on the [Co/Pt] n , which slightly deteriorates the following Ru texture. On the other hand, if Ru grows first, the (0002) texture is well established so the following Re can also have (0002) textured growth due to the proper Ru seed layer. Therefore, the different interfacial conditions may lead to various Ru crystallinity, which gives rise to different J ex for composite spacers of Ru/Re and Re/Ru.
To look into details of the changes after annealing, we plot the ratio of J ex changed after annealing, J ex (annealed)/J ex (as-deposited), shown in Fig. 3b, which may indicate the degree of degradation due to interdiffusion. A higher ratio was observed for thicker Re, suggesting the diffusion is less for thicker Re spacer. In addition, the sample with the upper insertion of Re (Ru/Re spacer) shows better performance, implying that the inter-diffusion may be more severe at the upper interface of spacer so that the suppression of inter-diffusion by upper Re works more effectively. www.nature.com/scientificreports/ We then compare the annealing temperature dependence of J ex for different spacers, as shown in Fig. 4. Compared to the single Ru spacer, the upper insertion (Ru/Re spacer) shows better thermal durability. Although the as-deposited sample with the Ru/Re spacer possesses a slightly lower J ex , J ex of the sample with Ru/Re spacer drops slowly and remains a higher value than that of the sample with a single Ru spacer after annealing. In contrast, the thermal durability of lower insertion (Re/Ru spacer) is not as good as Ru/Re. The degradation rate of J ex for the sample with the Re/Ru spacer is similar to that with the Ru spacer up to 400 °C annealing. In addition, the J ex of Re/Ru in the as-deposited state is much lower so that the J ex value of annealed samples is less than that with the Ru spacer for the annealing temperature lower than 450 °C. As we discussed earlier, the insertion of thin Re at the lower interface might deteriorate the Ru crystallinity, leading to the reduced J ex .
We also prepared the sandwich spacer composed of Re 0.18 nm/Ru 0.43 nm/Re 0.18 nm. The J ex value is 0.65 erg/cm 2 in the as-deposited state and becomes 0.40 erg/cm 2 after 450 °C annealing, slightly higher than the Ru spacer (0.35 erg/cm 2 ) but lower than that of the Ru/Re spacer (0.57 erg/cm 2 ). Although the insertion of both Re layers may further suppress the inter-diffusion, the deteriorated Ru crystallinity due to the lower insertion and reduced Ru thickness may compensate the gained thermal durability. Consequently, the SAF with the Ru/Re spacer, which can have strong RKKY interaction provided by Ru directly contacting to the ferromagnetic layer  www.nature.com/scientificreports/ at the lower interface and build a barrier to slow down the inter-diffusion at the upper interface, can achieve the highest J ex after high temperature annealing.
Effects of annealing on interfacial roughness and microstructure. In order to quantize the interfacial difference and the function of Re during annealing, we use X-ray reflectometry (XRR) analysis. Because of complicated multilayer structure, it is not easy to fully fit the experimental data of XRR. Therefore, we prepared samples in simplified structures to highlight the interface we would like to investigate. Three samples were grown: sub// Ta   www.nature.com/scientificreports/ sample structure is quite similar, after the spectra fitting, we mainly focus on the interfaces at Ru (Re) contacting the ferromagnetic layer. In sample A case, the fitted roughness at Ru/Co (top interface of Ru) is 0.39 nm for asdeposited state and 0.55 nm for annealed state, respectively, shown in Fig. 5a,b. The 0.16 nm increased roughness at the top interface can be considered as the average mixing thickness formed by annealing. For sample B, the fitted roughness of Re/Co (Re upper insertion) is 0.37 nm in the as-deposited state (Fig. 5c). Because the Re layer is quite thin so it grows conformally on Ru with similar roughness. After 450 °C annealing, Re/Co roughness becomes 0.45 nm, but Ru/Re interfacial roughness remains at 0.38 nm (Fig. 5d) Fig. S3 of Supporting Information. The increased roughness 0.09 nm is similar to sample C, revealing that the bottom insertion of Re does not significantly suppress diffusion. The fitted interfacial roughness from XRR data consists with our observation for the J ex variations with different spacer, shown in Fig. 3. The XRR data clearly reveal that the inter-diffusion at the top Ru/Co is more severe after 450 °C annealing; therefore, the upper insertion of Re (Ru/Re spacer) can effectively suppress the inter-diffusion, leading to larger J ex than the one with the Ru spacer after annealing.
To clarify the mechanism, which results in the difference between upper and lower interfaces, we took a close look into the microstructure by using STEM. By STEM-HAADF, shown in Fig. 6, we can clearly observe the atoms and how they stacked in the layers. In typical Ru-based perpendicular SAF system, [Co/Pt] multilayers grow along FCC [111] to generate large perpendicular anisotropy due to the interfacial interaction between Co and Pt [20][21][22] . Therefore, a high quality Pt seed layer is usually deposited first to provide strong (111) texture, leading to highly (111)-textured [Co/Pt] n . The STEM-HAADF images, shown in Fig. 6, reveal that the bottom Pt/ www.nature.com/scientificreports/ Co layers are well grown in a regular FCC order A-B-C, in which atoms are arranged in three kinds of atomic positions of the FCC structure. On top of Pt/Co, HCP Ru grows along [0001] with the close-packed plane parallel to FCC (111). The upper [Co/Pt] layers are expected to be perfectly aligned FCC (111) as well. However, for top Co/Pt, the first Co layer does not follow the regular A-B-C order of FCC, as shown in Fig. 6a, but forms a stacking fault, which is a kind of misalignment in a serial growth of lattice and may occur on the close-packed plane of FCC system. In this fault layer, atoms occupy the position of A-site instead of C-site, and form A-B-A ordering, which becomes HCP-like structure for the Co layer. It is known that Co can have two kinds of structure HCP and FCC and generally, a thin Co layer in Co/Pt prefers to form an FCC structure 23 . However, due to the existence of HCP Ru spacer in the SAF structure, Co is possible to initially grow with HCP sequence. The sample with the Ru/Re composite spacer also exhibits similar situation at the interface of Re/Co because Re is HCP as well. The fault layer stores additional energy, called stacking fault energy, compared to the well-ordered FCC structure. During annealing, the extra stored energy in the fault layer may provide additional driving force to make atoms migrate more easily. During the migration of Co atoms, some vacancies or wider atomic spacing might be temporarily induced so that Ru atoms have more chances to intermix with Co. Consequently, Ru and Co atoms may probably form substitutional diffusion so that the lattice or texture seems not to be deformed significantly but the interface of Ru/Co becomes intermixing after annealing, resulting in the degradation of RKKY strength. In addition, due to severe intermixing at Ru/Co interface, the first Co-layer atoms seem not successfully to recover their positions into FCC sequence but remains in the HCP structure instead, as shown in Fig. 6b. In contrast, when Re layer is inserted between Ru and Co, as shown in Fig. 6c, because Re is much heavy and stable than Ru, the migration is not as easy as Ru. During the annealing, the Re behaves as the diffusion barrier with much reduced migration; therefore, the intermixing is not so severe, leading to higher J ex than that of the Ru spacer. Notice that the Co atoms in the fault layer recover their positions back to the right FCC sequence, as shown in Fig. 6d

Conclusion
In this work, we demonstrate an interfacial engineering by Re insertion in Ru-based SAF structure to enhance the interlayer coupling after high temperature annealing. The composite spacer Ru/Re reveals significantly improved thermal tolerance compared to the single Ru spacer. We clearly show that much more severe inter-diffusion occurs during annealing at upper Ru/Co than that at lower Co/Ru interface possibly due to the formation of stacking fault at the upper interface. By inserting a monolayer of Re on top of Ru layer (Ru/Re spacer), the interdiffusion between spacer and Co can be substantially suppressed during annealing. On the other hand, although the lower Re insertion can also help the suppression of inter-diffusion for high temperature annealing, the thin Re layer at the bottom may slightly deteriorate the Ru texture, leading to reduced J ex . Therefore, the optimized spacer structure would be a bilayer composed of Ru/Re, which can effectively be against the inter-diffusion at the upper interface and remain lower Ru/Co interface to provide strong coupling strength. The interlayer coupling strength with Ru/Re spacer can remain more than half of the as-deposited value and reach 0.57 erg/cm 2 even after 1 h annealing at 450 °C, much higher than the value of single Ru spacer. The good performance upon 450 °C makes a wider process window for MTJs to be integrated to CMOS. Although our demonstrated coupling strength is still inferior to that of the Ir spacer, the cost of Re is much more affordable. Furthermore, our findings on the working mechanism based on the microstructure analysis can serve as a guideline for the material selection and layer structure design, not only for the SAF structure but also for other layers in the p-MTJs.
Re was inserted at top and/or bottom of Ru as the key player for interfacial engineering. The thickness of Ru and Re are varied as t 0 , t 1 , and t 2 for comparison. These samples were deposited on thermally oxidized Si (100) substrates by using a high vacuum DC magnetron sputtering system with base pressure of 8 × 10 −8 torrs. After deposition, samples were annealed at varied temperatures in a vacuum furnace with pressure better than 2 × 10 −5 torrs for 1 h.