Spin transfer-torque Magnetic Random Access Memory (STT-MRAM) is expected to play a major role in the future development of information and communication technologies. One of the drivers of the technology is the non-volatile character of this type of memory that allows to reduce the energy consumption compared to SRAM or DRAM. [1] Globally, we are seeking to develop technologies that help reduce carbon emissions. However, the nature of the materials involved in such technologies is also a concern both in terms of environmental impact and supply risk. [2,3] MRAM contains several materials classified as critical raw materials, in particular by the European Union (EU), being of high importance to the EU economy and whose supply is classified of high risk. [4] In order to improve the stability of the reference layer whose magnetic configuration has to remain fixed during the lifetime of the memory, a synthetic antiferromagnet (SAF) is typically used. This SAF structure is normally based on Co/Pt or Co/Pd multilayers antiferromagnetically coupled through a thin Ru layer. Platinum Group Metals (PGMs) have been classified as critical materials with high supply risk. In addition, obtaining these metals in their refined form requires extensive amounts of energy and CO2 emissions due to the low initial ore concentration. In this work, an assessment of the impact of the substitution of the Pt required in STT-MRAM for the SAF layer has been made. Alternative solutions can be envisaged such as the use of multilayers based on Co/Ni exhibiting perpendicular anisotropy [5]. A second more challenging approach based on the use of perpendicular shape anisotropy (PSA) does not rely on the use of PGMs. However, this PSA based approach is more challenging from technological point of view due to the requirement of a strict control of the cell diameter at sub-20nm dimensions during the etching process. The possibility of using Co/Ni multilayers for the SAF of MRAM has been already demonstrated [6,7]. We have observed how similar properties to Co/Pt SAF can be obtained by using Co/Ni multilayers (Figure 1). Our study has the added advantage of using a seed layer free of Hf, used in some previous studies to promote the required (111) texture for the Co/Ni multilayers growth [6,7]. Hf is also a material classified as critical and whose production is strongly concentrated in two countries, France and USA. In our study a Cu seed layer is used allowing for more diverse production sources. An evaluation of the environmental (energy and global warming potential) and economic impact of the substitution has been performed. Strong improvements in both economic and environmental aspects can be achieved by the substitution. However due to the tiny amounts of PGM materials used in STT-MRAM, in comparison to the mass of the silicon wafer itself, the environmental impact and price of the silicon wafer on which these memory devices are grown is much higher as shown in Figure 2. Our conclusion is that more attention should be focused on the process of fabrication of silicon wafers or find a substitution to this type of wafers whenever possible. Nevertheless, the high supply concentration of Pt as four mining companies represent the 80% of the global production [8] makes the presence of these alternatives a key aspect to ensure the survival of the technology in case of supply disruptions.

**References:**

1. B. Dieny and I. L. Prejbeanu, ‘Magnetic Random-Access Memory’, in Introduction to Magnetic Random-Access Memory, John Wiley & Sons, Ltd, pp. 101–164. (2017) doi:10.1002/9781119079415 2. European Commission. Critical materials for strategic technologies and sectors in the EU - a foresight study. (2020) doi: 10.2873/58081 3. Ku, A. Y. Sustainable Materials and Technologies, 15, 27-32. (2018) doi:10.1016/j.susmat.2017.10.001 4. European Commission. Study on the Review of the List of Critical Raw Materials - Critical Raw Materials Factsheets European Commission. (2017) doi: 10.2873/398823 5. Nuss P, Eckelman MJ. PLoS ONE 9(7): e101298. (2014) doi: 10.1371/journal.pone.0101298 6. Kar, G. S., Kim, W., Tahmasebi, T. et al., IEEE International Electron Devices Meeting. (2014) doi:10.1109/iedm.2014.7047080 7. Tomczak, Y., Lin, T., Swerts, J., et al. IEEE Transactions on Magnetics, 52(7), 1-4. (2016) doi:10.1109/tmag.2016.2515109 8. Sinsalo, Pia, and Mari Lundström. Metals, vol. 8, no. 4, p. 203. (2018) doi:10.3390/met8040203.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c8b57764016ecc1627245e1d0345fc1e.jpg)
Fig 1. VSM measurements on a MTJ with conventional SAF based on Co/Pt multilayers (red) versus a MTJ with SAF comprising Co/Ni multilayers (green) after annealing at 300°C for 10 minutes. Sample based on Co/Pt SAF has the following composition (thickness in nm) : Ta3/Pt25/[Co0.5/Pt0.25]6/Co0.5/Ru0.9/[Co0.5/Pt0.25]3/ Co0.5/Ta0.2/FeCoB1.2/MgO0.7/Ox/MgO0.5/FeCoB1.5/Ta3/Pt5. Sample based on (Co/Ni) multilayers is Pt-free and has the following composition Ta22/FeCoB0.8/Ta3/Cu3/[Co0.2/Ni0.4]6/Co0.5/Ru0.9/Co0.5/[Co0.2/Ni0.4]3/Ta0.2/FeCoB1.2/MgO0.7/Ox/MgO0.5/FeCoB1.5/Ta3/Al2.

![](https://assets.underline.io/uploads/markdown_image/1/image/471f429389b4483ff4e1a167fcaeea84.jpg)
Fig 2. Energy per 300mm wafer, global warming potential (GWP) and price in log scale required to obtain the amount of Pt comprised in a conventional SAF, of Co in the PSA based substitution, of Ni in the case of (Co/Ni) based SAF alternative and for the production of a 300mm bare Si wafer.

 Towards Spintronic Memory without Platinum Group Metals (PGMs) for Improved Sustainability

Due to the reduced switching current and easier scalability to smaller technology nodes than its in plane magnetized counterpart, Perpendicular Spin transfer-torque Magnetic Random Access Memory (p-STT-MRAM) had a quick industrial development in the last few years. The thermal energy barrier required to meet industrial data retention requirements is provided by a large interfacial anisotropy. For the free layer, this large perpendicular magnetic anisotropy (PMA) is induced at the interface of the ferromagnetic electrode and the oxide barrier. [1] The reference layer must have a thermal stability well above that of the free layer so that the free layer can change its magnetic configuration while the reference layer remains stable. Nowadays, this is typically achieved by coupling a synthetic antiferromagnet structure (SAF) as reference layer. The high thermal stability of the SAF comes from the interfacial magnetic anisotropy provided by magnetic multilayers such as Co/Pt or Co/Pd and the antiferromagnetic RKKY interaction between the two oppositely magnetized components of the SAF. However, the use of critical metals such as Pt is often undesired due to the high price, scarcity or the high supply risk associated to its highly concentrated production. [2,3] Therefore, alternatives based on more common materials such as Ni could help mitigate possible disruptions that might appear on more precious metals and reduce the vulnerability of STT-MRAM to such disruptions. A key factor to achieve a high perpendicular magnetic anisotropy (PMA) in Co/Ni multilayers is to induce a good (111) texture [4,5]. In this study, a smooth buffer layer has been developed first wherein reduced roughness was obtained thanks to the insertion of an amorphous FeCoB layer. Then a Cu seed layer has been used to induce the desired (111) texture with the added advantage of Cu being a very common non-critical metal. A low roughness value of 0.25nm could be obtained for a 5nm Cu seed, sufficiently thick to provide the required (111) texture. The coercive field of the Co/Ni SAF increases with the thickness of the Cu seed layer correlatively with the improved (111) texture, in agreement with previous results on Co/Ni multilayers [5] and SAF [6]. However, further increasing the Cu thickness yields a roughness increase at the tunnel barrier that can be detrimental to the optimal properties of the MTJ. Therefore, a tradeoff must be found on the Cu buffer thickness. Thin film measurements of the coercive field of the MTJ with Co/Ni SAF annealed at 300°C for 10 minutes show a switching field of 3.8 kOe, sufficiently high to meet the typical reference layer requirements. In nanopatterned devices, the reference layer is stable in both parallel and antiparallel magnetic configuration. However, during annealing at 400°C, the coercive field of the reference layer drastically decreases. This decrease of reference layer coercive field has already been observed when annealing Co/Ni Ultrathin SAF p-MTJ at high temperatures [7]. Increasing the Cu seed layer thickness, thus improving the (111) texture increases the switching field of the reference layer (Fig.1a) but can be detrimental as previously mention due to an increased roughness at the barrier level. In this study, we show that increasing the thickness of the Ta layer, used as a boron getter, separating the top part of the Co/Ni SAF from the FeCoB reference layer allows to keep higher coercive field of the (Co/Ni) SAF layer even after annealing at 400°C (Fig.1b). However, if this Ta layer is too thick (as in Fig.1b), the reference and SAF become insufficiently coupled. To circumvent this problem, we proposed here to replace the single Ta layer by a double Ta layer of composition (Ta 0.3nm/Co 1.8nm/Ta0.45nm). Such composite layer ensures a strong magnetic coupling while providing the benefit of a thicker diffusion barrier. As a result, a higher switching field of the reference layer of 5kOe after annealing at 400°C could be obtained (Figure 2) for an improved stability of the reference layer.

**References:**

1. Dieny, B., Prejbeanu, I.L., Garello, K. et al. Nat Electron 3, 446–459 (2020). doi : 10.1038/s41928-020-0461-5 2. European Commission. Study on the Review of the List of Critical Raw Materials - Critical Raw Materials Factsheets European Commission. (2017) doi: 10.2873/398823 3. Ku, A. Y. Sustainable Materials and Technologies, 15, 27-32. (2018) doi:10.1016/j.susmat.2017.10.001 4. Enlong Liu, J. Swerts, T. Devolder et al. Journal of Applied Physics 121, 043905 (2017); doi : 10.1063/1.4974885 5. Justin M. Shaw, Hans T. Nembach, and T. J. Silva. Journal of Applied Physics 108, 093922 (2010); doi : 10.1063/1.3506688 6. Arora, M., Lee-Hone, N. R., Mckinnon, T. et al. J. Phys. D: Appl. Phys. 50 505003 (11pp). (2017) doi:10.1088/1361-6463/aa97fa 7. Tomczak, Y., Lin, T., Swerts, J. et al. IEEE Transactions on Magnetics, 52(7), 1-4. (2016) doi:10.1109/tmag.2016.2515109

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ccef82b6b35bb013346bc9476ba5e5ab.jpg)
Figure 1. Coercive field (Hc) of the SAF structure increases with both Cu seed layer thickness and Ta getter thickness after annealing at 400°C for 10 minutes (a). MOKE results display the Hc dependence on the Ta getter thickness shown in (a) for a fixed 5nm Cu seed layer thickness (b). The full structure in nm: Ta 22/FeCoB 0.8/Ta 3/ Cu x /6x [Co 0.18/ Ni 0.55] /Co 0.5 / Ru 0.9 / Co 0.5 / 3x [Ni 0.55/ Co 0.18] / Ta Y / FeCoB 1.2 / MgO 1.2/ FeCoB 0.4 / Ta 3 /Pt 5.

![](https://assets.underline.io/uploads/markdown_image/1/image/e0640dbefefc13d33d46ca1c1340fc75.jpg)
Figure 2. Vibrating sample magnetometer (VSM) measurement of a full MTJ with double Ta composite B-getter layer (Ta 0.3nm/Co 1.8nm/Ta0.45nm) displaying the SAF switching at field as high as 5kOe. The full structure is in nm: Ta 22/FeCoB 0.8/Ta 3/ Cu 5 /6x [Co 0.18/ Ni 0.55] /Co 0.5 / Ru 0.9 / Co 0.5 / 3x [Ni 0.55/ Co 0.18] / double Ta composite B-getter layer /FeCoB 1.2 / MgO 1.2/ FeCoB 1 / Ta 0.2 /FeCoB 0.8/ MgO 1.2/ FeCoB 0.4/Ta 3 / Pt 5.

 SAF based on Co/Ni multilayers with improved annealing tolerance for sustainable Pt-free reference layer of STT-MRAM

The perpendicular Spin-Transfer-Torque Magnetic Random Access Memory (p-STT-MRAM) is one of the most promising emerging non-volatile memory technologies. As these devices are limited by their thermal stability factor at technological nodes smaller than 20 nm, their downsize capability is compromised. Indeed, as the device shrinks, there is a decrease in thermal stability due to a decrease in the total interfacial perpendicular anisotropy energy proportionally to the cell area. This decrease significantly reduces the retention time of the memory [1-3]. A promising solution to this problem relies on taking advantage of the shape anisotropy of the storage layer by increasing its thickness to values larger than its diameter. The shape anisotropy becomes out-of-plane, with strength proportional to the large volume of the thick cell, bringing an additional robust and tunable source of perpendicular anisotropy on top of the interfacial perpendicular anisotropy. These two sources of anisotropy allow to extend the downsize scalability of STT-MRAM towards sub-20 nm technological nodes [4, 5]. However, the storage layer thickness also affects the writing operation of the cell and should be examined. In this work, the magnetization reversal mechanism of the PSA-STT-MRAM induced by STT was numerically studied. Micromagnetic simulations were carried out (with and without the effect of thermal fluctuations), enabling the identification of different modes of magnetization reversal in this memory. An initial study comprises pillars with aspect-ratios (AR) between 0.8 and 3, with a fixed diameter of 20 nm. It was shown that, for an AR smaller than 1, the mechanism of reversal follows a macrospin-like reversal. When increasing the AR, a non-coherent reversal develops, which evolves from a buckling-like reversal to a transverse-domain wall nucleation-propagation at higher aspect ratios. This latter mode of reversal is associated with the slowing down of the switching dynamics, an effect which worsens as the layer thickness is increased [Fig. 1]. The inverse of the switching time follows a linear relationship with the applied voltage, a law conserved when considering thermal fluctuations. This demonstrates that, even though the nucleation process is assisted by thermal fluctuations, the reversal is controlled by the STT. Changing both the diameter and the thickness but maintaining a thermal stability factor of around 80 it is observed that the minimum voltage to reverse the storage layer increases [Fig. 2]. We interpret as the weakening influence of the strong interfacial STT of tunnelling electrons. This comes along with a transition from a macrospin-like reversal to a buckling-like reversal when AR is increased, despite the thermal stability value is in the same range. This study shows the possibility to obtain a macrospin-like reversal at low applied voltages at sub-20 nm magnetic tunnel junctions. With their switching time of the order of tens of ns, these PSA-STT-MRAM are not fast-enough memories suitable for Cache application but should be functional for dense DRAM type of applications. Also, the junction Resistance x Area product must be lowered compared to conventional STT-MRAM to maintain the write voltage significantly below the barrier breakdown voltage.

**References:**

[1] C. Yoshida et al, Jpn. J. Appl. Phys. 58, SBBB05 (2019). [2] H. Sato et al, Jpn. J. Appl. Phys. 58, 0802A6 (2017). [3] L. Thomas et al, J. Appl. Phys. 115, 172615 (2014). [4] N. Perrissin et al, Nanoscale 10, 12187-12195 (2018) [5] K. Watanabe et al, Nat. Com. 9, 663 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/f528241113b078615ca57fd5629207ff.jpg)
Fig 1: Time traces of the mean reduced magnetization 〈mz〉for different pillar thicknesses for an applied voltage of -3 V.

![](https://assets.underline.io/uploads/markdown_image/1/image/67e3ba17811e7244cfa1ee6d319e6030.jpg)
Fig 2: Voltage dependence of the time to reverse 50% of the magnetic layer (switching time - τswitch) for different pillar sizes.

 Magnetization reversal driven by spin-transfer-torque in sub-20 nm perpendicular shape anisotropy magnetic tunnel junctions

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Towards Spintronic Memory without Platinum Group Metals (PGMs) for Improved Sustainability

SAF based on Co/Ni multilayers with improved annealing tolerance for sustainable Pt-free reference layer of STT-MRAM

Magnetization reversal driven by spin-transfer-torque in sub-20 nm perpendicular shape anisotropy magnetic tunnel junctions

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