To increase the storage density, multi-level MRAM has been reported using complex magnetoresistive spin valve with perpendicular anisotropy []. The structure consists of two FLs and one reference layer, all based on Co/Pd multilayer exhibiting different four resistance levels. The selection of free-layer for storing data can be done by distinct level of resistance and their respective STT switching current []. Writing improvements As discussed earlier, spin-torque transfer (STT) principle has been widely accepted as the method to write information. Any new methods implemented in the design of MRAM must improve the writing speed, reduce the writing current and consumed power. This section discusses, in detail, the methods in which writability or speed can be increased. Equation (2) indicates that the critical current to switch the magnetization of the free layer depends on the damping factor  Therefore, it is essential to find materials that have a low damping factor, in order to reduce writing current. Materials such as FePt or Co/Pd, which were once researched for pMTJ applications due to their high Ku, have a large damping factor. Therefore, even though these materials could be scaled down to smaller devices with high thermal stability, writing information on these devices would consume significant power. In this context, the invention of interface-pMTJ based on Ta/CoFeB/MgO came as a boon for the STT-MRAM. CoFeB has a much lower damping factor and could be written with much lower currents, as compared to FePt or Co/Pd. Interface-pMTJ devices could be used down to 20 nm sizes, beyond which one may consider materials such as Co/Pd, Co/Pt or FePt, although the writing issues on such
VCMA effect relies on electric field induced change of atomic orbital occupancy at interface. This change is in synchronic connection with spin-orbital interaction, which consequently results in the change of anisotropy []. A qualitative explanation based on Rashba effect also has been given for VCMA, which relies on the Dzyaloshinskii–Moriya mechanism[]. In 2010, electric field effects on thickness dependent magnetic properties of CoFeB layers was investigated at room temperature. This study demonstrated the electric field manipulation of magnetic anisotropy in in-plane 2 nm thick CoFeB. On the other hand, for 1.16 nm and 1.02 nm thick perpendicularly magnetized CoFeB electric field induced alteration of coercivity was observed []. Wang et al. in 2012 demonstrated the switching in perpendicular MTJs []. The usual MTJs have thick ferromagnetic layers (>3 nm), where the magnetic anisotropy is independent of the ferromagnetic barrier oxide layer interface [, , , ]. The electric field effect in these layers is negligible due to the screening of electrons. In the case of electric field assisted switching, the thickness of CoFeB is relatively thin (1.6 nm). For this thickness, the PMA arises from the oxide layer interface. As a result of the applied field, the interface anisotropy is affected and the magnetic orientation is changed [, ]. The main advantage of using the VCMA based memory is the dramatic reduction in power dissipation. By reducing the required current to operate the device, Ohmic dissipation is significantly reduced, which in most cases of magnetic memory, is the predominant loss mechanism [, ]. Contrary to STT-MRAM for which the pulse polarity determines the switching direction, the write process in VCMA based memory is only one write pulse amplitude and width is used to achieve both 0-to-1 and 1-to-0 switching. Hence, a pre-read step is required to determine the initial state (0 or 1) before writing. On the other hand, the barrier being voltage dependent, reading error
34. Liu, L., et al., Spin-torque switching with the giant spin Hall effect of tantalum. Science, 2012. 336(6081): p. 555-558. 35. I.M., et al., Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature, 2011. 476(7359): p. 189-193. 36. Akerman, J., Applied physics. Toward a universal memory. Science, 2005. 308(5721): p. 508-10. 37. Marrows, C.H., L.C. Chapon, and S. Langridge, Spintronics and functional materials. Materials Today, 2009. 12(7-8): p. 70-77. 38. Dieny, B., et al., Magnetotransport properties of magnetically soft spin‐valve structures. Journal of Applied Physics, 1991. 69(8): p. 4774-4779. 39. Parkin, S. and D. Spin engineering: Direct determination of the far-field range function in ruthenium. Physical Review B, 1991. 44(13): p. 7131. 40. Zhu, J.-G., Spin and dual spin valve heads with synthetic antiferromagnets. Magnetics, IEEE Transactions on, 1999. 35(2): p. 655-660. 41. Sbiaa, R., H. Meng, and S. Piramanayagam, Materials with perpendicular magnetic anisotropy for magnetic random access memory. physica status solidi (RRL)-Rapid Research Letters, 2011. 5(12): p. 413-419. 42. Thomson, W., On the electro-dynamic qualities of metals:--effects of magnetization on the electric conductivity of nickel and of iron. Proceedings of the Royal Society of London, 1856. 8: p. 546-550. 43. Nickel, J., Magnetoresistance overview. 1995: Hewlett-Packard Laboratories, Technical Publications Department Palo Alto, CA, USA. 44. R., Magnetoresistive Read Heads: Fundamentals and Functionality. Developments in Data Storage: Materials Perspective, 2011: p. 97.
74. Yuasa, S., et al. Giant tunneling magnetoresistance in MgO-based magnetic tunnel junctions and its industrial applications. in Nanotechnology Materials and Devices Conference, 2006. NMDC 2006. IEEE. 2006. IEEE. 75. Yuasa, S., et al., High tunnel magnetoresistance at room temperature in fully epitaxial Fe/MgO/Fe tunnel junctions due to coherent spin-polarized tunneling. Japanese Journal of Applied Physics, 2004. 43(4B): p. L588. 76. Edmund, C.S., Ferromagnetism: magnetization curves. Reports on progress in physics, 1950. 13(1): p. 83. 77. Daughton, J., Magnetoresistive memory technology. Thin Solid Films, 1992. 216(1): p. 162-168. 78. Zhu, J.-G., Magnetoresistive random access memory: The path to competitiveness and scalability. Proceedings of the IEEE, 2008. 96(11): p. 1786-1798. 79. Prejbeanu, I., et al., Thermally assisted MRAM. Journal of Physics: Condensed Matter, 2007. 19(16): p. 165218. 80. R.S., et al., Curie point written magnetoresistive memory. Journal of Applied Physics, 2000. 87(9): p. 6403-6405. 81. Hosomi, M., et al., A novel nonvolatile memory with spin torque transfer magnetization switching: Spin-RAM. IEDM Tech. Dig, 2005. 459. 82. Tsymbal, E.Y., O.N. Mryasov, and P.R. LeClair, Spin-dependent tunnelling in magnetic tunnel junctions. Journal of Physics: Condensed Matter, 2003. 15(4): p. R109. 83. Parkin, S., et al., Magnetically engineered spintronic sensors and memory. Proceedings of the IEEE, 2003. 91(5): p. 661-680. 84. Yang, T., et al., Manipulating Spin Current in the Magnetic Nanopillar. Journal of nanoscience and nanotechnology, 2007. 7(1): p. 259-264.
105. Worledge, D., et al., Spin torque switching of perpendicular Ta form=. Applied Physics Letters, 2011. 98(2): p. 2501. 106. Tahmasebi, T. and S.N. Piramanayagam, Magnetoresistance Device. 2012, Google Patents. 107. Sato, H., et al., Perpendicular-anisotropy magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure. Applied Physics Letters, 2012. 101(2): p. 022414. 108. Sbiaa, R., et al., Spin transfer torque switching for multi-bit per cell magnetic memory with perpendicular anisotropy. Applied Physics Letters, 2011. 99(9): p. 092506. 109. Sbiaa, R., Magnetization reversal driven by a spin torque oscillator. Applied Physics Letters, 2014. 105(9): p. 092407. 110. Jourdan, M., Revival of Heusler compounds for spintronics. Materials 2014. 17(8): p. 362-363. 111. Diao, Z., et al., Spin transfer switching in dual MgO magnetic tunnel junctions. Applied Physics Letters, 2007. 90(13): p. 132508. 112. Hu, G., et al. STT-MRAM with double magnetic tunnel junctions. in Electron Devices Meeting (IEDM), 2015 IEEE International. 2015. IEEE. 113. Liu, H., et al., Precessional reversal in orthogonal spin transfer magnetic random access memory devices. Applied Physics Letters, 2012. 101(3). 114. Liu, H., et al., Ultrafast switching in magnetic tunnel junction orthogonal spin transfer devices. Applied Physics Letters, 2010. 97(24). 115. Sbiaa, R., S. Piramanayagam, and T. Liew, High speed in spin‐torque‐based magnetic memory using magnetic nanocontacts. physica status solidi (RRL)-Rapid Research Letters, 2013. 7(5): p. 332-335.
226. Schütte, C. and M. Garst, Magnon-skyrmion scattering in chiral magnets. Physical Review B, 2014. 90(9): p. 094423. 227. Zhang, X., et al., All-magnetic control of skyrmions in nanowires by a spin wave. Nanotechnology, 2015. 26(22): p. 225701. 228. Schwarze, T., et al., Universal helimagnon and skyrmion excitations in metallic, semiconducting and insulating chiral magnets. Nature materials, 2015. 14(5): p. 229. Schroeter, S. and M. Scattering of high-energy magnons off a magnetic skyrmion. Low Temperature Physics, 2015. 41(10): p. 817-825. 230. Kong, L. and J. Zang, Dynamics of an insulating skyrmion under a temperature gradient. Physical review letters, 2013. 111(6): p. 067203. 231. Lin, S.-Z., et al., AC current generation in chiral magnetic insulators and skyrmion motion induced by the spin Seebeck effect. Physical review letters, 2014. 112(18): p. 187203.