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Magnetoresistive random access memory (MRAM) based on spin transfer torque (STT) is entering volume production in the semiconductor industry. While STT-MRAM offers nonvolatile embedded memory operation with high endurance, its ultimate energy efficiency, speed and scalability are limited by its current-controlled write mechanism. In this talk we discuss novel device candidates, physics, and materials which may enable approaching the fundamental limits of speed and energy efficiency in spintronics. Building on the success of STT-MRAM, these emerging device candidates may not only address a broader cross-section of the memory hierarchy, but also enable new computing architectures with simultaneously ultralow-power and high-performance attributes, which are important for machine intelligence on both edge and cloud platforms.
We first review the recent progress of voltage-controlled ferromagnetic memory devices, which use interfacial magnetoelectric effects arising from spin-orbit interaction, to switch the magnetization in memory bits. We discuss progress in the development of magnetic tunnel junctions using voltage-controlled magnetic anisotropy (VCMA) for switching, which exhibit the lowest-energy MRAM cells to date (~ 5 fJ/bit with precessional switching times ~1 ns [1-6]). The device and materials-level challenges and opportunities are discussed, including biasing, VCMA coefficients, and write error rates.
As a strategy to further reduce switching time, and improve energy efficiency of VCMA-based MRAM towards picosecond and atto-Joules respectively, we then examine the VCMA effect in new free layer structures consisting of antiferromagnetic materials. We propose a method for switching the Néel vector of an antiferromagnetic thin film, by the application of an ultrashort electric field pulse [7]. The electric field induces a reorientation of the antiferromagnetic order parameter, due to the voltage-induced modification of the magnetic anisotropy. When the electric field pulse is timed to half the oscillation period of the Terahertz antiferromagnetic dynamics, it induces a picosecond time-scale reversal of the Néel vector. Importantly, the electric field required to induce this reversal is as small as ~ 100 mV/nm, comparable to fields used for switching of ferromagnetic tunnel junctions in earlier works. This electric field is determined by the anisotropy of the antiferromagnet, while the much larger exchange field determines the frequency of the resulting dynamics (and hence the switching time). Our simulation results indicate the possibility to switch a 50 nm circular antiferromagnetic element with an energy dissipation of 250 aJ, in less than 30 ps, and in the absence of any current-induced torque. The electric-field-induced switching of the Néel vector opens a new route towards energy-efficient and ultrafast magnetic memories and computing devices based on antiferromagnets.
1) P. Khalili Amiri et al., "Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling", IEEE Transactions on Magnetics, 51, 3401507, (2015).
2) C. Grezes et al., "Ultra-low switching energy and scaling in electric-field-controlled nanoscale magnetic tunnel junctions with high resistance-area product", Applied Physics Letters, 108, 012403 (2016).
3) W.-G. Wang et al., "Electric-field-assisted switching in magnetic tunnel junctions", Nature Materials, 11, 64–68 (2012).
4) Y. Shiota et al., "Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses", Nature Materials, 11, 39–43 (2012).
5) Y. Shiota et al., "Reduction in write error rate of voltage-driven dynamic magnetization switching by improving thermal stability factor", Applied Physics Letters, 111, 022408 (2017).
6) S. Kanai et al., "Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction", Applied Physics Letters, 101, 122403 (2012).
7) V. Lopez-Dominguez, H. Almasi, P. Khalili Amiri, "Picosecond electric-field-induced switching of antiferromagnets", Physical Review Applied, 11, in press (2019).
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