Wednesday August 07 at 11:00 we have the pleasure to welcome Cécile Grezes, Electrical Engineering department, UCLA, Los Angeles. She will give us a seminar at CEA/SPINTEC, Bat 1005, room 434 entitled :

Magnetic tunnel junctions using voltage control of the magnetic anisotropy for electric-field-controlled MRAM

Building on the advances of magnetoresistive random access memory (MRAM) based on spin transfer torque (STT) [1], a number of device proposals have emerged to overcome its limitations and expand its application areas. Among these, electric-field-controlled MRAM utilizing writing by voltage-control of the magnetic anisotropy (VCMA) [2] has potential for significant reduction of the write power consumption and enhanced bit density compared to STT-MRAM [3], while retaining the other desirable characteristics inherent to magnetic tunnel junctions (MTJs), such as high endurance, high read margin and non-volatility. In this seminar, I present our progress in the development of electric-field-controlled MRAM based on VCMA effect. In the first part, I will show results obtained from electric-field-controlled perpendicular MTJs (p-MTJs) test arrays with bit size down to 50 nm. We demonstrate bidirectional voltage-induced switching with write energies of 6 fJ/bit for switching speed of 0.5 ns [4], a dynamic power consumption significantly lower than that of STT-controlled devices. Write error rate, read disturbance and effects of external in-plane magnetic fields are also presented [5, 6], along with simulation of a 256 Kbit MRAM macro based on the measured MTJ device results. In the second part, I will present our progress on material development for improving performance and scalability. By engineering the MTJ stack, both the perpendicular magnetic anisotropy (PMA) and VCMA effect can be increased to achieve respectively longer retention time and lower write voltage. Along this line, we explore synthetic ferromagnet (SyF) free layer comprising at least two ferromagnetic layers separated by ultra-thin coupling metallic layers which are known for their large PMA properties [7]. Several configurations in single and double MgO-based p-MTJs are investigated with SyF free layer using single and multiple insertion of high spin-orbit coupling metals into different ferromagnetic materials. By combining SyF free layer with oxidation engineering at the MgO interface, we demonstrate VCMA and PMA enhancement by a factor 2.5 [8] compared to structures used in state-of-the-art VCMA-switching experiments [4, 9]. This achievement opens the way to sub-fJ write power consumption in 50 nm MTJ devices of 10 years retention time. The challenges for scaling down devices at advanced technology nodes will be discussed, along with possible future directions.

[1] Y. Lu et al., Proc. IEDM., pp 26.1.1 – 26.1.4 (2015).
[2] Y. Shiota et al., Nature Mater. 11 (1), 39 (2012).
[3] P. Khalili Amiri et al., IEEE Trans. Magn. 51, 3401507 (2015).
[4] C. Grezes et al., Appl. Phys. Lett. 108, 012403 (2016).
[5] C. Grezes et al., AIP Advances 6 (7), 075014 (2016).
[6] C. Grezes et al., IEEE Magnetics Letters 8, p 1-5 (2016).
[7] S. Yakata et al., Appl. Phys. Lett. 95, 242504 (2009).
[8] C. Grezes et al., to appear in J. Phys. D special issue on MRAM 2019.
[9] T. Yamamoto et al., Phys. Rev. Appl. 11, 014013 (2019).