PhD defence : Spin orbit torques for cache memory applications

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The development of a non-volatile memory with unlimited endurance that can be used at the processor clock frequency is a challenge for the microelectronics industry, currently facing energy over-consumption issues related to the miniaturization and the operating speed of components. The emergence of such a memory would modify the memory hierarchy by integrating non-volatility in the core of the processors.

The SOT-MRAM (magnetic random access memory using the spin orbit torques (SOT) for writing information), is a promising candidate as it combines the three required features: non-volatility, write reliability and sub-ns write time. Nevertheless, many challenges remain before the SOT-MRAM can be used for these purposes.

The objective of this thesis is to focus on two of these challenges: the study and understanding of the magnetization reversal by spin-orbit torques using sub-nanosecond current pulses, and the decrease of write currents using assistance from an electric field.

We have studied both the elementary brick of these memories, the Ta/CoFeB/MgO trilayer, as well as SOT-MRAM cells. We first demonstrated the ultra-fast write process, largely sub-ns, and showed that the reversal of the magnetization occurs through the nucleation of a small reversed domain followed by the propagation of the domain wall, while pointing at a lack in this microscopic understanding. We then modulated the magnetic anisotropy of these samples using an electric field and showed that its decrease leads to the SOT write current reduction. These results are very encouraging and point at the interest of further studies / optimizations in order to use this assistance in functional devices.

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