The evolution of computer memory has led to a multilevel hierarchy, balancing speed, storage density, and cost. As complementary metal oxide semiconductor (CMOS) memory limits are approached, interest grows in nonvolatile alternatives like magnetic random access memory (MRAM). Although perpendicular spin-transfer torque (p-STT-MRAM) is already in production, alternative technologies such as spin-orbit torque (SOT) are explored for high-speed and fast cache operation. However, SOT has drawbacks, such as a larger bit-cell area and the need for an applied magnetic field for deterministic switching. In the pursuit of faster, more ecient memory, all-optical switch- ing (AOS) technology seeks to achieve magnetization reversal through energy-ecient ultrafast writing with single-femtosecond laser pulses. This Ph.D. thesis addresses the objective of merg- ing helicity-independent HI-AOS with MRAM devices. The materials investigated are rare-earth transition-metal multilayer systems, with a particular focus on Tb/Co-based structures. Through a combination of experimental investigations and numerical simulations, we unraveled the intricate dynamics of HI-AOS and used them to develop functional nanometer-sized AOS magnetic tunnel junctions (AOS-MTJs). These results not only deepen our understanding of HI-AOS but also explore its applications in non-volatile data storage and energy-ecient computation at the nanoscale. The exploration of HI-AOS in thin films of rare-earth transition metal multilayer systems lays the foundation for reliable magnetization toggle reversal, exploring the response to different pulse durations and fluences. Key new findings include the observation of concentric rings with opposite magnetic orientations and the independence of fluence on pulse duration, challenging existing paradigms. A hypothesis emphasizing the role of local anisotropy decrease that favors a precessional type of HI-AOS is introduced. The development of a macrospin model, incorporating two-temperature model (2TM) principles, explains the precession-driven switching mechanism observed in the Tb/Co-based multilayer system. Simulations reveal dynamics influenced by parameters such as Gilbert damping, anisotropy tilt angle, and the quality factor Q. Atomistic simulations further validate the model’s predictions, emphasizing the significance of temperature-dependent magnetization and anisotropy e↵ects in the HI-AOS process. Practical applications require the fabrication and electrical char- acterization of MTJ devices. Optimization of tunnel magnetoresistance (TMR) values, reaching a maximum value of 74%, a significant improvement over previous demonstrations. The thesis main achievement was demonstrating field-free HI-AOS on 100 nm diameter patterned [Tb/Co] p-MTJ devices, using 50 fs laser pulses with an estimated absorbed energy of approximately 68.6 fJ per bit reversal. The research not only contributes to our understanding of HI-AOS but also opens future research paths. Recommendations for future directions include using atomistic and LLB models for a more comprehensive understanding, exploring alternative materials beyond Tb/Co, investigating deterministic switching using spin-transfer torque, and addressing device variability challenges. The thesis o↵ers a comprehensive exploration of HI-AOS, from its fundamental mechanisms to practi- cal applications in nonvolatile data storage and energy-ecient computation. The optimized TMR values and successful demonstration of AOS in nano-patterned devices are a first step toward a new memory technology. The use of both experimental and simulated methods strengthens the impor- tance of the obtained results for further progress in this area. In summary, this doctoral research not only has expanded the knowledge of helicity-independent all-optical switching but has also laid the foundation for innovative applications in nano-scale technology. The ongoing integration of pho- tonic and spintronic elements has the potential to bring about significant progress in data storage and computation.

Plus d'information :https://www.spintec.fr/phd-defense-all-optical-switching-of-spintronic-devices/​​
​

Pour suivre la soutenance ​​​en visioconférence : https://grenoble-inp.zoom.us/j/95554920195​

​
​​