Double barrier magnetic tunnel junctions for innovative spintronic devices

Abstract : One of the dilemmas faced by the present STT-MRAM technology is the reduction of the power consumption and increase of data access speed without jeopardizing the data retention. A possible solution lies on the double barrier magnetic tunnel junction (DBMTJ) where the amplitude of the spin transfer torque (STT) on the storage layer can be tuned through a proper magnetic configuration of the outer electrodes. Thus providing more reliable read/write operation modes for MRAM. Despite the reduction in half of the switching current, previous studies on DBMTJs with in-plane magnetization report undesired switchings in read mode associated with field-like torque. In this thesis, we further investigate the complex interplay between damping-like and field-like torques in these double barrierstructures. Measurements using DC current and short voltage pulses in DBMTJ with symmetric and asymmetric barriers have revealed a strong presence of the field-like torque both in write and read modes. Moreover, in DBMTJs with symmetric barriers set in read mode, we demonstrate pure field-like torque switching which is proportional to a quadratic voltage and adjusted by a b2 prefactor. Furthermore, this torque favors a antiparallel alignment between the storage layer magnetization and the two references’ magnetizations. The results obtained experimentally were in agreement with macrospin simulation performed with a proper tuning of the damping-like and field-like torque prefactors. In order to suppress the field-like torque and aiming for a further reduction of the writing currents and enhancedscalability of MTJs, we developed and realized DBMTJs with perpendicular anisotropy (p-DBMTJs). Novel seedless multilayers with improved perpendicular magnetic anisotropy to be used as top reference were designed and implemented in functional p-DBMTJs. The optimized p-DBMTJs were patterned into sub-300nm nanopillars and the spin transfer torque studied experimentally in write and read modes.The use of W instead of Ta as a spacer in the FeCoB/spacer/FeCoB composite storage layer showed a 3x improvement of STT efficiency. In write mode, p-DBMTJs have also demonstrated a considerable enhancement of STT efficiency when compared to single barrier p-MTJs. In read mode, switching has been prevented at the center of the bistable region but its thermal stability degraded with high voltage. Among several proposed explanations of this phenomenon, the reduction of the saturation magnetization and effective anisotropy with increasing temperature has been supported by macrospin simulations as the most probable one.
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Paulo Veloso Coelho. Double barrier magnetic tunnel junctions for innovative spintronic devices. Condensed Matter [cond-mat]. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAY048⟩. ⟨tel-02046789⟩

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