Application of Plasmonic Enhancement in Spin-Transfer Torque Random Access Memory (STT-RAM)
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Application of Plasmonic Enhancement in Spin-Transfer Torque Random Access Memory (STT-RAM)


Spin-transfer torque random access memory (STT-RAM) is a new memory technology which has attracted significant interest in recent years, due to its low power consumption, scalability, and high speed. Specifically for its potential use in space applications, it is important to quantify its robustness under ionizing radiation, e.g., gamma radiation. To this end, here, utilizing plasmonic sensing is proposed and theoretically demonstrated for in situ detection of defects caused by ionizing radiations. Plasmonic nanostructures are highly used for sensing purposes as they support strong field enhancement in nanoscale, making them highly sensitive to the refractive index change of their surrounding medium. STT-RAM structure in its simple form includes a dielectric barrier layer (MgO) sandwiched between two ferromagnetic layers (CoFeB), all placed between two metal layers for electrical contact (Au). The metal layers in STT-RAM structure can be utilized to support plasmonic resonance thus enable sensing of nanoscale changes that can be caused by irradiation. This is demonstrated via creating a nanohole array pattern in the STT-RAM multilayer. Moreover, the sensing performance is further improved by designing a dimer nanohole array.Fabrication of STT-RAM multilayer requires the presence of a dielectric substrate, onto which the layers are deposited. Therefore, there is a need for adhering the Au contact layer onto common dielectric substrate materials. However, as Au is a noble metal, an adhesion layer is necessary to achieve its firm bonding onto the substate. Conventional adhesion materials include metals like Cr, Ti, and Ta, which adversely affect the plasmonic resonance thus degrading the sensing performance. Here, MgO is proposed as an alternative low loss adhesion layer and tested optically and mechanically. It is shown that is has negligible effect on plasmonic resonance as well as strong adhesion performance. Finally, an opto-electronic readout scheme for STT-RAM cells is proposed and theoretically demonstrated. This method offers simplification of the readout circuitry as well as offering potential for high-speed readout of memory cells, without using conventional photodetectors. The proposed readout scheme is based on plasmonic enhancement of a nonlinear phenomenon called optical rectification. A single optical beam illuminates an array of STT-RAM cells, inducing electrical voltage in each cell, which is used to determine the memory state. Plasmonic enhancement is shown to provide approximately 20 times enhancement in variation of the photo-induced voltage with respect to the memory state. As a result, the proposed readout method offers potential for improving the memory readout rate, provided that proper supporting electronic circuitry is available.

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