Abstract
We demonstrate an optimized fabrication process for electric field (voltage gate) controlled nano-constriction spin Hall nano-oscillators (SHNOs), achieving feature sizes of <30 nm with easy to handle ma-N 2401 e-beam lithography negative tone resist. For the nanoscopic voltage gates, we utilize a two-step tilted ion beam etching approach and through-hole encapsulation using 30 nm HfOx. The optimized tilted etching process reduces sidewalls by 75% compared to no tilting. Moreover, the HfOx encapsulation avoids any sidewall shunting and improves gate breakdown. Our experimental results on W/CoFeB/MgO/SiO2 SHNOs show significant frequency tunability (6 MHz V-1) even for moderate perpendicular magnetic anisotropy. Circular patterns with diameter of 45 nm are achieved with an aspect ratio better than 0.85 for 80% of the population. The optimized fabrication process allows incorporating a large number of individual gates to interface to SHNO arrays for unconventional computing and densely packed spintronic neural networks.
Highlights
This has resulted in a new class of nanoscopic wide-band microwave oscillators known as spin Hall nano-oscillators (SHNOs),[13–25] which may be viewed as successors to the earlier spin torque nano-oscillators.[17,26]
Thanks to voltage controlled magnetic anisotropy (VCMA),[38–42] low-power manipulation of spintronic devices can be efficiently implemented[43–45] and the use of voltage gates has been studied in detail for faster magnetization switching,[40] control of spin–orbit torque[46] and spin accumulation.[44]
Further memristive control of SHNO arrays using resistive switching across voltage gates realizes synaptic neural network based on these oscillators and shows a path for neuromorphic computing.[34]
Summary
The discoveries of the spin Hall effect (SHE)[1–3] and the associated spin–orbit torque (SOT)[4–6] have played a crucial role in shaping recent research in spintronics.[7,8] Pure spin currents generated by the SHE in heavy metals (such as Pt, W, Ta etc.)[9–11] can generate anti-damping SOT in an adjacent ferromagnetic layer, counteract its Gilbert damping, and drive its magnetization into different types of auto-oscillatory precessional motion.[12–14] This has resulted in a new class of nanoscopic wide-band microwave oscillators known as spin Hall nano-oscillators (SHNOs),[13–25] which may be viewed as successors to the earlier spin torque nano-oscillators.[17,26] SHNOs have been studied in a wide range of geometries such as nano-pillars,[12] nano-gaps,[13,16] nano-wires[14,27,28] and nano-constrictions,[15,19] where the nano-constrictions stand out as promising and versatile thanks to their ease of fabrication, direct optical access to the magnetodynamical region,[15,23,29,30] a propensity for mutual synchronization in linear chains[31] and two-dimensional arrays,[32] affording them an order of magnitude higher quality factors, and easy implementation of neuromorphic computing concepts.[32–37]. Pure spin currents generated by the SHE in heavy metals (such as Pt, W, Ta etc.)[9–11] can generate anti-damping SOT in an adjacent ferromagnetic layer, counteract its Gilbert damping, and drive its magnetization into different types of auto-oscillatory precessional motion.[12–14] This has resulted in a new class of nanoscopic wide-band microwave oscillators known as spin Hall nano-oscillators (SHNOs),[13–25] which may be viewed as successors to the earlier spin torque nano-oscillators.[17,26]. Apart from this, HSQ resist forms poor quality SiO2 after the electron beam lithography (EBL) exposure and development,[50] which does not fit the scope of voltage-controlled devices To circumvent these problems, we have optimized the fabrication process, taking advantage of the widely used and easy to handle ma-N 2401 negative tone EBL resist.[51,52].
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