Spin-orbit torques (SOTs) in heavy metal (HM)/ferromagnet (FM) have gathered much attention due to its potential for non-volatile, high-speed, and low-power spintronic devices 1-3. While the majority of research has focused on the development of digital memories, previous studies on antiferromagnet (AFM)/FM structures showed field-free as well as analog-like (memristive) switching behavior 4,5, opening a new paradigm, the neuromorphic spintronics 6. For applications, one needs to address a challenge that the memristive characteristics diminish with a reduction of device dimensions; for the case of PtMn/Co/Ni system, it totally disappears at around 200 nm 7,8. In this study, we introduce a nanocomposite FM CoPtCrB towards SOT-based nanoscale memristors.
A stack with Si sub./ Ta(4)/ Pt(4)/ CoPtCrB(7)/ Ru(1) is processed into single nanodot devices with various dot diameters (Ddot: 30-1000 nm) through the electron beam lithography and Ar ion milling. Memristive characteristics are studied by switching experiments with perpendicular magnetic fields or pulse currents (Ich) under an in-plane external field (Hx).
Fig. 1 shows the results of SOT switching at Âµ0Hx = 90 mT for Ddot = 400 nm. Memristive behavior is observed, in contrast to the previous work with uniform FM exhibiting much fewer intermediate states 7. Fig. 2 shows the obtained relationship between the square root of the number of intermediate states NIS and Ddot. Ddot below which devices show binary switching is around 60 nm, which is much smaller than the previous study (~ 200 nm) 7. The present study demonstrates the potential of nanocomposite FM for nanoscale spintronic memristors.
The authors thank A. Kurenkov, T. Koga, K. K. Tham, and S. Saito for their support. This work is partly supported by JST-CREST JPMJCR19K3, JSPS Kakenhi 19H05622, and RIEC Cooperative Research Projects. K. V. D. Z. acknowledges WISE program for AIE for financial support.