Local bifurcation with spin-transfer torque in superparamagnetic tunnel junctions

Abstract

Modulation of the energy landscape by external perturbations governs various thermally-activated phenomena, described by the Arrhenius law. Thermal fluctuation of nanoscale magnetic tunnel junctions with spin-transfer torque (STT) shows promise for unconventional computing, whereas its rigorous representation, based on the Néel-Arrhenius law, has been controversial. In particular, the exponents for thermally-activated switching rate therein, have been inaccessible with conventional thermally-stable nanomagnets with decade-long retention time. Here we approach the Néel-Arrhenius law with STT utilising superparamagnetic tunnel junctions that have high sensitivity to external perturbations and determine the exponents through several independent measurements including homodyne-detected ferromagnetic resonance, nanosecond STT switching, and random telegraph noise. Furthermore, we show that the results are comprehensively described by a concept of local bifurcation observed in various physical systems. The findings demonstrate the capability of superparamagnetic tunnel junction as a useful tester for statistical physics as well as sophisticated engineering of probabilistic computing hardware with a rigorous mathematical foundation.

Review of Magnetic Tunnel Junctions for Stochastic Computing

“…Asynchronous measurements are typically obtained for thermally unstable MTJs with < 20, where thermal fluctuations drive random switching between states, as illustrated in Fig. 2(d) [54], [55], [56], [57], [58]. These devices typically show random fluctuations near the switching fields, as illustrated in Fig.…”

CMOS plus stochastic nanomagnets enabling heterogeneous computers for probabilistic inference and learning

“…Another advantage of sMTJ-based p-bits is the continuous generation of truly random bitstreams without the need to be reset in synchronous pulse-based designs 17 , 18 . The possibility of designing energy-efficient p-bits using low-barrier nanomagnets has stimulated renewed interest in material and device research with several exciting demonstrations from nanosecond fluctuations 19 – 21 to a better theoretical understanding of nanomagnet physics 22 – 25 and novel magnetic tunnel junction designs 26 , 27 .…”

Temperature dependence of the properties of stochastic magnetic tunnel junction with perpendicular magnetization

“…Several device-level studies clarified physical mechanisms that govern the properties of single s-MTJs, 7,[23][24][25][26] but all of them were performed only at RT. On the other hand, to expand the applicable operation range of the spintronicsbased p-computers, it is important to understand the temperature dependence of the properties of s-MTJs and design the device to suppress it.…”

“…The speed of p-computers scales with that of the switching dynamics (time-domain response) of the individual p-bits, viz., the relaxation rate 1/τ of s-MTJs. According to the Néel-Arrhenius law, 26,27)…”