Tracking spin-orbit torque switching in magnetic tunnel junctions in real time
Being able to fast switch magnetization by electrical currents is required for developing efficient non-volatile magnetic random access memories. By probing the real-time magnetization reversal, we show that spin-orbit torques can drive switching with a latency smaller than 0.2 ns.
A universal digital memory has to be fast, non-volatile, durable, and low-cost. While all these properties are difficult to combine in a single design, magnetic random access memories, so-called MRAMs, may go a long way in terms of scalability, data retention time, and endurance. MRAMs encode digital bits in the magnetization direction of nm-thick ferromagnetic layers embedded in a magnetic tunnel junction. The state of a bit is read out by a tunneling magnetoresistance measurement and written by current-induced spin transfer torque. In the last years, the optimization of the tunneling magnetoresistance, magnetic anisotropy, and spin transfer torque efficiency in MgO-based tunnel junctions has enabled large read-out signals, long retention times, and electrical switching at moderate current density. Owing to these advances and to their favorable scaling properties, MRAMs are being developed as a non-volatile replacement for SRAMs and eFlash in embedded cache memories, with potential applications also as a persistent DRAM technology. Ultimately, however, MRAMs will be limited to switching rates smaller than 1 GHz because of the relatively large switching latency of spin transfer torques and high currents required to induce magnetization reversal on the sub-ns timescale, which can damage the tunnel junction at the core of each bit cell.
The realization of faster memories requires fundamentally novel concepts to overcome the temporal bottlenecks of spin torque magnetization switching, namely the long incubation and broad variability of switching times. Current-induced spin-orbit torques, which arise from the transfer of angular momentum from the lattice to the spin system (as exemplified by the spin Hall and Rashba-Edelstein effects), offer a solution to the switching speed and reliability problem. Differently from spin-transfer torques, which arise from the transfer of spin angular momentum between two magnetic layers having noncollinear magnetization, spin-orbit torques apply also to uniform magnetic textures and do not require the electric current to flow inside a magnet. Further, in a perpendicular magnetic tunnel junction, the spin polarization giving rise to the spin-orbit torques is orthogonal to the quiescent magnetization of the free layer, which reduces the switching incubation time. These properties make spin-orbit torques extremely appealing for ultrafast and reliable MRAMs.
Our single-shot measurements of spin-orbit torque switching of a 3-terminal magnetic tunnel junction disclose the full time-dependent dynamics of magnetization reversal during current injection, unveiling both deterministic and stochastic events. By comparing magnetization switching induced by spin-orbit and spin-transfer torques in the same device, we evidence striking differences in the time scales and efficiency of the two reversal mechanisms. Further, by taking advantage of the multiple bias and magnetic configurations of a 3-terminal magnetic tunnel junction, we demonstrate novel strategies to achieve sub-ns reversal with statistical switching distributions as narrow as 150 ps, that is, at least one order of magnitude narrower compared to state-of-the-art devices. Finally, we analyze the interplay of spin torques, voltage control of magnetic anisotropy, and heat in promoting magnetization reversal, which holds promise for further optimization of the speed and efficiency of 3-terminal devices. Our study is a collaboration between imec, a leading R&D center in nanoelectronics and digital technologies located in Leuwen (Belgium) and ETH Zürich (Switzerland). All the results have been obtained on industrial-quality magnetic tunnel junction devices fabricated using fully CMOS-compatible processes on a 300 mm pilot MRAM line.
These results were recently published in Nature Nanotechnology:
Figure 1. a, Scanning electron microscope image of a 3-terminal magnetic tunnel junction device with injection electrodes for spin-orbit torque (SOT) and spin-transfer torque (STT) switching. b, Detail of the magnetic tunnel junction pillar and W current line. c, Electrical setup for the time-resolved measurements of the tunneling magnetoresistance during SOT and/or STT switching. d, Examples of ten different single-shot switching events induced by spin-orbit torques.