Proposal of an ultra-fast all spin logic device based on RE-TM ferrimagnetic material (2018)

Power consumption of logic operations has become the key bottleneck hindering the miniaturization of ICs. All Spin Logic Device (ASLD) using pure spin current, instead of charge current, is considered as a promising candidate for building future ultra-low power computing systems [1]. A typical ASLD based on ferromagnetic (FM) material is illustrated in figure 1a. Applying a voltage to the FM injector, a spin accumulation can be generated in non-magnetic (NM) channel. This spin accumulation can propagate spin angular momentum by spin diffusion, and switch the FM detector via Spin Transfer Torque (STT) effect [1, 2]. Exclusive of lower power consumption, higher computing speed is another significant requirement for future logic device [1, 3]. Recently, researchers have observed ultra-fast magnetic dynamics in ferrimagnetic (FI) alloys composed by rare-earth (RE) metal and transition metal (TM) [4–6]. Different from FM materials, the magnetic properties of RE-TM alloys in figure 1b are controlled conjointly by RE and TM sublattices [4]. Considering the different magnetizations and gyromagnetic ratios, the value of total angular momentum for RE-TM alloy can be calculated as the difference between angular momenta of RE element and TM element. In fact, this angular momentum can be controlled by adjusting the concentration of RE metal [4] or the temperature [5–6]. When the total angular momentum tends to vanish, FI alloys will arrive at an angular momentum compensated point (AMCP), where antiferromagnetic-like properties appear [4]. It has been demonstrated that laser or Spin Orbit Torque (SOT) effect assisted by external magnetic field can activate this mechanism [4–5, 7]. However, neither of these activating methods suits ASLD, since laser and external magnetic field will largely increase the power consumption and harm the integrability. We hence propose an alternative ASLD structure based on RE-TM FI material to realize ultra-fast speed. $\mathrm{Co}_{1-\mathrm{x}}\mathrm{Tb}_{\mathrm{x}}$ FI alloy is used to replace the FM layer in the proposed ASLD (figure 1b). Moreover, spin torque induced by SOT effect is applied to FI layer, driving the magnetization of $\mathrm{Co}_{1-\mathrm{x}}\mathrm{Tb}_{\mathrm{x}}$ to precess at an extremely high frequency [5]. Here we apply a reasonable condition $\mathrm {x}=0.165$, which is close to the value for experimentally proved AMCP, to implement the following analyses. As shown in figure 2a, the lowest energy state for precession under the sole impact of SOT is a circle in x-z plane, indicating that the precession caused by SOT effect doesn’t have a certain destination. In this occasion, magnetization switching in FI alloys is inaccessible. To break the precession balance, we introduce a spin current propagated from the injector which can exert STT effect on FI detector. Under the co-effect of these two external forces, the distribution of energy states changes, and the magnetization of FI detector can stabilize at the unique lowest energy state point shown in figure 2b. In order to describe the magnetic dynamics of FI layer, we use the typical Landau-Lifshitz-Gilbert (LLG) equation which contains effective magnetic field term, damping term, SOT term and STT term [8–9]. Figure 2c shows the switching process of Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.835</inf> Tb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.165</inf> layer. SOT current drives the initial magnetization to precess, while STT current prefers to stabilize the magnetization at the lowest energy state. Also, magnetizations for Co and Tb sublattices stay opposite during the switching. This process takes 2.4 ps, about 2 orders lower than the FM detector (0.2ns shown in figure 2d), which means a much quicker computing speed. Our work contributes to improve the performance of ASLD, and it is also a further step to explore the application of ferrimagnetic material in future potential spintronics devices.