Summary form only given. Efficient manipulation of magnetization direction in magnetic nanostructures is of crucial importance for realization of low-power and high-performance nonvolatile spintronics memory devices. Spin-transfer torque (STT) induced magnetization switching in magnetic nanopillars, or magnetic tunnel junctions, has been a leading technology in this field and is about to hit the market as a spin-transfer torque magnetoresistive random access memory (STT-MRAM). In STT-MRAM, the memory element typically has two terminals and magnetization is switched through a spin angular momentum transfer by applying vertical currents. In the meantime, recent researches have opened another possibility of nonvolatile spintronics devices with magnetic nanowires, where the magnetization is controlled by horizontal currents. Such devices typically form a three-terminal architecture [1] and magnetization can be controlled by current-induced domain wall (DW) motion [2, 3] or spin-orbit torque (SOT) induced magnetization switching [4, 5]. The three-terminal architecture provides a relaxed control of parameters and potential to operate at higher frequency at the cost of cell size compared with the two-terminal counterpart, making the devices an attractive option for integrated-circuits applications [1]. In terms of physics, moreover, the above two schemes complete magnetization switching with characteristic dynamics by torques with a different symmetry from the STT switching, offering unique attributes that are not seen in STT switching. In this presentation, we show recent advances in controlling the magnetization of nanowires via the DW motion and SOT switching. We particularly focus on the characteristic features of the SOT switching and discuss their impact on integrated circuit applications. One of the unique aspects of SOT is an orthogonal relation with the Gilbert damping torque when one utilizes perpendicular easy axis systems. For STT devices, because the damping torque acts in the antiparallel direction to STT, one needs to employ material systems with low damping, which in general has positive correlation with magnetic anisotropy that ensures thermal stability. For SOT devices, on the other hand, due to the orthogonal relation, one can employ high anisotropy materials even with high damping. As a result, electrically controllable memory elements with high thermal stability are expected. We have studied a SOT switching in nanowires with a Co/Pt multilayer having a high magnetic anisotropy and damping [6]. The fabricated nanowire devices show high thermal stability factor, given by a ratio of the energy barrier to the thermal fluctuation energy, of much greater than 100, and magnetization switching driven by current. In addition, we have found that a switching efficiency, defined as an anisotropy energy density divided by switching current density, increases with the stacking number of Co/Pt bilayer. This fact suggests that the SOT is generated in the Co/Pt multilayer despite a structural inversion symmetry, implying an unknown mechanism of SOT generation, as pointed out in a previous work [7]. The obtained results show promise for use in integrated circuits used in wide temperature ranges. The SOT acting in the orthogonal direction to the damping torque also provides a high-speed magnetization switching capability. The STT switching proceeds with a precessional motion and its threshold current is known to increase with an inverse of pulse duration below about a few nanoseconds. In contrast, for the case with SOT, a theory predicts an immediate magnetization switching as soon as the torque is exerted, resulting in a less pulse duration dependence of the switching current [8]. To elucidate this difference experimentally, we have studied a pulse duration dependence of the switching current density for two kinds of in-plane magnetized geometries; one shows switching as in the case for STT switching [5] and the other is expected to show a switching by orthogonal spins [9]. It was found that the latter scheme shows virtually constant switching current density with respect to the pulse duration down to 0.5 ns [10]. This result indicates the potential of SOT switching for high-speed memory operated at GHz class. We have also studied the pulse duration dependence of switching current density for perpendicular easy axis systems with W/CoFeB/MgO stacks [11]. In this experiment, we prepared several devices with various sizes to systematically examine the effect of an incoherent reversal with a DW propagation, which is expected to be significant in perpendicular easy axis systems. It was found that, for micrometer-scaled devices, the switching current density significantly decreases with increasing the pulse duration due to the incoherent reversal. The detailed investigation reveals that the nucleation events take place at various sites inside the magnetic dot or wires, and the switching completes by DW propagation among the sites. For devices with a CoFeB/MgO nanodot formed on a W nanowire, on the other hand, less pulse duration dependence of switching current density was observed as in the case for the in-plane system. Thus, the speed of SOT switching crucially depends on the employed device geometry for perpendicular easy axis systems.
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