Jung Hwan Moon

In Situ Observation of Voltage‐Induced Multilevel Resistive Switching in Solid Electrolyte Memory

Traditional charge-based memory technologies are approaching miniaturization limits as it becomes increasingly diffi cult to reliably retain suffi cient electrons in shrinking cells. [ 1 ] The capability of storing multi-bit information in a memory element without sacrifi cing scalability is one of the most important criteria that emerging memory technologies should fulfi ll. Resistive switching memories utilizing resistance change rather than charge storage have attracted considerable attention as potential alternatives to traditional charge-based memories. The phenomenon of resistive switching is based on the electrically induced change in the resistance state, observed in a variety of metal–insulator–metal structures. [ 1–21 ] Various resistive memories including solid electrolyte memories have displayed the capability of multilevel switching. [ 2–9 ] In spite of the great potential, however, the development of such devices has been delayed, largely because of the incomplete understanding of the switching mechanism and the physical structure for securing multilevel operation in nanometer-scale memory devices. Furthermore, the architectural innovation based on the switching property and fabrication process of each memory cell is required in order to overcome the limitations of conventional (FLASH) and other types of memory systems (PRAM (Phase-Change Random Access Memory), [ 22 ] MRAM (Magnetic Random Access Memory) [ 23 ] ) Here, we report in situ observation of voltage-induced changes in the microstructure of a solid electrolyte memory, revealing that the multilevel switching originates from the growth of multiple conducting fi laments with nanometer-sized diameter and spacing. Furthermore, we show that the main factor to determine the switching polarity is not electrode asymmetry but the non-uniform distribution of metal

Prediction of giant spin motive force due to Rashba spin-orbit coupling.

Magnetization dynamics in a ferromagnet can induce a spin-dependent electric field through a spin motive force. Spin current generated by the spin-dependent electric field can in turn modify the magnetization dynamics through spin-transfer torque. While this feedback effect is usually weak and thus ignored, we predict that in Rashba spin-orbit coupling systems with a large Rashba parameter α(R), the coupling generates the spin-dependent electric field [±(α(R)m(e)/eħ)(z[over ^]×∂m/∂t)], which can be large enough to modify the magnetization dynamics significantly. This effect should be relevant for device applications based on ultrathin magnetic layers with strong Rashba spin-orbit coupling.

Self-consistent calculation of spin transport and magnetization dynamics

Abstract A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting various types of current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, simultaneous, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization after the spin accumulation has been integrated out. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can make stationary states unstable and induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. A key aspect of this reduction is the suppression of the excitation of short wavelength spin waves by the non-local feedback. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque. These examples show that the self-consistent treatment of spin transport and magnetization dynamics is important for understanding the physics of the coupled dynamics and for providing a bridge between the ongoing research fields of current-induced magnetization dynamics and the newly emerging fields of magnetization-dynamics-induced generation of charge and spin currents.

Enhanced Nonadiabaticity in Vortex Cores due to the Emergent Hall Effect

We present a combined theoretical and experimental study, investigating the origin of the enhanced nonadiabaticity of magnetic vortex cores. Scanning transmission x-ray microscopy is used to image the vortex core gyration dynamically to measure the nonadiabaticity with high precision, including a high confidence upper bound. We show theoretically, that the large nonadiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture induced emergent Hall effect. This study demonstrates that the magnetic damping α and nonadiabaticity parameter β are very sensitive to the topology of the magnetic textures, resulting in an enhanced ratio (β/α>1) in magnetic vortex cores or Skyrmions.

Effect of spin diffusion on current generated by spin motive force

Spin motive force is a spin-dependent force on conduction electrons induced by magnetization dynamics. In order to examine its effects on magnetization dynamics, it is indispensable to take into account spin accumulation, spin diffusion, and spin-flip scattering since the spin motive force is in general nonuniform. We examine the effects of all these on the way the spin motive force generates the charge and spin currents in conventional situations, where the conduction electron spin relaxation dynamics is much faster than the magnetization dynamics. When the spin-dependent electric field is spatially localized, which is common in experimental situations, we find that the conservative part of the spin motive force is unable to generate the charge current due to the cancelation effect by the diffusion current. We also find that the spin current is a nonlocal function of the spin motive force and can be effectively expressed in terms of nonlocal Gilbert damping tensor. It turns out that any spin independent potential such as Coulomb potential does not affect our principal results. At the last part of this paper, we apply our theory to current-induced domain wall motion.

Magnon-mediated Dzyaloshinskii-Moriya torque in homogeneous ferromagnets

In thin magnetic layers with structural inversion asymmetry and spin-orbit coupling, a Dzyaloshinskii-Moriya interaction arises at the interface. When a spin wave current

Phase Diagram of a Single Skyrmion in Magnetic Nanowires

{\bf j}_m

Effect of Enhanced Damping Due to Spin-Motive Force on Field-Driven Domain Wall Motion

flows in a system with a homogeneous magnetization {\bf m}, this interaction produces an effective field-like torque on the form

Current-induced resonant motion of a magnetic vortex core: Effect of nonadiabatic spin torque

{\bf T}_{\rm FL}\propto{\bf m}\times({\bf z}\times{\bf j}_m)

Effect of enhanced damping caused by spin-motive force on vortex dynamics

as well as a damping-like torque,