In this paper, we review the recent progress in the resistive random access memory (ReRAM) technology, one of the most promising emerging nonvolatile memories, in which both electronic and electrochemical effects play important roles in the nonvolatile functionalities. First, we provide a brief historical overview of the research in this field. We also provide a technological overview and the epoch-making achievements, followed by an account of the current understanding of both bipolar and unipolar ReRAM operations. Finally, we summarize the challenges facing the ReRAM technology as it moves toward the beyond-2X-nm generation of nonvolatile memories and the so-called beyond complementary metal-oxide-semiconductor (CMOS) device.

A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Exotic features of a metal/oxide/metal sandwich, which will be the basis for a drastically innovative nonvolatile memory device, is brought to light from a physical point of view. Here the insulator is one of the ubiquitous and classic binary-transition-metal oxides (TMO), such as ${\mathrm{Fe}}_2{\mathrm{O}}_3$, $\mathrm{NiO}$, and $\mathrm{CoO}$. The sandwich exhibits a resistance that reversibly switches between two states: one is a highly resistive off state and the other is a conductive on state. Several distinct features were universally observed in these binary TMO sandwiches: namely, nonpolar switching, nonvolatile threshold switching, and current-voltage duality. From the systematic sample-size dependence of the resistance in on and off states, we conclude that the resistance switching is due to the formation of “electric faucet” at the interface, which shows up as a homogeneous to inhomogeneous transition of the current distribution.

The resistance switching properties in $\mathrm{Pt}∕\mathrm{Ni}–\mathrm{O}∕\mathrm{Pt}$ and $\mathrm{Pt}∕\mathrm{Co}–\mathrm{O}∕\mathrm{Pt}$ synthesized by the magnetron sputtering have been investigated. The oxygen partial pressure during sputtering and the post-thermal process are crucial to forming of the trilayer. By investigating x-ray photoemission spectroscopy spectra, the increase of initial resistance in Ni–O was caused by the variation of the stoichiometry, while that in the Co–O was accompanied by the phase transformation between CoO and ${\mathrm{Co}}_3{\mathrm{O}}_4$ . The resistance switching in $\mathrm{Pt}∕\mathrm{Co}–\mathrm{O}∕\mathrm{Pt}$ and $\mathrm{Pt}∕\mathrm{Ni}–\mathrm{O}∕\mathrm{Pt}$ exhibits the analogous electrode area and temperature dependences. As a result of the $I\text{-}V$ measurements at the elevated temperature, the assistance of Joule heating in the reset process is implied.

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the $\mathrm{Mn}\text{\hspace{0.05em}3d}$ partial density of states was similar to that of the diluted ferromagnetic semiconductor ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As},$ which also supports the fabrication of zinc-blende MnAs in the nanoscale.