Ferromagnetism modulation by ultralow current in a two-dimensional polycrystalline molybdenum disulphide atomic layered structure.

Layered materials, such as graphene and transition metal dichalcogenides, are able to obtain new properties and functions through the modification of their crystal arrangements. In particular, ferromagnetism in polycrystalline MoS2 is of great interest because the corresponding nonmagnetic single crystals exhibit spontaneous spin splitting only through the formation of grain boundaries. However, no one has reported direct evidence of this unique phenomenon thus far. Herein, we demonstrate ferromagnetism modulation by an ultralow current density < 103 A/cm2 in 7.5-nm-thick polycrystalline MoS2, in which magnetoresistance shows three patterns according to the current intensity: wide dip, nondip and narrow dip structures. Since magnetoresistance occurs because of the interaction between the current of 4d electrons in the bulk and localized 4d spins in grain boundaries, this result provides evidence of the current modulation of ferromagnetism induced by grain boundaries. Our findings pave the way for the investigation of a novel method of magnetization switching with low power consumption for magnetic random access memories.

Control of anisotropy of a redox-active molecule-based film leads to non-volatile resistive switching memory.

Control of the π-π interaction direction in a redox-active π-molecule based film led to the formation of new mechanistic nonvolatile resistive switching memory: a redox-active organic molecule, 2,5,8-tri(4-pyridyl)1,3-diazaphenalene, showed non-volatile bistable resistance states with a high on-off ratio, retention, and endurance only when the molecular orientation was anisotropic. Control experiments using redox-active/redox-inert organic molecules with isotropic/anisotropic molecular orientations implied that the formation of conductive oxidized π-π stacking layers from non-conductive neutral π-π stacking layers is responsible for resistive switching phenomena, indicating new mechanisms such as ReRAM. Our findings will give a comprehensive understanding of electron transport in organic solid materials based on the effects of redox-activity and molecular arrangement, leading to fabrication of a new class of ReRAM based on organic molecules.

Artificial control of the bias-voltage dependence of tunnelling-anisotropic magnetoresistance using quantization in a single-crystal ferromagnet.

A major issue in the development of spintronic memory devices is the reduction of the power consumption for the magnetization reversal. For this purpose, the artificial control of the magnetic anisotropy of ferromagnetic materials is of great importance. Here, we demonstrate the control of the carrier-energy dependence of the magnetic anisotropy of the density of states (DOS) using the quantum size effect in a single-crystal ferromagnetic material, GaMnAs. We show that the mainly twofold symmetry of the magnetic anisotropy of DOS, which is attributed to the impurity band, is changed to a fourfold symmetry by enhancing the quantum size effect in the valence band of the GaMnAs quantum wells. By combination with the gate electric-field control technique, our concept of the usage of the quantum size effect for the control of the magnetism will pave the way for the ultra-low-power manipulation of magnetization in future spintronic devices.

Sudden restoration of the band ordering associated with the ferromagnetic phase transition in a semiconductor.

The band ordering of semiconductors is an important factor in determining the mobility and coherence of the wave function of carriers, and is thus a key factor in device performance. However, in heavily doped semiconductors, the impurities substantially disturb the band ordering, leading to significant degradation in performance. Here, we present the unexpected finding that the band ordering is suddenly restored in Mn-doped GaAs ((Ga,Mn)As) when the Mn concentration slightly exceeds ∼0.7% despite the extremely high doping concentration; this phenomenon is very difficult to predict from the general behaviour of doped semiconductors. This phenomenon occurs with a ferromagnetic phase transition, which is considered to have a crucial role in generating a well-ordered band structure. Our findings offer possibilities for ultra-high-speed quantum-effect spin devices based on semiconductors.

Valence-band structure of the ferromagnetic semiconductor GaMnAs studied by spin-dependent resonant tunneling spectroscopy.

The valence-band structure and the Fermi level (E(F)) position of ferromagnetic-semiconductor GaMnAs are quantitatively investigated by electrically detecting the resonant tunneling levels of a GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant level from the heavy-hole first state is clearly observed in the metallic GaMnAs QW, indicating that holes have a high coherency and that E(F) exists in the band gap. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated in these double-barrier heterostructures.