X. J. Xing

Reverse-bias-induced bipolar resistance switching in Pt∕TiO2∕SrTi0.99Nb0.01O3∕Pt devices

Stoichiometric single-crystalline TiO2 thin films were grown on SrTi0.99Nb0.01O3 (Nb:STO) substrates by oxygen plasma-assisted molecular beam epitaxy. The Pt∕TiO2∕Nb:STO∕Pt devices showed extremely weak resistance switching hysteresis without applying reverse bias. However, when the reverse bias increased above −2V, the hysteresis became more and more prominent. Further, it was found that the low (high) resistance state can be set by applying sufficient reverse (forward) bias. The origin of the reverse-bias-induced bipolar switching behavior should be attributed to the modulation of Schottky-like barrier width by electrochemical migration of oxygen vacancies.

Structures and magnetic properties of p-type Mn:TiO2 dilute magnetic semiconductor thin films

The behavior of p-type conductivity in Mn-doped TiO2 films grown on LaAlO3 substrates by plasma-assisted molecular beam epitaxy has been investigated. Raman scattering, x-ray photoelectron spectroscopy, and x-ray diffraction studies indicate that the films are single phase, and Mn is successfully doped into the TiO2 matrix. Semiconducting behavior with p-type carriers was confirmed by Hall-effect measurements. The structural and electrical investigations demonstrate that the ferromagnetism observed at room temperature is an intrinsic property of the Mn:TiO2 films, and does not originate from any secondary phase. The magnetic properties of Ti1−xMnxO2 might be related to the formation of acceptor bound magnetic polarons, in which the spins of the holes and manganese are aligned via exchange interaction.

Resistive dependence of magnetic properties in nonvolatile Ti/Mn:TiO2/SrTi0.993Nb0.007O3/Ti memory device

We report the relationship of the magnetic properties of the Mn-doped TiO 2 ( Mn : TiO 2 ) thin film on SrTi 0.993 Ti 0.007 O 3 substrate to the different resistance states of the Ti / Mn : TiO 2 / Nb : SrTiO 3 / Ti device. Initially, the device showed paramagnetic behavior without applying voltage. When the device was switched to low resistance state after applying a positive voltage, robust ferromagnetism was observed. In contrast, when a negative voltage was applied to switch to high resistance state, the device exhibited weak ferromagnetism. This electrically controllable ferromagnetism should be attributed to the tunability of oxygen-vacancy concentration in the Mn : TiO 2 film by electric field.

N-derived signals in the x-ray photoelectron spectra of N-doped anatase TiO2

The plasma-assisted molecular beam epitaxial anatase TiO2−xNx (x<0.3) films were chosen to investigate the N-derived variation in the x-ray photoelectron spectroscopy (XPS). With increasing nitrogen concentration, the small chemical shift and the shoulder of the main peak emerge in the N 1s, O 1s, or Ti 2p3/2 core level spectra. In combination with the experimental results obtained from x-ray diffraction and atomic force microscopy, the variations in the spectra could be ascribed to the microstructural distortion. This distortion, induced by the N3− substitution for lattice O2−, could slightly decrease the average ionicity of the Ti–O (or N) bonds. In addition, the other N 1s features (at 399.8 and 401.8 eV) and the oxygen vacancy are also discussed. For the N-doped TiO2, this work introduces a correlation between the microstructural properties and the XPS signals.

Room-temperature ferromagnetism in (Mn, N)-codoped TiO2 films grown by plasma assisted molecular beam epitaxy

(Mn, N)-codoped TiO2 films have been fabricated by plasma assisted molecular beam epitaxy. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate the incorporation of both Mn and N ions into the TiO2 lattice but without Mn metal clusters. Ferromagnetism with clear hysteresis at 300 K is observed in (Mn, N)-codoped TiO2 films, whereas the films without nitrogen show paramagnetic behavior. It is demonstrated experimentally that the intentional nitrogen doping plays an important role in realizing dominant ferromagnetic ordering in Mn-doped TiO2 films.

Bloch-point-mediated magnetic antivortex core reversal triggered by sudden excitation of a suprathreshold spin-polarized current

We report on spin dynamics of single magnetic antivortices driven by sudden excitation of a spin-polarized direct current. Using micromagnetic simulations, we find that there is a critical current density where spin dynamics transition occurs. Above the critical value, the core is switched on a time scale of ∼200 ps through two Bloch points injection after fully suppressed by a quadruple-vortices array, not through the well-known process involving an antivortex-vortex pair creation and annihilation. As the current density decreases, the time required to switch the core increases. The state after full relaxation depends on the current density (the excitation parameter).

Surface reconstruction evolution and anatase formation in the process of oxidation of titanium nitride film

Titanium nitride film was grown on MgO(001) substrate by plasma-assisted molecular beam epitaxy and then oxidized by oxygen plasma. Reflection high-energy electron diffraction (RHEED) was employed to in situ monitor the process of growth and oxidation. After the TiN film was oxidized for a moment, spots among main streaks were observed in RHEED pattern, which should be attributed to the isolated surface reconstruction domains disorderedly distributing on flat surface. Subsequently, the spots gradually evolved to streaks so that more clear RHEED patterns of (2×1) surface reconstruction were observed. It was argued that the disordered and isolated reconstruction domains congregated to large domains or even perfect reconstruction surface with oxidation time evolving. After oxidation, a series of characterization methods were applied to study the TiO2 phase, which consistently confirmed that the phase of oxidized titanium nitride is anatase but not rutile.

Modulation of propagation characteristics of spin waves induced by perpendicular electric currents

We have theoretically and numerically investigated the effect of perpendicular currents on the propagation characteristics of spin waves on a wire composing the free layer of a spin valve. Our theory shows that the single Slonczewski’s spin-transfer torque can cause the spin-wave Doppler effect and modify the spin-wave attenuation and that the fieldlike torque makes negligible contribution due to its relatively small magnitude. Micromagnetic simulations confirm these theoretical predictions and further reveal that spin waves at suprathreshold currents are instable and become chaotic with increased time. Finally, selective tuning of the spin-wave attenuation is demonstrated by using local, individual currents.

Spin-transfer torque driven magnetic antivortex dynamics by sudden excitation of a spin-polarized dc

Spin dynamics of antivortices excited by sudden action of a spin-polarized dc is reported. Two main excitation modes are found with increased current density, involving a translational (gyrotropic) mode and a core reversal mode. The former mode can be described by Thiele’s equation, which accounts for the orbital distortion in view of the modified restoring force by nontrivial structures nucleated at sample edges. The final states of the system in the translational mode are obtained, being either a domain wall state or a vortex state, depending on the current density. The frequency of gyromotion is dependent on dot sizes. Within a threshold radius, the off-centered antivortex can freely relax back to the dot center.

Exchange-compelled vortices on magnetic core-shell cylinders and their spin-transfer torque driven dynamics

Magnetic core-shell cylinders comprising a hard dot and a soft ring are proposed. Due to energy redistribution, a compelled vortex is formed on the central dot. Current-driven vortex dynamics is clarified. Because of the confinement effect, the vortex-core gyrotropic motion, and further the vortex-antivortex pair mediated reversal are suppressed. Instead, the vortex-core reversal is achieved through a traveling Bloch-point mediated process. Depending on the current density, the Bloch-point-mediated reversal process adopts different modes (A-, B-, and C-modes). The switching time is a nonmonotonic function of the current density. For the 80-nm-wide cylinder, the A-mode gives a “clean” vortex-core reversal.