Lizhu Ren

Bipolar Resistance Switching in Transparent ITO/LaAlO3/SrTiO3 Memristors

We report reversible bipolar resistance switching behaviors in transparent indium-tin oxide (ITO)/LaAlO3/SrTiO3 memristors at room temperature. The memristors exhibit high optical transparency, long retention, and excellent antifatigue characteristics. The high performances are promising for employing ITO/LaAlO3/SrTiO3 memristors in nonvolatile transparent memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the migration of positively charged oxygen vacancies from the SrTiO3 substrate to the LaAlO3 film, resulting in Poole-Frenkel emission for the low resistance state and thermionic emission for the high resistance state.

Extrinsic anomalous Hall effect in epitaxial Mn4N films

Anomalous Hall effect (AHE) in ferrimagnetic Mn4N epitaxial films grown by molecular-beam epitaxy is investigated. The longitudinal conductivity σxx is within the superclean regime, indicating Mn4N is a highly conducting material. We further demonstrate that the AHE signal in 40-nm-thick films is mainly due to the extrinsic contributions based on the analysis fitted by ρAH=a′ρxx0+bρxx2 and σAH∝σxx. Our study not only provide a strategy for further theoretical work on antiperovskite manganese nitrides but also shed promising light on utilizing their extrinsic AHE to fabricate spintronic devices.

Direct evidences of filamentary resistive switching in Pt/Nb-doped SrTiO3 junctions

The first concerned question on the fundamental physics of the resistive switching (RS) effect in metal/Nb-doped SrTiO3 junctions is whether the RS does take place at the whole interface or at some local regions of the interface. Even though several investigations provide the clues of the filamentary nature of the RS, direct evidences are still required. Moreover, there is obvious inconsistency between the possible filamentary feature and the observed capacitance-voltage (C-V) hysteresis loops. Here, we report direct evidences of a filamentary RS effect in Pt/Nb-doped SrTiO3 junctions. The virgin Pt/NSTO junctions show an initial RS effect. The current-voltage (I-V) characteristic of the initial RS and the C-V characteristic in the virgin junctions are interpreted by using the theory of metal/semiconductor contacts. The correspondence between the initial RS and the C-V hysteresis loops is also discussed. The most important is that an electroforming process is observed at a large forward voltage, which is ...

Enlarged Mn 3s splitting and room-temperature ferromagnetism in epitaxially grown oxygen doped Mn2N0.86 films

Single-phase and oxygen doped Mn2N0.86 thin films have been grown on MgO (111) by plasma-assisted molecular beam epitaxy. The films grow under tensile strain and, remarkably, they show ferromagnetic-like interactions at low temperature and ferromagnetic ordering agreed well with the Bloch-law T3/2 at room-temperature. We further demonstrate the enlarged Mn 3s splitting (6.46 eV) and its possible relation to the observed ferromagnetism. Our study not only provide a strategy for further theoretical work on oxygen doped manganese nitrides, but also shed promising light on utilizing its room-temperature FM property to fabricate spintronic devices.

Magnetic properties of Mn3O4 film under compressive stress grown on MgAl2O4 (001) by molecular beam epitaxy

High quality single-crystalline Mn3O4 thin films were grown on MgAl2O4 (001) substrates by plasma-assisted molecular beam epitaxy. It is found that the films are compressed in the (001) plane and elongated in the perpendicular direction via in-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction, which is confirmed by frequency hardening of relevant Raman bands. Different from the bulk, the epitaxial film with a thickness of 65 nm shows more obvious magnetic anisotropy and higher magnetic phase transition temperatures (TN = 50 K, T1 = 40.5 K, and T2 = 36 K) than that of the bulk (TN = 42 K, T1 = 39 K, and T2 = 33 K). The variation of magnetic properties could be attributed to the changes of interplay among spin, orbital, and lattice degrees of freedom owing to the residual strain in the epitaxial film.

Write-once-read-many-times characteristics of Pt/Al2O3/ITO memory devices

Nonvolatile write-once-read-many-times (WORM) Pt/Al2O3/ITO memory devices prepared at room temperature were demonstrated. The WORM memory devices show irreversible transition from the initial low resistance (ON) state to the high resistance (OFF) state, high ON/OFF ratio, long data retention, and good reading endurance in air at room temperature. The high performances are promising for employing the Pt/Al2O3/ITO WORM memory devices in permanent storage of information. The nonvolatile memory behaviors could be attributed to the formation and permanent rupture of conductive filament consisting of positively charged oxygen vacancies.

Magnetic properties of Mn3O4 film with a coexistence of two preferential orientations

A Mn3O4 film with a coexistence of two preferential orientations has been grown on a Pt(111)//Si(100) substrate by plasma-assisted molecular beam epitaxy. The structural characteristics and chemical compositions of the film are investigated by using X-ray diffraction, Raman, and X-ray photoelectron spectra in detail. Together with the magnetic tests, the film is demonstrated to be a polycrystalline hausmannite Mn3O4 with no other impurities. Moreover, the hysteresis loops of the film are found to display a step or a characteristic shrinking at low fields. On the other hand, similar magnetic characteristics have also been discovered on the film with two phases grown on a MgAl2O4(001) substrate. In our opinion, considering the large magnetocrystalline anisotropy and shape anisotropy of the single crystal Mn3O4 film reported in previous works, the special structures and phases of the two films result in both of them as soft+hard magnetic composites, in agreement with some other reports.

Growth and magnetic property of antiperovskite manganese nitride films doped with Cu by molecular beam epitaxy

Manganese nitrides thin films on MgO (100) substrates with and without Cu-doping have been fabricated by plasma assisted molecular beam epitaxy. Antiperovskite compounds Mn3.6Cu0.4N have been grown in the case of Cu-doping, and the pure Mn3N2 single crystal has been obtained without Cu-doping. The Mn3.6Cu0.4N exhibits ferrimagnetism, and the magnetization of Mn3.6Cu0.4N increases upon the temperature decreasing from 300 K to 5 K, similar to Mn4N. The exchange bias (EB) effects emerge in the Mn3.6Cu0.4N films. The EB behavior is originated from the interfaces between ferrimagnetic Mn3.6Cu0.4N and antiferromagnetic metal Mn, which is verified to be formed by the data of x-ray photoelectron spectroscopy. The present results not only provide a strategy for producing functional antiperovskite manganese nitrides, but also shed promising light on fabricating the exchange bias part of spintronic devices.

Anomalous Hall effect in epitaxial ferrimagnetic anti-perovskite Mn4−xDyxN films

Anomalous Hall effect (AHE) has been studied for ferrimagnetic antiperovskite Mn4−xDyxN films grown by molecular-beam epitaxy. The introduction of Dy changes the AHE dramatically, even changes its sign, while the variations in magnetization are negligible. Two sign reversals of the AHE (negative-positive-negative) are ascribed to the variation of charge carriers as a result of Fermi surface reconstruction. We further demonstrate that the AHE current JAH is dissipationless (independent of the scattering rate), by confirming that anomalous Hall conductivity, σAH, is proportional to the carrier density n at 5 K. Our study may provide a route to further utilize antiperovskite manganese nitrides in spintronics.