Shuxiang Wu

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.

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.

Conducting nanofilaments formed by oxygen vacancy migration in Ti/TiO2/TiN/MgO memristive device

High-quality TiN and TiO2thin films were grown on MgO(100) substrate by plasma-assisted molecule beam epitaxy.X-ray photoelectron spectroscopy and x-ray diffraction studies indicate that the films are single phase. The memory cell composed of Ti/TiO2/TiN/MgO showed a bistable bipolar resistive switching behavior with either electrode grounded. The nature of the bipolar resistive switching phenomenon could derive from the formation and annihilation of filaments induced by oxygen vacancy and oxygen ion migration near/at the bias-applied electrode. In addition, a simple calculation of the filamentary resistivity in model might indicate that different devices made by TiO2thin films with different phases might share the similar resistive switching mechanism.

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.

Suppression of anomalous Hall effect by heavy-fermion in epitaxial antiperovskite Mn4-xGdxN films

Mn4-xGdxN films with x ranging from 0 to 0.48 have been grown by a plasma-assisted molecular beam epitaxy (PA-MBE) system. Analyses show that there is a competition between Kondo coupling and the Ruderman–Kittel–Kasuya–Yosida interaction in these films. The anomalous Hall effect (AHE) was investigated, and a multiple competing scattering mechanism was used to differentiate different contributions to the AHE. Fitting results using a multivariable scaling relation show that contribution of the skew-scattering mechanism to the AHE is suppressed and competition between different contributions is stronger in highly doped samples than that in undoped samples. Resistivity-temperature (ρ-T) curves in Gd-rich samples exhibit a typical behavior of heavy fermion (HF) materials. It shows a weak metal conducting behavior in a high temperature range, while Kondo coupling dominates the middle temperature range of 50 K–110 K. With a further decrease in the temperature to 5 K, a Fermi-liquid behavior is found in the range of 5 K–20 K. Comprehensive analyses indicate that Mn4-xGdxN with large x might be a new kind of HF material with room temperature ferromagnetism.Mn4-xGdxN films with x ranging from 0 to 0.48 have been grown by a plasma-assisted molecular beam epitaxy (PA-MBE) system. Analyses show that there is a competition between Kondo coupling and the Ruderman–Kittel–Kasuya–Yosida interaction in these films. The anomalous Hall effect (AHE) was investigated, and a multiple competing scattering mechanism was used to differentiate different contributions to the AHE. Fitting results using a multivariable scaling relation show that contribution of the skew-scattering mechanism to the AHE is suppressed and competition between different contributions is stronger in highly doped samples than that in undoped samples. Resistivity-temperature (ρ-T) curves in Gd-rich samples exhibit a typical behavior of heavy fermion (HF) materials. It shows a weak metal conducting behavior in a high temperature range, while Kondo coupling dominates the middle temperature range of 50 K–110 K. With a further decrease in the temperature to 5 K, a Fermi-liquid behavior is found in the range...

Thermal stimulated current response in cupric oxide single crystal thin films over a wide temperature range

Cupric oxide single crystal thin films were grown by plasma-assisted molecular beam epitaxy. X-ray diffraction, Raman spectrum and in situ reflection high-energy electron diffraction show that the thin films are 2x2 reconstructed with an in-plane compression and out-of-plane stretching. Thermal stimulated current measurement indicates that the electric polarization response presents in the special 2D cupric oxide single crystal thin film over a wide temperature range from 130 K to near-room temperature. We infer that the abnormal electric response involves the changing of phase transition temperature induced by structure distortion, the spin frustration and magnetic fluctuation effect of short-range magnetic order, or the combined action of both two factors mentioned above. This work suggests a promising clue for finding new room temperature single phase multiferroics or tuning phase transition temperature.

Abnormal magnetic properties of MnBi thin film synthesized by MBE

MnBi has attracted much attention due to its unusual magnetic properties. It has the potential as an alternative to the permanent magnets containing rare-earth elements. In this paper, the MnBi thin film was grown by plasma-assisted molecular beam epitaxial technique. The as-grown film shows ferromagnetic property and an anomalous magnetization dependence on temperature. The X-ray photoelectron spectrum reveals that there are regular and interstitial sites Mn atoms with two bonding states. The observed decrease of magnetization at ∼230 K would be ascribed to the movements of Mn atoms to the interstitial sites leaving behind Mn vacancies. Also at ∼90 K, it exhibits a spin reorientation due to the magnetization change from c -axis to ab -plane which is related to the abnormal temperature dependence of the lattice parameters.