Yufeng Tian

High TC ferromagnetism of Zn(1−x)CoxO diluted magnetic semiconductors grown by oxygen plasma-assisted molecular beam epitaxy

Co-doped wurtzite ZnO [Zn(1−x)CoxO] thin films have been grown on Al2O3(0001) substrates by using oxygen plasma-assisted molecular beam epitaxy at the low growth temperature of 450°C. The epitaxial films of Co concentration at 0⩽x⩽0.12 are single crystalline, which were examined by reflection high energy electron diffraction and x-ray diffraction. Both of optical transmission spectrum and in situ. x-ray photoelectron spectroscopy studies confirmed the incorporation of Co2+ cations into wurtzite ZnO lattice. Magnetic measurements revealed that the Zn(1−x)CoxO thin films are ferromagnetic with Curie temperature TC above room temperature, and the ferromagnetism shows intrinsic characteristic.

Magnetic and transport properties of homogeneous MnxGe1−x ferromagnetic semiconductor with high Mn concentration

Homogeneous MnxGe1−x ferromagnetic semiconductor films with high Mn concentration were prepared, contrasting with dilute inhomogeneous MnxGe1−x magnetic semiconductors. The saturation magnetization of Mn0.57Ge0.43 films is high, up to 327emu∕cm3 (1.04μB∕Mn) at 5K, and the Curie temperature is about 213K. The Mn0.57Ge0.43 films show semiconducting resistance, but the magnetoresistance is negligibly small. The anomalous Hall effect was observed below the Curie temperature, which is consistent with the magnetic measurements. The global ferromagnetism was discussed based on s,p-d exchange coupling between the weakly localized s,p hole carriers and the strongly localized d electrons of the Mn atoms.

Transformation of electrical transport from variable range hopping to hard gap resistance in Zn1−xFexO1−v magnetic semiconductor films

Transformation of the electrical transport from the Efros and Shklovskii [J. Phys. C 8, L49 (1975)] variable range hopping to the “hard gap” resistance was experimentally observed in a low temperature range as the Fe compositions in Zn1−xFexO1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ρ∝exp[TH∕T+(TES∕T)1∕2], was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.

Magnetism of amorphous Ge1−xMnx magnetic semiconductor films

Amorphous Ge1−xMnx magnetic semiconductor films with high Mn concentrations were prepared on liquid-nitrogen (LN2)-cooled glass substrates by ultrahigh vacuum thermal coevaporation. Hysteresis loops measured at 5K show coexistence of ferromagnetism and paramagnetism. The maximum Curie temperature of 220K was found in Ge0.48Mn0.52 film. Moreover, exchange bias occurs in magnetization hysteresis loops for samples with higher Mn concentrations, which can be explained by the antiferromagntic exchange coupling between ferromagnetic phase and antiferromagnetic phase. All the Ge1−xMnx magnetic semiconductor films show semiconducting transport behavior and anomalous Hall effects below the Curie temperature, indicating carrier-mediated ferromagnetism.

Spin memristive magnetic tunnel junctions with CoO-ZnO nano composite barrier

The spin memristive devices combining memristance and tunneling magnetoresistance have promising applications in multibit nonvolatile data storage and artificial neuronal computing. However, it is a great challenge for simultaneous realization of large memristance and magnetoresistance in one nanoscale junction, because it is very hard to find a proper spacer layer which not only serves as good insulating layer for tunneling magnetoresistance but also easily switches between high and low resistance states under electrical field. Here we firstly propose to use nanon composite barrier layers of CoO-ZnO to fabricate the spin memristive Co/CoO-ZnO/Co magnetic tunnel junctions. The bipolar resistance switching ratio is high up to 90, and the TMR ratio of the high resistance state gets to 8% at room temperature, which leads to three resistance states. The bipolar resistance switching is explained by the metal-insulator transition of CoO1−v layer due to the migration of oxygen ions between CoO1−v and ZnO1−v.

Strong anisotropy of magnetization and sign reversion of ordinary Hall coefficient in single crystal Ge1−xMnx magnetic semiconductor films

Epitaxial single-crystal Ge1−xMnx ferromagnetic-semiconductor films were fabricated on Ge(001) substrates by molecular beam epitaxy. All the samples are ferromagnetic and have strong magnetic anisotropy indicated by different magnetization in plane and out of plane. The electrical transport of the films obeys Efros variable range hopping law in the low temperature range. Interestingly, a negative coefficient of the ordinary Hall effect of p-type carriers was found in the variable range hopping at low temperature. Anomalous Hall effect in Ge0.949Mn0.051 film was observed below the Curie temperature, indicating the carrier-mediated intrinsic ferromagnetism.

Tunable rectification and giant positive magnetoresistance in Ge1−xMnx/Ge epitaxial heterojunction diodes

Ge1−xMnx/Ge single-crystal heterojunction diodes with p-type Ge1−xMnx ferromagnetic semiconductor were grown, respectively on Ge substrates of p-type, n-type, and intrinsic semiconductors by molecular beam epitaxy. The I-V curve of the p-Ge0.95Mn0.05/intrinsic-Ge diode can be greatly tuned by a magnetic field, which was indicated by a large positive magnetoresistance. The magnetoresistance shows a peak value of 700% under a +2 V bias voltage around the Curie temperature of 225 K of the Ge0.95Mn0.05 magnetic semiconductor, and it remains as high as 440% at room temperature. The origin of the positive magnetoresistance is discussed.

Large rectification magnetoresistance in nonmagnetic Al/Ge/Al heterojunctions.

Magnetoresistance and rectification are two fundamental physical properties of heterojunctions and respectively have wide applications in spintronics devices. Being different from the well known various magnetoresistance effects, here we report a brand new large magnetoresistance that can be regarded as rectification magnetoresistance: the application of a pure small sinusoidal alternating-current to the nonmagnetic Al/Ge Schottky heterojunctions can generate a significant direct-current voltage, and this rectification voltage strongly varies with the external magnetic field. We find that the rectification magnetoresistance in Al/Ge Schottky heterojunctions is as large as 250% at room temperature, which is greatly enhanced as compared with the conventional magnetoresistance of 70%. The findings of rectification magnetoresistance open the way to the new nonmagnetic Ge-based spintronics devices of large rectification magnetoresistance at ambient temperature under the alternating-current due to the simultaneous implementation of the rectification and magnetoresistance in the same devices.

Electrical transport properties of (CoxAl1−x)2O3−v oxide magnetic semiconductor and corresponding Co–Al2O3 granular films

(CoxAl1−x)2O3−v oxide magnetic semiconductor films were synthesized by introducing an impurity band in the insulating Al2O3 band gap, and the corresponding granular films were obtained by annealing. For both kinds of films, their electrical transport properties are well described by spin dependent variable range hopping mechanism instead of the usually expected intergrain tunnelling. The magnetoresistance was also discussed.

Reversible control of magnetization of Fe3O4 by a solid-state film lithium battery

The LiCoO2/LISICON/Fe3O4 structured solid-state film lithium battery is designed to realize a reversible control of the magnetization in Fe3O4 film. LISICON (Li1.5Al0.5Ge1.5P3O12) is selected to serve as the solid-state electrolyte owing to its excellent Li ion transport property at room temperature. A reversible and non-volatile control of the saturation magnetization of Fe3O4 film between the charged and discharged states was obtained, and the modulation ratio can reach as high as 10%. The redox of Fe ions, caused by the intercalation/deintercalation of lithium ion in Fe3O4 film, is responsible for the observed magnetic variation. The battery consisting entirely of solid films provides a promising strategy to control the magnetic properties electrically, which will be a very hopeful candidate for many practical applications.