Jian-Guo Zheng

Resistive Switching in Single Epitaxial ZnO Nanoislands

Resistive memory is one of the most promising candidates for next-generation nonvolatile memory technology due to its variety of advantages, such as simple structure and low-power consumption. Bipolar resistive switching behavior was observed in epitaxial ZnO nanoislands with base diameters and heights ranging around 30 and 40 nm, respectively. All four different states (initial, electroformed, ON, and OFF) of the nanoscale resistive memories were measured by conductive atomic force microscopy immediately after the voltage sweeping was performed. Auger electron spectroscopy and other experiments were also carried out to investigate the switching mechanism. The formation and rupture of conducting filaments induced by oxygen vacancy migration are responsible for the resistive switching behaviors of ZnO resistive memories at the nanoscale.

Multimode resistive switching in single ZnO nanoisland system.

Resistive memory has attracted a great deal of attention as an alternative to contemporary flash memory. Here we demonstrate an interesting phenomenon that multimode resistive switching, i.e. threshold-like, self-rectifying and ordinary bipolar switching, can be observed in one self-assembled single-crystalline ZnO nanoisland with base diameter and height ranging around 30 and 40 nm on Si at different levels of current compliance. Current-voltage characteristics, conductive atomic force microscopy (C-AFM), and piezoresponse force microscopy results show that the threshold-like and self-rectifying types of switching are controlled by the movement of oxygen vacancies in ZnO nanoisland between the C-AFM tip and Si substrate while ordinary bipolar switching is controlled by formation and rupture of conducting nano-filaments. Threshold-like switching leads to a very small switching power density of 1 × 103 W/cm2.

Chemical composition tuning of the anomalous Hall effect in isoelectronic L10FePd(1-x)Pt(x) alloy films.

The anomalous Hall effect (AHE) in L1(0)FePd(1-x)Pt(x) alloy films is studied both experimentally and theoretically. We find that the intrinsic contribution (σ(AH)(int)) to the AHE can be significantly increased, whereas the extrinsic side-jump contribution (σ(AH)(sj)) can be continuously reduced from being slightly larger than σ(AH)(int) in L1(0) FePd to being much smaller than σ(AH)(int) in L1(0) FePt, by increasing the Pt composition x. We show that this chemical composition tuning of the intrinsic contribution is afforded by the stronger spin-orbit coupling strength on the Pd/Pt site when the lighter Pd atoms are replaced by the heavier Pt atoms. Our results provide a means of manipulating the competing AHE mechanisms in ferromagnetic alloys for fully understanding the AHE and also for technological applications of ferromagnetic alloys.

In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy.

Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°.

Spin canting and spin-flop transition in antiferromagnetic Cr2O3 nanocrystals

The structure and magnetic properties of Cr2O3 and Cr nanoparticles have been investigated. The as-prepared Cr nanoparticles show a core/shell structure, consisting of a Cr core and a thin Cr oxide shell with a thickness of 1.5–2.5 nm. Annealing the Cr nanoparticles in air leads to the formation of Cr2O3 nanoparticles with spherical morphology. The temperature dependence of the magnetization indicates that these Cr2O3 nanocrystals are antiferromagnetic below their Neel temperature of 320 K and undergo an antiferromagnetic to weak-ferromagnetic transition below about 140 K due to spin canting at disordered surfaces. Below 100 K, a spin-flop transition is induced by an external field with a critical field lower than about 7 T.

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.

Magnetotransport in metal/insulating-ferromagnet heterostructures: Spin Hall magnetoresistance or magnetic proximity effect

We study the anomalous Hall-like effect (AHLE) and the effective anisotropic magnetoresistance (EAMR) in antiferromagnetic

Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

\ensuremath{\gamma}\ensuremath{-}{\mathrm{IrMn}}_{3}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}\phantom{\rule{0.28em}{0ex}}(\mathrm{YIG})

Memory characteristics of ordered Co/Al2O3 core-shell nanocrystal arrays assembled by diblock copolymer process

and Pt/YIG heterostructures. For

Structural and proximity-induced ferromagnetic properties of topological insulator-magnetic insulator heterostructures

\ensuremath{\gamma}\ensuremath{-}{\mathrm{IrMn}}_{3}/\mathrm{YIG}