G. Y. Wang

Strain engineering induced interfacial self-assembly and intrinsic exchange bias in a manganite perovskite film

The control of complex oxide heterostructures at atomic level generates a rich spectrum of exotic properties and unexpected states at the interface between two separately prepared materials. The frustration of magnetization and conductivity of manganite perovskite at surface/interface which is inimical to their device applications, could also flourish in tailored functionalities in return. Here we prove that the exchange bias (EB) effect can unexpectedly emerge in a (La,Sr)MnO3 (LSMO) “single” film when large compressive stress imposed through a lattice mismatched substrate. The intrinsic EB behavior is directly demonstrated to be originating from the exchange coupling between ferromagnetic LSMO and an unprecedented LaSrMnO4-based spin glass, formed under a large interfacial strain and subsequent self-assembly. The present results not only provide a strategy for producing a new class of delicately functional interface by strain engineering, but also shed promising light on fabricating the EB part of spintronic devices in a single step.

Conductance quantization in oxygen-anion-migration-based resistive switching memory devices

Quantized conductance was observed in an anion-migration-based resistive switching memory cell with the structure of (Ti, Ta, W)/Ta2O5/Pt. The conductance of the cell varies stepwise in units of single atomic conductance (77.5 μS), which is responsible for the formation and annihilation of atomic scale filament built from oxygen vacancies in Ta2O5 film. The quantized conductance behavior can be modulated by voltage pulses as fast as 100 ns. The demonstration of conductance quantization in Ta2O5 based memory device would open the door for quantized multi-bit data storage of anion-migration-based resistive switching nonvolatile memories.

Resistive switching and conductance quantization in Ag/SiO2/indium tin oxide resistive memories

The Ag/SiO2/indium tin oxide (ITO) devices exhibit bipolar resistive switching with a large memory window of ∼102, satisfactory endurance of >500 cycles, good retention property of >2000 s, and fast operation speed of <100 ns, thus being a type of promising resistive memory. Under slow voltage sweep measurements, conductance plateaus with a conductance value of integer or half-integer multiples of single atomic point contact have been observed, which agree well with the physical phenomenon of conductance quantization. More importantly, the Ag/SiO2/ITO devices exhibit more distinct quantized conductance plateaus under pulse measurements, thereby showing the potential for realizing ultra-high storage density.

Amino-acid-assisted synthesis and size-dependent magnetic behaviors of hematite nanocubes

This article reports the amino-acid-assisted synthesis and size-dependent magnetic properties of hematite nanocubes. The products were characterized using x-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM. The magnetic behavior of hematite nanocubes was studied using a vibrating sample magnetometer at room temperature. The sizes of hematite nanocubes were controlled by tuning the reaction parameters. The amino acid has double-hydrophilic functional groups, –NH2 and –COOH, which are utilized to control the growth and final size of hematite nanocubes. We show that utilizing biomolecules in chemical synthesis is a useful method for tailoring the physical properties of nanomaterials.

Electrical control of Co/Ni magnetism adjacent to gate oxides with low oxygen ion mobility

We investigate the electrical manipulation of Co/Ni magnetization through a combination of ionic liquid and oxide gating, where HfO2 with a low O2− ion mobility is employed. A limited oxidation-reduction process at the metal/HfO2 interface can be induced by large electric field, which can greatly affect the saturated magnetization and Curie temperature of Co/Ni bilayer. Besides the oxidation/reduction process, first-principles calculations show that the variation of d electrons is also responsible for the magnetization variation. Our work discloses the role of gate oxides with a relatively low O2− ion mobility in electrical control of magnetism, and might pave the way for the magneto-ionic memory with low power consumption and high endurance performance.

Resistive switching with self-rectifying behavior in Cu/SiOx/Si structure fabricated by plasma-oxidation

We report a resistive switching memory structure based on silicon wafers by employing both materials and processing fully compatible with complementary metal-oxide semiconductor technology. A SiOx nanolayer was fabricated by direct plasma-oxidation of silicon wafers at room-temperature. Resistive switching behaviors were investigated on both p- and n-Si wafers, whereas self-rectifying effect was obtained in the Cu/SiOx/n-Si structure at low-resistance state. The self-rectifying effect was explained by formation of the Schottky barrier between the as-formed Cu filament and the n-Si. These results suggest a convenient and cost-efficient technical-route to develop high-density resistive switching memory for nowadays Si-based semiconductor industry.

Tuning the entanglement between orbital reconstruction and charge transfer at a film surface

The interplay between orbital, charge, spin, and lattice degrees of freedom is at the core of correlated oxides. This is extensively studied at the interface of heterostructures constituted of two-layer or multilayer oxide films. Here, we demonstrate the interactions between orbital reconstruction and charge transfer in the surface regime of ultrathin (La,Sr)MnO3, which is a model system of correlated oxides. The interactions are manipulated in a quantitative manner by surface symmetry-breaking and epitaxial strain, both tensile and compressive. The established charge transfer, accompanied by the formation of oxygen vacancies, provides a conceptually novel vision for the long-term problem of manganites—the severe surface/interface magnetization and conductivity deterioration. The oxygen vacancies are then purposefully tuned by cooling oxygen pressure, markedly improving the performances of differently strained films. Our findings offer a broad opportunity to tailor and benefit from the entanglements between orbit, charge, spin, and lattice at the surface of oxide films.

Interlayer magnetostatic coupling and linear magnetoresistance in [Pd/Co]/MgO/Co junction sensor

We investigate interlayer magnetostatic coupling and linear magnetoresistance in [Pd/Co]/MgO/Co and [Pd/Co]/MgO/Co/MgO/[Co/Pd] tunnel junctions, where Co/Pd and Co ferromagnetic layers exhibit out-of-plane and in-plane magnetic anisotropy, respectively. Because of the magnetostatic interaction between Co moments and the stray field from Co/Pd stripe domains, the Co layer shows a significant enhancement of coercivity and exchange bias. Linear magnetoresistance is observed in both junctions in the field up to 15 kOe with the current perpendicular to the film plane, due to coherent rotation of the ferromagnets, making junctions with MgO barrier and orthogonal magnetization configuration a promising high magnetic field sensor.

Modulating resistive switching by diluted additive of poly(vinylpyrrolidone) in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

Here we report a convenient and economic method to modulate resistive switching of a bipolar resistive memory based on Al/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Al. We found that diluted additive of poly(vinylpyrrolidone) (PVP) in PEDOT:PSS could enlarge the ON/OFF ratio from 103 to 105 and keep a long retention time over 105 s. Besides, the effects of PVP on switching process were studied by detailed analyses of I-V curves, atomic force microscopy images, X-ray photoelectron spectroscopy, and Raman spectra. The enhancement of the resistive window is due to the loss of PEDOT, charge traps, and conformation change induced by PVP.

Giant coercivity in perpendicularly magnetized cobalt monolayer

We report giant coercivity (HC) up to 35 kOe at 4 K, measured by the anomalous Hall effect, in perpendicularly magnetized Co (∼0.3 nm) films, where Co is approximately one monolayer. The HC is dramatically reduced with huge applied current, due to Joule heating rather than Rashba effect. It is also sensitive to temperatures, producing almost zero HC at 200 K. The Curie temperature of the Co monolayer is ∼275 K, far lower than that of bulk Co. The giant HC could be explained by the strong interaction at Co/Pd interface, providing a promising paradise: one monolayer, one permanent magnet.