Issei Sugiyama

Ferromagnetic dislocations in antiferromagnetic NiO

Crystal lattice defects often degrade device functionality, but engineering these defects may have value in future electronic and magnetic device applications. For example, dislocations--one-dimensional lattice defects with locally distinct atomic-scale structures--exhibit unique and localized electrical properties and can be used as a template for producing conducting nanowires in insulating crystals. It has also been predicted that spin-polarized current may flow along dislocations in topological insulators. Although it is expected that the magnetic properties of dislocations will differ from those of the lattice, their fundamental characterization at the individual level has received little attention. Here, we demonstrate that dislocations in NiO crystals show unique magnetic properties. Magnetic force microscopy imaging clearly reveals ferromagnetic ordering of individual dislocations in antiferromagnetic NiO, originating from the local non-stoichiometry of the dislocation cores. The ferromagnetic dislocations have high coercivity due to their strong interaction with the surrounding antiferromagnetic bulk phase. Although it has already been reported that nanocrystals of rock-salt NiO show ferromagnetic behaviour, our study characterizes the ferromagnetic properties of individual lattice defects. We discuss the origin of the unexpected ferromagnetism in terms of the physical properties of the atomic-scale core structures of single dislocations, and demonstrate that it is possible to fabricate stable nanoscale magnetic elements inside crystalline environments composed of these microstructures.

Atomic structures and oxygen dynamics of CeO2 grain boundaries

Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

Spatially-resolved mapping of history-dependent coupled electrochemical and electronical behaviors of electroresistive NiO

Bias-induced oxygen ion dynamics underpins a broad spectrum of electroresistive and memristive phenomena in oxide materials. Although widely studied by device-level and local voltage-current spectroscopies, the relationship between electroresistive phenomena, local electrochemical behaviors, and microstructures remains elusive. Here, the interplay between history-dependent electronic transport and electrochemical phenomena in a NiO single crystalline thin film with a number of well-defined defect types is explored on the nanometer scale using an atomic force microscopy-based technique. A variety of electrochemically-active regions were observed and spatially resolved relationship between the electronic and electrochemical phenomena was revealed. The regions with pronounced electroresistive activity were further correlated with defects identified by scanning transmission electron microscopy. Using fully coupled mechanical-electrochemical modeling, we illustrate that the spatial distribution of strain plays an important role in electrochemical and electroresistive phenomena. These studies illustrate an approach for simultaneous mapping of the electronic and ionic transport on a single defective structure level such as dislocations or interfaces, and pave the way for creating libraries of defect-specific electrochemical responses.

Pulsed laser deposition of oxide thin films by the fifth harmonic of a Nd:Y3Al5O12 (Nd:YAG) laser

We report the pulsed laser deposition (PLD) of anatase TiO2, LaAlO3, and La0.7Ca0.3MnO3 epitaxial thin films by a fifth-harmonic Nd:Y3Al5O12 (Nd:YAG) laser. High-quality anatase Ti0.996Nb0.004O2 epitaxial thin films were deposited and showed transparent conducting properties, with a maximum mobility of 94 cm2V−1s−1 at 80 K. These results are consistent with those obtained for films deposited by excimer lasers. Furthermore, we report the deposition of droplet-free flat LaAlO3 and La0.7Ca0.3MnO3 films. The latter films exhibited an insulator-to-metal transition at ∼240 K, accompanied by a large magnetoresistive effect, in agreement with previous studies using excimer lasers. These results indicate that the fifth harmonic of a Nd:YAG laser may be a potential alternative to excimer lasers for PLD.We report the pulsed laser deposition (PLD) of anatase TiO2, LaAlO3, and La0.7Ca0.3MnO3 epitaxial thin films by a fifth-harmonic Nd:Y3Al5O12 (Nd:YAG) laser. High-quality anatase Ti0.996Nb0.004O2 epitaxial thin films were deposited and showed transparent conducting properties, with a maximum mobility of 94 cm2V−1s−1 at 80 K. These results are consistent with those obtained for films deposited by excimer lasers. Furthermore, we report the deposition of droplet-free flat LaAlO3 and La0.7Ca0.3MnO3 films. The latter films exhibited an insulator-to-metal transition at ∼240 K, accompanied by a large magnetoresistive effect, in agreement with previous studies using excimer lasers. These results indicate that the fifth harmonic of a Nd:YAG laser may be a potential alternative to excimer lasers for PLD.

Relative Li-ion mobility mapping in Li0.33La0.56TiO3 polycrystalline by electron backscatter diffraction and electrochemical strain microscopy

Li-ion conductivity in a solid-state electrolyte has so far been measured by impedance spectroscopy. In this method, however, it is difficult to obtain microstructural information because of the absence of spatial resolution. Here, we show the relationship between the Li-ion mobility and the crystal orientation in Li0.33La0.56TiO3 polycrystalline by electrochemical strain microscopy combined with electron backscatter diffraction. On the experimentally constructed multivariable regression model, we obtained a qualitative Li-ion mobility map of sub-millimeter width with a 100 nm spatial resolution, which is impossible to achieve by only atomic force microscopy. The proposed method must be useful for identifying the Li-ion diffusion pathway in three dimensions.

Fabrication of atomically abrupt interfaces of single-phase TiH2 and Al2O3

We report the fabrication of atomically abrupt interfaces of titanium dihydride (δ-TiH2) films and α-Al2O3(001) substrates. With the assistance from reactive hydrogen in plasma, single-phase δ-TiH2 epitaxial thin films were grown on α-Al2O3(001) substrates using the reactive magnetron sputtering technique. Scanning transmission electron microscopy measurements revealed an atomically abrupt interface at the δ-TiH2(111) film and Al2O3(001) substrate. These results indicate that the reactive magnetron sputtering has great potential to deposit various epitaxial thin films of hydrides restricted by the hydrogenation limit. The fabrication of high-quality hydride epitaxial thin films with atomically controlled interfaces paves the way for future hydride electronics.

A nonvolatile memory device with very low power consumption based on the switching of a standard electrode potential

We prepared a nonvolatile memory device that could be reversibly switched between a high and a low open-circuit voltage (Voc) regime. The device is composed of a solid electrolyte Li3PO4 film sandwiched between metal Li and Au electrodes: a Li/Li3PO4/Au heterostructure, which was fabricated at room temperature on a glass substrate. The bistable states at Voc ∼ 0.7 and ∼0.3 V could be reversibly switched by applying an external voltage of 2.0 and 0.18 V, respectively. The formation and deformation of an ultrathin Au–Li alloy at the Li3PO4/Au electrode interface were the origin of the reversible switching.