Physical Review B

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Antiferromagnetic spintronics

Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.

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Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications. Magnetoelectric multiferroics, where magnetic properties are manipulated by electric field and vice versa, could lead to improved electronic devices. Here, advances in materials, characterisation and modelling, and usage in applications are reviewed.

Advances in magnetoelectric multiferroics.

This study introduces a predictive model involving the concept of the dipole neutrality point (DNP) for dipoles at the high-k dielectric/semiconductor interface. The DNP allows a simple and intuitive prediction of interface dipoles using electronegativity (EN). This concept is formulated based on a negative correlation between the dielectric work function and EN observed for many high-k oxides using both our measured electron affinity values and those available from the literature. Good agreement in the interface dipoles prediction is observed for our measured dipoles and the flatband voltage shifts from other reports. The simple model will be beneficial for investigations involving threshold voltage adjustment in metal-oxide–semiconductor devices using high-k dielectric materials, which is important for advanced gate stacks development.

An interface dipole predictive model for high-k dielectric/semiconductor heterostructures using the concept of the dipole neutrality point

The electronic properties of SrRuO3/LaAlO3 (SRO/LAO) superlattices with different interlayer thicknesses of SRO layers were studied. As the thickness of SRO layers is reduced, the superlattices exhibit a metal-insulator transition implying transformation into a more localized state from its original bulk metallic state. The strain effect on the metal-insulator transition was also examined. The origin of the metal-insulator transition in ultrathin SRO film is discussed. All the superlattices, even those with SRO layers as thin as 2 unit cells, are ferromagnetic at low temperatures. Moreover, we demonstrate field effect devices based on such multilayer superlattice structures.

Tailoring the electronic properties of SrRuO3 films in SrRuO3/LaAlO3 superlattices

Using an electric field instead of an electric current (or a magnetic field) to tailor the electronic properties of magnetic materials is promising for realizing ultralow energy-consuming memory devices because of the suppression of Joule heating, especially when the devices are scaled to the nanoscale. In the review, we summarize recent results on the giant magnetization and resistivity modulation in a metamagnetic intermetallic alloy - FeRh, which is achieved by electric-field-controlled magnetic phase transitions in multiferroic heterostructures. Furthermore, the approach is extended to topological antiferromagnetic spintronics, which is currently receiving attention in the magnetic society, and the antiferromagnetic order parameter has been able to switch back and forth by a small electric field. In the end, we envision the possibility of manipulating exotic physical phenomena in the emerging topological antiferromagnetic spintronics field via the electric-field approach.

Electric-Field Control of Magnetic Order: From FeRh to Topological Antiferromagnetic Spintronics

Localization of electrons in the two-dimensional electron gas at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO${}_{3}$ were grown on NdGaO${}_{3}$ (110) substrates and capped with LaAlO${}_{3}$. When the SrTiO${}_{3}$ thickness is \ensuremath{\le}6 unit cells, most electrons at the interface are localized, but when the number of SrTiO${}_{3}$ layers is 8--16, the free carrier density approaches 3.3\ifmmode\times\else\texttimes\fi{}10${}^{14}$ cm${}^{\ensuremath{-}2}$, the value corresponding to charge transfer of 0.5 electrons per unit cell at the interface. The number of delocalized electrons decreases again when the SrTiO${}_{3}$ thickness is \ensuremath{\ge}20 unit cells. The \ensuremath{\sim}4 nm conducting channel is therefore located significantly below the interface. The results are explained in terms of Anderson localization and the position of the mobility edge with respect to the Fermi level.

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Zhiqi Liu

Papers

An interface dipole predictive model for high-k dielectric/semiconductor heterostructures using the concept of the dipole neutrality point

Tailoring the electronic properties of SrRuO3 films in SrRuO3/LaAlO3 superlattices

Electric-Field Control of Magnetic Order: From FeRh to Topological Antiferromagnetic Spintronics

Conducting channel at the LaAlO 3 /SrTiO 3 interface

原子的にテーラー加工したSrTiO 3 を用いた系における二軸歪み誘起輸送特性変化