Nonlocal Anomalous Hall Effect Zhang, Steven S-L; Vignale, Giovanni
Physical review letters,
2016-Apr-01, 2016-4-00, 20160401, Letnik:
116, Številka:
13
Journal Article
Recenzirano
Odprti dostop
The anomalous Hall (AH) effect is deemed to be a unique transport property of ferromagnetic metals, caused by the concerted action of spin polarization and spin-orbit coupling. Nevertheless, recent ...experiments have shown that the effect also occurs in a nonmagnetic metal (Pt) in contact with a magnetic insulator yttrium iron garnet (YIG), even when precautions are taken to ensure that there is no induced magnetization in the metal. We propose a theory of this effect based on the combined action of spin-dependent scattering from the magnetic interface and the spin-Hall effect in the bulk of the metal. At variance with previous theories, we predict the effect to be of first order in the spin-orbit coupling, just as the conventional anomalous Hall effect-the only difference being the spatial separation of the spin-orbit interaction and the magnetization. For this reason we name this effect the nonlocal anomalous Hall effect and predict that its sign will be determined by the sign of the spin-Hall angle in the metal. The AH conductivity that we calculate from our theory is in order of magnitude agreement with the measured values in Pt/YIG structures.
It is well known that the photovoltaic effect produces a direct current (DC) under solar illumination owing to the directional separation of light‐excited charge carriers at the p–n junction, with ...holes flowing to the p‐side and electrons flowing to the n‐side. Here, it is found that apart from the DC generated by the conventional p–n photovoltaic effect, there is another new type of photovoltaic effect that generates alternating current (AC) in the nonequilibrium states when the illumination light periodically shines at the junction/interface of materials. The peak current of AC at high switching frequency can be much higher than that from DC. The AC cannot be explained by the established mechanisms for conventional photovoltaics; instead, it is suggested to be a result of the relative shift and realignment between the quasi‐Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes. By virtue of this effect, the device can work as a high‐performance broadband photodetector with extremely high sensitivity under zero bias; it can also work as a remote power source providing extra power output in addition to the conventional photovoltaic effect.
An alternating current (AC) photovoltaic effect, different from known photovoltaic effects, that produces a large AC at a p–n junction is demonstrated. It is suggested that this new effect is due the relative shift and realignment between the quasi‐Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes.
The marine internet of things (MIoT), an increasingly important foundation for ocean development and protection, consists of a variety of marine distributed sensors under water. These sensors of the ...MIoT have always been highly dependent on batteries. To realize in situ power supply, a flexible seaweed-like triboelectric nanogenerator (S-TENG) capable of harvesting wave energy is proposed in this study. The flexible structure, designed with inspiration from the seaweed structure, processes extensive marine application scenarios. The bending and recovering of the S-TENG structure under wave excitations are converted to electricity. As the output performance increases with the number of parallel connected S-TENG units, an S-TENG system with multiple units could serve for floating buoys, coastal power stations, and even submerged devices. Through the demonstration experiments performed in this study, the flexible, low-cost S-TENG could become an effective approach to achieve a battery independent MIoT.
Recent experiments have revealed nonlinear features of the magnetoresistance in metallic bilayers consisting of a heavy metal (HM) and a ferromagnetic metal (FM). A small change in the longitudinal ...resistance of the bilayer has been observed when reversing the direction of either the applied in-plane current or the magnetization. We attribute such nonlinear transport behavior to the spin-polarization dependence of the electron mobility in the FM layer acting in concert with the spin accumulation induced in that layer by the spin Hall current originating in the bulk of the HM layer. An explicit expression for the nonlinear magnetoresistance is derived based on a simple drift-diffusion model, which shows that the nonlinear magnetoresistance appears at the first order of the spin Hall angle, and changes sign when the current is reversed, in agreement with the experimental observations. We also discuss possible ways to control sign of the nonlinear magnetoresistance and to enhance the magnitude of the effect.
Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ...ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi
Se
and an insulating ferromagnet BaFe
O
. A charge current in Bi
Se
can switch the magnetization in BaFe
O
up and down. When the magnetization is switched by a field, a current in Bi
Se
can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe
O
. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.
Rapid progress in nanotechnology allows us to develop a large number of innovative wearables such as activity trackers, advanced textiles, and healthcare devices. However, manufacturing processes for ...desirable nanostructure are usually complex and expensive. Moreover, materials used for these devices are mainly derived from nonrenewable resources. Therefore, it poses growing problems for living environment, and causes incompatible discomfort for human beings with long‐time wearing. Here, a self‐powered cellulose fiber based triboelectric nanogenerator (cf‐TENG) system is presented through developing 1D eco‐friendly cellulose microfibers/nanofibers (CMFs/CNFs) into 2D CMFs/CNFs/Ag hierarchical nanostructure. Silver nanofibers membrane is successfully introduced into the cf‐TENG system by using CMFs/CNFs as template, which shows excellent antibacterial activity. Enabled by its desirable porous nanostructure and unique electricity generation feature, the cf‐TENG system is capable of removing PM2.5 with high efficiency of 98.83% and monitoring breathing status without using an external power supply. This work provides a novel and sustainable strategy for self‐powered wearable electronics in healthcare applications, and furthermore paves a way for next‐generation flexible, biocompatible electronics.
The introduction of 1D eco‐friendly cellulose microfibers/nanofibers (CMFs/CNFs) into 2D CMFs/CNFs/Ag hierarchical nanostructure for a self‐powered triboelectric nanogenerator system enables the device with high efficiency for PM2.5 removal (98.83%), excellent antibacterial activity, and monitoring breathing without an external power supply, due to desirable pores/nanofibers design in the hierarchical nanostructure and a unique electricity generation feature of the device.
Ship draft measurement is of great significance for ensuring navigation safety and facilitating ship control. In this work, a self‐powered water level sensor based on a liquid–solid tubular ...triboelectric nanogenerator (LST‐TENG) is proposed and analyzed. The LST‐TENG is made of multiple copper electrodes uniformly distributed along a polytetrafluoroethylene (PTFE) tube. When water flows into the PTFE tube, it induces alternating flows of electrons between the main electrode and the distributed bottom electrodes. The obvious peaks in the derivative of open‐circuit voltage with respect to time are found to correspond with the electrode distribution. Then it can be utilized as a robust and sensitive indicator for detecting the water level as the number of obvious peaks in the derivative of open‐circuit voltage is directly related to the water level height. The ship draft is successfully detected using the LST‐TENG with an accuracy of 10 mm. It shows that the water level sensor has stable performance for liquid–solid interface monitoring. Therefore, this LST‐TENG is self‐powered, robust, and accurate for extensive applications in marine industry.
A self‐powered water level sensor based on a liquid–solid tubular triboelectric nanogenerator (LST‐TENG) is proposed and analyzed. It shows that the water level sensor has stable performance for liquid–solid interface monitoring. The ship draft is successfully detected using the LST‐TENG with high accuracy. This LST‐TENG is self‐powered, robust, and accurate for extensive applications in the marine industry.
The first contact‐mode triboelectric self‐powered strain sensor using an auxetic polyurethane foam, conductive fabric, and polytetrafluroethylene (PTFE) is fabricated. Utilizing the auxetic ...properties of the polyurethane foam, the auxetic polyurethane foam would expand into the PTFE when the foam is stretched, causing contact electrification. Due to a larger contact area between the PTFE and the foam as the foam is stretched, this device can serve effectively as a strain sensor. The sensitivity of this method is explored, and this sensor has the highest sensitivity in all triboelectric nanogenerator devices that are used previously as a strain sensor. Different applications of this strain sensor are shown, and this sensor can be used as a human body monitoring system, self‐powered scale to measure weight, and a seat belt to measure body movements inside a car seat.
The first contact‐mode triboelectric self‐powered strain sensor is fabricated using auxetic materials. Utilizing the auxetic properties of polyurethane foam, the polyurethane foam will expand when it is stretched, causing contact electrification. Different applications are realized and the triboelectric self‐powered strain sensor can be used for monitoring human body movement.
The interplay between electronic transport and antiferromagnetic order has attracted a surge of interest as recent studies show that a moderate change in the spin orientation of a collinear ...antiferromagnet may have a significant effect on the electronic band structure. Among numerous electrical probes to read out such a magnetic order, unidirectional magnetoresistance (UMR), where the resistance changes under the reversal of the current direction, can provide rich insights into the transport properties of spin-orbit-coupled systems. However, UMR has never been observed in antiferromagnets before, given the absence of intrinsic spin-dependent scattering. Here, we report a UMR in the antiferromagnetic phase of aFeRh/Ptbilayer, which undergoes a sign change and then increases strongly with an increasing external magnetic field, in contrast to UMRs in ferromagnetic and nonmagnetic systems. We show that Rashba spin-orbit coupling alone cannot explain the sizable UMR in the antiferromagnetic bilayer and that field-induced spin canting distorts the Fermi contours to greatly enhance the UMR by 2 orders of magnitude. Our results can motivate the growing field of antiferromagnetic spintronics and suggest a route to the development of tunable antiferromagnet-based spintronics devices.
Vibration is a common mechanical phenomenon and possesses mechanical energy in ambient environment, which can serve as a sustainable source of power for equipment and devices if it can be effectively ...collected. In the present work, a novel soft and robust triboelectric nanogenerator (TENG) made of a silicone rubber‐spring helical structure with nanocomposite‐based elastomeric electrodes is proposed. Such a spring based TENG (S‐TENG) structure operates in the contact‐separation mode upon vibrating and can effectively convert mechanical energy from ambient excitation into electrical energy. The two fundamental vibration modes resulting from the vertical and horizontal excitation are analyzed theoretically, numerically, and experimentally. Under the resonant states of the S‐TENG, its peak power density is found to be 240 and 45 mW m−2 with an external load of 10 MΩ and an acceleration amplitude of 23 m s−2. Additionally, the dependence of the S‐TENG's output signal on the ambient excitation can be used as a prime self‐powered active vibration sensor that can be applied to monitor the acceleration and frequency of the ambient excitation. Therefore, the newly designed S‐TENG has a great potential in harvesting arbitrary directional vibration energy and serving as a self‐powered vibration sensor.
A novel soft and robust triboelectric nanogenerator (TENG) made of a silicone rubber‐spring helical structure with nanocomposite based elastomeric electrodes is designed. Such a spring based TENG structure operates in contact‐separation mode upon vibrating and can effectively convert mechanical energy from arbitrary direction excitation into electrical energy. It can also serve as a self‐powered sensor for vibration monitoring.
PDF
