mV mW Research Articles

High-frequency ultra-sensitive spin-torque diode based on Co-rich magnetic tunnel junction with in-plane magnetized free layer

Spin-torque diodes (STDs) based on magnetic tunnel junctions (MTJs) are newly developing spintronic devices. However, so far, the highly sensitive STDs reported in experiments have a low detection frequency, generally lower than 2 GHz, which limits the application of STDs in HF fields. Here, we develop Co-rich MTJs to enhance in-plane magnetic anisotropy and demonstrate an in-plane magnetized HF ultra-sensitive STD. By selecting the proper field and using an injection locking mechanism, HF microwave detection close to 5 GHz can be realized with a sensitivity greater than 3700 mV mW−1. This result may pave the way to developing spin-torque microwave detectors that are HF and highly sensitive.

Harvesting microwave energy using pyroelectricity of nanostructured graphene/zirconium-doped hafnium oxide ferroelectric heterostructures

In this work, we present the design, atomistic/circuit/electromagnetic simulations, and the experimental results for graphene monolayer/zirconium-doped hafnium oxide (HfZrO) ultra-thin ferroelectric-based field effect transistors fabricated at the wafer scale, regarding the pyroelectricity generation directly from microwave signals, at room temperature and below it, namely at 218 K and at 100 K. The transistors work like energy harvesters, i.e. they collect low-power microwave energy and transform it into DC voltages with a maximum amplitude between 20 and 30 mV. The same devices function as microwave detectors in the band 1–10.4 GHz and at very low input power levels not exceeding 80 μW when they are biased by using a drain voltage, with average responsivity values in the range 200–400 mV mW−1.

The microwave properties of tin sulfide thin films prepared by RF magnetron sputtering techniques

In this paper we present the microwave properties of tin sulfide (SnS) thin films with the thickness of just 10 nm, grown by RF magnetron sputtering techniques on a 4 inch silicon dioxide/high-resistivity silicon wafer. In this respect, interdigitated capacitors in coplanar waveguide technology were fabricated directly on the SnS film to be used as both phase shifters and detectors, depending on the ferroelectric or semiconductor behaviour of the SnS material. The ferroelectricity of the semiconducting thin layer manifests itself in a strong dependence of the electrical permittivity on the applied DC bias voltage, which induces a phase shift of 30 degrees mm−1 at 1 GHz and of 8 degrees mm−1 at 10 GHz, whereas the transmission losses are less than 2 dB in the frequency range 2–20 GHz. We have also investigated the microwave detection properties of SnS, obtaining at 1 GHz a voltage responsivity of about 30 mV mW−1 in the unbiased case and with an input power level of only 16 μW.

Tunable Resonant‐Photopyroelectric Detector Using Chalcogenide−Metal−Fluoropolymer Nanograting

AbstractPyroelectric detectors are often broadband and require external filters for wavelength‐specific applications. This paper reports a tunable, narrowband, and lightweight pyroelectric infrared detector built upon a flexible membrane of As2S3−Ag−P(VDF‐TrFE) with subwavelength grating, which is capable of both on‐chip filtering and photopyroelectric energy conversion. The top surface of this hybrid membrane is a corrugated As2S3−Ag film contributing to narrowband light absorption in the near‐infrared (NIR) regime, and the bottom part is a polyvinylidene fluoride‐trifluoroethylene (PVDF‐TrFE) membrane for the conversion of the absorbed light to an electrical signal. Uniquely, applying a bias voltage to the PVDF‐TrFE membrane enables the tuning of the device's absorption and pyroelectric characteristics owing to the piezoelectrically induced mechanical bending. The resonator exhibited a resonant absorption coefficient of 80% and a full‐width‐half‐maximum of 15 nm within the NIR, a responsivity of 1.4 mV mW−1, and an equivalent noise power of 13 µW Hz−1/2 at 1560 nm. By applying a 15‐V bias to the PVDF‐TrFE membrane, the absorption coefficient decreased to 18% due to the change in the grating period and incident angle. The narrowband and tunable features of the As2S3−Ag−P(VDF‐TrFE) pyroelectric detector will benefit a variety of potential applications in sensors, optical spectroscopy, and imaging.

Size effects in a hybrid plasmonic waveguide based power monitor with a bolometric method

A silicon based hybrid plasmonic waveguide (HPW) configuration has been widely utilized to monitor the power flowing in the plasmonic circuit. Due to the approximately linear relationship between resistance and temperature change (the temperature coefficient of resistance, TCR), the voltage-signal variation directly leads to the change of resistance and the corresponding change of coupled-in power. However, the size effect is rarely explored and demonstrated, which plays a substantial role while the metal-strip thickness approaches to its electron mean free path. It exhibits different electric properties, such as reduced TCR and thermal conductivity, resulting in deviations of resistivity. On the contrary, due to the increase of boundary scattering and narrow-strip absorption, an improvement of the responsivity and excepted sensitivity is found. Therefore, considering a trade-off associated with size effects, 50 nm silver-strip and gold-strip were utilized as the conductor layer in the HPW configuration with different geometries, respectively. Meanwhile, a 50 nm × 300 nm gold-strip HPW provides an improvement performance, up to 13.7 mV mW V−1, 0.05 μW sensitivity at bias 0.1 V. Its responsivity is about 1.4 times larger than that of silver, which contradicts to the macroscopic model.

Dynamics properties of skyrmion based microwave detectors under external field

We designed a new structure microwave detectors based on skyrmion, which has a very high theoretical limit sensitivity reaching over 100 000 mV mW−1 without any external bias fields. Moreover, the response of the detector to external variables such as bias current and bias magnetic field were also investigated. Our investigations offer a new way for the design of skyrmion based microwave detectors, which are useful in the developments of spintronic devices.

Spin‐Torque Diodes: From Fundamental Research to Applications

AbstractAmong a rich variety of emerging spintronic devices, spin‐torque diodes (STDs) are among the most interesting, from both a fundamental and an applied perspective. The spin‐torque diode effect occurs when a microwave alternating electric current injected into a magnetic tunneling junction (MTJ) is rectified due to the simultaneous actions of tunneling magnetoresistance and spin‐transfer torque. While the sensitivity of STDs observed in the initial research is rather modest (about 1.4 mV mW−1), after only a few years researchers have increased it to 200 kV W−1 and demonstrated rectification at nanowatt input powers, which radically exceeds the capabilities of mainstream Schottky diodes. This impressive progress is based on a deep understanding of the complex STD physics and on recent advances in MTJ fabrication technology. Herein, the experimental pathways toward increasing the sensitivity and extending the frequency range of STDs and theoretical works aimed at explaining the complex nonlinear dynamics are analysed, and a wide variety of possible STD applications are discussed.

Analysis of nonlinear characteristics of a graphene based four-terminal ballistic rectifier using a drift-diffusion model.

In this study, rectification behavior and noise spectra of a graphene based four-terminal ballistic rectifier are reported utilizing semi-classical drift-diffusion 3D modeling. The room temperature DC and RF characteristics of the novel rectifier are demonstrated considering the traps in the material similar to a real device, reducing the rectification efficiency from 0.5% to 0.35%. The responsivity and noise equivalent power of about 89.21 mV mW−1 and 97.52 pW Hz−1/2, respectively, are obtained for different frequencies varying from 50 Hz to 1 THz. Furthermore, the noise spectral analysis of the device predicts a minimum low frequency noise, which depends upon the carrier concentration inside the device active region rather than mobility, and hence enables potential applications as THz detectors for imaging.

Graphene ballistic nano-rectifier with very high responsivity.

Although graphene has the longest mean free path of carriers of any known electronic material, very few novel devices have been reported to harness this extraordinary property. Here we demonstrate a ballistic nano-rectifier fabricated by creating an asymmetric cross-junction in single-layer graphene sandwiched between boron nitride flakes. A mobility ∼200,000 cm2 V−1 s−1 is achieved at room temperature, well beyond that required for ballistic transport. This enables a voltage responsivity as high as 23,000 mV mW−1 with a low-frequency input signal. Taking advantage of the output channels being orthogonal to the input terminals, the noise is found to be not strongly influenced by the input. Hence, the corresponding noise-equivalent power is as low as 0.64 pW Hz−1/2. Such performance is even comparable to superconducting bolometers, which however need to operate at cryogenic temperatures. Furthermore, output oscillations are observed at low temperatures, the period of which agrees with the lateral size quantization.

Giant spin-torque diode sensitivity in the absence of bias magnetic field.

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW−1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

Highly sensitive nanoscale spin-torque diode

Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.

Modeling of the terminating-type power sensors fabricated by GaAs MMIC process

In this paper, a two-dimensional (2D) model of the terminating-type power sensor is established under different input powers. The 2D heat transfer equation is applied to describe the temperature distribution, and Fourier series is used to obtain the solution based on the boundary conditions. In order to demonstrate the validity of the 2D model, finite-element method (FEM) simulation using ANSYS software was performed. The sensitivity from the 2D model and FEM is 0.25 mV mW−1 and 0.28 mV mW−1 respectively, while the sensitivity from the 1D model is 0.34 mV mW−1, which indicates that the presented 2D model is closer to the simulation than the 1D model. The terminating-type power sensor was designed and fabricated by MEMS technology and the GaAs MMIC process. The measured return loss is less than −26 dB for a frequency up to 10 GHz. The power measurement was performed and a good linearity of the output thermovoltage with respect to the input power is obtained. The measured sensitivity is close to 0.26 mV mW−1, 0.23 mV mW−1 and 0.16 mV mW−1 at 0.1, 1 and 10 GHz, respectively. Moreover, the frequency dependence measurement demonstrates that the measured thermovoltage decreases with increasing the frequency. The measurements demonstrate that the measured results agree with the presented 2D model for low frequency while the measured thermovoltage deviates from the expectation at high frequency. The reason is that the electromagnetic coupling loss of the coplanar waveguide and the parasitic loss of the load resistor become higher at high frequency.

A 35 GHz wireless millimeter-wave power sensor based on GaAs micromachining technology

A novel MEMS wireless millimeter-wave power sensor based on GaAs MMIC technology is presented in this paper. The principle of this wireless millimeter-wave power sensor is explained. It is designed and fabricated using MEMS technology and the GaAs MMIC process. With the millimeter-wave power range from 0.1 to 80 mW, the sensitivity of the wireless millimeter-wave power sensor is about 0.246 mV mW−1 at 35 GHz. In order to verify the power detection capability, this wireless power sensor is mounted on a PCB which influences the microwave performance of the CPW-fed antenna including the return loss and the radiation pattern. The frequency-dependent characteristic and the degree-dependent characteristic of this wireless power sensor are researched. Furthermore, in addition to the combination of the advantages of CPW-fed antenna with the advantages of the thermoelectric power sensor, another significant advantage of this wireless millimeter-wave power sensor is that it can be integrated with MMICs and other planar connecting circuit structures with zero dc power consumption. These features make it suitable for various applications ranging from the environment or space radiation detection systems to radar receiver and transmitter systems.

Enhanced lateral photovoltaic effect in the p-n heterojunction composed of manganite and silicon by side irradiation for position sensitive detecting

Lateral photovoltaic effect has been studied in p-La0.67Ca0.33MnO3/n-Si heterojunction. Under illumination of continuous 808 nm laser beam on the film surface, a transient photovoltaic overshoot accompanied with the steady signal was observed when the laser turned off and on. The open-circuit photovoltage had a linear dependence on illuminated position, and the sensitivity reached 0.75 mV mW(-1) mm(-1) for steady value and 6.25 mV mW(-1) mm(-1) for the transient peak value. Especially, an enhancement in position detecting sensitivity was observed when the interface of this heterojunction was irradiated, which were 1.25 mV mW(-1) mm(-1) (steady value) and 26.0 mV mW(-1) mm(-1) (peak value). This work demonstrates a novel way to increase sensitivity for manganite-based position sensitive detectors.

Ka-band quasioptical detectors based on antenna-integrated planar Schottky diodes

This letter presents a quasioptical monolithic detector for Ka-band applications.It consists of an antenna-integrated planar Schottky diode and an extended semispherical lens. A three-dimensional electromagnetic model is built for evaluating its radiation performance. The responsivity of the detector ranges from 712 to 1483 mV mW−1 over 26.5∼ 40 GHz under the bias voltage of 0.8 V, corresponding to the estimated noise equivalent power between 2.5E−11 and 5.2E−11 W Hz−1/2. The radiation patterns are measured and agree well with the simulated ones, showing the validity of the proposed model. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26168

A terminating-type MEMS microwave power sensor and its amplification system

A terminating-type MEMS microwave power sensor and its amplification system are presented in this paper. A SPICE model is introduced to simulate temperature distribution of this power sensor, and the model has a reference value to estimate the sensitivity of the power sensor. This power sensor is designed and fabricated using MEMS technology and the GaAs MMIC process. It is measured in the frequency range up to 20 GHz with an input power in the 0 to 50 mW range. Over the 50 mW dynamic range, the sensitivity is about 0.29, 0.24 and 0.22 mV mW−1 at 5, 10 and 15 GHz, respectively. The output voltage of the power sensor ranges from 0.64 to 15.2 mV at 5 GHz. After amplification, the output voltage ranges from 0.19 to 6.56 V. The amplification gain is about 437.5, and the sensitivity is increased to 1274 mV mW−1. At 15 GHz, the output voltage of the power sensor ranges from 0.57 to 11.69 mV. After amplification, the output voltage ranges from 0.146 to 5.02 V. The amplification gain is about 438, and the sensitivity is increased to 974.8 mV mW−1. The measurement results show that the amplification system can amplify the output weak signal of the power sensor well and have good linearity.

A novel symmetrical microwave power sensor based on GaAs monolithic microwave integrated circuit technology

A novel symmetrical microwave power sensor based on GaAs monolithic microwave integrated circuit (MMIC) technology is presented in this paper. In this power sensor, the left section inputs the microwave power, while the right section inputs the dc power. Because of the symmetrical structure, this power sensor is created to provide more accurate microwave power measurement capability without mismatch uncertainty and restrain temperature drift. The loss model is built and the loss voltage is 0.8 mV at 20 GHz when the input power is 100 mW. This power sensor is designed and fabricated using GaAs MMIC technology. And it is measured in the frequency range up to 20 GHz with the input power in the −20 dBm to 19 dBm range. Over the 19 dBm dynamic range, the sensitivity can achieve about 0.2 mV mW−1. The difference between the input powers in the two sections is below 0.1% for equal output voltages. For an amplitude modulation measurement, the carrier frequency is the main factor to influence the measurement results. In short, the key aspect of this power sensor is that the microwave power measurement can be replaced by a dc power measurement with precise wideband.

Enhanced photovoltage in perovskite-type artificial superlattices on Si substrates

We have fabricated a three-component perovskite-type superlattice (SL) consisting of La0.9Sr0.1MnO3, SrTiO3 and LaAlO3 with atomic scale control by laser molecular beam epitaxy on Si substrates. When a He–Ne laser irradiated the superlattice by side illumination, a stable photovoltage was produced and the responsivity reached 46.7 mV mW−1 which is six times higher than that of a similarly grown La0.9Sr0.1MnO3 single layer on Si substrates. This work demonstrates the potential of the present SL in photo-detectors operating at room temperature.

Enhancement of photovoltaic effect in La0.9Sr0.1MnO3/Si heterojunction by side illumination

The side-illumination-enhanced photovoltaic effect has been studied in La0.9Sr0.1MnO3/Si heterojunctions fabricated by laser molecular beam epitaxy. When a He–Ne laser illuminated the La0.9Sr0.1MnO3 film surface, a stable photovoltage was produced and the responsivity is ∼0.17 mV mW−1. An enhancement of the photovoltaic effect occurred when the La0.9Sr0.1 MnO3/Si interface was illuminated directly by side illumination, and the steady responsivity reached ∼6.87 mV mW−1. This work demonstrates an effective way to improve the photovoltaic responsivity of the perovskite oxide heterojunctions.

A spot pricing mechanism for voltage stability

Payment for reactive power services is a part of generator revenue in most established electricity markets today. However, such payments do not have a sound technical basis taking into account the critical role dynamic sources of reactive power play in maintaining voltage stability of the power system. It is also expected that the reactive power will eventually be traded under a spot market arrangement as the markets mature. This paper proposes a spot pricing mechanism for reactive power that takes into account the contributions made by generators both by providing reactive power to meet the current demand and also holding reactive power ‘reserve’ to maintain the system voltage stability in the face of a potential contingency. An OPF based dispatch/pricing model is proposed that optimally allocates real and dynamic reactive reserve among the generators to meet a pre-specified voltage stability margin. Nodal MW and MV Ar spot prices are affected by the desired stability margin because an increase in MV Ar and/or MW demand has the cascading effect of increasing the dynamic MV Ar/MW reserve requirement. Simplistic illustrative examples are presented to develop insights on the theoretical premise. Experiments conducted for a reduced New Zealand North Island power system reveal some interesting impacts of the voltage stability constraint on real and reactive power prices.