We have studied microwave emission from a current-perpendicular-to-plane pseudo spin valve nanopillars with Heusler alloy Co 2 Fe(Ga 0.5 Ge 0.5 ) electrodes. Large emission amplitude exceeding 150 nV/Hz 0.5 , partly owing to the large magnetoresistance, and narrow generation linewidth below 10 MHz are observed. We also find that the linewidth shows significant dependence on the applied field magnitude and its angle within the film plane. A minimum in the linewidth is observed when the slope of the frequency versus current becomes near zero.This agrees with theoretical prediction that takes into account non-linear phase noise as a source for linewidth broadening.* E-mail address: hayashi.masamitsu@nims.go.jp 2Since the prediction that spin transfer torque (STT) 1-2 can induce magnetization dynamics, numerous experimental and theoretical studies have been performed 3 . Spin torque oscillators (STO) 4-6 , a nanoscale frequency tunable microwave source, can be applied to microwave sources for electro-communication devices and oscillating field sources for microwave assisted recording heads 7 . To obtain narrow spectral linewidth and large microwave power, various ferromagnetic materials and different geometries have been investigated.Geometry-wise, it has been demonstrated that it is possible to further increase the output power and achieve narrow linewidth when multiple STOs are synchronized [8][9][10] . Fig. 1(a).A ground-signal-ground (GSG) contact pads are patterned to allow high frequency signal detection from the pillar. Microwave probes (bandwidth: DC-18 GHz) are used to contact the device and the signal is fed to a spectrum analyzer (bandwidth: 9 kHz to 20 GHz) via a bias tee and an amplifier (bandwidth: 0.1-15 GHz). The gain of the amplifier is assumed constant throughout the entire frequency range and the measured amplifier gain (~50 dB) is divided out to obtain the power from the pillar device. Negative current corresponds to flow of electron from the thin to thick Co 2 Fe(Ga 0.5 Ge 0.5 ) layer. The sample is subjected to in plane magnetic field which is generated by a two axis vector magnet. We define 0˚ of the field angle as the magnetic field pointing towards the +X direction in the inset of Fig. 1(a) and the sense of rotation is set to counter clockwise. We performed microwave emission measurements for a number of devices and here we show results from two representative devices of size 130×90 nm 2 (device A) and 140×140 nm 2 (device B).To study the crystalline anisotropy of the film, in Fig. 1(b), we show the magnetization hysteresis loops of a continuous film with the same film stack structure on which the nanopillar devices were fabricated. Magnetization measurements are carried out using a vibrating sample magnetometry (VSM) and the applied field angle is varied within the film plane. From the hysteresis curves we infer that the film is isotropic, i.e. the shape of the magnetization hysteresis loop remains nearly unchanged at different applied field angle.Thus in the patterned pillar devices, the ...
This paper is intended to aid to bridge the gap between chemistry and electronic engineering. In this work, the fabrication of chemical vapour deposited graphene field-effect transistors employing silicon-nitride (Si3N4) gate dielectric is presented, showing originally p-type channel conduction due to ambient impurities yielding uncontrollable behaviour. Vacuum annealing has been performed to balance off hole and electron conduction in the channel, leading to the observation of the Dirac point and therefore improving controllability. Non-covalent functionalisation by methylamine has been performed for passivation and stability reasons yielding electron mobility of 4800 cm2/V s and hole mobility of 3800 cm2/V s as well as stabilised controllable behaviour of a bottom-gated transistor. The introduction of interface charge following the non-covalent functionalisation as well as the charge balance have been discussed and analysed.
All-chemical vapor deposited silicon nitride / monolayer graphene TFTs have been fabricated. Polychromatic Raman spectroscopy shows high quality monolayer graphene channels with uniform coverage and significant interfacial doping at the source-drain contacts. Nominal mobilities of approximately 1900 cm 2 V -1 s -1 have been measured opening up a potentially useful platform for analogue and RFR-based applications fabricated through allchemical vapor deposition processes.Graphene is a one atomic thick layer of carbon atoms, where their sp 2 bonds are arranged in a honeycomb crystal lattice. It is being touted as the new "miracle material" for electronics and photonics in the 21 st Century. This is because of its unique properties such as the very high carrier mobility at room temperature, large free mean path for ballistic transport, high Young's modulus and near zero-gap semiconductor (semi-metal). These make it potentially unique for nano-electronics, biosensors and information and computer technologies. Early graphene transistor configurations consisted of back-gated metal-oxide-semiconductor field effect transistors. Alternative top-gate configurations include the use of channels formed from high-quality natural and Kish exfoliated graphene, CVD grown and polymer transferred graphene and epitaxial deposited samples. SiO 2 , Al 2 O 3 , and HfO 2 have been investigated as viable dielectrics in zero band gap devices. Graphene transistors exhibit linear transfer characteristics without saturation or weakly saturate followed by a secondary linear regime [1]. Due to the low on/off current ratio, graphene-based devices are unsuitable for logic applications through various routes to bandgap opening have been proposed, such as the use of sub-5 nm-wide nanoribbons [2], mechanically strained free-standing channels, and biased bilayers [3].The dielectric is known to affect the charge carrier type in graphene devices [4]. SiN x has been successfully used as a dielectric for TFTs within flat-panel displays for many years, yielding a mature and widely accepted technology, which is, for example, routinely used in a-Si backplanes for organic light-emitting diode displays [5]. In this work, we show TFTs based on monolayer graphene as the active channel material with SiN x as the insulator where both materials have been produced using chemical vapor deposition (CVD) processes. Though low on/off ratios mitigate against their use in logic circuits they are well suited for analog RF applications, in body area networks and some flat panel applications.Bottom gated graphene TFTs were produced on 300 nm thick SiN dielectric layers deposited by PECVD onto degenerately doped silicon substrates. SiN x was deposited at 150 o C using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) 10.1149/05008.0217ecst ©The Electrochemical Society ECS Transactions, 50 (8) 217-221 (2012) 217 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.59.222.12 Downlo...
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