Spin dynamics excited by spin-polarized current in magnetic tunnel junctions (MTJs) is potentially useful in nanoscale electrical oscillation sources and detection devices. A spin oscillator/detector should work at a high frequency, such as that of a millimeter-wave, where the quality of a semiconductor device is restricted by carrier mobility, the CR time constant, and so on. Developers of spin systems for practical use need to find out how to excite spin dynamics (i) in the millimeter-wave region, (ii) with low power consumption (ex: no external magnetic field, low damping material), and (iii) for broad frequency modulation. Here L10-ordered FePd alloy with perpendicular magnetocrystalline anisotropy (PMA) and a low damping constant, 0.007, was used for the free layer in the MTJs, and a homodyne-detected ferromagnetic resonance (FMR) signal was obtained at around 30 GHz together with the possibility of one-octave frequency modulation. The FMR signal in out-of-plane magnetized L10-ordered FePd free layer could be excited without an external magnetic field by injecting in-plane spin polarized alternating current. This study shows the potential utility of L10-ordered alloy materials such as FePt, CoPt, MnAl, and MnGa in a variety of millimeter-wave spin devices.
We make a 2-D analysis of breakdown characteristics of field-plate AlGaN/GaN HEMTs with a high-k passivation layer, and the results are compared with those having a normal SiN passivation layer. As a result, it is found that the breakdown voltage is enhanced particularly in the cases with relatively short field plates because the reduction in the electric field at the drain edge of gate effectively improves the breakdown voltage in the case with the high-k passivation layer. In the case with the moderate-length field plate, the enhancement of breakdown voltage due to the high-k passivation layer occurs because the electric field profiles between the field-plate edge and the drain become more uniform. It is also studied how the breakdown voltage depends on a deep-acceptor density in the Fe-doped semiinsulating buffer layer when a high-k passivation layer is used. It is shown that the breakdown voltage increases with increasing the relative permittivity of the passivation layer ε r and with increasing the deep-acceptor density N DA . When ε r = 60 and N DA = 2-3 × 10 17 cm −3 at the gate length of 0.3 μm, the breakdown voltage becomes about 500 V at a gate-to-drain distance of 1.5 μm, which corresponds to an average electric field of about 3.3 MV/cm between the gate and the drain. Index Terms-2-D analysis, breakdown characteristics, buffer layer, GaN HEMT, high-k passivation layer. I. INTRODUCTION N OWADAYS, AlGaN/GaN HEMTs are attractive for applications to high-power microwave devices and highpower switching devices [1], [2]. It is well known that introducing a field plate enhances the power performance of AlGaN/GaN HEMTs as well as GaAs FETs [3]-[5]. This occurs because by introducing a field plate, the current collapse is reduced [6], [7], and the breakdown voltage increases [8]-[10]. The increase in breakdown voltage occurs because the electric field at the drain edge of gate is reduced by introducing a field plate.
Current-induced microwave spectra were measured in small-sized giant magnetoresistance devices composed of a NiFe vortex free layer and an out-of-plane magnetized Co/Pd multilayer polarizer. The influence of a large direct current (DC) and a bias field on the excited mode of the free layer is systematically investigated. For small current values, microwave spectra due to the vortex core oscillation were observed around 1 GHz, while the frequency abruptly changed to 4–4.5 GHz at certain DC values. The experimental data were reproduced by micromagnetic simulation, which indicates that the mode change of the vortex core oscillation in the free layer is dominated by the Oersted field from the large DC.
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