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We have investigated this magneto-switching behavior in GaAs structure. Exposing these devices to a small magnetic field (< 0.5 Tesla) induced a change in conductance that was above 100,000% at 10 K and up to 1,000% at 300 K. We also observed a variety of transport mechanisms in a single device that were clearly distinguished according to the bias voltage and their characteristic magnetoconductance can be specified. This work represents important progress in understanding the physics of the magneto-switching function and achieving large ON/OFF ratios.

Most studies of the Rashba effect have focused on interfacial Rashba spin–orbit coupling. Recently, bulk Rashba materials have attracted considerable interest owing to their potential to enhance the Rashba spin–orbit torque. By employing a bulk Rashba material, GeTe, as a spin–orbit channel in GeTe/NiFe bilayers, a large field-like spin–orbit torque up to 15.8 mT/(107 A cm−2) is measured. This value is one of the largest reported field-like torques and is attributed to the interfacial spin–orbit coupling being enhanced by the bulk Rashba effect in the GeTe channel. Furthermore, the large field-like torque is maintained even for a 20-nm-thick NiFe layer. This unconventional dependence on the thickness of both the GeTe and NiFe layers cannot be described by conventional theory, but it is believed to stem from the additional bulk Rashba effect-induced term. The large field-like torque over a wide range of ferromagnet thicknesses results in scalable in-plane spin–orbit torque devices. This result calls for a further theoretical study on spin transport in heterostructures, including bulk Rashba materials. We find a very large field-like spin-orbit torque in a bulk Rashba channel/ferromagnet structure and this large value of spin-orbit torque is attributed to the interfacial spin-orbit coupling enhanced by the well-grown bulk Rashba material.

In order to investigate the effective mass of a single-crystalline WTe2 layer, we measured the temperature dependence of the Shubnikov-de Haas oscillation. In this method, the magnetoresistance with a perpendicular magnetic field is monitored by changing the temperature from 1.9 K to 6 K. The extracted effective mass of WTe2 for magnetic fields ranging from 6.8 T to 8.7 T is m* = 0.327m0 which is almost constant in this range. The theoretical expectation values of m*/m0 for the valence and the conduction bands are 0.58 and 0.26, respectively; the experimental value of the effective mass is between these two values. Thus, our results clearly show that single-crystalline WTe2 has both electron- and hole-like pockets that simultaneously contribute to electrical transport in the channel.

An InAs heterostructure-based micro-Hall sensor was used to study the magnetic properties of a superconducting Nb micro-ring, enabling observation of magnetic phenomena such as diamagnetism onset and magnetic flux trapping in the 20-μm-diameter sample. The superconducting diamagnetism of the micro-ring was observed to develop slowly from T = 7.5 K down to 5 K and showed a notably sharp and substantial drop at 7.0 K, the zero-resistivity temperature obtained from transport measurements on a strip-patterned sample. The observed superconducting transition is discussed in terms of a percolation scenario. In magnetic-field-cooling measurements, the Hall signal from the magnetic flux trapped in the Nb ring at 4.5 K was detected at a sufficiently high level for quantitative comparison with the estimate.

The LaAlO3/SrTiO3 (LAO/STO) heterostructure has an inherent space inversion asymmetry causing an internal electric field near the interface. The Rashba spin-orbit coupling arising from this structural characteristic has a considerable influence on spin transport. With application of an external magnetic field, we observed conductance change in the LAO/STO interface which depends on the sign and magnitude of the field. Our systematic study revealed that these results come from spin dependent transport, by which we obtained quantitative strength of the Rashba effect. The Rashba strength in this system depends on the temperature: it varies from 2.6 x 10(-12) eVm to negligible value in the temperature range of 1.8 K-12 K. This method for detecting Rashba effect covers a wider temperature range in comparison with those obtained from Shubnikov-de Haas oscillation or weak antilocalization measurements.

In the field of spintronics, charge-to-spin conversion without a ferromagnetic material has attracted intense interest from researchers seeking to realize a fully electrical spin device because this design obviates the need for magnetic field control of magnetization. Instead of spin injection from a ferromagnetic source, spin-Hall-induced pure spin current has recently attracted considerable interest for transferring spin information into the semiconductor channel. In the present work, the spin is injected from a platinum electrode via the direct spin Hall effect and is subsequently detected in a strong Rashba channel via the inverse spin Hall effect. Before being detected, the spin state is modulated by a gate voltage; the signal observed with various channel lengths and gate voltages demonstrates this Rashba precessional modulation. The addition of Zeeman precession induced by an external magnetic field provides the signal-elucidating definite spin motion in the channel and clear interplay between the Rashba and Zeeman processions. Our approach opens a fascinating possibility for realizing a ferromagnet-free system for use in low-power and high-temperature spin transistors.

We experimentally investigated the transport properties near metal electrodes installed on a conducting channel in a LaAlO3/SrTiO3 interface. The local region around the Ti and Al electrodes has a higher electrical conductance than that of other regions, where the upper limits of the temperature and magnetic field can be well defined. Beyond these limits, the conductance abruptly decreases, as in the case of a superconductor. The samples with the Ti- or Al-electrode have an upper-limit temperature of approximately 4 K, which is 10 times higher than the conventional superconducting critical temperature of LaAlO3/SrTiO3 interfaces and delta-doped SrTiO3. This phenomenon is explained by the mechanism of electron transfer between the metal electrodes and electronic d-orbitals in the LaAlO3/SrTiO3 interface. The transferred electrons trigger a phase transition to a superconductor-like state. Our results contribute to the deep understanding of the superconductivity in the LaAlO3/SrTiO3 interface and will be helpful for the development of high-temperature interface superconductors.

Perpendicular magnetic anisotropy was observed in sputtered FeZr/CoFeB/MgO multilayers. A thin paramagnetic amorphous FeZr layer was used as a buffer layer and perpendicular anisotropy was obtained by annealing the samples without an external magnetic field. The critical CoFeB thickness for perpendicular anisotropy was 1.8 nm; the anisotropy changes from out-of-plane to in-plane as the CoFeB thickness increases beyond this point. Perpendicular anisotropy was also enhanced when a Ta layer was capped on top of the MgO layer. The amorphous buffer provided better perpendicular anisotropy than previously reported Ta buffer, and it may be applied to perpendicular magnetization MRAM devices where good uniformity of tunnel junctions is required.

The performance of CdZnTe (CZT) detectors is limited not only by conventional carrier-trapping to point defects but also by trapping at macroscopic Te secondary-phase defects, such as Te inclusions and Te precipitates. The aim of this research is to remove these secondary-phase defects via thermomigration, and to obtain high resistivity of the material by creating Te antisites through annealing in a high Te overpressure. We annealed Te-rich CZT samples in the temperature range between 500 and 700 °C under Te overpressure with a temperature gradient of 50–60 °C/cm. We investigated the effects of annealing under these conditions by IR transmission microscopy, current–voltage measurements, photoluminescence, and white-beam X-ray diffraction topography (WBXDT) measurements comparing the findings with those from isothermally annealed CZT samples under Cd overpressure. We proved experimentally that Te inclusions present in Te-rich CZT melts contain void fractions. We attributed the complex defect of Te antisites with Cd vacancies in our annealing experiment as the deep level defect that pins the Fermi near the middle of the bandgap, rather than the Te antisite.

The spin polarization of a high g-factor bulk semiconductor is theoretically investigated in the presence of a magnetic field parallel to a driving electric field. Calculations have been carried out using the energy-dependent relaxation time approximation in association with spin-flip scattering. As the magnitude of the magnetic field increases, the spin-polarized current alternates between the spin-up and spin-down states for the low spin-scattering system. This implies that the current polarization can be tuned by controlling the magnetic field strength, suggesting possible applications to spintronic devices. An experimental method for investigating alternative current polarization is also considered.

Semi-insulating Cd 0.9 Zn 0.1 Te:Pb crystals were grown by the vertical Bridgman method. Measurements of the current deep level transient spectroscopy (I-DLTS) revealed three trap levels in this material. Unlike other compensating dopants, CdZnTe:Pb samples do not show any Cd-vacancies defects and A-center levels. We subjected them to temperature-gradient annealing in Cd overpressure at 490–717 °C, and recorded an exponential relationship between the annihilation time of Te precipitates and the annealing temperature. The energy resolution of an annealed CdZnTe:Pb detector, tested using a 137 Cs radioactive source, gave an energy resolution of 2.5%.

We fabricated a magnetic field sensor made of an InAs quantum-well. Various numbers of magnetic particles were located on the sensor and the stray field generated by the magnetic particles was detected using micro-Hall magnetometry at room temperature. A numerical calculation carried out for this device successfully explained the experimental results and a useful equation relating the signal of the device and the number of particles was deduced.

Magnetic tunneling transistors (MTT) with double tunneling barriers are fabricated. The structure of the transistor is AFM/FM/I/FM/I/FM/AFM, and ferromagnetic layers serve as the emitter, base and collector. This double-barrier tunneling transistor (DBTT) has an advantage of controlling the potential between the base and collector, compared to the Schottky-barrier-based base and collector of MTT. We found that the collector current density of DBTT is at least 10 3 times larger than that of conventional MTT, since tunneling through AlO x barrier provides much larger current density than that through Schottky barrier.

The transport properties of ferromagnet/Al2O3/semiconductor tunnel junctions were investigated. Si and GaAs substrates were used, and CoFe and NiFe were taken for ferromagnets. Diode characteristics were observed in current-voltage curves. The dependences on the Al2O3 thickness and temperature were also studied. It is found that the Al2O3 layer plays a crucial role in preventing inter-diffusion at the interface. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two kinds of mechanisms for magnetic field sensors are discussed, namely, intrinsic and geometric magnetoresistance (IMR and GeMR). Two extreme cases are experimentally obtained in InAs and HgCdTe, respectively, and the results are in good agreement with numerical analysis by the finite difference method (FDM). By adjusting the aspect ratio of rectangular samples, we show that the magnetoresistance (MR) can be enhanced several times. It is also found that MR becomes almost independent of geometric factors in the strong IMR case. These results can be applied to optimize semiconductor magnetic sensor devices.

We present experimental and theoretical results for the propagation of 2D electrons through simple magnetic barriers. We show that it is not possible to use a ballistic picture to understand the barrier conductance even when the width of the barrier inthe directionof curren t propagationis small compared to the electronmeanfree path. ? 2002 Elsevier Science B.V. All rights reserved.

Cd1−xZnxTe grown by high-pressure Bridgman method (HPB) has been known to be a good candidate for gamma-ray detection material. The metal contacting conditions are very important for the performance of gamma-ray detector. Therefore, it is essential to find proper contact technology. For these purposes, various contact methods (electroless deposition, thermal vacuum evaporation) and materials (Au, Pt) are attempted. From the measurement of current–voltage characteristics, gamma-ray spectroscopy and auger electron spectroscopy (AES), the physical properties of metal contact on the CdZnTe are analyzed and contact methods compared with one another. Electroless Pt deposition is found to have better properties than others in many respects.

A model of surface recombination velocity for n-type Hg 1− x Zn x Te is presented. It has been shown that, both experimentally and theoretically, the surface recombination velocity is dominated by Shockley–Read recombination mechanism at higher temperature and dominated by Auger process at low temperature. We have further developed a surface recombination theory for the narrow-gap semiconductor. It is concluded that the optimum conditions to reduce recombination velocity is about 2×10 11 cm −2 in the fixed charge density and the Cd 0.8 Zn 0.2 Te passivation layer thickness is between 140 and 160 nm.

The apparatus of the modulated Hall effect using laser-chopping technique was set up to measure the minority carrier mobility of p-type HgCdTe grown by Rotating technique. The measurements for the Fourier transform infrared (FTIR) spectroscopy and the Hall effect of p-type HgCdTe was carried out. One of the as-grown HgCdTe samples was converted into n-type through Hg annealing to see if the value of the minority carrier mobility measured by the modulated Hall effect is the same with the majority carrier mobility of the n-type HgCdTe. This converted sample was characterized by FTIR and Hall effect measurement. The modulated Hall effect was measured by changing the magnetic field of 0-0.45 T over the temperature range from 77 to 150 K.

We have fabricated hybrid ferromagnet-semiconductor devices which have magnetic barriers in a form of the serial junctions of positive and negative magnetic-field regions. These magnetic barriers are controlled by a magnetization of micromagnets on the surface of the device. The large variation of longitudinal resistance as a function of the magnitude of magnetic barriers has been observed, and its functional form is similar to Hall resistance. At low temperature the measured longitudinal resistance of our device shows negative value. This device can be a good candidate for spintronic devices using spin up/down junction.

We have observed the negative value of longitudinal resistance in a InAs two-dimensional electron gas (2DEG) system in which magnetic barriers have been made in the form of the serial junctions of positive and negative magnetic-field regions. When the transport mechanism is ballistic (at low temperature), the measured longitudinal resistance of our device shows negative or positive values depending on the sign of the externally applied magnetic field.

The spin susceptibility of HgCdTe 2DEG system was measured in this study with two independent methods which yielded the same value. It is remarkably large compared to any other 2DEG system. The large value of spin susceptibility as well as the large g-factor are the unique properties of HgCdTe which can be utilized as spintronic material.

Tunneling magnetoresistance (TMR) dependence on the Al2O3 barrier thickness was investigated for CoFe/Al2O3/CoFe magnetic tunnel junctions (MTJs). MTJs with very thin Al2O3 layers were grown by inserting an amorphous FeZr buffer layer whose role is only to reduce the roughness of bottom electrode. The TMR decreased as the thickness of the Al2O3 layer was reduced. The results are analyzed with the dependence of the spin-filtering effect on the Al2O3 thickness. It was found that a simple model of separating sp- and d-like electrons does not work, and it may suggest that the tunneling electrons are in rather hybridized state.

In order to investigate the surface states of electron which can tunnel through the sufficiently thin insulator, the electron tunneling spectroscopy is used to measure the tunneling current in function of the bias voltage, and directly to get the subbands of accumulation layer. The tunneling current through ZnS barrier in the Hg1-xCdxTe-ZnS-In junction structure is measured by various applied bias at 77 K and 4.2 K. From the measurement at 77 K, the subband energy levels in the electron accumulation layer at the surface of n-type Hg1-xCdxTe are found to be located at -59 meV for the ground state and -13 meV for the first excited state relative to the Fermi level of Hg1-xCdxTe. At low temperature when the applied bias is larger than the difference between the work function of In and the electron affinity of ZnS, a negative differential resistance is measured. On the calculation using transfer matrix method it is understood that this negative conductance is attributed to Fowler-Nordheim tunneling which is caused by the variation of ZnS barrier according to the various applied bias.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The ferromagnetic layer in magnetic tunnel junctions (MTJs) was oxidized with varying O2 concentrations, and the corresponding effect on spin-dependent transport was studied. As expected from our previous results for MTJs with an over-oxidized AlOx tunnel barrier, a partially oxidized ferromagnetic layer plays an important role in spin-dependent transport. As the temperature is lowered, the junction resistance increases dramatically, and the tunneling magnetoresistance (TMR) is strongly suppressed. Increasing the O2 concentration enhances the increase of resistance and suppression of TMR. This work supports our previous conclusion that oxidizing the ferromagnetic layer generates localized magnetic moments, which act as a scattering center for spin-polarized tunneling electrons.

An abrupt change of conductance at a threshold magnetic field was observed in a device consisting of a nonmagnetic narrow-gap semiconductor. The conductance varies more than 25 times as the magnetic field increases. The threshold magnetic field can be tuned using a bias voltage from zero to several hundred Gauss. This large magnetoconductance effect is caused by the magnetic field-dependent impact ionization process. A theoretical model is proposed, and calculations based on this model simulate the experimental results closely. This device may be a good candidate for an electrical switching device controlled by a magnetic field.

Spin accumulation in magnetic tunnel junctions and lateral all-ferromagnetic spin valves is theoretically investigated by using a spin-dependent diffusive transport model. When electric current tunnels through a barrier, spin accumulation occurs in the ferromagnetic layer and can be detected with nonlocal spin valve measurements. Depending on the relative orientation of magnetization, the nonlocal spin signal has three different values and can determine the bulk spin polarization of the ferromagnetic material.