Nb/Al/AlOx/Nb superconducting tunnel junctions as x-ray detectors

Nb/Al/AlOx/Nb junctions have been shown to be effective x-ray detectors that are robust to thermal cycling. We compare results from two junctions, one with a fine-grained base-layer electrode and counterelectrode and the other with an epitaxial base-layer electrode and a fine-grained counterelectrode. For a 6 keV x-ray, at 0.4 K, the epitaxial sample had a FWHM resolution of 157 eV and the fine-grained junction had a FWHM resolution of 300 eV. The differences between the junctions will be discussed.     

Spectroscopy of short electron beam pulses by superconducting Nb/AlOx/Nb tunnel junctions

We have used superconducting Nb/AlO_x/Nb tunnel junctions (STJs) for the spectroscopy of short electron beam pulses, which contain only a few electrons with energy ranging between 6 and 35 keV. The spectroscopy was performed by Low Temperature Scanning Electron Microscopy (LTSEM) at T=2.6 K. The spectra show that the number of beam electrons within each single pulse follows the Poisson statistics and agree well with the theoretically expected spectra. Increasing the average number of beam electrons per pulse shows that the STJ-detector response is linear up to a deposited energy of more than 500 keV. For a large average number of beam electrons the achievable energy resolution of the detector is limited by the statistics of the beam electrons and the influence of the backscattered electrons.     

High tunnel magnetoresistance in epitaxial Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al/MgO/CoFe tunnel junctions

We have fabricated fully epitaxial magnetic tunnel junctions (MTJs) using a Co-based full-Heusler alloy Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al (CCFA) thin film and an MgO tunnel barrier. The CCFA thin film for the lower ferromagnetic electrode was deposited by magnetron sputtering on an MgO-buffered MgO single-crystal substrate, and the MgO tunnel barrier was formed by electron beam evaporation. The microfabricated epitaxial CCFA/MgO/CoFe MTJs showed high tunnel magnetoresistance ratios of 42% at room temperature and 74% at 55K.     

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.     

Tunneling magnetoresistance modulation in a magnetic tunnel junction with a ferroelectric barrier

A simple model is developed to investigate the potential profile changes due to mechanical stress at the ferromagnetic/ferroelectric interfaces of ferromagnetic-ferroelectric-ferromagnetic tunnel junctions with an ultrathin ferroelectric barrier. The potential changes associated with the polarization variation have significant effects on the tunneling conductance of the junctions. The discovered effect is illustrated by the example of a multiferroic tunnel junction in which approximately four orders of changes of the tunneling conductance and several-fold changes of the tunneling magnetoresistance (TMR) are observed due to the spin-flip induced nanomechanical stress. The TMR modulation effect is essential for realization of novel spintronics nano-devices as well as being useful for investigating fundamental aspects of the spin transfer.     

Negative magnetoresistance in SiC heteropolytype junctions

In this study, we carried out for the first time a galvanomagnetic investigation of 3C–SiC/6H–SiC heterostructures at liquid-helium temperatures and observed in n-3C–SiC low resistance of the samples and the appearance of a negative magnetoresistance in weak fields (~1 T). Analysis of the results we obtained shows that the low resistance is in all probability due to a metal—insulator transition in 3C–SiC epitaxial films. It was also found that the negative magnetoresistance magnitude decreases as the density of intertwine boundaries in a 3C–SiC epitaxial film becomes lower.     

Y-Ba-Cu-O/Nb Josephson tunnel junctions

The authors fabricated Y-Ba-Cu-O/Au/AlO_x/Nb and Y-Ba-Cu-O/AlO_x/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Sintered Y-Ba-Cu-O was used as the base electrode. Superconducting Josephson current and hysteresis of the current-voltage characteristics, which are typical features of Josephson tunnel junctions, have been observed at 4.2 K. RF-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed, and RF-induced subharmonic steps have also appeared. The superconducting Josephson current was modulated by the magnetic field     

具有覆盖层的Co/Cu/Co三明治的巨磁电阻效应研究

研究了覆盖层为铁磁性的Fe和非铁磁性的Ti、Cu的Co/Cu/Co三明治在室温和低温下的巨磁电阻效应。实验结果表明,室温下有覆盖层时,Co/Cu/Co三明治的巨磁电阻效应值没有明显变化,但以Fe为覆盖层的样品的矫顽力和饱和场明显减小,而Ti、Cu覆盖层对三明治样品的矫顽力和饱和场无太大的影响。温度降低时,覆盖层使Co/Cu/Co三明治的巨磁电阻值显著增加,表明样品的巨磁电阻效应与覆盖层及其与上层Co所形成的界面密切相关。     

Electrical conduction through self-assembled monolayers in molecular junctions: Au/molecules/Au versus Au/molecule/PEDOT:PSS/Au

We fabricated and characterized a large number of octanedithiol (denoted as DC8) molecular devices as vertical metal–molecule–metal structure with or without using an intermediate conducting polymer layer of poly (3,4-ethylenedioxythiophene) stabilized with poly(4-styenesulfonic acid) (called as PEDOT:PSS). The electronic transport properties of DC8 molecular devices with and without PEDOT:PSS layer were statistically compared in terms of current density and device yield. The yields of the working molecular devices were found to be ~ 1.75% (84 out of 4800 devices) for Au/DC8/Au junctions and ~ 58% (74 out of 128 devices) for Au–DC8/PEDOT:PSS/Au junctions. The tunneling decay constants were obtained with the Simmons tunneling model and a multibarrier tunneling model for two kinds of molecular devices with and without PEDOT:PSS layer.     

Anomalous anisotropic magnetoresistance in topological insulator films

Topological insulators are insulating in the bulk but possess spin-momentum locked metallic surface states protected by time-reversal symmetry. The existence of these surface states has been confirmed by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Detecting these surface states by transport measurements, which might at first appear to be the most direct avenue, was shown to be much more challenging than expected. Here, we report a detailed electronic transport study in high quality Bi₂Se₃ topological insulator thin films. Interestingly, measurements under an in-plane magnetic field, along and perpendicular to the bias current show anomalous opposite magnetoresistance.      

Surface Plasmon Resonance Magneto-Optical Kerr Effect in Au/Co/Au Magneto-Plasmonic Multilayer

Surface plasmon resonance magneto-optical Kerr effect is studied in magneto-plasmonic multilayer as Au (11 nm)/Co (11 nm)/Au (11 nm). Our experimental setup is consists of spectral magneto-optical rotation in Kretschmann-based attenuated total reflection condition as surface plasmon resonance magneto-optical Kerr effect. Based on this new experimental setup, the sample exposed under external magnetic filed at surface plasmon resonance angle. Our results show sufficient surface plasmon resonance magneto-optical Kerr effect in visible region, thanks to the resonant excitation of surface plasmons which is very suitable for miniaturized and controllable magneto-optical imaging systems, memory, and also magneto-optical isolators.     

NbN/MgO/NbN superconducting tunnel junctions as x-ray detectors

We report on our work using NbN/MgO/NbN junctions for the detection of x-rays. Detectors based on superconducting tunneling junctions offer the prospect of energy resolution over an order of magnitude higher than is obtainable with the current generation of semiconductor-based detectors. NbN is interesting due to the possibility of its use in trapping layer devices. The junctions were fabricated at the NASA Jet Propulsion Laboratory (JPL) with an area of 4 µm by 4 µm. They were tested at the Naval Research Laboratory (NRL) in an applied magnetic field of approximately 250 gauss, a current bias of several hundred nanoamps and an operating temperature of 1.7 K.     

Seebeck effect in magnetic tunnel junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K?1 from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K?1. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.     

Electric-field-assisted switching in magnetic tunnel junctions

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.     

Spin filtering effects in monocristalline Fe/MgO/Fe magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport.     

Colloidal domain lithography for regularly arranged artificial magnetic out-of-plane monodomains in Au/Co/Au layers

Regularly arranged magnetic out-of-plane patterns in continuous and flat films are promising for applications in data storage technology (bit patterned media) or transport of individual magnetic particles. Whereas topographic magnetic structures are fabricated by standard lithographical techniques, the fabrication of regularly arranged artificial domains in topographically flat films is difficult, since the free energy minimization determines the existence, shape, and regularity of domains. Here we show that keV He(+) ion bombardment of Au/Co/Au layer systems through a colloidal mask of hexagonally arranged spherical polystyrene beads enables magnetic patterning of regularly arranged cylindrical magnetic monodomains with out-of-plane magnetization embedded in a ferromagnetic matrix with easy-plane anisotropy. This colloidal domain lithography creates artificial domains via periodic lateral anisotropy variations induced by periodic defect density modulations. Magnetization reversal of the layer system observed by magnetic force microscopy shows individual disc switching indicating monodomain states.     

退火对NiCo薄膜各向异性磁电阻(AMR)的影响

用磁控溅射法制备了以NiFeCr和Ta分别为缓冲层的两种NiCo薄膜样品,在200、300、400℃温度下对两种样品退火.结果表明,不同的退火温度导致样品AMR值不同,在退火200℃时样品AMR值最大,超过200℃退火磁电阻急剧下降.XRD和摇摆曲线的结果都表明,退火后晶粒的平均晶粒尺寸都会长大,晶内缺陷减少.振动样品磁强计(VSM)测量的结果显示,退火后样品的磁矩有所减小,矫顽力增大,表明缓冲层与NiCo磁薄膜之间有扩散现象.     

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.