Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions.

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Phase Engineering of 2D Tin Sulfides.

Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase-selective growth of both hexagonal tin (IV) sulfide SnS2 and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO2 substrates through atmospheric pressure vapor-phase method in a conventional horizontal quartz tube furnace with SnO2 and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations.

Sidewall oxide effects on spin-torque- and magnetic-field-induced reversal characteristics of thin-film nanomagnets.

The successful operation of spin-based data storage devices depends on thermally stable magnetic bits. At the same time, the data-processing speeds required by today's technology necessitate ultrafast switching in storage devices. Achieving both thermal stability and fast switching requires controlling the effective damping in magnetic nanoparticles. By carrying out a surface chemical analysis, we show that through exposure to ambient oxygen during processing, a nanomagnet can develop an antiferromagnetic sidewall oxide layer that has detrimental effects, which include a reduction in the thermal stability at room temperature and anomalously high magnetic damping at low temperatures. The in situ deposition of a thin Al metal layer, oxidized to completion in air, greatly reduces or eliminates these problems. This implies that the effective damping and the thermal stability of a nanomagnet can be tuned, leading to a variety of potential applications in spintronic devices such as spin-torque oscillators and patterned media.

Effects of amorphous layers on ADF-STEM imaging.

A study of high-resolution ADF imaging in uncorrected and aberration-corrected STEMs was carried out by multislice simulation. The presence of amorphous layers at the surface of a crystalline specimen is shown to significantly alter the visibility of the atomic columns. After propagating through an amorphous layer a portion of the beam passes without any alteration while scattered electrons introduce a Gaussian background. The dependence of the image contrast on the crystal structure, orientation and the types of the atoms present in the crystal was studied. In the case of uncorrected probes an amorphous layer thicker than 200 A is necessary to achieve considerable reduction of the visibility of the atomic columns, but with aberration-corrected probes only 60 A is necessary. With changes in defocus, crystalline specimens with amorphous layers on the top can also be imaged and high-resolution ADF images can be obtained. An amorphous layer at the beam entry surface affects the ADF image more than that of an amorphous layer at the exit surface. Approximately linear reduction of the contrast (with a slop of 1) is expected with increased thickness of amorphous layer.

Critical role of inelastic interactions in quantitative electron microscopy.

A semiquantitative correlation between experimental observations and theoretical prediction in electron microscopy is achieved. Experiments conducted on amorphous silicon in the convergent beam electron diffraction mode provide measurements of the reduction of the central-disk intensity. In addition to elastic scattering the effects of multiple inelastic scattering of the probe electrons were incorporated into the theory describing beam propagation through the specimen. With incorporation of the dominant plasmon scattering a better than 10% match of the theory with experiment is observed indicating the critical role of multiple inelastic scattering in quantitative electron diffraction and imaging.

Effects of tilt on high-resolution ADF-STEM imaging.

A study of the effects of small-angle specimen tilt on high-resolution annular dark field images was carried out for scanning transmission electron microscopes with uncorrected and aberration-corrected probes using multislice simulations. The results indicate that even in the cases of specimen tilts of the order of 1 degree a factor of 2 reduction in the contrast of the high-resolution image should be expected. The effect holds for different orientations of the crystal. Calculations also indicate that as the tilted specimen gets thicker the contrast reduction increases. Images simulated with a low-angle annular dark field detector show that tilt effects are more pronounced in this case and suggest that these low-angle detectors can be used to correct specimen tilt during scanning transmission electron microscopes operation.

Separation of bulk and surface-losses in low-loss EELS measurements in STEM.

To identify major features in low electron energy loss spectra, the different excitations (bulk plasmons, interband transitions, surface plasmons, Cherenkov and surface guided modes) must be delineated from each other. In this paper, this process is achieved by noting the linear thickness dependence of bulk processes contrasted with the constant thickness behavior of surface excitations. An alternative approach of analyzing bulk plasmon-loss is also introduced. Using a new algorithm, the parameters of plasma generation-plasmon energy E(P,0), a damping parameter DeltaE(P) and the coefficient of the dispersion relation gamma were obtained from a single curve fitting on the example of Si. The ability to separate surface-losses from the rest of the data permitted identification of the fine structure of the surface-losses. The strong peak at 8.2 eV characteristic of non-radiative surface plasmon excitations was measured for Si. Analysis of surface excitations indicates that a 10ASiO2 surface coating layer is still present despite careful cleaning the specimen. Dielectric functions deduced from the EELS data prove to be considerably affected by the presence of the surface-losses for samples as thick as 800A.

Direct determination of local lattice polarity in crystals.

With current advances in sub-angstrom resolution scanning transmission electron microscopy (STEM), it is now possible to image directly local crystal structures of materials where dramatically different atoms are separated from each other at distances about or less than 1 angstrom. We achieved direct imaging of atomic columns of nitrogen in close proximity to columns of aluminum in wurtzite aluminum nitride by using annular dark field imaging in an aberration-corrected STEM. This ability allows direct determination of the local polarity in nanoscale crystals and crystal defects.

Full recovery of electron damage in glass at ambient temperatures.

An unusually complete recovery of extensive electron-beam-induced damage in a thin film of a CaO-Al2O3-SiO2 glass was discovered. Nanoscale measurements show that the Ca ions migrate about 10 nm away during irradiation and return during recovery. Oxygen atoms are trapped largely as molecular oxygen and do not migrate. Electron energy loss measurements demonstrate that the glass returns completely to the original compositional and structural state thus indicating that the glass is in a deep thermodynamic energy minimum.