K. A. Mkhoyan

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 todays 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.

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.

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.

Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions

Structural, magnetic, and transport studies have been performed on perpendicular magnetic tunnel junctions (pMTJ) with Mo as the buffer and capping layers. After annealing samples at 300 °C and higher, consistently better performance was obtained compared to that of conventional pMTJs with Ta layers. Large tunneling magnetoresistance (TMR) and perpendicular magnetic anisotropy (PMA) values were retained in a wide range of samples with Mo layers after annealing for 2 h at 400 °C, in sharp contrast to the junctions with Ta layers, in which superparamagnetic behavior with nearly vanishing magnetoresistance was observed. As a result of the greatly improved thermal stability, TMR as high as 162% was obtained in junctions containing Mo layers. These results highlight the importance of the heavy-metal layers adjacent to CoFeB electrodes for achieving larger TMR, stronger PMA, and higher thermal stability in pMTJs.

Electron-beam-induced damage in wurtzite InN

Knock-on type damage with ejection of nitrogen atoms from a sample was observed in wurtzite InN during irradiation by 100 keV electron beam in scanning transmission electron microscope. Comparison of the measured integrated intensities of nitrogen K and indium M4,5 edges with calculated mass-loss provided a method to measure the energy of vacancy-enhanced displacement in InN for nitrogen atoms, which was found to be 4.6 eV. The results were also applied to predict the rate of electron beam induced damage that will occur in InN specimens with different thicknesses.

Measuring electronic structure of wurtzite InN using electron energy loss spectroscopy

The electronic structure of wurtzite InN has been investigated by electron energy loss spectroscopy (EELS). Spectra of the nitrogen K edge and the indium M4,5 edge have been measured and were compared with calculated partial, N 2p and In 5p conduction band density of states in InN. Excellent agreement on the relative positions of the characteristic peaks were obtained. From low-loss EELS the bulk plasmon energy at 15.5 eV, location of the In 4d deep valence states at about 16.3 eV below the conduction band maximum and strong interband transitions with 6.2 eV excitation energy are also found.

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.

Formation of a quasi-two-dimensional electron gas in GaN/AlxGa1−xN heterostructures with diffuse interfaces

Calculations of the electronic energy levels and the distribution of the quasi-two-dimensional electron gas (Q2DEG) at the GaN/AlxGa1−xN interface that take into account the graded nature of the interface are presented in this article. Mapping of the interface using scanning transmission electron microscopy annular dark-field imaging, the changes in the N K-edge and the integrated intensity of the Al L2,3-edge revealed that the interface can be up to 20 A wide. Self-consistent calculations in the local density approximation estimate the sensitivity of the Q2DEG formed at the interface to various parameters, including the width of the interface, the concentration of bound charge, ambient temperature, and the geometrical sizes of the structure.

Atomic level scanning transmission electron microscopy characterization of GaN/AlN quantum wells

GaN quantum wells in an AlN matrix are characterized using scanning transmission electron microscopy. The width of the quantum wells and sharpness of the interfaces are measured with composition sensitive annular dark field imaging and electron energy-loss spectroscopy. The effects of beam broadening inside the specimen are discussed and mechanisms to minimize it are suggested. The quantitatively measured intensity of the N K-edge versus position is compared with the propagating beam intensity obtained from multislice calculations. Possible effects of strain in the structure on its electronic states and energy-loss spectra are also discussed.

Nonuniformities in GaN/AlN quantum wells

Composition sensitive annular dark field imaging and electron energy-loss spectroscopy were used to determine long-range uniformities of GaN quantum wells and the sharpness of their interfaces grown in AlN matrix by molecular beam epitaxy. Low magnification annular dark field images reveal waviness along the growth plane with a period of ∼50 nm and a height ∼20 nm in one sample and significant changes of the long-range uniformity in the other. Measurements of the changes in energy-loss spectra of the Al L2,3, Ga L2,3, and N K edge across quantum well indicate that the interfaces between the quantum wells and the barriers are in most cases almost atomically sharp.