Mahesh G. Samant

Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.

Suppression of Metal-Insulator Transition in VO2 by Electric Field–Induced Oxygen Vacancy Formation

Mind the Vacancies Varying the carrier density of solid-state systems to manipulate their electrical properties usually involves chemical doping, which can lead to disorder. Recently, ionic liquids have been used to form an electronic double layer on the surface of a material, tuning its carrier density by the application of an electric field. Jeong et al. (p. 1402) used liquid gating on VO2, which undergoes a metal-insulator transition close to room temperature. The liquid gating suppressed the transition to lower and lower temperatures; however, the material remained in the metallic state, even when the gating fluid was washed off. It appears that, instead of a simple electrostatic effect, the properties of VO2 are modulated by the introduction of oxygen vacancies, an electrochemical consequence of high electric fields. The results imply that careful interpretation of liquid gating experiments in condensed matter physics is needed. Electrochemistry plays a role in the ionic liquid gating of a strongly correlated oxide. Electrolyte gating with ionic liquids is a powerful tool for inducing novel conducting phases in correlated insulators. An archetypal correlated material is vanadium dioxide (VO2), which is insulating only at temperatures below a characteristic phase transition temperature. We show that electrolyte gating of epitaxial thin films of VO2 suppresses the metal-to-insulator transition and stabilizes the metallic phase to temperatures below 5 kelvin, even after the ionic liquid is completely removed. We found that electrolyte gating of VO2 leads not to electrostatically induced carriers but instead to the electric field–induced creation of oxygen vacancies, with consequent migration of oxygen from the oxide film into the ionic liquid. This mechanism should be taken into account in the interpretation of ionic liquid gating experiments.

Magnetically engineered spintronic sensors and memory

The discovery of enhanced magnetoresistance and oscillatory interlayer exchange coupling in transition metal multilayers just over a decade ago has enabled the development of new classes of magnetically engineered magnetic thin-film materials suitable for advanced magnetic sensors and magnetic random access memories. Magnetic sensors based on spin-valve giant magnetoresistive (GMR) sandwiches with artificial antiferromagnetic reference layers have resulted in enormous increases in the storage capacity of magnetic hard disk drives. The unique properties of magnetic tunnel junction (MTJ) devices has led to the development of an advanced high performance nonvolatile magnet random access memory with density approaching that of dynamic random-access memory (RAM) and read-write speeds comparable to static RAM. Both GMR and MTJ devices are examples of spintronic materials in which the flow of spin-polarized electrons is manipulated by controlling, via magnetic fields, the orientation of magnetic moments in inhomogeneous magnetic thin film systems. More complex devices, including three-terminal hot electron magnetic tunnel transistors, suggest that there are many other applications of spintronic materials.

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

First experimental results from IBM/TENN/TULANE/LLNL/LBL undulator beamline at the advanced light source

The IBM/TENN/TULANE/LLNL/LBL Beamline 8.0 at the advanced light source combining a 5.0 cm, 89 period undulator with a high‐throughput, high‐resolution spherical grating monochromator, provides a powerful excitation source over a spectral range of 70–1200 eV for surface physics and material science research. The beamline progress and the first experimental results obtained with a fluorescence end station on graphite and titanium oxides are presented here. The dispersive features in K emission spectra of graphite excited near threshold, and found a clear relationship between them and graphite band structure are observed. The monochromator is operated at a resolving power of roughly 2000, while the spectrometer has a resolving power of 400 for these fluorescence experiments.

The adsorption structure of glycine adsorbed on Cu(110); comparison with formate and acetate/Cu(110)

The molecular orientation of an ordered monolayer of glycine adsorbed on Cu(110) has been studied using X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Photoelectron Diffraction (XPD), Low-Energy Electron Diffraction (LEED) and theoretical calculations. In particular, the NEXAFS results are discussed in terms of the spectra of the related molecules ammonia (NH3), formate (HCOO), and acetate (CH3COO) on Cu(110). Whereas the latter two molecules chemisorb in similar geometries, glycine is found to assume a very different chemisorption geometry. Formate and acetate bond through two equivalent oxygen atoms with the molecular plane oriented nearly perpendicular to the surface, aligned along the [110]-azimuth. In the case of adsorbed glycine (NH2CH2COO), the azimuthal orientation is still present, i.e. the bonding oxygen atoms are aligned along the [110]-azimuth, but the molecule is found to bend towards the surface. A second chemisorption bond is formed at the nitrogen end of the molecule, involving copper atoms in the neighboring [110]-row. We therefore have the interesting case of a chemisorption bond involving different functional groups in the same molecule.

Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS

Using grazing incidence geometry and fluorescence detection, surface EXAFS of a monolayer of underpotentially deposited copper on epitaxially deposited gold (111) on mica was observed. Both Cu–Au and Cu–O scattering are observed. The results are interpreted in terms of models in which the Cu–O distance is 2.08±0.03 A and the Cu–Au distance is 2.5±0.06 A. The copper and three gold atoms at the metallic surface form an elongated tetrahedron, with the oxygen on top of the copper. Two plausible models for the oxygen arrangement are proposed, one involving only one oxygen, the other with a sulfate ion adsorbed. This second model yields a slightly better fit of the data.

Liquid crystal alignment by rubbed polymer surfaces: a microscopic bond orientation model

We discuss the microscopic origin of a previously poorly understood phenomenon, the alignment of a nematic liquid crystal (LC), consisting of rod-like molecular units, when placed on a rubbed polymer surface. After giving a brief review of the phenomenon and its technological utilization in flat panel displays we discuss the use of surface sensitive, polarization dependent near edge X-ray absorption spectroscopy for the study of rubbed polymer surfaces. These measurements are shown to provide a microscopic picture for the origin of the alignment process. It is shown that the LC orientation direction is set by an asymmetry in the molecular bonds, i.e. of the charge, at the rubbed polymer surface. The experimental results are explained by a general theory, based on tensor order parameters, which states that the minimum energy state of the interaction between the LC and oriented polymer surface corresponds to maximum directional overlap of the respective anisotropic charge distributions. q 1998 Elsevier Science B.V. All rights reserved.

In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on platinum electrodes: Part 1. Sulfuric acid

Abstract The adsorption of bisulfate and sulfate ions on Pt in sulfuric acid has been studied by observing their asymmetric S-O stretching vibrations using potential difference Fourier transform infrared reflection absorption spectroscopy. It has been found that the two anions are coadsorbed over the potential range studied. A rather clear band of the bending mode of water was observed in the same spectra. This has been attributed to water molecules weakly bonded to the surface through H in the hydrogen region and O in the oxygen region. The band intensity ratio of sulfate against bisulfate ions is much larger than the calculated activity ratio in 0.5 M sulfuric acid solution, although the intensity ratio becomes closer to the calculated activity ratio in 0.05 M sulfuric acid. A large frequency shift with potential of 100 cm−1/V is observed for the adsorbed bisulfate ions in the double layer potential region, while the frequency shift is much smaller for sulfate ions. In the oxide formation region above 0.85 V (RHE), the sulfate ions appear to be replaced by bisulfate ions; this is interpreted as resulting from the formation of a hydrogen bond between HSO−4 and the negatively charged oxygen atoms which are present on the Pt surface.

Thermal stability of IrMn and MnFe exchange-biased magnetic tunnel junctions

The thermal stability of exchange-biased magnetic tunnel junctions is explored using near-edge x-ray absorption fine-structure spectroscopy. Structures with Ir–Mn antiferromagnetic exchange bias layers are much more thermally stable than similar structures with Fe–Mn exchange bias layers. In both cases, diffusion of Mn from the antiferromagnetic layer through thin exchange-biased ferromagnetic layers to the tunnel barrier is observed at elevated temperatures. This observation explains the diminished magnetoresistance of these structures on annealing even though the resistance of the tunnel junctions is hardly changed.