Malcolm G. Thomas

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.

Subnanometer-Diameter Wires Isolated in a Polymer Matrix by Fast Polymerization

The preparation and analysis of inorganic-organic polymer nanocomposites consisting of inorganic nanowires and multiwire “cables” in a random-coil organic polymer host is reported. Dissolution of inorganic (LiMo3Se3)n wires in a strongly coordinating monomer, vinylene carbonate, and the use of a rapid polymerization in the presence of a cross-linking agent produce nanocomposites without phase separation. Polymerization of dilute solutions yields a material containing mostly (Mo3Se3−)n mono- and biwires, 6 to 20 angstroms in diameter and 50 to 100 nanometers long. Polymerization of more concentrated liquid crystalline solutions yields a nanocomposite containing oriented multiwire cables, 20 to 40 angstroms in diameter and up to 1500 nanometers long, that display optical anisotropy and electrical conductivity.

A high efficiency annular dark field detector for STEM

A new high efficiency annular dark field (ADF) detector for an HB501 STEM (Scanning Transmission Electron Microscope) has been constructed and tested. This detector uses a single crystal YAP scintillator and a solid quartz light pipe extending from the scintillator (inside the vacuum) to the photomultiplier tube (outside the vacuum). A factor of approximately 100 improvement in signal relative to the original detector has been obtained. This has substantially improved the signal to noise ratio in the recorded high resolution ADF-STEM images.

Imaging "invisible" dopant atoms in semiconductor nanocrystals

Nanometer-scale semiconductors that contain a few intentionally added impurity atoms can provide new opportunities for controlling electronic properties. However, since the physics of these materials depends strongly on the exact arrangement of the impurities, or dopants, inside the structure, and many impurities of interest cannot be observed with currently available imaging techniques, new methods are needed to determine their location. We combine electron energy loss spectroscopy with annular dark-field scanning transmission electron microscopy (ADF-STEM) to image individual Mn impurities inside ZnSe nanocrystals. While Mn is invisible to conventional ADF-STEM in this host, our experiments and detailed simulations show consistent detection of Mn. Thus, a general path is demonstrated for atomic-scale imaging and identification of individual dopants in a variety of semiconductor nanostructures.

Analytical electron microscopy study of growth mechanism for smoothing of metallic multilayer thin films

We have used scanning transmission electron microscopy and electron energy-loss spectroscopy techniques to study the nature of interfacial interactions in a Ta∕CuOx stack that give rise to a smooth surface morphology, which can be utilized for seeding thin magnetic multilayer devices. Our measurements reveal that the interfacial smoothing is mainly due to the preferential reaction of Ta with O at the Ta∕CuOx interface assisted by grain boundary diffusion of oxygen which thereby acts to smooth out the surface roughness created by the large crystalline grains of Cu.

Detection of CdS Nanoparticles and Implications for Cadmium Yellow Paint Degradation in Edvard Munch’s The Scream (c. 1910, Munch Museum)

1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA. 2. Department of Chemistry and Biochemistry & UD Surface Analysis Facility, University of Delaware, Newark, DE, USA. 3. Cornell Center for Materials Research, Cornell University, Ithaca, NY, USA. 4. The Munch Museum, Tøyen, Oslo, Norway. 5. Department of Conservation, Rijksmuseum, Amsterdam, NL. 6. Department of Materials Science and Engineering, University of Delaware, Newark, DE, USA. 7. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY, USA.

Reverse Engineering Cadmium Yellow Paint from Munch’s “The Scream” with Correlative 3-D Spectroscopic and 4-D Crystallographic STEM

1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA. 2. Department of Chemistry and Biochemistry & UD Surface Analysis Facility, University of Delaware, Newark, DE, USA. 3. Cornell Center for Materials Research, Cornell University, Ithaca, NY, USA. 4. The Munch Museum, Tøyen, Oslo, Norway. 5. Department of Conservation, Rijksmuseum, Amsterdam, NL. 6. Department of Materials Science and Engineering, University of Delaware, Newark, DE, USA. 7. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY, USA.