David Kirkwood

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

Propagation of exchange bias in CoFe∕FeMn∕CoFe trilayers

CoFe∕FeMn, FeMn∕CoFe bilayers and CoFe∕FeMn∕CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field HEB depends strongly on the order of depositions and is much higher at CoFe∕FeMn than at FeMn∕CoFe interfaces. By combining the two bilayer structures into symmetric CoFe∕FeMn(tFeMn)∕CoFe trilayers, HEBt and HEBb of the top and bottom CoFe layers, respectively, are both enhanced. Reducing tFeMn of the trilayers also results in enhancements of both HEBb and HEBt. These results evidence the propagation of exchange bias between the two CoFe∕FeMn and FeMn∕CoFe interfaces mediated by the FeMn antiferromagnetic order.

Electrodeposited CoNiP Films with Perpendicular Magnetic Anisotropy

CoNiP alloy films have been grown by electrodeposition from a chloride bath under galvanostatic control. Film growth, structure, composition, magnetic properties, and morphology have been found to depend significantly on deposition current density. Hard magnetic films were successfully grown at the current density of 7 mA/cm 2 with thickness up to 10 μm, at which a perpendicular coercivity of 188 kA/m was achieved. The magnetic hardness and perpendicular anisotropy of these films are the combined result of a microstructure composed of columnar grains and of a hexagonal phase oriented with the c axis perpendicular to the film plane. Preliminary investigations of magnetization reversal processes suggest the presence of pinning sites located probably at grain boundaries.

Evolution of Surface Roughness in Electrodeposited Co–Ni–P and Co–Ni Films

Dynamic scaling analysis was applied to the investigation of the roughening kinetics of Co-Ni and Co-Ni-P films electrodeposited from acidic (pH 3) chloride solutions. Co-Ni films exhibit uninhibited growth with the formation of well-defined crystal facets, while Co-Ni-P films exhibit a granular microstructure with a much smaller apparent grain size. Correspondingly, the rate at which Co-Ni-P films roughen is much slower than that of the Co-Ni films. This is ascribed to autocatalytic processes during Co-Ni-P deposition occurring in parallel with electrochemical processes, inducing growth inhibition due to the ongoing nucleation, incorporation of phosphorus, and enhanced hydrogen evolution. The slow roughening of Co-Ni-P films is advantageous for microfabrication processes involving the formation of thick magnetic films and microstructures.

Properties of vanadium and tantalum granular oxide-metal tunnel junctions fabricated by electrochemical anodization

Localized electrochemical anodization has been used to prepare lateral vanadium (V) and tantalum (Ta) tunnel junctions. Electrical transport properties of these junctions were investigated at various temperatures ranging from 25 to 135 °C. A strong nonlinear current-voltage (I-V) curve indicates nonohmic transport which we believe is due to tunnel junction behavior. The metal-insulator transition was observed in the V junction at ∼80 °C. The microstructure of these junctions explored by transmission electron microscope is consistent with metallic grains embedded in an oxide matrix and we therefore expect tunneling between the metallic grains to be the dominant transport mechanism.

AFM Studies of Ni Electrodeposits on GaAs

In this study, the surface morphology of electrodeposited Ni on single crystal GaAs (001) was investigated by atomic force microscopy (AFM). The images show granular deposits with stepped contours typical of single-crystalline grains. The correlation length correlates very well with the size of the grains, indicating that the layers grow as columns with diameter increasing with thickness. This growth mechanism is observed for layers with thicknesses in the range of 10 to 500 nm at a deposition rate of ~0.5 nm/s.

Spintronics and Novel Magnetic Materials for Advanced Spintronics

This chapter contains both the description of advanced spintronic devices for logic and memory applications and the synthesis and characterization of some new magnetic materials that would lead to new paradigms in spintronics. The first part gives a brief introduction to spintronics and its history. First-generation spintronics has entered the mainstream of information technology through its utilization of the magnetic tunnel junction in applicable devices such as read head sensors for hard disk drives and magnetic random access memory. We also discuss the conceptual spintronic devices, including spin torque transfer random access memory, spin-polarized field-effect transistor, and spin-based qubit quantum processor, and their potential impacts on information technology. The future of spintronic devices requires next-generation spintronic materials. The second part of the chapter is dedicated to the synthesis and characterization of some novel magnetic materials, including ferromagnetic oxides and diluted magnetic Group IV semiconductors.