Z. G. Khim

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

The magnetic and structural properties of cobalt-implanted ZnO single crystals are reported. High-quality, (110)-oriented single-crystal Sn-doped ZnO substrates were implanted at ∼350 °C with Co to yield transition metal concentrations of 3–5 at. % in the near-surface (∼2000 Å) region. After implantation, the samples were subject to a 5 min rapid thermal annealing at 700 °C. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs H behavior at T=5 K. Coercive fields were ⩽100 Oe at this measurement temperature. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K, although hysteresis in the M vs H behavior was not observed at room temperature. Four-circle x-ray diffraction results indicate the presence of (110)-oriented hexagonal phase Co in the ZnO matrix. From the 2θ full width at half maximum (FWHM) of the Co (110) peak, the nanocrystal size is estimated to be ∼3.5 nm, which is below the superparamagnetic limit at room temperature. In-plane x-ray diffraction results show that the nanocrystals are epitaxial with respect to the ZnO host matrix. The magnetic properties are consistent with the presence of Co nanocrystals, but do not preclude the possibility that a component of the magnetism is due to Co substitution on the Zn site in the ZnO matrix.

A dynamic contact mode operation of electrostatic-force microscopy (EFM) with an ac modulation has been developed and used to investigate the domain strucutre and dynamics of a triglycine sulfate single crystal. Well-separated topographic and domain contrast images have been obtained by detecting the force instead of the force gradient in the dynamic contact mode operation of EFM. Surface charge density and the anisotropic domain wall thickness have been measured. The evolution of domains embedded in an oppositely polarized larger domain indicates the existence of a significant interaction between domains of the same polarity.

Dynamic contact mode electrostatic force microscopy (DC-EFM) was developed as a new operation mode of scanning probe microscope (SPM). By operating EFM in a contact mode with an ac modulation bias, we have improved the spatial resolution and also achieved a complete separation of the topographic effect from other electrostatic force effect overcoming the mixing problem of a topographic effect with other electrostatic effects frequently encountered in the conventional noncontact EFM measurement. DC-EFM can be utilized either as a force microscopy for the surface hardness, or as a potentiometry for the surface potential distribution, or as a charge densitometry for the surface charge density study. This is also applicable to the measurement and control of the domain structure in ferroelectric materials that have a bound surface charge.

We report on the magnetic and structural properties of epitaxial metal organic chemical vapor deposition grown p-GaN:Mg/Al2O3 implanted with Co, Cr, and V ions at varying high doses at 350 °C. Magnetic and structural properties were investigated after a short anneal at 700 °C to remove implantation damage. Magnetic properties determined from superconducting quantum interference device magnetometer measurements indicate ferromagnetic-like ordering for Co and Cr doped samples up to 320 K, while V doped samples show paramagnetic behavior for all temperatures considered. For all samples studied, structural characterization techniques such as x-ray diffraction, high-resolution cross-sectional transmission electron microscopy, and selected area diffraction pattern, indicate no second phases that may contribute to the magnetic properties measured. Transport measurements (resistivity as a function of temperature) reveal all samples to show insulating-like behavior.