A. Melnikov

Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN

The structural and the magnetic properties of Gd-focused ion-beam-implanted GaN layers are studied. Gd3+ ions were uniformly implanted in molecular beam epitaxy grown GaN layers at room temperature with an energy of 300keV at doses ranging from 2.4×1011to1.0×1015cm−2 which corresponds to an average Gd concentration range of 2.4×1016–1.0×1020cm−3. The implanted samples were not subjected to any annealing treatment. No secondary phase related to Gd was detected by x-ray diffraction in these layers. Magnetic characterization with superconducting quantum interference device reveals a colossal magnetic moment of Gd and ferromagnetism with an order temperature above room temperature similar to that found in epitaxially grown Gd-doped GaN layers. The effective magnetic moment per Gd atom in these samples is, however, found to be an order of magnitude larger than that found in epitaxially grown layers for a given Gd concentration which indicates that the defects play an important role in giving rise to this effect.

Effect of annealing on the magnetic properties of Gd focused ion beam implanted GaN

The authors have studied the effect of annealing on the magnetic and the structural properties of Gd focused ion beam implanted GaN samples. Molecular beam epitaxy grown GaN layers, which were implanted with 300keV Gd3+ ions at room temperature at doses 2.4×1011 and 1.0×1015cm−2, are rapid thermally annealed in flowing N2 gas up to 900°C for 30s. X-ray diffraction results indicate the presence of Ga and N interstitials in the implanted layers. Their densities are also found to reduce upon annealing. At the same time, magnetic measurements on these samples clearly show a reduction in the saturation magnetization as a result of the annealing for the lowest Gd incorporated sample, while in the highest Gd incorporated sample it does not change. These findings suggest that Gd might be inducing magnetic moment in Ga and/or N interstitials in giving rise to an effective colossal magnetic moment of Gd and the associated ferromagnetism observed in Gd:GaN.

Structural, magnetic, and optical properties of Co- and Gd-implanted ZnO(0001) substrates

ZnO(0001) substrates were ion implanted with 100 keV of Co and 300 keV of Gd at different fluences ranging from 5×1013–1×1015/cm2. The resulting Co:ZnO and Gd:ZnO samples were analyzed with respect to their structural, magnetic, and optical properties. The effect of annealing at 350 °C on the structure and the resulting magnetic and optical properties were investigated as well. For Co:ZnO hardly any changes were observable, neither in the structural nor in the magnetic properties, even though the existence of substitutional Co2+ in the ZnO lattice could be shown by means of low temperature photoluminescence especially for Zn-annealed samples. For the much larger Gd ion the implantation leads to a changed crystal structure, which leads to a ferromagneticlike behavior for higher implantation doses, which could even be enhanced by annealing in vacuum. Ferromagnetic behavior in annealed Gd:ZnO is corroborated by ferromagnetic resonance measurements at low temperatures. The distinct behavior of Gd- and Co-impl...

Focused ion beam implantation induced site-selective growth of InAs quantum dots

The site-selective growth of InAs quantum dots (QDs) by a combined focused ion beam (FIB) and molecular beam epitaxy (MBE) process has been demonstrated. An array of FIB modified spots on MBE grown GaAs was fabricated. Thereafter, an in situ annealing step followed by InAs deposition was performed. The InAs QDs were preferentially formed in the holes generated by the FIB. The influences of ion dose, annealing parameters, and InAs amount were investigated. With optimized parameters, the authors observe more than 50% single dot occupancy per holes. Photoluminescence spectra confirm the good optical quality of the QDs.

Magnetic and structural properties of Gd-implanted zinc-blende GaN

Zinc-blende GaN layers grown by molecular beam epitaxy were uniformly focused-ion-beam implanted with 300keV Gd3+ ions for doses ranging from 1×1012to1×1015cm−2, and their structural and magnetic properties were studied. The implanted samples were not subjected to any annealing treatment. Only Gd incorporation into zinc-blende GaN was observed by x-ray diffraction. Magnetic investigations using superconducting quantum interference device magnetometry reveal a (super)paramagneticlike behavior with an ordering temperature around 60K for the sample with the highest implantation dose. Our experimental studies indicate that the spontaneous electric polarization in wurtzite GaN is the crucial mechanism for its ferromagneticlike behavior upon Gd doping.

Scaling of the low temperature dephasing rate in Kondo systems

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations n_{s}. Because of the relatively high Kondo temperature T_{K} approximately 4.3 K of this system, we are able to explore a temperature range from above T_{K} down to below 0.01T_{K}. We show that the magnetic contribution to the dephasing rate gamma_{m} per impurity is described by a single, universal curve when plotted as a function of T/T_{K}. For T>0.1T_{K}, the dephasing rate is remarkably well described by recent numerical results for spin S=1/2 impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for S>1/2, we discuss possible explanations for the observed deviations.

Localization and nonlinear transport in single walled carbon nanotube fibers

Electrical and magnetotransport properties of single walled carbon nanotube (SWCNT) fibers are reported. The dependencies of resistance on temperature can be approximated by the Mott law for three-dimensional variable range hopping (VRH) below 80 K and by typical law for fluctuation induced tunneling model within the range of 80–300 K. Both negative and positive magnetoresistances (MRs) were observed. At low fields, MR is negative. Positive upturn was observed on the MR curves, which shifted to the high field’s values with temperature increase. The upturn field of the MR effect was shifted from 1.5 T at 2 K to a value of about 20 T at 40 K. The value of positive MR varies as exp(B2), which changes to B1/3 at sufficiently high fields as expected for the VRH transport. The model of VRH transport is illustrated by the influence of strong microwave field and terahertz radiation induced photocurrent manifestation at low temperatures.

Magnetotransport in Gd-implanted wurtzite GaN∕AlxGa1−xN high electron mobility transistor structures

GaN∕AlxGa1−xN heterostructures containing a two-dimensional electron gas (2DEG) 27nm underneath the surface were focused-ion-beam implanted with 300keV Gd ions at room temperature. At 4.2K, current-voltage characteristics across implanted rectangles showed that the structures remained conducting up to a Gd dose of 1×1012cm−2. Extraordinary Hall effect and anisotropic magnetoresistance were observed at T=4.2K for structures implanted with 3×1011cm−2 Gd. This dose corresponds to a 23% reduction in electron concentration and a decrease in the mobility by a factor of 14 at 4.2K. However, the still-conducting 2DEG is now embedded in a ferromagnetic semiconductor which opens the possibility to polarize its spins.

Low-temperature dephasing in irradiated metallic wires

≈ 4K, used in recent experiments on dephasing in Kondo systems [F. Mallet et al.,Phys. Rev. Lett. 97, 226804 (2006); G. M. Alzoubi and N. O. Birge, Phys. Rev. Lett. 97, 226803(2006)]. We show that the phase coherence time is not affected by the implantation procedure,clearly proving that ion implantation process by itself does not lead to any extra dephasing at lowtemperature.

Structural, optical, and magnetic properties of Ho-implanted GaN thin films

Ho ions were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a 100kV focused-ion-beam implanter at room temperature (RT). The implantation doses of Ho ions ranges from 1014 to 1016 cm−2. Without thermal annealing, the structural, optical, and magnetic properties of the Ho-implanted thin films were investigated. Structural properties studied by x-ray diffraction revealed Ho incorporation into GaN matrix without secondary phase. The overall photoluminescence of any implanted sample is weaker than that of the non-implanted one. The spectra show neutral-donor-bound exciton emission and defect-related blue luminescence. Blocked superparamagnetic behavior was identified from Ho-implanted samples at temperatures below RT by measurements with a superconducting quantum interference device. The highest ordering temperature is 100 K.