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We measure the spin Hall angle in Au layers of 5--100 nm thicknesses by spin pumping from ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$ epitaxial films grown by ultrahigh vacuum, off-axis sputtering. We observe a striking increase in the spin Hall angle for Au layers thinner than the measured spin diffusion length of 12.6 nm. In particular, the 5 nm Au layer shows a large spin Hall angle of 0.087, compared to those of 0.016 and 0.017 for the 50 and 100 nm Au layers, respectively, suggesting that the top surface plays a dominant role in spin Hall physics when the spin current is able to reach it. Other spin pumping related parameters, including Gilbert damping enhancement, interfacial spin mixing conductance, and spin current are also determined for Au layers of various thicknesses. Given the pervasive role of ultrathin films in electrical and spin transport applications, this result emphasizes the importance of considering the impact of the top surface and reveals the possibility of tuning critical spin parameters by film thickness.

B20 phase magnetic materials, such as FeGe, have been of significant interests in recent years because they enable magnetic skyrmions, which can potentially lead to low energy cost spintronic applications. One major effort in this emerging field is the stabilization of skyrmions at room temperature and zero external magnetic field. We report the growth of phase-pure FeGe epitaxial thin films on Si(111) substrates by ultrahigh vacuum off-axis sputtering. The high crystalline quality of the FeGe films was confirmed by x-ray diffraction and scanning transmission electron microscopy. Hall effect measurements reveal strong topological Hall effect after subtracting out the ordinary and anomalous Hall effects, demonstrating the formation of high density skyrmions in FeGe films between 5 and 275 K. In particular, substantial topological Hall effect was observed at zero magnetic field, showing a robust skyrmion phase without the need of an external magnetic field.

The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. As reported previously Sr2CrOsO6 crystallizes with R 3 ¯ symmetry due to a−a−a− octahedral tilting. A decrease in the tolerance factor leads to a−a−b+ octahedral tilting and P21/n space group symmetry for Ca2CrOsO6. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2CrOsO6 orders at 660 K while Ca2CrOsO6 orders at 490 K. Variable temperature magnetization measurements suggest that the magnetization of the Cr3+ and Os3+ sublattices have different temperature dependences in Sr2CrOsO6. This leads to a non-monotonic temperature evolution of the magnetic moment. Similar behavior is not seen in Ca2CrOsO6. Both compounds have similar levels of Os/Cr antisite disorder, with order parameters of η=80.2(4)% for Sr2CrOsO6 and η=76.2(5)% for Ca2CrOsO6, where η=2θ−1 and θ is the occupancy of the osmium ion on the osmium-rich Wyckoff site.

Epitaxial films of the pyrochlore Nd2Ir2O7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd2Ir2O7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd2Ir2O7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. The epitaxial relationship between the YSZ and Nd2Ir2O7 is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.

Semi-insulating halide vapor phase epitaxial β-Ga2O3 films without intentional dopants introduced during growth are demonstrated. The sheet resistance measured in the 340–480 K range yielded 268–134 kΩ/◻ and an activation energy of 0.81 eV. Room temperature capacitance-voltage measurements at 1 MHz showed evidence of an ultra-low free carrier concentration n-type film with a free carrier concentration near flatband (VFB ∼ 4.4 V) estimated to be

High critical field strength and saturation electron velocity of gallium nitride (GaN) makes it a promising material for future compact, high voltage, and high-speed photoconductive switches. Photoconductive semiconductor switches (PCSSs) have advantages for pulsed power applications since it is a device that turns on when illuminated with fast rise time, low jitter, and allows for high repetition rate. Semi-insulating GaN is necessary to sustain the high blocking voltages required for such devices. This is accomplished by compensation doping with doped with iron, carbon, or magnesium. However, a strong photoresponse is also necessary to enable optical triggering of the switch. We have found that carbon-doped GaN (GaN:C) simultaneously reduces the off-state leakage current and extends the blocking voltage, while still allowing for high on-state photocurrent compared to unintentionally doped GaN. However, forming a low resistance contact to highly insulating GaN:C is challenging. Our approach is to study both Si ion implantation and Si-doped confined epitaxial growth as techniques to achieve low resistance N+ contacts. Implantation and activation is a planar process that is promising for formation of contact regions while mitigating high electric field points within the device. In this work, we implant Si with a box profile to a depth of 0.4 μm into GaN:C with carbon concentrations of 5x1017 and 1x1018 cm-3 grown on SiC substrates. Then, a sputtered AlN capping layer is introduced before annealing at various temperatures to preserve the surface from decomposition during annealing. Activation anneals are performed using rapid thermal annealing as well as the previously reported multicycle rapid thermal annealing processes. The AlN capping layer is then selectively wet etched and Ti/Al/Ni/Au contacts are deposited and alloyed at 850 °C. As a comparison, Si-doped low temperature and high temperature GaN layers were grown by MOCVD using a LPCVD SiO2 mask. Top-to-top lateral PCSSs are fabricated and characterized to identify the impact of the contact processing on the ON-resistance, off-state leakage, blocking voltage, and photocurrent. Finally, as the device is a top-side illuminated lateral device, the electric fields at the surface must be properly managed to realize reliable high voltage operation. The appropriate surface passivation and encapsulation processes to realize field plate structures while simultaneously enabling top-side UV illumination will be discussed.

One major goal of GaN based technology studies is the fabrication of rugged power switching devices. While this is best achieved through vertical GaN technology, the process development of GaN substrates is not yet mature, and the substrates for lateral power switch are more cost effect than GaN. A common way of doping lateral GaN devices is through ion implantation followed by rapid thermal annealing for ion activation. Activation of most p-type dopants requires temperatures above 1300 °C; however, silicon will activate as an n-type dopant at lower temperatures. This research reports on the electrical properties of devices such as junction field effect transistors and Van der Pauw structures containing silicon doped GaN activated by ion implantation.

The availability of high quality, free-standing GaN substrates opens windows for new device applications in III-nitrides, especially in vertical structures. With the introduction of these native substrates, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow homoepitaxial devices to achieve their full potential. In order to enable device-quality epitaxial layers, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for metal organic chemical vapor deposition (MOCVD) growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation at the interface and in the films. Although the initial substrate factors influencing the epi can be subtle, they can have far reaching impact on device performance. By studying these effects using wafers from several different vendors, with substrates from both hydride vapor phase epitaxy (HVPE) and ammonothermal techniques, an understanding of the requirements for device quality MOCVD homoepitaxy can be determined.

A convenient process for the synthesis of glyoxylate-derived chloroformates has been developed. The approach involves the reaction of glyoxylate esters with triphosgene and pyridine in various solvent systems. These novelglyoxylate-derived chloroformates are multifunctional, possessing a chloroformate, ester, and haloalkyl moiety. Further elaboration via a thiocarbonate route afforded highly functionalized carboxylic acid-derived alkoxycarbonyl-chlorocarbonyloxy-methyl esters. Application of a benzyl glyoxylate-derived chloroformate to halofenozide was also accomplished, affording halofenozide-N-carbonyloxy-chloro/iodoacetic acid benzyl ester derivatives, which were further derivatized to carboxylic acid-derived (halofenozide-N-carbonyloxy)-benzyloxycarbonyl-methyl esters. The trifunctional nature of the glyoxylate moiety was demonstrated through conversion of the benzyloxycarbonyl moiety of a carboxylic acid-derived (halofenozide-N-carbonyloxy)-benzyloxycarbonyl-methyl ester to its respective diethylamide.

The synthesis of a series of substituted benzaldehyde-derived chloroformates and their application towards the synthesis of a diverse series of novel insecticidally active carboxylic acid [ N ′- tert -butyl- N ′-(3,5-dimethylbenzoyl)- N -(3-methoxy-2-methylbenzoyl)hydrazinocarbonyloxy]phenylmethyl esters, prepared in a parallel synthesis fashion, is reported.

We report structural and magnetic properties of magnetostrictive Fe100−xGax (x ≈ 15) alloys when deposited onto antiferromagnetic manganese nitride and non-magnetic magnesium oxide substrates. From X-ray diffraction measurements, we find that the FeGa films are single crystalline. Scanning tunneling microscopy imaging reveals that the surface morphologies are dictated by the growth temperature, composition, and substrate. The magnetic properties can be tailored by the substrate, as found by magnetic force microscopy imaging and vibrating sample magnetometry measurements. In addition to pronounced tetragonal deformations, depositing FeGa onto manganese nitride leads to the formation of stripe-like magnetic domain patterns and to the appearance of perpendicular magnetic anisotropy.

The saturation magnetization of Y3Fe5O12 (YIG) epitaxial films 4 to 250 nm in thickness has been determined by complementary measurements including the angular and frequency dependencies of the ferromagnetic resonance fields as well as magnetometry measurements. The YIG films exhibit state-of-the-art crystalline quality, proper stoichiometry, and pure Fe3+ valence state. The values of YIG magnetization obtained from all the techniques significantly exceed previously reported values for single crystal YIG and the theoretical maximum. This enhancement of magnetization, not attributable to off-stoichiometry or other defects in YIG, opens opportunities for tuning magnetic properties in epitaxial films of magnetic insulators.

A method for the synthesis of three-dimensionally ordered macroporous crystals from materials that exhibit additional functionality using a hetero-metallic precursor is reported.

The synthesis of a series of substituted benzaldehyde-derived chloroformates and their application towards the synthesis of a diverse series of novel insecticidally active carboxylic acid [ N ′- tert -butyl- N ′-(3,5-dimethylbenzoyl)- N -(3-methoxy-2-methylbenzoyl)hydrazinocarbonyloxy]phenylmethyl esters, prepared in a parallel synthesis fashion, is reported.

A convenient process for the synthesis of glyoxylate-derived chloroformates has been developed. The approach involves the reaction of glyoxylate esters with triphosgene and pyridine in various solvent systems. These novelglyoxylate-derived chloroformates are multifunctional, possessing a chloroformate, ester, and haloalkyl moiety. Further elaboration via a thiocarbonate route afforded highly functionalized carboxylic acid-derived alkoxycarbonyl-chlorocarbonyloxy-methyl esters. Application of a benzyl glyoxylate-derived chloroformate to halofenozide was also accomplished, affording halofenozide-N-carbonyloxy-chloro/iodoacetic acid benzyl ester derivatives, which were further derivatized to carboxylic acid-derived (halofenozide-N-carbonyloxy)-benzyloxycarbonyl-methyl esters. The trifunctional nature of the glyoxylate moiety was demonstrated through conversion of the benzyloxycarbonyl moiety of a carboxylic acid-derived (halofenozide-N-carbonyloxy)-benzyloxycarbonyl-methyl ester to its respective diethylamide.

Research results from solar-dynamo models show the northern and southern hemispheres may evolve separately throughout the solar cycle. The observed phase lag between the hemispheres provides information regarding the strength of hemispheric coupling. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 - 23, we determine how out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric phase differences range from 0 - 11, 0 - 14, and 2 - 19 months for the rising, maximum, and declining periods, respectively. The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar cycle. An analysis of the Gnevyshev gap is conducted to determine if the double-peak is caused by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which the hemispheres couple, we measured the magnetic flux crossing the Equator by examining magnetograms for Solar Cycles 21 - 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the Equator, length of solar minima, and phase lag between the hemispheres., 15 pages, 7 figures

We have experimentally realized unidirectional or one-way coupling in a mechanical array by powering the coupling with flowing water. In cyclic arrays with an even number of elements, solitonlike waves spontaneously form but eventually annihilate in pairs, leaving a spatially alternating static attractor. In cyclic arrays with an odd number of elements, this alternating attractor is topologically impossible, and a single soliton always remains to propagate indefinitely. Our experiments with 14- and 15-element arrays highlight the dynamical importance of both noise and disorder and are further elucidated by our computer simulations.