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Optical and electrical properties of Hf- and Zn-doped β-Ga2O3 samples, which are n-type and insulating, respectively, were altered via high-energy electron irradiation at 2.5 or 0.5 MeV. The β-Ga2O3:Hf samples irradiated with 2.5 MeV electrons experienced a color change from blue to yellow and a large drop in conductivity, attributed to the creation of gallium vacancies, which compensate donors. This irradiation resulted in the absence of free carrier absorption and the presence of Cr3+ photoluminescence (PL). PL mapping prior to irradiation revealed optically active ZnO precipitates that formed during the growth of β-Ga2O3:Zn. These precipitates have a 384 nm (3.23 eV) stacking fault emission in the core; in the outer shell of the precipitate, the PL blue-shifts to 377 nm (3.29 eV) and a broad defect band is observed. After 0.5 MeV electron irradiation, the defect band broadened and increased in intensity. The blue PL band (435 nm) of β-Ga2O3 was enhanced for both Hf- and Zn-doped samples irradiated with 0.5 MeV. This enhancement is correlated with an increase in oxygen vacancies. [ABSTRACT FROM AUTHOR]

The effects of pressure on single crystals of Cr-doped gallium oxide (β-Ga2O3:Cr3+) and aluminum gallium oxide [(Al0.1Ga0.9)2O3] were examined by measuring the wavelength shift in the spectral R lines. Photoluminescence (PL) spectra of these materials were collected from samples in diamond anvil cells at pressures up to 9 GPa. The β-Ga2O3:Cr3+R lines were found to shift linearly under hydrostatic pressure. The (Al0.1Ga0.9)2O3R lines also show a linear shift but the R1 line shifted less than for β-Ga2O3:Cr3+. The ratio of R2 to R1 peak areas vs pressure is dominated by nonradiative recombination. X-ray diffraction measurements of (Al0.1Ga0.9)2O3 indicate that its equation of state is similar to that of β-Ga2O3. β-Ga2O3:Cr3+ was examined under non-hydrostatic conditions by using mineral oil as a pressure transmitting medium. Similar to the case in ruby, the R1 line is much more sensitive to non-hydrostatic stress than R2. Spatially resolved PL of a sample at 8 GPa in mineral oil showed significant variations in the R1 emission wavelength. These results suggest that the R1 line can serve as a sensitive probe of alloy composition and non-hydrostatic stress, while the R2 line is insensitive to these perturbations. [ABSTRACT FROM AUTHOR]

This Review summarizes a range of analytical techniques that can be used to detect, quantify, and/or distinguish between isotopes of iodine (e.g., long-lived 129I, short-lived 131I, stable 127I). One reason this is of interest is that understanding potential radioiodine release from nuclear processes is crucial to prevent environmental contamination and to protect human health as it can incorporate into the thyroid leading to cancer. It is also of interest for evaluating iodine retention performances of next-generation iodine off-gas capture materials and long-term waste forms for immobilizing radioiodine for disposal in geologic repositories. Depending upon the form of iodine (e.g., molecules, elemental, and ionic) and the matter state (i.e., solid, liquid, and gaseous), the available options can vary. In addition, several other key parameters vary between the methods discussed herein, including the destructive vs nondestructive nature of the measurement process (including in situ vs ex situ measurement options), the analytical data collection times, and the amount of sample required for analysis. [ABSTRACT FROM AUTHOR]

In this work, bulk Czochralski-grown single crystals of 10 mol. % Al2O3 alloyed β-Ga2O3—monoclinic 10% AGO or β-(Al0.1Ga0.9)2O3—are obtained, which show +0.20 eV increase in the bandgap compared with unintentionally doped β-Ga2O3. Further, growths of 33% AGO—β-(Al0.33Ga0.67)2O3—and 50% AGO—β-(Al0.5Ga0.5)2O3 or β-AlGaO3—produce polycrystalline single-phase monoclinic material (β-AGO). All three compositions are investigated by x-ray diffraction, Raman spectroscopy, optical absorption, and 27Al nuclear magnetic resonance (NMR). By investigating single phase β-AGO over a large range of Al2O3 concentrations (10–50 mol. %), broad trends in the lattice parameter, vibrational modes, optical bandgap, and crystallographic site preference are determined. All lattice parameters show a linear trend with Al incorporation. According to NMR, aluminum incorporates on both crystallographic sites of β-Ga2O3, with a slight preference for the octahedral (GaII) site, which becomes more disordered with increasing Al. Single crystals of 10% AGO were also characterized by x-ray rocking curve, transmission electron microscopy, purity (glow discharge mass spectroscopy and x-ray fluorescence), optical transmission (200 nm–20 μm wavelengths), and resistivity. These measurements suggest that electrical compensation by impurity acceptor doping is not the likely explanation for high resistivity, but rather the shift of a hydrogen level from a shallow donor to a deep acceptor due to Al alloying. Bulk crystals of β-(Al0.1Ga0.9)2O3 have the potential to be ultra-wide bandgap substrates for thin film growth, with a lattice parameter that may even allow higher Al concentration β-Ga2O3 single crystal thin films to be grown. [ABSTRACT FROM AUTHOR]

Several acceptor dopants have been explored in β-Ga2O3 to produce semi-insulating substrates and epitaxial films. Fe and Mg make up the majority of research thus far; however, other transition metals provide potential alternatives for optimized performance. β-Ga2O3 bulk single crystals were grown by the Czochralski and vertical gradient freeze methods with a nominal dopant concentration of 0.25 at. % Mn. Ultraviolet-visible-near infrared spectroscopy and photoluminescence revealed polarization- and orientation-dependent optical absorptions (pleochroism) coupled with an orange luminescence. All samples were electrically insulating, on the order of 109–1011 ohm cm at room temperature, indicative of acceptor doping. Actual dopant concentrations of the intentionally doped transition metal and background impurities were determined via glow discharge mass spectrometry, indicating the macroscale segregation behavior. High-temperature resistivity measurements indicated an experimental acceptor level of 1.7 ± 0.2 eV. Hydrogenation of samples resulted in an increase in the orange luminescence and O–H stretching modes observable in the infrared spectrum. Density functional theory calculations were performed to determine the likely site-occupancy and acceptor level of Mn in the bandgap. [ABSTRACT FROM AUTHOR]

In this study, the formation and character of gallium vacancies (VGa) and their complexes in near surface and bulk regions of single crystal β-Ga2O3 were explored using unintentionally doped single crystals grown by the Czochralski method. As-grown and O2 annealed (up to 1550 °C) samples were investigated using positron annihilation spectroscopy (PAS) to study the top 0.05–6 μm, and also current–voltage measurements and infrared (IR) spectroscopy, with hydrogenated samples to probe VGa, to study the bulk. After annealing in O2 > 1000 °C, the β-Ga2O3 resistivity begins increasing, up to ∼109 Ω cm for 1550 °C treatment, with the top 0.5 mm being many orders of magnitude more resistive. PAS measurements of the top 6 μm (S values) and very near surface 200 nm (diffusion length, L) indicate differential behavior as a function of peak annealing temperature. At least four temperature regimes of behavior are described. VGa are present in the bulk after growth, but considerable changes occur upon annealing at a temperature ≈1000 °C, where L and S decrease simultaneously, suggesting an increasing defect concentration (L) but a decreasing defect volume (S). Annealing at a temperature ≈1400 °C increases S again, showing an increasing volume concentration of VGa, with IR absorption showing a large signature of VGa-2H, indicative of increased VGa formation that was not present when annealing at a temperature ≈1000 °C. These results suggest that defect changes from annealing in oxygen are depth dependent, and that VGa configuration may not be the same near the oxygen-exposed surface of the sample and in the bulk. [ABSTRACT FROM AUTHOR]

β-Ga2O3 has several soluble deep acceptors that impart insulating behavior. Here, we investigate Zn doping (0.25 at. %) in bulk Czochralski and vertical gradient freeze β-Ga2O3. Representative crystals were assessed for orientation (electron backscatter diffraction and Raman spectroscopy), purity (glow discharge mass spectrometry and secondary ion mass spectrometry), optical properties (ultraviolet to near infrared absorption), and electrical properties (resistivity and current–voltage). Purity measurements indicate that Zn evaporation is insufficient to inhibit doping of Zn into β-Ga2O3. Hybrid functional calculations show Zn substitutes nearly equally on tetrahedral and octahedral sites, with less than ∼0.1 eV preference for the octahedral (GaII) site. Furthermore, calculations show that ZnGa acts as a deep acceptor with trapping levels ∼1.3 and ∼0.9 eV above the valence band for one and two holes, respectively. The solubility and electronic behavior of Zn dopants are consistent with measured concentrations >1 × 1018 atoms/cm3 and electrical measurements that show resistivity 1011–1013 Ω cm, with no p-type conduction. [ABSTRACT FROM AUTHOR]

Bulk undoped ZnS materials exhibit relatively bright yet diverse luminescence behavior, which has, in recent years, been attributed to intrinsic defects. However, the luminescence also resembles that of doped materials, implying a role of impurities. Luminescence features have also been attributed to oxygen impurities causing defect clusters or energy band anti-crossing. Thus, this study couples optical and electrical techniques, such as band edge transmission, photoluminescence (PL), PL excitation, radioluminescence, thermoluminescence, optical deep level transient spectroscopy, and photoinduced current transient spectroscopy, to explore the identity of defect levels. ZnS materials studied are commercial single crystals made by physical vapor transport, high-pressure Bridgman, and powder processing. These undoped bulk ZnS exhibit luminescence behavior similar to that of reported doped ZnS powders (10−4 to 10−2 mol. % doping for luminescent materials). Dopants (such as Al, Cl, Cu, and Ag) are also commonly found impurities in ZnS; hence, it is reasonable to believe they have a role in the luminescence of nominally undoped ZnS. By comparing the variation in optical and electrical properties between samples to the processing method and the rich literature on intentionally doped ZnS, this study shows a possible dominant contribution of impurities and impurity-containing defects on the luminescence of bulk "undoped" ZnS. Hence, there is no need to resort to complex mechanisms to explain the luminescence, but rather the metal and halide impurities and their defect complexes determine the main characteristics of luminescence in this wide-bandgap semiconductor. [ABSTRACT FROM AUTHOR]

Alloying β-Ga2O3 with Al2O3 to create (AlxGa1−x)2O3 enables ultra-wide bandgap materials suitable for applications deep into ultraviolet. In this work, photoluminescence (PL) spectra of Cr3+ were investigated in monoclinic single crystal β-Ga2O3, and 10 mol. % Al2O3 alloyed with β-Ga2O3, denoted β-(Al0.1Ga0.9)2O3 or AGO. Temperature-dependent PL properties were studied for Cr3+ in AGO and β-Ga2O3 from 295 to 16 K. For both materials at room temperature, the red-line emission doublet R1 and R2 occurs at 696 nm (1.78 eV) and 690 nm (1.80 eV), respectively, along with a broad emission band at 709 nm (1.75 eV). The linewidths for AGO are larger for all temperatures due to alloy broadening. For both materials, the R-lines blue-shift with decreasing temperature. The (lowest energy) R1 line is dominant at low temperatures due to the thermal population of the levels. For temperatures above ∼50 K, however, the ratio of R2 to R1 peak areas is dominated by nonradiative combination. [ABSTRACT FROM AUTHOR]

Multiphase magnetic systems are common in nature and are increasingly being recognized in technical applications. One characterization method which has shown great promise for determining separate and collective effects of multiphase magnetic systems is first order reversal curves (FORCs). Several examples are given of FORC patterns which provide distinguishing evidence of multiple phases. In parallel, a visualization method for understanding multiphase magnetic interaction is given, which allocates Preisach magnetic elements as an input "Preisach hysteron distribution pattern" to enable simulation of different "wasp-waisted" magnetic behaviors. These simulated systems allow reproduction of different major hysteresis loops and FORC patterns of real systems and parameterized theoretical systems. The experimental FORC measurements and FORC diagrams of four commercially obtained magnetic materials, particularly those sold as nanopowders, show that these materials are often not phase pure. They exhibit complex hysteresis behaviors that are not predictable based on relative phase fraction obtained by characterization methods such as diffraction. These multiphase materials, consisting of various fractions of BaFe12O19, ε-Fe2O3, and γ-Fe2O3, are discussed. [ABSTRACT FROM AUTHOR]

Zinc-doped monoclinic gallium oxide (β-Ga2O3:Zn) has semi-insulating properties that could make it a preferred material as a substrate for power devices. Infrared and UV/Visible spectroscopy were used to investigate the defect properties of bulk β-Ga2O3:Zn crystals. As-grown crystals contain a single O-H stretching mode at 3486.7 cm−1 due to a neutral ZnH complex. A deuterium-annealed sample displays the corresponding O-D stretching mode at 2582.9 cm−1, confirming the O-H assignment. A strong Ir4+ electronic transition at 5147.6 cm−1 is also observed, along with sidebands attributed to ZnIr pairs. These sidebands show distinct differences compared with Mg-doped samples; most importantly, several peaks are attributed to Ir4+ paired with a Zn on the tetrahedral Ga(I) site. Annealing under an oxygen atmosphere produced an insulating material with a resistance above 1 TΩ. [ABSTRACT FROM AUTHOR]

Iron oxide films were produced using ion-beam-assisted deposition, and Raman spectroscopy and x-ray diffraction indicate single-phase magnetite. However, incorporation of significant fractions of argon in the films from ion bombardment is evident from chemical analysis, and Fe/O ratios are lower than expected from pure magnetite, suggesting greater than normal disorder. Low temperature magnetometry and first-order reversal curve measurements show strong exchange bias, which likely arises from defects at grain boundaries, possibly amorphous, creating frustrated spins. Since these samples contain grains ~6 nm, a large fraction of the material consists of grain boundaries, where spins are highly disordered and reverse independently with external field. [ABSTRACT FROM AUTHOR]

Glass-ceramic (GC) materials have the formability advantages of glasses and the opportunity for controlled crystallization of functional ceramic phases. Here, we obtain GCs containing the hard magnetic phase Sr-hexaferrite (SrFe12O19) from a borate glass. Ten compositions in the B2O3-Fe2O3-SrO system were explored, varying B2O3 and Sr/Fe ratio. Compositions forming glass on quenching, according to X-ray diffraction (XRD), were subsequently heat treated to promote crystallization. Three selected compositions were investigated with vibrating sample magnetometry (VSM), thermal analysis, and electron probe microanalysis (EPMA). Phases identified by XRD after air heat treatment included α-Fe2O3, Fe3O4, SrB2O4, and SrFe12O19. Glasses were also crystallized in situ in a VSM in an argon environment, measuring magnetic properties during heating to 650°C after in situ heat treatment at 800°C. In samples with SrFe12O19, wasp-waisted loops were observed. First order reversal curve (FORC) measurements confirmed low (magnetite) and high (Sr-hexaferrite) coercivity phases. Room temperature VSM measurements of argon in situ treated samples were compared with two ex situ air heat treated protocols. The microstructures of the three investigated air heat treated GC materials were completely different, and compositional position on the phase diagram appeared to influence crystallization progress. These results suggest that careful control of composition as well as heat treatment protocol including atmosphere is necessary for crystallization of desired magnetic phases. Also, VSM was confirmed to be sensitive to magnetic phases at low concentrations not visible to XRD. [ABSTRACT FROM AUTHOR]

A cluster deposition method was used to produce films of loosely aggregated nanoclusters (NCs) of Fe core-Fe3O4 shell or fully oxidized Fe3O4. Films of these NC on Si(100) or MgO(100)/Fe3O4(100) were irradiated to 1016 Si2+/cm2 near room temperature using an ion accelerator. Ion irradiation creates structural change in the NC film with corresponding chemical and magnetic changes which depend on the initial oxidation state of the cluster. Films were characterized using magnetometry (hysteresis, first order reversal curves), microscopy (transmission electron, helium ion), and x-ray diffraction. In all cases, the particle sizes increased due to ion irradiation, and when a core of Fe is present, irradiation reduces the oxide shells to lower valent Fe species. These results show that ion irradiated behavior of the NC films depends strongly on the initial nanostructure and chemistry, but in general saturation magnetization decreases slightly. [ABSTRACT FROM AUTHOR]

Ion irradiation effects are studied on the Fe-based core-shell nanocluster (NC) films with core as Fe and shell as Fe3O4/Fe3N. These NC films were deposited on Si substrates to thickness of ~0.5 μm using a NC deposition system. The films were irradiated at room temperature with 5.5MeV Si2+ ions to ion fluences of 1015 and 1016 ions/cm2. It is found that the irradiation induces grain growth, Fe valence reduction in the shell, and crystallization or growth of Fe3N. The film retained its Fe-core and its ferromagnetic properties after irradiation. The nature and mechanism of oxide shell reduction and composition dependence after irradiation were studied by synthesizing additional NC films of Fe3O4 and FeO+Fe3N and irradiating them under the same conditions. The presence of nanocrystalline Fe is found to be a major factor for the oxide shell reduction. The surface morphologies of these films show dramatic changes in the microstructures due to cluster growth and agglomeration as a result of ion irradiation. [ABSTRACT FROM AUTHOR]

Optical and electrical properties of Hf- and Zn-doped β-Ga2O3 samples, which are n-type and insulating, respectively, were altered via high-energy electron irradiation at 2.5 or 0.5 MeV. The β-Ga2O3:Hf samples irradiated with 2.5 MeV electrons experienced a color change from blue to yellow and a large drop in conductivity, attributed to the creation of gallium vacancies, which compensate donors. This irradiation resulted in the absence of free carrier absorption and the presence of Cr3+ photoluminescence (PL). PL mapping prior to irradiation revealed optically active ZnO precipitates that formed during the growth of β-Ga2O3:Zn. These precipitates have a 384 nm (3.23 eV) stacking fault emission in the core; in the outer shell of the precipitate, the PL blue-shifts to 377 nm (3.29 eV) and a broad defect band is observed. After 0.5 MeV electron irradiation, the defect band broadened and increased in intensity. The blue PL band (435 nm) of β-Ga2O3 was enhanced for both Hf- and Zn-doped samples irradiated with 0.5 MeV. This enhancement is correlated with an increase in oxygen vacancies. [ABSTRACT FROM AUTHOR]

Magnetic Barkhausen noise (MBN), hysteresis measurements, first order reversal curves (FORC), Vickers microhardness, and Transmission Electron Microscopy (TEM) analyses were performed on Fe-1wt.%Cu (Fe-Cu) samples isothermally aged at 700°C for 0.5 - 25 hours to obtain samples with different sized Cu precipitates and dislocation structures. Fe-Cu is used to simulate the thermal and irradiation-induced defects in copper-containing nuclear reactor materials such as cooling system pipes and pressure vessel materials. The sample series showed an initial increase followed by a decrease in hardness and coercivity with aging time, which is explained by Cu precipitates formation and growth as observed by TEM measurements. Further, the MBN envelope showed a continuous decrease in its magnitude and the appearance of a second peak with aging. Also, FORC diagrams showed multiple peaks whose intensity and location changed for different aging time. The changes in FORC diagrams are attributed to combined changes of the magnetic behavior due to Cu precipitate characteristics and dislocation structure. A second series of samples aged at 850°C, which is above the solid solution temperature of Fe-Cu, was studied to isolate the effects of dislocations. These samples showed a continuous decrease in MBN amplitude with aging time although the coercivity and hardness did not change significantly. The decrease of MBN amplitude and the appearance of the second MBN envelope peak are attributed to the changes in dislocation density and structure. This study shows that the effect of dislocations on MBN and FORC of Fe-Cu materials can vary significantly and should be considered in interpreting magnetic signatures. [ABSTRACT FROM AUTHOR]

DC electrical conductivity, Seebeck and Hall coefficients are measured between 300 and 450 K on single crystals of monoclinic bismuth vanadate that are doped n-type with 0.3% tungsten donors (W:BiVO4). Strongly activated small-polaron hopping is implied by the activation energies of the Arrhenius conductivities (about 300 meV) greatly exceeding the energies characterizing the falls of the Seebeck coefficients' magnitudes with increasing temperature (about 50meV). Small-polaron hopping is further evidenced by the measured Hall mobility in the ab-plane (10-1 cm²V-1 s-1 at 300 K) being larger and much less strongly activated than the deduced drift mobility (about 5 x 10-5cm²V-1 s-1 at 300 K). The conductivity and n-type Seebeck coefficient is found to be anisotropic with the conductivity larger and the Seebeck coefficient's magnitude smaller and less temperature dependent for motion within the ab-plane than that in the c-direction. These anisotropies are addressed by considering highly anisotropic next-nearest-neighbor (≈5 Å) transfers in addition to the somewhat shorter (≈4 Å), nearly isotropic nearest-neighbor transfers. [ABSTRACT FROM AUTHOR]

A series of microwave resonant oscillator sensors were designed and characterized using bandpass planar and volumetric electrical resonators having loaded quality factor (Q) values in the range of 2 to 20. The use of these resonators in positive feedback circuits yielded sensor Q-factors of up to 2×107, demonstrating Q-factor amplifications on the order of 106. It is shown that the Q-factor amplification can be increased in a positive feedback system through the selection of feedback loop group delay, allowing use of resonators with lower static, loaded Q-factor values. A low-frequency electromagnetic interference sensing application is demonstrated for two resonant oscillator configurations, showing considerable frequency sensitivity to 45 kHz emitters. [ABSTRACT FROM AUTHOR]

A prototype regenerative feedback resonant circuit has been developed for measuring the transient spectral response due to perturbations in properties of various electromagnetic materials. The circuit can accommodate a variety of cavity resonators, shown here in the 8 GHz range, with passive quality factors (Qstat) as high as 7000 depending upon material loading. The positive feedback enhanced dynamic quality factors (Qdyn) of resonator/material combinations in the regenerative circuit are on the order of 107-108. The theory, design, and implementation of the circuit is discussed along with real-time monitored example measurements of effects due to photon-induced charge carriers in high-resistivity silicon wafers and magnetic-field induced perturbations of yttrium-iron garnet. [ABSTRACT FROM AUTHOR]

Highly conductive thin films of ZnO doped with Ga were grown by pulsed laser deposition with 10 mTorr of H2 in the growth chamber. Compared with a more conventional method of producing conductive films of ZnO, i.e., growth in O2 followed by annealing in forming gas (5% H2 in Ar), the H2 method requires no postgrowth anneal and also produces higher carrier concentrations and lower resistivities with better depth uniformity. As an example, a 65-nm-thick sample had a room-temperature mobility of 32 cm2/V s, a concentration of 6.8×1020 cm-3, and a resistivity of 2.9×10-4 Ω cm. From a scattering model, the donor and acceptor concentrations were calculated as 8.9×1020 and 2.1×1020 cm-3, respectively, as compared to the Ga and H concentrations of 11×1020 and 1×1020 cm-3. The authors conclude that growth in H2 produces higher Ga-donor concentrations but that H-donors themselves do not play a significant role. [ABSTRACT FROM AUTHOR]

The electrical resistivity values for a series of pure and doped (Co, Mn, Al) ZnO epitaxial films grown by pulsed laser deposition were measured with equipment designed for determining the direct current resistivity of high resistance samples. Room-temperature resistances ranging from 7×101 to 4×108 Ω/sq were measured on vacuum-reduced cobalt-doped ZnO, (Al,Co) co-doped ZnO, pure cobalt-doped ZnO, Mn-doped ZnO, and undoped ZnO. Using a four-point collinear geometry with gold spring-loaded contacts, resistivities were measured from 295 to 5 K for resistances of <∼1012 Ω/sq. In addition, magnetoresistance and Hall effect were measured as a function of temperature for select samples. Throughout the investigation, samples were also measured on commercially available instrumentation with good agreement. The challenges of transport measurements on high resistivity samples are discussed, along with some offered solutions to those challenges. [ABSTRACT FROM AUTHOR]

We have investigated the effects of domain and grain anisotropy on spin-resonance in magnetic permeability, implementing a Monte-Carlo approach and a coupled Landau-Lifshitz-Gilbert equation. The Monte-Carlo approach provides great flexibility by employing different probability density functions, allowing modeling of material texture differences that may occur due to different preparation methods. Changes in the permeability tensor result from variations in grain demagnetization and domain demagnetization as well as the anisotropy field relative to saturation magnetization. Experimental permeability measurements on demagnetized polycrystalline yttrium iron garnet show for the first time that the best fit to measured data requires a complex distribution of both grain and domain demagnetization factors. Assuming that grain and domain demagnetizations are decoupled, it was found that the grain structure (i.e., grain demagnetization distribution) has a smaller effect on the frequency-dependent permeability than does the same distribution of domains (i.e., domain demagnetization distribution). Implications for modeling experimental data assuming particular phenomenological loss coefficients or linewidths are also offered. [ABSTRACT FROM AUTHOR]

Mn1.68Co0.24Ni0.48Cu0.6O4 was produced via slip casting and sintering of spray-pyrolysis produced powders. The magnetic properties of this composition were measured for the first time, as a function of the processing temperature (900 °C or 1000 °C sintering), in order to study the effects of Cu and Mn valence and site preference. Quantitative x-ray photoelectron spectroscopy showed that Cu+ site occupancy changed from tetrahedral to a mix of tetrahedral and octahedral with increasing sintering temperature. X-ray diffraction demonstrated that the materials had a cubic spinel structure devoid of tetragonal Jahn-Teller distortion. ac magnetic susceptibility indicated ferrimagnetic behavior below ∼109 K and spin glass behavior below ∼66 to 74 K, depending on the measurement frequency. ac susceptibility freezing temperatures were modeled with the Vogel-Fulcher law and showed intermediate characteristics, between those of canonical spin glasses and cluster glasses. [ABSTRACT FROM AUTHOR]

Complex magnetic permeability and dielectric permittivity of micro- and nano-sized powdered barium (BaFe12O19) and strontium (SrFe12O19) hexaferrites have been studied in a broadband millimeter wave frequency range (30-120 GHz). Transmittance measurements have been performed using a free-space quasi-optical millimeter wave spectrometer, equipped with a set of high-power backward wave oscillators. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high-precision transmittance spectra. Frequency dependences of the magnetic permeability have been obtained from Schlömann's equation for partially magnetized ferrites. These materials show promise as tunable millimeter wave absorbers, based on their size-dependent absorption. [ABSTRACT FROM AUTHOR]

Zinc-doped monoclinic gallium oxide (β-Ga2O3:Zn) has semi-insulating properties that could make it a preferred material as a substrate for power devices. Infrared and UV/Visible spectroscopy were used to investigate the defect properties of bulk β-Ga2O3:Zn crystals. As-grown crystals contain a single O-H stretching mode at 3486.7 cm−1 due to a neutral ZnH complex. A deuterium-annealed sample displays the corresponding O-D stretching mode at 2582.9 cm−1, confirming the O-H assignment. A strong Ir4+ electronic transition at 5147.6 cm−1 is also observed, along with sidebands attributed to ZnIr pairs. These sidebands show distinct differences compared with Mg-doped samples; most importantly, several peaks are attributed to Ir4+ paired with a Zn on the tetrahedral Ga(I) site. Annealing under an oxygen atmosphere produced an insulating material with a resistance above 1 TΩ. [ABSTRACT FROM AUTHOR]