Fabrication and properties of Li-doped ZnCoO diluted magnetic semiconductor thin films

Li-doped ZnCoO (ZnCoO:Li) diluted magnetic semiconductor thin films were prepared on SiO₂ substrates by pulsed laser deposition. In ZnCoO:Li films, Co²⁺ substituted Zn²⁺ and Li occupied the interstitial sites behaving as donors. The ZnCoO:Li films are of high electron concentration in the 10²⁰ cm⁻³ order and acceptable crystal quality with a hexagonal wurtzite structure. No cluster, precipitate, or second phase was found from the X-ray diffraction pattern and Co k-edge X-ray absorption near-edge structure measurements. The sp-d exchange interactions between the band electrons and the localized d electrons of Co ions substituting Zn ions were observed. The magnetization of ZnCoO:Li film is 0.61 μB/Co, higher than that of the ZnCoO film (0.49 μB/Co). The enhanced defect density and electron concentration due to the introduced Li donors may answer for the improvement of ferromagnetism at room temperature.     

Comparison of ferromagnetism in n- and p-type magnetic semiconductor thin films of ZnCoO

Both n- and p-type diluted magnetic semiconductor ZnCoO are made by magnetron co-sputtering with, respectively, dopants of Al and dual dopants of Al and N. The two sputtering targets are compound ZnCoO with 5% weight of Co and pure metal Al. Sputtering gases for n- and p-type films are pure Ar and N₂, respectively. These films are magnetic at room temperature and possess free electron- and hole-concentration of 5.34×10²⁰ and 5.27×10¹³ cm⁻³. Only the n-type film exhibits anomalous Hall-effect signals. Magnetic properties of these two types of films are compared and discussed based on measurements of microstructure and magneto-transport properties.     

Dependence of substrate temperature of Zn0.8Co0.2O films deposited by pulsed DC magnetron sputtering

We have investigated the microstructure, electrical and magnetic properties of the ZnCoO thin films, which were prepared by the asymmetrical bipolar-pulsed DC magnetron sputtering as a function of substrate temperature. The structural properties of ZnCoO films were characterized with a high resolution XRD. The XRD patterns of the ZnCoO films showed a strong (0 0 2) preferential orientation. The average crystallite size was 23–35 nm, which was estimated from full width at half maximum of XRD results. The electrical resistivity of the films were measured by the van der Pauw method through Hall measurement and showed below 10⁻¹ Ω cm above 300 °C. The magnetic properties of the ZnCoO films were analyzed by the alternating gradient magnetometer at room temperature. All of the films were exhibited the ferromagnetic nature. The high conductivity and room temperature ferromagnetism of the ZnCoO films above 300 °C suggested that the possibility for the application to diluted magnetic semiconductors.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO_3（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3（STO）（110） substrates.Inclined ZnO and cobalt-doped ZnO（ZnCoO） thin films are grown on unetched STO（110） substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO（110） surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E（2G）→ 4A2（4F） transition of Co 2＋ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2＋ ions at the lattice positions of the Zn 2＋ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO₃（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3(STO)(110) substrates.Inclined ZnO and cobalt-doped ZnO(ZnCoO) thin films are grown on unetched STO(110) substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO(110) surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E(2G)→ 4A2(4F) transition of Co 2+ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2+ ions at the lattice positions of the Zn 2+ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature     

Magnetic properties of single crystalline Zn1−xCoxO thin films

We report on the magnetic properties of single crystalline thin films of Zn_1−xCo_xO (x=0.003–0.14) grown by plasma-assisted molecular beam epitaxy. In order to understand the role of intermediate charge carriers in the magnetic properties of this material two types of films were fabricated, with and without Ga-codoping. Magnetic measurements were made between 2 and 300 K in fields up to 5 T with a Quantum Design SQUID magnetometer. We found that all the tested films exhibit paramagnetic behavior following the Curie–Weiss law, χ=C/(T−θ), with negative Curie–Weiss temperatures and that this behavior holds even under strong n-doping. We show that the magnetization data, M(H), in function of the Co content provide additional evidence in favor of the antiferromagnetic Co–Co interaction in this material. We also observe that these data exhibit an ‘easy plane’ magnetic anisotropy for all the studied Co concentrations. Finally, we develop a simple cluster model, in order to describe the magnetic properties of ZnCoO, which is found to be in good agreement with our experiments.     

Electron paramagnetic resonance and 1H nuclear magnetic resonance study of Y-doping effect on the hydrogen shallow donors in ZnO nanoparticles

Hydrogen shallow donors in sol-gel-derived pristine and rare-earth Y-doped ZnO nanoparticles have been investigated by electron paramagnetic resonance (EPR) and high-resolution ¹H nuclear magnetic resonance (NMR). It is shown by EPR measurements that the energy level of the hydrogen shallow donors in the Y-doped ZnO is much deeper (E ～ 174 meV) than in the pristine ZnO (E ～ 75 meV). The temperature-dependent ¹H NMR chemical shift and linewidth measurements of the pristine and the Y-doped ZnO systems indicated that Y-doping effectively modifies the lattice environment and hinders the hydrogen motions in the ZnO nanoparticles.     

Origin of the ferromagnetism in ZnCoO from chemical reaction of Co3O4

We investigated conversion of Co₃O₄ to Co nanoclusters through hydrogen reduction. Quantitative analysis is performed on the conversion of Co₃O₄ to Co metal as a function of hydrogen-injection conditions. Our results reveal that Co₃O₄ must be completely eliminated to avoid formation of the metal phase in ZnCoO. We also propose a new M–T curve based method for detecting nano-sized Co clusters which are below the detection limit of diffraction techniques. It is also found that the Co phase can be transformed back to the Co₃O₄ phase through oxygen annealing and that, as a result, the ferromagnetism can be eliminated. These findings are discussed in the context of the origin of ferromagnetism in ZnCoO.     

Measurement of the fine structure of the n = 4 state of hydrogen

A time-resolved spectroscopy technique has been used to measure the fine structure of the n = 4 state of hydrogen. The excited hydrogen atoms were formed by the use of an electron beam to dissociate molecular hydrogen. The resonance transitions were observed by keeping the radio frequency field fixed and sweeping the magnetic field. Measurements on deuterium were used to study the effects of pressure and motional electric fields. The ²S_1/2 → ²P_1/2 Lamb shift transition , the ²P_3/2 → ²S_1/2 fine structure interval ΔE − L, and the ambient electric field in the measurement volume were determined by using measurements on three separate transitions made at fixed magnetic field. The measurements gave and     

Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform²H nuclear magnetic resonance (NMR), two-dimensional²H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.     

SMILETRAP — Atomic mass measurements with ppb accuracy by using highly charged ions

In the SMILETRAP facility externally produced highly charged ions are captured in a Penning trap and utilized for high precision measurements of atomic masses. Accuracy tests on a ppb level have been performed, using highly charged carbon, oxygen and neon ions. In all cases hydrogen ions served as a reference for the calibration and monitoring of the magnetic field in the trap. Deviations smaller than 3 ppb from the expected results were found in mass measurements of the¹⁶O and²⁰Ne atomic masses. The proton atomic mass, determined from the reference measurements on hydrogen ions, is in good agreement with the accepted value [1]. A direct mass measurement on the⁸⁶Kr-isotope, using trapped⁸⁶Kr²⁹⁺-ions is reported.     

Hydrogen spectroscopy with a Lamb-shift polarimeter

A Lamb-shift polarimeter, which has been built for a fast determination of the polarization of protons and deuterons of an atomic-beam source and which is frequently used in the ANKE experiment at COSY-Jülich, is shown to be an excellent device for atomic-spectroscopy measurements of metastable hydrogen isotopes. It is demonstrated that magnetic and electric dipole transitions in hydrogen can be measured as a function of the external magnetic field, giving access to the full Breit-Rabi diagram for the 2²S_1/2 and the 2²P_1/2 states. This will allow the study of hyperfine structure, g factors and the classical Lamb shift. Although the data are not yet competitive with state-of-the-art measurements, the potential of the method is enormous, including a possible application to anti-hydrogen spectroscopy.     

Interaction of hydrogen and deuterium with intrinsic atomic defects in high-purity electron-irradiated nickel

Abstract

High-purity nickel was irradiated with 2 MeV electrons at temperatures below 80 K to a dose of 1 × 10²³ e^?/m² in the as-prepared state and after charging with H or D. By means of magnetic after-effect measurements relaxations of anisotropic radiation-induced defects and of defect-hydrogen complexes were investigated in the temperature range between 4.2 and 500 K. The isochronal annealing behaviour of these relaxations and the isochronal recovery of the residual resistivity was measured simultaneously on the same specimens. At temperatures below the hydrogen mobility (< 160 K) in charged irradiated specimens relaxation maxima are observed at 45, 100, 115 and 140 K which show no isotope shift for H and D charging. The maxima below 160 K are explained by defect-hydrogen complexes, where radiation-induced defects reorient around immobile hydrogen atoms. Above 160 K, where hydrogen atoms get mobile, in charged irradiated specimens a broad relaxation maximum appears at 170 K which shows an inverse isotope shift for H and D charging. This 170 K maximum anneals in Stage III. A hydrogen diffusion maximum observed in charged specimens at 215 K prior to irradiation is missing after electron irradiation. The 170 K relaxation is explained by defect-hydrogen complexes, where hydrogen atoms reorient around immobile radiation-induced defects while the long-range hydrogen diffusion is suppressed by these defects. In such relaxation measurements hydrogen and deuterium atoms are used as a “probe” to investigate radiation-induced defects.     

Parametric dependence of ion temperature and electron density in the summa hot-ion plasma using laser light scattering and emission spectroscopy

Hot-ion plasma experiments were conducted in the NASA Lewis SUMMA facility. A steady-state modified Penning discharge was formed by applying a radially inward d.c. electric field of several kilovolts near the magnetic mirror maxima. Results are reported for a hydrogen plasma covering a wide range in midplane magnetic flux densities from 0.5 to 3.37 T. Input power greater than 45 kW was obtained with water-cooled cathodes. Steady-state plasmas with ion kinetic temperatures from 18 to 830 eV were produced and measured spectroscopically. These ion temperatures were correlated with current, voltage, and magnetic flux density as the independent variables. Electron density measurements were made using an unusually sensitive Thomson scattering apparatus. The measured electron densities ranged from 2.1 x 10¹¹ to 6.8 x 10¹²1/cm³.     

感应磁场压缩下氢等离子体中巴耳末系谱线的斯塔克加宽

本工作研究感应磁场压缩下,氢等离子体中的巴耳末系谱线的斯塔克加宽。电容器对围绕在放电管外部的线圈放电,产生交变轴向磁场,使放电管中氢气离化、压缩与加热。放电周期14微秒,放电电压20千伏,最大电流165千安,初始工作气压2.0×10^-1毫米汞高。用照相方法研究H_β和H_γ的轮廓。这些谱线有显著的加宽。在整个发光阶段中,H_β的平均半宽度为14—16(埃),H_γ为18—19(埃),相应的离子密度为1.7—2.5×10¹⁶厘米^-3。实验轮廓与Griem,Kolb和Shen的理论较为接近,与Holtsmark理论相差较远。光电测量进一步表明:H_β的轮廓随着时间而改变,相应的离子密度亦在改变。在放电的第三半周期初级电流极大时,离子密度最大,达到3.2×1O¹⁶厘米^-3,为初始氢原子密度的2.2倍。     

Mechanism of protonic conductivity in an NH4HSeO4 crystal

The chemical exchange of deuterons in a partly deuterated ammonium hydrogen selenate crystal is investigated by deuteron magnetic resonance (²H NMR) spectroscopy over a wide range of temperatures. The changes observed in the line shape of the NMR spectra at temperatures above 350 K are characteristic of chemical exchange processes. The exchange processes are thoroughly examined by two-dimensional ²H NMR spectroscopy. It is established that, over the entire temperature range, only deuterons of hydrogen bonds are involved in the exchange and the rates of exchange between deuterons of all types are nearly identical. No deuteron exchange between the ND₄ groups and hydrogen bonds is found. A new model of proton transport in ammonium hydrogen selenate is proposed on the basis of the experimental data. This model makes it possible, within a unified context, to explain all the available experimental data, including macroscopic measurements of the electrical conductivity.     

Spectroscopic measurements on a titanium plasma gun with hydrogenated electrodes

The plasma produced by a plasma gun which consists of concentric titanium and tantalum washers has been observed in the plasma gun as well as in a drift tube in which the plasma particles have been injected. Axial drift velocities of hydrogen and impurities have been determined in the drift tube by optical time-of-flight measurements. Maximum hydrogen velocities between 9·10⁶ to 1·10⁸ cm/sec could be determined, in agreement with magnetic probe-, Faraday cup-, microwave-, and electrostatic energy analyser measurements. The plasma which is produced within the plasma gun by a powerful discharge (vacuum arc) has been observed with a framing camera, spectrographs and photomultipliers. From end-on observed time resolved spectrograms the temperatures and electron densities have been determined as a function of time. The measurements indicate that thermal equilibrium is reached about 2·5 μsec after the firing of the main discharge. 4 μsec after the beginning of the main discharge a maximum temperature of about 72000° K is reached. The measurements further indicate that the high-energy part of the ejected particles is produced before an equilibrium state in the plasma gun has been established.     

Effect of defects on room-temperature ferromagnetism in Co and Na co-doped ZnO

The role of defects in the room temperature ferromagnetism of the Co–ZnO based diluted magnetic semiconductor (DMS) was investigated by co-doping the DMS with Na. The structure characterizations indicate that both Na and Co ions enter into the ZnO lattice without the formation of secondary phase. The oxygen vacancy of ZnCoNaO increased while the carrier concentration decreased compared with that of ZnCoO, leading to the enhancement of the ferromagnetic property in the ZnCoNaO. The observed ferromagnetism introduced by Na ions is attributed to the exchange interaction via the electron trapped oxygen vacancies coupled with the magnetic Co ions.     

Hydrogen absorption in CeFe2 and ThFe3

The effect of hydrogen absorption on the magnetic properties of CeFe₂ and ThFe₃ was studied by magnetization measurements and by the ⁵⁷Fe Mössbauer effect. In both cases an increase in the transition metal moment is observed. In CeFe₂ the long range periodicity of the crystal lattice is lost whereas in ThFe₃ a structural change occurs. Surprisingly, the magnetic anomaly observed in ThFe₃ around 250 K is still present in the hydride.     

Magnetic characterization of a hydrogen phase trapped inside deep dislocation cores in a hydrogen-cycled PdH x (x ≈ 4.5 × 10−4) single crystal

The magnetic characterization of Pd single crystals deformed by cycling in a hydrogen atmosphere has been performed. Based on evidence obtained from thermal desorption analysis, it is shown that the condensed hydrogen phase formed inside deep dislocation cores in PdH _x (x = H/Pd ≈ 4.5 × 10⁻⁴) is tightly bound with a Pd matrix. The activation energy of hydrogen desorption from these cores was found to be as high as e = 1.6eV/H-atom, suggesting the occurrence of a strong band overlapping between Pd and H atoms. SQUID measurements carried out in a weak magnetic field (H < 5.0 Oe) showed an anomalous diamagnetic contribution to the DC and AC magnetic susceptibilities of the PdH _x sample at T < 30 K resulting in the presence of the hydrogen phase. It is suggested that the anomalous diamagnetic response in PdH _x is caused by the presence of a hydrogen dominant phase, tightly bound with a Pd matrix inside the dislocation cores (nanotubes). The text was submitted by the authors in English.