Structural and magnetic properties of GaGdN/GaN superlattice structures

GaGdN/GaN superlattices (SLs) having various GaGdN and GaN layer thicknesses were grown on Al₂O₃ (0001) substrates by radio frequency molecular beam epitaxy and their structural and magnetic properties were investigated. By changing the thickness of GaGdN and GaN layer in the GaGdN/GaN SL, we find that the lattice constant in the c-direction of GaGdN is larger than that of GaN. All SLs samples exhibited ferromagnetic characteristics at 10 K and 300 K. Comparing with the GaGdN single layer sample, the magnetization of SL samples is larger at the same unit volume of GaGdN. Magnetization data show that the SL samples having relatively thinner GaGdN layer and thicker GaN layer have larger magnetic moment per Gd atom. This phenomenon can be explained by considering that the carriers (electrons) flow from the GaN layers and are accumulated in the GaGdN layers; thinner GaGdN layer has higher carrier density, suggesting the carrier-enhanced magnetization in SL structure.     

衬底温度对脉冲激光沉积在Si衬底上Zn0.95Co0.05O薄膜的影响

在不同的衬底温度下在n型Si(111)衬底上采用脉冲激光沉积的方法生长了(002)择优取向的具有室温铁磁性的Zn0.95Co0.05O薄膜。X射线衍射显示所生长的薄膜呈六方纤锌矿结构。X射线光电子能谱测试表明薄膜中出现的室温铁磁性不是由于Co团簇产生的。发现薄膜生长过程中产生的间隙锌、氧空位以及晶格缺陷对铁磁性有显著的影响。通过改变衬底温度可以控制薄膜中间隙锌、氧空位及晶格缺陷的数量,薄膜的铁磁性同时也可以被明显地改变,这是缺陷与薄膜的室温铁磁性相关的直接证据。     

Electrical and Magnetic Characteristics of MBEGrown GaMnN

We present magnetometry and charge transport data for a GaMnN film with approximately 7% (atomic) Mn grown by molecular beam epitaxy. Measurements of magnetization vs. applied magnetic field show hysteresis consistent with the existence of ferromagnetism up to 300 K. Magnetization curves as a function of temperature indicate a phase transition near 170 K. Temperature-dependent Hall effect measurements show n-type characteristics with high carrier concentration and low mobility, which are both only weakly dependent on temperature. Piezoresistance measurements under hydrostatic pressure yield pressure coefficients that show little variation for temperatures of 77, 194, and 300 K.     

Sonochemical Synthesis and Magnetism in Co-doped ZnO Nanoparticles

The understanding and control of ferromagnetism in diluted magnetic semiconducting oxides (DMO) is a special challenge in solid-state physics and materials science due to its impact in magneto-optical devices and spintronics. Several studies and mechanisms have been proposed to explain intrinsic ferromagnetism in DMO compounds since the theoretical prediction of room-temperature ferromagnetism. However, genuine and intrinsic ferromagnetism in 3d-transition metal-doped n-type ZnO semiconductors is still a controversial issue. Furthermore, for DMO nanoparticles, some special physical and chemical effects may also play a role. In this contribution, structural and magnetic properties of sonochemically prepared cobalt-doped ZnO nanoparticles were investigated. A set of ZnO samples was prepared varying cobalt molar concentration and time of ultrasonic exposure. The obtained results showed that single phase samples can be obtained by the sonochemical method. However, cobalt nanoclusters can be detected depending on synthesis conditions. Magnetic measurements indicated a possible ferromagnetic response, associated to defects and cobalt substitutions at the zinc site by cobalt. However, ferromagnetism is depleted at higher magnetic fields. Also, an antiferromagnetic response is detected due to cobalt oxide cluster at high cobalt molar concentrations.     

Defect induced room temperature ferromagnetism in well-aligned ZnO nanorods grown on Si (100) substrate

In this work, we report the fabrication of high quality single-crystalline ZnO nanorod arrays which were grown on the silicon (Si) substrate using a microwave assisted solution method. The as grown nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photo-luminescence (PL) and magnetization measurements. The XRD results indicated that the ZnO nanorods are well oriented with the c-axis perpendicular to the substrate and have single phase nature with the wurtzite structure. FE-SEM results showed that the length and diameter of the well aligned rods is about ~ 1 μm and ~ 100 nm respectively, having aspect ratio of 20-30. Room-temperature PL spectrum of the as-grown ZnO nanorods reveals a near-band-edge (NBE) emission peak and defect induced green light emission. The green light emission band at ~ 583 nm might be attributed to surface oxygen vacancies or defects. Magnetization measurements show that the ZnO nanorods exhibit room temperature ferromagnetism which may result due to the presence of defects in the ZnO nanorods.     

Variation of structural,optical, dielectric and magnetic properties of SnO<Subscript>2</Subscript> nanoparticles

We report effect of oxygen vacancies on band gap narrowing, enhancement in electrical conductivity and room temperature ferromagnetism of SnO₂ nanoparticles synthesized by simple chemical precipitation approach. As the calcination temperature is elevated from 400 to 800 °C, the average particle size increases from 12.26 to 34.43 nm, with enhanced grain growth and crystalline quality. At low temperatures, these nanoparticles are in a rather oxygen-poor state revealing the presence of many O vacancies and Sn interstitials in SnO₂ nanoparticles as in this case Sn⁺² is not oxidized completely to Sn⁺⁴ and small sized nano particles have more specific surface area, hence defects are more prominent. The oxygen content increases steadily with increasing temperature, with the Sn:O atomic ratio very near to the stoichiometric value of 1:2 at high temperatures suggesting the low density of defects. The optical band gap energies of all SnO₂ nanoparticles are in the visible light region, decreasing from 2.89 to 1.35 eV, while room temperature ferromagnetism and electrical conductivity are enhanced with reduced temperatures. The dielectric constant (ε_r) exhibited dispersion behaviour and the Debye’s relaxation peaks were observed in tanδ. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell–Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺⁴. The narrowed band gap energies and enhanced ferromagnetism are mainly attributed to the increase of defects density (e.g., oxygen vacancies). The presence of oxygen vacancies is confirmed by EDX, Raman, PL, XPS, and UV–Vis spectra. The band gap of 1.35 eV is the smallest value for SnO₂ reported so far. This rather small band gap, enhanced conductivity and room temperature ferromagnetism demonstrate that SnO₂ nanoparticles are very promising in the visible light photo catalysis and optoelectronic devices.     

Ferromagnetism in C60 polymers: pure carbon or contamination with metallic impurities?

A review of ferromagnetism in C60 polymeric materials synthesized by high pressure high temperature (HPHT) treatment is presented. Analysis of published data proves that the reported ferromagnetism cannot be assigned to polymeric structure in either perfect or defect states. Most recent experimental studies have not confirmed previously reported levels of magnetization in polymeric samples while it appears that ferromagnetism of "magnetic carbon" is preserved above the depolymerization point of any C60 polymer. Identical ferromagnetic properties in some samples of fullerene polymer and graphite like hard carbon phase also show that the effect is most likely not connected to fullerenes at all. Most of the data published previously as an evidence of ferromagnetism in C60 polymers synthesized at HPHT conditions can be explained by contamination with magnetic impurities. Formation of iron carbide (Fe3C) due to reaction of metallic iron with fullerene molecules allows to explain observed Curie temperature of approximately 500 K and levels of magnetization reported for "magnetic carbon".     

Mössbauer studies and magnetic properties of SnO2 doped with Fe

Transparent conducting SnO₂ powders doped with 10% Fe content were prepared by a polymerized complex method under acidic solutions, and annealed finally at 550 °C, and at 600 °C. These samples were characterized by X-ray diffraction, magnetization, and Mössbauer spectrometry at room temperature. Rutile SnO₂ phase was obtained for both samples, and the crystallite sizes were in the range of 13-14 nm. Both samples exhibit magnetization and the saturation magnetization was smaller for the sample annealed at 600 °C than for sample annealed at 550 °C. Room temperature Mössbauer spectra for both samples showed the presence of two different paramagnetic iron sites but no magnetic sextets. These results suggest that ferromagnetism originates from magnetic defects and not directly from iron ions.     

The Transition from Paramagnetic to Ferromagnetic States as Influenced by Evolving Microstructure of Ni0.5Zn0.5Fe2O4

In this paper the effect of sintering temperature on Ni_0.5Zn_0.5Fe₂O₄ is examined closely. The evolution of toward magnetically ordered materials was to be tracked with the parallel evolving microstructure subjected to sintering temperatures in an ascending order. The starting powder of Ni_0.5Zn_0.5Fe₂O₄ was prepared via mechanical alloying and later molded into toroidal samples. After each sintering, we observed the resulting changes in the materials. The XRD data showed a single phase being formed as early as 600 °C and the peak intensity was increasing with the sintering temperature indicating an increase in the degree of crystallinity. The BH hysteresis loops showed the evolution from paramagnetism to moderate ferromagnetism to strong ferromagnetism with microstructural changes. For lower sintering temperatures, the samples showed paramagnetic behavior dominating the samples. As sintering temperature increased, paramagnetic states decreased and, at 900 °C, a moderately ferromagnetic state appeared. Sintering at 1000 °C produced a strongly ferromagnetic state giving a well-formed sigmoid-shape hysteresis loop.     

Properties of N-doped ZnO grown by DBD-PLD

N-doped ZnO thin films have been grown on sapphire substrates by dielectric barrier discharged pulsed laser deposition (DBD-PLD). Low temperature photoluminescence spectra of N-doped ZnO film verified the p-type doping status to find the acceptor-bound exciton peaks with the high resolution detection. At low temperature growth, the major defects in the N-doped ZnO film were the oxygen interstitials that can combine with N, so that the N played the role as an acceptor. On the other hand, the major defects in the samples processed at high temperature were oxygen vacancies with which N doesn't play the role as an acceptor. The acceptor binding energy of N acceptor was estimated to be about 105 meV.     

Study of Influence of Annealing Time on Some Physical Properties of ZnO:Cu Nanorods Grown by a Simple Chemical Bath Deposition Method

A simple chemical bath deposition method was used to prepare high density ZnO nanorods on ZnO seeded Si substrates. Upon the nanorods growth, Cu-doping was achieved by diffusion process at 500 ^°C for different times in vacuum ambient. The structural, optical, and magnetic properties of the obtained ZnO:Cu nanorods were then examined. XRD analysis showed that undoped and ZnO:Cu samples were highly c-axis oriented with a hexagonal wurtzite structure. SEM analysis indicated that ZnO:Cu nanorods had diameters of ～200 nm and lengths of ～1.5 μm. X-ray photoemission spectroscopy demonstrated that Cu was successfully doped into ZnO in a divalent state. Photoluminescence results showed that Cu-doping caused a decrease in the green band intensity of nanorods compared to undoped ZnO. Room temperature magnetic measurements showed that pure ZnO nanorods exhibited ferromagnetism that might be ascribed to defect-induced d⁰ ferromagnetism. All the ZnO:Cu nanorods also showed the room temperature ferromagnetism that was attributed to the bound magnetic polarons (BMPs).     

Structural and magnetic study of a diluted magnetic semiconductor: Fe-doped CeO2 nanoparticles

This paper reports the effect of Fe doping on the structure and room temperature ferromagnetism of CeO2 nanoparticles. X-ray diffraction and the selective area electron diffraction measurements performed on the Ce(1-x)Fe(x)O2 (0 < or = x < or = 0.07) nanoparticles showed a single-phase nature with a cubic structure, and none of the samples showed the presence of any secondary phase. The mean particle size, which was calculated using transmission electron microscopy, increased with the increase in the Fe content. The DC magnetization measurements that were performed at room temperature showed that all the samples exhibited ferromagnetism. The saturation magnetic moment increased with the increase in the Fe content.     

Griffiths Phase and Reduced Magnetization of La0.5Ca0.5MnO3 with Different Annealing Temperature

In this paper, the existence of Griffiths phase and variation of ferromagnetism have been studied for La_0.5Ca_0.5MnO₃ nanoparticles annealed at different temperatures. It is detected by x-ray diffraction patterns that the increasing of annealing temperature induces the increment of grain size. The magnetization as a function of temperature demonstrates that magnitude of magnetization at low temperature is decreased as the annealing temperature increases, which is explained by the core-shell model. The temperature dependence of inverse susceptibility of those samples manifests that the Griffiths phase occurs for the samples annealed at temperatures 900?°C and 1150?°C and it disappears for the sample annealed at 1350?°C. The reduction of Griffiths phase with increasing of annealing temperature indicates the suppression of quenched disorder after annealing, which may be related to the lower degree of disorder, redistribution of magnetic ions, and weaker hybridization of?O?2p states with its near-neighbor atoms. As to the disappearance of Griffiths phase, competition between paramagnetic, ferromagnetic, and antiferromagnetic phases plays a very important role.     

Cu掺杂ZnO纳米粒子的光学性能及铁磁性

利用溶胶凝胶法制备了纳米结构的Cu掺杂ZnO基稀磁半导体,通过X射线衍射分析表明,样品为纯相ZnO纤锌矿结构,磁性测量表明样品在室温下呈室温铁磁性,铁磁性来源为氧化锌晶格中的缺陷与Cu2+离子之间的交换作用。室温光致发光(PL)谱观察到紫外带边和可见光区两个发射峰,且随着Cu掺杂量增加,紫外峰淬灭,可见峰发射增强。     

Annealing Effect in GaDyN on Optical and Magnetic Properties

The annealing effect on the optical and magnetic properties of the GaDyN layers was studied. The PL intensities of yellow and green bands as well as the intra-4f orbital transition of Dy³⁺ ions were found to decrease for the samples annealed at 900 and 1000 °C. It is supposed that the intra-4f orbital transition is related to the broad peak luminescence coming from defects. Increasing the annealing temperature, the magnetization becomes smaller. It is considerable that the number of electrons coming from defects was reduced by the annealing treatments and that the ferromagnetism in GaDyN is attributed as carrier induced ferromagnetism.     

Co-doped ZnO epitaxial films: from a Brillouin-like paramagnet to a phase-separated superparamagnetic ensemble

Co-doped ZnO films are epitaxially grown on sapphire by reactive magnetron sputtering. The preparation conditions such as temperature and the composition of the sputtering gas are systematically varied. For optimized growth conditions virtually all Co dopant atoms are located on substitutional Zn lattice sites as revealed by X-ray linear dichroism (XLD). The material behaves as a Brillouin-like paramagnet with S = 3/2 and L = 1 as revealed by integral and element specific magnetometry. Reducing the oxygen content during preparation leads to the onset of phase separation as revealed by X-ray diffraction, and more clearly by a strong reduction of the XLD signal. Such samples behave like a blocked superparamagnetic ensemble. In the entire range of preparation conditions no signs of intrinsic ferromagnetism are found.     

Unusual ferromagnetism in high purity ZnO sintered ceramics

We report our novel result regarding the observation of unusual but clean ferromagnetic (FM) signature at room temperature (RT) in high pure (99.999%) ZnO bulk ceramics processed by slow step sintering schedule (SSSS). All the sintered samples showed ferromagnetic signature at room temperature. However, sample sintered at 850 °C showed more prominent M-H loop and saturation magnetization with respect to samples sintered at 500 °C and 1300 °C. Observation of FM behavior in sintered ceramics may be attributed to the presence of defects like cation (Zn)/anion (O) vacancies and interstitials created during the processing of sample. It is anticipated that SSSS promotes enhanced physical bulk as well as surface densification and at the same time confined the outgassing of free oxygen and Zn from the bulk which are acting as suitable defect sites inside the sintered grains and grain boundary junctions. Our micro-Raman and X-ray photoelectron studies revealed the existence of these defects which are believed to be the origin of the unconventional ferromagnetism (FM) of ZnO bulk ceramic at room temperature.     

Magnetization study of Fe-doped ZnO co-doped with Cu: Synthesized by wet chemical method

Fe- and Cu-doped ZnO of nominal compositions Zn_0.95Fe_0.05O and Zn_0.94Fe_0.05Cu_0.01O were synthesized by a wet chemical route. X-ray diffraction analysis of the samples annealed at 575 K showed that they are single phase without any secondary phases. DC magnetization measurements of Cu co-doped samples (Zn_0.94Fe_0.05Cu_0.01O) as a function of field at room temperature showed ferromagnetic signature while the samples without Cu co-doping (Zn_0.95Fe_0.05O) are paramagnetic in nature. On increasing the temperature of annealing from 575 K to 1,075 K an impurity phase emerges in both the samples, which has been identified as a variant of ZnFe₂O₄. Both the samples heated at and above 1,075 K are found to be paramagnetic at room temperature. These observations, the absence of room temperature ferromagnetism in Zn_0.95Fe_0.05O and the disappearance of ferromagnetism in Zn_0.94Fe_0.05Cu_0.01O on raising the temperature of annealing clearly rules out the likelihood of room temperature ferromagnetism arising from the impurity phases like γ-Fe₂O₃ and/or ZnFe₂O₄ that might have been formed during the synthesis. Our results strongly suggest that room temperature ferromagnetism in Zn_0.94Fe_0.05Cu_0.01O can be attributed to the formation of a secondary phase of Cu-doped ZnFe₂O₄.     

Structural, optical and magnetic properties of Co-doped ZnO nanorods with hidden secondary phases

Co-doped ZnO nanorods (composition: Zn(0.955)Co(0.045)O) were grown by a simple surfactant-assisted hydrothermal technique. The morphological, structural, optical and magnetic properties of the as-prepared nanorods were investigated by means of scanning electron microscopy, high-resolution transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, micro-Raman spectroscopy, micro-cathodoluminescence, and vibrating sample magnetometry (VSM). The results showed that the sample had rod-like morphology and that the preferential growth direction was along the c axis. While Co was successfully doped into the ZnO wurtzite lattice structure as revealed by several characterization techniques, hidden secondary phases of Zn(y)Co(3-y)O(4) (0≤y≤1) were also clearly detected by the micro-Raman spectroscopic technique. We propose that the predominant diffusion-limited Ostwald ripening crystal growth mechanism under the hydrothermal coarsening yielded such phase segregation. VSM results showed that the nanorods displayed relatively weak room-temperature ferromagnetism. We suggest that the origin of the ferromagnetism is probably due to the presence of the mixed cation valence of Co via a d-d double-exchange mechanism rather than the real doping effect. It is essential to control the crystal growth mechanism and defect states associated with the ferromagnetism in order to realize the intrinsic diluted magnetic semiconductors.     

探究GSMBE制备GaAsBi薄膜中生长条件对Bi浓度的影响

为了探究GaAsBi薄膜生长中生长条件与Bi浓度的关系,我们利用气态源分子束外延(GSMBE)技术,通过改变每个GaAsBi单层的生长温度、AsH_3压和Bi源温度,在半绝缘GaAs(100)衬底上生长GaAsBi的多层结构。通过二次离子质谱(SIMS)及透射电镜X光谱(EDX)测试得出:GaAsBi中Bi的浓度随着生长温度的升高而降低,且生长速率越慢表面偏析和再蒸发严重,可导致Bi浓度下降趋势更明显;Bi浓度随着AsH_3压的升高而减小,在As_2和Ga束流比在0.5~0.8之间几乎成线性变化,远不如固态源MBE敏感;此外,Bi源温度升高,Bi掺入的浓度也会增大,但是当生长温度大于420℃时,Bi就很难凝入。