掺银氧化锌纳米棒的水热法制备研究

利用水热法在直流磁控溅射制备的掺铝氧化锌 (AZO) 种子层上制备了不同形貌和光学性能的掺银ZnO纳米棒, 并采用XRD、扫描电镜、透射谱、光发射谱和EDS谱详细研究了Ag离子与Zn离子的摩尔百分比 (R_Ag/Zn) 及AZO种子层对掺银ZnO纳米棒的结构和光学性质的影响. 随着R_Ag/Zn的增加, 掺银ZnO 纳米棒的微结构和光学性质的变化与银掺杂诱导的纳米棒的端面尺寸变化有关. 平均端面尺寸的变化归结于种子层颗粒大小和颗粒数密度不同导致掺入的Ag离子的相对比例不同. 溅射15 min的AZO种子层上生长的ZnO纳米棒由于缺陷增多导致在可见光区的发光峰明显强于溅射10 min 的AZO种子层上、相同R_Ag/Zn 条件下生长的ZnO纳米棒. Ag掺杂产生的点缺陷增多导致可见光区PL波包较宽. 纯ZnO纳米棒的微结构与种子层厚度导致的结晶度和颗粒大小有关. 关键词： ZnO纳米棒 水热法 Ag掺杂 直流磁控溅射     

Experimental evidences and driving mechanisms for anisotropic misalignments in exchange coupled systems

Experimental evidence for misalignments between F anisotropy axes, AF anisotropy axes and the exchange bias field direction is shown in a CoFe/Ni_0.38O_0.62 system. The angular dependence of the remanent magnetization, the exchange bias field and the coercive field is studied as a function of the diluted NiO thickness. The exchange coupling leads to misalignments between the applied field during growth, the exchange bias field and the coercive field directions. It shows that two different interfacial spin frustrations are present, corresponding to pinned and unpinned spins contributions of the diluted NiO.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Hydrothermal synthesis and magnetic properties of gadolinium-doped CoFe2O4 nanoparticles

CoFe_2−xGd_xO₄ (x=0-0.25) nanoparticles were synthesized via a simple hydrothermal process at 200 °C for 16 h without the assistance of surfactant. The as-synthesized powders were characterized by X-ray diffraction, transmission electron microscopy, and a vibrating sample magnetometer. The X-ray diffraction results showed that the as-synthesized powders were in the pure phase with a doping amount of ≤0.25, and the peaks could be readily indexed to the cubic spinel cobalt ferrite. Transmission electron microscopy and high resolution transmission electron microscopy observations revealed that the gadolinium-doped cobalt ferrite nanoparticles were single crystal, roughly spherical, uniformly distributed, and not highly agglomerated. The room temperature magnetic field versus magnetization measurements confirmed a strong influence of gadolinium doping on the saturation magnetization and coercivity due to large lattice distortion and grain growth of small particles.     

The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers

Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Co-doped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field H_E at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field H_E and blocking temperature T_B decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers.     

The effect of Ru doping on the alkali-doped manganite Pr0.8Na0.2MnO3

Polycrystalline manganites Pr_0.8Na_0.2MnO₃ doped by ruthenium (0.0≤x_Ru≤0.2) were prepared by the sol-gel process. The magnetic field induced metamagnetism was observed to occur with a large resistivity drop at 3 K for x_Ru≤0.02 samples, which can be sorted into the kind of CMR phenomenon. It was found that the 0.01 Ru doping increased T_C and decreased the metamagnetic critical field than that of the undoped sample. The doped manganites show a quick increase in their magnetic moments as x_Ru increases from 0.01 to 0.04, but the larger fraction of Ru doping (0.04≤x_Ru≤0.2) reduces their M and T_C. All the five doped samples have larger magnetic moments than that of the host sample. Ru doping of x_Ru>0.01 results in a rapid disappearance for the observation of long range spin and charge ordering in the samples’ M-T curves, which is characteristic of the undoped sample. It was found that larger low-temperature MR favored decrease in the metamagnetic critical field. Finally, the phase diagram of Ru doping vs. transition temperatures is presented to summarize the experiments.     

Magnetic properties of substituted strontium ferrite nanoparticles and thin films

SrFe_12−x(Zr_0.5Mg_0.5)_xO₁₉ nanoparticles and thin films with x=0-2.5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). Structural and magnetic characteristics of synthesized samples were studied employing x-rays diffraction (XRD), transmission electron microscopy (TEM), magnetic susceptometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). TEM micrographs display that the narrow size distribution of ferrite nanoparticles with average particle size of 50 nm were fabricated. Fitting obtained data of effective magnetic susceptibility by Vogel-Fulcher law confirms the existence of strong magnetic interaction among fine particles. XRD patterns and FE-SEM micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. AFM micrographs exhibited that the surface roughness increases with an increase in Zr-Mg content. It was found from the VSM graphs that with an increase in substitution contents the coercivity decreases, while the saturation of magnetization increases. The Henkle plots confirms the existence of exchange coupling among nano-grain in ferrite thin films.     

Nanocrystallite interface effect and exchange bias phenomenology in Cr2O3 and NiO nanoparticles

Nanocrystalline Cr₂O₃ and NiO are prepared using high-energy ball milling. Average sizes of the particles obtained from Scanning Electron Microscopy and crystallite sizes obtained from X-ray diffraction are larger for Cr₂O₃ than NiO particles. At low temperature, large high-field magnetization and small coercivity lead to a weak exchange bias for Cr₂O₃, whereas small high-field magnetization and large coercivity lead to a considerable exchange bias for NiO. The training effect is observed for NiO at 4 K which could be described with a recursive formula constructed in the framework of the spin configurational relaxation model. The results suggest that the pinning mechanism at the interface between the antiferromagnetic and the weak ferromagnetic component ascribed to uncompensated spins leads to the exchange bias effect.     

Exchange bias in Co nanoparticles embedded in an Mn matrix

Magnetic properties of Co nanoparticles of 1.8 nm diameter embedded in Mn and Ag matrices have been studied as a function of the volume fraction (VFF). While the Co nanoparticles in the Ag matrix show superparamagnetic behavior with T_B=9.5 K (1.5% VFF) and T_B=18.5 K (8.9% VFF), the Co nanoparticles in the antiferromagnetic Mn matrix show a transition peak at ∼65 K in the ZFC/FC susceptibility measurements, and an increase of the coercive fields at low temperature with respect to the Ag matrix. Exchange bias due to the interface exchange coupling between Co particles and the antiferromagnetic Mn matrix has also been studied. The exchange bias field (H_eb), observed for all Co/Mn samples below 40 K, decreases with decreasing volume fraction and with increasing temperature and depends on the field of cooling (H_fc). Exchange bias is accompanied by an increase of coercivity.     

Tailoring the exchange bias of Ni/NiO nanogranular samples by the structure control

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H₂, at selected temperatures (200≤T_ann≤300 °C), NiO powder previously milled for 5, 10, 20 and 30 h. Both the as-milled NiO powders and the Ni/NiO samples have been analyzed by X-ray diffraction and the exchange bias properties have been investigated in the 5-200 K temperature range. The structure and the composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both T_ann and the milling time of the precursor NiO powders. In particular, by increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites induced by the H₂ treatments, so that, T_ann being equal, the amount and the mean grain size D_Ni of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field H_ex: the highest value (∼940 Oe) has been measured at T=5 K in a sample containing ∼7 wt% Ni and with D_Ni=19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of H_ex at T=5 K, the EB effect vanishes at the same temperature (∼200 K) and the same thermal dependence of H_ex is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and T_ann.     

Ultrasound-assisted microwave preparation of Ag-doped CdS nanoparticles

Ag-doped CdS nanoparticles were synthesized by an ultrasound-assisted microwave synthesis method. The X-ray diffraction patterns reveal a structural evolution from cubic to hexagonal with increasing molar ratios of Ag⁺/Cd²⁺ from 0% to 5%. It shows that the Ag-doped hexagonal CdS nanoparticles are polycrystal. The X-ray photoelectron spectroscopy of the CdS nanoparticles doping with 5% Ag⁺ shows that the doped Ag in CdS is metallic. Simultaneously, the characteristic Raman peaks of the CdS nanoparticles enhance with increasing Ag⁺ concentrations. The photocatalytic activity of different Ag-doped samples show a reasonable change due to different ratios of Ag which doped into CdS.     

Role of inter-site exchange and hybrid interactions in itinerant ferromagnetism

In this work, we study the magnetic properties of itinerant electron systems using the Hubbard-like tight binding Hamiltonian along with inter-site exchange and hybrid interactions. We have used the mean-field approximation to deal with the exchange and hybrid interactions. It is found that hybrid interaction is more effective than exchange interaction for the on-set of ferromagnetic state. We have studied the effect of hybrid interaction on various physical quantities at different temperatures. The effective mass (m*/m) of up spin electrons increases slowly as the temperature decreases but below the critical temperature (T_c), it decreases rapidly. For down spin electrons effective mass increases slowly as the temperature decreases and below T_c, it increases more rapidly. Spectral weight (n/m*) for up spin electrons decreases slowly upto T_c and below T_c, it increases rapidly. For down spin electrons spectral weight decreases slowly upto T_c and below T_c, it decreases rapidly. Our results for both the effective mass and spectral weight are in good agreement with recently observed experimental behaviour in itinerant ferromagnet Ga_1−xMn_xAs [Phys. Rev. Lett. 89 (2002) 097203]. We have also studied variation of the spectral weight and optical absorption with temperature in presence of magnetic field. We found that these two quantities for up spin electrons increase as applied magnetic field increases at all temperatures (∼4T_c). For down spin electrons these two quantities decrease as applied magnetic field increases.     

Synthesis of Fe-doped NiO nanofibers using electrospinning method and their ferromagnetic properties

To make p-type diluted magnetic semiconductor (DMS), Ni_1−xFe_xO nanofibers with different Fe doping concentrations have been successfully synthesized by electrospinning method using polyvinyl alcohol (PVA) and Ni(CH₃COO)₂·4H₂O as starting materials. The nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, superconductivity quantum interference device (SQUID) and X-ray photoelectron spectroscopy (XPS) test. The results show that Fe doping has no influence on the diameter and surface morphology of NiO nanofibers, and the nanofibers are polycrystalline with NaCl structure. All Fe-doped samples show obvious ferromagnetic properties and the saturation magnetization is enhanced with increase of the doping concentration of Fe, which indicates that the doped Fe has been incorporated into the NiO host and results in room-temperature ferromagnetism in the Ni_1−xFe_xO nanofibers.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

A study of the properties of core/shell/shell Ag/FeCo/Ag nanoparticles

The properties of heterophase core/shell/shell Ag/FeCo/Ag nanoparticles synthesized via a plasma method that are promising for biological applications are studied. As is established, the core/shell/shell Ag/FeCo/Ag nanoparticles exhibit a superparamagnetic state at room temperature that allows one to manage the hyperthermia process. The magnetic characteristics of core/shell/shell Ag/FeCo/Ag nanoparticles are interpreted by assuming partial oxidation of the surface layer of a ferromagnetic FeCo shell and formation of the antiferromagnetic Co_xFe_1–xО layer on the FeCo surface. The interaction between the surface antiferromagnetic Co_xFe_1–xО layer and the ferromagnetic FeCо shell causes the emergence of the exchange bias in Ag/FeCo/Ag nanoparticles.     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

Ca3－x AgxCo4O9(x=0—0.05)氧化物的电输运性能

Ca位掺杂可以优化Ca₃Co₄O₉复合氧化物的电输运性能. 采用柠檬酸溶胶凝胶结合放电等离子烧结制备了Ca位微量掺杂Ag的Ca_3-xAg_xCo₄O₉(x=0—005)氧化物块体试样,通过X射线衍射、扫描电子显微镜、电参数测试仪分析了所得试样. 试验结果表明: 产物呈单一物相,Ca位微量掺杂Ag降低了Ca_{关键词： 3}Co₄O₉')" href="#">Ca₃Co₄O₉ 掺杂 电输运     

Nanostructural,magnetic and electrical properties of Ag doped Mn-ferrite nanoparticles

Nano-crystalline MnFe_2−xAg_xO₄ (x = 0, 0.1, 0.2, 0.3 and 0.6) samples with average grain size of 4–7 nm were synthesized by a simple method based on decomposition of metal nitrates in presence of citric acid. The samples were characterized by different structural, magnetic and electrical measurements. Rietveld refinement of X-ray diffraction data confirmed cubic spinel structure of the samples. Results show that Ag doping decreases the crystallite size, magnetization and coercivity of nanoparticles. By increasing the Ag content in the samples the saturation magnetization shows interesting temperature dependent behavior. It was realized that magnetization of smaller particles show higher sensitivity to temperature variations than larger particles. DC electrical resistivity measurements in the temperature range of 300–650 K show that the resistivity first increases and then decreases by increasing the Ag content in the samples. Curie temperature (T_c) and polaron activation energy in ferromagnetic and paramagnetic regions were estimated by using resistivity curves.     

Indium doping effect on the magnetic properties of Y-type hexaferrite Ba0.5Sr1.5Zn2(Fe1-xInx)12O22

The effect of indium doping on structural and magnetic properties of Y-type hexaferrite Ba_0.5Sr_1.5Zn₂(Fe_1-xIn_x)₁₂O₂₂ (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) prepared by the solid state reaction method was investigated. The Rietveld refinement method was used to analyze the X-ray diffraction patterns. The magnetic transition temperatures associated with the proper-screw spin phase to the collinear ferrimagnetic spin phase transition can be efficiently modulated by varying indium content. The magnetic transition temperature increases to a maximum with indium content x = 0.04 and then decreases with x, suggesting the possibility that electrically controlled magnetization reversal can be can be effectively tailored by varying indium content. The saturation magnetization at room temperature was decreased as increasing indium content, which can be explained as the metal ions occupation. It is worthy to note that the coercivity of In-doped samples was decreased drastically compared that of undoped sample, which is probably resulted from the reduction in anisotropy field with substitution of In³⁺ for Fe³⁺. The In-doped hexaferrite Ba_0.5Sr_1.5Zn₂(Fe_1-xIn_x)₁₂O₂₂ may be potential candidates for application in magnetoelectric devices.