Spin-axis-dependent magnetic properties of FePt and CoPt

A theoretical study of the magnetic properties of the CoPt and FePt ordered alloys has been performed. The calculation is done as a function of the spin-quantization axis by means of both the local spin density and the generalized-gradient approximations in conjunction with the full-potential linear muffin–tin orbital method. Both approximations produced similar results for the FePt and CoPt compounds. The band structure and the total density of states have been calculated and it was confirmed that all electronic states contribute to the magneto-crystalline anisotropy energy; the magnetization axis is along the [0 0 1] direction. The Fe and Co orbital magnetic moments decrease with respect to the angle γ between the [0 0 1] axis and the spin quantization axis, but for the [1 0 0] axis the orbital moment is comparable to the [0 0 1] moment. The Pt orbital moments are of the same order of magnitude as those of Fe and Co moments due to the large spin–orbit splitting parameter of Pt and show a similar behavior with the angle γ.     

Ni掺杂对ZnO磁光性能的影响

在掺杂浓度范围为2.78%—6.25%(物质的量分数)时,Ni掺杂ZnO体系吸收光谱分布的实验结果存在争议,目前仍然没有合理的理论解释.为了解决存在的争议,在电子自旋极化状态下,采用密度泛函理论框架下的第一性原理平面波超软赝势方法,构建不同Ni掺杂量的ZnO超胞模型,分别对模型进行几何结构优化和能量计算.结果表明,Ni掺杂量越大,形成能越高,掺杂越难,体系稳定性越低,掺杂体系带隙越窄,吸收光谱红移越显著.采用LDA(局域密度近似)+U方法调整带隙.结果表明,掺杂体系的铁磁性居里温度能够达到室温以上,磁矩来源于p-d态杂化电子交换作用.Ni掺杂量越高,掺杂体系的磁矩越小.另外还发现Ni原子在ZnO中间隙掺杂时,掺杂体系在紫外光和可见光区的吸收光谱发生蓝移现象.     

Effect of vanadium doping on structural, magnetic and optical properties of ZnO nanoparticles

V-doped ZnO nanoparticles were synthesized by heating metal acetates in organic solvent. All synthesized samples were annealed in air and reducing gas atmosphere at 600 °C for 8 h. The XRD patterns of both samples annealed in air and reducing atmosphere indicate that samples have polycrystalline wurtzite structure with increase in lattice constant with increase in V-doping. The particle sizes were calculated by using Scherrer's equation which lies in the range of 25-30 nm. The SEM images show that particles annealed in air and under reducing environment are spherical in nature. The EDX results reveal that samples contain V, Zn, and O contents only. The TPR results indicate that the system contains isolated VO_x, ZnO_x and bimetallic Zn: V (O_x) sites and indication of electronically excited bimetal sites. There is no signature of ferromagnetism in all samples annealed in air while room temperature ferromagnetism has been observed only under reducing atmosphere annealing. There is monotonically increase in saturation magnetization with V-doping concentration. UV-vis spectroscopy study shows that there is a linear increase in band gap energy with increase in V-doping, a direct evidence of change in magnetic properties due to V-doping and under reducing environment.     

Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

The ZnO:Ni²⁺ nanoparticles of mean size 2-12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional NiO-associated diffraction peak was observed above 15% of Ni²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Ni²⁺ doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic behavior, however, at higher doping percentage of Ni²⁺, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Ni-Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni²⁺.     

Temperature-dependent magnetic and structural ordering of self-assembled magnetic array of FePt nanoparticles

Silicon nanoparticles (SiNPs) can be synthesized by a variety of methods. In many cases these routines are non-scalable with low product yields or employ toxic reagents. One way to overcome these drawbacks is to use one-pot synthesis based on the chemical reduction of micelles. In the following study trichloroalkylsilanes of differing chain lengths were used as a surfactant, and the level of capping, surface bonding and size of the nanoparticles formed has been investigated. FTIR results show that the degree of alkyl capping for SiNPs with different capping layers was constant, although SiNPs bound with shorter chains display a much higher level of Si–O owing to the reaction of the ethanol used in the method with uncapped sites on the particle. SiNPs with longer chain length capping show a sharp Si–H peak on the FTIR, these were heated at reflux with the corresponding 1-alkene to fully cap these particles, resulting in a reduction/disappearance of this peak with a minimal change in the intensity of the Si–O peak. Other techniques used to analyze the surface bonding and composition, XPS, ¹H-NMR, and TEM/EDX, show that alkyl-capped SiNPs have been produced using this method. The optical properties showed no significant changes between the different capped SiNPs.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

Effect of iron doping on the structural and magnetic properties of ZnO nanoparticles prepared by pulsed electron beam evaporation

Nanocrystalline ZnO, ZnO-Zn, and ZnO-Zn-Fe powders with a specific surface area up to 45 m²/g and a low Fe concentration (no more than 0.619 wt %) have been prepared using pulsed electron beam evaporation. The crystal structure, morphology, and size of the nanoparticles have been determined using X-ray powder diffraction, transmission electron microscopy, and scanning electron microscopy. It has been found that the magnetization of the ZnO-Zn and ZnO-Zn-Fe nanopowders increases after annealing in an oxidizing atmosphere. An elemental mapping with energy-dispersive X-ray analysis has revealed the absence of Fe clusters in the ZnO-Zn-Fe sample. A thermal analysis has demonstrated that dopants of Fe in ZnO increase the temperature of complete oxidation of Zn nanoparticles to 600°C, which creates favorable conditions for an increase in the density of structural defects upon oxidation of Zn to ZnO. The absence of clusters and secondary magnetic Fe phases in pure and doped ZnO-based nanopowders indicates the intrinsic nature of ferromagnetism at room temperature in nanopowders prepared by pulsed electron beam evaporation.     

Layer resolved structural relaxation at the surface of magnetic FePt icosahedral nanoparticles

The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of Fe52Pt48. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich Fe/Pt core.     

Structural,magnetic and optical properties of diluted magnetic semiconductor (DMS) phase of Ni modified CuO nanoparticles

Control on the size of copper oxide (CuO) in the nano range is a highly motivating approach to study its multifunctional nature. The present investigation reports a sol-gel derived Ni doped CuO nanoparticles (Cu_1-xNi_xO). Rietveld refinement of the XRD spectra confirms the formation of single monoclinic phase of Cu_1-xNi_xO nanoparticles having crystallite size within the range of 19–21 nm. Raman spectra show the presence of characteristics Raman active modes and vibrational bands in the Cu_1-xNi_xO samples that corroborate the monoclinic phase of the samples as revealed by refinement of XRD data. The estimated band gap of pure CuO is found to be ∼1.43 eV, which decreases with the increase of dopant concentration into CuO matrix. This result is in line with estimated crystallite size. Magnetization curves confirm the weak ferromagnetic nature of Cu_1-xNi_xO nanoparticles which reveal the DMS phase. This weak magnetic nature may be induced in the samples due to the exchange interaction between the localized magnetic d-spins of Ni ions and carriers (holes or electrons) from the valence band of pristine CuO lattice. Replacement of Cu⁺² by Ni⁺² ions into the host CuO lattice induces the magnetization. The quantified value of squareness ratio (S < 0.5) confirms the inter-grain magnetic interactions in the Cu_1-xNi_xO nanoparticles which is also the reason of weak induced magnetization.     

Effect of hexane on magnetic blocking behavior of FePt nanoparticles

In this work effect of the carrier fluid, hexane, on the magnetic properties of 4.7 nm sized FePt nanoparticles is investigated. Nanoparticles are synthesized by chemical method. Structural and magnetic characterizations confirmed that samples are monodispersed with disordered face centered cubic (fcc) crystal structure and, magnetically, exhibit two blocking behaviors; the first is at 27 K and second at 110 K. Carrier fluid of particles, hexane, is found to influence the blocking of 7% of the total magnetic moments in the system by freezing at low temperatures resulting in a two blocking phenomena even for nanoparticles that are monodispersed with narrow particle size distribution.     

Effect of Mn doping on the structural and optical properties of sol-gel derived ZnO nanoparticles

Un-doped and Mn-doped ZnO nanoparticles were successfully synthesized in an ethanolic solution by using a sol-gel method. Material properties of the samples dependence on preparation conditions and Mn concentrations were investigated while other parameters were controlled to ensure reproducibility. It was observed that the structural properties, particle size, band gap, photoluminescence intensity and wavelength of maximum intensity were influenced by the amount of Mn ions present in the precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. The diffraction peaks of doped samples are slightly shifted to lower angles with an increase in the Mn ion concentration, signifying the expansion of the lattice constants and increase in the band gap of ZnO. All the samples show the absorption in the visible region. The absorbance spectra show that the excitonic absorption peak shifts towards the lower wavelength side with the Mn-doped ZnO nanoparticles. The PL spectra of undoped ZnO consist of UV emission at 388 nm and broad visible emission at 560 nm with varying relative peak intensities. The doping of ZnO with Mn quenches significantly the green emission while UV luminescence is slightly affected.     

Effect of Pb on structural and magnetic properties of Ba-hexaferrite

Lead was doped in barium hexaferrite by co-precipitation method and the Pb-doped Ba-hexaferrite with compositions of Ba_1−xPb_xFe₁₂O₁₉ was investigated for the first time at x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The molar ratio (Fe³⁺/Ba²⁺) of the solutions was kept 12 while pH was maintained at 13 by using NaOH (M=5) as precipitating agent. Prepared samples were sintered at 965±5 °C for three hours. Structural and morphological studies were done by X-ray diffractometer (XRD) and the scanning electron microscope (SEM). SEM micrographs confirmed the formation of hexagonal plate like structures and particle size was observed to be increased with the increase in Pb concentration. The hysteresis loops obtained from the magnetometer showed that with the increase in Pb concentration, the coercivity decreased while magnetic induction and remanence increased, which in turn increased the maximum energy product (BH)_max. Lower coercivity and the moderate increase in saturation magnetization obtained from Pb doping makes the material useful for magnetic recording media and other frequency based applications.     

Effect of cobalt doping on structural,optical and redox properties cerium oxide nanoparticles

Cobalt-doped ceria nanoparticles were synthesized using the polyol method under co-precipitation hydrolysis. The structural, morphological, optical and redox properties were observed to investigate the influence of different concentration of cobalt ion doping on the prepared CeO₂ nanomaterials in terms of X-ray diffraction, field-emission transmission electron microscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, UV/vis absorption spectroscopy and temperature program reduction techniques. The optical band gap energy was calculated from the optical absorption spectra for doped ceria nanoparticles, which have been found to be 2.68, 2.77, and 2.82 eV for the 2, 4, and 7 mol% Co ion-doped CeO₂ nanoparticles, respectively. As observed, the band gap energies increases as the doping Co ion concentrations increased, which could be due to significant increased oxygen vacancies with Co doping. The synergistic interaction between Co and CeO₂ was the main factor responsible for high catalytic activity of cobalt-doped CeO₂ model catalysts.     

Influence of organic precursor on the structural and magnetic properties of Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles were synthesized by the coprecipitation method. With the addition of the sucrose as chelating agent (sucrose) the size of the particles was reduced from 54 nm to 19 nm. The Co₃O₄ nanoparticles exhibit a cubic spinel structure identified for X-ray diffraction (XRD) and confirmed by Rietveld refinement. Scanning Electron Microscopy (SEM) images exhibit a spherical-like morphology and confirm the decrease of the particle size observed by XRD. The magnetic measurements as a function of temperature using a superconducting quantum interference device (SQUID) show a large surface anisotropy for samples obtained with the addition of sucrose accompanied by an exchange Bias effect indicating also the existence of a weak ferromagnetism. A decrease of the Néel temperature from the bulk (and other nanostructures-type) was observed, which can be associated with finite-size effect in the nanoparticles' shape.     

Effect of magnetic doping on the electronic states of Ni

Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with Fe and Cr. Well-defined spin and k states are selected using high energy and k resolution combined with single crystal alloys. Iron suppresses the mean free path of minority spins only, while chromium suppresses both spins and decreases the magnetic splitting. The strong variation of these effects from one impurity to the other supports the concept of magnetic doping.     

Effect of magnetic field annealing on microstructure and magnetic properties of FePt films

FePt (20 nm) films were annealed in a magnetic field (along the normal direction of the films) at a temperature around the Curie temperature of L1₀ FePt. The influence of magnetic filed annealing on texture and magnetic properties of FePt films were investigated. The results indicate that preferential (0 0 1) orientation and perpendicular anisotropy can be obtained in L1₀ FePt films by using magnetic field annealing around the Curie temperature of L1₀ FePt. This is one of the potential methods to obtain (0 0 1) orientation and thus to improve the perpendicular anisotropy in FePt films.     

Study of magnetic and structural properties of ferrofluids based on cobalt-zinc ferrite nanoparticles

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 30 nm, dispersed in a liquid carrier. Magnetic Co_(1−x)Zn_xFe₂O₄ (x=0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive X-ray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy (TEM) studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer (VSM); and finally, a magnetic force microscope was used to visualize the magnetic domains of Co_(1−x)Zn_xFe₂O₄ nanoparticles. X-ray diffraction patterns of Co_(1−x)Zn_xFe₂O₄ show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe₂O₄. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5±0.3) nm to (5.4±0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co-Zn ferrite nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer's formula. The magnetic properties investigated with the aid of a VSM at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co_(1−x)Zn_xFe₂O₄ magnetic nanoparticles, the crystal and nanoparticle sizes determined by X-ray Diffraction and TEM, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is small and for this reason Co_(1−x)Zn_xFe₂O₄ nanoparticles are considered as soft magnetic material.     

Effect of Mn doping on structural and magnetic properties of GaFeO3

GaFe_1−xMn_xO₃ polycrystalline materials have been prepared by a solid state reaction (SR) and by the sol-gel (SG) method. The maximum Mn content amounts up to 10% and 40% for SR and SG preparation, respectively. All compounds in these composition ranges crystallize in space group P c 2₁ n derived from Rietveld refinement of X-ray powder patterns. The gradual incorporation of manganese is accompanied by a decrease in the cell volume. The ferrimagnetic transition temperature of T_c=282 K for GaFeO₃ decreases with Mn content and reaches T_c=149 K for x=0.4.     

Ti掺杂对高熵合金FeCrVMoTix(x=0-1)结构和力学性能的影响

采用第一性原理方法研究了Ti对高熵合金FeCrVMoTi_x结构、弹性和塑性的影响.分别用混合焓、原子尺寸差、价电子浓度、相对熵效应和结合能评价FeCrVMoTi_x结构的稳定性,研究发现BCC型的FeCrVMoTi_x在x=0.0-0.9范围内都是结构稳定的.另外,弹性和塑性的计算发现,块体模量随Ti的增加而线性减小.剪切模量和杨氏模量首先随x的增加而变大,然后达到最大值,最后随x的进一步增加而快速减小. Ti的掺杂有助于提高FeCrVMoTi_x的屈服强度和抗拉强度,其中FeCrVMoTi_0.57的拉伸性能最好.热力学的计算表明压强能够抑制FeCrVMoTi_0.57的体膨胀系数,并增强其德拜温度.