The variation of Mn-dopant distribution state with x and its effect on the magnetic coupling mechanism in Znl-xMnxO nanocrystals

Zn1-xMnxO （x = 0.0005, 0.001, 0.005, 0.01, 0.02） nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and its X-ray absorption fine structure. It is found that the solubility of substitutional Mn in a ZnO lattice is very low, which is less than 0.4%. Mn ions first dissolve into the substitutional sites in the ZnO lattice, thereby forming Mn2＋O4 tetrahedral coordination when x ≤ 0.001, then entering into the interstitial sites and forming Mn3＋O6 octahedral coordination when x ≥ 0.005. All the samples exhibit paramagnetic behaviors at room temperature, and antiferromagnetic coupling can be observed below 100 K.     

稀释磁性半导体Zn1—xFexSe薄膜的光学特性

用真空蒸镀法制备了稀释磁性半导体Zn_(1-x)Fe_xSe多晶薄膜,用X射线衍射和电子扫描电镜测定了薄膜结构和成份.其光吸收数据表明:光学能隙E_g随着Fe~(2+)成分x增加而线性减小,用线性回归法拟合得其关系.E_g=2.722-2.2x(eV).     

Study of magnetic and optical properties of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni) diluted magnetic semiconductors:First principle approach

We present structural,magnetic and optical characteristics of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni and x = 6.25%),calculated through Wien2 k code,by using full potential linearized augmented plane wave(FP-LAPW) technique.The optimization of the crystal structures have been done to compare the ferromagnetic(FM) and antiferromagnetic(AFM) ground state energies,to elucidate the ferromagnetic phase stability,which further has been verified through the formation and cohesive energies.Moreover,the estimated Curie temperatures T_c have demonstrated above room temperature ferromagnetism(RTFM) in Zn_(1-x)TM_xTe(TM =Mn,Fe,Co,Ni and x= 6.25%).The calculated electronic properties have depicted that Mn- and Co-doped ZnTe behave as ferromagnetic semiconductors,while half-metallic ferromagnetic behaviors are observed in Fe- and Ni-doped ZnTe.The presence of ferromagnetism is also demonstrated to be due to both the p-d and s-d hybridizations between the host lattice cations and TM impurities.The calculated band gaps and static real dielectric constants have been observed to vary according to Penn's model.The evaluated band gaps lie in near visible and ultraviolet regions,which make these materials suitable for various important device applications in optoelectronic and spintronic.     

Manipulating coupling state and magnetism of Mn-doped ZnO nanocrystals by changing the coordination environment of Mn via hydrogen annealing

Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H_2/Ar atmosphere with different H_2/Ar ratios.It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO.Mn ions initially enter into interstitial sites and a Mn~(3+)O_6 octahedral coordination is produced in the prepared Mn-doped ZnO sample,in which the nearest neighbor Mn~(3+) and O~2 ions could form a Mn~(3+)-O~(2-)-Mn~(3+) complex.After H_2 annealing,interstitial Mn ions can substitute for Zn to generate the Mn~(2+)O_4tetrahedral coordination in the nanocrystals,in which neighboring Mn~(2+) ions and H atoms could form a Mn~(2+)-O~(2-)-Mn~(2+)complex and Mn-H-Mn bridge structure.The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn~(3+)-O~(2-)-Mn~(3+) complex,while the annealed samples exhibit their ferromagnetism,which originates from the Mn-H-Mn bridge structure and the Mn-Mn exchange interaction in the Mn~(2+)-O~(2-)-Mn~(2+)complex.     

单晶和孪晶的Zn0.96Co0.04O稀磁半导体薄膜的制备与研究

采用分子束外延技术分别在不同晶面的蓝宝石（sapphire Al₂O₃）基片上制备了沿c轴生长的Zn_0.96Co_0.04O稀磁半导体薄膜.发现在Al₂O₃（1120）晶面（a面）上薄膜是二维层状外延生长的高质量单晶薄膜,而在Al₂O₃（0001）晶面（c面）上薄膜却具有有趣的孪晶结构,部分区域相互之间有一个30°的面内转动来减少和基片之间的失配度.在孪晶薄膜中存在的这些相互旋转形成的区域界面上会引起载流子强烈的散射作用,导致载流子迁移率的下降和平均自由程的缩短.利用X射线吸收精细结构技术证明了无论单晶还是孪晶的Zn_0.96Co_0.04O薄膜中所有的Co都以+2价替代进入了ZnO的晶格,而没有形成任何杂相.而对其磁性研究发现,孪晶的薄膜样品比高质量的单晶薄膜样品具有大得多的饱和磁矩.这充分说明孪晶薄膜中的铁磁性来源与缺陷有关.我们还对铁磁性耦合机制进行了探讨. 关键词： Co掺杂ZnO 稀磁半导体 X射线吸收精细结构 单晶和孪晶薄膜     

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW₂O₈ magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy³⁺ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy³⁺ ions. The behavior of the C ( T ) dependencies above and below T _N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002     

O1s and Mn2p NEXAFS on single-layered La1-xSr1+xMnO4: crystal field effect versus orbital coupling mechanism

O1s and Mn2p near-edge X-ray absorption spectroscopy on La_1-xSr_1+xMnO₄ (0 ≤x ≤0.5) single crystals shows that Sr doping does not only provide holes to the system but also induces a continuous transfer of electrons from out-of-plane d_3z2-r2 to in-plane d_3x2-r2/d_3y2-r2 orbitals. Furthermore, a non-vanishing electron occupation of in-plane d_x2-y2 and out-of-plane d_3z2-r2 orbitals is observed up to relatively high doping contents. These findings demonstrate that the energy difference between all these orbital types has to be very small and manifest that the orbital degree of freedom is determined not just by crystal field effects but also by orbital coupling. Moreover, the doping-dependent transfer of spectral weight observed in the current data identifies La_1-xSr_1+xMnO₄ as a charge-transfer insulator.     

Impact of composition ratio on the structure and optical properties of (1-x)MnFe2O4/(x)ZnMn2O4 nanocomposite

($1-x$)MnFe$_{2}$O$_{4}$ (MFO)/$x$ZnMn$_{2}$O$_{4}$ (ZMO) ($x=0$, 0.2, 0.5, 0.8, and 1.0) nanocomposite samples were prepared using co-precipitation procedure. The phase percentage, cell parameters, and crystallite size of MFO and ZMO phases in each nanocomposite sample were calculated using Rietveld refinement procedure. The x-ray diffraction (XRD) analysis and Fourier-transform infrared spectroscopy techniques established the variation in the lattice parameters of each phase are due to permutation of all cations among the octahedral and tetrahedral sites of MFO and ZMO. The different oxidation states of different ions in all samples were determined using x-ray photoelectron spectroscopy (XPS) technique. The variation in absorbance of the nanocomposite samples with composition parameter ($x$) is dependent on the wavelength region. The optical bandgap of the nanocomposite samples is decreased as the content of ZMO phase increased. The effect of alloying on the refractive index, extinction coefficient, dielectric constant, optical conductivity, and the nonlinear optical behaviors of all samples were studied in detail. The nanocomposite sample $x=0.5$ disclosed upgraded optical parameters with the highest refractive index, optical conductivity, and PL intensity, which nominate it to be functional in various application fields.     

Study of the local micro-structure and magnetic and transport properties of CrxGe1−x thin films

The local micro-structure as well as the magnetic and transport properties of Cr_xGe_1−x films prepared by means of magnetron sputtering have been investigated. Structural analysis shows that Cr atoms are situated in substitutional sites in the Ge lattice. Electrical transport properties indicate that Cr introduces a shallow acceptor level at 0.016 eV from the valence band implying Cr substituting for Ge. The low temperature ferromagnetism observed in the films is mediated mainly by ferromagnetic superexchange interactions between diluted Cr ions.     

Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Θ_Fe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Θ_N < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe-Fe (or Fe-Cu) bond length values varying between R₁ = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R₁ = 2.58 ± 0.01 Å at Θ_Fe = 0.5 ML, Θ_N = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe (R = 2.48 Å), and compatible with fcc Fe (R₁ = 2.52 Å) and fcc Cu (R₁ = 2.55 Å). At the Fe lowest coverage (Θ_Fe = 0.5 ML) the dependence of R₁ on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (Θ_N = 0.5 ML) the XAFS analysis has to keep into account the Fe-N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.     

Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in Eu(1-x)Gd(x)O

Raman scattering studies as functions of temperature, magnetic field, and Gd substitution are used to investigate the evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in Eu(1-x)Gd(x)O. These studies reveal a spin-fluctuation-dominated paramagnetic (PM) regime for T>T*>T(C), and a coexistence regime for T相似文献   

Neutron powder diffraction investigation on the crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O3

The crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O(3) (x = 0.00, 0.01, 0.02, 0.03) has been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling an orthorhombic to monoclinic phase transition occurs on account of the charge and orbital ordering taking place in the Mn sub-lattice; at low temperature phase separation takes place and the main monoclinic phase coexists with a secondary orthorhombic phase, whose amount slightly increases with the increase of Cr content. Cr(3+) is not involved in orbital ordering or superexchange interactions. The charge and magnetic ordering are decoupled: the Mn moments order according to a CE-type structure in all samples.     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001     

Electronic structure of octane on Cu(1 1 1) and Ni(1 1 1) studied by near edge X-ray absorption fine structure

The electronic structure of an octane film grown on Cu(1 1 1) and Ni(1 1 1) was studied using C K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the 1 ML thick octane films on the metal substrates. The pre-peak originated from metal induced gap states (MIGS) in the band gap of octane. The intensity of the pre-peak for octane/Ni(1 1 1) was the same as that of octane/Cu(1 1 1), suggesting that there was little difference in the density of unoccupied MIGS between the octane film on Ni(1 1 1) and Cu(1 1 1). We discuss the metal dependence of the density of unoccupied MIGS on the band structure of the metals.     

Li(AlxCo1-x)O2晶体中Co3+电子态的变化及对结构演化的影响

研究了应用于锂二次电池正极的新型高能量密度存贮材料Li(AlxCo1-x)O2 (x=01—05)的磁性.发现Al3+的掺杂可导致Co3+中d电子自旋态发生变化，即有部分d电子进入高自旋态.伴随Co3+中电子状态的改变，材料结构演化也发生了相应变化，表现为c/a比增大明显减缓，较好地解释了材料结构对Vegard定律的正偏离.这对材料的微观结构与性能设计具有重要意义. 关键词： 锂电池材料 Li（AlxCo1-x）O2 磁性 自旋态 结构演化     

Adsorption and structure of methyl mercaptoacetate on Cu(1 1 1) surface by XPS and NEXAFS spectroscopy

Methyl mercaptoacetate (MA) on Cu(1 1 1) surface was investigated using synchrotron radiation-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. MA adsorbs on the surface via thiolate formation and weak interaction of the carbonyl group with the surface. Two different adsorption states previously reported for methanethiolate and ethanethiolate were confirmed, besides atomic sulfur. NEXAFS measurements support gauche-type conformation of MA whose skeleton lies on the surface.     

Comment on “Effect of annealing temperature on structure, magnetic properties and optical characteristics in Zn0.97Cr0.03O nanoparticles” by Liu et al., [Appl. Surf. Sci. 256 (2010) 3559]

The authors of a recent paper (Appl. Surf. Sci. 256 (2010) 3559) studied the effect of annealing temperature on structural, magnetic and optical properties in Cr-doped zinc oxide (Zn_0.97Cr_0.03O) nanoparticles synthesized by sol-gel method. The authors have verified that Cr ions, in 3+ valence state, substitute the Zn ions in the ZnO lattice. They concluded that the ferromagnetism observed in the samples is an intrinsic property of the Cr-doped ZnO. However, we noticed an unusual point in this article. The Cr 2p XPS spectrum shown by them is against the general trend of the 2p XPS spectra of the transition metals. In this light, we re-measured the high-resolution 2p XPS spectrum of Cr for a 3% Cr-doped ZnO sample that is entirely different than theirs. The spectrum presented by them therefore demands proper interpretations or it might mislead the researchers in this developing field.     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

Novel dielectric anomaly in the hole-doped La(2)Cu(1-x)Li(x)O(4) and La(2-x)Sr(x)NiO(4) insulators: signature of an electronic glassy state

The low-frequency dielectric response of hole-doped insulators La(2)Cu(1-x)Li(x)O(4) and La(2-x)Sr(x)NiO(4) shows a large dielectric constant epsilon(') at high temperature and a steplike drop by a factor of 100 at a material-dependent low temperature T(f). T(f) increases with frequency, and the dielectric response shows universal scaling in a Cole-Cole plot, suggesting that a charge-glass state is realized both in the cuprates and in the nickelates.     

The effect of the sputtering process ambient on the magnetic state and phase composition of the film nanocomposites (Fe0.45Co0.45Zr0.10) x (Al2O3)1−x

The effect of oxygen-containing ambient arising at sputtering of granular nanocomposites (Fe_0.45Co_0.45Zr_0.10) _x (Al₂O₃)_1−x (30 at.%≤x≤65 at.%) on their magnetic state and phase composition has been investigated. It was shown that the presence of oxygen resulted in the formation of oxide shells preventing the ferromagnetic interaction between Co_0.45Fe_0.45Zr_0.10 nanoparticles and also the formation of metallic percolative net beyond the percolation threshold (as opposed to the films prepared in pure argon atmosphere).