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1.
朱小溪  刘敬华  徐翔  蒋成保 《中国物理 B》2011,20(7):77501-077501
A method based on the measurement of Fe average atomic magnetic moment to identify the structural transition caused by the increase of Ga content in quenched Fe1 - xGax alloys (0.15 ≤ x ≤ 0.30) is proposed. The quenched Fe1 - xGax alloys show a change of the Fe average atomic magnetic moment from 2.25 μB to 1.78 μB and then to 1.58 μB, which corresponds to the structural transition from A2 to D03 and then to B2. The relationship between the structure and the magnetostriction is clarified, and the maximum magnetostriction appears in the A2 phase. The variation tendency of the magnetostriction is well characterized, which also reflects the structural transition.  相似文献   

2.
The Mn cation distribution in the seven sublattices of theW-type structure has been determined from high temperature neutron diffraction data of a SrMn2Fe16O27 powder sample. The Mn2+ ions enter the tetrahedral sites of blockS with a preference for site 4e. Mn3+ ions in small amounts are found in the octahedral sites, substituting for Fe3+, while 0.3 Fe2+ has been found in site 6g. The sample has the formula SrMn2.4Fe15.6O27. The crystal structure has been described in the non-centrosymmetric Pˉ62c space group, instead of the usual P63/mmc one. The magnetic structure, obtained from low temperature (4.2 K) data, is collinear, with the spins parallel to thec axis, according to the Gorter model. The magnetic moments of the sites give an experimental magnetization valueM s=28.4(1.5) μB/f.u., in good agreement with magnetic measurements and the theoretical value.  相似文献   

3.
The structural and magnetic properties of R2AlFe16-xMnx(0≤x≤8 for Y,Ho,0≤x≤10 for Pr, 0≤x≤16 for Nd)compounds have been investigated by means of X-ray diffraction and magnetization measurements.The R2AlFe16-xMnx compounds have a hexagonal Th2Ni17-type structure for R=Y and Ho, and a rhombohedral Th2Zn17-type structure for Nd and Pr.With increasing x,the unit-cell volumes have a small increase initially,followed by a greater linear increase.The Curie temperature and the saturation magnetizati on of these compounds show marvelous drop with increasing x.Compensation point was observed in the temperature dependence of the magnetization for Ho2AlFe12Mn4 compound.  相似文献   

4.
Neutron diffraction and M?ssbauer measurements have been carried out on the cubic Laves phase intermetallic TbMnFe. The magnetic moment on the transition metal atom is found to be low, 0.2μ B, at room temperature. This moment is temperature independent down to 10 K. Magnetic moment on the rare earth atom varies from 2.5μ B at 296 K to 7.27μ B at 10 K. M?ssbauer spectra recorded at 298 K and 78 K have magnetic character but there is a large distribution of hyperfine field values. Both these features arise due to magnetic frustration created in the sample due to the competing ferro and antiferromagnetic interactions between the transition metal atoms.  相似文献   

5.
The formation of induced 5d magnetic moment on Ir in Fe100−x Ir x (x=3, 10 and 17) and Co100−x Ir x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.  相似文献   

6.
李成仁  李淑凤  董斌  程宇琪  殷海涛  杨静  陈宇 《中国物理 B》2011,20(1):17803-017803
This paper reports that a series of Nd3+:Er3+:Yb3+ co-doped borosilicate glasses have been prepared and their absorption spectra measured. The J--O intensity parameters Ωk (k=2, 4, 6), spontaneous radiative lifetime τrad, spontaneous transition probability A, fluorescence branching ratio β and oscillator strength fed of the Nd3+ ions at room temperature are calculated based on Judd--Ofelt (J--O) theory. The temperature dependence of the up-conversion photoluminescence characteristics in a Nd3+:Er3+:Yb3+ co-doped sample is studied under a 978 nm semiconductor laser excitation, and the energy transfer mechanisms among Yb3+, Er3+ and Nd3+ ions are analysed. The results show that the J--O intensity parameters Ω2 increase when the Nd3+ concentration of the Nd3+:Er3+:Yb3+ co-doped borosilicate glasses increases. The possibility of spontaneous transition is small and lifetimes are long at levels of 4F5/2 and 4F3/2. The intensity of Nd3+ emissions at 595, 691, 753, 813 and 887 nm are markedly enhanced when the sample temperature exceeds 400 K. The reasons being the cooperation of the secondary sensitization from Er3+ to Nd3+ and the contribution of a multi-phonon.  相似文献   

7.
耿遥祥  特古斯  毕力格 《中国物理 B》2012,21(3):37504-037504
The structural and magnetocaloric properties of Mn1.35Fe0.65P1-xSix compounds are investigated. The Si-substituted compounds, Mn1.35Fe0.65P1-xSix with x = 0.52, 0.54, 0.55, 0.56, and 0.57, are prepared by high-energy ball milling and the solid-state reaction. The X-ray diffraction shows that the compounds crystallize into the Fe2P-type hexagonal structure with space group P62m. The magnetic measurements show that the Curie temperature of the compound increases from 253 K for x = 0.52 to 296 K for x = 0.56. The isothermal magnetic-entropy change of the Mn1.35Fe0.65P1-xSix compound decreases with the Si content increasing. The maximal value of the magnetic-entropy change is about 7.0 J/kg稫 in the Mn1.35Fe0.65P0.48Si0.52 compound with a field change of 1.5 T. The compound quenched in water possesses a larger magnetic entropy change and a smaller thermal hysteresis than the non-quenched samples. The thermal hysteresis of the compound is less than 3.5 K. The maximum adiabatic temperature change is about 1.4 K in the Mn1.35Fe0.65P0.45Si0.55 compound with a field change of 1.48 T.  相似文献   

8.
汤沛哲  刘海涛  朱洁  王山鹰  段文晖 《中国物理 B》2012,21(2):27104-027104
The structural and magnetic properties of Fen-mGam (n=3~6, m=0~2; n=13, m=0~3) alloy clusters have been studied using density functional theory. The substitutional doping is favourable for small clusters with up to six atoms at low Ga concentration and substitutional Ga atoms in 13-atom clusters prefer surface sites. The Ga-doping generally could reduce the energetic stability but enhance the electronic stability of Fe clusters, along with a decrease of the local magnetic moments of Fe atoms around Ga dopants. These findings provide a microscopic insight into Fe-Ga alloys which are well-known magnetostriction materials.  相似文献   

9.
Activated carbon supported Mo-based catalysts were prepared and reduced under different activation atmospheres, including pure H2, syngas (H2/CO=2/1), and pure CO. The catalysts structures were characterized by X-ray diffraction, X-ray absorption fine structure, and in situ diffuse reflectance infrared Fourier transform spectroscopy. The catalytic performance for the higher alcohol synthesis from syngas was tested. The pure H2 treatment showed a high reduction capacity. The presence of a large amount of metallic Co0. and low valence state Moφ+ (0<φ<2) on the surface suggested a super activity for the CO dissociation and hydrogenation, which promoted hydrocarbons formation and reduced the alcohol selectivity. In contrast, the pure CO-reduced catalyst had a low reduction degree. The Mo and Co species at the catalyst mainly existed in the form of Mo4+ and Co2+. The syngas-reduced catalyst showed the highest activity and selectivity for the higher alcohols synthesis. We suggest that the syngas treatment had an appropriate reduction capacity that is between those of pure H2 and pure CO and led to the coexistence of multivalent Co species as well as the enrichment of Moδ+ on the catalyst''s surface. The synergistic effects between these active species provided a better cooperativity and equilibrium between the CO dissociation, hydrogenation and CO insertion and thus contributed beneficially to the formation of higher alcohols.  相似文献   

10.
The present work establishes a systematic approach based on the application of in-situ Fourier transform infrared spectroscopy (FTIR) for the investigation of the crystal structure, thermal stability, redox behavior (temperature-programmed reduction/temperatureprogrammed re-oxidation) as well as the catalytic properties of Co3O4 thin films. The syntheses of Co3O4 were achieved by chemical vapor deposition in the temperature range of 400-500℃. The structure analysis of the as-prepared material revealed the presence of two prominent IR bands peaking at 544 cm-1 (υ1) and 650 cm-1 (υ2) respectively, which originate from the stretching vibrations of the Co-O bond, characteristic of the Co3O4 spinel. The lattice stability limit of Co3O4 was estimated to be above 650℃. The redox properties of the spinel structure were determined by integrating the area under the emission bands υ1 and υ2 as a function of the temperature. Moreover, Co3O4 has been successfully tested as a catalyst towards complete oxidation of dimethyl ether below 340 ℃. The exhaust gas analysis during the catalytic process by in situ absorption FTIR revealed that only CO2 and H2O were detected as the final products in the catalytic reaction. The redox behavior suggests that the oxidation of dimethyl ether over Co3O4 follows a Mars-van Krevelen type mechanism. The comprehensive application of in situ FTIR provides a novel diagnostic tool in characterization and performance test of catalysts.  相似文献   

11.
The crystal and magnetic structures of the composite compound Nd2Co6Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce2Ni7-type structure consisting of Nd(Co,Fe)2 and Nd(Co,Fe)5 structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)5 structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd2Co7−xFex compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)5 blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)2 and Nd(Co,Fe)5 blocks are reduced significantly.  相似文献   

12.
于洪飞  张鲁山  吴小会  郭永权 《物理学报》2011,60(10):107306-107306
利用非自耗真空电弧熔炼法制备了NdNi2Ge2化合物样品,采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其晶体结构. 结果显示该化合物的空间群为I4/mmm,点阵参数为:a=4.120(1),c=9.835(0),Z=2,Nd原子占据2a晶位,Ni原子占据4d晶位,Ge原子占据4e晶位. NdNi2Ge2化合物呈现顺磁性,应用居里-外斯定律拟合计算得到居里-外斯常数为25.8,居里-外斯温度为6.24 K. 有效势磁矩为3.69μB,这与理论计算Nd3+的磁矩相符,表明磁矩主要源于Nd3+. 电阻率变化范围为0.3 Ω ·μm-1-1 Ω ·μm,电阻曲线拟合显示NdNi2Ge2呈半金属性. 关键词: 2Ge2')" href="#">NdNi2Ge2 Rietveld结构精修 电磁输运  相似文献   

13.
It is difficult to obtain the crystallographic alignment for stoichiometric Nd2Fe14B alloys by applying the melt-spun and subsequent hot-pressing and hot-deformation techniques. However, the enhanced alignment and magnetic properties of die-upset nano-crystal Nd2Fe14B magnets have been obtained by Nb addition in the present paper. The magnetic properties studies show that Nb addition leads to the remarkable increase of remanence Br and intrinsic coercivity Hci, which is due to the improvement of c-axis texture and refinement of microstructure. Microstructure studies using transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that Nb atoms are enriched at grain boundary and the NbFeB phase is observed with increasing Nb content. Since some Fe atoms in the Nd2Fe14B phase participate in the formation of NbFeB phase, the excessive Nd atoms may be enriched at grain boundary, which may improve the physical property of grain boundary and provide a mass transport pass for preferential growth of oriented Nd2Fe14B grains, thus leading to the enhanced alignment and magnetic properties.  相似文献   

14.
The electronic structure of hexagonal Gd3Co11B4 compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd3Co11B4 compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μB/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd3Co11B4 compound with those of Nd3Co11B4 compound.  相似文献   

15.
吴文霞  郭永权  李安华  李卫 《物理学报》2008,57(4):2486-2492
应用固体与分子经验电子理论计算了Nd2Fe14B的价电子结构、磁矩和居里温度,计算结果与实验值相符.计算表明:该合金的磁性与3d磁电子数成正比.从Fe(c)晶位到Fe(k2)晶位磁矩增加,其机理源于价电子、哑对电子和3d磁电子之间的转化,有78%的哑对电子和18%的3d共价电子转化成了磁电子.居里温度和磁矩与Fe原子配位数成正比,与加权等同键数Iσ成反比,Nd原子 关键词: 2Fe14B')" href="#">Nd2Fe14B 价电子结构 居里温度  相似文献   

16.
郝延明  王玲玲  严达利  安力群 《物理学报》2009,58(10):7222-7226
通过X射线衍射、磁测量等手段对电弧炉制备的不同热处理条件的Sm2Fe17-xCrxx=1—3)化合物的结构和磁性进行了研究.结果表明1050 ℃下退火5 d的Sm2Fe17-xCrxx=1—3)化合物具有菱方相的Th2Zn17型结构,同样温度下退 关键词: 2Fe17-xCrx化合物')" href="#">Sm2Fe17-xCrx化合物 磁体积效应 居里温度 磁晶各向异性  相似文献   

17.
Magnetometric and neutron diffraction studies of polycrystalline NdCo2GE2, ErCo2Ge2 and PrFe2Ge2 compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ~ 4.2 and 13 K, respectively. The RECo2Ge2 compounds have collinear antiferromagnetic order of +?+? type. For PrFe2Ge2 a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE3+ free ion value. In ErCo2Ge2 the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.  相似文献   

18.
The crystallochemical and magnetic nature of ternary Sc1+δFe4−δAl8 intermetallic with a small Sc excess δ=0.1 was investigated by scanning electron microscope, X-ray powder diffraction, neutron diffraction, Mössbauer effect and superconducting quantum interference device techniques. The sample crystallizes in a tetragonal ThMn12 type structure. The excess of Sc atoms substitute Fe at the (8f) positions and have a pronounced effect on the magnetic properties. The experiments carried out in temperature range 4-320 K show that below 120 K the magnetic structure of the alloy forms a double cycloid with magnetic moments rotating according to the incommensurate in-plane wave vector, which is temperature independent up to 160 K. The value of Fe magnetic moment is close to 0.9 μB atom−1 at 4 K. Temperature dependence of unit cell dimensions can be explained within the Debye-Grüneisen approximation.  相似文献   

19.
钱萍  刘九丽  申江  白丽君  冉琼  王云良 《中国物理 B》2010,19(12):126001-126001
This paper investigates the structural stability of intermetallics R3Ni13-xCoxB2(R=Y,Nd and Sm) with Nd 3 Ni 13 B 2-type structure and the site preferences of the transition element Co by using a series of interatomic pair potentials.The space group remains unchanged upon substitution of Co for Ni in R3Ni13-xCoxB2 and the calculated lattice constants are found to agree with reports in literatures.The calculated cohesive energy curves show that Co atoms substitute for Ni with a strong preference for the 3g sites and the order of site preference is 3g,4h and 6i.Moreover,the total and partial phonon densities of states are first evaluated for the R 3 Ni 13 B 2 compounds with the hexagonal Nd 3 Ni 13 B 2-type structure.  相似文献   

20.
杨育奇  高庆庆  李冠男 《物理学报》2013,62(1):16103-016103
在金属间化合物的结构演变中,原子尺寸因素起着重要的作用.由于密堆积效应,不同原子半径比的元素往往形成不同的结构.而自由电子填充于原子构成的晶体结构的间隙中,它对化合物的结构也有影响.基于组合结构化合物Ho2Ni7-xFex,结合原子尺寸与自由电子对晶体结构的不同影响,文章探讨一种单位体积内自由电子浓度的经验方法来判断Ho2Ni7-xFex化合物中两种异构体间的转变.随着Fe含量的增加,Ho2Ni7-xFex化合物先结晶成Gd2Co7型三方结构,然后结晶成Ce2Ni7型六方结构.利用Rietveld精修技术和磁测量,获得了化合物的晶体结构参数和饱和磁化强度.化合物晶胞常数随Fe含量增加而增加,饱和磁化强度则随之减少(dMs/dx=-2).分析结果表明,单位体积内自由电子浓度更高,化合物形成三方结构,反之则形成六方结构.  相似文献   

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