Structural and magnetic properties of Co50Ni50 powder mixtures

In the present work, morphological, structural, thermal and magnetic properties of nanocrystalline Co₅₀Ni₅₀ alloy prepared by high energy planetary ball milling have been studied by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. The coercivity and the saturation magnetization of alloyed powders were measured at room temperature by a vibration sample magnetization. Morphological observations indicated a narrow distribution in the particle and homogeneous shape form with mean average particle size around 130 μm². The results show that an allotropic Co transformation hcp→fcc occurs within the three first hours of milling and contrary to what expected, the Rietveld refinement method reveals the formation of two fcc solid solutions (SS): fcc Co(Ni) and Ni(Co) beside a small amount of the undissolved Co hcp. Thermal measurement, as a function of milling time was carried out to confirm the existence of the hcp phase and to estimate its amount. Magnetic measurement indicated that the 48 h milled powders with a steady state particles size have the highest saturation (105.3 emu/g) and the lowest coercivity (34.5 Oe).     

A 59Co NMR study to observe the effects of ball milling on small ferromagnetic cobalt particles

To demonstrate the potential of nuclear magnetic resonance (NMR) spectroscopy for investigating detailed structural properties in ferromagnetic materials, three different particle sized cobalt (Co) powders have been ball milled for 24 h are accurately characterised by internal-field ⁵⁹Co NMR. The ⁵⁹Co NMR spectra show distinct resonance bands corresponding to the different Co sites, face-centred-cubic (fcc), hexagonal-close-packed (hcp) and stacking faults (sfs), in Co metal powders. The hcp+fcc→hcp phase transition encouraged by ball-milling was observed and quantitative values for each Co environment were obtained.     

Phase transformations in the manganese hydride

Abstract

Pure fcc phase of manganese hydride has been synthesized at high temperature from α-Mn and the H₂ gas compressed to 1.2 GPa. This fcc phase transforms into the hcp modification at 6 GPa and 298K.     

Thermal expansion behaviors of hcp and fcc Co nanowire arrays

The lattice parameters of as-prepared and annealed Co nanowires with hcp and fcc structures have been measured using the in situ high-temperature x-ray diffraction method. The hcp and fcc Co nanowires have been fabricated within the porous anodic alumina membranes by a direct-current electrodeposition technique. The results indicate that the variational quantity of the interplanar spacing for hcp Co nanowire arrays is bigger than that for fcc Co nanowire arrays in spite of as-prepared and annealed samples. The structural difference between hcp and fcc Co nanowires results in the different thermal expansion behaviors.     

Facile synthesis and characterization of Co nanoplatelets with tunable dimensions via solution reduction process

Plate-like Co nanoparticles with different sizes were synthesized by solution reduction process. The size of Co nanoplatelets can be tuned by varying the concentration of CoCl₂·6H₂O and the dosage of N₂H₄·2H₂O. The Co nanoplatelets with the different size all exist in both fcc and hcp crystal structures. The normal direction of the nanoplatelets is perpendicular to the (002) planes of hcp phase or to the (111) planes of fcc phase. The saturation magnetizations of the samples are lower than the corresponding bulk value. The coercivities of the samples vary with the phase content and the particle size. The shape control mechanism was discussed.     

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.     

A New Insight into Cobalt Metal Powder Internal Field 59Co NMR Spectra

The authors present a new approach of internal field ⁵⁹Co NMR spectra assignment leaving apart from “usual” decomposition on “pure” hcp and fcc stackings, and a set of stacking faults (sfs) sf₁–sf₅ with a certain lines position. The authors propose including into consideration not only cobalt structural features as well as its magnetic nature due to the strong ferromagnetism in Co metal. The last fact supposes an existence of different magnetic species such as magnetic domains, domain walls, and single-domain particles, thereby helping to spectral lines assignment according to both structural and magnetic origin. The examined sample contains fcc and hcp resonance peaks in both domains and domain walls giving the hcp to fcc ratio equal to 1.9, as well a significant amount of Co sfs, or Co in loose coordination, up to 10 %. The research exhibits a good agreement of all implemented techniques.     

New beta(fcc)-cobalt to 210 GPa

We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.     

Crystal Structure Transformations of Rare-Gas Solids Under Pressure

Pressure-induced structural changes in solid krypton (Kr) and xenon (Xe) have been studied using angle dispersive X-ray diffraction in a diamond-anvil cell (DAC) up to 50 GPa. The analysis of the results shows that in solid Kr (Xe) the phase transition from fcc to hcp starts below 3.2 GPa (1.5 GPa). Albeit the hcp/fcc ratio increases under pressure, both phases coexist up to the highest pressure reached in this study. Room temperature (RT) equations of state (EOS) are determined.     

A study of the electrodeposition of Co–Cu alloys thin films on FTO substrate

In this work, the early stages and the properties of the electrodeposition process of Co–Cu alloys thin films on a fluorine-doped tin oxide (FTO)-coated conducting glass substrate from a sulfate bath were investigated using conventional electrochemical techniques and X-ray diffraction technique (XRD). FTO was chosen as a foreign substrate because of its high transparence and its properties as inert material. Within the potential range analyzed, the kinetics of the Co–Cu electrodeposition corresponded to a model including instantaneous nucleation on active sites and diffusion controlled cluster growth. The number of active sites of the substrate, N ₀, and the diffusion coefficient, D, were determined from the analysis of potentiostatic current transients on the basis of existing theoretical models. XRD patterns of the Co–Cu alloys thin films display fcc and hcp phase, with peaks quite close to those of the Co phase (fcc and hcp). Therefore, the variation of the composition of thin films alloy is possible depending on the deposition potential.     

Stability of Ferromagnetism in Fe, Co, and Ni Metals under High Pressure with GGA and GGA+U

The stability of the ferromagnetic state in Fe, Co, and Ni metals under high pressure is investigated using generalized gradient approximation (GGA) and GGA+U within the density functional theory (DFT). It is found that the ferromagnetic state under pressure is very different for Fe, Co, and Ni metals, and is closely associated with the crystal structure. In the case of Fe, a ferromagnetic bcc ground state is obtained at ambient pressure and a nonmagnetic hcp ground state is found at pressure around 12 and 115 GPa for GGA and GGA+U, respectively. For Co, the phase transition from a ferromagnetic hcp to a nonmagnetic fcc is found around 107 GPa for GGA. In contrast to Fe and Co, a ferromagnetic fcc state in Ni is maintained even at 200 GPa. The calculated results suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced decrease of the density of state at the Fermi level.     

Stacking-fault nucleation on Ir(111)

Variable temperature scanning tunneling microscopy experiments reveal that in Ir(111) homoepitaxy islands nucleate and grow both in the regular fcc stacking and in the faulted hcp stacking. Analysis of this effect in dependence on deposition temperature leads to an atomistic model of stacking-fault formation: The large, metastable stacking-fault islands grow by sufficiently fast addition of adatoms to small mobile adatom clusters which occupy in thermal equilibrium the hcp sites with a significant probability. Using parameters derived independently by field ion microscopy, the model accurately describes the results for Ir(111) and is expected to be valid also for other surfaces.     

室温下Fe62Ni27Mn11（wt%）合金的压致fcc-hcp相变

 本文采用Mao-Bell型金刚石对顶砧（DAC）及高压在位（in situ）粉末X光衍射照相方法研究了Fe₆₂Ni₂₇Mn₁₁（wt%）合金在0~43.2 GPa压力范围内的压致结构相变和等温压缩行为，实验结果表明，该合金在低压时为fcc结构，在19.4 GPa压力附近出现压致fcc→hcp结构相变，直到43.2 GPa一直保持fcc、hcp二相共存；相变过程中，二相的molar体积相同；高压hcp相得晶格参数比值c/a基本上不随压力而变，可以表示为c/a=1.630±0.006；在卸压过程中，hcp相可保持到5.8 GPa，当卸压到常压时，该合金完全恢复到fcc结构；用Murnaghan等温固体状态方程对其压缩数据进行最小二乘法拟合，得到B₀=(166±12) GPa，B₀'=5.2±0.5；本文还给出了该合金的压致fcc→hcp结构相变模型，并对存在很宽的二相共存区间问题进行了初步探讨。     

Electrodeposited cobalt films: hcp versus fcc nanostructuring and magnetic properties

The crystallographic structure and morphology of electrodeposited cobalt films on Au(111) is found to be very sensitive on the electrolyte pH value and on the overpotential applied during deposition. The samples, 2 to 500 nm thick, where characterized by nuclear magnetic resonance (NMR), atomic force microscopy (AFM) and electron diffraction. The latter technique shows that the Co films grow in registry with the gold underlayer, reproducing the Au(111) texture. During the first stage of growth and depending on overpotential and pH value, either continuous hcp Co films or hcp Co islands are formed. Only the latter growth mode leads to an out of plane magnetization with 100% of remanence. Increasing the thickness, fcc Co becomes the prevailing phase. Eventually the fcc to hcp ratio saturates at the same value regardless the overpotential. The thickness for which the equilibrium fcc to hcp ratio is obtained as well as the sample structure and morphology before saturation, depend strongly on the overpotential value. In any case, the predominance of the fcc Co phase leads to an in plane magnetization of the thick samples. This study opens up new opportunities of engineering the properties of electrodeposited cobalt films. Received 29 February 2000 and Received in final form 3 July 2000     

Atomic force microscopy study on microstructural changes by 'training' in Fe-Mn-Si-based shape memory alloys

Microstructural changes after several cycles of the thermomechanical treatment consisting of a small deformation by stress-induced martensitic transformation (fcc to hcp) and subsequent reversion to austenite by heating (referred to as 'training') have been studied by atomic force microscopy in Fe-Mn-Si based shape memory alloys. Well-trained samples contain a uniform distribution of thin martensite plates of the same variant, the widths of which decrease with increasing number of the training cycles, and their distribution becomes more uniform. Such microstructural development by training originates mainly from extremely thin plates (about 1 nm thick) of hcp phase that are still retained together with stacking faults in the austenite even after heating far above the reverse transformation temperature. In the reverse transformation on heating, a martensite plate that looks as though it is apparently one plate is, in fact, split into very thin plates, which indicates that the plate actually consists of extremely thin martensite plates and these thin plates are reverse-transformed one after another by reverse movement of the Shockley partial dislocations at their tips. This mode of reverse transformation ensures a perfect shape memory effect.     

在不同的退火温度下钴注入二氧化钛结构与磁性

选用能量为180 keV,温度为623 K,注入4×10¹⁶ cm^-2剂量的钴离子束注入TiO₂样品.在不同的退火温度下,用高分辨的扫描电镜、同步辐射X射线衍射、卢瑟福背散射/沟道实验和超导量子干涉仪,分别对样品进行结构与磁性的测试. SR-XRD和HRTEM测试结果表明: 在Co注入TiO₂后,形成了钴的体心立方(hcp)相和面心立方(fcc)相,且在TiO₂中,钴-纳米粒子也已经形成. 随着退     

Ball milling of Fe40Ni40P20-xSix (x = 6, 10 and 14): production and characterization

Progress in the ball milling amorphization of elemental powders with the overall composition Fe₄₀Ni₄₀P_{20 ? x}Si_x (X = 6, 10 and 14) and thermally induced crystallization of obtained alloys were characterized by differential scanning calorimetry, X-ray diffraction and transmission Mössbauer spectroscopy (TMS). Diffusion of Si into Fe and Ni alloys promotes the formation of the amorphous phase, via previous formation of (Fe, Ni) phosphides. After milling for 32–64 h, most of the powders are amorphous but bcc Fe(Si) crystallites remain (about 5% in volume). TMS results indicate that homogenization of the amorphous phase occurs by interdiffusion of Ni and Fe in Fe(Si,P)-rich and Ni(Si,P)-rich zones respectively. Annealing induces structural relaxation of stresses induced by milling, growth of bcc Fe(Si) crystallites, precipitation of bcc Fe(Si) and fcc Ni–Fe, and minor phases of Ni-rich silicides and (Fe, Ni) phosphides. The main ferromagnetic phase is bcc Fe(Si) for Fe₄₀Ni₄₀P₁₀Si₁₀ powders obtained after milling for 32 h. However, it is fcc Fe–Ni for the same alloy after milling for 64 h. In the later powders, as well as for alloys with x = 6 and 14 milled for 32 h, the fcc Fe–Ni shows the Invar magnetic collapse.     

57Fe Mössbauer effect studies on the magnetism of iron-rich bcc,fcc and hcp phases of Fe100−c Mn c

Iron-rich Fe_100?c Mn _c -alloys (0<c _Mn<35 at%) are found in three different phase structures depending on manganese concentration: bcc, fcc and hcp. Mössbauer effect studies on these phases reveal different magnetic properties: ferromagnetism, antiferromagnetism and paramagnetism. The evaluation of poorly resolved Mössbauer spectra of a sample with FeNi-Invar analogous composition (c _Mn=35%) and coexisting fcc and hcp phases was possible with the help of the Afanas'ev-Tsymbal sharpening method introduced recently [2,3].     

XAFS和XRD研究高能球磨对Fe70Cu30合金结构的影响

利用XRD和XAFS方法研究机械合金化Fe₇₀Cu₃₀二元金属合金随球磨时间的结构变化.XRD结果表明，球磨2 h后，部分金属Fe与Cu生成Fe-Cu合金；球磨20h后，金属Fe与Cu已完全合金化生成Fe-Cu合金，并只在2θ=44°处出现一个宽化的弱衍射峰，认为是在球磨20h后的Fe₇₀Cu₃₀合金中共存着fcc和bcc结构的Fe-Cu合金相.XAFS结果进一步表明，在球磨的初始阶段(2h)，fcc结构的Cu颗粒的晶 关键词： XAFS XRD 70Cu₃₀合金')" href="#">Fe₇₀Cu₃₀合金 机械合金化