Atomic structure and thermal stability of nanostructured Y-Fe alloys

We present results of an X-ray and EXAFS study of nanostructured Y-Fe alloys with 0.05 ≤ _XFe ≤ 0.77. Because the atomic size mismatch implies a large elastic contribution to the heat of solution of Fe in the Y lattice, the alloy system is a potential candidate for the stabilization against grain-growth by grain boundary segregation. For _XFe ≤ 0.3, the majority of Fe is segregated to the grain boundaries of crystalline Y. The grain-size of the Y-crystals in asprepared and annealed samples decreases with increasing x_Fe ⩾ 0.45, the alloys are amorphous. Upon annealing, the grain-size of the low-x_Fe crystalline samples increases significantly, whereas high-x_Fe crystalline samples show little grain-growth. Annealing is accompanied by a progressive increase of the concentration of Fe dissolved in the Y crystal lattice to values considerably beyond the equilibrium solubility.     

Electron-microscopical observation of crystal growth in amorphous Fe75B25 alloy

Amorphous ribbons of Fe₇₅B₂₅ alloy were made by melt-spinning and their crystallization process was observed in situ under a transmission electron microscope (TEM). Especially, the growth rate of b c t Fe₃B crystals was measured with high precision at various temperatures, and the kinetics of the movement of the amorphous-crystalline interface was examined. Differing from the amorphous Fe₈₆B₁₄ alloy studied earlier and other Fe-B alloys with compositions not close to the 31 stoichiometry, crystal growth in the Fe₇₅B₂₅ alloy showed a perfect linearity against time, with no relaxation phenomena being observed. The linear growth was not disturbed by the small concentration deviations or the crystal orientations. When the annealing temperature was changed stepwise up and down between 190° C to 380° C, the linear growth was maintained at each temperature and no temperature hysteresis of the growth rate appeared. The Arrhenius plots of the growth rates gave an apparent activation energy of about 2 eV, but the plotted lines were not exactly straight but upward concave and/or convex. Theoretical analysis of the reaction kinetics of the amorphous-crystalline boundary movement was based on either the concept of multiple processes connected in series or in parallel or of the temperature-dependent activation energy of the process. Good agreement between the calculated and experimental curves was obtained and underlying mechanisms of the growth process were considered and discussed. It was concluded that the crystal growth process of amorphous Fe₇₅B₂₅ alloy is not controlled by long-range diffusion but presumably by small concentration deviations and local fluctuations of concentration and structural order.     

Effects of Annealing on High-Magnetoresistance Tunnel Junctions Using Co75Fe25 Ferromagnetic Electrodes

1. IntroductionSpin tunneling magnetoresistive effect in ferromagnet / insulat or / ferromagnet (FM / I / F M ) j "net ionshas been the subject of intense study since the discovery of large tunneling magnetoresistance (TMR) inFe/Alzos/Fe and CoFe/Alzos/Co junctions at roomtemperature (RT)[',']. TMR ratio of FM/I/FM junctions at the early period 'was achieved only a fewpercent at RT due to the restriction of fabricationtechnology and TMR junction structure and area[3-6].Recently, re…     

Atomic structure and dynamic behavior of small interstitial clusters in Fe and Ni

Atomic structure and dynamic behavior of small interstitial clusters (dislocation loops) in Fe and Ni have been studied by means of both the static relaxation method and the molecular dynamics (MD) method in order to clarify their role in the evolution of damage structure during irradiation, especially in the so-called production bias effect through one-dimensional migration and sink efficiency to dislocations. Model crystals were constructed by using N-body potentials and stable atomic structures of small interstitial clusters, i.e., bundle of crowdions were obtained. It was found that each crowdion in the cluster has a split structure when the number of crowdions in the cluster is larger than 10. Dynamic behavior of the loop, e.g., the interaction with a crowdion on a 1 1 1 loop axis was also investigated as a function of time by the MD method. It was shown that a small interstitial cluster, e.g., I₁₉ in Fe is very mobile under the interaction with a crowdion, which shows that this interstitial cluster I₁₉ has already the property of a dislocation loop of edge character and low Peierls potential for the motion of this loop, which is consistent with the straight edge dislocation in Fe.     

Atomic structure and reactivity of ferromagnetic Fe deposited on Si(001)

This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 Å Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 °C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe₃Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.     

Mg:Sc:Fe:LiNbO_3晶体缺陷结构和全息存储性能

采用提拉法,从Li/Nb变化(0.94,1.05,1.20,1.38)的熔体中生长出Mg:Sc:Fe:LiNbO_3晶体。Li/Nb=1.05的晶体OH~-振动吸收峰在3504cm~(-1)处出现的主吸收峰,且在3466cm~(-1)、3481m~(-1)处有两附加峰。Li/Nb=1.38的晶体OH~-振动吸收峰在3535 cm~(-1)处出现附加吸收峰。红外光谱结果表示Li/Nb=1.05的晶体是近化学剂量比的,且Sc掺质优先于Mg掺质达到阈值浓度。采用透射光斑畸变法测得Mg:Sc:Fe:LiNbO_3晶体(Li/Nb=1.05)的抗光损伤能力为2.0×10~4W/cm~2,比Fe:LiNbO_3提高了三个数量级。采用波长为632.8nm的He—Ne激光器作为光源,通过二波耦合方法测试晶体全息存储性能。实验结果表明:随着Li/Nb的增加,晶体的写入时间缩短,晶体的衍射效率降低,光折变灵敏度增加,动态范围减少。在一系列晶体中,Mg:Sc:Fe:LiNbO_3晶体(Li/Nb=1.05)更加适合作为全息存储介质。     

Atomic structure,electronic properties,and thermal stability of diamond-like nanowires and nanotubes

Density functional tight-binding calculations are used to investigate the structure, electronic properties, energy stability, and thermal behavior (0–1500 K) of extended monolithic (nanowires) and hollow (nanotubes) diamond-like carbon nanostructures. The results indicate that diamond-like nanowires and nanotubes may be both metallic and semiconducting, depending on their morphology and size. A new type of hybrid (sp ³ + sp ²) nanostructure is identified, which has the form of a monolithic diamond-like (sp ³) wire inside a graphite-like (sp ²) shell. Diamond-like nanowires are shown to be more stable than nanotubes of comparable size.     

Enhancement of structural stability of nanosized amorphous Fe2O3 powders by surface modification

The surface of low-dimensional solids plays a key role in their phase transition. In the present study, to enhance the structural stability of nanosized amorphous Fe₂O₃ powders their surfaces were modified by employing NaOH solution, which leads to an increase in both the crystallization temperature from 364 °C to 411 °C and the crystallization activation energy from 81.5 kJ/mol to 156.8 kJ/mol. The surface-modified amorphous Fe₂O₃ powders show an entirely different crystallization behavior as compared with the as-prepared amorphous powders. The enhanced structural stability is attributed to the increase of the amount of hydroxide groupings at the surfaces of amorphous powders, which lowers their surface energy.