Effect of temperature on the elastic anisotropy of pure Fe and Fe0.9Cr0.1 random alloy

The elastic properties of pure iron and substitutionally disordered 10 at.?% Cr Fe-Cr alloy are investigated as a function of temperature by using first-principles electronic-structure calculations by the exact muffin-tin orbitals method. The temperature effects on the elastic properties are included via the electronic, magnetic, and lattice expansion contributions. We show that the degree of magnetic order in both pure iron and Fe(90)Cr(10) alloy mainly determines the dramatic change of the elastic anisotropy of these materials at elevated temperatures. The effect of lattice expansion is found to be secondary but also very important for quantitative modeling.     

Mechanical properties of bcc Fe-Cr alloys by first-principles simulations

The effect of chromium content on the fundamental mechanical properties of Fe-Cr alloys has been studied by first-principles calculations. Within a random solid solution model, the lattice constants and the elastic constants of ferromagnetic bcc Fe_1?x Cr _x (0? · ?0.156) alloys were calculated for different compositions. With addition of Cr content, the lattice parameters of Fe-Cr alloys are larger than that of pure Fe solid, and the corresponding Young??s modulus and shear modulus rise nonmonotonically with the increasing Cr content. All alloys (except 9.4 at% Cr) exhibit less ductile behavior compared with pure bcc Fe. For the Fe_1?x Cr _x (0? · ?0.156) alloys, the average magnetic moment per atom decreases linearly with the increasing Cr concentration.     

Mn/Cr掺杂对Ni-Fe-Ga合金相变和磁性转变的影响

系统研究了Mn和Cr元素掺杂对Ni_(55)Fe_(18)Ga_(27)合金马氏体相变温度和居里温度的影响.研究表明:随着Mn含量的增加,Ni_(55-x)Mn_xFe_(18)Ga_(27)系列合金的马氏体相变温度逐渐降低,居里温度有所增加;Ni_(55)Fe_(18-x)Mn_xGa_(27)系列合金的马氏体相变温度也逐渐降低,但居里温度变化并不明显.随着Cr含量的增加,Ni_(55-x)Cr_xFe_(18)Ga_(27)系列合金的马氏体相变温度明显降低,居里温度则小幅度降低;Ni_(55)Fe_(18-x)Cr_xGa_(27)系列合金的马氏体相变温度和居里温度均有规律的降低.     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².     

Molecular-Dynamics Approach to the Magnetic Structure of Competing Magnetic Alloys: Fe-Cr Alloys

A molecular dynamics (MD) approach which determines automatically the complex magnetic structures in itinerant electron systems is applied to Fe-Cr alloys with use of 250 atoms in a MD unit cell (5×5×5 bcc lattice). It is demonstrated that the Fe-Cr alloys show various complex magnetic structures due to competing interactions: the collinear ferromagnetism (F) of matrix Fe with antiparallel Cr moments beyond 80 at.% Fe, the coexistence of non-collinear structure of Cr and collinear F of Fe between 50 and 75 at.% Fe, the coexistence of broken antiferromagnetism (AF) of Cr and the F of Fe between 25 and 45 at.% Fe, the coexistence of F of Fe and antiferromagnetic long-range order of Cr around 20 at.% Fe, the AF of Cr matrix with non-collinear Fe moments (spin-glass like structure) between 5 and 15 at.% Fe, and the AF below 5 at.% Fe. In the concentration region between 5 and 20 at.% Fe, ferromagnetic Fe pairs which are stabilized with different amplitudes of local moments are found. The magnetic phase diagram and calculated magnetic moments are shown to be consistent with the neutron, Mössbauer, and photoemission experiments.     

Development of an engineering model for ferromagnetic shape memory alloys

This paper presents a relationship among stress, temperature and magnetic properties of a ferromagnetic shape memory alloy. In order to derive an engineering model of ferromagnetic shape memory alloys, we have developed a measuring system of the relationship among stress, temperature and magnetic properties. The samples used in this measurement are Fe68–Ni10–Cr9–Mn7–Si6 wt% ferromagnetic shape memory alloy. They are thin ribbons made by rapid cooling in air. In the measurement, the ribbon sample is inserted into a sample holder winding consisting of the B-coil and compensation coils, and magnetized in an open solenoid coil. The ribbon is stressed with attachment weights and heated with a heating wire. The specific susceptibility was increased by applying tension, and slightly increased by heating below the Curie temperature.     

Effect of Fe addition on the crystallization behaviour and Curie temperature of CoCrSiB-based amorphous alloys

The crystallization behaviour and Curie temperature of melt-spun Co_{71? x} Fe _x Cr₇Si₈B₁₄ (x?=?0, 2, 3.2, 4, 6, 8 and 12?at.%) amorphous alloys have been studied. Differential scanning calorimetry (DSC) showed two stages of crystallization. The first stage of crystallization (T _X1) in the alloy with 6?at.%?Fe was the highest and it had the highest activation energy. X-ray diffraction studies revealed that the primary crystalline phase is hcp-(CoCr)₂Si for Fe-free alloy, whereas (CoFeCr)₂Si and (CoFeCr)₃Si phases were formed with the addition of Fe. hcp-Co was also formed along with these phases. The secondary crystalline phases were fcc-Co and various boron-rich phases. The Curie temperature of the alloys also changed with the addition of Fe to the system. Like the primary crystallization temperature, the Curie temperature of the alloys did not vary systematically with the Fe content. The addition of Fe to the Co-based system changes the nearest-neighbour interaction. This changes the exchange interaction between the transition metal elements. Due to the asymmetry in the Bethe–Slater curve, a systematic variation with Fe addition was not observed in the Curie temperature measurement.     

Magnetic and electrical transport properties of amorphous Zr-Fe alloys

Amorphous Zr_1?xFe_x samples were prepared in the composition range 0.2 ? x ? 0.9 either by means of vapour deposition or melt spinning. The electrical resistivity was determined in the range 4.2–300 K. Negative temperature coefficients were observed in the whole concentration range. The extended Ziman theory (diffraction model) was found to be able to explain these results only if the effective valence of the Fe atoms involves not only s electrons but also d electrons. The magnetic properties and the ⁵⁷Fe Mössbauer effect of the Zr_1?xFe_x alloys were studied in the range 4.2–300 K. The Fe-rich alloys are ferromagnetic. The Fe moment vanishes in alloys of an Fe concentration lower than about 50 at%. In most alloys (x ? 0.8) the Curie temperature is below room temperature and continuously decreases with Zr concentration. By means of Mössbauer spectroscopy and magnetic measurements it is shown that compositional short-range order (CSRO) is present to a higher degree in melt-spun alloys than in vapour-deposited alloys. The effect of sign and magnitude of the heat of solution on CSRO and the magnetic properties is discussed.     

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 1: Damage evolution in thin-foils at lower doses

The evolution of radiation damage in Fe and Fe–Cr alloys under heavy-ion irradiation was investigated using transmission electron microscopy. Thin foils were irradiated with 100 or 150 keV Fe⁺ and Xe⁺ ions at room temperature (RT) and 300°C. Dynamic observations followed the evolution of damage and the early stages in damage development are reported. Small (2–5 nm) dislocation loops first appeared at doses between 10¹⁶ and 10¹⁷ ions m^?2 in all materials. Loop number densities depended strongly on the foil orientation in pure Fe but not in Fe–Cr alloys. Number densities did not depend strongly on Cr content. For a given material, defect yields were higher for Xe⁺ ions than for Fe⁺ ions, and were higher at RT than at 300°C. Loops with both ?100? and ½?111? Burgers vectors were identified. The proportion of ?100? loops was larger, especially in pure Fe. Dynamic observations showed that: the contrast of some new loops developed over intervals as long as 0.2 s; hopping of ½?111? loops was induced by the ion and electron beams and was pronounced in ultra-pure iron; and many loops were lost during and after ion irradiation by glide to the foil surface. The number of loops retained was strongly dependent on the foil orientation in Fe, but less so in Fe–Cr alloys. This is due to lower loop mobility in Fe–Cr alloys, probably due to pinning by Cr atoms. Reduced loop loss probably explains the higher loop number densities in Fe–Cr alloys compared with pure Fe.     

Co和稳定元素对Nd3(Fe,Co,M)29(M=Ti,V,Cr) 化合物结构和磁性的影响

利用x射线衍射和磁测量研究了不同稳定元素Co以及Ti,V和Cr替代对Nd₃Fe_29-x-yCo_xM_y(M=Ti,V,Cr)化合物结构和磁性的影响.研究发现:每一个稳定元素都有一替代量极限,在此极限以内所有化合物均为Nd₃(Fe,Ti)₂₉型结构,A2/m空间群.不同稳定元素的溶解极限不同.Co的替代量与稳定元素有关,当以Cr作为稳定元素时,Cr的替代量随着Co含量的提高而提高 关键词： 3(Fe')" href="#">Nd₃(Fe Co 29')" href="#">M)₂₉ 结构 磁性     

Ferromagnetic-spin glass transition in amorphous and crystalline Fe-Cr

Through a direct comparison of experimental results in amorphous and crystalline forms of Fe-Cr alloys we examine the effect of topological and compositional disorder on magnetic properties. Both types of disorder result in a decrease in the Curie temperature, the magnetic moment at T = 0 and the exchange stiffness constant. These results are discussed in the framework of several models.     

Invar effects on some iron-base amorphous alloys

The thermal expansion and magnetic properties of Fe-B and Fe-P amorphous alloys prepared from melts have been investigated. These amorphous alloys show distinct Invar characteristics. heir magnetic properties are also very similar to those of Fe-Ni crystalline Invar alloys; that is, the high-field susceptability and forced-volume magnetostriction are remarkably large, the magnetic moment per Fe atom does not increase linearly, the Curie temperature decreases with a decrease in concentration of B or P, and their reduced their magnetization curves are much flatter than those of crystalline pure Fe.     

Magnetic structure of critical Fe-Ni alloys as derived from Arrott plots and Mössbauer spectra

New evidence of magnetic phase transition brought about by an external magnetic field acting on critical composition bulk Fe-Ni alloys was found by using iron-nickel alloys containing 30 wt% Ni and 31.3 wt% Ni, respectively (balance Fe). The split in the central peak of Mössbauer spectrum of the Fe-Ni alloy containing 29.7 wt% Ni, at its Curie temperature in a weak external magnetic field of 3100 Oe, is a conclusive evidence of its superparamagnetic structure.     

Atomic and Magnetic Structure of the Interface in Multilayers

Temperature dependence of the magnetic properties of Fe/Ag vacuum evaporated multilayers was studied in a wide range of layer thickness. For Fe thickness larger than 1 nm continuous magnetic layers can be found, but its hyperfine field is significantly lower than that of pure α-Fe at elevated temperatures. It is attributed to a decrease of the Curie temperature due to Ag impurities in the Fe layer. Below 1 nm Fe thickness magnetic relaxation and the formation of a granular alloy with 35 T average hyperfine field was observed. Magnetoresistance results indicate the presence of Fe clusters in the Ag matrix, as well. This revised version was published online in August 2006 with corrections to the Cover Date.     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L21结构Co2FeSi高有序合金之间的Co50Fe50-xSix合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似（GGA＋U）方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co2FeSi合金的分子磁矩和居里温度分别达到5.92μB和777℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co2FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内.     

Gd3Co29-xCrx新相化合物的结构与磁性

制备出具有室温单轴磁晶各向异性的非间隙型Co基Gd3Co29-xCrx化合物(x=65和70)，x射线衍射和磁性测量表明所有单相化合物均属于单斜晶系，Nd3(Fe,Ti)29型结构和A2/m空间群.Gd3Co29-xCrx化合物的居里温度在x=65时为412 K，x=70时为359 K. Gd3Co29-xCrx化合物在x=65 时磁化强度随温度的变化曲线表明，在居里温度以下的某一温度处有一补偿点，在补偿点处求得晶格分子场系数nRT=33 T f.u./μB. 关键词： Gd3Co29-xCrx化合物 x射线衍射 磁晶各向异性     

Resistance of nanocrystalline vis-à-vis microcrystalline Fe–Cr alloys to environmental degradation and challenges to their synthesis

This paper presents a hypothesis and its experimental validation that a nanostructure can bring about dramatic improvements in the oxidation/corrosion resistance of iron–chromium alloys. More specifically, a nanocrystalline Fe–10 wt% Cr alloy was found to undergo oxidation at a rate that was an order of magnitude lower than its microcrystalline counterpart. Importantly, the oxidation resistance of nanocrystalline Fe–10 wt% Cr alloy was comparable with that of the common corrosion-resistant microcrystalline stainless steels (having 18–20 wt% chromium). The findings have the potential of leading to the next generation of oxidation-resistant alloys. However, due to poor thermal stability of nanocrystalline structure, synthesis/processing of such alloys is a challenge. Discs of nanocrystalline Fe–10% Cr alloy were produced by ball-milling of Fe and Cr powders and compaction of the powder without considerable grain growth by processing within a suitable time–temperature window. The paper also presents a theoretical treatise to arrive at the minimum chromium content required for establishing a protective layer of chromium oxide in an Fe–Cr alloy of a given nanometric grain size.     

Low temperature magnetic and magnetostrictive properties in Pr(Fe_(1-x)Co_x)_(1.9) cubic Laves alloys

The structures,spin reorientations,magnetic,and magnetostrictive properties of the polycrystalline Pr(Fe_(1-x)Co_x)_(1.9)(x=0–1.0)cubic laves phase alloys between 5 K and 300 K are investigated.Large low-field magnetostrictions are observed at 5 K in the alloys with x=0.2 and 0.4 due to the low magnetic anisotropies of these two alloys.A large negative magnetostriction of about-1130 ppm is found in PrCo_(1.9) alloy at 5 K.The magnetizations of the alloys with 0≤x≤0.6decrease abnormally at the spin reorientation temperature T_(sr),and an abnormity is detected in the alloy with x=1.0 at its Curie temperature T_c(45 K).The substitution of Fe by Co increases the value of T_(sr) in the alloy with x value increasing from 0.0 to 0.4,and then reduces the value of Tsr with x value further increasing to 0.6.     

Electronic structure and magnetic and optical properties of double perovskite Bi2 FeCrO6 from first-principles investigation

Double perovskite Bi2 FeCrO6 , related with multiferroic BiFeO3 , is very interesting because of its strong ferroelectricity and high magnetic Curie temperature beyond room temperature. We investigate its electronic structure and magnetic and optical properties by using a full-potential density-functional method. Our optimization shows that it is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to previous studies. Spin exchange constants and optical properties are calculated. Our Monte Carlo magnetic Curie temperature is 450 K, much higher than any previously calculated value and consistent with experimental results. Our study and analysis reveal that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful in understanding such perovskite materials and exploring their potential applications.     

Magnetic and structural properties of FeCr alloys

Synergistic synchrotron x-ray absorption experiments using imaging magnetic microspectroscopy, x-ray magnetic circular dichroism, and ab initio calculations on FeCr alloys reveal that the Cr content strongly influences the ferromagnetic microstructure and the Fe magnetic moments. The Cr local structure resolved by extended x-ray absorption fine structure (EXAFS) is also found to be affected by the alloy's composition. Both EXAFS and ab initio calculations show a change in the Cr local atomic structure above 10 at.% Cr content from the distance contraction of the first two coordination shells around the Cr absorbing atom. These results indicate the strong dependence of magnetic and structural properties of these alloys on Cr concentration.