Hyperfine fields of Laves phase compounds

The hyperfine fields were calculated by the LMTO method in the framework of the LSD approximation. Among the transition metal compounds, YMn₂ is a remarkable compound because the contribution from the valence electrons to the hyperfine field is positive and anomalously large.     

Transferred hyperfine interactions in ferrimagnetic RCo2 cubic Laves phase compounds

The ferrimagnetic system (Gd_xY_1−x)Co₂ has been studied by the Mössbauer effect of ¹⁵⁵Gd and by the NMR of ¹⁶⁵Ho as a substitutional impurity. The data are interpreted in terms of a simple model which involves transferred hyperfine fields from the lanthanide and cobalt ions and which allows for crystal field quenching of the holmium moment.     

Electronic structure and magnetovolume effect of AFe2 compounds with C14 and C15 Laves structures

The density of states of d-electrons is calculated by the recursion method for AFe₂ with the C14 and C15 structures in the para-, ferro- and antiferromagnetic states. The trends of the crystal structures are discussed. Magnetic moment and volume magnetostriction are calculated on the basis of the Stoner theory.     

Mg-Al-Ca合金中二元Laves相的电子结构和力学性能

采用基于密度泛函理论的第一性原理方法,研究了Mg-Al-Ca合金中AB2型二元Laves相(如CaMg2,CaAl2和MgAl2)在不同形态结构(C14,C15和C36)下的晶体结构、电子结构和力学性能.计算所得的晶格常数与实验数据吻合很好.形成能与相关能的计算表明C15-CaAl2 Laves相具有很好的合金化能力和结构稳定性.态密度和电荷密度的计算从微观上进一步分析了C15-CaAl2具有很好的合金化能力和稳定性的根源.弹性常数的计算和力学性能的分析表明C14- MgAl2具有很好的延展性,C15- MgAl2具有很好的塑性.     

Electronic structure and magnetic properties of Laves phase LuFe2

The electronic structure and magnetic properties of the Laves phase of LuFe₂ with C14, C15, and C36 structures has been investigated using the full-potential linearized augmented plane wave method. In order to study the stability of magnetic phases, nonmagnetic and spin-polarized calculations for ferromagnetic ordering were performed. It is found that the ferromagnetic hexagonal C14 phase is the ground-state structure and the C15 phase is an intermediate state between the C14 and C36 structures. There is an increase in the average magnetic moment on the Fe sites in the order of C15 →C14 →C36 structures, whereas the Lu-moment is not significantly different. We also find that there exist both localized and itinerant d electrons, resulting in antiferromagnetic ordering in the three structures. Their density-of-states, equilibrium volumes, and elastic properties are discussed, which is important for the understanding of the physical properties of LuFe₂ and may inspire future experimental research.     

Magnetic hyperfine field at s-p impurities on Laves phase compounds

Recent experimental results for the magnetic hyperfine field B_hf at the nuclei of s-p impurities such as ¹¹⁹Sn in intermetallic Laves phases RM₂ (R=Gd, Tb, Dy, Ho, Er; M=Fe, Co) and ¹¹¹Cd in R Co₂, the impurity occupying a R site indicate that the ratio B_hf/μ_3d exhibits different behavior when one goes from RFe₂ to RCo₂. In this work, we calculate these local moments and the magnetic hyperfine fields. In our model, B_hf has two contributions: one arising from the R ions, and the other arising from magnetic 3d-elements; these separate contributions allow the identification of the origin of different behavior of the ratio mentioned above. For ¹¹¹Cd in RCo₂ we present also the contributions for B_hf in the light rare earth Pr, Nd, Pm, Sm compounds. For the sake of comparison we apply also the model to ¹¹¹Cd diluted in R Ni₂. Our self-consistent magnetic hyperfine field results are in good agreement with those recent experimental data.     

Ab initio calculation of the phase stability,mechanical properties and electronic structure of ZrCr2 Laves phase compounds

Ab initio calculations have been used to investigate the phase stability, mechanical properties and electronic structure of ZrCr₂ Laves phase compounds, based on the method of augmented plane waves plus local orbitals with the generalized gradient approximation. The calculated lattice constants for the C15, C36 and C14 structures are in good agreement with experimental values. The calculation of heats of formation showed that C15 is a ground-state phase, whereas C36 is an intermediate phase and C14 the high-temperature phase. The elastic constants and elastic moduli for the C15 structure were calculated systematically and compared with experiments and previous theoretical calculations. The intrinsic and extrinsic stacking fault energies are found to be 112 and 98?mJ?m^?2, respectively. The equilibrium separations between Schockley are also predicted using the calculated elastic moduli and stacking fault energies. Finally, the calculated electronic structures of these Laves phases are discussed based on these results.     

Influence of Tb on easy magnetization direction and magnetostriction of ferromagnetic Laves phase GdFe_2 compounds

The crystal structure,magnetization,and spontaneous magnetostriction of ferromagnetic Laves phase Gd Fe2 compound have been investigated.High resolution synchrotron x-ray diffraction(XRD) analysis shows that Gd Fe2 has a lower cubic symmetry with easy magnetization direction(EMD) along [100] below Curie temperature TC.The replacement of Gd with a small amount of Tb changes the EMD to [111].The Curie temperature decreases while the field dependence of the saturation magnetization(Ms) measured in temperature range 5–300 K varies with increasing Tb concentration.Coercivity Hc increases with increasing Tb concentration and decays exponentially as temperature increases.The anisotropy in Gd Fe2 is so weak that some of the rare-earth substitution plays an important role in determining the easy direction of magnetization in GdFe_2.The calculated magnetostrictive constant λ100 shows a small value of 37×10~(-6).This value agrees well with experimental data 30×10~(-6).Under a relatively small magnetic field,GdFe_2 exhibits a V-shaped positive magnetostriction curve.When the field is further increased,the crystal exhibits a negative magnetostriction curve.This phenomenon has been discussed in term of magnetic domain switching.Furthermore,magnetostriction increases with increasing Tb concentration.Our work leads to a simple and unified mesoscopic explanation for magnetostriction in ferromagnets.It may also provide insight for developing novel functional materials.     

Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

Characterization of Laves phase in Crofer 22 H stainless steel

This study investigated the effect of annealing temperature on the precipitation behavior of Crofer^® 22 H at 600 °C, 700 °C, and 800 °C. The grain size distribution, precipitate phase identification, and microstructure were analyzed using electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). The morphology of Laves phase (Fe,Cr,Si)₂(Nb,W) precipitates having the Cr₂Nb structure changed from strip-like to needle-shaped as the annealing temperature was increased. The precipitates of the Laves phase also shifted from the grain boundaries to the grain interiors when the temperature was increased. However, the average grain size (150 μm) of the ferritic matrix did not significantly change at 600 °C, 700 °C, and 800 °C for 10 h.     

Heat capacities of Laves phase compounds RMn2 (R = Y, Gd and Er)

Measurements of heat capacity indicate that the Mn moment in YMn₂ has an itinerant character and an additional C_M is observable even above T_N. In GdMn₂, the Gd moments are in disorder at T_N simultaneously with the Mn moments. The CEF contributions in ErMn₂ are observed and are calculated using a single-ion Hamiltonian.     

Electronic spectra and electronic structures of [2.2]paracyclophane and related compounds

The absorption spectra of [2.2]paracyclophane (PC), tetramethyl [2.2]paracyclophane (DD), and triple- and quadrupole-layered cyclophanes (DDD and DDDD) were measured with the n-heptane and 3-methylpentane solutions in the range of 25000–59000 cm^?1. The polarized absorption spectra of the [2.2]paracyclophane single crystal were measured in the range of 30000–50000 cm^?1 for the ab plane and 30000–37500 cm^?1 for the ac plane. Strong bands corresponding to the 180 nm band of benzene were newly observed at 53000, 49750, 49600, and 50000 cm^?1 for PC, DD, DDD, and DDDD, respectively.The electronic structures of PC, DD, DDD, and DDDD were studied semiempirically by considering the configuration interaction among the ground, locally excited and charge-transfer configurations.From the band positions, the absorption intensities, and the band polarization obtained experimentally and theoretically, we made the more reasonable and more reliable band assignments compared with those by the previous authors.     

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

The⁵⁷Fe Mössbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe₂ (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe–Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The⁵⁷Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the⁵⁷Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.This paper is based on a paper presented at the 5th Int. Conf. on hyperfine interactions, Berlin, July 21–25, 1980.     

Magnetostriction and spin reorientation in ferromagnetic Laves phase Pr(Ga_xFe_(1-x))_(1.9) compounds

The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga_xFe_(1-x))_(1.9) alloys have been investigated by high-precision x-ray diffraction(XRD) step scanning, magnetization, and Mo¨ssbauer spectra measurements. Ga substitution reduces the magnetostriction(λ_(||)) with magnetic field H ≥ 8 kOe(1 Oe = 1.33322×10~2 Pa), but it also increases the λ|| value when H ≤ 8 kOe at 5 K. Spin-reorientations(SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mo¨ssbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature(T_(SR)) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(T_x Fe_(1-x))_(1.9)(T = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction(EMD) or T_(SR) for developing an anisotropic compensation system.     

Spin fluctuations in hydrogen-stabilized Laves phase UTi2H5

Uranium Laves phase UTi₂ does not exist in a pure form, but can be stabilised by the presence of hydrogen, which can be absorbed in concentration exceeding 5?H atoms/f.u. Low temperature specific heat, magnetic susceptibility, and resistivity indicate that UTi₂H₅ is a spin fluctuator close to the verge of magnetic ordering. Its susceptibility follows at high temperatures the Curie–Weiss law with U effective moment µ_eff[ ?= 3.1?µ_B/U and paramagnetic Curie temperature Θ_p = ?200 K. The temperature dependence of specific heat exhibits a pronounced and weakly field dependent upturn in C_p/T versus T below 10 K reflecting the effect of spin fluctuations. It can be described by an additional T^½ term. The Sommerfeld coefficient γ = 256?mJ/mol K² classifies the compound as a mid-weight heavy fermion. Spin fluctuations are affecting also electrical and thermal transport and thermoelectric power, which all resemble UAl₂. A lattice anomaly at ≈ 240?K, attributed to the melting of hydrogen sublattice, reflects in most of bulk properties.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。     

Antiferromagnetism of the hexagonal Laves phase ThMn2

Magnetization, thermal expansions and neutron diffraction measurements have been performed on ThMn₂. Below T_N, one Mn atom (2a) in four remains nonmagnetic whilst the others order in a triangular configuration. The large anistropic thermal expansions effects minimize the frustration of the Mn-Mn interactions.     

Structure imaging and vanadium substitution in cubic TiCr2 Laves phase

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr₂ Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces.     

Helimagnetism in the cubic Laves phase YMn2

Neutron diffraction experiments on YMn₂ using a wave length of λ_N = 2.483 Å show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T_N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T_N, it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.     

First-Principles Study of Electronic Structure of the Laves Phase ZrFe2

We perform the ab initio calculation for obtaining the density of states and magnetic properties of ZrFe2 Laves phase compound based on the method of augmented plane waves plus local orbital The results indicate that the ferromagnetic state is more stable than the paramagnetic one, but with a slightly larger volume. The 3d - 4d exchange interactions between Fe and Zr electrons lead to the antiparallel coupling for Fe 3d and Zr 4d states, which is responsible for the ferrimagnetic ordering of the compound. The resulting magnetic moment of about 1.98μB for Fe is spatially localized near the Fe site, while around Zr a small but extended negative spin states causes a moment of about -0.44 μB. Moreover, the resulting magnetic moments with the generalized gradient approximation are more consistent with experimental values than that of the local-spin density approximation.