Surface energy and magnetism of the 3d metals
Abstract
We present an ab initio study of surface energies, surface magnetism and work functions of the 3d transition metals. The calculations are performed by means of a spin-polarized Green's function technique based on the tight-binding linear muffin-tin orbitals method within the atomic sphere approximation. In addition to the conventional paramagnetic and spin-polarized calculations we use the fixed spin-moment method to clarify the effect of magnetism on the surface energies. The results are shown to be consistent with a Friedel model of d-electron bonding combined with spin-split state densities as well as with a Stoner-type description. It is established that the anomaly in the surface energy of the 3d metals deduced from surface tension measurements is purely a magnetic solid state effect. In addition, it is found that magnetism reduces the surface energy of open surfaces, e.g. the (001) crystal faces, to the extent that the usual anisotropy of the surface energy is reversed. Thus a complete realization of the surface energy anomaly only takes place in the less close-packed surface facets.
References (50)
- O. Jepsen et al.
J. Magn. Magn. Mater.
(1980)Phys. Rev. B
(1982) - O. Eriksson et al.
Phys. Rev. B
(1992) - S.H. Vosko et al.
Can. J. Phys.
(1980) - H.L. Skriver
J. Phys. F
(1981) - V.L. Moruzzi et al.
Phys. Rev. B
(1989) - J. Friedel
Ann. Phys.
(1976) - A. Zangwill
Physics at Surfaces
- J. Mathon
Rep. Prog. Phys.
(1988) - S. Ohnishi et al.
Phys. Rev. B
(1983) - E. Wimmer et al.
Phys. Rev. B
(1984)
J. Magn. Magn. Mater.
Phys. Rev. B
Phys. Rev. Lett.
Phys. Rev. Lett.
Phys. Rev. B
Phys. Rev. B
J. Phys. F
Phys. Rev. Lett.
Phys. Rev. B
Phys. Rev. B
Phys. Rev. B
Phys. Rev. B
The LMTO Method
Phys. Rev. Lett.
Cited by (115)
Temperature-dependent magnetism in Fe foams via spin-lattice dynamics
2022, Computational Materials ScienceSpin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature dependence of the magnetization in foams is determined as a function of the fraction of surface atoms in foams, . The Curie temperature of foams decreases approximately linearly with , while the critical exponent of the magnetization increases considerably more strongly. If the data are plotted as a function of the fraction of surface atoms, reasonable agreement with recent data on vacancy-filled Fe crystals and novel data on void-filled crystals is observed for the critical temperature. Critical temperature and critical exponent also depend on the coordination of surface atoms. Although the decrease we find is relatively small, it hints to the possibility of improved usage of topology to taylor magnetic properties.
Influence of vacancies on the temperature-dependent magnetism of bulk Fe: A spin-lattice dynamics approach
2022, Computational Condensed MatterThe effect of random vacancies on the magnetic properties of a bcc Fe sample is studied using classical coupled spin-lattice dynamics. This simulation technique is based on molecular dynamics of the atoms coupled to a classical dynamics of the spin system. We show that the presence of vacancies lowers the Curie temperature of the system. This result is in qualitative agreement with studies of Ising systems with random non-magnetic defects. However, magnetic moments are increased in the vicinity of vacancies. When this effect is included in our simulation using a core-bulk model, the vacancy effects are reduced, albeit without reaching the values of an ideal lattice.
Effective Work Functions of the Elements: Database, Most probable value, Previously recommended value, Polycrystalline thermionic contrast, Change at critical temperature, Anisotropic dependence sequence, Particle size dependence
2022, Progress in Surface ScienceAs a much-enriched supplement to the previous review paper entitled the “Effective work functions for ionic and electronic emissions from mono- and polycrystalline surfaces” [Prog. Surf. Sci. 83 (2008) 1–165], the present monograph summarizes a comprehensive and up-to-date database in Table 1, which includes more than ten thousands of experimental and theoretical data accumulated mainly during the last half century on the work functions (, and ) effective for positive-ionic, electronic and negative-ionic emissions from mono- and polycrystalline surfaces of 88 kinds of chemical elements (1H–99Es), and also which includes the main experimental condition and method employed for each sample specimen (bulk or film) together with 490 footnotes. From the above database originating from 4461 references published to date in the fields of both physics and chemistry, the most probable values of , and for substantially clean surfaces are statistically estimated for about 600 surface species of mono- and polycrystals. The values recommended for together with and in Table 2 are much more abundant in both surface species and data amount, and also they may be more reliable and convenient than those in popular handbooks and reviews consulted widely still today by great many workers, because the latter is based on less-plentiful data on published generally before 1980 and also because it covers no value recommended for and . Consequently, Table 1 may be more advantageous as the latest and most abundant database on work functions (especially ) for quickly referring to a variety of data obtained under specified conditions. Comparison of the most probable values of recommended for each surface species between this article and other literatures listed in Tables 2 and 3 indicates that consideration of the recent work function data accumulated particularly during the last 40 years is very important for correct analysis of these surface phenomena or processes concerned with either work function or its changes. On the basis of our simple model about the work function of polycrystal consisting of a number of patchy faces (1–i) having each a fractional area (F) and a local work function (), its values of both and are theoretically calculated and also critically compared with a plenty of experimental data. In addition, the “polycrystalline thermionic work function contrast” () well-known as the thermionic peculiarity inherent in every polycrystal is carefully analyzed as a function of the degree of monocrystallization () corresponding to the largest (F) among F’s (Tables 4–6 and Fig. 1), thereby yielding the conclusions as follows: (1) const (>0) holds for the generally called “polycrystalline” surfaces (usually < 50%), (2) ranges from 0.3 eV (Pt) to 0.7 eV (Nb) depending upon the polycrystalline surface species, (3) in the case of the “submonocrystal” (50 < < 100%) tentatively named here, decreases parabolically down to zero as increases from 50% up to 100% (monocrystal), (4) applies to a clean and smooth monocrystalline surface ( 100%) alone, (5) regarding negative ion emission, on the other hand, our theoretical prediction of is experimentally verified to hold for any surface species under any surface conditions (Table 7), (6) every polycrystal (usually, < 50%) may be concluded in general to have a unique value of characteristic of its species with little dependence upon , (7) this conclusion affords us first a sound basis for supporting theoretically the experimental fact (Table 2) that every species of polycrystal has a nearly constant value of as well as (usually within the uncertainty of 0.1 eV) depending little upon the difference in the surface components (F and ) among specimens so long as < 50%, (8) on the contrary to polycrystal ( < 50%), any submonocrystal (50 < < 100%) has such an anomaly that it does not possess the unique value of work function characteristic of the surface species itself, because its as well as changes considerably depending upon , (9) consequently, submonocrystal must be taken as another type (category) different from both poly- and monocrystals, (10) in this way, acts as the key factor mainly governing the work functions in the different mode between poly- and submonocrystals with lower and higher than the “critical point” of 50%, respectively, (11) on the contrary to , belonging to has a differential effect on both and , but their values remain nearly constant so long as < 50% and, thus interestingly, (12) the complicate governance of and by both and and also the anomaly of submonocrystal (cf. (8) above) observed first by our theoretical analysis may be considered as a new contribution to the work function studies developed to date. Together with brief comments and experimental conditions, typical data on and/or are summarized from the various aspects of (1) examination of the work function dependence upon the surface atom density of low-Miller-index monocrystals of typical metals such as Al, Ni, W and Re (Table 8), (2) demonstration of the above dependence usually called the “anisotropic work function dependence sequences” of both (110) > (100) > (111) and (110) > (100) > (111) for various bcc-metals (e.g., Nb, Mo, Ta and W) exactly obeying the Smoluchowski rule (Table 9), (3) substantiation of both (111) > (100) > (110) for a variety of fcc-metals (except Al and Pb) and (111) > (100) > (110) for Ni strictly following the above rule (Table 10), (4) verification of the quantitative relations between work function and surface energy and also melting point of the three low index planes of several metals (typically, Ni), (5) examination of the work function change () due to allotropic transformation from to or to phase (Table 11) together with a concise outline of the Burgers orientation relationship, (6) evaluation of due to liquefying (Table 12), (7) estimation of due to transformation from ferro- to paramagnetic state (Table 13) in addition to a brief description of the Curie point dependence upon metastable metal film thickness above one monolayer, (8) estimation of due to transition from normal to superconducting state (Table 14), (9) study of the work function dependence on the Wigner–Seitz radius and also comparison between its theoretical values (by Kohn) and experimental data (Fig. 2), (10) inspection of the annealing effect on work function for layers or films, (11) verification of the coincidence of work function values among different experimental methods, and (12) inquisition of the work function dependence upon the size of fine particles (20–100 Å in radius) studied by theory and experiment.
Surface termination and thickness dependent magnetic coupling of Cr adlayers on Ni<inf>2</inf>MnGa(001) surfaces: An ab initio study
2021, Journal of Magnetism and Magnetic MaterialsCr adlayers on magnetic and non-magnetic substrates have attracted attention in the recent years due to the interesting magnetic properties of these systems. In this work, we explore the magnetic properties of Cr adlayers on the (001) surface of a ferromagnetic Heusler alloy, namely Ni2MnGa, using density functional theory based electronic structure calculations. The magnitudes and orientation of the magnetic moments of the Cr adlayers on both the Mn-Ga and Ni terminated Ni2MnGa (001) surfaces have been investigated and we find that the magnetic ordering is sensitive to the substrate atoms and the adlayer thickness as well. For one monolayer of Cr adlayer on Mn-Ga (Ni) surface, the Cr atoms are ferromagnetically (anti-ferromagnetically) coupled within the layer. However, for two monolayers of Cr, the Cr adatoms are ferromagnetically coupled, irrespective of the surface termination. The ferromagnetic ordering of the Cr atoms has been explained from the Stoner criterion. The origin of different magnetic orderings of the Cr adatoms with the substrate has been understood by analyzing the Heisenberg exchange coupling parameters between the adatoms and the substrate atoms beneath the adlayer. Further, to obtain a chemical understanding of the observed magnetic behavior, we have employed the crystal orbital hamilton population bonding analyses. The results corroborate well with the results of Heisenberg exchange coupling parameters as well as spin-polarized atom-derived density of states.
Chemisorption mechanism of defluorinated fluorine on bcc Fe surface during formation of PTFE transfer film
2021, Applied Surface ScienceSince defluorination is most likely to occur during the formation of polytetrafluoroethylene (PTFE) transfer film, understanding the adsorption mechanism of defluorinated fluorine can help describe the interfacial tribochemical reaction. XPS analysis showed that the defluorinated fluorine was chemisorbed on the Fe surface to generate iron fluoride. Density functional theory (DFT) calculations revealed that the adsorption of fluorine on the Fe (1 0 0) surface at the bridge site with 0.25 monolayer coverage exhibited the lowest adsorption energy (−4.71 eV), indicating a preferable adsorption configuration. At an identical adsorption site, the adsorption strength decreases with increasing coverage, which is attributed to the repulsive interaction of adsorbed fluorine. The chemisorption of fluorine on the Fe surface induces a considerable redistribution of charges, i.e. electrons are transferred from the Fe surface and accumulate around the adsorbed fluorine. The adsorbed fluorine mainly interacted with the topmost Fe layer. The two Fe atoms located at the bridge sites of the topmost layer play significant roles in the formation of Fe–F bonds that have high degrees of ionic character. The electronic states of Fe–F bond are derived from the hybridisation of Fe 4s, Fe 4p, Fe 3d, F 2s, and F 2p orbitals.
Ab-initio investigation of the effect of adsorbed salts on segregation of Cr at the Ni (100) surface
2021, Applied Surface ScienceCitation Excerpt :Auger electron spectroscopy (AES) experiments carried out under a vacuum atmosphere confirm that Ni should segregate to the surface [16]. The preference for Ni segregation in the first layer is generally attributed to the average relative surface energies of Cr and Ni [17]; that of Cr being higher implies less surface stability. Other more complicated factors also contribute to this oscillatory segregation behavior.
The dissolution and depletion of chromium (Cr) in salt facing nickel (Ni) alloy surfaces is one of the predominant degradation mechanisms of structural components in molten salt technology. In this work, we use density functional theory to investigate the role of electronic level interactions that may underlie the depletion phenomenon of Cr in a Ni surface exposed to various adsorbed salt species. Our results show that, under vacuum, Ni preferentially segregates to the surface layer. Conversely, in the presence of adsorbed anionic salt species (e.g., chlorine (Cl), fluorine (F) or the impurity oxygen (O)) Cr segregation becomes more favorable. In these cases, Cl has the weakest effect on segregation, while O has the strongest effect. Our analysis reveals the strong correlation between Cr segregation and the amount of valence charge transferred between the Cr atom and surface adsorbate: the greater the charge transfer, the lower the segregation energy. We also show that, when considered, secondary cations screen Cr-anion interactions, which in turn reduce the magnitude of the anions effect on segregation. These results shed light on the role of salt impurities likely play in the overall corrosion phenomena in molten salt environments. This work provides insights into the atomic level interactions fundamental to molten salt corrosion and on the importance of maintaining salt purity.