Stoner Theory Research Articles

Relation between Magnetism and Electronic Structure of 3d-Metal Alloys in the Stoner Theory and in the DSFT

Relation between Magnetism and Electronic Structure of 3d-Metal Alloys in the Stoner Theory and in the DSFT

Spin asymmetry originating from densities of states: Criterion for ferromagnetism, structures and magnetic properties of 3d metals from crystal field based DOSs

Ferromagnetism in 3d metals is re-examined in a simple band model. It is shown that the molecular field model cannot account for the low values of coercive field in ferromagnetic pure metals, and that the standard Stoner theory of band ferromagnetism incorrectly evaluates the total electronic energy and cannot predict reasonable Curie temperatures. A simple band model for magnetism is formulated, yielding a criterion for ferromagnetism even in absence of a Hubbard term, involving only the density of states (DOS) at the Fermi level, its derivative and the filling of the 3d band. By introducing a double-peaked DOS, one may explain the occurrence of ferromagnetism in bcc Fe, hcp-fcc Co and fcc Ni, the stabilization of fcc-hcp or bcc structures across all 3d elements, the occurrence of antiferromagnetism in chromium, and derive reasonable Curie temperatures. ‘Re-entrant’ ferromagnetism is predicted at ultrahigh temperatures, suggesting an alternate origin for the geomagnetic field.

Sustainability policy paradox: coping with changing environmental priorities in municipal waste management

ABSTRACTThe impacts of climate change are apparent, but the US federal government continues to deny climate science, deregulate, and obstruct international environmental consensus. Municipal governments are thus, with increasing urgency, modifying infrastructure to achieve a broad range of environmental goals. Much of this action occurs within a sustainability framework. Commonly framed as a balance of economic, environmental, and social interests that ensures the viability of future generations, sustainability is conflictual in practice. As cities attempt bold environmental action, understanding how these conflicts play out is crucial. This study uses Deborah Stone's theory of policy paradox combined with insights from sustainability policy and discards studies literatures to analyze how policy makers respond to competing interests and values in the case of waste management in Washington DC. Though DC policy actors agree about the need for sustainability, stakeholders have clashed repeatedly over the city's reliance on waste-to-energy (WTE) incineration for disposal. I argue that WTE in DC can best be understood as a ‘sustainability policy paradox’: a policy paradox that results not only from predictable tensions between environmental and economic interests, but conflicts between environmental objectives that surface in the contemporary context of technocratic uncertainty, the neoliberal state, and climate change.

Evolution from B_{2g} Nematics to B_{1g} Nematics in Heavily Hole-Doped Iron-Based Superconductors.

Recent experiments reported an unusual nematic behavior of heavily hole-doped pnictides AFe_{2}As_{2}, with alkali A=Rb, Cs. In contrast to the B_{2g} nematic order of the parent AeFe_{2}As_{2} compounds (with alkaline earth Ae=Sr, Ba), characterized by unequal nearest-neighbor Fe-Fe bonds, in the hole-doped systems nematic order is observed in the B_{1g} channel, characterized by unequal next-nearest-neighbor Fe-Fe (diagonal Fe-As-Fe) bonds. In this Letter, using density functional theory, we attribute this behavior to the evolution of the magnetic ground state along the series Ae_{1-x}A_{x}Fe_{2}As_{2}, from single stripes for small x to double stripes for large x. Our simulations using the reduced Stoner theory show that fluctuations of Fe moments are essential for the stability of the double-stripe configuration. We propose that the change in the nature of the magnetic ground state is responsible for the change in the symmetry of the vestigial nematic order that it supports.

EFEKTIVITAS FUNGSI PENGAWASAN INSPEKTORAT TERHADAP KINERJA PEGAWAI ( STUDI KASUS BADAN KEPEGAWAIAN DAN DIKLAT KABUPATEN OGAN KOMERING ILIR TAHUN 2016)

The results of this study indicate that the function of OKI's inspectorate supervision on the performance of BKD OKI which has not been effective. This is due to its non-fulfillment of elements associated with Stoner's theory of Accurate, timely, focused on strategic monitoring points. Where BKD data at the time of collecting documents is incomplete as well as the existence of delays. The data that has not been completed by BKD is LP2P data and SOP. As well as in conducting inspection inspectorate apparatus has not determined the areas of irregularities that often occur, so that the impact is not corrective in performing performance in the BKD. Factors that affect the implementation of the inspectorate supervision of the performance of Personnel and Training Agency OKI Regency is an internal factor, namely as the supervisory apparatus and external factors, that are facilities and infrastructure and some negligence by the object inspectionKeywords : Effectiveness, Supervision, Inspectorate, BKD

Spontaneous distortion via the appearance of ferromagnetism in Pd ultrathin films: Observation of an inverse mechanism for the Stoner criterion

We study the crystal structure of Pd(100) ultrathin films, which show ferromagnetism induced by the quantum confinement effect, using in-situ X-ray crystal truncation rod measurement and density functional calculation. The energy gain due to the appearance of ferromagnetism in Pd results in flatter and uniform film growth of ferromagnetic Pd films compared with paramagnetic Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to suppress the increase in kinetic energy of electrons accompanied by the occurrence of exchange splitting. Although the traditional theory of magnetism in metals indicates that the increase in density of states that induces ferromagnetism (Stoner criterion), our present finding reveals a mechanism of modulation in the density of states via the appearance of ferromagnetism, i.e., the inverse mechanism of Stoner's theory.

Muon contact hyperfine field in metals: A DFT calculation

In positive muon spin rotation and relaxation spectroscopy it is becoming nowadays customary to take advantage of Density Functional Theory (DFT) based computational methods to aid the experimental data analysis. DFT aided muon site determination is especially useful for measurements performed in magnetic materials, where large contact hyperfine interactions may arise. Here we present a systematic analysis of the accuracy of the ab initio estimation of muon's hyperfine contact field on elemental transition metals, performing state of the art spin-polarized plane wave DFT and using the projector augmented pseudopotential approach, which allows to include the core state effects due to the spin ordering. We further validate this method in not-so-simple, non-centrosymmetric metallic compounds, presently of topical interest for their spiral magnetic structure giving rise to skyrmion phases, such as MnSi and MnGe. The calculated hyperfine fields agree with experimental values in all cases, provided the spontaneous spin magnetization of the metal is well reproduced within the approach. To overcome the known limits of the conventional mean field approximation of DFT on itinerant magnets, we adopt the so-called reduced Stoner theory [L. Ortenzi et al.,Phys. Rev. B 86, 064437 (2012)]. We establish the accuracy of the estimated muon contact field in metallic compounds with DFT and our results show improved agreement with experiments compared to those of earlier publications.

Magnetocaloric effect in itinerant magnets around a metamagnetic transition

The phase diagram and magnetocaloric effect in itinerant magnets is explored within the Stoner theory, which yields a reasonable description of the metamagnetic transition observed in various compounds. We obtain the phase diagram as a function of temperature and magnetic field, identifying the region of metastability around the first-order ferromagnetic transition. The impact on the magnetocaloric properties has been verified through the calculation of the isothermal entropy change ΔS, which is computed from two alternative methods based on specific heat or magnetization data. From the direct comparison between the two methods, we observe that the second one is strongly dependent on the process, and we explain under what conditions they become equivalent by using the Clausius-Clapeyron equation. We also discuss the effect of metastable states on the curves of ΔS. The evolution of the transition from first to second order is in good agreement with the phenomenological approach based on the Landau expansion. The results can be applied to different magnetic compounds such as RCo2, MnAs1−xSbx, and La(FexSi1−x)13.

CuO nanosheets/ZnO nanorods synthesized by a template-free hydrothermal approach and their optical and magnetic characteristics

Abstract Nanostructured CuO/ZnO heterojunctions were fabricated via a template-free hydrothermal reaction. The prepared CuO/ZnO nanocomposites are composed of one dimensional (1D) hexagonal ZnO nanorods and two-dimensional (2D) monoclinic CuO nanosheets. The single crystal nature of both ZnO and CuO was confirmed. Optical spectra show the widened absorption range from UV to visible light, which suggests the potential of the obtained CuO/ZnO composites for visible-light-driven photocatalyst. The point defects at the interface play the leading role in triggering ferromagnetism of CuO/ZnO composites. Fundamentally, the ferromagnetism can be understood by the charge-transfer mechanism according to Stoner theory. The findings can give a further insight into the ferromagnetic origin of nonmagnetic composites and the integration of ferromagnetic and photocatalytic properties into an identical sample.

Computer Modeling of Nickel–Iron Alloy in Power Electronics Applications

Rotational magnetizations of an Ni-Fe alloy are simulated using two different computer modeling approaches, physical and phenomenological. The first one is a model defined using a single hysteron operator based on the Stoner and Wohlfarth theory and the second one is a model based on a suitable system of neural networks. The models are identified and validated using experimental data, and, finally, an example of their application for a finite-element analysis is given.

Isotope shift of the ferromagnetic transition temperature in itinerant ferromagnets

We present a theory of the isotope effect of the Curie temperature Tc in itinerant ferromagnets. The isotope effect in ferromagnets occurs via the electron–phonon vertex correction and the effective attractive interaction mediated by the electron–phonon interaction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction, which results in a positive isotope shift of Tc when the mass M of an atom increases. Following this picture, we evaluate the isotope effect of Tc by using the Stoner theory and a spin-fluctuation theory. When Tc is large enough as large as or more than 100 K, the isotope effect on Tc can be measurable. Recently, precise measurements on the oxygen isotope effect on Tc have been performed for itinerant ferromagnet SrRuO3 with Tc∼160 K. A clear isotope effect has been observed with the positive shift of Tc∼1 K by isotope substitution (O16→O18). This experimental result is consistent with our theory.

Correlation effect in Sr1−xLaxRuO3 studied by soft x-ray photoemission spectroscopy

To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.

Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials.

We report a combination of Fe Kβ x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2(As1-xPx)2. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1-xPx)2. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2(As1-xPx)2 (x=0.055) and Ca0.78La0.22Fe2As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

Charge transfer induced magnetism in sol–gel derived nanocrystalline BaTiO3

Nanocrystalline BaTiO3 is prepared by sol–gel synthesis. The charge transfer and different state mixing effects are used in explaining the observed magnetism by the modified Stoner theory. The magnetic coercivity is found to decrease from 125 to 43Oe as temperature increases from 5K to 300K. The optical absorption shows band narrowing arising from exchange–correlation and many-body effects of free carriers created on the surface of nanoparticles, as supported also by the non-adiabatic small polaron conductivity derived from impedance spectroscopy.

Room-temperature ferromagnetism in nanocrystalline Cu/Cu2O core-shell structures prepared by magnetron sputtering

Cu/Cu2O core-shell nanoparticles with diameters around 8–9 nm have been fabricated by magnetron sputtering pure Cu targets with subsequent annealing in oxygen. Room-temperature ferromagnetism (FM) was observed in the samples annealed at 150 °C for 10–120 min. The maximum of saturated magnetization is as high as 19.8 emu/cc. The photoluminescence spectra show solid evidence that the FM originates from Cu vacancies in the Cu2O shell of the Cu/Cu2O core-shell nanoparticles. Furthermore, the FM can be modulated by the amount of Cu vacancies through the Cu/Cu2O core-shell interface engineering. Fundamentally, the FM can be understood by the charge-transfer ferromagnetism model based on Stoner theory.

Magnetism of Mg atomic chains on the NaCl(100) surface

Based on the first-principles calculations within the density-functional theory, we have investigated the magnetism of 1D Mg nanowires, both unsupported and supported by the NaCl(100) surface. It is shown that Mg can exhibit magnetism in both the geometry of linear and zigzag chains. The freestanding Mg linear chain shows magnetization when the bonds are compressed. For an Mg zigzag chain, however, ferromagnetism is predicted at the equilibrium bond length. It is found that the magnetism of Mg chains can be maintained when deposited on the NaCl(100) surface. We have also analyzed the results by using the Stoner theory and the calculated electronic band structures.

Ferromagnetic Instability and Finite-Temperature Properties of Two-Dimensional Electron Systems with van Hove Singularities

We study a ferromagnetic tendency in the two-dimensional Hubbard model near van Hove filling by using a functional renormalization-group method. We compute temperature dependences of magnetic susceptibilities including incommensurate magnetism. The ferromagnetic tendency is found to occur in a dome-shaped region around van Hove filling with an asymmetric property: incommensurate magnetism is favored near the edge of the dome above van Hove filling whereas a first-order-like transition to the ferromagnetic ground state is expected below van Hove filling. The dome-shaped phase diagram is well captured in the Stoner theory by invoking a smaller Coulomb interaction. Triplet p-wave superconductivity tends to develop at low temperatures inside the dome and extends more than the ferromagnetic region above van Hove filling.

Stability of ferromagnetism within the time‐dependent Gutzwiller approximation for the Hubbard model

AbstractWe study the problem of itinerant ferromagnetism (FM) in one‐, two‐ and infinite‐dimensional Hubbard models. Our investigations are based on the time‐dependent Gutzwiller approximation (TDGA) which allows for the derivation of a generalized ‘GA Stoner criterion’ for the instability towards ferromagnetic, but also antiferromagnetic and incommensurate magnetic order. After having established magnetic phase boundaries we investigate the stability of FM by including the effect of transverse fluctuations.We also compute the spin‐wave velocity in the ferromagnetic phase and compare our results with other approaches like HF + RPA Stoner theory. Our investigations reveal that even in the fully polarized ferromagnetic phase the TDGA yields a renormalized excitation spectrum with respect to Stoner theory although the double occupancy vanishes in this case.

On the multi‐orbital band structure and itinerant magnetism of iron‐based superconductors

AbstractThis paper explains the multi‐orbital band structures and itinerant magnetism of the iron‐pnictide and chalcogenide superconductors. We first describe the generic band structure of a single, isolated FeAs layer. Use of its Abelian glide‐mirror group allows us to reduce the primitive cell to one FeAs unit. For the lines and points of high symmetry in the corresponding large, square Brillouin zone, we specify how the one‐electron Hamiltonian factorizes. From density‐functional theory, and for the observed structure of LaOFeAs, we generate the set of eight Fe d and As p localized Wannier functions and their tight‐binding (TB) Hamiltonian, h (k) . For comparison, we generate the set of five Fe d Wannier orbitals. The topology of the bands, i. e. allowed and avoided crossings, specifically the origin of the d6 pseudogap, is discussed, and the role of the As p orbitals and the elongation of the FeAs4 tetrahedron emphasized. We then couple the layers, mainly via interlayer hopping between As pz orbitals, and give the formalism for simple tetragonal and body‐centered tetragonal (bct) stackings. This allows us to explain the material‐specific 3D band structures, in particular the complicated ones of bct BaFe2As2 and CaFe2As2 whose interlayer hoppings are large. Due to the high symmetry, several level inversions take place as functions of kz or pressure, and linear band dispersions (Dirac cones) are found at many places. The underlying symmetry elements are, however, easily broken by phonons or impurities, for instance, so that the Dirac points are not protected. Nor are they pinned to the Fermi level because the Fermi surface has several sheets. From the paramagnetic TB Hamiltonian, we form the band structures for spin spirals with wavevector q by coupling h (k) and h (k + q). The band structure for stripe order is studied in detail as a function of the exchange potential, Δ, or moment, m, using Stoner theory. Gapping of the Fermi surface (FS) for small Δ requires matching of FS dimensions (nesting) and d‐orbital characters. The interplay between pd hybridization and magnetism is discussed using simple 4 × 4 Hamiltonians. The origin of the propeller‐shaped Fermi surface is explained in detail. Finally, we express the magnetic energy as the sum over band‐structure energies and this enables us to understand to what extent the magnetic energies might be described by a Heisenberg Hamiltonian, and to address the much discussed interplay between the magnetic moment and the elongation of the FeAs4 tetrahedron.

First-principles calculations of magnetism of Fe atomic sheet

The electronic and the magnetic properties of Fe single-layered atomic shees separately with two-dimensional square and hexagonal structures are calculated by the first-principles method based on the spin-polarized density functional theory. The calculations show that planar square and hexagonal as well as the bcc structures manifest their magnetisms at their equilibrium lattice constants. The magnetic moments for these structures are 2.65, 2.54 and 2.20μВ, respectively. The calculated magnetic properties for the elongated and the compressed bond lengths suggest that when the bond is stretched to a length larger than 4.40, the bond should be broken and the magnetic moments of the systems reach the magnetic moment of an independent Fe atom, 4μВ. When the bond lengths are reduced, the magnetic moments of all the systems studied decrease correspondingly. At the critical bond lengths (1.80 for planar square lattice, and 1.75 for hexagonal lattice), the magnetisms of the two planar lattices disappear. Using the Stoner theory, the change from magnetism to non-magnetism for the lattice compression is elucidated.