Ferromagnetic Bulk Research Articles

From Magnets to Metals: The Response of Tetragonal Bisdiselenazolyl Radicals to Pressure

The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).

Ferromagnetism Induced by Vacancies in Bulk and the (1010) Surfaces of ZnO: Density Functional Theory Calculations

We studied the vacancy-induced ferromagnetism in bulk and at the (1010) surface of wurtzite ZnO using density functional theory calculations. It is found out that surface vacancies are energetically favored to bulk vacancies and that the ferromagnetism is induced by Zn vacancies but not by O vacancies. Moreover, local magnetic moments induced at O atoms adjacent to the Zn vacancy are calculated to be larger on the surface than in bulk. These findings indicate that Zn vacancies at surfaces are responsible for the experimentally observed room-temperature ferromagnetism in undoped ZnO thin films and nanoparticles.

ChemInform Abstract: Electron Microscopy and the Study of Microstructure and Domain Structure of Magnetic Materials

Abstract Five different electron optical techniques to characterize magnetic domains and domain walls in ferromagnetic materials are reviewed. It is shown that one can choose one or more of these techniques to solve a given problem depending on the nature of the information needed, the resolution level desired and the sophistication of the instrument available. Lorentz microscopy in a transmission electron microscope is a clear choice for most problems although spin-polarized scanning electron microscopy or magnetic force microscopy are better suited to non-destructive surface studies, and differential phase contrast microscopy is preferable for quantitative study of domains in thin specimens. Electron holography is capable of providing contour maps corresponding to lines of force distribution as well as quantitative data on flux distribution. In the near future, it should become possible to combine a few of these techniques with computer simulation to obtain three-dimensional domain wall distributions in bulk ferromagnets.

Longitudinal polarization analysis in small-angle neutron scattering

Due to recent progress in the development of 3He spin filters, it has only now become possible to perform routinely longitudinal (one-dimensional) neutron-spin analysis (POLARIS) in small-angle neutron scattering (SANS) experiments. It is the purpose of this article to provide a brief introduction into the technique and to discuss first experimental data. In particular, for the most common scattering geometry where the applied magnetic (guide) field is perpendicular to the incident neutron beam, we write down the equations for the non-spin-flip and spin-flip SANS cross sections of a bulk ferromagnet, and we discuss the various angular anisotropies and asymmetries along with some selected experimental results on an FeCr based soft magnetic nanocrystalline alloy. In particular, we show that the analysis of the spin-flip data allows one to obtain the magnitude-squares of the three vector (Fourier) components of the magnetization.

Tuning of Gadolinium Based Compounds for Potential Application in Self-Controlled Hyperthermia Treatment of Cancer

We report on the synthesis and investigation of magnetic properties of several new Gd based alloys suitable for their potential use in self-controlled hyperthermia treatment of cancer. Self-controlled hyperthermia is a non-invasive technique that employs heating of magnetic materials (ferromagnetic bulk sized thermo-seeds or magnetic nanoparticles) in a. c. fields to cure cancer cells. Magnetic materials with Curie temperature (Tc) in the range of 40-46oC are desired, as decreased magnetic coupling in paramagnetic regime (above Tc) provides a safeguard against overheating of normal cells. The need for developing such materials was dictated by the lack of existing magnetic materials with magnetic ordering temperature in the suitable range of hyperthermia applications. This study shows that these materials have high magnetization values and their Tc values can be varied linearly over a broad range by adjusting the composition of the constituent elements.

57Fe implantation effect of Sb doped SnO2 films

We implanted 57Fe with 5x1016 ions/cm2 into SnO2 films containing 0.1% Sb and 3% Sb at the substrate temperature of 500°C in vacuum. As a result, four kinds of subspectra were observed in the depth selective CEM spectra by using a back scattered type of gas counter. Two doublets are assigned to paramagnetic Fe3+ Fe2+ species and two sextets are assigned to site A and site B of magnetite. Magnetite formation was found to prefer the top layer, whereas paramagnetic Fe2+ species were included in the deeper layer of the films. SnO2 (with 0.1% and 3% Sb) films doped with 5x1016 57Fe ions/cm2 at 500°C and post-annealed at 400°C for 6 h showed bulk ferromagnetism at room temperature although the 57Fe implanted SnO2 (3% Sb) film showed smaller Kerr effect than the 57Fe implanted SnO2 (0.1% Sb) film. This phenomenon was attributed mainly to the amount of magnetite in the as prepared samples and to the maghemite in the post-annealed samples, respectively, in addition to magnetic defects.

A Convergence Test of the Full-potential Linearized Augmented Plane Wave (FLAPW) Method: Ferromagnetic Bulk BCC Fe

The convergence behavior of the all-electron full-potential linearized augmented plane-wave (FLAPW) method with the explicit orthogonalization (XO) scheme is tested on ferromagnetic bulk body-centered-cubic Fe. Applying a commonly used criterion relating the plane-wave and angular momentum cutos, l max = R MT K max , where R MT is the mun-tin (MT) sphere radius and K max is the plane-wave cuto for the basis—the total energy is converged and stable for K max R MT = 10. The total energy convergence dependence on the star function cuto, G max , is minimal and so a G max of 3K max or a large enough G max is a reasonable choice. We demonstrate that the convergence with respect to l max or a fixed large enough G max and K max are independent, and that K max provides a better measure of the convergence than R MT K max . The dependence of the total energy on R MT is shown to be small if the core states are treated equivalently, and that the XO scheme is able to treat systems with significantly smaller R MT than the standard LAPW method. For converged systems, the calculated lattice parameter, bulk modulus, and magnetic moments are in excellent agreement with the experimental values.

Domain branching in uniaxial ferromagnets: asymptotic behavior of the energy

In this article we analyze the ground state energy of a ferromagnetic bulk sample with strong uniaxial anisotropy in a regime featuring domain branching. We derive the convergence of the micromagnetic energy towards a sharp interface energy in the spirit of Γ-limits. Then we use this convergence to rigorously justify the notion of a minimal energy per cross-section area. Compared to known results, the scaling bounds for the minimal energy are improved to an asymptotic equality.

Room Temperature Ferromagnetism in Dilute Iron-Doped Yttrium Aluminum Garnet Polycrystals

Powder samples of Y3Al5−xFexO12 and Y3(YAl4−x)BFexO12 garnets with iron concentrations between 0.02 ≤ x ≤ 0.4 were investigated by means of magnetization measurements and Mossbauer spectroscopy. The results show that paramagnetism, antiferromagnetism, and ferromagnetism coexist in all samples at room temperature. The relative area of the ferromagnetic component was found to show a maximum at x = 0.04 and to diminish with raising the level of iron doping to x ≥ 0.1. Postannealing the samples in air did not result in a significant alteration of the Mossbauer spectra, but it decreased the bulk ferromagnetism considerably due to elimination of oxygen vacancies. We propose that the induced ferromagnetism at low iron doping rates can be discussed in the frame of the dilute ferromagnetism model for insulators, and so iron-doped yttrium aluminum garnets may represent one of novel room temperature dilute ferromagnetic materials.

Synthesis, Structure, and Magnetic Properties of Na3LnMn3O3(AsO4)3(Ln = La, Sm, and Gd): A New 3d-4f Series Exhibiting One-Dimensional Manganese(III) Oxide Chains Connected via LnO9Units

In attempts to synthesize compounds containing mixed lanthanide and transition metal oxide magnetic nanostructures, a new 3d-4f arsenate phase, Na(3)GdMn(3)O(3)(AsO(4))(3), was isolated using high-temperature, molten-salt methods. The X-ray single-crystal structure analysis shows that Na(3)GdMn(3)O(3)(AsO(4))(3) crystallizes in a hexagonal space group with a = 11.091(2) A, c = 5.954(1) A, and V = 634.2(2) A(3); P6(3)/m (No.176); Z = 2. The structure has been refined by full-matrix, least-squares methods to a final solution with R1 = 0.0466, wR2 = 0.1318 and GOF = 1.101. This new three-dimensional framework consists of ferromagnetic one-dimensional [MnO(4)](infinity) chains, composed of edge-shared MnO(6) units propagating along c. The GdO(9) polyhedra connect three neighboring [MnO(4)](infinity) chains via oxygen atoms. Na(3)LaMn(3)O(3)(AsO(4))(3) (1), and Na(3)SmMn(3)O(3)(AsO(4))(3) (2), the isostructural derivatives of Na(3)GdMn(3)O(3)(AsO(4))(3) (3), were synthesized to acquire additional insight into the 3d-4f magnetic couplings. The Na(3)LnMn(3)O(3)(AsO(4))(3) series reveals an increase in chiT at low temperatures upon substitution of smaller Ln(3+) ions ranging from La(3+) to Gd(3+). The increase in chiT is likely due to the magnetic nature and size of the lanthanide ions present. It was expected that the Mn-O-Mn ferromagnetic interactions would dominate as 3d-4f magnetic interactions are generally weak. The magnetic data shows that the paramagnetic Ln(3+) ions in 2 (Sm) and 3 (Gd) do significantly enhance the bulk ferromagnetism compared to that of 1 (La), suggesting sizable contributions from the 4f magnetic ions.

Evolution of the electronic structure of a ferromagnetic metal: Case ofSrRuO3

Using density-functional calculations we have examined the evolution of the electronic structure of ${\text{SrRuO}}_{3}$ films grown on ${\text{SrTiO}}_{3}$ substrates as a function of film thickness. At the ultrathin limit of two monolayers (${\text{RuO}}_{2}$-terminated surface) the films are found to be at the brink of a spin-state transition which drives the system to an antiferromagnetic and insulating state. Increasing the film thickness to four monolayers, one finds the surprising result that two entirely different solutions coexist. An antiferromagnetic insulating solution coexists with a metallic solution corresponding to an antiferromagnetic surface and a ferromagnetic bulk. The electronic structure found at the ultrathin limit persists for thicker films and an unusual result is predicted. Thicker films are found to be metallic as expected for the bulk but the magnetism does not directly evolve to the bulk ferromagnetic state. The surface remains antiferromagnetic while the bulk exhibits ferromagnetic ordering.

Bulk and surface sp half-metallic ferromagnetism in alkali metal pnictides with rocksaltstructure: a first-principles calculation

Recently, sp half-metallic ferromagnetism in alkali metal/alkaline earth pnictides(I/II–V) with zinc blende structure was predicted by first-principles calculations(Sieberer et al 2006 Phys. Rev. B 73 024404). To search for half-metallicferromagnets with a more stable structure in alkali metal pnictides MX (M = Li, Na,K; X = N, P, As), we apply here first-principles all-electron and pseudopotential-method-baseddensity functional theory to investigate the electronic and magnetic propertiesof these compounds with rocksalt structure. Five rocksalt compounds arefound to be half-metallic ferromagnets with a magnetic moment of 2.00μB/f.u., and they are more stable energetically than the ones with zinc blende structure, whichmakes them more promising candidates for the possible growth of half-metallic films orultrathin films on suitable semiconductor substrates. Furthermore, we show that bothlithium- and nitrogen-terminated (111) surfaces of rocksalt lithium nitride retain the bulkhalf-metallic characteristic, and the surface atomic magnetic moments are enhancedcompared to the bulk values.

Magnetism inNb1−yFe2+y: Composition and magnetic field dependence

We present a systematic study of transport and thermodynamic properties of the Laves phase system ${\text{Nb}}_{1\ensuremath{-}y}{\text{Fe}}_{2+y}$. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for $|y|\ensuremath{\ge}0.02$), show bulk ferromagnetism at low temperature. For stoichiometric ${\text{NbFe}}_{2}$, on the other hand, magnetization, magnetic susceptibility, and magnetoresistance results point toward spin-density wave (SDW) order, possibly helical, with a small ordering wave vector $Q\ensuremath{\sim}0.05\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$. Our results suggest that on approaching the stoichiometric composition from the iron-rich side, ferromagnetism changes into long-wavelength SDW order. In this scenario, $Q$ changes continuously from 0 to small, finite values at a Lifshitz point in the phase diagram, which is located near $y=+0.02$. Further reducing the Fe content suppresses the SDW transition temperature, which extrapolates to zero at $y\ensuremath{\approx}\ensuremath{-}0.015$. Around this Fe content magnetic fluctuations dominate the temperature dependence of the resistivity and of the heat capacity which deviate from their conventional Fermi-liquid forms, inferring the presence of a quantum critical point. Because the critical point is located between the SDW phase associated with stoichiometric ${\text{NbFe}}_{2}$ and the ferromagnetic order which reemerges for very Nb-rich ${\text{NbFe}}_{2}$, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.

Behaviour of Thin Magnetic Films at Low Temperatures

The problem of the analysis of low temperature magnetization is exposed in detail. The application of exact Bose representation of spin operators to the bulk ferromagnets is justified. By means of this representation the magnetization of ultrathin films was analysed. It turned out that thin films magnetization contains exponentially small terms characterized by Dyson’s functions even in harmonical approximation. The main conclusions of this work are that magnetic lattice of thin film is more rigid than the macroscopic lattice and that the autoreduction process (the three layer film divides onto three two layer subfilms) takes place in the film.

Exotic spin textures show up in diverse materials

A binary semiconductor, an insulating alloy, and a bulk ferromagnet can each be coaxed into manifesting new and different forms of spin coherence.

The Kondo effect in ferromagnetic atomic contacts

Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano-Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

Spin-resolved photoemission of a ferromagnetic Mn5Ge3(0001) epilayer on Ge(111)

Here we present a study of the electronic structure of epitaxial ferromagnetic Mn5Ge3(0001) films on Ge(111) by means of x-ray absorption spectroscopy, x-ray photoelectron spectroscopy, and spin-resolved photoelectron spectroscopy. Spin-polarization (P) value of +(15±5)% at the Fermi energy (EF) is measured with a photon energy of hν=21.2 eV. Our findings are in contrast to recent band structure calculations, predicting P=−41% at EF for the ferromagnetic bulk Mn5Ge3.

Surface phase transitions of the transverse Ising model with modified surface–bulk coupling

The phase diagram of the three-dimensional transverse Ising system with antiferromagnetic intrasurface J S and surface–bulk J ⊥ interactions modified with respect to the ferromagnetic bulk exchange interaction J B is studied by the use of an effective field method in the framework of a single-site cluster theory. In this investigation, the state equations are derived by means of differential operator technique. Using these coupled equations within the four-layer approximation, it has been shown that the new parameter J ⊥ strongly affects the phase diagram obtained recently in the case J ⊥ = J B. Indeed, our calculations reveal some qualitatively interesting features both in the absence and in the presence of the surface and bulk transverse fields.

Thermospin effects in a quantum dot connected to ferromagnetic leads

We study a system composed of a quantum dot in contact with ferromagnetic leads held at different temperatures. Spin analogs to the thermopower and thermoelectric figure of merit are defined and studied as a function of junction parameters. It is shown that in contrast to bulk ferromagnets, the spin-thermopower coefficient in a junction can be as large as the Seebeck coefficient, resulting in a large spin figure of merit. In addition, it is demonstrated that the junction can be tuned to supply only spin current but no charge current. We also discuss experimental systems where our predictions can be verified.

Spatial Dependence of Magnetization and Gaussian Fluctuations of Molecular Field in an Ising Nanopyramid Interacting with the Substrate

We study thermodynamic properties of an Ising model of a ferromagnetic nanoscopic pyramid deposited on a ferromagnetic bulk substrate. The influence of the interaction between the pyramid and the substrate is calculated in terms of the reduced-state (density) operator used for description of thermodynamic properties of nanoscopic systems. The spatial distribution of magnetization in the nanopyramid is obtained in the Gaussian fluctuations approximation.