Iron Magnetic Moments Research Articles

Temperature-induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi0.9Sm0.1FeO3 driven by f−d induced magnetic anisotropy

In multiferroic BiFeO$_3$ a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that by substituting samarium for 10% of the bismuth ions the periodicity of the room temperature cycloid is increased, and by cooling below $\sim15$ K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from $f-d$ exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order - a result of general significance regarding the stability of non-collinear magnetic structures in the presence of multiple magnetic sublattices.

Comparative study of structural phase transitions in bulk and powdered Fe–27Ga alloy by real-time neutron thermodiffractometry

Phase transformations in an iron–gallium alloy have been analyzed by in situ real-time neutron diffraction in the temperature range from 293 to 1223 K. Two compositionally identical samples were studied: the first was in the as-cast bulk state, and the second was ground into a powdered state. In both samples, the same sequence of structural transitions was recorded on heating with a constant heating rate (D03 → A2 → L12 → D019 → A2), and the same structural state (D03 + L12) was recorded after slow cooling to room temperature. Owing to strong texture in the bulk sample, only diffraction patterns of the powdered sample were treated with the Rietveld method to determine the volume fractions of the coexisting phases, the coefficients of thermal expansion, and the thermal and static atomic disorder parameters. The occupancy of Ga positions and the ordered iron magnetic moment were refined at selected temperatures. The level of microstrain in the crystallites in the initial as-quenched state is small, but it sharply increases in the course of phase transitions when the alloy is heated. The microstrains are high and strongly anisotropic after slow cooling. Generally, phase transformations occur similarly in the powdered and bulk samples, but with a noticeable difference in details. The fulfilled analysis of the bulk and powdered samples allowed the real possibilities of the quantitative neutron diffraction analyses of phase transitions in ferromagnetic ordered alloys to be assessed.

Structural properties of multiferroic 0.5BiFeO3–0.5Pb(Fe0.5Nb0.5)O3 solid solution

Magnetoelectric multiferroics are very promising materials because of their practical applications and fundamental interests. The most widely studied magnetoelectric oxides are ABO3 perovskites. In the paper structural properties of BiFeO3 and Pb(Fe0.5Nb0.5)O3 solid solution are described. The material crystallizes in rhombohedral R3c crystal structure which parameters are presented. Mössbauer spectroscopy was used to study local changes in an iron environment due to Fe/Nb substitution and hyperfine interaction parameters of different local surroundings of iron atoms are presented. The random distribution of B-site sublattice cations was confirmed. Ab initio calculations of the studied solid solution were conducted and theoretical crystal structure parameters were compared with the experimental data. The theoretical magnetic and electric properties are discussed. The local iron magnetic moments were estimated and their dependence on the local surrounding changes is shown. The calculated electrons densities and Bader's topological analysis were used to describe chemical bonding properties.

Evidence for a collinear easy-plane magnetic structure of multiferroic EuFe3(BO3)4 : Spectroscopic and theoretical studies

We performed high-resolution polarized optical transmission spectroscopy and theoretical studies of multiferroic $\mathrm{EuF}{\mathrm{e}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ single crystals in the three phases: paramagnetic $R32\phantom{\rule{0.28em}{0ex}}(Tg{T}_{\mathrm{s}}=84\phantom{\rule{0.28em}{0ex}}\mathrm{K})$ and $P{3}_{1}21\phantom{\rule{0.28em}{0ex}}({T}_{\mathrm{s}}gTg{T}_{\mathrm{N}}=34\phantom{\rule{0.28em}{0ex}}\mathrm{K})$, and antiferromagnetic $(Tl{T}_{\mathrm{N}})$ ones. The analysis of the spectra reveals interesting details of the magnetic structure, namely, a collinear arrangement of the iron magnetic moments along the ${C}_{2}$ symmetry axis in the $ab$ crystallographic plane of $\mathrm{EuF}{\mathrm{e}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ below ${T}_{\mathrm{N}}$. Spectral signatures of the phase transitions and the spin-phonon interaction are observed and discussed. Reliable crystal-field and exchange-interaction parameters are obtained and used to model the magnetic susceptibility of the compound. The results of detailed calculations of the electric polarization of $\mathrm{EuF}{\mathrm{e}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ in the $R32$ phase are presented, and mechanisms of the magnetoelectric response are discussed. We detect a strong effect of impurities (that enter the crystal from a flux in the course of the crystal growth) on the structural phase-transition temperature and demonstrate a coexistence of both $R32$ and $P{3}_{1}21$ phases down to the lowest temperatures in a $\mathrm{EuF}{\mathrm{e}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ crystal grown with the $\mathrm{B}{\mathrm{i}}_{2}\mathrm{M}{\mathrm{o}}_{3}{\mathrm{O}}_{12}$ based flux, due to inhomogeneous distribution of impurity $\mathrm{B}{\mathrm{i}}^{3+}$ ions. Our study can be considered as a demonstration of the abilities of optical spectroscopy in delivering new information on a magnetic compound, even in the case when other methods fail.

Orientation of the electric field gradient and ellipticity of the magnetic cycloid in multiferroic BiFeO3

The paper deals with the hyperfine interactions observed on the 57Fe nucleus in multiferroic BiFeO3 by means of the 14.41-keV resonant transition in 57Fe and for transmission geometry applied to the random powder sample. Spectra were obtained at 80 K, 190 K and at room temperature. It was found that iron occurs in the high-spin trivalent state. Hyperfine magnetic field follows distribution due to the elliptic-like distortion of the magnetic cycloid. The long axis of the ellipse is oriented along 〈1 1 1〉 direction of the rhombohedral unit cell. The hyperfine magnetic field in this direction is about 1.013 of the field in the perpendicular direction at room temperature. This ratio diminishes to 1.010 at 80 K. Axially symmetric electric field gradient (EFG) on the iron atoms has the principal axis oriented in the same direction and the main component of the EFG is positive. Our results are consistent with the finding that iron magnetic moments are confined to the crystal plane.

Determination of hyperfine fields orientation in nuclear probe techniques

One of the most popular nuclear probes, 57Fe is used for the investigation of orientations of hyperfine fields and also for the determination of other important properties. In particular, the orientation of iron magnetic moments can be unambiguously determined, including its signs. Experiments with polarized radiation are presented with regard to selected systems. Orientation of electric field gradient is used for acquiring information about the shape of the texture-free spectra. Applications on the analysis of iron-based superconductors are presented.

Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2as seen by Raman light scattering and band structure calculations

We report Raman light scattering in the phase separated superconducting single crystal Rb0.77Fe1.61Se2 with Tc = 32 K. The spectra have been measured in a wide temperature range 3K -500K. The observed phonon lines from the majority vacancy ordered Rb2Fe4Se5 (245) antiferromagnetic phase with TN= 525 K demonstrate modest anomalies in frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed Rb{\delta}Fe2Se2 (122) conductive phase. The superconducting gap with dx2-y2 symmetry is also detected in our spectra. In the range 0-600 cm-1 we observed the low intensive but highly polarized B1g-type background which becomes well structured under cooling. The possible magnetic or multiorbital origin of this background has been discussed. We argue that phase separation in M0.8+xFe1.6+ySe2 has pure magnetic origin. It occurs below Neel temperature when iron magnetic moment achieves some critical magnitude. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab-initio spin polarized band structure calculations we demonstrate that compressed vacancy ordered Rb2Fe4Se5 phase can be conductive and therefore may serve as a protective interface spacer between the pure metallic Rb{\delta}Fe2Se2 phase and the insulating Rb2Fe4Se5 phase providing the percolative Josephson-junction like superconductivity in the whole sample of Rb0.8+xFe1.6+ySe2 Our lattice dynamics calculations show significant difference in the phonon spectra of the conductive and insulating Rb2Fe4.Se5 phases.

Annealed FINEMET ribbons: Structure and magnetic anisotropy as revealed by the high velocity resolution Mössbauer spectroscopy

The high velocity resolution 57Fe Mössbauer spectroscopy was used in order to elucidate structural and compositional details of FINEMET (Fe73.5Si15.5Nb3B7Cu1) alloys obtained via the annealing (with and without external magnetic field) of rapidly quenched ribbons. The analysis of the measured Mössbauer spectra was carried out, on one hand, by considering the possibility of a random distribution of iron atoms substituting Si at the D sites in the well crystallized DO3 Fe-Si phase, on the other hand, by allowing for an arbitrary-shape hyperfine magnetic field distribution for the case of the amorphous matrix. The results refer to the influence of the next-nearest-neighbor configurations on the magnitude of iron magnetic moments at the D sites in the precipitated nanocrystalline Fe-Si phase. The applied analysis method enables us to draw conclusions regarding the relative occurrence of the various iron microenvironments in the nanocrystalline phase and amorphous matrix, and the associated Si concentration of the precipitated nanocrystalline DO3 Fe-Si phase. The studied samples provide further evidence concerning the correlation between the induced magnetic anisotropy and the magnetic permeability in annealed FINEMET ribbons.

Magnetic effects induced by self-ion irradiation of Fe films

Iron magnetic moment enhancement is observed following the irradiation of iron films with 490 keV Fe$^+$ at room temperature. The iron magnetic moment enhancement increases to saturation with irradiation dose. Theenhanced magnetic moment decays exponentially to its value before the irradiation with a time constant of 5.2 months. The iron magnetic moment enhancement is attributed to the creation of vacancy clusters having a concentration of about 20 %, whereas the relaxation effects is attributed to the dissociation of these clusters.

Synthesis of nanocrystalline Ni3Fe powder by mechanical alloying using an extreme friction mode

Extreme conditions for ball milling were created in order to have a complete friction mechanical milling/alloying mode: the vials were fulfilled with milling balls having almost 100% filling degree and so the shock collisions were avoided and the powder were processed only by mechanical energy transferred by friction. Nanocrystalline Ni3Fe powder has been obtained from elemental powders using the above mentioned specific conditions. The influence of friction mode on the formation of Ni3Fe intermetallic compound was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), particles size analysis and magnetic measurement. The Ni3Fe intermetallic compound was completely formed after 50h of milling in this extreme friction milling mode and it is in nanocrystalline state with a mean crystallites size of 8nm. The spontaneous magnetization increases by progressive formation of the Ni3Fe intermetallic compound, due to the iron magnetic moment enhancing by the Ni atoms vicinity.

Magnetism in the YxGd1−xFe3 system

The series of polycrystalline YxGd1−xFe3 compounds with a PuNi3-type of crystal structure have been obtained. Based on wide-ranging SQUID magnetometer series of different magnetic measurements have been carried out. Moreover, the magnetic properties in the paramagnetic range has been studied by means of Faraday type magnetic balance. The partial replacement of Gd by Y atoms is reflected in the increase within the total saturation magnetic moment MS from 1.61μB/f.u (x=0.0) to 5.32μB/f.u (x=1.0) and the decrease of the Curie temperature TC from 721K (x=0.0) to 533K (x=1.0). Exchange coupling parameters of R–R (ARR), T–T (ATT) and R–T (ART) have been evaluated from M(T) magnetization curves based on the mean field theory (MFT) calculation. The valence band spectra as well as the core level lines obtained by the use of X-ray photoemission spectroscopy (XPS) at room temperature have been analyzed as the influence of Gd/Y substitution on the electronic structure. The valence bands near the Fermi level are dominated by Fe3d states. A quite good agreement between the values of the iron magnetic moment estimated from M(H) curves, from MFT calculations as well as from fitting of Fe3s core level lines has been obtained.

A new approach in the synthesis of La1−xGdxFeO3perovskite nanoparticles – structural and magnetic characterization

A series of highly crystalline orthoferrite nanoparticles (type La(1-x)Gd(x)FeO3, where x = 0 to 1) were prepared using the self-combustion method. Extensive studies including X-ray diffraction, Rietveld refinement and Fourier transform infrared spectroscopy as well as Raman spectroscopy confirmed the orthorhombic space group Pnma of the obtained materials. The calculated average grain size for powders is in the range of 30 to 80 nm. Magnetic characterization of the La(1-x)Gd(x)FeO3 series, performed at 1.72 K, indicated an antiferromagnetic state characterized by some canting of iron magnetic moments, in good agreement with the data reported for similar fine-particle systems.

Magnetic anisotropy of van der Waals absorbed iron(II) phthalocyanine layer onBi2Te3

A self-assembled iron(II) phthalocyanine single layer adsorbed on the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ was investigated by spin-polarized scanning tunneling microscopy and density functional theory calculations. Although the molecule-substrate interaction is dominated by a relatively weak van der Waals force, the local density of states of Fe was found to strongly depend on the adsorption sites, resulting in a supermolecular lattice. Spin-polarized measurements show that the magnetic moment of iron(II) phthalocyanine persists with an in-plane magnetic easy axis, which was further confirmed by density functional calculations.

Enhancement of magnetocaloric effect in B-rich FeZrBCu amorphous alloys

Magnetic properties and magnetocaloric effect were studied in the Fe92−xZr7BxCu1 amorphous alloy series (x=0–23at.%). Enhancement of the magnetocaloric effect was observed in the B-rich side (x>15at.%) which correlates well with the anomalous increase of the saturation magnetic moment per Fe atom in this concentration range. Research into Fe2(B1−yETMy) amorphous alloys (ETM=Zr, Nb, Ti and V; y=0–0.55 atomic fraction for Zr and to 0.25 atomic fraction for the rest) reveals an unexpected increase of the iron magnetic moment when early transition metals are substituted for B up to y=0.1–0.25 atomic fraction. This behavior is attributed to the highly attractive B–ETM interaction. Similar mechanism is thought to explain the anomalous increase of the iron magnetic moment and hence the magnetocaloric effect in the boron-rich Fe1−xZr7BxCu1 amorphous alloys. Universal scaling of the magnetic entropy change curves is also used to detect differences between Fe92−xZr7BxCu1 alloys.

Analysis of the structure and magnetic properties of an interface in multilayered (Fe/Si) N nanostructures with the surface-sensitive XMCD method

The structural and magnetic properties of (Fe/Si)N nanostructures obtained by successive deposition on the SiO2/Si(100) surface at a temperature of the substrate of 300 K have been studied. The thicknesses of all Fe and Si layers have been determined by transmission electron microscopy measurements. The magnetic properties have been studied by the X-ray magnetic circular dichroism (XMCD) method near the Fe L3, 2 absorption edges. The orbital (ml) and spin (mS) contributions to the total magnetic moment of iron have been separated. The thicknesses of magnetic and nonmagnetic iron silicide on the Si/Fe and Fe/Si interfaces have been determined with the surface sensitivity of the XMCD method and the model of the interface between the nonmagnetic and weakened magnetic phases.

Negative magnetoresistance and spin filtering of spin-coupled di-iron-oxo clusters

Spin dependent transport has been investigated for an {\it open shell singlet} diiron-oxo cluster. Currents and magnetoresistances have been studied, as a function of spin state, within the non-equilibrium Green's function approach. The applied bias can be used for tuning the sign of the observed magnetoresistance. A colossal magnetoresistance ratio has been determined, on the order of to 6000$%$, for hydrogen anchoring. Applied biases lower than 0.3 V, in conjunction with sulfur anchoring, induce a negative magnetoresistance due to lowering of the anchor-scatterer tunneling barrier. In addition, the diiron-oxo cluster displays nearly perfect spin filtering for parallel alignment of the iron magnetic moments due to energetic proximity, relative to the Fermi level, of its highest occupied molecular orbitals.

Impact of Fe/NaCl (0 0 1) interface structure on electronic, magnetic and spin-polarized transport properties of Fe/NaCl/Fe (0 0 1) heterojunctions: An ab initio study

Structural, electronic and spin-dependent transport properties of Fe/NaCl/Fe (001) tunnel junctions with c(2×2) structure are studied by means of first-principles calculations. Several Fe/NaCl (001) interfaces are considered and their stability is analyzed. The interfacial charge transfer has an important role in stabilizing Fe/NaCl (001) interfaces. Interfacial iron magnetic moments are enhanced over the bulk value. Fe induced gap states are observed in barrier layers on both Na and Cl ions. Small exchange couplings with exponential decays as function of the barriers thickness are evidenced. Resonant tunnelling plays a major role on the spin-polarized transport properties. High magnetoresistance ratios comparable to Fe/MgO/Fe junctions are predicted. Total energy calculations show that interfacial interdiffusion is not favourable but the disorder at interfaces may strongly affect the electronic transport properties. Absence of interfacial interdiffusion together with the small lattice mismatch between Fe and NaCl layers are important advantages of Fe/NaCl/Fe (001) junctions for the purpose of spintronic applications.

Colloquium: The unexpected properties of alkali metal iron selenide superconductors

The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi Surface nesting of hole and electron pockets when in the presence of weak Hubbard $U$ interactions. This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets. However, recent results in the context of alkali metal iron selenides, with generic chemical composition $A_x$Fe$_{2-y}$Se$_2$ ($A$ = alkali element), have drastically challenged those previous ideas since at particular compositions $y$ the low-temperature ground states are insulating and display antiferromagnetic magnetic order with large iron magnetic moments. Moreover, angle resolved photoemission studies have revealed the absence of hole pockets at the Fermi level in these materials. The present status of this exciting area of research, with the potential to alter conceptually our understanding of the iron-based superconductors, is here reviewed, covering both experimental and theoretical investigations. Other recent related developments are also briefly reviewed, such as the study of selenide two-leg ladders and the discovery of superconductivity in a single layer of FeSe. The conceptual issues considered established for the alkali metal iron selenides, as well as the several issues that still require further work, are discussed in the text.

Structural, electronic, magnetic and spin dependent transport properties of Fe/CaS/Fe (001) heterostructures

Structural, electronic, and magnetic properties of Fe/CaS (001) interfaces and Fe/CaS/Fe (001) heterostructures have been studied by means of a self-consistent Green's function technique for surface and interfaces implemented within the tight-binding linear muffin-tin orbital formalism. Spin dependent transport properties of the Fe/CaS/Fe (001) tunnel junctions with thin and intermediate barriers, in the current-perpendicular-to-plane geometry, have been determined by means of Kubo-Landauer approach implemented within the tight-binding linear muffin-tin orbital formalism. A small charge rearrangement is evidenced at the Fe/CaS (001) interfaces. The iron interfacial magnetic moments are enhanced over the bulk value. A small exchange coupling with the sign depending on the Fe/CaS (001) interface geometric structure and the strength decaying exponentially with the barrier is evidenced. Interfacial charge transfer, interface iron magnetic moments, and tunneling currents are sensitive to the interfacial structure. Interface resonant states have a decisive role in the tunneling process and the main contribution to the current in the ferromagnetic state of the junction is given by the minority-spin electrons.

Homologous Series of Layered Perovskites An+1BnO3n–1Cl: Crystal and Magnetic Structure of a New Oxychloride Pb4BiFe4O11Cl

The nuclear and magnetic structure of a novel oxychloride Pb(4)BiFe(4)O(11)Cl has been studied over the temperature range 1.5-700 K using a combination of transmission electron microscopy and synchrotron and neutron powder diffraction [space group P4/mbm, a = 5.5311(1) Å, c = 19.586(1) Å, T = 300 K]. Pb(4)BiFe(4)O(11)Cl is built of truncated (Pb,Bi)(3)Fe(4)O(11) quadruple perovskite blocks separated by CsCl-type (Pb,Bi)(2)Cl slabs. The perovskite blocks consist of two layers of FeO(6) octahedra located between two layers of FeO(5) tetragonal pyramids. The FeO(6) octahedra rotate about the c axis, resulting in a √2a(p) × √2a(p) × c superstructure. Below T(N) = 595(17) K, Pb(4)BiFe(4)O(11)Cl adopts a G-type antiferromagnetic structure with the iron magnetic moments confined to the ab plane. The ordered magnetic moments at 1.5 K are 3.93(3) and 3.62(4) μ(B) on the octahedral and square-pyramidal iron sites, respectively. Pb(4)BiFe(4)O(11)Cl can be considered a member of the perovskite-based A(n+1)B(n)O(3n-1)Cl homologous series (A = Pb/Bi; B = Fe) with n = 4. The formation of a subsequent member of the series with n = 5 is also demonstrated.