Iron Magnetic Moments Research Articles

Temperature-dependent evolution of micromagnetic structure of B20-type Fe1-xCoxGe studied by Mössbauer spectroscopy

The non-collinear magnetic structures hosted in magnets with broken central inversion symmetry, some of which are topologically non-trivial, have unique physical properties and application prospects. This paper presents a detailed study of the temperature evolution of the micromagnetic structures of B20-type Fe1-xCoxGe (x = 0, 0.05, 0.10) polycrystalline samples using 57Fe Mössbauer spectroscopy. We take the relatively weak electric quadrupole interaction as a perturbation of the hyperfine magnetic interaction to obtain the distribution of the hyperfine magnetic field Hhf (φ) in the plane perpendicular to the magnetic propagation vector ks. It is found that the geometric features of Hhf (φ) are sensitive to the relative orientation of ks regarding the electric field gradient at the iron nucleus, as well as to the spin texture. The temperature dependence of Hhf (φ) provides a distinct signal of first-order phase transition when ks shifts from [100] to [111]. The doping of cobalt atoms leads to out-of-plane component and in-plane component aggregation of the neighboring iron magnetic moments. These results are instructive for understanding the dynamical behavior of spiral magnets with long-periods.

Orthogonal magnetic structures of Fe4O5: representation analysis and DFT calculations.

The magnetic and electronic structures of Fe4O5 have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe4O5 corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k, two spin-ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by ferromagnetic ordering of iron magnetic moments at prismatic sites along the b-axis and antiferromagnetic ordering of iron moments at octahedral sites along the c-axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b-axis and the moments in the octahedra are still antiferromagnetically ordered along the c-axis. Under high pressure, Fe4O5 exhibits magnetic transitions with the corresponding electronic transitions of the metal-insulator type. At a critical pressure PC ∼ 60 GPa, the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ∼4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

Opticheskiy lineynyy dikhroizm v ab-ploskosti ferroborata NdFe3(BO3)4

he high-resolution optical absorption spectra of NdFe3(BO3)4 single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd3+ ion. Light linearly polarized at different angles to the C2 axes in the basal plane has been passed along the trigonal C3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (TN ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C2 axes up to about 17 K, the number of domains with variously directed C2 axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C2 axis within the ±10° interval.

Effect of Substitution on Magnetic Moments of Iron and Chromium Atoms in MAX Phases of Type (Cr4 – xFex)0.5SiC: Theoretical Calculation

Effect of Substitution on Magnetic Moments of Iron and Chromium Atoms in MAX Phases of Type (Cr4 – xFex)0.5SiC: Theoretical Calculation

Intricate magnetic behavior of Fe6Ge5 and its origin within a complex iron framework: The magnetic and 57Fe Mössbauer study

Fe6Ge5 was synthesized from the elements both as single crystals and a polycrystalline powder. Its crystal structure was reinvestigated in a wide temperature range using high-resolution powder and single-crystal X-ray diffraction. The crystal structure of Fe6Ge5 is connected with those of other iron germanides and features an alternation of iron-rich and germanium-rich layers. The magnetic behavior was studied by magnetization measurements on a single crystal and 57Fe Mössbauer spectroscopy. The results of both methods showed that a peculiar two-step antiferromagnetic ordering (TN1 = 110 K, TN2 = 330 K) takes place, with the part of iron magnetic moments remaining disordered between the transitions. Such behavior originates from the large number of both symmetrically and locally distinct iron atoms that form a weakly connected iron framework and yield the competition between ferromagnetic and antiferromagnetic interactions.

Magnetic behavior and site occupancy in Fe-doped In2O3 nanoparticles

(In1-xFex)2O3 monophasic nanoparticles were prepared by a freeze-drying process followed by heat treatment after which they were characterized by magnetization measurements and Mössbauer spectroscopy at temperatures ranging from 300 to 4.2 K. The iron cations were trivalent and substituted for the indium cations in both cationic sites, 24d and 8b, of the oxide matrix, showing an occupancy preference that depends on x. Two different magnetic regimes were observed after varying the temperature of the doped samples, and both correlated with the iron magnetic moments. At room temperature, the moments at the site 24d were partially ordered with ferromagnetic alignment, whereas those at the site 8b showed paramagnetic behavior. After decreasing the temperature, both sublattices become progressively ordered and more robustly below 50 K.

Magnetic, structural and magnetocaloric effect investigations on the substituted spinel Mg1−xZnxFe2O4 (0 ≤ x ≤ 1) prepared by sol-gel method

Polycrystalline Mg1−xZnxFe2O4 (0 ≤ x ≤ 1) ferrites were prepared with sol-gel method. The obtained samples were characterized by powder X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mössbauer spectrometry. The X-ray diffraction analysis combined with the Rietveld refinement confirmed that the samples crystallize in a single-phase cubic spinel structure with (SG: Fd3−m) and an average grain size in the range of 29.1–40 nm. Lattice parameter increases with increasing Zn concentration, due to the larger ionic radius of Zn2+ ions in regard of Mg2+ ions. The Curie temperature TC decreases linearly with substitution. Saturation magnetization (MS), magnetic moment of iron (µFe) and magnetic entropy change (-ΔSM) increase for x ≤ 0.5 and decrease for x ≥ 0.5, this behavior is attributed to the cationic distribution of the ions on tetrahedral [A] and octahedral [B] sites, where the structure turns from partially inverse spinel to normal spinel, affecting the remanent magnetization and the coercitive field. Moreover, a new empirical relation is acquired performing linear relation between magnetic moment and the inversion parameter δ. Mössbauer spectra show a transformation from sextuple to an only a doublet which indicates a transformation from ferrimagnetic to paramagnetic phase, this ferrimagnetism degradation is confirmed by the negative values of -ΔSM and the transition of the magnetocaloric effect from normal to inverse through substitution.

A new iron-phosphate compound (Fe7P11O38) obtained by pyrophosphate stoichiometric glass devitrification

Iron phosphates are a wide group of compounds that possess versatile applications. Their properties are strongly dependent on the role and position of iron in their structure. Iron, because of its chemical character, is able to easily change its redox state and accommodate different chemical surroundings. Thus, iron-phosphate crystallography is relatively complex. In addition, the compounds possess intriguing magnetic and electric properties. In this paper, we present crystal structure properties of a newly developed iron-phosphate compound that was obtained by devitrification from iron-phosphate glass of pyrophosphate stoichiometry. Based on X-ray diffraction (XRD) studies, the new compound (Fe7P11O38) was shown to adopt the hexagonal space group P63 (No. 173) in which iron is present as Fe3+ in two inequivalent octahedral and one tetrahedral positions. The results were confirmed by Raman and Mössbauer spectroscopies, and appropriate band positions, as well as hyperfine interaction parameters, are assigned and discussed. The magnetic and electric properties of the compound were predicted by ab initio simulations. It was observed that iron magnetic moments are coupled antiferromagnetically and that the total magnetic moment of the unit cell has an integer value of 2 µB. Electronic band structure calculations showed that the material has half-metallic properties.

Structural and magnetic properties of ordered inverse spinel LixFe5O8

The inverse spinel lithium ferrite compound LixFe5O8 has attracted a lot of attention because of its highest magnetoelectric coupling temperature. Here, we have investigated the effect of Lithium substitution on the structural and magnetic properties of ordered LixFe5O8−y (x = 0.91 and 1.02) using the X-ray diffraction, Mössbauer spectroscopy, dc magnetization, and neutron diffraction techniques. The Rietveld refinement of X-ray diffraction patterns confirms single phase formation of these compounds in cubic structure (space group P4332). Lithium composition for both compounds was determined using the atomic emission spectrometry (ICP-AES) measurements. The Rietveld refinement of neutron diffraction patterns reveal that both compounds order in a ferrimagnetic structure, where the magnetic moments of iron at the octahedral (12d) and tetrahedral (8c) sites are aligned anti-parallel. The refined values of the ordered moments for Fe3+ ions at 12d (octahedral) and 8c (tetrahedral) sites are 4.185 µB and 4.545 µB, respectively, for x = 0.91; and the corresponding values for x=1.02 are 4.950 µB and 5.274 µB, respectively. An increase in the value of saturation magnetization (in the M vs H study) has been observed with the increasing concentration of the Li. The observed values of the saturation magnetization at 5, 50, 100, 200 and 300 K are 3.91, 3.88, 3.85, 3.76, and 3.51 μB per f.u. for x=0.91 whereas for x=1.02 the corresponding values are 4.76, 4.70, 4.68, 4.56 and 4.33 μB per f.u. The observed increase in the saturation magnetization and ordered moment with increasing the Li concentration has been ascribed to the presence of iron (~9%) at the lithium site (4b) and similar amount of iron vacancies at 12d (octahedral) and 8c (tetrahedral) sites, for the x = 0.91 compound. Our results show that the magnetic properties of these compounds are tunable. The ferrimagnetic order determined using the microscopic neutron diffraction technique in the present study will provide a new pathway to build theoretical models for the magnetoelectric coupling.

Magnetic behavior of Fe-doped zirconia studied by synchrotron radiation measurements and first-principles simulations

Exploiting first-principles simulations and x-ray absorption near edge spectroscopy (XANES) in high magnetic fields, we investigated the magnetic properties of thin films of zirconia doped with Fe impurities. In our ${\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{2\ensuremath{-}y}$ samples, grown by atomic layer deposition (ALD), the Fe dopants are uniformly distributed, ranging from diluted ($x\ensuremath{\simeq}2\text{--}3%$) up to high ($x\ensuremath{\simeq}25%$) atomic concentration. By x-ray magnetic circular dichroism (XMCD), we carefully analyzed, for samples having different Fe concentration, the magnetic moments as a function of temperature, in the range from 5 K up to 150 K, studying the best dopant concentration range maximizing the magnetic signal. Surprisingly, the iron magnetic moment measured for diluted concentrations degrades as the concentration of magnetic dopant increases. On the basis of ab initio simulations, we propose that the microscopic mechanisms responsible for the peculiar magnetic properties of this compound can be explained by oxygen-mediated superexchange mechanism between the Fe dopants producing, at high dopant concentration, an antiferromagnetic coupling between two Fe atoms. We identify and discuss the role of O vacancies to control such microscopic mechanisms.

High-resolution optical spectroscopy and modeling of spectral and magnetic properties of multiferroic ErFe3(BO3)4

We carried out the high-resolution broadband temperature-dependent polarized optical spectroscopy and theoretical studies of $\mathrm{ErF}{\mathrm{e}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ single crystals in the paramagnetic and antiferromagnetic $(Tl{T}_{N}=39\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ phases. On the basis of the experimentally determined 45 crystal-field (CF) levels of $\mathrm{E}{{\mathrm{r}}^{3}}^{+}$ ions at sites with the ${C}_{2}$ point symmetry, CF calculations were performed, a set of physically grounded CF parameters was obtained and used to model the temperature dependences of the Er magnetic moments measured in neutron-scattering experiments, as well as the magnetic susceptibility and magnetization of the compound; the contributions of the quasi-one-dimensional iron magnetic subsystem were calculated in the frame of the previously developed self-consistent four-particle cluster model. The modeling strongly supports an easy-plane collinear structure of iron magnetic moments and excludes earlier proposed additional magnetic phase.

Spiral magnetic structures with various helix parameters in langasite family compounds tested by Mössbauer spectroscopy

A physico-mathematical model for processing the Mössbauer spectra of the langasite family compounds with a helical magnetic structure was developed. It was shown that the Mössbauer spectra demonstrate high information content and sensitivity to details of the magnetic structure even in the case of polycrystalline samples and in the absence of an external magnetic field. In addition, they make it possible to monitor the dynamics of the magnetic structure in the entire temperature range below the Néel point. As an example, the helical magnetic structure in the langasite Ba3SbFe3Si2O14 was experimentally investigated. It was established that during translation along the c axis, the magnetic moments of iron in the (ab) planes rotate at an angle of 72°, forming a spiral with a period of 5c. The directions of the main axis of the electric field gradient (EFG) and the local axis of magnetic anisotropy were determined, which are very important characteristics of the frustrated magnetic structure of langasites.

Magnetic and electronic properties of CaFeO2Cl

Abstract CaFeO2Cl is a unique exception in the family of the iron oxyhalides. Its crystal structure is not related to the Ruddlesden-Popper phases as known for the other members, but contains layers of edge-sharing FeO2/2O3/3 pyramids without Fe–Cl contacts. The iron atoms form a distorted honeycomb substructure. Magnetization measurements on single crystals show an unexpected weak anisotropy and indicate antiferromagnetic ordering of the iron magnetic moments already at room temperature. 57Fe Mössbauer spectra confirm the trivalent oxidation state of iron and the presence of magnetic order. DFT calculations using the LDA + U approach support a Mott-insulating antiferromagnetic ground state and indicate a Néel-type ordered antiferromagnetic state in the honeycomb layer. The band gap from optical measurements is 1.3 eV and agrees with the red-brown colour as well as with the theoretical calculations.

Forming a ferrimagnetic-like structure in the PbMn1−xFexBO4 (x ≈ 0.1) single crystal upon partial substitution

The PbMn1−xFexBO4 (x ≈ 0.1) orthoborate single crystals have been grown for the first time by spontaneous crystallization and their magnetic and resonance properties and specific heat have been examined. It has been established that partial substitution of iron ions for manganese ones leads to an increase in the Curie temperature to 34.2 K from its value of 30.3 K in the unsubstituted crystal, enhances the magnetic anisotropy, and reduces the saturation magnetization. The magnetization drop is explained in the framework of the model of a ferrimagnetic-like structure, in which the magnetic moments of iron and manganese ions form ferromagnetic subsystems coupled by the antiferromagnetic exchange.It has been found that under magnetization along the rhombic b axis the magnetic moments switch stepwise to the magnetic field direction in a certain critical field. The spin-reorientation transition is the first-order one. This feature of the crystal magnetization does not allow the experimental ferromagnetic resonance frequency-field dependence for the rhombic b axis to be described using the calculation for a simple rhombic ferromagnet. It has been established that the increase in the magnetic anisotropy of the crystal upon substitution leads to an increase in the energy gap in the ferromagnetic resonance spectrum to 121.5 GHz at T = 4.2 K.

Mössbauer Spectral Study of the Low-Temperature Electronic and Magnetic Properties of α-FePO4 and the Mixed Valence Iron(II/III) Phosphate SrFe3(PO4)3.

The Mössbauer spectra of trigonal α-FePO4, measured between 4.2 and 300 K, exhibit hyperfine parameters characteristic of high-spin iron(III) in a pseudotetrahedral oxygen environment. Between 24.5 and 300 K, the spectra show a paramagnetic quadrupole doublet and at 24.0 K the spectrum reveals the onset of antiferromagnetic exchange. At 4.2 and 16 K, a single magnetic sextet is observed with hyperfine fields of 51.36(1) and 42.74(1) T, respectively, with an angle, θ, of 90° between the principal axis of the electric field gradient tensor in the basal plane of the trigonal unit cell and the hyperfine field along the c axis. The spectra obtained between 21 and 18 K have been fitted with two equal-area magnetic sextets with θ angles of 25 and 85°, angles which indicate that the iron(III) magnetic moments are canted away from the c axis. The reduced hyperfine field versus reduced temperature plot indicates a departure from a Brillouin S = 5/2 behavior, as a result of some magnetostriction at the Néel temperature. The Mössbauer spectra of class 1 mixed-valence SrFe3(PO4)3, measured between 4.2 and 300 K, exhibit hyperfine parameters characteristic of two high-spin iron(II) ions and one high-spin iron(III) ion in a pseudooctahedral oxygen environment. At and above 40 K, the spectra show two paramagnetic quadrupole doublets, whereas at 39.0 K the spectrum reveals the onset of ferrimagnetic exchange. Between 4.2 and 30 K, the spectra have been fitted with two magnetic sextets with θ angles of 85 and 10° for the iron(II) and iron(III) sites, respectively. The reduced hyperfine field versus reduced temperature plots for the iron(II) and iron(III) sites show a distinct departure from Brillouin S = 2 and S = 5/2 behavior, respectively, a departure that suggests a first-order magnetic transition at 39.5(5) K with differing magnetostrictions at the iron(II) and iron(III) sites.

Magnetic Anisotropy of Graphene-Coated Thin Iron Films

The ab initio calculations of the magnetic anisotropy of thin (to six monolayers) nickel films and iron films with unusual fcc structures formed upon intercalation of graphene with Fe atoms are presented. The data have been obtained for both the pure-surface films and the films coated with graphene. The density functional theory and the pseudopotential method are used to calculate the magnetic moments of atoms of all the layers and to determine the total energies of the structures with different orientations of magnetic moments of iron and nickel atoms with respect to the film surface. A strong influence of graphene on the magnetic properties contacting iron films is demonstrated.

Spin ordering and hyperfine interactions in langasite-like ferrite Ba3SbFe3Si2O14: 57Fe Mössbauer reinvestigation and ESR measurements

The present work reports new analysis and reinterpretation of the 57Fe Mössbauer data for the ferrite Ba3SbFe3Si2O14 with langasite-like crystal structure. The Mössbauer measurements show a two-step magnetic ordering process. Relaxation behavior, for temperatures below TN ≈ 32 K, persisting down to T* ≈ 20 K, indicates the coexistence of the short-range ordering spin clusters within a helically ordering state. Below T*, Mössbauer spectra are consistent with a modulated helical magnetic order, where iron magnetic moments rotate within the (ab) plane with propagation vector along c axis. All experimental spectra at T < TN were analyzed assuming the modulation of hyperfine interactions and anisotropy of the magnetic hyperfine field Bhf at the 57Fe nuclei when Fe3+ magnetic moments rotate with respect to the principal axis of the EFG tensor. The broadening of the electron spin resonance lines upon approaching TN from above was explained in terms of the short-range correlations near magnetic phase transition. The present results evidence a 3D character of magnetic correlations for high-temperatures T ≥ T0, which changes for rather 2D character of magnetic correlations below T0 ≈ 60 K. Such spin dynamics in Ba3SbFe3Si2O14 probably triggered by quasi-2D crystal structure and non-trivial helical magnetic structure.

Blocking of iron magnetic moments and Spin density wave in FeTi2S4 and Fe2TiS4

Iron-titanium sulfides FeTi2S4 and Fe2TiS4 have been structurally and magnetically characterized using powder X Ray Diffraction with Rietveld refinement method and 57Fe Mössbauer spectroscopy at variable temperature. Both sulfides have the same crystallographic phase, based on the monoclinic Cr3S4 type structure and vary in atomic coordinates; FeTi2S4 retains the ideal atomic positions proposed for the Cr3S4 phase, while in Fe2TiS4 the metal displacements from ideal sites are noticeable. Mössbauer spectra reveal different magnetic behaviors; in FeTi2S4 there is a transition from paramagnetic to magnetic ordering at temperature Tc = 145 K, giving rise to unusually low hyperfine magnetic field of 2.5 T (at 77 K) if compared with values of iron magnetic moments reported previously, this behavior is explained on the basis of blocking Fe localized magnetic moments by the Spin density wave (SDW) originated from 3d Ti atoms. In Fe2TiS4 a transition from paramagnetic to (SDW) arises at Tc = 290 K, the SDW is spread in both Fe and Ti metals through [1 0 1] crystallographic plane and undergoes a transition of first order from Incommensurate SDW (ISWD) to Commensurate SDW (CSDW) at TIC = 255 K. The atomic positions in the unit cell are correlated to the magnetic behavior in both sulfides.

Pressure-Induced Decrease in the Curie Temperature of Gd2Fe17: LSDA+U Calculations

To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R2Fe17 (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd2Fe17 ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature TC at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd2Fe17 is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature TC of Gd2Fe17 is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd2Fe17 during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level ϵF rather than to a change in the overlapping of wavefunctions, which play a minor role.

Local microscopic properties and annealing effect of Rb0.85Fe1.9Se2 single crystals

Investigation of mesoscopically phase-separated Rb0.85Fe1.9Se2 single crystals has been performed and two iron sites: nonmagnetic and magnetic ones, were observed by Mössbauer spectroscopy. The softening of the nonmagnetic one, having clearly more soft dynamics, was found to be gained further by the annealing of the single crystals at phase separation temperature, Tp, leading to the reduction of size of initially separated domains and their more homogenous distribution in the tetragonal matrix of the studied sample. The magnetic Fe sites of Rb0.85Fe1.9Se2 show strong magnetic texture, indicating the perpendicular to the ab-plane orientation of the iron magnetic moments. It was found that the annealing at Tp causes a systematic decrease of the isomer shift of the doublet by 0.02(1) mm s−1.