Iron Spins Research Articles

Structural and Magnetic Effects of meso-Substitution in Alkyl-Substituted Metalloporphyrinate π-Cation Radicals: Characterization of [Fe(TalkylP•)(Cl)]SbCl6 (alkyl = ethyl and n-propyl)

We report the preparation and characterization of two meso-alkyl substituted porphyrin pi-cation radical derivatives, [Fe(TalkylP(*))(Cl)]SbCl(6) (alkyl = ethyl or propyl). Both complexes have been characterized by UV/vis/near-IR, IR, and Mossbauer spectroscopy, temperature-dependent solid-state magnetic susceptibility measurements, and X-ray structure determinations. All data for both oxidized species are consistent with the formulation of the complexes as ring-oxidized iron(III) porphyrin species. The molecular structures of the two five-coordinate species have the typical square-pyramidal coordination group of high-spin iron(III) derivatives. The crystal structures also reveal that the species form cofacial pi-pi dimers with lateral shifts of 1.44 A and 3.22 A, respectively, for the propyl and ethyl radical derivatives. Both radicals exhibit porphyrin cores with alternating bond distance patterns in the inner 16-membered ring. In addition, [Fe(TEtP(*))(Cl)]SbCl(6) and [Fe(TPrP(*))(Cl)]SbCl(6) have been characterized by temperature-dependent (6-300 K) magnetic susceptibility studies, the best fitting of the temperature-dependent moments reveal strong coupling between iron spins and porphyrin radical, and a smaller magnitude of antiferromagnetic coupling between ring radicals, which are opposite to those found in the five-coordinate iron(III) OEP radicals. The differences in structure and properties of the cation radical meso-alkyl and beta-alkyl derivatives possibly reflect differences in properties of a(1u)- and a(2u)-forming radicals.

Magnetic and specific heat properties ofYFe3(BO3)4 andErFe3(BO3)4

The present paper reports on the specific heat and magnetization of theYFe3(BO3)4 andErFe3(BO3)4 single crystals. In both compounds, antiferromagnetic order of the iron spins evolves atTN = 38 K. The experimental data suggest that the magnetic moments are in the basal plane ofthe trigonal crystal for both compounds. In the magnetically ordered state thecrystal is subdivided into three types of domains, the magnetic moments of theFe3 + ions being alignedalong the a axis withineach domain. For ErFe3(BO3)4, two non-equivalent magnetic positions of theEr3 + ions in each domain are observed.

Electronic and magnetic structure of pyroxenes: II. Orthoferrosilite

The electronic and magnetic structure of the chain silicate orthoferrosilite Fe22+Si2O6 has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d–d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mossbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm−1. Between these ribbons within the (b, c)-plane a weak ferromagnetic coupling of about +2 cm−1 is obtained. Neighboured (b, c)-planes are coupled antiferromagnetically via chains of SiB-tetrahedra but ferromagnetically via chains of SiA-tetrahedra. Such a theoretically determined "double-plane antiferromagnetic" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Neel temperature currently not available.

Investigating the formation and magnetic properties of the Dy 0.75Fe 1.25O 3 orthoferrite prepared by sol–gel method

In this paper, the Dy 0.75Fe 1.25O 3 orthoferrite nanoparticles were synthesized successfully by sol–gel method. Dy 0.75Fe 1.25O 3 orthoferrite nanoparticles are obtained by calcining the flakes at 600 and 700 °C. The magnetic properties of the different samples are investigated using Quantum Design MPMS SQUID magnetometer and MS-500 Mössbauer spectrometer. Magnetic phase γ-Fe 2O 3 coexists in the samples calcined at 600 °C and orthoferrite phase is completely recovered in the samples calcined at 700 °C. Although excessive Fe 3+ ions were introduced, none of these iron spins couple magnetically with Dy 3+ ions.

Effect of Imidazole and Phenolate Axial Ligands on the Electronic Structure and Reactivity of Oxoiron(IV) Porphyrin π-Cation Radical Complexes: Drastic Increase in Oxo-Transfer and Hydrogen Abstraction Reactivities

To study the effect of axial ligands on the electronic structure and reactivity of compound I of peroxidases and catalases, oxoiron(IV) porphyrin pi-cation radical complexes with imidazole, 2-methylimidazole, 4(5)-methylimidazole, and 3-fluoro-4-nitrophenolate as the axial ligands were prepared by ozone oxidation of iron(III) complexes of 5,10,15,20-tetramesitylporphyrin (TMP) and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). These complexes were fully characterized by absorption, (1)H, (2)H, and (19)F NMR, electron paramagnetic resonance (EPR), and electrospray ionization mass spectrometry (ESI-MS) spectroscopy. The characteristic absorption peak of compound I at approximately 650 nm was found to be a good marker for estimation of the electron donor effect from the axial ligand. The axial ligand effect did not change the porphyrin pi-cation radical state, the a(2u) state of the TMP complexes, or the a(1u) radical state of both the TMTMP complexes and compound I. The ferryl iron and porphyrin pi-cation radical spins were effectively transferred into the axial ligands for the a(2u) complexes but not for the a(1u) complexes. Most importantly, the reactivity of the oxoiron(IV) porphyrin pi-cation radical complex was drastically increased by the imidazole and phenolate axial ligands. The reaction rate for cyclooctene epoxidation was increased 100- to 400-fold with axial coordination of imidazoles and phenolate. A similar increase was also observed for the oxidation of 1,4-cyclohexadiene,N,N-dimethyl-p-nitroaniline and hydrogen peroxide. These results suggest extreme enhancement of the reactivity of compound I by the axial ligand in heme enzymes. The functional role of axial ligands on the compound I in heme enzymes is discussed.

Density Functional Theory Study of the Structural, Electronic, and Magnetic Properties of a μ-oxo Bridged Dinuclear FeIV Complex Based on a Tetra-Amido Macrocyclic Ligand

Recently, the synthesis, crystallographic structure, and Mossbauer characterization of the first example of an [(Fe(IV)TAML)2O](2-) (TAML = tetra-amido microcyclic ligand) complex were reported. Here, we elucidate the prominent structural, electronic, and magnetic properties of this complex on the basis of density functional theory (DFT) calculations. While the torsion between the molecular halves is caused by hydrogen bonding between the TAML moieties, the bending of the Fe-O-Fe unit is an intrinsic property of the bridge. The values for the (57)Fe isomer shift and quadrupole splitting obtained with DFT are in good agreement with experimental results and indicate that the irons have intermediate spin states (S1 = S2 = 1). The iron spins are coupled by strong antiferromagnetic exchange to yield a ground state with system spin S = 0. The Fe-O distances in the excited S > 0 states are significantly longer than in the ground state. Since the wave function of the ground state, in which the iron spins are antiferromagnetically coupled to give system spin S = 0, is a linear combination of Slater determinants that cannot be treated with existing DFT codes, the Fe-O distance for the S = 0 state has been estimated by extrapolation from the optimized geometries for the ferromagnetic state (S = 2) and the broken symmetry state to be 1.748 A, in good agreement with the crystallographic distance 1.728 A. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: H(ex) = c' + j0S1 x S2 + j1(S1 x S2)(2). A computational scheme is presented for estimating the exchange parameters, yielding the values j0 = 199 cm(-1) and j1 = -61 cm(-1) for [(Fe(IV)B*)2O](2-). Two mechanisms for biquadratic exchange are discussed.

Magnetic order of the iron spins in NdFeAsO

Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the $c$ axis are antiparallel like LaFeAsO. The ordered moment is $0.25(7)\phantom{\rule{0.2em}{0ex}}{\ensuremath{\mu}}_{B}$, which is the smallest moment found so far in these systems.

A comparative Mössbauer study of the mineral cores of human H-chain ferritin employing dioxygen and hydrogen peroxide as iron oxidants

Ferritins are ubiquitous iron storage and detoxification proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidize and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilizes both O 2 and H 2O 2 as oxidants for Fe 2+ where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. The present study was undertaken to determine whether the nature of the mineral core formed depends on the protein ferroxidase center versus mineral surface mechanism and on H 2O 2 versus O 2 as the oxidant. The data reveal that similar cores are produced in all instances, suggesting that the structure of the core is thermodynamically, not kinetically controlled. Cores averaging 500 Fe/protein shell and diameter ∼ 2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H 2O 2 and O 2 oxidized samples, respectively. The observed blocking temperatures are consistent with the unexpectedly large effective anisotropy constant K eff = 312 kJ/m 3 recently reported for ferrihydrite nanoparticles formed in reverse micelles [E.L. Duarte, R. Itri, E. Lima Jr., M.S. Batista, T.S. Berquó and G.F. Goya, Large Magnetic Anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles, Nanotechnology 17 (2006) 5549–5555.]. All ferritin samples exhibited two magnetic phases present in nearly equal amounts and ascribed to iron spins at the surface and in the interior of the nanoparticle. At 4.2 K, the surface spins exhibit hyperfine fields, H hf, of 436 and 445 kOe for the H 2O 2 and O 2 samples, respectively. As expected, the spins in the interior of the core exhibit larger H hf values, i.e. 478 and 486 kOe for the H 2O 2 and O 2 samples, respectively. The slightly smaller hyperfine field distribution DH hf for both surface (78 kOe vs. 92 kOe) and interior spins (45 kOe vs. 54 kOe) of the O 2 sample compared to the H 2O 2 samples implies that the former is somewhat more crystalline.

Magnetism in the magnetoelectric hexaferrite system(Ba1−xSrx)2Zn2Fe12O22

The hexaferrite system ${({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x})}_{2}{\mathrm{Zn}}_{2}{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ possesses a rich magnetic phase diagram and for strontium content $x=0.75$ exhibits a magnetoelectricity, the existence of which is intimately connected to the noncollinear arrangement of the iron spins. The system comprises seven cation sublattices, two of which have tetrahedral coordination. The tetrahedral sublattices are partly occupied by Fe and partly by Zn. Using ab initio--calculated exchange interactions we show that the magnetism in the ${({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x})}_{2}{\mathrm{Zn}}_{2}{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ system is sensitive to the Fe and Zn distributions. The region of existence of the noncollinear spin structure and hence of ferroelectricity is determined and found to be rather narrow. The critical temperature of magnetic ordering is calculated using molecular field and random phase approximations. The results are in good agreement with the experiment.

Exchange interaction and conductivity in ferroelectric hexaferrite

Ba 0.5Sr 1.5Zn 2Fe 12O 22 is a new promising multiferroic compound in which the existence of electric polarization is conditioned by the noncollinear arrangement of the iron spins. In principle, ferroelectricity might be present above the room temperature, but electrical conductivity prevents its observation at temperatures higher than ≈130 K. Our recent calculations showed that the electronic structure and in particular the conductivity is strongly influenced by relative fractions of iron and zinc in the tetrahedral sublattice of the spinel block in corresponding Y hexaferrite structure. In this paper, we show that the same holds for the exchange interactions. Dependence of the exchange integrals on the distribution of iron and zinc in the tetrahedral sublattice of the spinel block and on the content of Sr in Ba 2− x Sr x Zn 2Fe 12O 22 system is calculated using the ab initio approach with an improved treatment of strongly correlated 3d electrons of iron ions. The interaction between the two magnetic blocks that is decisive for the phase diagram is analysed in details.

NEW NON-COOPERATIVE QUANTUM PHENOMENON IN A FERRIMAGNET WITH ANTIFERROMAGNETIC IMPURITY

The differential magnetic susceptibility of Gd0 01Y2.99Fe5O12 polycrystalline compounds is studied at the temperature range from 0.6 to 4.2 K in pulsed magnetic fields up to 50 T. It was observed that the magnetization of the compound with x = 0.01 shows a quantum (stepwise) behavior at T = 0.6 K in magnetic fields around 33 T. The Heisenberg model of a ferrimagnet with weakly coupled antiferromagnetic impurities is used to explain the obtained results. Following this model, the energy levels of the impurity ions become not equally spaced due to the magnon-magnon interaction between the iron spins and the rare earth spins. Increase of the external field leads to the crossover of the excited states with the ground one, which takes place in different fields. In result, several peaks of the magnetic susceptibility appear under a variation of magnetic field in the vicinity of a field, which is equal to the effective field of the Gd-Fe exchange interaction.

A M�ssbauer Study of DyFe2, YFe2 and HoFe2 Thin Films and Multilayers

We have performed room temperature Conversion Electron Mössbauer Spectroscopy (CEMS) on thin films and multilayers of DyFe2, YFe2 and HoFe2 under applied magnetic fields of 0 T and 0.25 T in plane. The samples were constructed by M.R. Wells and R.C.C. Ward using Molecular Beam Epitaxy on sapphire substrates giving single crystals with the [110] plane parallel to the surface. The samples produced and studied are: thin films of DyFe2, YFe2 and HoFe2; multilayers of DyFe2/YFe2 with respective layer thicknesses of 50 Å/50 Å, 40 Å/20 Å, 20 Å/40 Å and 20 Å/20 Å. In all spectra the fits obtained are different to those found in bulk samples: all the iron sites in the cubic Laves Phase unit cell are magnetically inequivalent and the spins on the iron atoms always have a component perpendicular to the layers. With a field of 0.25 T applied in the plane of the sample the iron spins in the YFe2 thin film are rotated into the plane by a large amount (8.5°) compared to only a minor change in the DyFe2 thin film (1°). In multilayer systems the iron atom spins in the YFe2 layers appear to be locked to those in the DyFe2 layers.

Spin Dynamics and Quantum Tunneling in Molecular Magnet Fe8

Spin dynamics and quantum tunneling of magnetization in single crystals of the molecular magnet Fe8 have been studied by 1 H-NMR experiment. The ground state of Fe8 has a total spin S = 10. NMR spectra show that below about 3 K the iron spins freeze in the double potential wells separated with the energy barrier of 25 K, while they are superparamagnetic above that temperature. The spin lattice relaxation rates above 400 mK are strongly temperature dependent. The relaxation is dominated by the thermal transition of the iron spin states between the energy levels discretely separated by the single ion-type anisotropy. The rates become to be temperature independent and very long over 100 sec below 300 mK. In this temperature range we have observed the fast recovery of 1 H-NMR spectra at the level crossing fields. This phenomenon is caused by the resonant quantum tunneling of the iron magnetization and is analyzed by the Landau-Zener transition.

Single-ion versus dipolar origin of the magnetic anisotropy in iron(III)-oxo clusters: a case study.

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

Temperature dependence of spin-lattice relaxation in rare-earth iron garnets

The spin-lattice relaxation rate in iron garnets containing ions of the rare-earth series features a peak in the temperature range from 30 to 100 K that is consistent throughout the group. This effect is important in microwave applications at liquid nitrogen temperatures because magnetic losses increase directly with the intrinsic ferrimagnetic resonance linewidth ΔH, which in turn is proportional to the relaxation rate τ−1. The decrease in X-band linewidth with lowering temperatures is accounted for by the Orbach exponential relation for τ−1 of individual paramagnetic rare-earth ions. To explain the decrease in ΔH above the peak, the Orbach function is multiplied by the magnetic exchange energy between the iron and rare-earth spins. Computations of the linewidth as a function of temperature for Dy3+, Ho3+, Er3+, and Yb3+ are made by utilizing the multiple sublattice molecular-field theory. By fitting theory to data, values are extracted for excited state energy splitting D that enters the exponent (−D/kT) of the Orbach function. Measurement data from high purity yttrium iron garnet also confirm that iron relaxation rates decrease monotonically with reducing temperatures, indicating that microwave magnetic loss at cryogenic temperatures can be reduced below room temperature values if offending rare-earth impurities are removed.

Incommensurate Nuclear and Magnetic Structure of the Oxygen-Deficient Perovskites (Ba2−3xBi3x−1)(Fe2xBi1−2x)O2+3/2x (0.43≤x≤0.50)

The actual nuclear and magnetic structures of the oxygen perovskites [Ba2−3xBi3x−1][Fe2xBi1−2x]O2+3x/2 (0.40<x≤0.50) were studied combining XRPD and NPD data. The incommensurate modulated structures of the members x=0.45 and x=0.48 were determined in the superspace formalism. It is shown that bismuth and barium cations are not distributed at random over a single site but exhibit a particular ordering which can be described by a substitutional modulation. In such structures, the displacement of the cation from its average position is much larger for bismuth than for barium. The modulated four-fold superstructure of Bi2Ba2Fe4O11 (x=0.50), “ap×ap×4ap” is closely related to those of incommensurate phases, with a “Bi1/4Ba3/4Ba1/4” ordering of the cations. Nevertheless, a certain disordering of these cations and oxygen vacancies are evidenced in the superstructure. The antiferromagnetic (AF) ordering is the same for all the members: iron spins are antiferromagnetically coupled along [110] and along [001] forming (001) AF planes.

Weakly(x=0)and randomly(x=0.033)coupled Ising antiferromagnetic planes in(Li1−3xFex)NiPO4compounds

Neutron diffraction and magnetic susceptibility studies of $({\mathrm{Li}}_{1\ensuremath{-}3x}{\mathrm{Fe}}_{x}){\mathrm{NiPO}}_{4} (x=0$ and $x=0.033)$ compounds reveal remarkable differences between the magnetic properties of pure ${\mathrm{LiNiPO}}_{4}$ and those of its lightly iron-doped derivative. The spin system associated with the ${\mathrm{Ni}}^{2+}$ ions $(S=1)$ in the pure compound undergoes a collinear antiferromagnetic ordering at ${T}_{N}=19.1\ifmmode\pm\else\textpm\fi{}0.5 \mathrm{K},$ with the characteristics of weakly coupled two-dimensional (2D) Ising square planes. By contrast, randomly intercalated iron spins (in Li sites) between ${\mathrm{Ni}}^{2+}$ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temperatures ${T}_{N}=25.2\ifmmode\pm\else\textpm\fi{}0.5 \mathrm{K},$ and significantly modifies the critical behavior of the 2D ${\mathrm{Ni}}^{2+}$ system. It is argued that the doped compound can serve as a model system for studying the randomly coupled planar Ising model.

Hyperfine fields in Fe–Cr thin films

We have calculated the electronic structure of the 2Cr/3Fe/2Cr slab in bcc(110) geometry using FLAPW method in the local spin density approximation (LDA). The main aim of this work was to compare the calculated values of the hyperfine fields (Hf) with these obtained in the experiment carried out by Żukrowski et al. (J. Magn. Magn. Mater, 1995, 145, 97).Our calculation shows the influence of the ordering on the Fe hyperfine fields and for the measured Cr/3.3Fe/Cr film we suggest the existence of two different Hf values in each Fe plane, differing by 0.4–0.8 T. The magnetic moments of chromium are near 0.7μB on the surface and oppositely oriented in the plane, while subsurface moments are small and anti-parallel to iron spins. The moments of iron atoms on the interface are slightly reduced.

High-field Mössbauer spectrometry study of the iron spin structure in Fe-R-B (R = Ho, Dy) amorphous ribbons

The magnetic structure of the iron in melt-spun amorphous ribbons with the compositions and was investigated using high-field Mössbauer spectrometry. The low-temperature variation of the hyperfine parameters under fields ranging from 0 to 8 T is reported. A geometrical model was developed that accounts quantitatively for the experimental results. It describes the evolution of the spin structure in the framework of angular deformation of the cone formed by the iron magnetic moments. It is shown that exhibits a collinear spin structure of the iron from 2 T, while in , the iron spins remain distributed at random within a cone up to 8 T. At 8 T, the semi-angle of the cone apex amounts to about .

Atomic-volume dependence of the spin state in Fe-rich fcc Fe-Pt-Ru alloys

Fe 100- xPt x-y Ru y ( x = 20, 25 and 28, y = 0 to 20) alloys with FCC structure are structurally stable down to very low temperature. The values of 57Fe hyperfine field of this system are summarized as a function of Wigner—Seitz radius r w in atomic units. As a result, the iron spins in this system are in a zero-spin state for r w ⩽ 2.712 a.u., and in a low-spin state for 2.712 a.u. < r w ⩽ 2.745 a.u., and then transform to a high-spin state just above r w = 2.745 a.u.