Low Magnetic Moment Research Articles

Magnetic and magnetocaloric properties of Gd3−xTbxGa5O12 (x=0, 1, 2, 3) garnets

The structure and magnetocaloric properties of Tb substituted Gd3Ga5O12 garnets have been investigated. The structural refinement of X-ray diffraction patterns show that Gd3−xTbxGa5O12 (x=0, 1, 2, 3) are isostructural, with the symmetry of a cubic space group Ia3¯d. The temperature dependence of magnetization studies indicate that all the samples follow Curie–Weiss paramagnetic behavior above 25K and the effective magnetic moment increases with Tb concentration. The low temperature magnetization studies suggest that there exists a field dependent antiferromagnetic ordering of rare earth ions in Tb substituted Gd3Ga5O12, which in turn leads to a lower magnetic moment in Gd3−xTbxGa5O12 (x=1, 2, 3), compared to that of Gd3Ga5O12. Magnitude of field corresponding to the antiferromagnetic ordering is found to be dependent on temperature as well as on Tb concentration. Magnetocaloric effect analysis confirms that Tb substitution enhances the magnetic entropy change at higher temperatures and Gd3−xTbxGa5O12 (x=1, 2, 3) are better candidates than Gd3Ga5O12 for low fields and low temperature magnetic refrigeration applications.

Identification and Location of Iron Species in Fe/SBA-15 Catalysts: Interest for Catalytic Fenton Reactions

We have used a single SBA-15 silica batch (mesopore diameter of 7.1 nm) and the two-solvent impregnation technique with either pentane or cyclohexane to prepare model samples in which an initial aqueous solution of salt precursor (iron nitrate or iron chloride) is deposited mainly inside silica pores. Detailed characterization by SAXS, WAXS, Ar sorption, TEM (SAED), and UV–visible–NIR spectroscopy is described. Molecular iron species and iron oxide particles are identified. Special emphasis is given to the spatial location of the oxide particles (trapped inside silica pores or dispersed outside silica grains). The formation of hematite external particles is favored when iron chloride is used as a precursor and pentane as a solvent. Spectral signatures that can be attributed either with iron oxide particles or with iron dispersed molecular species (isolated Fe(II)/(III) ions, pairs and/or clusters, films of iron silicate) are detected by UV–visible–NIR spectroscopy. Additional neutron diffraction measurements performed on a sample particularly enriched in external hematite particles indicate a very low magnetic moment of 3.9 (1) μB per iron atom and show that external hematite particles are not significantly polluted by Si atoms. Model catalysts, all containing molecular species but particularly enriched in either external or internal iron oxide particles, were selected and tested for the catalytic Fenton decomposition (with H2O2 and diluted O2 and under artificial white light) of sodium formate and of an organic copper-containing dye (Reactive Violet 2) in water. Samples enriched in external hematite particles are significantly less active and react less rapidly than samples containing mainly iron species remaining trapped inside silica grains. Preliminary UV–visible–NIR spectra suggest that strong interactions between molecular species and internal replicated oxide particles are at the origin of an enhanced reactivity.

Stable organic radical stacked by in situ coordination to rare earth cations in MOF materials

With the correct choice of the solvothermal conditions, we have achieved the unprecedented in situ formation of the free radical form of the anthraquinone-1,5-disulfonate molecule, and its favorable organization. The semiquinone radicals are coordinated to rare-earth cations to produce a 2D framework with a very high charge mobility and electric conductivity through the π–π-interactions. The existence of AQDS3−˙ anion radicals is proven on the base of: i) the electrical neutrality: elemental analyses for the different lanthanide RPF-8 bulks, the maximum residual electron densities in the structure, rule out the existence of any other neutralizing ion, ii) the geometrical modifications in the antraquinone molecules, and iii) although less definitive, due to the low magnetic moment μ = 0.39 μB, the exhibited paramagnetism for the La (3+) with no unpaired electrons.

Facile synthesis of zinc ferrite nanoparticles as non-lanthanide T1 MRI contrast agents

Monodisperse ZnFe2O4 nanoparticles were synthesized using a simple and low-cost polyol process based on thermal decomposition of the precursors of Fe(acac)3 and Zn(acac)2 in triethylene glycol without any surfactant. The as-prepared ZnFe2O4 nanoparticles are highly crystalline, uniform in size, superparamagnetic and can be easily dispersed in aqueous media due to being coated by a layer of hydrophilic polyol ligands in situ. Magnetic study shown that the ZnFe2O4 nanoparticles had a low magnetic anisotropy and low magnetic moment compared to the conventional Fe3O4 nanoparticles. As a result, the as-prepared ZnFe2O4 nanoparticles provide an optimized r2/r1 ratio for T1-weighted magnetic resonance imaging (MRI) in the clinical field strength. A preliminary in vitro cytotoxicity test suggests that the zinc ferrite nanoparticles possess a good safety profile. Therefore, the as-prepared ZnFe2O4 nanoparticles have great potential to serve as a novel non-lanthanide T1 MRI contrast agent.

Structural, electrical, optical and magnetic properties of chromium substituted Co–Zn nanoferrites Co0.6Zn0.4CrxFe2−xO4 (0⩽x⩽1.0) prepared via sol–gel auto-combustion method

Abstract Chromium substituted Co–Zn nanoferrites having the formula Co 0.6 Zn 0.4 Cr x Fe 2− x O 4 ( x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been synthesized using the sol–gel auto-combustion method. Characterized using FT-IR spectroscopy, powder X-ray diffraction, electrical and magnetic properties. XRD patterns indicated the formation of single-phased cubic spinel structure having space group Fd-3m for all the annealed samples. The lattice parameter has been observed to decrease with increasing substitution of Cr 3+ ions because of the smaller ionic radii of Cr 3+ (0.68 Ǻ) as compared to Fe 3+ (0.73 Ǻ). The electrical resistivity increases with increasing Cr 3+ concentration attributing to the reason that electrical conductivity decreases with decrease in Fe concentration because of the decrease in polaron hopping of Fe 2+ –Fe 3+ ions. The resistivity of all the annealed samples decreases with increasing temperature showing the semiconductor nature of the samples. The drift mobility also increases with increasing temperature because of the enhanced mobility of charge carriers due to thermal activation. The saturation magnetization ( M s ) decreases with increasing chromium content, due to the lower magnetic moment of Cr 3+ ion (3 μ B ) than that of Fe 3+ ion (5 μ B ).

Density functional study on size-dependent structures, stabilities, electronic and magnetic properties of Au n M (M = Al and Si, n = 1–9) clusters: comparison with pure gold clusters

The density functional theory (DFT) method has been employed to systematically investigate the geometrical structures, relative stabilities, and electronic and magnetic properties of Au(n)M (M = Al and Si, n = 1-9) clusters for clarifying the effect of Al(Si) modulation on the gold nanostructures. Of all the clusters studied, the most stable configurations adopt a three-dimensional structure for Au(n)Al at n = 4-8 and Au(n)Si at n = 3-9, while for pure gold systems, no three-dimensional lowest energy structures are obtained. Through a careful analysis of the fragmentation energy, second-order difference of energy, HOMO-LUMO energy gap, and magnetic moment as a function of cluster size, an odd-even alternative phenomenon has been observed. The results show that the clusters with even-number valence electrons have a higher relative stability, but lower magnetic moments. Furthermore, Al(Si) doping is found to enhance the stabilities of gold frameworks. In addition, the charge analysis has been given to understand the different effects of individual doped atom on electronic properties and compared further.

Structure and Magnetic Properties of ${\hbox {LiFePO}}_{4}$ Under Pressure

LiFePO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> has attracted considerable attention as cathode materials for rechargeable lithium batteries. Here, the structural, electronic, and magnetic properties of LiFePO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> under pressure are investigated based on density functional theory. When pressure is applied to the LiFePO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , a transition to a Cmcm phase is identified, agreeing well with experimental results. Upon compression, the magnetic moment of Fe ion decreases steadily without abrupt change, being different from iron nitride. With increasing pressure the band broadening induces the spin flipping, which consequently results in the lower magnetic moment of Fe ion. By further applying pressure LiFePO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> undergoes magnetic transition from antiferromagnetic state to nonmagnetic state at 52 GPa. Hence pressure can be used as efficient way to manipulate even eliminate the magnetism of LiFePO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> .

Distinguishing magnetic and electrostatic interactions by a Kelvin probe force microscopy–magnetic force microscopy combination

The most outstanding feature of scanning force microscopy (SFM) is its capability to detect various different short and long range interactions. In particular, magnetic force microscopy (MFM) is used to characterize the domain configuration in ferromagnetic materials such as thin films grown by physical techniques or ferromagnetic nanostructures. It is a usual procedure to separate the topography and the magnetic signal by scanning at a lift distance of 25–50 nm such that the long range tip–sample interactions dominate. Nowadays, MFM is becoming a valuable technique to detect weak magnetic fields arising from low dimensional complex systems such as organic nanomagnets, superparamagnetic nanoparticles, carbon-based materials, etc. In all these cases, the magnetic nanocomponents and the substrate supporting them present quite different electronic behavior, i.e., they exhibit large surface potential differences causing heterogeneous electrostatic interaction between the tip and the sample that could be interpreted as a magnetic interaction. To distinguish clearly the origin of the tip–sample forces we propose to use a combination of Kelvin probe force microscopy (KPFM) and MFM. The KPFM technique allows us to compensate in real time the electrostatic forces between the tip and the sample by minimizing the electrostatic contribution to the frequency shift signal. This is a great challenge in samples with low magnetic moment. In this work we studied an array of Co nanostructures that exhibit high electrostatic interaction with the MFM tip. Thanks to the use of the KPFM/MFM system we were able to separate the electric and magnetic interactions between the tip and the sample.

Magnetic properties and X‐ray photoelectron spectroscopy of nanocrystalline Ho3Al5O12

AbstractNanocrystalline holmium aluminium garnet (Ho3Al5O12) has been prepared for the first time by modified Pechini's reaction after sintering the precursor gel at 1223 K. The nanomaterial has been characterized by X‐ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The XRD pattern confirms the formation of single‐phase Ho3Al5O12; the average size of the nanoparticles has also been determined. X‐ray photoelectron spectroscopy (XPS) has been used to study the chemical composition and bonding in the as‐prepared samples. The binding energies of core‐level electrons in Ho, Al and O in the title material have been found slightly shifted compared to the values of the respective elements. DC magnetic susceptibility has been measured in the temperature range 2 – 260 K. Low effective magnetic moment of Ho3+, μeff = 1.35 µB and Weiss constant have been derived from the inverse magnetic susceptibility–temperature linear plot. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis, Characterization, and Multielectron Reduction Chemistry of Uranium Supported by Redox-Active α-Diimine Ligands

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

Magnetoelastic effects in Jahn–Teller distortedCrF2 andCuF2 studied by neutron powder diffraction

We have studied the temperature dependence of the crystal and magneticstructures of the Jahn–Teller distorted transition metal difluoridesCrF2 andCuF2 by neutron powder diffraction in the temperature range 2–280 K. The lattice parametersand the unit cell volume show magnetoelastic effects below the Néel temperature. Thelattice strain due to the magnetostriction effect couples with the square of the orderparameter of the antiferromagnetic phase transition. We also investigated the temperaturedependence of the Jahn–Teller distortion, which does not show any significant effect at theantiferromagnetic phase transition but increases linearly with increasing temperature forCrF2, and remains almost independent of temperature inCuF2. The magnitude of magnetovolume effect seems to increase with the low temperaturesaturated magnetic moment of the transition metal ions but the correlation is not at allperfect.

Low-magnetic moment and strong anisotropy in non-doped iron-based superconductors

We perform first-principles calculations with a specific LDA+U scheme for non-doped iron based superconductor LaFeAsO, which exhibits an antiferromagnetic ordering with an unexpected low-magnetic moment compared to the one predicted by standard first-principles calculations. Consequently, we find two stable electronic structures, one of which has the high magnetic moment ∼2.0μB equivalent with the previous calculation results, and another of which reproduces the low magnetic moment ∼0.3μB as obtained experimentally. The former is stable in U=0, whereas the latter becomes stable in an intermediate U range. The latter well explains various anomalous features as observed experimentally.

Synthesis, spectroscopic, and biological activities of two binuclear complexes [Cu(II)(1-phenylamidino-O-methylurea)2(H2O)]2(Cl2)2 and [Cu(II)(1-phenylamidino-O-i-butylurea)tmen]2(Cl2)2·2H2O

EPR spectra of two copper(II) binuclear complexes, [Cu(II)(1-phenylamidino-O-methylurea)2(H2O)]2(Cl2)2 (1) and [Cu(II)(1-phenylamidino-O-i-butylurea)tmen]2(Cl2)2 · 2H2O (2), at room temperature showed fine structure transitions (ΔM s = ±1) and a very weak half-field signal corresponding to forbidden transitions (ΔM s = ±2). The spectrum of 1 showed disappearance of normal and half-field transitions when cooled to 77 K, suggesting antiferromagnetical coupling dicopper complex which is also supported by the low magnetic moments (µ eff = 1.64 B.M.). The isotropic exchange interaction constant J (41 cm−1) for 2 indicated that interaction between the two spins of the binuclear complex is ferromagnetic, confirmed from the high magnetic moment value (µ eff = 2.25 B.M.). The binding of these complexes with calf thymus DNA suggested that these complexes interact with DNA by electrostatic or groove binding, not by intercalation. The two complexes have good antibacterial activity against tested bacteria responsible for urinary tract infection.

Synthesis, structural and magnetic study of polycrystalline LaNi 1− xMn xO 3 films

Polycrystalline double perovskite LaNi 1− x Mn x O 3 ( x=0.3, 0.4, 0.5 and 0.7, which is defined as Mn03, Mn04, Mn05 and Mn07, respectively) thin films are successfully deposited on Si (1 0 0) substrates via chemical solution deposition method. Their structural and magnetic properties are measured. All the thin films are of single phase. Raman spectra indicate that relative intensity of Mn05 is stronger than that of others that can be attributed to the higher degree of B-site ordering. The low temperature magnetic moment of Mn05 is about 500 emu/cm 3, which is obviously larger than that of Mn03 and Mn07 because of the long-range ordering of Mn and Ni ions in Mn05.

Loops, Chains, Sheets, and Networks from Variable Coordination of Cu(hfac)2 with a Flexibly Hinged Aminoxyl Radical Ligand

One pair of reactants, Cu(hfac)(2) = M and the hinge-flexible radical ligand 5-(3-N-tert-butyl-N-aminoxylphenyl)pyrimidine (3PPN = L), yields a diverse set of five coordination complexes: a cyclic loop M(2)L(2) dimer; a 1:1 cocrystal between an M(2)L(2) loop and an ML(2) fragment; a 1D chain of M(2)L(2) loops linked by M; two 2D M(3)L(2) networks of (M-L)(n) chains cross-linked by M with different repeat length pitches; a 3D M(3)L(2) network of M(2)L(2) loops cross-linking (M-L)(n)-type chains with connectivity different from those in the 2D networks. Most of the higher dimensional complexes exhibit reversible, temperature-dependent spin-state conversion of high-temperature paramagnetic states to lower magnetic moment states having antiferromagnetic exchange within Cu-ON bonds upon cooling, with accompanying bond contraction. The 3D complex also exhibited antiferromagnetic exchange between Cu(II) ions linked in chains through pyrimidine rings.

Enhanced iron magnetic moment in the ThFe11C2 intermetallic compound

Detailed theoretical investigations on the electronic and magnetic properties of the ThFe11C2 compound have been performed using both the linear muffin-tin orbital and Korringa–Kohn–Rostocker methods of band structure calculation. The structure of the ThFe11C2 compound has three inequivalent iron sites with different local environment. A strongly enhanced magnetic moment is observed on certain Fe positions, coexisting with much lower magnetic moments on other iron positions of the lattice. Band structure calculations indeed show that the Fe magnetic moments depend strongly on the local environment. The average Fe magnetic moment obtained from these calculations is in good agreement with the experimental average Fe moment obtained from magnetization measurements. The orbital contribution to the magnetic moment is found to be especially large on the Fe 4b position. Comparing calculated hyperfine fields with experimental results, it is found that the calculated and experimental hyperfine fields are correlated. However, similarly to the results reported before for elemental Fe, the magnitude of all calculated Fe hyperfine fields is about 25% smaller. The agreement with the Mössbauer measurements is improved by scaling the core polarization contribution and by estimating the orbital valence d-electrons contribution to the magnetic hyperfine fields using the local spin density approximation + dynamical mean field theory calculated orbital moments.

Transient electromagnetic study of basin fill sediments in the Upper San Pedro Basin, Mexico

The Upper San Pedro River Basin in Mexico and the United States is an important riparian corridor that is coming under increasing pressure from growing populations and the associated increase in groundwater withdrawal. Several studies have produced three-dimensional maps of the basin fill sediments in the US portion of the basin but little work has been done in the Mexican portion of the basin. Here, the results of a ground-based transient electromagnetic (TEM) survey in the Upper San Pedro Basin, Mexico are presented. These basin fill sediments are characterized by a 10–40 m deep unsaturated surficial zone which is composed primarily of sands and gravels. In the central portion of the basin this unsaturated zone is usually underlain by a shallow clay layer 20–50 m thick. Beneath this may be more clay, as is usually the case near the San Pedro River, or interbedded sand, silt, and clay to a depth of 200–250 m. As you move away from the river, the upper clay layer disappears and the amount of sand in the sediments increases. At 1–2 km away from the river, sands can occupy up to 50% of the upper 200–250 m of the sediment fill. Below this, clays are always present except where bedrock highs are observed. This lower clay layer begins at a depth of about 200 m in the central portion of the basin (250 m or more at distances greater than 1–2 km from the river) and extends to the bottom of most profiles to depths of 400 m. While the depth of the top of this lower clay layer is probably accurate, its thickness observed in the models may be overestimated due to the relatively low magnetic moment of the TEM system used in this study. The inversion routine used for interpretation is based on a one-dimensional geologic model. This is a layer based model that is isotropic in both the x and y directions. Several survey soundings did not meet this requirement which invalidates the inversion process and the resulting interpretation at these locations. The results from these locations were rejected.

The role of SrRuO3 bottom layer in strain relaxation of BiFeO3 thin films deposited on lattice mismatched substrates

The present study deals with BiFeO3 and SrRuO3 thin films deposited on large lattice mismatched substrates like LaAlO3, MgO, and SrTiO3. The 80 nm thickness BiFeO3 films deposited directly on LaAlO3 substrate are highly constrained, while those on SrTiO3 substrate are only somewhat constrained. The BiFeO3 films deposited with 50 nm SrRuO3 bottom layer are fully relaxed on all the three substrates. The separate SrRuO3 layers deposited under identical conditions on these substrates are also found to be relaxed. These films exhibit different morphological features in accordance with strain relaxation process. The BiFeO3 thin films show characteristic low magnetic moment resulting from antiferromagnetic ordering irrespective of substrate induced strain.

Metamagnetic DyAlO3 nanoparticles with very low magnetic moment

Nanocrystalline dysprosium monoaluminate (DyAlO3) has been synthesized by modified sol–gel method after sintering the precursor gel at 950 °C. The micro-structural features have been verified by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy. The XRD pattern confirms the formation of single-phase DyAlO3; the average size of the nanoparticles is 50 nm. X-Ray photoelectron spectroscopy has been used to study the chemical composition and bonding in the samples. The binding energies of core-level electrons in Dy, Al and O in DyAlO3 nanopowder have been found slightly shifted compared to the respective values of the same elements. Both AC and DC magnetic susceptibilities have been measured in the temperature range 2–300 K. Unusually low effective magnetic moment of Dy3+, μeff = 0.38, has been derived from the inverse magnetic susceptibility–temperature plot between 4 and 252 K. The Neel temperature, TN = 3.920 K and exchange interaction constant J/k = −1.74 K, have been also determined.

Low-lying states of valence-hole nuclei in the 208Pb region

Systematic calculations of low-lying states for Ir, Pt, Au, Hg and Tl isotopes with neutron numbers between 120 and 125 have been performed within the framework of the SDG-pair approximation of the shell model. We employ a monopole and quadrupole pairing plus quadrupole–quadrupole-type interaction with optimized parameters, which are assumed to be constants for nuclei with the same proton number or neutron number. We calculate binding energies of the ground states, low energy level schemes, electric quadrupole and magnetic dipole moments, and E2 transition rates. Our results are reasonably consistent with the available experimental data as well as previous theoretical studies, in particular, for low-lying yrast states. We also demonstrate that low-lying states of nuclei studied here are usually well represented by very simple configurations in collective nucleon-pair basis.