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Two series of chlorine or fluorine derivatives containing monooxamato ligands result in diamagnetic complexes of Y3+ and La3+ and paramagnetic complexes of Nd3+ and Ho3+ that behave as single-ion magnets at lower temperatures.

Herein a 1D Co(II) coordination polymer of formula [Co(η1-L1)(η2-L1)(py)2(H2O)]n (CoCP) has been synthesised using the rigid H2L1 proligand, containing a long spacer bearing two triple bonds. Single-crystal X-ray diffraction showed that Co(II) adopts a distorted octahedral geometry. The state-averaged complete active self-consistent field (SA-CASSCF) calculation showed that the ground state of CoCP is a high spin quartet with a highly multiconfigurational character of its electronic structure. Due to the large intra- and intermolecular distances between the spin carriers, the magnetic interactions are negligible and the zero-field splitting (ZFS) effects of cobalt(II) ions are predominant. This behavior was confirmed by direct current (DC) magnetic measurements and theoretical calculations using the broken-symmetry approach. Quantum chemical calculations indicate that CoCP has a negative axial component possessing mixed tri-axial anisotropy. The DC magnetic susceptibility data were fitted with a Griffith-Figgis Hamiltonian and the obtained parameters are in good agreement with those simulated by the ab initio calculation. Alternating current (AC) magnetic measurements showed a field induced slow magnetic relaxation in CoCP, which is attributed to the hyperfine interaction effects.

In this work, we present the synthetic pathway, a refined structural description, complete solid-state characterization and the magnetic properties of four new cobalt(II) compounds of formulas [Co(H2O)6][Co2(H2mpba)3]·2H2O·0.5dmso (1), [Co(H2O)6][Co2(H2mpba)3]·3H2O·0.5dpss (2), [Co2(H2mpba)2(H2O)4]n·4nH2O (3), and [Co2(H2mpba)2(CH3OH)2(H2O)2]n·0.5nH2O·2ndpss (4) [dpss = 2,2′-dipyridyldisulfide and H4mpba = 1,3-phenylenebis(oxamic) acid], where 2 and 4 were obtained from [Co(dpss)Cl2] (Pre-I) as the source of cobalt(II). All four compounds are air-stable and were prepared under ambient conditions. 1 and 2 were obtained from a slow diffusion method [cobalt(II) : H2mpba2− molar ratio used 1 : 1] and their structures are made up of [Co2(H2mpba)3]2− anionic helicate units and [Co(H2O)6]2+ cations, exhibiting supramolecular three-dimensional structures. Interestingly, a supramolecular honeycomb network between the helicate units interacting with each other through R22(10) type hydrogen bonds occurs in 2 hosting one co-crystallized dpss molecule. On the other hand, for the first time, linear (3) and zigzag (4) cobalt(II) chains were isolated by slow evaporation of stirred solutions of mixed solvents with cobalt(II) : H2mpba2− in 1 : 2 molar ratio at room temperature. Magnetic measurements of Pre-I revealed a quasi magnetically isolated S = 3/2 spin state with a significant second-order spin–orbit contribution as expected for tetrahedrally coordinated cobalt(II) ions. The analysis of the variable temperature static (dc) magnetic susceptibility data through first- (1 and 3) and second-order spin–orbit coupling models (2 and 4) reveals the presence of magnetically non-interacting high-spin cobalt(II) ions with easy-axis (1 and 4)/easy-plane magnetic anisotropies (2 and 4) with low rhombic distortions. Dynamic (ac) magnetic measurements for Pre-I and 1–4 below 8.0 K show that they are examples of field-induced Single-Ion Magnets (SIMs).

The reaction between the ligand di(4-pyridyl)sulfide (dps) and two salts of divalent first row transition metals (M = Co2+, and Cu2+) resulted in three new compounds with formula: [Co(dps)4(H2O)2](ClO4)2⋅H2O (1), [Cu(dps)2(dmso)2]n(ClO4)2n (2) and [{Cu(dps)2(dmso)2}{Cu(dps)2(dmso)(H2O)}]n(ClO4)4n⋅2nH2O⋅n(dmso) (3). Crystal structures of 1–3 were determined using single-crystal X-ray diffraction. Crystal structure of 1 consists of a mononuclear complex, in which the dps ligand acts in a monodentate mode through one of the pyridyl nitrogen atoms. Compounds 2 and 3 present the dps ligands bridging metal centers leading to bidimensional coordination polymers. Magnetic properties in the polycrystalline samples of 1–3 in the 300–2 K temperature range were investigated. Complex 1 exhibits a field-induced slow magnetization behavior and behaves as a single-ion magnet with an effective energy barrier for the reversal of magnetization of 22.9 (1.1) K and τ0 = 5.3(1.2) × 10−7 s.

A series of lanthanide(III) oxamate complexes of formula Na[Eu(4-HOpa)4(H2O)]∙2H2O (1), Na[Gd(4-HOpa)4(H2O)]∙2H2O (2), Na[Dy(4-HOpa)4(H2O)]∙2H2O (3), and Na[Ho(4-HOpa)4(H2O)]∙2H2O (4) (4-HOpa = N-4-hydroxyphenyloxamate) were synthesized and characterized. The crystal structures of complexes 1–4, as well as the oxamate ligand in the ethyl ester acid form (Et[4-HOpa]), were elucidated by single crystal X-ray diffraction. 1–4 are isostructural, and they crystallize in the P21/n space group of the monoclinic system. The crystal structures of 1–4 consist of mononuclear lanthanide(III) complex anions, [Ln(4-HOpa)4(H2O)]− (Ln = Eu3+, Gd3+, Dy3+, and Ho3+), coordinated sodium(I) countercations, and crystallization water molecules, resulting in a 2D NaILnIII supramolecular coordination network. The coordination sphere of Ln3+ ions consists of square-face capped square antiprism (SAPRS-9). Magnetic properties were investigated for 1–4 in the 2–300 K temperature range using polycrystalline samples. Complex 2 exhibits an unusual and rare slow-relaxation of the magnetization for Gd3+ ions under DC applied field of 1.0 kOe, behaving as a field-induced single-ion magnet as occurs for the Dy3+ complex 3. The Eu3+ complex 1 has a nonmagnetic ground state, while the Ho3+ compound 4 behave as a paramagnet, and do not exhibit relaxation of the magnetization with or without an applied field.

Two air-stable, isostructural, mononuclear six-coordinate manganese(II) and cobalt(II) oxamate complexes, [M(4-HOpa)2(H2O)2] [4-HOpa = N-4-hydroxyphenyloxamate; M= Mn2+ (1) or Co2+ (2)], exhibit field-induced slow magnetic relaxation. A bottleneck process is observed throughout the temperature range of 2-20 K for 1, while for 2, it dominates only at low temperatures (2-4 K). Additionally, the Raman process [n = 6.9(2)] is responsible for an increase in the relaxation time at higher temperatures to 2.

This work has investigated the microstructure and magnetic properties of supersaturated melt-spun Cu–10%Co alloys by means of magnetization, magnetotransport studies, electron microscopy and electron holography. Our results demonstrate that the microstructure of the annealed supersaturated alloys is constituted by two different types of Co-rich phases: spherical Co-rich precipitates as a result of spinodal decomposition, and Co rod-like precipitates, as a result of discontinuous grain-boundary precipitation. The aspect ratio of such precipitates is quite high, exhibiting magnetic anisotropy along their long axes, which generates global anisotropy of the alloy sample. We verified that the highest giant magnetoresistance occurs concurrently with an optimum discontinuous precipitate development. Electron holography reveals an antiferromagnetic coupling of the Co rods in the remanent magnetization state. The magnetization and magnetoresistance curves as a function of applied magnetic field are well described by an independent-moment approach considering the sum of the average magnetic moments of the spherical and the rod-like Co precipitates.

Magnetic properties of Fe30Ni70 nanoalloys dispersed in a silica matrix are reported. X-ray diffraction patterns and TEM images are consistent with a fcc crystalline structure (space group Fm 3 m ). Thermogravimetric data and its derivative reveal important features about the kinetics formation of the nanoalloy. Magnetization data as a function of an applied magnetic field and temperature indicate that nanoalloys are superparamagnetic with blocking effects appearing around T = 10 K. However, the unusual magnetic-field-dependence of the blocking temperature suggests that interaction effects must be taken into account. In this sense, as the power law usually employed to describe noninteracting particles system do not fit our experimental data, we have used a simple modification of the random anisotropy model (RAM). Finally, once the size distribution of nanoparticles should also play a crucial role in determining the magnetic behavior of the sample, we have analyzed the T-dependence of coercive field using a generalized model, which takes into account this parameter.

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition CoxNi1-xFe2O4 was varied over the range 0 ≤ x ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a "checkerboard" ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results.

Three new coordination compounds of formula (edap)2[Cu(opba)]2·4H2O (1), (edap)[{Na2(H2O)4}{Cu2(opba)2}]·2H2O (2), and (edap)[{K2(H2O)2}{Cu2(opba)2}]·3H2O (3) (edap = 1,1′-ethylenebis(4-aminopyridinium) and opba = 1,2-phenylenebis(oxamate)) were synthesized through the metathesis reaction involving A2[Cu(opba)] (A = Li+, Na+, and K+) and (edap)Cl2·2H2O. Crystal structures of 1–3 and edap(IO3)2·4H2O compound were elucidated by single crystal X-ray diffraction. Compounds 1–3 are built up from dinuclear copper(II) entities, {[Cu(opba)]2}4– with an asymmetric bis(monatomic oxygen) bridge resulting from the parallel “out-of-plane” disposition of the planar mononuclear [Cu(opba)]2–. They possess distinct supramolecular arrangements of varying dimensionality (nD with n = 0 (1), 1 (2), and 2 (3)) in the solid state depending on the nature of the coordinated alkaline ion present alongside edap2+ counterions. While the {[Cu(opba)]2}4– building blocks are well-isolated in 1, they form either double chains or corruga...

Ultra-low-density BiFeO3 nanoparticles have been prepared by pulsed laser deposition and their structure and magnetic properties have been studied. Annealing increases crystallinity and the size of the particles leading to an alteration of magnetic properties, observed from magnetic studies and evaluated using high-resolution transmission electron microscopy , selected area electron diffraction and x-ray diffraction patterns analysis. Transmission electron microscopy results show that the BiFeO3 as-deposited nanoparticles annealed up to 400 °C exhibit a orthorhombic distorted perovskite structure without secondary phase and with diameters varying from 9 nm (as-deposited) to 17 nm (annealed at 400 °C). Magnetic data exhibit exchange bias and magnetic blocking effects at low temperatures and typical superparamagnetic behavior at high temperatures. Meanwhile, the BiFeO3 nanoparticles annealed at 500 °C exhibit a rhombohedrally distorted perovskite structure with typical antiferromagnetic properties and diameter of about 56 nm. The analysis of magnetic relaxation time using the Arrhenius equation suggests a superparamagnetic blocking process of ferromagnetic clusters on the surface of the nanoparticles at low temperature. The magnetic properties are discussed considering the interactions between nanoparticles and the co-existence of different magnetic phases within the nanoparticles: an ordered antiferromagnetic core and ferromagnetic clusters on the surface.

Five new complexes of formula K4[Pd2(mpba)2] · 4H2O (1), {[K4(H2O)(dmso)][Pd2(mpba)2]} (2), {[Cu(bpca)]4[Pd2(mpba)2]} · 6H2O (3), {[Cu(bpca)]2[Pd(opba)]} · 1.75dmso · 0.25H2O (4), {[Cu(bpca)]2[Pd(opba)]}n · ndmso (5) [H4mpba =1,3-phenylenebis(oxamic acid), H4opba = 1,2-phenylenebis(oxamic acid), Hbpca = bis(2-pyridylcarbonyl)amide, and dmso = dimethyl sulfoxide] have been prepared and investigated by infrared spectroscopy, thermal analysis, single crystal X-ray diffraction, and magnetic susceptibility techniques. The structure of 2 consists of a [Pd2(mpba)2]4– anionic entity in which the palladium(II) cations are coordinated by two mpba ligands resulting in a dipalladium(II) unit that acts as a ligand toward potassium(I) cations leading to a neutral three-dimensional network. Compound 3 is a neutral hexanuclear complex where the dinuclear [Pd2(mpba)2]4– unit adopts a tetrakis(bidentate) coordination mode toward four [Cu(bpca)]+ end-cap entities. This compound can be viewed as a “dimer of trimers” in which...

The synthesis, crystal structure and preliminary magnetic characterization of a new heterobimetallic [MnII5CuII5] wheel containing a flexible bis-oxamate type ligand are described. This decanuclear compound exhibits a relatively strong intra-wheel antiferromagnetic interaction leading to a ground spin state S = 10.

Low temperature measurements of specific heat and magnetic properties were performed on granular Cu90Co10 ribbons prepared by melt spinning technique. The thermal and magnetic behavior of an as-quenched sample is compared to that of an annealed sample at 500 °C for 1 h. It was found that the electronic specific heat γ decreases about 50% and the magnetic properties change from spin-glass to superparamagnetic with the annealing process. These results are interpreted and discussed considering the role of isolated Co atoms in the RKKY interaction among small nanoparticles.

Magnetic effects caused by dipolar interactions in single-domain magnetic ensembles at finite temperatures are described. A modified superparamagnetic approach based on the mean field theory and random anisotropy model has been developed to describe the magnetization curves of nanoparticle assemblies. The model was used to fit experimental zero-field-cooled and field-cooled magnetization curves in Fe 3O 4 nanoparticles embedded in paraffin. The fitting parameters were based on structural properties of the materials and the strength of the interactions between nanoparticles. The model provides a quantitative description of the effects of the nanoparticle interaction with good agreement with the experiment. In addition, the model was adapted to describe magnetic properties of a NiFe thin film patterned into a nanodot array, showing potential to be used as a framework to predict magnetic interaction effects in high-density 2D arrays such as bit patterned media.

In order to investigate the effect of the addition of magnetite in layered double hydroxide (LDH) from a structural and catalytic point of view, the synthesis of LDH containing cobalt and chromium (CoCr-LDH) was performed with or without Fe3O4. These materials were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 adsorption Isotherms, Fourier transform infrared spectroscopy (FTIR), and evaluated as adsorbents and heterogeneous catalysts in Fenton-like and photo-Fenton-like reactions. The results showed that the magnetite was prepared and incorporated to the LDH with success, forming a composite providing magnetic properties and promoting alteration in the morphology, quantity and size of the LDH pores. The evaluation of the adsorption and oxidation processes utilizing the Fe3O4/LDH composite showed to be more efficient than for pure LDH. This better performance can be explained by higher surface area and larger pores for Fe3O4/LDH which allow a more effective contact with the dye solution and more proximity of large molecules like those of Reactive Black with the sites where the OH• radicals are formed.

Two new mononuclear oxamato-containing palladium(II) complexes of formula K2[Pd(opba)]·2H2O (1) and (PPh4)2[Pd(opba)]·2H2O (2) and the heterodimetallic palladium(II)–cobalt(II) chain {[Co(H2O)2Pd(opba)]·dmso}n (3) [opba = 1,2-phenylenebis(oxamate), PPh4+ = tetraphenylphosphonium cation and dmso = dimethyl sulfoxide] have been prepared, and the structures of two of them (compounds 2 and 3) were determined by X-ray diffraction analysis of single crystals. The structure of 2 consists of discrete anions of [Pd(opba)2]2– and PPh4+ cations. Each PdII ion in 2 is surrounded by two oxamate nitrogen atoms and two carboxylate oxygen atoms in a square-planar surrounding. Compound 3 is a neutral chain with regular alternating PdII and CoII ions, the [Pd(opba)]2– entity acting as a bis(bidentate) ligand towards trans-diaquacobalt(II) fragments, and dmso molecules of crystallization. Compound 3 exhibits a single-crystal-to-single-crystal phase transition between monoclinic C2/c (α phase) and triclinic P (β phase) space groups. Within the α phase the dmso molecule of crystallization is disordered, but it becomes ordered below 215 K in the β phase. The ordering process of the dmso molecule is followed by a reduction in the unit cell volume. The magnetic properties of 3 are barely affected by the structural phase transition, and they indicate an important spin–orbit coupling of the high-spin octahedral CoII ion [α = 1.245, λ = –151 cm–1, Δ = 494 cm–1, the spin Hamiltonian being defined as H = –αλLS + Δ(Lz2–2/3) + βH(–αL + geS)] with an almost negligible intra- [through the diamagnetic square-planar PdII centre] and interchain antiferromagnetic interactions (θ = –0.32 K).

This work describes the synthesis and characterization of two new compounds with ligand opy (N-(2-pyridyl)oxamate): the copper(II) precursor [Me4N]2[Cu(opy)2].5H2O and CoII CuII coordination polymer {[CoCu(opy)2]}n×4nH2O. This latter compound was obtained by reaction of [Me4N]2[Cu(opy)2].5H2O and CoCl2.6H2O in water. The heterobimetallic CoII CuII chain was characterized by elemental analysis, IR spectroscopy, thermogravimetry and magnetic measurements. Magnetic characterization revealed typical behavior of one-dimensional (1D) ferrimagnetic chain as shown in the curves of temperature (T) dependence of magnetic susceptibility (χM), in the form of χMT versus T, and dependence of magnetization (M) with applied field (H).

We have studied the magnetic properties of NiO nanoparticles dispersed into mesoporous silica glass with three particle size distributions obtained using the bottom-up nanofabrication technique. The samples were characterized by XRD, Raman, and TEM and found to consist of monodispersed rock salt structure NiO nanoparticles. The magnetic results indicate that the particles can be considered as an antiferromagnetically ordered core with an uncompensated magnetic moment, and a magnetically disordered surface. The magnetization loops exhibit a loop shift with intriguing temperature dependence. Furthermore, an increase in the average blocking temperature is observed with increases in the number of impregnation−decomposition cycles. These results are discussed considering the influence of the magnetic interaction on the core−shell structure of the nanoparticles.

A detailed study of the magnetic properties of colloidal Ni nanoparticles (∼5.2 nm) that do not present long range order is presented. Two magnetic contributions were observed at high temperatures (>25 K), a dominating paramagnetic one and a ferromagnetic contribution, detected by ferromagnetic resonance and low field magnetic measurements. Interestingly, the ferromagnetic behavior does not follow a classical superparamagnetic description. The effective anisotropy increases at low temperatures due to interactions among the ferromagnetic clusters, which leads the system to a frustrated state.

We present low temperature specific heat and ac-susceptibility measurements of Fe8 powdered sample. Below 1.3 K, super-paramagnetic blocking effects and an excess specific heat contribution are observed. The latter is attributed to a splitting of the ground state doublet in an inhomogeneous local field of hyperfine and dipolar origin. The local field contributions are evident at the resonances observed in the field dependent ac-susceptibility and specific heat below 0.5 K. The low temperature Schottky contribution is in agreement with known crystal field parameters of effective spin Hamiltonians proposed to simulate EPR and inelastic neutron scattering experiments.

The ferrimagnetic system CoCu(opba) [opba = ortho-phenylenebis(oxamato)] was employed to prepare a traditional chain [CoCu(opba)]·4H2O (1) and a nanocomposite by incorporation in porous Vycor glass (PVG). This nanocomposite was made by first anchoring [Bu4N]2[Cu(opba)] on PVG [PVG-Cu (2)] and then treating it “in situ” with cobalt(II) acetate to obtain the nanomagnet PVG-CuCo (3). Magnetic measurements show that 1 consists of a one-dimensional ferrimagnet with strong intrachain antiferromagnetic coupling and weak interchain interactions that result in spin-glass behavior below 3.5 K. Nanocomposite 3 presents ferrimagnetic chains limited by the nanopore size, which leads to a slow relaxation of the magnetization following Arrhenius' law, frequency dependence for in-phase and out-of-phase susceptibility, and hysteresis below the blocked regime temperature (< 6 K). These features are characteristics of single-chain magnets (SCM).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Nanocrystalline CoFe2O4 with an average grain size of about 40 nm was successfully prepared by a modified citrate-gel method. At temperatures of 3 and 300 K, the measured coercive fields are 0.43 and 0.07 T and the magnetizations at 7 T are 89 and 83 emu/g, respectively. At room temperature, the longitudinal and transversal magnetostriction values are −130 and 70 ppm, respectively. The contribution of a disordered magnetic phase was detected by the occurrence of a peak in the ac-susceptibilities curves at around 250 K. The temperature dependence of the field-cooled and zero field-cooled low-field magnetization showed a larger irreversibility below this temperature. This disordered phase behaves like a spin-glass, which is coexisting with the ferrimagnetically ordered main phase

The synthesis of one-dimensional, molecule-based magnets and the investigation of their structure and physical properties are described. The one-dimensional CoCu(opba) system, where opba [ ortho -phenylenebis(oxamato)] bridges the metal ions resulted in antiferromagnetic coupling, and was studied by insertion of electron donating and electron withdrawing groups in the organic ligands’ aromatic rings. In order to verify the influence of these modifications, three compounds of the formula [CoCu(opba- xy )], where x = y =CH 3 (compound 1 ), x = y =Cl (compound 2 ), x =H and y =NO 2 (compound 3 ) are described. The electronic structure of the oxamato bridge group has been studied using calculations at the density functional theory (DFT) level for [Cu(opba- xy )] 2− building blocks. Magnetic measurements in polycrystalline samples showed ferrimagnetic behavior for the three compounds. Least-squares fits of the experimental data indicate antiferromagnetic couplings J CoCu of −35.0, −32.9 and −24.2 cm −1 for 1 , 2 and 3, respectively.

An overview on magnetic of nanostructured magnetic materials is presented, with particular emphasis on the basic features displayed by granular nanomagnetic solids. Besides a review of the basic concepts and experimental techniques, the role of structural disorder (mainly the distribution of grain sizes), interparticle magnetic interactions and surface effects are also discussed with some detail. Recent results, models and trends on the area are also discussed.

In this work we report on the study of the magnetic properties of 2D arrays and 3D dispersions of colloidal iron oxide nanoparticles prepared by Langmuir–Blodgett technique and by dilution in paraffin wax solid solution, respectively. The influence of magnetic interparticle coupling on the superparamagnetic relaxation behavior was investigated by means of DC magnetization measurements. A quantitative analysis of the field dependence of the blocking temperature and the role of the interparticle coupling is presented. We explain our results using a phenomenological model based on the random anisotropy and micromagnetic theories that account for particle coupling effects in the superparamagnetic properties.

The synthesis, crystal structure and preliminary magnetic characterization of a new heterobimetallic [Mn₅(II)Cu₅(II)] wheel containing a flexible bis-oxamate type ligand are described. This decanuclear compound exhibits a relatively strong intra-wheel antiferromagnetic interaction leading to a ground spin state S = 10.

A new precursor of molecule-based magnetic systems, [Fe(H2opba-i)(dmso)2]Cl (1), with opba = ortho-phenylenebis(oxamato) in an iminoalcohol tautomeric form, was obtained as a product from the reaction between H4opba and FeCl3. Data from elemental analysis, IR and Mössbauer spectroscopies and magnetic measurements indicate that this precursor is composed of a mixture of trans (83%) and cis (17%) isomers. The chiMT value at 298K (2.1 emu K mol-1) corresponds to FeIII with spin state (S) between 3/2 and 5/2. Theoretical calculations (PBE/DZVP2) of trans- and cis-[Fe(H2opba-i)(dmso)2 ]+ show that both isomers have spin S = 1/2 in the ground state and S = 3/2 for the trans and S = 5/2 for the cis in the first excited state. The combination of these results leads to chiMT values of 0.375 and 2.3 emu K mol-1, at low and high temperature respectively, which are in accordance with the experimental data for 1. Um precursor inédito de sistemas magnéticos moleculares, [Fe(H2opba-i)(dmso)2]Cl (1), onde opba = orto-fenilenobis(oxamato) na forma tautomérica iminoálcool, foi obtido como produto da reação entre H4opba e FeCl3. Os dados de análise elementar, espectroscopias de absorção no infravermelho e Mössbauer e medidas magnéticas indicam que este precursor é constituído por uma mistura de isômeros trans (83%) e cis (17%). O valor de chiMT a 298 K (2,1 emu K mol-1) corresponde ao FeIII com estado de spin (S) entre 3/2 e 5/2. Cálculos teóricos (PBE/DZVP2) de trans- e cis-[Fe(H2opba-i)(dmso)2 ]+ mostram que ambos os isômeros têm spin S = 1/2 no estado fundamental e S = 3/2 para o trans e S = 5/2 para o cis no primeiro estado excitado. A combinação destes resultados leva a valores de chiMT de 0,375 emu K mol-1 e 2,3 emu K mol-1 a baixas e altas temperaturas, respectivamente, os quais são concordantes com os dados experimentais para 1.

In order to study the influence of magnetic interactions on the blocking temperature of magnetic nanoparticles, magnetization measurements were carried out on a discontinuous Co/SiO 2 multilayer. The observed field dependence of the blocking temperature does not fit any of the non-interacting laws commonly used to describe this behavior. We applied a generalized model which considers a field-dependent magnetic correlation length. This model explains well the experimental results and can be used in other nanoparticulate systems.

Interactions in magnetic granular systems are difficult to address in real systems. Several experimental results have been obtained along the last 50 years, but few theoretical approaches have been developed to account for these extremely complex systems. A brief review on the role of interactions on the magnetic properties of granular magnetic systems is given, with a focus on a novel approach known as interacting superparamagnetic model. Two different systems are tested, and the results are discussed in terms of such phenomenological model.

In this work, we investigated the effects of postdeposition heat treatment on structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition. Structural analysis by X-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy indicate the formation of a single phase cobalt ferrite nanoparticles with the size ranging from 4.3 to 33.3 nm depending on the annealing temperature. The magnetic properties of the samples were investigated in a wide temperature range (50-400 K). Noticeable effects of the cubic magnetocrystalline anisotropy on the magnetization process of nanoparticles were observed for samples annealed at 450 °C and 600 °C, while for samples as-deposited and annealed at 300 °C the magnetization properties were dominated by a uniaxial effective anisotropy. ΔM technique was used to investigate the magnetic interaction among the nanoparticles. Only demagnetizing interactions were observed for the sample annealed up to 300 °C, while for the samples treated at 450 °C and 600 °C, both magnetizing and demagnetizing interactions were observed. The results are discussed considering the evolution of the nanoparticles' nanostructure with anneals and its effects on the magnetic properties. Fil: Fabris, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidade Federal Fluminense; Brasil Fil: Xing, Y.T.. Universidade Federal Fluminense; Brasil Fil: Franceschini, D.F.. Universidade Federal Fluminense; Brasil Fil: Sanchez, D.R.. Universidade Federal Fluminense; Brasil Fil: Alzamora, M.. Universidade Federal do Rio de Janeiro; Brasil Fil: Nunes, W.C.. Universidade Federal Fluminense; Brasil

We have investigated the magnetic properties of Ni nanoaggregates produced by ion implantation in ZnO single crystals. Several deviations from classical models usually adopted to describe the magnetic properties of nanoparticle systems were found. The strain between host and Ni nanoaggregates induces a magnetic anisotropy with a preferred direction. We show that these anisotropy effects can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among the nanoaggregates similar to that of an oriented, interacting nanocrystal ensemble.

Preparation of size-controllable magnetite (Fe3O4) nanoparticles by alkaline oxidation of ferrous ion adsorbed in sulfonated mesoporous styrenedivinylbenzene copolymer is described. It was observed that the magnetite nanoparticle size increases by ion-charging the sulfonated polymeric template with ferrous aqueous solution at increasing iron concentration. A simple model describing the amount of iron incorporation in the polymeric template is proposed. The magnetite-based composite was investigated by atomic absorption, transmission electron microscopy, Mo 1ssbauer spectroscopy, X-ray diffraction, and magnetization data.

Measurements of magnetization loops on melt-spun CuCo ribbons revealed a minimum in the temperature dependence of the coercivity. A coherent interpretation was given through Monte Carlo simulations of a dispersed system of noninteracting, uniaxial magnetic granules embedded in a nonmagnetic matrix. The coercivity is implicitly defined by the balance between the negative magnetization of superparamagnetic granules and the remaining magnetization of blocked granules after saturation in the positive field direction. When the temperature rises in a system made of a large amount of small granules and a small amount of big granules, unblocking predominates over thermal fluctuations and the coercivity decreases until a certain temperature at which most of the small granules are superparamagnetic; above this temperature, thermal fluctuations predominate, and the coercivity increases almost linearly with the temperature until the final unblocking of the big granules.

We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe73.5Nb3CuSi13.5B9 ribbons (FINEMET) were submitted to the thermal treatments of several times (1 ⩽ t ⩽ 240 min) at 570 °C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe3Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 °C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase.

The magnetic properties of arrays of magnetic nanowires, prepared by electrodeposition in nanopores of alumina membranes, were investigated. The hysteresis loops were measured using a magnetic field applied both perpendicularly and parallel to the nanowire axes for temperatures between 4 and 300 K. The results show a preferred direction of magnetization parallel to the nanowire axis at room temperature, but perpendicular to the nanowire axis below a certain crossover temperature. We observe that such crossover temperature increases by increasing the nanowire length. The results are discussed considering strains caused by the different expansion coefficients of the nanowires and the matrix.

In this study, we investigate the temperature dependence of the zero-field-cooled magnetization of a quasi-monodisperse dextran-coated magnetite-based magnetic fluid. The well-defined maximum in the magnetization versus temperature curve and its downshift with the applied external field is explained by a simple model considering thermally activated dynamics of the nanoparticles magnetic moment and the temperature dependence of the saturation magnetization.

This paper reports the use of muon spin relaxation spectroscopy to study how the aggregation behavior of magnetic surfactants containing lanthanide counterions may be exploited to create spin glass-like materials. Surfactants provide a unique approach to building in randomness, frustration and competing interactions into magnetic materials without requiring a lattice of ordered magnetic species or intervening ligands and elements. We demonstrate that this magnetic behavior may also be manipulated via formation of micelles rather than simple dilution, as well as via design of surfactant molecular architecture. This somewhat unexpected result indicates the potential of using novel magnetic surfactants for the generation and tuning of molecular magnets.

We present the specific heat measurements at very low temperatures of the single molecule magnets Mn12ac and Fe8. Below 3 K, both systems present superparamagnetic blocking effects in zero external magnetic field and an excess specific heat contribution around 1 K. The experimental results are explained considering a distribution of internal fields of dipolar and hyperfine origin as well as quantum tunneling of the magnetization.

The magnetization curves of Co 10 Cu 90 as-cast and annealed melt-spun ribbons were measured for 4.2 < T < 600 K. The narrow hysteresis found in these systems present an unusual temperature dependence of the coercive field H c (T), in most samples with a decrease in the low T region due to superparamagnetic unblocking followed by an increase in Tand a maximum around room temperature. These results are explained in analogy to soft nanocrystalline alloys using the random anisotropy model and the existence of interparticle interactions.

An overview on magnetic of nanostructured magnetic materials is presented, with particular emphasis on the basic features displayed by granular nanomagnetic solids. Besides a review of the basic concepts and experimental techniques, the role of structural disorder (mainly the distribution of grain sizes), interparticle magnetic interactions and surface effects are also discussed with some detail. Recent results, models and trends on the area are also discussed.

This work focuses on synthetic methods to produce monodisperse Ni colloidal nanoparticles (NPs), in the 4−16 nm size range, and their structural characterization. Narrow size distribution nanoparticles were obtained by high-temperature reduction of a nickel salt and the production of tunable sizes of the Ni NPs was improved compared to other methods previously described. The as-synthesized nanoparticles exhibited spherical shape and highly disordered structure, as it could be assigned by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Annealing at high temperature in organic solvent resulted in an increase of nanoparticle atomic ordering; in this case, the XRD pattern showed an fcc-like structure. Complementary data obtained by X-ray absorption spectroscopy confirmed the complex structure of these nanoparticles. Temperature dependence of the magnetic susceptibility of these highly disordered Ni NPs showed the magnetic behavior cannot be described by the conventional superparamagnetic theory, claiming the importance of the internal structure in the magnetic behavior of such nanomaterials. Fil: Winnischofer, Herbert. Brazilian Synchrotron Light Laboratory; Brasil Fil: Rocha, Tulio C. R.. Universidade Estadual de Campinas; Brasil Fil: Nunes, Wallace C.. Universidade Estadual de Campinas; Brasil Fil: Socolovsky, Leandro Martín. Universidade Estadual de Campinas; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Knobel, Marcelo. Universidade Estadual de Campinas; Brasil Fil: Zanchet, Daniela. Brazilian Synchrotron Light Laboratory; Brasil

In this paper, the exchange bias effect in Fe/EuTe(111) bilayers prepared by molecular beam epitaxy (MBE) is investigated by SQUID. For cooling fields from 2 to 70 kOe, only positive shifts was found of the magnetic hysteresis loops. This result indicates antiferromagnetic coupling between the ferromagnetic (F) and the antiferromagnetic (AF) layer at the Fe/EuTe(111) interface independently of the applied cooling field. Fe/EuTe bilayers prepared on EuTe(111) surfaces with stoichiometry (Eu:Te = 2.1) and, consequently, magnetic order altered by Ar-sputtering do not present exchange bias.

Magnetic properties of Co-SiO2 granular films are displayed and explained by means of a phenomenological model that takes into account the effects of magnetic interparticle coupling on the superparamagnetic relaxation behavior. The model is based on the analysis of coupled nanoparticles through a modified random anisotropy model that takes into account the concentration and size of the nanoscopic particles, as well as the field dependence of the correlation length. The proposed model leads to an accurate description of the field dependence of the blocking temperature, substantially better than the power law usually employed to describe noninteracting particles.

Spherical magnetic nanoparticles with narrow size distribution and organic capping were diluted in paraffin with different concentrations to verify the role of dipolar interactions on the macroscopic magnetic behavior. Increasing concentration of magnetic nanoparticles leads to higher blocking temperatures. The experimental data were analyzed by means of a recently proposed model that takes into account magnetic interactions of dipolar origin, and an excellent agreement was found. Considering the magnetic interaction among particles it was possible to obtain the real magnetic moment and estimate structural parameters that are consistent with the ones obtained by small angle x-ray scattering and transmission electron microscopy.

In this paper, we study the effect of long-range dipolar interaction on the magnetic ordering of water-based cadmium ferrite fluids (MF).

The magnetic properties of Cu97Co3 and Cu90Co10 granular alloys were measured over a wide temperature range (2 to 300K). The measurements show an unusual temperature dependence of the coercive field. A generalized model is proposed and explains well the experimental behavior over a wide temperature range. The coexistence of blocked and unblocked particles for a given temperature rises difficulties that are solved here by introducing a temperature dependent blocking temperature. An empirical factor gamma arise from the model and is directly related to the particle interactions. The proposed generalized model describes well the experimental results and can be applied to other single-domain particle system., Comment: 7 pages, 8 figures, revised version, accepted to Physical Review B on 29/04/2004