A simple model for the magnetocrystalline anisotropy in mixed ferrite nanoparticles

A simple model, based on the relative occupancy of tetrahedral and octahedral sites by different cations, is proposed for the magnetocrystalline anisotropy of mixed ferrite nanoparticles. According to this model, the total magnetocrystalline anisotropy is the weighted average of the contributions of the anisotropies of Fe³⁺ and M²⁺ ions in A and B sites. The model predictions are confirmed in the case of cobalt-zinc ferrite.     

Vibrational dynamics of biological molecules: Multi-quantum contributions

High-resolution X-ray measurements near a nuclear resonance reveal the complete vibrational spectrum of the probe nucleus. Because of this, nuclear resonance vibrational spectroscopy (NRVS) is a uniquely quantitative probe of the vibrational dynamics of reactive iron sites in proteins and other complex molecules. Our measurements of vibrational fundamentals have revealed both frequencies and amplitudes of ⁵⁷Fe vibrations in proteins and model compounds. Information on the direction of Fe motion has also been obtained from measurements on oriented single crystals, and provides an essential test of normal mode predictions. Here, we report the observation of weaker two-quantum vibrational excitations (overtones and combinations) for compounds that mimic the active site of heme proteins. The predicted intensities depend strongly on the direction of Fe motion. We compare the observed features with predictions based on the observed fundamentals, using information on the direction of Fe motion obtained either from DFT predictions or from single crystal measurements. Two-quantum excitations may become a useful tool to identify the directions of the Fe oscillations when single crystals are not available.     

An estimation of the magnetic disorder at Fe/Cr interfaces in coupled and non-coupled Fe/Cr multilayers

The antiferromagnetic coupling at the Fe/Cr interfaces, inferred from the orientation of the Cr magnetic moments, is used to estimate the magnetic disorder resulting from the interfacial roughness in Fe/Cr multilayers. A crossover from in-plane to out-of-plane orientation of Cr moments depends on the energy cost in either case: (i) to break the interfacial Fe–Cr antiferromagnetic coupling or (ii) having sites with frustrated Cr–Cr antiferromagnetic coupling in the Cr interlayers. A quantitative model of the magnetic frustration due to interfacial disorder in Fe/Cr multilayer systems is described. The step edge density, or terrace size, required to break the interfacial Fe–Cr coupling and destroy the Fe–Fe interlayer exchange coupling is estimated.     

Concerning the cation distribution in MnFe2O4 synthesized through the thermal decomposition of oxalates

A single phase manganese ferrite powder have been synthesized through the thermal decomposition reaction of MnC₂O₄·2H₂O-FeC₂O₄·2H₂O (1:2 mole ratio) mixture in air. DTA-TG, XRD, Mössbauer spectroscopy, FT-IR and SEM techniques were used to investigate the effect of calcination temperature on the mixture. Firing of the mixture in the range 300-500 °C produce ultra-fine particles of α-Fe₂O₃ having paramagnetic properties. XRD, Mössbauer spectroscopy as well as SEM experiments showed the progressive increase in the particle size of α-Fe₂O₃ up to 500 °C. DTA study reveals an exothermic phase transition at 550 °C attributed to the formation of a Fe₂O₃-Mn₂O₃ solid solution which persists to appear up to 1000 °C. At 1100 °C, the single phase MnFe₂O₄ with a cubic structure predominated. The Mössbauer effect spectrum of the produced ferrite exhibits normal Zeeman split sextets due to Fe³⁺ions at tetrahedral (A) and octahedral (B) sites. The obtained cation distribution from Mössbauer spectroscopy is (Fe_0.92Mn_0.08)[Fe_1.08Mn_0.92]O₄.     

Magnetism in nanoparticles of semiconducting FeSi2

Iron disilicide has been found to exhibit superparamagnetism in nanoparticles, even though no magnetic ordering occurs in bulk. The unexpected behavior was attributed, based on magnetic studies, to chemical disorder. A lack of sextet-type signals in Mössbauer spectra supports that the observed magnetic order is confined to only a very small fraction of magnetic Fe ions. Moreover, quadrupole-splitting and isomer-shift parameters reveal a significant amount of Fe in a short-range α-FeSi₂ structure, while XRD suggests an overall β-FeSi₂ structure. Such a compositional heterogeneity is also reflected in a calorimetrically obtained spin-glass-like anomaly at low temperatures. Meanwhile, as the particle size decreases, specific heat and Sommerfeld constant are enhanced due to lattice softening and the emergence of surface charge density of states, respectively, in nanoparticles.     

Study of magnetic phases in mechanically alloyed Fe50Zn50 powder

The mechanosynthesis of Fe₅₀Zn₅₀ alloy resulted in the formation of the bcc Fe(Zn) solid solution after 20 h of milling. Structural transformations induced by mechanical alloying and heating, and magnetic properties of the powders were studied by Mössbauer spectroscopy, X-ray diffraction, Faraday balance and vibrating sample magnetometry techniques. All alloys studied exhibit strong magnetic ordering with Curie temperatures close to 900 K. Room temperature Mössbauer measurements revealed distinguished magnetic environments in the samples. The decrease of coercivity with prolonged milling time was attributed to the reduction or averaging of local magnetic anisotropies.     

Effect of indium addition on the structure and magnetic properties of YIG

In this work we report the structure and magnetic properties of a series of single-phase indium-substituted yttrium iron garnet (In-YIG) nanoparticles with nominal composition of Y₃In_xFe_5−xO₁₂ (x=0.1, 0.2, 0.3 and 0.4) prepared by conventional mixed oxide route. Based on XRD results, the lattice parameters of the samples increased with increase in In³⁺ content due to its larger ionic radius. Mössbauer results confirmed the substitution of In³⁺ for Fe³⁺ in [a] site of YIG structure. Further, the magnitudes of the magnetic hyperfine field (MHF) were seen to reduce due to indium substitution. Moreover, a rising trend was observed for saturation magnetization (M_S) of the samples with x>0.2 owing to the substitution of non-magnetic In³⁺ for Fe³⁺. However, the observed initial drop of M_S for the sample with x=0.2 compared to that with x=0.1 is possibly attributed to the dominance of spin canting over the net magnetization rise caused by In³⁺ in [a] sites.     

Structural and Magnetic Properties of NiFe2−2xSnxCuxO4

The substituted nickel ferrite (NiFe_2−2xSn_xCu_xO₄, x=0, 0.1, 0.2, 0.3) was prepared by the conventional ceramic method. The effect of substitution of Fe³⁺ ions by Sn⁴⁺ and Cu²⁺ cations on the structural and magnetic properties of the ferrite was studied by means of ⁵⁷Fe Mössbauer spectroscopy, alternating gradient force magnetometry (AGFM) and Faraday balance. Whereas undoped NiFe₂O₄ adopts a fully inverse spinel structure of the type (Fe)[NiFe]O₄, Sn⁴⁺ and Cu²⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite. Based on the results of Mössbauer spectroscopic measurements, the crystal-chemical formula of the substituted ferrite may be written as (Fe)[NiFe_1−2xSn_xCu_x]O₄, where parentheses and square brackets enclose cations in tetrahedral (A) and octahedral [B] coordination, respectively. The Néel temperature and the saturation magnetization values of the NiFe_2−2xSn_xCu_xO₄ samples were found to decrease with increasing degree of substitution (x). The variation of the saturation magnetization with x measured using the AGFM method and that calculated on the basis of the Mössbauer spectroscopic measurements are in qualitative agreement.     

Mössbauer Spectroscopic Studies of an Oxidized Ordinary Chondrite Fallen at Itawa-Bhopji,India

Mössbauer studies of the Itawa-Bhopji meteorite fallen on May 30, 2000 in Rajasthan, India, show that the main iron minerals in it are Fe–Ni (kamacite/taenite), troilite, olivine and pyroxene. These provide characteristic signatures of an ordinary chondrite. Mössbauer absorption areas corresponding to different phases favour its classification as L/LL-type ordinary chondrite. The iron in the olivine is unusually high and the metallic iron is quite low, showing that it has faced oxidizing conditions prior to fall.     

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni-Mn-Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using ⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni-Mn-Ga-Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.     

Mössbauer and magnetic studies in nickel ferrite nanoparticles: Effect of size distribution

The magnetic properties of nickel ferrite nanoparticles in the form of powders, prepared by the sol-gel process and subjected to different annealing temperatures, were investigated using both static and dynamic measurements namely hysteresis, zero field cooled-field cooled magnetization (ZFC-FC) measurements and Mössbauer spectroscopy. The Transmission Electron Microscopy (TEM) studies reveal particle sizes for the as-prepared particles which increases upto 52 nm with annealing. A bimodal distribution, upto an annealing temperature of was observed. ZFC-FC measurements for the as-prepared samples reveal twin peaks, indicative of the bimodal size distribution. ZFC-FC measurements performed for fields varying from 100 Oe to 3 kOe show a superparamagnetic phase with blocking temperatures between 320 and . Numerical simulations for the ZFC-FC studies indicate that the signature of the bimodal size distribution can be seen only at very low fields. The variation of coercivity with particle size, as determined from the hysteresis measurements, shows a transition from a single domain to a multi domain state for particle sizes larger than 35 nm. Mössbauer measurements performed at room temperature for the as-prepared sample shows a six finger pattern for the samples with higher particle size and a doublet pattern for the samples with smaller particle size, which is indicative of their superparamagnetic nature.     

Spin excitations in a K0.84Fe1.99Se2 superconductor as studied by Mössbauer spectroscopy

Mössbauer spectroscopy was used to probe the site-specific information of a K_0.84Fe_1.99Se₂ superconductor. A spin excitation gap, ΔE ≈5.5 meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using the simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated iron-selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.     

Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

On the Role of Aluminum in Finemet

The role of aluminum with respect of its influence on some intrinsic magnetic properties of the nanocrystalline Finemet, such as saturation magnetization, Curie temperature, spin-wave stiffness constant and hyperfine magnetic fields was investigated. The strengthening effect of Al on the exchange interaction was observed for the alloys with small Al content (up to 3 at.%). Higher concentration of Al (5-7 at.%) led to considerably suppressed exchange interaction.     

Magnetization behavior and magnetocaloric effect in bulk amorphous Fe60Co5Zr8Mo5W2B20 alloy

Microstructure by X-ray diffraction and Mössbauer spectroscopy, and isothermal magnetic entropy changes in the bulk amorphous Fe₆₀Co₅Zr₈Mo₅W₂B₂₀ alloy in the as-quenched state and after annealing at 720 K for 15 min are studied. The as-cast and heat treated alloy is paramagnetic at room temperature. The quadrupole splitting distribution is unimodal after annealing indicating the more homogenous structure in comparison with that for the as-cast alloy. Curie temperature slightly increases after annealing from 265±2 K in the as-quenched state to 272±2 K and the alloy exhibits the second order magnetic phase transition. The maximum of isothermal magnetic entropy changes appears at the Curie points and is equal to 0.30 and 0.42 J/(kg·K) for the alloy in the as-quenched state and after annealing, respectively. In the paramagnetic region the material behaves as a Curie-Weiss paramagnet.     

Effects of annealing temperature on structure and magnetic properties of CoAl0.2Fe1.8O4/SiO2 nanocomposites

CoAl_0.2Fe_1.8O₄/SiO₂ nanocomposites were prepared by sol–gel method. The effects of annealing temperature on the structure and magnetic properties of the samples were studied by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy. The results show that the CoAl_0.2Fe_1.8O₄ in the samples exhibits a spinel structure after being annealed. As annealing temperature increases from 800 to 1200 °C, the average grain size of CoAl_0.2Fe_1.8O₄ in the nanocomposites increases from 5 to 41 nm while the lattice constant decreases from 0.8397 to 0.8391 nm, the saturation magnetization increases from 21.96 to 41.53 emu/g. Coercivity reaches a maximum of 1082 Oe for the sample annealed at 1100 °C, and thereafter decreases with further increasing annealing temperature. Mössbauer spectra show that the isomer shift decreases, hyperfine field increases and the samples transfer from mixed state of superparamagnetic and magnetic order to the completely magnetic order with annealing temperature increasing from 800 to 1200 °C.     

Effects of substitution of Mo for Nb on less-common properties of Finemet alloys

Particular properties of Fe-Nb/Mo-Cu-B-Si rapidly quenched ribbons were examined. Apart from minor variation, no significant difference due to the Mo for Nb substitution was observed in alloy density and its annealing-induced changes. The same holds for the anisotropic thermal expansion of as-cast ribbon when annealed and for induced anisotropy when annealed under stress. The Mo-substituted ribbons show only slightly higher crystallinity and lower coercivity if annealed in inert gas ambience than in vacuum. Some diversity in surface to interior heterogeneity of the differently annealed ribbons can still be distinguished. Preserving a minor percentage of Nb together with Mo does not seem substantiated to obtain favorable soft magnetic properties of ribbons annealed in inert gas.