Magnetic properties of nanocrystalline CoFe2O4 synthesized by thermal plasma in large scale

The paper reports the large scale synthesis of nanoparticles of CoFe₂O₄ using thermal plasma reactor by gas phase condensation method. The yield of formation was found to be around 15 g h⁻¹. The magnetic properties of CoFe₂O₄, synthesized at different reactor powers, were investigated in view of studying the effect of operating parameters of plasma reactor on the structural reorganization leading to the different cation distribution. The values of saturation magnetization, coercivity and remanent magnetization were found to be influenced by input power in thermal plasma. Although the increase in saturation magnetization was marginal (61 emu g⁻¹ to 70 emu g⁻¹) with increasing plasma power; a significant increase in the coercivity (552 Oe to 849 Oe) and remanent magnetization (16 emu g⁻¹ to 26 emu g⁻¹) were also noticed. The Mössbauer spectra showed mixed spinel structure and canted spin order for the as synthesized nanoparticles. The detailed analysis of cation distribution using the Mössbauer spectroscopy and X-ray photoelectron spectroscopy leads to the conclusion that the sample synthesized at an optimized power shows the different site selective states.     

Effect of boron substitution on the properties of NiZnCu ferrite for multilayer chip inductors

The initial permeability of the toroidal core sample with boron substituted NiZnCu ferrite (Ni_0.36Zn_0.44Cu_2.2Fe_1.96 − xB_xO₄(x = 0.0, 0.2, 0.4, 0.6, and 0.8)) which was annealed at 900 °C for 2 h decreased from 162.4 to 123.2 as boron concentration x is increased from 0.2 to 0.8. The quality factor and density of 0.2 mol% boron substituted NiZnCu ferrites sintered at 900 °C for 2 h was about 240.0 and 4.84 g/cm³, respectively. The density, shrinkage and saturation magnetization were increased with increasing annealing temperature. It was shown that the boron substituted NiZnCu ferrite systems were promising as a high quality factor material in the fabrication of multilayer chip inductors (MLCIs).     

From Magnetite to Cobalt Ferrite

We synthesized Fe_3–x Co _x O₄ (x = 0–1) using the hydrothermal method in order to demonstrate the compositional modulation of magnetite to cobalt ferrite. Our Mössbauer spectroscopy results provided direct evidence for the presence of the Co substitution in the B sublattice, which was found to be accompanied by a systematic increase of the hyperfine magnetic field at these sites. The mechanism we propose relies on the substitution of Fe²⁺ by Co²⁺ in the B sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on content x of cobalt substitution. The X-ray diffraction (XRD) determinations demonstrated a linear increase in the lattice parameter when going from magnetite to cobalt ferrite. For the particular value x = 0.1, we report that the two sublattices of magnetite become equally populated with Fe. For this particular value of the cobalt content, we obtained a thin film sample by laser ablation deposition and characterized its properties by XRD and conversion electron Mössbauer spectroscopy (CEMS).     

Enhanced magnetic behavior of Al substituted LaFeO3 (La(1 − x)AlxFeO3, x = 0.10 and 0.30)

La_(1 − x)Al_xFeO₃ (x = 0, 0.1, 0.3) has been prepared by solid state reaction method. The formation of pure crystallographic phase of LaFeO₃ and the substitution of Al³⁺ in all the doped samples have been confirmed by Rietveld analysis. The magnetic measurements viz., magnetization curves, hysteresis loops etc. in the temperature range 300-5 K showed that magnetization of the doped samples has been appreciably enhanced compared to that of the pristine LaFeO₃. The maximum enhancement factor of ~ 19 for saturation magnetization measured at room temperature has been found in La_0.7Al_0.3FeO₃.     

Magnetic moment distribution study of Fe antidot arrays

Fe antidot arrays have been fabricated by ion beam sputtering method onto the porous alumina templates. Conversion electron Mössbauer spectroscopy was used to describe the magnetic moment distribution of the Fe antidot arrays. A component of the magnetic moment was found in the perpendicular direction of the film plane. The experimental results of magnetic moment distribution can be well explained by a simple model.     

The Structure and Magnetic Properties of Fe-Si-Cu-Nb-B Powder Prepared by Mechanochemical Way

We have studied the influence of the long-term (1700 h) milling on the structure and magnetic properties of Fe_73.5Si_13.5Cu₁Nb₃B₉ powder prepared in a vibratory mill. Three samples (in amorphous state, partially crystallized state, and mixed of pure elements) of the same chemical content were prepared. We found that the structure and magnetic properties significantly depend on the milling time. The coercivity of the sample prepared from pure elements monotonously increases with increased milling time. The coercivity of the samples milled from the ribbon increases to its maximum for milling time of 800 h and then decreases.     

Structural and magnetic properties of glass-ceramics containing silver and iron oxide

Glass-ceramics with a nominal composition of 25SiO₂–(50 − x)CaO–15P₂O₅–8Fe₂O₃–2ZnO–xAg (where x = 0, 2 and 4 mol%) have been prepared. Structural features of glass-ceramics have been investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Magnetic properties were studied using vibrating sample magnetometer and Mössbauer spectroscopy. Ca₃(PO₄)₂, hematite and magnetite are formed as major crystalline phases. The microstructure reveals the formation of 25–30 nm size particles. Mössbauer spectroscopy has shown the relaxation of magnetic particles. Saturation magnetization value is increased with an increase of Ag content up to 4 mol%, which has been attributed to the formation of magnetically ordered particles. The antibacterial response was found to depend on Ag ions concentration in the glass matrix and samples with 4 mol% Ag in glass matrix have shown effective antibacterial activity against Escherichia coli.     

Structural properties and Mössbauer spectra of metastable NdFe12B6

Metastable ternary NdFe₁₂B₆ compound was prepared by the annealing of the amorphous Nd-Fe-B ribbons and has a main phase of NdFe₁₂B₆ with some impurities of α-Fe and Nd₅Fe₁₈B₁₈. The NdFe₁₂B₆ was found to have a SrNi₁₂B₆ structure with space group R3¯m. Mössbauer spectra of the sample were measured between 300 K and 20 K. The result indicated that an abrupt drop with increasing temperature took place at about 205 K for the hyperfine field at both Fe(18 g) and Fe(18 h) sites. The corresponding hyperfine fields were found nearly constant at ferromagnetic state about 16.9(6) T and 19.3(5) T for Fe(18 g) and Fe(18 h) sites, respectively. Accordingly, the relative size of the Fe moments was deduced.     

Collapse of the Magnetic Hyperfine Structure of Barium Hexaferrite by Mechanochemical Activation

The collapse of the magnetic hyperfine structure of barium hexaferrite as an effect of mechanochemical activation was investigated. The various inequivalent sites and phases present in the milled material were studied by transmission Mössbauer spectroscopy. Barium hexaferrite nanoparticles were obtained after only 2 h of milling time and were found to coexist with iron oxide particles for longer times of exposure to mechanochemical activation. The milled samples were then subjected to thermal annealing, in order to test the reversibility of the collapse of the hyperfine structure. The thermal annealing performed (700°C, 1 h) was partly successful in restoring the magnetic hyperfine structure of milled hexaferrite. Complementary information was obtained using transmission electron microscopy (TEM) and electron diffraction (ED).     

Mössbauer Studies in the Mechanochemistry of Spinel Ferrites

Mössbauer spectroscopy can provide information on a large variety of mechanochemical processes in solids. In this overview, selected examples are presented of Mössbauer studies of homogeneous and heterogeneous mechanochemical reactions in spinel ferrites. Several interesting features are involved in the work, e.g., mechanically induced cation redistributions, changes of nearest-neighbor configurations, superparamagnetic relaxation, spin-canting effects, as well as formation and redox reactions. The considerable literature on mechanically treated spinel ferrites is reviewed. Despite numerous efforts, the understanding of the nonequilibrium mechanochemical processes in spinel ferrites is considered to be far from complete.     

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy were investigated according to martensitic transformation kinetics, morphology, magnetism of both austenite and martensite phases and also in terms of martensitic transformation start temperatures (M_s) for different austenite grain sizes of alloy. Kinetics of the transformation was found to be athermal. Also only lenticular martensite morphology was observed during investigations. On the other hand, Mössbauer spectra revealed a paramagnetic character for austenite phases and a ferromagnetic character for thermally induced martensitic phases. Determined M_s temperatures were found to be at − 128 °C for large grained samples and − 135 °C for small grained samples.     

Magnetic structure of the modulated martensite phase in Ni-Mn-Ga ferromagnetic shape memory alloys

The magnetic structure of the seven layered (7 M) modulated martensite phases in Mn-rich Ni-Mn-Ga alloys was studied using Mössbauer spectroscopy. The Mössbauer results clearly demonstrate that in contrast to the non-modulated tetragonal structure two new magnetic sublattices exists for the 7 M orthorhombic martensite phase. Based on the unit cell symmetry and atomic coordination, the additional magnetic sublattices have been assigned to the Ni site. The variation in the magnetic properties of the martensite phases has been related to the underlying magnetic structure.     

Microwave assisted preparation of Fe doped β-dicalcium silicate by sol-gel method

Fe³⁺ doped and undoped β-dicalcium silicates (β-Ca₂SiO₄ or β-C₂S) were prepared by sol-gel method. The gels formed were heated in a microwave oven and subsequently in a muffle furnace. The particle sizes of doped and undoped samples were found to be about 150 nm and 600 nm respectively. The materials were characterized by scanning electron microscopic, powder X-ray diffraction, BET specific area measurements, infrared spectroscopic and Mössbauer spectroscopic techniques. IR spectroscopic studies showed the possibility of distorted tetrahedral symmetry in Fe³⁺ doped β-C₂S. Calorimetric studies have shown that iron doped β-C₂S is highly reactive. From Mössbauer spectroscopic studies it is found that electronic environment of Fe³⁺ in β-C₂S is changed due to hydration. Degree of hydration calculated from non-equilibrium water content measurements have shown that Fe³⁺ doped β-C₂S hydrates at a much higher rate. Scanning electron microscopic studies have revealed that the hydration products in both the cases are almost identical. Formation and hydration of β-C₂S in the presence of Fe³⁺ have been understood.     

Interfacial atomic diffusion in AF/Fe/Cu/Fe (AF = Fe50Mn50 and Ir50Mn50) multilayer systems

Spin valve like AF/Fe/Cu/Fe (AF = Fe₅₀Mn₅₀ and Ir₅₀Mn₅₀) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the ⁵⁷Fe isotope were artificially grown at the AF/Fe and Fe/Cu interfaces and the interfacial atomic diffusion was observed via ⁵⁷Fe conversion electron Mössbauer spectroscopy. The results show that the atomic interdiffusion at all involved interfaces is lower in the IrMn based structures as compared to the FeMn based ones.     

Synthesis,crystal structure and electrochemical properties of the manganese-doped LiNaFe[PO4]F materials

The new compounds LiNaFe_1−xMn_x[PO₄]F (x ≤ 1/4) were synthesized by a solid state reaction route. The crystal structure of LiNaFe_3/4Mn_1/4[PO₄]F was determined from single crystal X-ray diffraction data. LiNaFe_3/4Mn_1/4[PO₄]F crystallizes with the Li₂Ni[PO₄]F-type structure, space group Pnma, a = 10.9719(13), b = 6.3528(7), c = 11.4532(13) Å, V = 798.31(16) Å³, and Z = 8. The structure consists of edge-sharing (Fe_3/4Mn_1/4)O₄F₂ octahedra forming (Fe_3/4Mn_1/4)FO₃ chains running along the b-axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The manganese-doped phases show poor electrochemical behavior comparing to the iron pure phase LiNaFe[PO₄]F.     

NiMn2−xFexO4 prepared by a reverse micelles method as conversion anode materials for Li-ion batteries

Synthesis by reverse micelles was used to produce NiMn_2−xFe_xO₄ with nanometric particle sizes for their use as conversion anode materials for lithium ion batteries. The hydroxide precursor was characterized by infrared spectroscopy and the decomposition was followed by thermal analysis. Cation distribution in the spinel structure of pristine samples was evaluated by Mössbauer spectroscopy evidencing that octahedral Fe³⁺ is substituted by Mn³⁺ ions in NiMnFeO₄. Capacity values of 750 mA h g⁻¹ were retained for 50 cycles for NiMnFeO₄ and NiFe₂O₄, respectively. A good kinetic response was observed in NiMnFeO₄ at 2C.     

Interpretation of Mössbauer spectra of YBa2Cu3−x Fe x O7−d

We have studied the Mössbauer spectra of YBa₂Cu_3?x Fe _x O_7?d (11 compositions) and present here the results obtained from quantitative analyses of the paramagnetic and magnetic spectra.     

Mössbauer and magnetic study of Mn, Zr and Cd substituted W-type hexaferrites prepared by co-precipitation

BaCo_2−xMn_xFe_16−2y(Zr–Cd)_yO₂₇ (x = 0–0.5 and y = 0–1.0) hexaferrite nanocrystallites of average sizes in the range of 33–42 nm are synthesized by the chemical co-precipitation method. The synthesized materials are characterized using different techniques including X-ray diffraction (XRD), energy dispersive X-ray florescence (ED-XRF), scanning electron microscope (SEM), Mössbauer spectrometer and vibrating-sample magnetometer (VSM). Based on analysis of the data obtained from Mössbauer spectral studies, doping is believed to have occurred preferably in the vicinity of 12k sub-lattice, i.e. f_IV (4e, 4f_IV), 2b (6g, 4f) and 2d site. Variations in the saturation magnetization (77.1–60.9 emu g⁻¹), remanent magnetization (22.08–31.23 emu g⁻¹) and coercivity (1570.1–674.7 Oe) exhibit tunable behavior with dopant content and therefore can be useful for application in various magnetic devices.     

Crystallization and Hardness of Melt Spun Fe73Si13B9Nb4Cu1 Alloy

An alloy having composition Fe₇₃Si₁₃B₉Nb₄Cu₁ was synthesized by melt spinning to investigate the kinetics of crystallization. Techniques of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Mössbauer spectroscopy were employed to characterize the phases produced due to annealing at various temperatures. High temperature DSC revealed two stage crystallization reactions. First stage, crystallization occurs at temperature around 514°C with the production of α-Fe (bcc) and Fe₃Si phases. In the second stage, Fe₂B and α-Fe (Si,Nb) phases were produced. Mössbauer results revealed the formation of Fe₃Si, Fe₁₃Si₃ and Fe₇Si₁ in the first stage and Fe₃Si, Fe₁₃Si₃, Fe₂B and α-Fe (Si,Nb) phases in the second stage of crystallization. An abrupt change in average internal magnetic field was observed at 500°C. The maximum hardness value was found for the sample heat-treated at 500°C.     

The Morin transition in nanostructured pseudocubic hematite: Effect of the intercrystallite magnetic exchange

The Morin transition in α-Fe₂O₃ particles with homogeneous pseudocubic morphology of about 1.8 μm side has been studied by different techniques. The particles are made up of nanometric crystallites with a narrow size distribution. Samples annealed at 673, 773 and 873 K, yielded Morin transition temperatures, T_M, of about 230, 241, and 245 K, respectively, which are lower than the bulk value, T_M ≈ 263 K. In addition, the temperature range ΔT_M through which these samples undergo the transition is very narrow with respect to the values reported in the literature. Ranges of 20, 10 and 8 K, were obtained for the samples annealed at 673, 773 and 873 K. In this work, through the estimation of the exchange magnetic energy, we demonstrate that the existence of the Morin transition, and its shift and breadth are related to the presence of intercrystallite interactions within the pseudocubic particles. These intercrystallite interactions are modified by the annealing treatments, which help to remove non-stoichiometric hydroxyl groups and incorporated water molecules.