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.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

On the magnetic properties of mechanosynthesized Co-Fe-Ni ternary alloys

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used as complementary methods to obtain structural data and to determine magnetic properties of the mechanically synthesized and subsequently thermally treated Co-Fe-Ni alloys. New, however approximate, phase diagrams were established on the basis of X-ray diffraction investigations. Mössbauer spectroscopy and magnetization measurements allowed to reveal practically linear correlation between the average values of the hyperfine magnetic field induction, 〈B_hf〉, and the effective magnetic moments, μ_eff, of the alloys. The decrease in 〈B_hf〉 with the number of electrons per atom, e/a, was observed. Moreover, the dependence of μ_eff on the valence 3d and 4s electrons per atom follows the Slater-Pauling curve. Thermal treatment of mechanosynthesized Co-Fe-Ni alloys led to some changes in the phase diagrams, increase in the grain size and decrease of the level of internal strains in alloys. Dependencies of lattice constants, average hyperfine magnetic fields, effective magnetic moments and Curie temperatures on the number of electrons per atom have the same trends for mechanically synthesized as well as for thermally treated alloys.     

Effect of Nb addition on the structure and soft magnetic properties of melt-spun Co69Fe7Si14B10 alloy

Melt-spun ribbons of Co₆₉Fe₇Si_14−xNb_xB₁₀ alloys with x=0, 2 and 4 have been prepared and characterized for structure and soft magnetic properties. Ribbons with x=0 and x=2 are found to be completely amorphous whereas the ribbon with x=4 contains irregular shaped faulted Co₂Si orthorhombic phase with grain size of about 100 nm. Nb addition is found to decrease the degree of amorphicity and induce perpendicular anisotropy, deteriorating the soft magnetic and magnetoimpedance properties.     

Effect of Cu-Cr co-substitution on magnetic properties of nanocrystalline magnesium ferrite

This study deals with the temperature and composition dependence of magnetization and magnetic anisotropy of Cu²⁺-Cr³⁺ co-substituted magnesium ferrite, Mg_1−xCu_xCr_xFe_2−xO₄ (x=0.0-0.5). The synthesized materials are characterized using thermo gravimetric analysis, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, Mössbauer spectrometer, superconducting quantum interference device magnetometer and vibrating sample magnetometer. The M-H loops measured up to 50 kOe at 300, 200 and 100 K, revealed narrow hysteresis curves with a coercive field and saturation magnetization varying for different compositions. The high field regimes of these loops are modeled using the Law of Approach to saturation to extract anisotropy information and saturation magnetization. Both the saturation magnetization and the anisotropy constant are observed to increase with the decrease in temperature while decrease with the Cu-Cr co-substituents for all the samples. Explanation of the observed behavior is proposed in terms of the preference of the co-substituent ions of Cu²⁺ and Cr³⁺ and their predominant choice to substitute into the octahedral sites of the cubic spinel lattice.     

Mössbauer studies and magnetic studies of Ni doped orthoferrites PrFe1−xNixO3 (x≤0.3)

The magnetic properties of polycrystalline PrFe_1−xNi_xO₃ (x≤0.3) system were studied using Mössbauer spectroscopy and magnetization measurements. The Mossbauer spectra exhibit six line spectra which loses its sharpness as the Ni substitution increases within the system. As the Ni concentration in the system increases, the hyperfine field and isomer shift shows decrease, which is vivid from the sluggish nature of the sextets. The small value of quadrupole splitting confirms the octahedral environment of the Fe⁺³ ions. The magnetization curves show the reversible behavior and represent the fall in negative molecular field leading to AFM frustration. From these results, we conclude that sagging in the spectra reveals the change from antiferromagnetic state to ferromagnetic state, which can be attributed to mixed state of Fe⁺³ ions i.e. high spin (HS) and low spin (LS) which is a consequence of progressive collapse of Hund’s rule due to HS→LS transition. These results confirm the weak ferromagnetic component due to canted-AFM spin arrangement of Fe³⁺ magnetic moments.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Recoilless fraction, structural, magnetic and thermal properties of Fe73.5Cu1Nb3Si13.5B9 alloy

Differential scanning calorimetry, X-ray diffraction and room temperature Mössbauer spectrum measurements of Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The DSC profile of as-quenched Finemet showed two exothermic peaks at 530 and 702 °C, corresponding to two crystallization processes. The Finemet alloy remains amorphous at 450 °C with one broad peak in XRD pattern and one broad sextet in Mössbauer spectrum. When the Finemet alloy was annealed at 550 °C, only well indexed body-center-cubic phase was detected. After being annealed at 650 and 750 °C, the XRD patterns showed the coexistence of α-Fe(Si) and Fe-B intermetallic phases with the increase in XRD peak intensities, indicating the growth of crystallites and the decomposition of Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at elevated temperatures. The Mössbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements.     

Spin- and charge density perturbations and short-range order in Fe–Cu and Fe–Zn BCC alloys: A Mössbauer study

A model used to describe the ⁵⁷Fe Mössbauer spectra for the binary BCC iron alloys rich in iron has been extended to account for the alloy crystallographic ordering. The ordering is accounted for by introducing single order parameter. Extension of the model is described in detail. The model has been tested applying it to the Fe–Cu alloys obtained by the arc melting and to the Fe–Zn alloys prepared by the solid state reaction. Random alloys are obtained up to ∼2 at% of Cu, and up to ∼8 at% of Zn. For higher impurity (minor alloy component) concentration it has been found that Cu atoms try to avoid Fe atoms in the iron matrix as nearest neighbors, while the opposite happens to the Zn atoms, albeit at much lesser scale, i.e., Zn–Zn interactions are much weaker than Fe–Zn interactions at the nearest neighbor distance. Perturbations to the iron magnetic hyperfine field (spin density) and electron (charge) density on the iron nucleus have been obtained for both series of alloys versus impurity concentration.     

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.     

Synthesis and magnetic properties of ferrites spinels MgxCu1−xFe2O4

Polycrystalline Mg_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and magnetic measurements.Using mean field theory and high-temperature series expansions (HTSE), extrapolated with the padé approximants method, the magnetic properties of Mg_1−xCu_xFe₂O₄ have been studied. The nearest neighbor super-exchange interactions for intra-site and inter-site of the Mg_1−xCu_xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_c is calculated as a function of Mg concentration. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements.     

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.     

Medium range ordering and some magnetic properties of amorphous Fe90Zr7B3 alloy

High-resolution electron microscopy (HREM) reveals in the as-quenched Fe₉₀Zr₇B₃ alloy the existence of medium range ordered (MRO) regions 1-2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones. Above the Curie point of the amorphous phase (T_C=(257±2)K) they behave like non-interacting superparamagnetic particles with the magnetization decreasing linearly with the temperature. For these particles the average magnetic moment of 390μ_B and the average size of 1.7 nm, in excellent agreement with HREM observations, were estimated. The maximum of the isothermal magnetic entropy change at the maximum magnetizing field induction of 2 T occurs at the Curie temperature of the amorphous phase and equals to 1.05 Jkg⁻¹ K⁻¹. The magnetic entropy changes exhibit the linear dependence on the maximum magnetizing field induction in the range 0.5-2 T below, near and above T_C. Such correlations are attributed to superparamagnetic behavior of α-Fe MRO regions.     

Surface magnetic disorder in nanostructured Ni0.5Zn0.5Fe2O4 particles

Nanostructured ferroxide particles with initial formula Ni_0.5Zn_0.5Fe₂O₄ are investigated. The aim was to explore the monodomain and the superparamagnetic states of the ferrospinel and the impact of the surface magnetic disorder on the magnetization processes. Mössbauer spectroscopy (MöS) demonstrated that the ion distribution follows the general formula (Zn_0.5Fe_0.5)_A[Ni_0.5Fe_1.5]_BO₄, where A is the tetrahedral and B, the octahedral sublattice. MöS in an external magnetic field (5 T) at 4.2 K shows non-collinearity of the sublattices’ magnetic moments and deviations in the hyperfine magnetic field that could be related to a canting effect. Magnetic measurements were applied to characterize the temperature behavior of the magnetic properties and the a.c. complex magnetic susceptibility.     

Magnetic hysteresis loop shift in NiFe2O4 nanocrystalline powder with large grain boundary fraction

Magnetic properties of nanocrystalline NiFe₂O₄ spinel mechanically processed for 350 h have been studied using temperature dependent from both zero-field and in-field ⁵⁷Fe Mössbauer spectrometry and magnetization measurements. The hyperfine structure allows us to distinguish two main magnetic contributions: one attributed to the crystalline grain core, which has magnetic properties similar to the NiFe₂O₄ spinel-like structure (n-NiFe₂O₄) and the other one due to the disordered grain boundary region, which presents topological and chemical disorder features (d-NiFe₂O₄). Mössbauer spectrometry determines a large fraction for the d-NiFe₂O₄ region (62% of total area) and also suggests a speromagnet-like structure for it. Under applied magnetic field, the n-NiFe₂O₄ spins are canted with angle dependent on the applied field magnitude. Mossbauer data also show that even under 120 kOe no magnetic saturation is observed for the two magnetic phases. In addition, the hysteresis loops, recorded for scan field of 50 kOe, are shifted in both field and magnetization axes, for temperatures below about 50 K. The hysteresis loop shifts may be due to two main contributions: the exchange bias field at the d-NiFe₂O₄/n-NiFe₂O₄ interfaces and the minor loop effect caused by a high magnetic anisotropy of the d-NiFe₂O₄ phase. It has also been shown that the spin configuration of the spin-glass like phase is modified by the consecutive field cycles, consequently the n-NiFe₂O₄/d-NiFe₂O₄ magnetic interaction is also affected in this process.     

Deposition and properties of high-carbon iron films

Thin high-carbon iron films were deposited by pulsed laser deposition onto grids for transmission electron microscopy using pre-combined carbon/iron targets with equal area ratio. The deposited films of about 20 nm in thickness were directly characterized by transmission electron microscopy. The films showed a variety of phases, surprisingly also including the NaCl-type FeC phase, which was theoretically predicted in the literature. For comparison, thin high-carbon stainless-steel films were deposited onto oxidized Si wafers with different carbon ratios in the targets (10, 20, 40 and 50 at.%). These films were characterized by means of Mössbauer Spectroscopy, the magneto-optical Kerr-effect, grazing incidence X-ray diffraction and Rutherford backscattering spectrometry. With these methods clearly defined multilayer-structures were observed which could lead to interesting magneto-resistance phenomena if the thickness of the multilayers can be controlled by the processing parameters.     

Shaping distinct magnetic interactions in molecular compounds

Oxalates containing various 3d transitional elements and positive NH₄ or negative OH groups were newly synthesized. Each above-mentioned component has directly influenced the structure, the electronic or interaction properties, while some unexpected behaviors were revealed by various magnetic and Mössbauer measurements. The main magnetic parameters, the long-range anti-ferromagnetic couplings observed at very low temperature and, particularly the uncompensated moment are discussed in detail. The induced lower spin states for bivalent ions and especially the anti-parallel arrangement of the spins belonging to trivalent and bivalent iron inside the molecule are also emphasized.     

Magnetic properties of RE0.7Ca0.3Mn0.95Fe0.05O3 (RE=Sm and Gd) manganites at low temperature

The magnetic properties of RE_0.7Ca_0.3Mn_0.95Fe_0.05O₃ perovskite with rare-earth cations (RE=Sm and Gd) were investigated by means of X-ray diffraction, Mössbauer spectroscopy, and low temperature (4.2-266 K) magnetization measurements. Structural characterization of these compounds shows that they both have orthorhombic (Pbnm) structure. The Mössbauer spectra show clear evidence of local structural distortion of the Mn(Fe)O₆ octahedron, which is based on the non-zero nuclear quadrupole interactions for high-spin Fe³⁺ ions. It was found that the local structural distortion increases significantly when Sm³⁺ is replaced by Gd³⁺. This distortion is attributed to the Jahn-Teller coupling strength as estimated from the Mössbauer effect results. The magnetic results indicate that the Curie temperature decreases as a result of replacing Sm by Gd. This is due to the decrease of the average A-site cationic radius 〈r_A〉. The rapid increase of magnetization at low temperature indicates the magnetic ordering of rare earth ions at the A-site.     

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Nanocrystalline zinc ferrite (ZnFe₂O₄) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mössbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 Å and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mössbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.     

Tailoring of structural, electrical and magnetic properties of BaCo2 W-type hexaferrites by doping with Zr-Mn binary mixtures for useful applications

Single-phase W-type hexaferrite, BaCo₂Fe_16−2x(ZrMn)_xO₂₇ (x=0.0-1.0), has been synthesized by the chemical co-precipitation technique. Mössbauer analysis indicates substitution of Zr ions on tetrahedral (4e and 4f_IV) sites and Mn ions on the octahedral ‘4f_VI site’ at low-doped concentration when the concentration is increased Mn ions but show preference for the octahedral ‘2b site’. The highest enhancement in the value of the room temperature resistivity of 2.82×10⁹ Ω cm has been obtained by doping with Zr-Mn content of x=0.6. The dissipation factor increases from 6.49×10³ to 9.97×10³ at 10 kHz with the addition of Zr-Mn dopants. Such materials are potentially suitable for electromagnetic attenuation purposes, for microwave absorption and as radar absorbing material. High values of saturation magnetization (67 emu/g) and remanent magnetization (34.7 emu/g) are obtained for substitution level of x=0.4 making them suitable for data processing devices.