Magnetic Hyperfine Field Research Articles

Disorder in Co-Cu granular alloys studied by 59Co NMR

We have performed 59Co zero field nuclear magnetic resonance (NMR) experiment at 4 K on CoxCu1-x (x = 0.10, 0.32 and 0.76) nanogranular alloys. The NMR signal arises from Co rich ferromagnetic clusters formed within the Cu matrix. The analysis of NMR spectra shows that the resonance peaks arise from both fcc and hcp phases of Co. However, Co nuclei experience magnetic hyperfine field from Co only environment as well as other environments in which nearest-neighbor (NN) Co atoms are replaced by Cu. Assuming that these two environments constitute the core and the shell of the Co clusters, it is estimated using previous magnetization measurements that the contribution of core dominates in the ferromagnetism of the particles though it occupies only about 20% of the volume of Co clusters even for high Co containing alloys.

Synthesis and magnetic properties of spinel Zn1−xNixFe2O4 (0.0 ≤ x ≤ 1.0) nanoparticles synthesized by microwave combustion method

Nanoparticles of Zn1−xNixFe2O4 (x = 0.0–1.0 in step of 0.2) ferrites are synthesized by microwave combustion method using glycine as a fuel. The structure, site occupancy and ferrimagnetic behavior have been characterized by X-ray diffraction (XRD), Mössbauer Spectroscopy (MS) and Vibrating Sample Magnetometer (VSM). Special emphasis is given to the information provided by each of these techniques concerning the impact of the site occupancy on the magnetic properties. The results reveal the formation of single phase cubic spinel with crystallite sizes in the range of 30–42 nm. The lattice constants and the lattice strains decrease gradually with increasing Ni-content. Two well-defined Mössbauer sextets in the spectra at 78 K belong to octahedral [B] and tetrahedral (A) sites, respectively, confirming the formation of Zn1−xNixFe2O4 spinel. The isomer shift (δFe) values were assigned to Fe3+ ions distributed in both A and B sites. The quadrupole splitting (Δ) values showed also that the orientation of the magnetic hyperfine field (Hint) with respect to the principle axes of the electric field gradient (EFG) is random. The nanosize samples In the presence of an externally applied magnetic field prepared exhibit the hysteresis loops of the ferrimagnetic nature. The value of saturation magnetization increases with introducing Ni up to x = 0.8 and then it decreases. The magnetic data parameters of this structure are discussed as a function of the content of Ni ions.

Structural, magnetic, and mechanical hardness characterization of the alloy Nd16(Fe76−xNix)B8 with x = 0, 10, 20, and 25

In this work, samples of the alloy Nd16 (Fe76−x Nix) B8 with x = 0, 10, 20, and 25 were produced by arc melting under an Ar atmosphere and heat treated at 1073 K during 30 min, and then quenched in ice–water mixture. X-ray diffraction (XRD), Mossbauer spectroscopy (MS), and vibrating sample magnetometry (VSM) were used to characterize the properties of the obtained samples. Microhardness tests were performed too. XRD results found Nd2Fe14B and Nd1.1Fe4B4 as main phases. It is found for x = 0, 10, 20, and 25, lattice parameters a and c, and the volume of Nd2Fe14B and Nd1.1Fe4B4 phases, which decrease with the increase of nickel content. The parameter a of the Nd1.1Fe4B4 phase has a tendency to increase in value compared to the parameter c which has a tendency to decrease with the concentration of nickel; this behavior causes the lattice to dilate in a and it is compressed in c, indicating that Ni atoms are preferably entering along c. The two phases parallel to crystallite sizes are greater than the perpendicular ones, indicating that the crystallite shape is not spherical but elongated in that direction. XRD results for all samples doped with Ni indicate that the crystallite sizes are of the nanometer order and range between 5 and 90 nm. It is evident the addition of this element does not damage the structural formation of Nd2Fe14B phase. MS shows seven magnetic sites (16k1, 16k2, 8j1, 8j2, 4c, 4e, and sb d) and a paramagnetic phase (doublet) corresponding to the phases found by XRD. The mean hyperfine magnetic field of the samples decreases with the increase in the concentration of Ni. According to the decrease of the percentage of the area and the hyperfine magnetic field of 16k2, 8j2, and 4c sites’ phase Nd2Fe14B in Ni-doped samples, preference of nickel atoms to replace iron atoms at these sites was established. It can see also the relative area of the site d (Nd1.1Fe4B4) which decreases with Ni concentration. VSM results showed that the major phase Nd2Fe14B is the hard magnetic phase, while the paramagnetic phase Nd1.1Fe4B4 is the minority phase. There is a general trend of microhardness increase with nickel content, attributable to nickel doping which reduces the lattice parameters and porosities in the sample improving its hardness.

Structural and magnetic study of Mn0.5Zn0.5CuxFe2-xO4 nanoferrites synthesized via solution combustion method

In the present work, Mn0.5Zn0.5CuxFe2-xO4 (x = 0.0, 0.1, 0.2 & 0.3) nanoferrites have been synthesized via solution combustion technique. X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mössbauer spectroscopy techniques have been employed to explore the structural, cation distribution and magnetic behaviour of the synthesized nanoferrites. Rietveld refined XRD confirmed the cubic spinel phase of the nanoferrites with Fd3m space group. A reduction in particle size (51-40 nm) has been observed with the addition of copper ions. The Nelson–Riley plots have been used to estimate the lattice parameter and the same has been observed to increase (8.40–8.46 Å) with the increasing substitution of copper content. The addition of copper ions has been observed to decrease the saturation magnetization (0.89–0.76 emu/g) and magnetic hyperfine field. Magnetization method has been used to predict the distribution of cations. The distribution of cations has been further utilized to investigate various structural parameters.

Effect of annealing temperature on structure and magnetic properties of sol–gel synthesized Co0.8Fe2.2O4/SiO2 nanocomposites

Using SiO2 as amorphous matrix, Co0.8Fe2.2O4/SiO2 nanocomposites were synthesized by sol–gel method. The effect of annealing temperature on structure, cation distribution, and magnetic properties was investigated by X-ray diffraction (XRD), Mossbauer spectroscopy, and vibrating sample magnetometer (VSM). All Co0.8Fe2.2O4 in obtained composites exhibit cubic spinel structure after annealed at 500 °C and above. The particle size of Co0.8Fe2.2O4 increases from 5 to 25 nm as the annealing temperature increases from 500 to 1100 °C. Analysis of Mossbauer and VSM reveal that higher annealing temperature induces the migration of Fe3+ from octahedral B to tetrahedral A sites, which results in the enhancement in magnetic hyperfine field Hin. While the increasing content of Co2+ at B sites and the weakened ‘pinning’ effect of domain wall result in the increase of coercivity Hc. In addition, larger Co0.8Fe2.2O4/SiO2 particles exhibit higher saturation magnetization Ms due to the relative decrease in magnetically disordered content on the Co0.8Fe2.2O4 surface.

Structural and magnetic studies of Mg substituted cobalt composite oxide catalyst Co1−xMgxFe2O4

The article synthesized the Co1−xMgxFe2O4 sample of single phase spinel structure with sol-gel auto-combustion. The lattice parameter increases, the X-ray density and average crystal sized decrease with an increase in the Magnesium content. The average crystallite size increases with an increase in the calcinating temperature. The ferrimagnetic behaviour of all samples is confirmed by room temperature Mössbauer spectra, and the magnetic hyperfine field decrease with an increase in Mg substitution, which can be attributed to the decrease of the A-B super-exchange interaction. The calcining temperature influences the magnetic hyperfine field and the distribution of Fe3+ ions at the tetrahedral A site and the octahedral B site. The saturation magnetization and coercivity have significant change with increasing Mg2+ ions, the decrease in the coercivity suggests that the sample towards soft magnetic material transformation. Magnesium substituted for Cobalt both having preference to occupy B sites (octahedral sites) reduces. Besides that increased particle size with Mg substitution causes increased distance, and reduces magnetic moment of the B sublattice.

Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4 solid solutions synthesized by sol-gel auto-combustion method

Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion method followed by annealing at 1000 °C for 1 h in air. Scanning electron micrographs of all the samples show crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160 nm). From the analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase in aluminium concentration due to the smaller ionic radius of the Al3+ ions substituting the other cations such as Co2+, Ni2+ and Fe3+ ions in the compounds. The room temperature Mössbauer spectra of the samples show Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe3+) cations. The observed magnetic hyperfine field (Bhf) decreases with increase in Al-concentration due to the distribution of diamagnetic Al3+ in the environment of 57Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of Bhf. The distributions of the cations obtained from the Rietveld refinement and Mössbauer spectroscopy results indicate an increase in Fe3+(Th)/Fe3+(Oh) occupancy-ratio on increasing Al3+ concentration, and Ni2+ cations prefer the octahedral site, whereas Co2+ and Al3+ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.

Synthesis and Characterization of Ag2MnSnS4, a New Diamond‐like Semiconductor

Phase pure Ag2MnSnS4 was synthesized from the elements using standard high‐temperature solid‐state methods. Its crystal structure was solved from single‐crystal X‐ray diffraction data collected from a pseudo‐merohedrally twinned crystal as well as by Rietveld refinement of X‐ray powder diffraction data. Ag2MnSnS4 crystallizes in the monoclinic space group Pc (no. 7) with the unit cell parameters a = 6.651(1), b = 6.943(1), c = 10.536(2) Å, β = 129.15(1)°, V = 337.3(1) Å3, and Z = 2. The tetrahedraly compound crystallizes in a superstructure of the wurtzite type and the tetrahedra volumes are in good agreement with the model for diamond‐related compounds derived from the wurtzite structure type. The red semiconductor Ag2MnSnS4 has an optical bandgap of Eg = 2.0 eV and is stable up to its peritectic decomposition temperature of approximately 700 °C. Ag2MnSnS4 is a Curie–Weiss paramagnet with an experimental magnetic moment of μexp = 5.4(1) μB per manganese atom. Antiferromagnetic ordering is detected at a Néel temperature of TN = 8.8(1) K. 119Sn Mößbauer spectra at 78 K underline the single tetrahedrally coordinated SnIV site (δ = 1.34(1) mm·s–1). The 6 K spectrum (magnetically ordered state) reveals a small transferred magnetic hyperfine field of 1.02(1) T.

Manganese-doped feroxyhyte nano-urchins produced by chemical methods

Mn-doped feroxyhyte (δ-FeOOH) nanoparticles were synthesized using a wet chemical method, starting from a mixture of iron and manganese salts. The particles obtained were needle-like, around 100 nm in length, and formed had a nano-urchin structure. The compositions of the four samples obtained were calculated to be δ-Fe0.92Mn0.08OOH, δ-Fe0.75Mn0.25OOH, δ-Fe0.56Mn0.44OOH, and δ-Fe0.32Mn0.68OOH. The superparamagnetic behavior of the nanoparticles was determined from their room-temperature Mossbauer spectra. The hyperfine magnetic field of the Mn-doped δ-FeOOH nanoparticles decreased when iron atoms were substituted by Mn atoms. Furthermore, Mn atoms were locally doped into δ-FeOOH.

Cluster-glass-like behavior in zinc ferrite nanograins

Structural and magnetic properties of ZnFe2O4 nanograins, prepared by high-energy ball milling and annealed, were systematically studied by X-ray diffraction, 57Fe Mössbauer spectroscopy and AC magnetic susceptibility measurements. Disordered spinel-like structure, with a grain size of 12 nm, is established after 200 h of milling. While the 300 K Mössbauer spectrum of the as-milled sample (200 h) displays broad magnetic absorption lines, characteristic of a disordered system, the magnetization data do not show a magnetic phase transition between 4 and 300 K. At low temperatures, the Mössbauer spectra suggest the presence of two distinct ferrite magnetic phases: one attributed to the grain core (crystalline-like phase), with magnetic ordering temperature of about 90 K, and one showing a magnetic hyperfine field distribution; the latter is associated with a chemically disordered phase (grain boundary contributions). Annealing the 200 h sample at 973 K leads to an improvement of atomic ordering of the spinel structure (reduction of cationic inversion) and average grain size of about 17 nm. AC magnetic susceptibility shows a cusp at about T≈30K, whilst Mössbauer experiments in the same sample reveal magnetic blocking in the same temperature range. The frequency dependence of susceptibility suggests the formation of a cluster-glass-like state. High temperature susceptibility can be described with a Fulcher law of interacting magnetic clusters.

Thermodynamic Properties, Mössbauer Study, and First-Principles Calculations of TlFe(MoO4)2

We report the results of magnetization and specific heat measurements, a 57Fe Mössbauer study of hyperfine interactions and density functional theory calculations in TlFe(MoO4)2 demonstrating two magnetic phase transitions at TN2 = 5.7 K and TN1 = 6.2 K. In the asymptotical critical region TN2 ≤ T ≤ TN1, the temperature dependence of the average hyperfine magnetic field ⟨Bhf(T)⟩ at the 57Fe nuclei is well-described by the power law ⟨Bhf(T)⟩ ∝ tβ with the static critical exponent β ≈ 0.37 inherent for a three-dimensional magnet. In this range, the Mössbauer spectra are described in terms of collinear spin-density-wave with the inclusion of high-order harmonics. The spectra of the low-temperature phase, below TN2, are well-fitted by a single six-line Zeeman pattern reflecting the equivalence of magnetic sites occupied by Fe3+ ions in an antiferromagnetic structure. The first-principles calculations allowed estimation of the strength of main exchange interactions.

Microstructural, Optical, and Magnetic Properties of Vanadium-Substituted Nickel Spinel Nanoferrites

The current study investigates the impact of vanadium substitution on the structural, magnetic, and optical properties of NiFe2−xVxO4 (x ≤ 0.3) nanoparticles (NPs) produced by the cost-effective sol-gel route. The as-prepared spinel ceramic powders were examined by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). The functional groups, spinel phase, and crystal structure were confirmed by XRD and FT-IR, respectively. The crystallites size decreased from 45.24 to 36.56 nm as the doping process increases. The plots of Tauc were drawn to determine optical band gap magnitudes of 1.291, 1.302, and 1.312 eV for x = 0.0, 0.2, and 0.3, respectively. The estimated saturation magnetization is maximum for pristine NiFe2O4 NPs and decreases to minimum for NiFe1.7V0.3O4 NPs. The σ-H hysteresis loops have finite coercivity (between 125 and 169 Oe) and retentivity (between 9.36 and 14.04 emu/g) values. The calculated σr/σs ratios are lower than 0.500, assigning the uniaxial anisotropy for NiFe2−xVxO4. The effective anisotropy constants (Keff) are in the range of 0.824 × 105 and 1.303 × 105 Erg/g. The magnetocrystalline anisotropy field (Ha) values are around 5.0 kOe. The characteristics of hysteresis (σ-H) curves and the order of magnetic data reveal the soft ferrimagnetic feature of as-prepared nanoparticle samples. From Mossbauer analysis, the variations in hyperfine magnetic field, quadrupole splitting, line width, and isomer shift have been evaluated. The distribution of cations showed that the octahedral B sites are occupied by all the ions of V3+. Mossbauer spectra are composed of four Zeeman sextets and one doublet.

Synthesis and properties of 1D manganese-doped hematite particles

The properties of Mn-doped hematites and their Mn-doped precursors were investigated using the 57Fe Mössbauer spectroscopy, XRD, FT-IR, UV/Vis/NIR and FE SEM. Mn-doped goethite precursors were synthesized in a high alkaline pH medium starting with iron choline citrate. The XRD analysis of all Mn-doped goethites and Mn-doped hematites showed only the presence of a goethite or hematite crystal structure. The RT Mössbauer spectrum of reference goethite showed typical features of precipitated goethite. With increased Mn-doping the spectral lines of the goethite precursor were broadening, the hyperfine magnetic field decreased and the relative intensity of central quadrupole doublets increased. The RT Mössbauer spectra of Mn-doped hematite showed features of the hematite phase and increased relative intensity of a central quadrupole doublet which disappeared in the spectra recorded at liquid N2 temperature. The presence of this superparamagnetic component in the RT Mössbauer spectra is a direct consequence of Mn-doping. In the FT-IR spectra the characteristic IR bands sensitive to Mn-doping were identified. The UV/Vis/NIR spectra showed shifts of optical absorption bands and a strong absorption increase in the visible part of the spectrum. FE SEM images of Mn-doped goethite precursor showed a change in shape from rod to star-like particles. Upon heating at 300 °C for 4 h in air, the obtained Mn-doped hematite particles preserved the original morphologies of Mn-doped goethite precursors. Taking into account the superparamagnetic spectra (RT and liquid N2 temperature), XRD and the corresponding FE SEM images of Mn-doped hematite particles it can be concluded that these particles contained substructure. In this substructure of Mn-doped hematites the fraction of superparamagnetic crystallites (domains) is given by calculated value for the superparamagnetic doublet present.

The Properties of Mn, Ni, and Al Doped Cobalt Ferrites Grown by Sol-Gel Method

The manganese-, nickel-, and aluminum-doped cobalt ferrite powders, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4, and Al0.2CoFe1.8O4, are fabricated by the sol-gel method, and the crystallographic and magnetic properties of the powders are studied in comparison with those of CoFe2O4. All the ferrite powders are nano-sized and have a single spinel structure with the lattice constant increasing in Mn0.2Co0.8Fe2O4 but decreasing in Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4. All the Mössbauer spectra are fitted as a superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the Fe3+ ions. The values of the magnetic hyperfine fields of Ni0.2Co0.8Fe2O4 are somewhat increased in the A and B sites, while those of Mn0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 are decreased. The variation of Mössbauer parameters is explained using the cation distribution equation, superexchange interaction and particle size. The hysteresis curves of the ferrite powders reveal a typical soft ferrite pattern. The variation in the values of saturation magnetization and coercivity are explained in terms of the site distributions, particle sizes and the spin magnetic moments of the doped ions.

AC susceptibility and hyperfine interactions of vanadium substituted barium nanohexaferrites

Vanadium ions substituted BaFe12O19 nanohexaferrites, BaFe12-xVxO19 (0.0 ≤x ≤ 0.1), were produced through the sol-gel auto-combustion route. The structure, morphology and the elemental compositions of various products were examined using X–ray powder diffraction, scanning electron microscopy coupled with EDX and EDS elemental mapping. These techniques confirmed the formation of the desired Ba-nanohexaferrite phases. The crystallites size was found to be 55–58 nm range for all products. The magnetic properties of BaFe12-xVxO19 nanohexaferrites were investigated by Mossbauer spectroscopy, ZFC-FC magnetizations and AC susceptibility. The evolutions in the values of hyperfine magnetic field, isomer shift, quadrupole splitting, and line width were deduced via Mossbauer analysis. The experiments of ZFC and FC magnetizations indicated that no blocking temperature is observed in the temperature interval 2–400 K, which signals the typical ferromagnetic (FM) behavior for the produced nanohexaferrites. A super-spin glass like behavior is noticed at lower temperatures. The experiments AC susceptibility confirmed that the strength of magnetic interactions is enhanced for lower content of V3+ (x = 0.02). For higher amount of V3+, the magnetic interactions are weakened. The obtained results are mainly accredited to the substitutions of Fe3+ ions by V3+ ions.

Synthesis of Cu-doped δ-FeOOH nanoparticles by a wet chemical method

Cu-doped maghemite (γ-Fe2O3) and Cu-doped feroxyhyte (δ-FeOOH) nanoparticles were synthesized using a wet chemical method. The synthesis was performed starting from a mixture of iron and copper salts. Cu-doped γ-Fe2O3 and Cu metal nanoparticles were obtained from a solution with a high Cu/Fe ratio, whereas Cu-doped δ-FeOOH nanoparticles were obtained from a solution with a low Cu/Fe ratio. The size of the nanoparticles was below 10 nm. The superparamagnetic behavior of the nanoparticles was determined from their Mossbauer spectra. The hyperfine magnetic field of Cu-doped δ-FeOOH nanoparticles was larger than that of undoped δ-FeOOH nanoparticles. The Cu content of the intermediate species plays an important role in the oxidation process. The intermediate species in the precipitates before rinsing was speculated to be Cu1-xFeIIxFeIII2O4 (x = 0.4–0.7), which was subsequently oxidized in air to form Cu-doped γ-Fe2O3 or Cu-doped δ-FeOOH, depending on the amount of Cu.

Effect of Ce3+ Ion on Structural and Hyperfine Interaction Studies of Co0.5Ni0.5Fe2−xCexO4 Ferrites: Useful for Permanent Magnet Applications

Nanoparticles of Co0.5Ni0.5Fe2−xCexO4 (where x = 0.0, 0.01, 0.015 and 0.02) ferrites are prepared by the modified solution combustion method using a mixture of fuels and are characterized to understand their structural, microstructural and magnetic properties. The X-ray diffraction is used to confirm the formation of a single-phase cubic spinel structure. The average crystallite sizes are calculated using the Scherrer formula and are found to be less than 50 nm. The microstructural features are obtained by the scanning electron microscopy, and the compositional analysis is done by using the energy-dispersive spectroscopy. The transmission electron microscopy (TEM) investigations show that the synthesized ferrites are made up of very fine spherical nanoparticles. The influence of a rare-earth element (Ce3+) on the magnetic properties of the samples was studied using the Mossbauer spectroscopy. The Mossbauer spectroscopy reveals the formation of broadened Zeeman lines and quadrupole-split lines and the presence of the Fe3+ charge state at B sites in the samples. The quadrupole splitting shows that the orientation of the magnetic hyperfine field with respect to the principle axes of the electric field gradient was random. The magnetic hyperfine field values indicate that the A sites have more A-O-B superexchange interactions than the B sites. The coexistence of magnetic sextet and a doublet component on the room-temperature spectra suggests superparamagnetic properties of the nanoparticles. The low-temperature (15 K) Mossbauer spectroscopy explores the paramagnetic relaxation in the nanoparticles. The area under the sextet refers to Fe3+ concentrations in the tetrahedral and octahedral sites of the ferrite. This study confirms that the Ce3+ substitution of Fe3+ only for octahedron sites causes the decrease in Fe-O-Fe arrangement. The effect of Ce3+ doping on the magnetic properties of Co0.5Ni0.5Fe2O4 is examined from the vibrating sample magnetometry (VSM) spectra. Saturation magnetization values are decreased initially and then increased, as result of Ce3+ doping. This can be explained by Neel’s two-sub-lattice model. Further, the value of coercivity is found to be increasing with increasing Ce3+ concentration. The obtained results of M-H loop with improved coercivity (from 851 to 1039 Oe) by Ce3+ doping of Co0.5Ni0.5Fe2O4 demonstrate the usefulness for permanent magnet applications.

Substitution effect of Cr3+ on hyperfine interactions, magnetic and optical properties of Sr-hexaferrites

Chromium-substituted strontium nano-hexaferrites, SrCrxFe12-xO19 (0.0 ≤ x ≤ 1.0), were produced by the hydrothermal route. The structure, morphology, optical and magnetic properties were studied. Crystal structure, spinel phase and functional groups were verified using X-ray diffraction and Fourier Transform Infrared spectroscopy. The average crystallites size is ranging between 50 and 67 nm. Scanning electron microscopy observations indicated that the multi-grains of the particles are aggleromated and are hexagonal in shape. The isotherm plots obtained from the nitrogen physisorption experiments showed that the mesopore area and the total pore volume decreased progressively with increasing the Cr3+ content. Besides, the band gap energy (Eg) decreases from 1.75 to 1.43 eV. The magnetic hysteresis loops showed that the produced nano-hexaferrites exhibit hard ferromagnetic-like (FM) behavior. Compared to the pristine SrFe12O19 product, the deduced saturation (Ms) and remanent (Mr) magnetizations, the coercivity (Hc) and the magnetocrystalline anisotropy field (Ha) increase for lower Cr3+ content (x ≤ 0.4) and then decrease as the Cr3+ content further increases. The squareness ratio Mr/Ms are ranging between 0.5 and 0.6, suggesting the single domain nano-sized particles with uniaxial anisotropy for various synthesized products. Mossbauer analysis was done to determine the quadrupole splitting, the isomer shift, and the hyperfine magnetic field values of all products.

Structure, magnetism and magnetic induction heating of NixCo(1-x)Fe2O4 nanoparticles

NiFe2O4 and CoFe2O4 nanoparticles present different efficiencies as magnetic heating agents due to their respectively soft and hard magnetic properties. The substitution of Ni by Co to form mixed NixCo(1-x)Fe2O4 nanoparticles is employed, along with thermal treatment, to tailor their heating efficiency by modification of their structural and magnetic properties. For that purpose, nanostructured NixCo(1-x)Fe2O4 (with x = 0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by coprecipitation and annealed at 400, 700 and 1000 °C. The substitution of Ni by Co allows obtaining particles with progressively lower saturation magnetization and magnetic anisotropy. Heat treatment allows modifying the saturation magnetization and coercivity differently, by enhancing hyperfine magnetic fields and increasing the particle size beyond the transition from monodomain to multidomain behaviour, respectively. Samples annealed at 700 °C exhibit the largest coercivity and anisotropy values, whilst magnetization values increase progressively with the annealing temperature. The AC induced heating efficiency of NixCo(1-x)Fe2O4 increases with Ni content due to increased minor hysteresis losses. For a given time of exposure and concentration, selection of the nanoparticle size and Ni content allows to obtain different efficiencies, which can be selected according to the heating applications involved.

Mössbauer spectroscopy measurements on the 35.5 K superconductor Rb1−δEuFe4As4

The results of x-ray diffraction and $^{57}\mathrm{Fe}$ and $^{151}\mathrm{Eu}$ M\"ossbauer spectroscopy measurements, supplemented with ab initio hyperfine-interaction parameter calculations, on the new 35.5 K superconductor ${\mathrm{Rb}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{EuFe}}_{4}{\mathrm{As}}_{4}$ are presented. The superconductor crystallizes in the tetragonal space group $P4/mmm$ with the lattice parameters $a=3.8849(1)$ \AA{} and $c=13.3370(3)$ \AA{}. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature ${T}_{\mathrm{C}}=16.54(8)$ K is determined from the temperature dependence of both the hyperfine magnetic field at $^{151}\mathrm{Eu}$ nuclei and the transferred hyperfine magnetic field at $^{57}\mathrm{Fe}$ nuclei that is induced by the ferromagnetically ordered Eu sublattice. The Eu magnetic moments are demonstrated to be perpendicular to the crystallographic $c$ axis. The temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a ${T}^{3/2}$ power-law relation. Good agreement between the calculated and measured hyperfine-interaction parameters is observed. The Debye temperature of ${\mathrm{Rb}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{EuFe}}_{4}{\mathrm{As}}_{4}$ is found to be 391(8) K.