Nickel-induced magnetic behaviour of nano-structured α-Fe2O3, synthesised by facile wet chemical route

We report the synthesis of pristine and nickel containing iron oxide (α-Fe₂O₃) nanocrystallites by facile environmentally benign wet chemical process. The magnetic behaviour of the samples has been found to change progressively with nickel content. The Mössbauer spectra revealed the precipitation of secondary phase of nickel ferrite (NiFe₂O₄) at ～2?wt% nickel contents. The transmission electron micrographs together with asymmetric magnetic hysteresis loop have confirmed the formation of core–shell structure. The Morin temperature of nanostructured α-Fe₂O₃ as estimated by superconducting quantum interference device has been found to be 257, 245, 247 and 242?K at nickel content of 0, 1, 2 and 4?wt%, respectively. The similar trends of increase/decrease in Morin temperature have been noticed by Mössbauer analysis. Furthermore, below Morin temperature, the temperature range of coexisted antiferromagnetic and ferromagnetic states has been found to increase with increase in nickel content.     

Study of low temperature ferromagnetism,surface paramagnetism and exchange bias effect in α-Fe1.4Ga0.6O3 oxide

The compound α-Fe_1.4Ga_0.6O₃ has been prepared by mechanical alloying of α-Fe₂O₃ and β-Ga₂O₃ and subsequent heating under vacuum condition. X-ray diffraction, Raman and Mössbauer spectroscopy confirmed stabilization of the material in rhombohedral phase. A remarkable change in the magnetic ordering has been found by substitution of non-magnetic Ga atoms in α-Fe₂O₃. The ferromagnetism in α-Fe_1.4Ga_0.6O₃ is found to be more soft and enhanced in comparison to α-Fe₂O₃. The samples also exhibited features of exchange bias, low temperature surface paramagnetism, and suppression of Morin transition. Nano-sized grains and alloying process of the material affected these features up to certain extent. The results of magnetic measurements are highly interesting for designing metal doped α-Fe₂O₃ based ferromagnet for room temperature applications.     

Structural,electrical and magnetic properties of Zr–Mg cobalt ferrite

Spinel cobalt ferrite, CoFe_2−xM_xO₄ has been synthesized by substitution of the combination of metallic elements M=Zr–Mg by the microemulsion method using polyethylene glycol as a surfactant. Powder X-ray diffraction analysis reveals that the substitution results in shrinkage of the unit cell of cobalt ferrite due to higher binding energy of the synthesized samples. The energy-dispersive X-ray fluorescence analysis confirms the stoichiometric ratios of the elements present. The thermogravimetric analysis shows that the minimum temperature required for the synthesis of these substituted compounds is 700 °C. A two-point probe method was employed for the measurement of the electrical resistivity in a temperature range of 293±5 to 673±5 K. It appears that there is a decrease in the number of Fe²⁺/Fe³⁺ pairs at the octahedral sites due to the substitution and corresponding migration of some of the Fe³⁺ ions to tetrahedral sites, consequently increasing the resistivity and the activation energy of hopping of electron at the octahedral sites. The susceptibility data also suggest migration of Fe³⁺ to tetrahedral site in the initial stage, which results in an increase in A–B interactions leading to large increase in the blocking temperature (T_B) as observed in samples having dopant content x=0.1.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

The modulated magnetic structure of α-Fe2O3: Ga weak ferromagnet

The domain structure of a diluted weak α-Fe₂O₃: Ga ferromagnet is considered. Magnetic fields of specific amplitudes and orientations applied in the basal plane of the crystal are found to produce nonuniform magnetic states due to a periodic deviation of the antiferromagnetic vector from the easy axis of crystallographic anisotropy. A phase diagram of the modulated magnetic state with the triad axis is constructed, and a dependence of the spatial period of modulation on an external magnetic field is studied. A phenomenological model and the nature of the magnetic superstructure discovered are discussed.     

Thermally Induced Solid-State Syntheses of γ-Fe2O3 Nanoparticles and Their Transformation to α-Fe2O3 via ε-Fe2O3

The thermally induced solid-state syntheses of -Fe₂O₃ nanoparticles from iron-bearing materials (FeSO₄, Fe₂(C₂O₄)₃ and almandine garnet) are described. Magnetic properties, particles size and the mechanism of the structural change of -Fe₂O₃ nanoparticles have been investigated using ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction (XRD) and atomic force microscopy (AFM). -Fe₂O₃ nanoparticles are transformed into hematite via -Fe₂O₃ as the intermediate.     

Structural and electrical characterization of BiFeO3–NaTaO3 multiferroic

Using a standard high-temperature solid-state reaction technique, polycrystalline samples of (Bi_1?x , Na _x ) (Fe_1?x , Ta _x ) O₃ (x = 0.0, 0.5) were prepared. The formation of the desired materials was confirmed by X-ray diffraction. The surface texture of the prepared materials recorded by scanning electron microscope exhibits a uniform grain distribution with small voids suggesting the formation of high-density pellet samples. The impedance and dielectric properties of the materials were investigated as a function of temperature and frequency. The relative dielectric constant and loss tangent of BiFeO₃ decrease on addition of NaTaO₃ (x = 0.5). The effect of addition of NaTaO₃ on grain and grain boundary contributions in the resistive and capacitive components of BiFeO₃ was studied using complex impedance spectroscopy. The value of activation energy due to both grain and grain boundary of both the samples is nearly same. The nature of variation of dc conductivity confirms the Arrhenius behavior of the materials. Study of frequency dependence of ac conductivity suggests that the materials obey Jonscher’s universal power law and the presence of ionic conductivity.     

Structural and magnetic heterogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.3 − x Bi x Mn1.1O3

The structural, resistive, magnetic, and magnetoresistive properties of the La_0.6Sr_{0.3 ? x} Bi _x Mn_1.1O₃ ceramics have been studied. The substitution of Bi ions for Sr ions increases the lattice parameter of the rhombohedral perovskite structure, decreases the metal-semiconductor and ferromagnet-paramagnet phase transition temperatures and the peak of the magnetoresistive effect, and increases the resistivity, approaching the system to the ferroelectric state. The ⁵⁵Mn NVR study indicates on the high-frequency Mn³⁺ ? Mn⁴⁺ superexchange and heterogeneity of the valence and magnetic states of manganese due to the nonuniformity of distribution of all ions and defects. The phase diagram has been constructed, which shows a strong correlation between the structural, magnetic, and magnetoresistive properties.     

57Fe Mössbauer spectra study of coated α-Fe2O3 nanoparticles

Nano-sized -Fe₂O₃ particles coated with polar organic molecules have been studied using the Mössbauer spectroscopy method. The -Fe₂O₃ nanoparticles were prepared by the microemulsion method. The average particle size of the Fe₂O₃ particles is about 24 Å. Because the particle size is so small that the Mössbauer spectra of the -Fe₂O₃ samples only consist of a quadrupole-split central line. It was proved that the Isomer Shifts (DIS) and the Quadrupole Splitting (DQS) changed as the refluxing time prolongs and the refluxing temperature increases during the preparation of the Fe₂O₃ nanoparticles, which implied an enhancement of the surface electrofield gradient formed by the surface coated polar molecules during the refluxing process.     

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La_{0.6 ? x} Pr _x Sr_0.3Mn_1.1O₃ (x = 0?0.6) have been studied using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R $\bar 3$ c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature T _mi and the ferromagnetic-paramagnetic phase transition temperature T _c with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn³⁺ ? Mn⁴⁺. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn²⁺ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn³⁺ Mn⁴⁺ in the B-positions and a local heterogeneity of their surrounding by other ions (La²⁺, Pr³⁺, Sr²⁺) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures T _mi and T _c , and magnetoresistive properties.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Synthesis,characteristics and sonocatalytic activities of calcined γ-Fe2O3 and TiO2 nanotubes/γ-Fe2O3 magnetic catalysts in the degradation of Orange G

In this work, γ-Fe₂O₃ and TiO₂ NTs/γ-Fe₂O₃ composites with good magnetism and sonocatalytic activity were prepared by a facile polyol method and utilize the principle of isoelectric point method, respectively. The structural and magnetic features of the prepared calcined γ-Fe₂O₃ and composite catalysts were investigated by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), surface analysis, UV–Vis diffuse reflectance spectra (UV–Vis DRS), vibrating sample magnetometry (VSM) and zeta potential analysis. The effects of calcination temperature on γ-Fe₂O₃ phase variation, physical properties and sonocatalytic properties were investigated. The porosity, specific surface area, band gap energy and sonocatalytic activity of γ-Fe₂O₃ were gradually decreased with calcination temperature increased. TiO₂ NTs/γ-Fe₂O₃ with appropriate composition and specific structural features possess synergetic effects such as efficient separation of charge carriers and hydroxyl radicals produced by heterogeneous fenton and fenton-like reactions. This enhanced the sonocatalytic activity for the degradation of Orange G under ultrasonic irradiation. The sonocatalytic reactions obeyed pseudo first-order kinetics. All these information provide insight into the design and development of high-efficiency catalyst for wastewater treatment.     

Structural and magnetic inhomogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Nb x O3 ceramics

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Nb _x O₃ (x = 0–0.3) annealed at temperatures of 1260 and 1500°C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic (χ_ac, ⁵⁵Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter a of the rhombohedral R $\bar 3$ c structure and the average ionic radius with increasing niobium concentration x and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C with an increase in the niobium concentration x and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn³⁺/Mn⁴⁺, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn³⁺ ? Mn⁴⁺. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn²⁺ and Nb³⁺ in distorted A-positions. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.     

Investigation of the parametric excitation and relaxation of phonons in antiferromagnetic α-Fe2O3

Parametric excitation of magnetoelastic waves was investigated in the easy-plane antiferromagnet α-Fe₂O₃ by parallel and perpendicular microwave pumping over a wide range of frequencies, magnetic fields, and temperatures, and the parametric resonance thresholds were measured. The frequencies of the natural magnetoelastic vibrations of the sample were investigated as a function of the magnetic field and temperature. The results of the measurements were used to calculate the parameters of the magnetoelastic wave spectrum and the rate of relaxation of the excited quasi-phonons. Possible mechanisms for quasi-phonon damping were analyzed.     

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf (→¹⁸¹Ta)-implanted corundum α-Fe₂O₃ single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Mössbauer effect study of the Β-FeOOH to α-Fe2O3 phase transformation

-FeOOH was precipitated from a chloride solution. Mössbauer spectra were taken at room temperature (RT) and at 4.2 K in zero field and in external magnetic fields. Samples aged for successively longer time periods were studied. They contain -FeOOH and -Fe₂O₃, the Mössbauer spectra of which show superparamagnetic behavior. The transformation into -Fe₂O₃ is already induced by aging in the mother solution at 100 C. A final heat treatment at 350 C leads to the complete transformation into -Fe₂O₃ and to an increase of the crystallite size.     

High-pressure characteristics of α-Fe2O3 using DFT+U

We have calculated the structural, magnetic and electronic properties of corundum-type α-Fe₂O₃ from first principles using density-functional theory (DFT) and the DFT?+?U method to account for correlation effects in this material. Although the correct magnetic ground state is obtained by pure DFT, the magnetic moments and the band gap are too small, and the predicted structural phase transition coupled with a transition from the insulating high-spin to a metallic low-spin phase at a pressure of 14?GPa is not observed experimentally. We find that considering the Coulomb interaction directly by including a Hubbard-like term U in the density functional greatly improves the results with respect to band gap and magnetic moments. The phase transition is shifted to higher pressures with increasing values of U and disappears for U?>?3?eV. The best overall agreement of structural, magnetic and electronic properties with experimental data is obtained for U?=?4?eV.     

Mössbauer study of carbon-supported α-Fe catalysts

Mössbauer spectra of carefully reduced carbon-supported iron catalysts show superparamagnetic -Fe at 80 K. The results indicate that the particle size depends on the reduction temperature. Effects of evacuation and CO chemisorption are discussed.