Electrical and magnetic properties of nonstoichiometric Ca2MnxFe2−xO5+δ

Electrical resistance and magnetic susceptibility of nonstoichiometric Ca₂Mn_xFe_2-xO_5+δ were measured in the range from room temperature to 1100°C and from the liquid helium temperature to 600°C, respectively. The high electrical resistance and activation energy for the brownmillerite phase changed scarcely with increase of δ. The Néel temperature decreased slightly. Electrical and magnetic properties changed abruptly in the phase transition from brownmillerite-type to ordered-type which was derived from the slight increase of δ. The ordered phase had properties similar to those of oxygen-deficient perovskite-type ferrates (i.e. low electrical resistance, low activation energy, very low Néel temperature). The range of two-phase mixture was discussed from the result of electrical resistance measurement.     

Structural and electrical characterization of Bi9?xTi3Mn5+xO27

Solid solutions of bismuth layer structured Bi_9?xTi₃Mn_5+xO₂₇ (x?=?0–3) system were prepared by the solid-state reaction technique using component oxides. X-ray powder diffraction, scanning electron microscopy, dielectric constant, variation of magnetization with magnetic field measurement techniques are used to characterize the compounds. The substitution of Mn³⁺ at Fe³⁺ site in Bi_9?xTi₃Mn_5+xO₂₇ was found to shift the Curie temperature towards the low-temperature region. The relationship between atomic displacement of B site ions (Ti⁴⁺, Fe³⁺, Mn³⁺) and the Curie temperature is discussed. Diffused type of phase transition was observed with a very high loss leading to the formation of soft ferroelectrics.     

Low-temperature thermoelectric and magnetic characteristics of Ca2.9Bi0.1Co4−xFexO9+δ (0 ≤ x ≤ 0.10)

The effect of Fe ion doping on the low-temperature thermoelectric and magnetic properties of Ca_2.9Bi_0.1Co_4?xFe_xO_9+δ (x = 0.00, 0.025, 0.05 and 0.10) have been investigated. The samples were prepared by conventional solid-state synthesis. The X-ray diffraction patterns revealed that all the samples are single phase. The electrical resistivity results indicated that all the samples obey the variable range hopping in the low temperature regime. The T^* (transition temperature from Fermi liquid metal to incoherent metal) was increased and the slope of A value (Fermi-liquid transport coefficient) was decreased with increasing Fe content. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity, thermopower and total thermal conductivity were decreased with increasing Fe content. Among the doped samples, Ca_2.95Bi_0.10Co_3.90Fe_0.10O_9+δ had the highest dimensionless figure of merit of 0.056 at 300 K. Magnetic measurements indicated that all the samples exhibit a low-spin state of cobalt ion. The effective magnetic moments were decreased with increasing Fe content.     

Magnetic properties of Bi(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript> (M = Mn,Ti)

The magnetic properties of Bi(Fe_{1 − x} M _x )O₃(M = Mn, Ti) solid solutions have been studied in magnetic fields of up to 11.2 × 10³ kA/m, and the composition stability range of the R3c ferroelectric phase has been determined. The results indicate that partial Ti⁴⁺ substitution for Fe³⁺ leads to a transition from a modulated antiferromagnetic state to a homogeneous weakly ferromagnetic ferroelectric state (x = 0.08), whereas the Bi(Fe_1−x ³⁺Mn _x ³⁺)O₃ solid solutions do not exhibit weak ferromagnetism. Charge compensation in is assumed to be ensured by cation vacancies.     

A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method

M-type hexaferrites; barium hexaferrite BaFe₁₂O₁₉ and strontium hexaferrite SrFe₁₂O₁₉ powders have been successfully prepared via the co-precipitation method using 5 M sodium carbonate solution as alkali. Effects of the molar ratio and the annealing temperature on the crystal structure, crystallite size, microstructure and the magnetic properties of the produced powders were systematically studied. The results indicated that a single phase of barium hexaferrite was obtained at Fe³⁺/Ba²⁺ molar ratio 12 annealed at 800–1,200 °C for 2 h whereas the orthorhombic barium iron oxide BaFe₂O₄ phase was formed as a impurity phase with barium M-type ferrite at Fe³⁺/Ba²⁺ molar ratio 8. On the other hand, a single phase of strontium hexaferrite was produced with the Fe³⁺/Sr²⁺ molar ratio to 12 at the different annealing temperatures from 800 to 1,200 °C for 2 h whereas the orthorhombic strontium iron oxide Sr₄Fe₆O₁₃ phase was formed as a secondary phase with SrFe₁₂O₁₉ phase at Fe³⁺/Sr²⁺ molar ratio of 9.23. The crystallite sizes of the produced nanopowders were increased with increasing the annealing temperature and the molar ratios. The microstructure of the produced single phase M-type ferrites powders displayed as a hexagonal-platelet like structure. A saturation magnetization (53.8 emu/g) was achieved for the pure barium hexaferrite phase formed at low temperature 800 °C for 2 h. On the other hand, a higher saturation magnetization value (M _s = 85.4 emu/g) was obtained for the strontium hexaferrite powders from the precipitated precursors synthesized at Fe³⁺/Sr²⁺ molar ratio 12 and thermally treated at 1,000 °C for 2 h.     

Studies on the structural,electrical and magnetic properties of LaCrO3, LaCr0.5Cu0.5O3 and LaCr0.5Fe0.5O3 by sol–gel method

The structural, electrical and magnetic properties of LaCr_0.5M_0.5O₃ (M = Cr³⁺, Cu²⁺ and Fe³⁺) synthesized by a sol–gel technique were studied. The X-ray diffraction pattern shows the structure to be orthorhombic and the size of the particles is around 100 nm as seen from the TEM images. The effects of Cu²⁺ and Fe³⁺ on the electrical properties of LaCrO₃ were studied using impedance spectroscopy at room temperature (RT). The properties of LaCr_0.5Cu_0.5O₃ were studied over a wide range of temperature from RT to 533 K. A maximum conductivity of 1.7 × 10^?3 S cm^?1 was observed for LaCr_0.5Cu_0.5O₃ at a measured temperature of 533 K. The impedance spectra indicate a negative temperature coefficient of resistance (NTCR) and also imply the conduction is through bulk of the material. The magnetic studies performed using a SQUID magnetometer interpret the antiferromagnetically ordered LaCrO₃ to behave ferromagnetically on the addition of Cu²⁺ and Fe³⁺, and the magnetization was found to be enhanced in the LaCr_0.5Fe_0.5O₃.     

Magnetoresistance Study of Y3Ba5Cu8O18 Superconducting Phase Substituted by Nd3+ and Ca2+ Ions

Superconducting samples of type Y_3?x Nd _x Ba_5?x Ca _x Cu₈O₁₈ with 0.0 ≤ x ≤ 0.4 have been prepared via the solid-state reaction technique. The prepared samples were characterized using X-ray powder diffraction (XRD) technique for phase analysis. The elemental content of the prepared samples was determined using particle-induced X-ray emission (PIXE). In addition, the oxygen content of these samples was obtained using non-Rutherford backscattering spectroscopy (RBS) at 3 MeV proton beam. The results indicate that these substitutions do not affect the orthorhombic structure, while they decrease the oxygen content of Y-358 phase. The electrical resistivity of the prepared samples was measured by the conventional four-probe technique from room temperature down to the zero superconducting transition temperature (T ₀). A slight change in the superconducting transition temperature (T _c) is observed for 0 ≤ x ≤ 0.1, and then it decreases linearly with further increase in x. The linear decrease in T _c is attributed mainly to the partial substitution of Ba²⁺ ions by Ca²⁺ ions rather than the partial substitution of Y³⁺ ions by Nd³⁺ ions. The effect of magnetic fields up to 4.44 kG on the electrical resistivity has been studied to investigate the vortex dynamics for the prepared samples. The experimental data, in the second stage of superconducting transition, fit well with the thermally activated flux creep (TAFC) model, and the activation energy U(B) shows a power law dependence on magnetic field as B ^?β . Also, the transition width is related to the magnetic field according to the relation ΔT α B ⁿ . The values of β and n are strongly dependent on the Nd³⁺ and Ca²⁺ ion substitution. The magnetic field and temperature dependence of the activation energy U(B, T) is found to be U(B, T)? ΔT B ^?η , where η = β + n. Furthermore, the critical current density at zero temperature, J _c(0), as a function of the applied field was calculated for all the prepared samples. The result shows an enhancement in J _c(0) of Y-358 phase at x equals 0.4 at different applied fields.     

Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Synthesis and Properties Investigation of Non-equivalent Substituted W-Type Hexaferrite

A novel composed W-type hexaferrite Ba_1?x La _x Co₂Fe₁₆O₂₇ was rapidly synthesized via a sol–gel self-combustion reaction. The effects of lanthanum ions on the oxidation state of iron ions and cobalt ions in hexaferrite were explored by X-ray photoelectron spectroscopy. The changes of the Fe 2p X-ray absorption spectra indicated that the nonequivalent substitution can lead to the transition Fe³⁺→ Fe²⁺ in Ba_1?x La _x Co₂Fe₁₆O₂₇. However, the oxidation state of cobalt ions was maintained as Co²⁺. Moreover, the effects of La content on the phase composition, structural parameters, morphology, and static magnetic properties were also investigated in detail by using the X-ray diffractometer, scanning electron microscope, and vibrating sample magnetometer. The results indicated that the structural parameters decreased regularly with increasing the La content, and the magnetic properties were enhanced after substitution, which is beneficial for their application in various electrical devices employed for industrial and military applications.     

Systematics in the electron transport and magnetic properties of LnBO3 perovskites

Systematics in the electrical and magnetic properties of transition metal perovskites LnBO₃ (Ln=rare-earth ion, B=3d transition metal) with the variation of Ln and B ions are reviewed. The electrical resistivity and activation energy of LnBO₃ compounds increase with the decreasing size of the Ln³⁺ ion for a given B ion. The low-spin to high-spin transition temperature of Co³⁺ ion in LnCoO₃ similarly increases with the decrease in size of Ln³⁺ while the magnetic ordering temperatures in LnVO₃, LnFeO₃, LnCrO₃ and LnSrCo₂O₆ decreases with decreasing size of the rare-earth ion. These results may be understood in terms of the increasing acidity of the rare earth ion with decreasing size and the competition between the Ln³⁺ and the B³⁺ ions for covalency with the oxygen ions. The effect of this competition on the metal oxygen covalency and crystal field parameter is discussed in relation to the results obtained and Goodenough’s phase diagram. The possibility ofd-f exchange interaction in La_1−xLn_x NiO₃ is also discussed in the light of ESR results. Communication No. 66 from the Solid State and Structural Chemistry Unit.     

Synthesis and characterizations of Nd doped SrFe12O19 nanoparticles

This is the first report ever on Nd³⁺ doped M-type hexaferrite nanoparticles: SrNd_xFe_12−xO₁₉ (0 ≤ x ≤ 1) prepared by citrate precursor using the sol–gel technique followed by gel to crystallization. The influence of the Nd³⁺ substitution, Fe³⁺/Sr²⁺ molar ratio and the calcination temperature on the crystallization of ferrite phase have been examined using powder X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), inductance capacitance resistance meter bridge (LCR) and vibrating sample magnetometer (VSM). The structural analysis reveals that the Nd³⁺ ions rearrange themselves in the host lattice without disturbing the parent lattice and Fe³⁺/Sr²⁺ molar ratio less than 12 is more favorable to achieve single phase hexaferrite at calcination temperature 900 °C for 4 h. Mid-IR analysis confirms that Nd³⁺ occupies the octahedral site. Detailed studies of electrical properties of prepared materials have been investigated in the frequency range 100–1000 Hz at room temperature by LCR meter and two probe technique. The result shows that the electrical properties strongly depend upon the frequency of applied field and dopant concentration. The magnetic measurements showing a considerable improvement in coercivity with the substitution of Nd³⁺ on iron sites, while the unsubstituted hexaferrites have highest value of specific saturation magnetization.     

Influence of synthesis variables on the properties of barium hexaferrite nanoparticles

Barium hexaferrite (BaFe₁₂O₁₉) nanoparticles were synthesized by sol–gel auto-combustion route. Prepared samples were sintered at 950 and 1100 °C with Fe³⁺/Ba²⁺ = 12 and 20 mol ratio. The formation mechanism of barium hexaferrite was investigated by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. In addition, the effect of temperature and Fe³⁺/Ba²⁺ mole ratio on BaFe₁₂O₁₉ formation and magnetic properties, and the effect of increasing the Fe³⁺/Ba²⁺ upon gel ignition and subsequent phase development were investigated. Finally the magnetic behavior was monitored with VSM. DSC studies showed that pure barium hexaferrite phase was formed from maghemite (γ-Fe₂O₃), without the formation of hematite (α-Fe₂O₃). Also, XRD results confirmed the formation of barium hexaferrite phase in non stoichiometric Fe/Ba ratio. VSM results showed that the saturation magnetization was decreased and coercivity increased with decreasing the grain size.     

Structural and Magnetic Properties of Pr0.6Sr0.4Mn1−x Fe x O3 (0≤x≤0.3) Manganites Oxide Prepared by the Ball Milling Method

We present the structural and magnetic properties of Pr_0.6Sr_0.4Mn_1?x Fe _x O₃ (x=0, 0.1, 0.2, and 0.3) compounds. Samples have been prepared by the ball milling method. Rietveld refinements of the X-ray powder diffraction data show that all our synthesized samples are single phase and crystallize in the orthorhombic symmetry with the Pnma space group. The unit cell volume increases with increasing the Fe content. The infrared spectrum shows two active bands, which can be ascribed to the internal stretching and bending modes. The magnetization measurements versus temperature showed that Fe doping leads to a weakening of the ferromagnetic ordering at low temperature, the Curie temperature T _C decreases from 300 K for x=0.0 to 88 K for x=0.2. The magnetization versus applied magnetic measurements at low temperature lead to conclude that the substitution of Mn³⁺ ions by Fe³⁺ ions triggers antiferromagnetic interactions between the Fe³⁺ and Mn⁴⁺ spins, and also the magnetization versus applied magnetic measurements at room temperature shows a small hysteresis loop and a low coercive field, which indicates that the samples are superparamagnetic.     

A mössbauer study of hyperfine interactions in the boroferrite Fe3−x Ga xBO6(x=0, 0·12)

Fe₃BO₆ shows no thermal hysteresis around the phase transition temperatureT _SR=418 K; Fe_2·88Ga_0·12BO₆ undergoes spin transition atT _SR=403 K resulting in a reduction of theT _SR. Both these features are different from the reported observations on α-Fe₂O₃. The two Fe³⁺ sites show difference in their magnetic interactions. Ga³⁺ replaces Fe³⁺ in a random manner.     

Charge disproportionation in CaFeO3 studied with the Mössbauer effect

The Mössbauer effect spectrum of stoichiometric CaFeO₃ at 4 K consisting of two magnetic hyperfine patterns with nearly the same intensities is explained assuming a charge disproportionation 2Fe⁴⁺→Fe³⁺ + Fe⁵⁺. The disproportionation is supposed to set in at or in the vicinity of the Nèel temperature. The single magnetic hyperfine pattern for SrFeO₃ at 4 K, on the other hand, indicates a rapid electron exchange between Fe³⁺ and Fe⁵⁺ ions, for the center shift and the hyperfine field coincide approximately with the average values of the corresponding parameters for CaFeO₃.     

Investigations of Sr3Fe2−xMnxO7−δ the n=2 Ruddlesden–Popper phases with d3/d4 interactions

We have prepared a series of Mn substituted strontium ferrates, Sr₃Fe_2−xMn_xO_7−δ where x=2/3, 1 and 4/3, and investigated their properties with X-ray diffraction, X-ray absorption near edge spectroscopy (XANES), Mössbauer spectroscopy, variable-temperature resistivity and magnetic susceptibility. These compounds are metastable and have to be prepared at high temperatures, >1250°C, with solid state techniques followed by quenching in air to room temperature. As a consequence the compounds are oxygen deficient and contain some Fe³⁺. They are insulators and exhibit spin-glass like behavior at low temperatures due to frustrated magnetic interactions between the disordered array of transition metal ions on the B site. Mössbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe⁴⁺ in these compounds are charge-disproportionated into Fe³⁺ and Fe⁵⁺ at low temperature.     

Synthesis and characterization of Fe3+-modified PLZT ferroelectrics

Fe³⁺-modified nanocrystalline PLZT, i.e., Pb_0.92[La_1–z Fe _z ]_0.08[Zr_0.60Ti_0.40]_0.98O₃ (PLFZT) (z=0.0, 0.3, 0.6, 0.9, 1) materials were synthesized by a high-temperature solid-state reaction technique. X-ray studies of the compounds confirmed the formation of single-phase, ultrafine (nano-sized) and homogeneous materials. Microstructural scanning electron microscopy (SEM) study shows the uniform distribution of smaller grains on the surface of the samples. Detailed dielectric studies of the compounds as a function of temperature (30–450 °C) show that the broadening of the permittivity peak and transition temperature depends on Fe³⁺-ion concentration. Analysis of diffuseness () of the broadened dielectric peaks of the materials gave its value between 1 and 2, indicating the different degrees of substitutional disorder in the system. The increase in Fe³⁺-substitution at the La-site of PLZT shows many interesting and unusual dielectric relaxor behaviors of the compounds. The transition temperature T _c of PLZT (8/60/40) ferroelectric shifts towards a higher temperature region on increasing Fe³⁺ concentration. The variation of d.c. and a.c. electrical conductivity of the material with temperature shows its semiconducting behavior; and hence the materials can be used for some electronic devices.     

Effect of Cr doping on the phase structure,surface appearance and magnetic property of BiFeO<Subscript>3</Subscript> thin films prepared via sol–gel technology

Multiferroic BiFeO₃ (BFO) and BiFe_1?xCr_xO₃ (BF_1?xC_xO, x?=?0.05, 0.1, 0.15 and 0.2) thin films were successfully synthesized on silicon (111) substrates via sol–gel technology. The effect of Cr³⁺ ion doping on the phase structure, surface morphology, valence states for Fe element and magnetic property was investigated. The introduction for simulation images of ionic space arrangement was to better comprehend the substitution site and superexchange interaction between the Fe³⁺ (Cr³⁺) and O^2? ions. The phase structure of Cr-doping thin films transition from rhombohedral to orthorhombic was confirmed by the X-ray diffraction (XRD) and Raman measurements, and the obtained results also demonstrated that the Cr³⁺ ions successfully located in Fe²⁺ and Fe³⁺ ions sites of BFO lattice system. The Field Emission Scanning Electron Microscopy (FESEM) patterns clearly exhibited that the grains sizes were remarkably decreased by Cr³⁺ ions doping, and the surfaces textures got glossier and smoother judging from the Atomic Force Microscope (AFM) images. The dense surface structure can restrict the O^2? ions escaping from the lattices system, which is beneficial for the release of magnetic property due to superexchange interaction of improvement. It was found that the saturation magnetization (Ms) was significantly linearly increased accompanying the adding of Cr-doping due to destroying of spatial modulation helical structure and enhancing of superexchange interaction. Moreover, the Hall-effect results firstly revealed that the carrier concentration and mobility rate played significant roles in magnetoelectric effect behaviors.     

Magnetic and Mössbauer studies of FeNi2BO5 and FeNiBO4

Single crystals of boron ferrites Fe^IIINi^II₂BO₅ and Fe^IIINi^IIBO₄ were grown by CVT method using HCl carrier gas. The measurements of magnetic susceptibility and Fe⁵⁷ Mössbauer effect were performed on the powdered samples in the temperature range from 4.2 K to 295K. FeNi₂BO₅ (FeNiBO₄) exhibits antiferromagnetic transition at T_N=54 K (13 K) giving asymptotic Curie temperature of ?220 K (?900 K) and the Curie constant agrees well with what is expected from Fe³⁺ and Ni²⁺ ions in the compounds. However, Zeeman splitting appears in the Mössbauer spectrum of FeNi₂BO₅ (FeNiBO₄) below ca. 102 K (26 K), far above the T_N. Further, the Zeeman pattern of FeNi₂BO₅ shows the presence of spin relaxation in the temperature range of ca. 94 ～ 107 K, and the quadrupole splitting decreases an order in magnitude and the line width becomes broader compared to the paramagnetic pattern. The origin for these remarkable features are discussed relating to the crystal structures and the magnetic interactions in the compounds.     

Magnetization, magnetic susceptibility, and effective magnetic moment of the Fe3+ ions in Bi2Fe4O9

Bi₂Fe₄O₉ with an orthorhombic structure and lattice parameters a = 7.9595 Å, b = 8.4297 Å, c = 5.9912 Å, and V = 401.987 ų has been prepared by solid-state reactions method. Its molar magnetic susceptibility measured as a function of temperature in the range 5–950 K indicates that Bi₂Fe₄O₉ is an antiferromagnet with a Néel temperature of 258 K. In the range 280–750 K, its molar magnetic susceptibility exhibits Curie-Weiss behavior, which allowed us to determine the Weiss constant (Θ = ?1468 K) of this material and the effective magnetic moment of the Fe³⁺ ions $\left( {\mu _{eff}^{Fe^{3 + } } = 6.37\mu _B } \right)$ . Magnetization versus magnetic field data show no magnetic hysteresis, indicating that the Bi₂Fe₄O₉ sample studied exhibits no weak ferromagnetism.