Synthesis and characterization of K2NiF4-type CaLnCoO4 (Ln = Sm and Gd)

K₂NiF₄-type CaLnCoO₄ (Ln = Sm and Gd) has been synthesized at 1173 or 1223 K in air using citric acid (CA) and ethylene glycol (EG). CaLnCoO₄ (Ln = Sm and Gd) has an orthorhombic structure with the space group Bmab. The average particle sizes are approximately 300 nm for CaSmCoO₄ and approximately 170 nm for CaGdCoO₄, respectively. The global instability index (GII) indicates that the crystal structure of CaGdCoO₄ is more stable than that of CaSmCoO₄. CaLnCoO₄ (Ln = Sm and Gd) is a p-type semiconductor and shows paramagnetic behavior above 5 K. The 1/χ-T curve of CaSmCoO₄ deviates from the Curie-Weiss law, whereas the 1/χ-T curve of CaGdCoO₄ follows the Curie-Weiss law in the temperature range of 5 ≤ T ≤ 300 K. From the values of the observed effective magnetic moment (μ_eff) of CaLnCoO₄ (Ln = Sm and Gd), it is considered that the spin state of the Co³⁺ ion is low.     

Physicochemical properties of solid-solid interactions in nanosized NiO-substituted Fe2O3/TiO2 system at 1200 °C

The solid-solid interactions between nanosized pure and NiO-substituted ferric and titanium(IV) oxides have been investigated using XRD technique and microstructure studies, also magnetic properties were studied using vibrating samples magnetometer (VSM). The amounts of substituting Ni²⁺ were x = 0, 0.2, 0.4, 0.6, 0.8 and 1 mole. A mixture equimolar proportions of finely powdered Fe₂O₃ and TiO₂ were mixed with NiO, ball milled, compressed at 250 kg/cm² and fired at 1200 °C for 4 h.The obtained results showed that with substituting Ni²⁺ concentration x = 0 only Fe₂TiO₅ phase is present (∼80 nm) which showed a very small saturation magnetic flux density (B_s), remnant magnetic flux density (B_r) and the maximum energy product (BH)_max. By the addition of x = 0.2 NiO, new phases were observed NiTiO₃ and NiFe₂O₄ of crystallite sizes 160 and 110 nm, respectively. By the increase of substituting Ni²⁺ concentration the NiTiO₃ and NiFe₂O₄ phases increased on the expense of Fe₂TiO₅ up to x = 0.4, then the increase in substituting Ni²⁺ concentration led to a decrease in Fe₂TiO₅ and NiTiO₃ while NiFe₂O₄ increases which results in a great improvement of magnetic properties.All samples exhibit a catalytic activity towards H₂O₂ decomposition and the values of rate constant increase with increasing amount of Ni²⁺ substituting. The most acidic active sites are shown by specimens substituted with x = 0 this concludes that H₂O₂ decomposition is not favored on acidic active sites.     

Enhancement of the magnetic properties of Ni-Cu-Zn ferrites with the substitution of a small fraction of lanthanum for iron

The effect of lanthanum ion substitution for iron on the structural and magnetic properties of Ni-Cu-Zn ferrite is reported. The (Ni_0.25Cu_0.20Zn_0.55)La_xFe_2−xO₄ ferrite compositions with x = 0.0, 0.025, 0.050 and 0.075, were synthesized by nitrate-citrate auto-combustion route. Rietveld structure refinement was carried out to evaluate; La solubility in spinel, residual stress in sintered core, quantity of secondary LaFeO₃ phase formed and change in lattice parameters, etc. Density, crystallite size, grain size, residual macrostress and initial permeability were directly affected by the substitution. A significant increase in initial permeability was achieved by a small fraction of La substitution. The La solubility in the Ni-Cu-Zn ferrite lattice was found very low (∼0.1 atom/unit cell). Co-relations between magnetic properties and measured physical/structural properties were discussed.     

Controllable synthesis and magnetic property of BiMn2O5 crystals

Uniform submicron BiMn₂O₅ particles were prepared via a facile one-step hydrothermal route at low temperature. Bi(NO₃)₃, MnCl₂·4H₂O and KMnO₄ were used as starting materials; KOH as a pH adjustor and also as a mineralizer. Single-crystalline orthorhombic BiMn₂O₅ sample with controllable morphology was obtained. The microstructure strongly depends on the molar ratio of the starting materials, KOH concentration and reaction temperature. X-ray photoelectron spectroscopy shows the existence of Mn⁴⁺ state. Magnetic measurement indicates Néel temperature T_N at 44 K. The susceptibility above T_N obeys the Curie-Weiss law, χ = C/(T − θ), with θ = −350 K. The effective paramagnetic moment μ_eff = 4.66 μ_B/Mn, demonstrating the coexistence of mixed Mn³⁺ and Mn⁴⁺ valences.     

Magnetic properties of NiCuZn ferrite nanoparticles synthesized using egg-white

A new process to prepare single-phase nano-sized ferrites, Ni_0.8−xCu_0.2Zn_xFe₂O₄ with x = 0.1-0.7, was developed using egg-white precursors. TG measurement showed that, the precursors must be calcined at 550 °C. XRD patterns indicated the formation of single-phase cubic ferrites with particle size in the range 28.7-48.4 nm. TEM image gave particle size agrees well with that estimated using XRD. FT-IR spectroscopy showed the characteristic ferrite bands. Hysteresis loops measurements exhibited an increase in the saturation magnetization value (M_s) up to zinc content of 0.2 followed by unexpected decrease, which suggests the preference of Zn²⁺ ions to occupy octahedral sites. The decrease in the coercivity (H_c) with increasing zinc content is attributed to the lower magneto-crystalline anisotropy of Zn²⁺ ions compared to Ni²⁺ ions. Temperature dependence of the molar magnetic susceptibility (χ_M) suggested a ferrimagnetic behavior of the investigated samples and showed a decrease in the value of the Curie temperature (T_C) with increasing zinc.     

Neutron diffraction study of magnetic ordering of the manganese bismuth chloro-sulfide: MnBiS2Cl

In quaternary compounds of Mn²⁺PnQ₂X (Pn = Sb, Bi; Q = S, Se; X = Cl, Br, I), Mn atoms in octahedral coordination (4 Q and 2 X) form waved layers separated by Pn atoms. The magnetic structure of the manganese bismuth chloro-sulfide MnBiS₂Cl has been determined by neutron powder diffraction, revealing a magnetic ordering with an incommensurate wave-vector along b-axis (k = [0, 0.3978, 0]) at 1.6 K. Two modulation models, sinusoidal and helicoidal, give quite equivalent magnetic reliability factors (R_mag = 0.0450 and 0.0481, respectively). The magnetic moment decreases with increasing temperature, to zero at T_N = 32 K. The evolution of the propagation wave-vector shows an irregularity at about 28 K. It could evidence two-phase transitions in agreement with the specific heat measurements. These results are compared to those of manganese antimony chloro-sulfide MnSbS₂Cl, isotypic with MnBiS₂Cl.     

Synthesis of single-phase Sr3Co2Fe24O41 Z-type ferrite by polymerizable complex method

Synthesis of single-phase Sr₃Co₂Fe₂₄O₄₁ Z-type (Sr₃Co₂Z) ferrite was realized by adopting the polymerizable complex method. Crystal structure of samples has been investigated by powder X-ray diffraction (XRD). Single-phase Sr₃Co₂Z ferrite was obtained by heating at 1473 K for 5 h in air. Magnetic properties were discussed by measurements of M-H curves with vibrating sample magnetometer (VSM). Sr₃Co₂Z ferrite prepared by polymerizable complex method showed typical M-H curve of soft ferrite, with a saturation magnetization of 21.5μ_B/formula unit (50.5 emu/g) and a coercive force of 0.014 T at room temperature.     

Effect of Al-ion substitution on structural and magnetic properties of Co–Ni ferrites nanoparticles prepared via citrate precursor method

Nanocrystalline Co_0.8Ni_0.2Al_yFe_2−yO₄ (y = 0.00, 0.15, 0.30, 0.45, 0.60, 0.75) powders have been synthesized by the citrate precursor auto-combustion method. The effect of the nonmagnetic aluminum ion substitution on the structural and magnetic properties has been studied. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the obtained powders have a single phase of cubic spinel structure. The crystallite sizes estimated from XRD data have been confirmed using high-resolution transmission electron microscopy (HRTEM). Photographs showed powders consisting of nanosized grains with an average size ranging from 10 to 45 nm, depending on the Al content. Magnetic hysteresis loops were traced at room temperature using vibrating sample magnetometer (VSM). It was found that, due to the aluminum substitution, the values of magnetic losses and saturation and remanent magnetization were decreased, indicating a reduction in the ferrimagnetic behavior. This reduction of magnetization, compared to the undoped sample, was attributed to the increase of spin noncollinearity with increasing Al content. Samples with toroidal forms were used to measure both the initial permeability (μ_i) and Curie temperature (T_C). The obtained results showed a decrease in the values of both μ_i and T_C as the Al content increases. This behavior was explained in view of the weakness in the interaction of A and B sublattices with increasing Al³⁺ ion concentration.     

High frequency properties of Ni75Fe25-SiO2 granular thin films with very high resistivity

Excellent soft magnetic properties have been achieved in low metal volume fraction(x ∼ 0.52) of (Ni₇₅Fe₂₅)_x(SiO₂)_1−x granular film fabricated by magnetron sputtering. The coercivity of 247 A m^− 1 with very high resistivity ρ over 1.18 × 10⁴ μΩ cm has been obtained. At the frequency lower than 1 GHz, the real part μ′ of the complex permeability keeps about 55 and the imaginary part μ? is less than 2. A combined study of TEM and XRD indicates that the sample consists of Ni₇₅Fe₂₅ particles uniformly embedded in insulating SiO₂ matrix. The excellent soft magnetic properties are ascribed to the magnetic dipole interaction between particles. The investigations of field cooled and zero field cooled curves as well as Henkle Plot prove the existence of this intergranular interaction.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Structural and magnetic characterization of LiMn1.825Cr0.175O4 spinel obtained by ultrasonic spray pyrolysis

Quaternary spinel oxide LiMn_1.825Cr_0.175O₄ powder was synthesized by using an ultrasonic spray pyrolysis method, without additional annealing. The crystal structure of the as-prepared powder was revealed by X-ray powder diffraction and identified as a single spinel phase with Fd3m space group. The powders had a spherical morphology with extremely smooth surface appearance and densely congested interior structure. Transmission electron microscopy confirmed that the particle consisted by the cohesion of the primary particles. Magnetic measurements performed in DC field in both zero-field-cooled and field-cooled regimes, as well as AC susceptibility experiments, show that system undergoes spin-glass transition at the freezing temperature T_f = 20 K. The value of the effective magnetic moment μ_eff = 4.34 μ_B obtained from the Curie-Weiss fit in the high temperature region confirms the substitution of Mn³⁺ ions with Cr³⁺ ions.     

High-temperature anomalies of dielectric constant in TiO2 thin films

Anatase and rutile TiO₂ thin films were prepared by chemical vapor deposition with precursors Ti(OPrⁱ)₄ and Ti(dpm)₂(OPrⁱ)₂ (dpm = 2,2,6,6-tetramethylheptane-3,5-dione and Prⁱ = isopropyl), respectively. The dielectric properties of TiO₂ thin films have been studied in 20-1100 K temperature range in air, in controlled Ar/O₂ atmospheres, and in vacuum with silicon-based metal-insulator-semiconductor Au/TiO₂/Si capacitors. High-temperature (T_c ∼ 980 K) anomalous behavior of dielectric constant was observed in both anatase and rutile TiO₂ thin films.     

Vanadium dioxide nanobelts: Hydrothermal synthesis and magnetic properties

VO₂ (B) nanobelts were prepared by a hydrothermal method at 180 °C using V₂O₅·nH₂O sol and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 50 to 100 nm in width, 20-30 nm in thickness with lengths up to 1.5 μm. Measurements of the static magnetic susceptibility provide evidence for two phase transitions at T₁ = 225 K and T₂ = 290 K, respectively. Below T₁, the data suggest the presence quasi-free as well as of strongly antiferromagnetic correlated spins associated to V⁴⁺-ions.     

Preparation of porous magnetic nanocomposites using corncob powders as template and their applications for electromagnetic wave absorption

Strong absorption, low density, and thin matching thickness are important parameters for electromagnetic (EM) wave absorbers. In this study, we prepared novel porous magnetic nanocomposites using corncob powders as template. The presence of corncob will significantly decrease the bulk density of samples from more than 4.0 to about 0.55 g cm⁻³. The porous structures remarkably decreased the permittivity (ε) and permeability (μ) and enhanced the impendence matching between the absorber and air. The porous magnetic nanocomposites exhibit enhanced absorption for EM waves at thin matching thickness. The optimum thickness is only 1.0–1.4 mm, with bandwidth of RL < −5 dB of about 8 GHz, covering the half X-band and the whole K_u-band. The areal density of magnetic absorbers at this study is only about 0.7–1.0 kg m⁻² at thickness of 1.0–1.4 mm, much lower than the reported values of other magnetic absorbers. Due to the strong absorption at low density and thin matching thickness, the porous magnetic nanocomposites prepared using corncob powders as template are promising light-weight EM wave absorbers.     

The effect of calcining temperature on the magnetic properties of the ultra-fine NiCuZn-ferrites

In this study, the ultra-fine NiCuZn-ferrite was prepared by a coprecipitation method. The magnetic properties were investigated in terms of calcining temperature. The ferrite powders, Ni_0.206Cu_0.206Zn_0.618Fe_1.94O_4−δ, were initially heat treated at various temperatures of 300-750 °C, and then sintered at the final temperature of 900 °C. The average particle size calculated by a XRD pattern and confirmed by a transmission electron microscope (TEM) micrograph was 7.5 nm. The calcining temperature was an important factor for microstructures and magnetic properties of the sintered ferrite. Scanning electron microscope (SEM) micrographs showed a uniform grain growth with small pores and high densification at the calcining temperature of 450 °C. From the results of magnetic property measurements, the ferrite calcined at 450 °C showed higher initial permeability (170) and quality factor (72) than those of other calcining temperatures.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Investigation of BiFeO3 modified PbTiO3-Bi(MgZr)O3-based complex perovskite ceramics

Bismuth containing crystalline solutions of (1 − x)Bi(MgZr)_0.5O₃-xPbTiO₃ (BMZ-PT) and [(BiFeO₃)_y − (BiMg_0.5Zr_0.5)_1−y]_x − [PbTiO₃]_1−x (BMZ-BF-PT) have been developed using conventional ceramic technology. X-ray diffraction analysis reveals that both the systems possess a perovskite structure, in which tetragonal to rhombohedral phase transformation appears for x = 0.55 in BMZ-PT and y = 0.20 in BMZ-BF-PT systems. SEM photographs reveal a uniform grain size distribution in the solid solution matrix with the presence of ferroelectric domains in few of the compositions. Ferroelectric hysteresis (polarization-electric field, P-E) loops reveal that increase in BiFeO₃ in BMZ-PT systems results in a decrease in residual polarization of the system with change and distortion in the shape of the (P-E) loops.     

Effect of bead milling on heat generation ability in AC magnetic field of FeFe2O4 powder

Nano-sized FeFe₂O₄ ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0 μm in particle size showed a temperature enhancement (ΔT) of 3 °C in an AC magnetic field (powder weight 1.0 g, 370 kHz, 1.77 kA m⁻¹) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (ΔT = 26 °C) in the AC magnetic field in ambient air was for the fine FeFe₂O₄ powder with a 4.7 nm crystallite size (the samples were milled for 6 h using 0.1 mm? beads). However, the heat generation ability decreased for the excessively milled FeFe₂O₄ samples having average crystallite sizes of less than ca. 4.0 nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B–H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Néel relaxation of the superparamagnetic material. The hysteresis loss in the B–H magnetic curve would be generated as the magnetic moment rotates (Néel relaxation) within the crystal. The heat generation ability (W g⁻¹) can be estimated using a 1.07 × 10⁻⁴fH² frequency (f, kHz) and the magnetic field (H, kA m⁻¹) for the samples milled for 6 h using 0.1 mm? beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (ΔT = 59 °C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6 nm for the sample calcined at 500 °C. The heat generation ability (W g⁻¹) for this heat treated sample was 2.54 × 10⁻⁴fH².     

High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.