Structural,magnetic properties,and electronic structure of Cr1−xMnxO2 solid solution

Bulk Cr_1-xMn_xO₂ samples are prepared by high pressure synthesis technology. The crystal structure, magnetic properties and electronic structure of the samples are investigated by experiments and theoretical calculation. The crystal structure of the samples are indexed to a rutile structure with space group P4₂/mnm. The lattice parameter a of the samples remains basically unchanged in accordance with Vegard's law, but the lattice parameter c decreases due to increasing Mn dopant content (x) as well as strong Metal–Metal bonding along the c-axis. The saturation magnetization of the Cr_1-xMn_xO₂ samples decreases with an increase in x. According to XPS analysis, there is electron transfer between Mn and Cr in Cr_1-xMn_xO₂. Mn exists as Mn²⁺ and Mn³⁺ions, and part of Cr is oxidized to Cr⁶⁺. Based on the XPS analysis, the magnetic moment of Cr_1-xMn_xO₂ is calculated and its value is in accordance with the experimental data.     

The microstructure and magnetic properties of Ca2+ ion doped GdCrO3

In this paper, the crystal structure, vacancy defect, local electron density and magnetic properties of Gd_1-xCa_xCrO₃ (0 ≤ x ≤ 0.3) polycrystalline samples were investigated systematically. The crystal structural analyses show that all the samples are orthorhombic phase and a structural distortion happens around x = 0.3. Due to the formation of Cr⁴⁺ ions, both the lattice constant and the Cr–O bond length decrease. The results of positron annihilation spectrum reveals that the vacancy defect concentration increases and the local electron structure changes with the introduction of Ca²⁺ ions. The field-cooled (FC) and zero-field cooled (ZFC) curves of Gd_1-xCa_xCrO₃ samples measured under H = 100 Oe exhibits negative magnetization characteristics due to the interaction between Gd³⁺ and Cr³⁺ ions, and the magnetism can be affected by the structural distortion.     

Modulating the structure,vacancy defects,magnetic and dielectric properties in multiferroic GdMnO3 ceramics by alkaline earth ion substitution

This investigation highlights the significant influences of alkaline earth ion substitution for Gd on the structure, vacancy defects, magnetic and dielectric properties in GdMnO₃ (GMO) ceramics synthesized using the solid-state reaction method. The structure measurements indicate that all of the Gd_0.90A_0.10MnO₃ (A = Ca, Sr, Ba) samples show a single phase structure, and the introduction of A²⁺ ion induces structure distortion. A²⁺ ions substitution increases the Mn⁴⁺ ion concentration in GMOs, but has no significant effect on the oxygen vacancy concentration. All samples with irregular grain shapes have dense microstructures, and A²⁺ ion substitution inhibits grain growth. Positron annihilation experimental results indicate that A²⁺ ion substitution can increase the vacancy size and concentration, while the vacancy concentration increases first and then decreases with increasing A²⁺ ion radius. The evolution of the temperature- and magnetic field-dependent magnetization curves shows that A²⁺ ion substitution could obviously affect the magnetic state of GMOs, and improve the magnetic transition temperature and magnetization of Gd_0.90A_0.10MnO₃. The dielectric measurements reveal that the A²⁺ ion substituted samples exhibit giant dielectric constant characteristics over a broad frequency range. It is found that the enhanced magnetization of Gd_0.90A_0.10MnO₃ has a close relationship with the vacancy concentration, and the giant dielectric constant behaviors in Gd_0.90A_0.10MnO₃ ceramics can be associated with the mixed-valent structure of Mn³⁺/Mn⁴⁺.     

Structure and magnetic properties of Cr-Doped tin monoxide prepared by hydrothermal method

Ball- and sheet-like Sn_1-xCr_xO were successfully prepared by one-step hydrothermal method. The SEM and TEM results show that the diameter of ball-like unit is about 15 μm and the unit was assembled by several nanosheets with a thickness of about 0.5 μm. Raman pattern displays that there are two intense Raman mode E_g (~114 cm⁻¹) and A_1g (~211 cm⁻¹) in the SnO, and the A_1g peak shifted significantly. UV–Visible spectrum illustrates that the bandgap of the Cr-doped SnO firstly decreases and then increases as the Cr concentration increases, which is attributed to the sp-d exchange interactions and Burstein-Moss theory. The analysis of XPS also provides evidence for the incorporation of Cr into the system. Furthermore, it is found that oxygen vacancy (V_O) decreases with the increase of Cr content. Based on Krӧger-Vink notation system, the competition mechanism between V_O and V_Sn could explain the origin and variation of magnetism of Sn_1-xCr_xO from structural defects.     

A comparative study of the structure,defects, optical,dielectric, and magnetic properties of GdMnO3 multiferroic ceramics synthesized by solid-state reaction and sol-gel methods

In this work, GdMnO₃ ceramics were synthesized by solid state reaction and sol-gel methods, and the structure, defects and optical, dielectric and magnetic properties of the synthesized samples were comparatively investigated. The samples synthesized by different methods show a single phase structure without any detectable impurities. The SEM results suggest that the particle size of the specimen obtained by the solid phase route is on the micron scale, while that of the specimen fabricated by the sol-gel route is on the nanometer scale. Compared with the ceramic fabricated by solid-state reaction technology, the specimen synthesized by sol-gel technique possesses lower oxygen vacancies and Mn²⁺ concentration, and Mn³⁺ concentration. The positron annihilation analyses show that the cation vacancy concentration of the specimen synthesized by the solid phase approach is higher than that of the specimen synthesized via the sol-gel approach. The compound obtained by the solid phase reaction has better dielectric properties than that obtained with the sol-gel method. The magnetic transition temperature and the effective magnetic moment are influenced by the Mn ion valence state in GdMnO₃. The stronger magnetization of the ceramic synthesized via the sol-gel approach is associated with the lower concentration of cation vacancies.     

Structural and magnetic evaluation of substituted NiZnFe2O4 particles synthesized by conventional sol–gel method

The aim of this study is to evaluate the structural and magnetic properties of Ni–Zn doped ferrite with trivalent Al³⁺ and Cr³⁺ cations substitution in Ni_0.6Zn_0.4Fe_2−xCr_x/2Al_x/2O₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) synthesized by employing conventional sol–gel method. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), Mössbauer spectroscopy (MS) and vibrating sample magnetometer (VSM) analysis were carried out in order to characterize the structural and magnetic properties of particles. The XRD results confirmed the formation of single phase of spinel ferrite particles for a whole series of samples. The results of FTIR analysis indicated that the functional groups of Ni–Zn spinel ferrite were formed during the sol–gel process. Furthermore, FE-SEM micrographs revealed that the distribution of particles size is narrow. According to Mössbauer spectra,the doped cations are replaced in iron site occupancy of octahedral sites. It was found that with an increase in substitution contents magnetization decreased due to occupation of Al and Cr cations at low level substitutions in octahedral sites.     

Structural,electrical, and magnetic transport properties of La0.72Ca0.28Mn1−xCrxO3 (0 ≤ x ≤ 0.06) ceramics

To obtain comprehensive materials with both high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at low magnetic fields, polycrystalline La_0.72Ca_0.28Mn_1?xCr_xO₃ (x = 0, 0.02, 0.04, 0.06) ceramics were prepared herein by sol–gel method. Electronic configuration of Cr³⁺ ions is similar to that of Mn⁴⁺ ions, therefore, successive substitution of Mn with Cr increases electrical resistivity and decreases metal–insulator transition temperature of ceramics, even yielding hump-like feature for Cr-rich (x = 0.06) samples. The best TCR (28.50%·K^?1) and MR (72.37%) values were obtained simultaneously at Cr dopant content of 0.02 (La_0.72Ca_0.28Mn_0.98Cr_0.02O₃). Strong response of the material to temperature and magnetic field was caused by minimal symmetry of orthorhombic structure and the most robust Jahn–Teller distortion. With increasing Cr content, Mn³⁺/Mn⁴⁺ or Mn³⁺/Cr³⁺ double exchange was diluted, and Cr³⁺/Cr³⁺ or Cr³⁺/Mn⁴⁺ superexchange was promoted. However, the internal competition effect was not conducive to the improvement of material properties.     

Design,fabrication, and magnetic properties of novel (Ni,Cr,Zr)-co-doped M-type barium ferrite ceramics

Multicomponent co-doping is an effective method to balance the counteracting magnetic properties of ferrite ceramics. In this work, novel (Ni,Cr,Zr)-co-doped M-type barium hexaferrites (BaFe_12-3xNi_xCr_xZr_xO₁₉, x = 0–0.8) were designed and synthesized by traditional solid-state reaction. Thermogravimetric analysis indicated that NiO would participate in the formation of secondary phase NiFe₂O₄ in the as-synthesized powder. Through traditional solid-state sintering, by using the synthesized pure-phase magnetic powders, almost full-dense ceramics were fabricated. Visual high-temperature deformation analysis revealed that there was no obvious difference in the sintering behavior and densification temperature of the ceramics with different compositional x, due to the low sintering activity of the as-synthesized magnetic powders. And X-ray diffraction analysis indicated that all the fabricated ceramics are of pure-phase M-type barium ferrite, and the lattice parameter c/a firstly increased as x raised up to 0.4 and then remained almost unchanged with further increased x, even if the lattice distortion became heavier. Microstructure examination revealed that the grain size monotonously decreased as the quantity of the substituent ions increased. The remnant magnetization and coercivity of the fabricated ceramics decreased monotonously as x increased, while the saturated magnetization could be maintained till the samples with x ≤ 0.4. Taking all the parameters into consideration, the samples with x = 0.4 might be a good candidate for transformer cores.     

Influence of Ti ions valence states on the optical and magnetic properties of GdCr1-xTixO3 (0≤x≤0.05)

The vacancy defects, optical and magnetic properties of GdCr_1-xTi_xO₃ (0≤x ≤ 0.05) polycrystalline samples were investigated in this research. The crystal structural analyses show that the orthorhombic perovskite structure exists in all the samples. The positron annihilation spectra reveal that the vacancy defects and local electron density vary with the change of Ti ions valence states. The magnetic measurements show that the antiferromagnetic transition is shifted to lower temperature with the replacement of Ti ions in Cr sites, and the value of spin reorientation caused by the exchange interaction between Gd³⁺ ion and Cr³⁺ ion can be influenced by the change of Ti ions valence states, which rises from x = 0 to 0.04 and then decreases at x = 0.05 sample. Simultaneously, the optical band gap decreases from 2.7 eV to 0.8 eV with the introduction of Ti ions into the lattice, the result can provide a reference for improving optical activity of rare-earth chromium oxides.     

Effect of Nb substitution on magnetic,ferroelectric and photocatalytic properties of Bi0.95Sm0.05Fe1-xNbxO3 (0 ≤ x ≤ 0.1) nanoparticles

The single phase Bi_0.95Sm_0.05Fe_1-xNb_xO₃ (0 ≤ x ≤ 0.1) nanoparticles were synthesized by the sol-gel route, and the effect of Nb substitution on their magnetic, ferroelectric and photocatalytic properties were studied. X-ray diffractometry confirms a phase transformation from rhombohedral to orthorhombic with an increase in Nb substitution. The grain size decreases significantly, and the morphology of grains becomes homogeneous with the increase of Nb concentration. The maximum remnant magnetization (0.014 emu/g), coercivity (565 Oe) and polarization (0.592 μC/cm²) are observed in Bi_0.95Sm_0.05Fe_0.9Nb_0.1O₃. It has been observed that the energy band gap has been slightly reduced from 2.14 to 2.03 eV with Nb substitution, indicating an improvement of photocatalytic activity. The methylene blue degradation is used to represent the photocatalytic ability of Bi_0.95Sm_0.05Fe_1-xNb_xO₃ nanoparticles. The highest degradation efficiency (~74%) of methylene blue is obtained in Bi_0.95Sm_0.05Fe_0.93Nb_0.07O₃, which is much higher than that of Bi_0.95Sm_0.05FeO₃ (~51%) and can be attributed to the optimum particle size and the smallest energy band gap.     

Tailoring the room temperature ferromagnetism in TiO2 nanoparticles by modulating the concentration of surface oxygen vacancy via La incorporating

La-incorporated TiO₂ nanoparticles (Ti_1-xLa_xO₂) were synthesized by a sol-gel method followed by vacuum annealing at 500 °C for 4 h, and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy, and vibrating sample magnetometer (VSM), respectively. The results indicated that La ions were incorporated with TiO₂ without distorting the tetragonal anatase structure, and a small amount of La ions were doped into TiO₂ lattice by substituting the surface Ti sites, whereas most of the La atoms evenly distribute into TiO₂ matrix in the form of La₂O₃. The incorporation of La ions with TiO₂ matrix modulates the concentration of surface oxygen vacancy in Ti_1-xLa_xO₂ nanoparticles. The room temperature ferromagnetism (RTFM) in Ti_1-xLa_xO₂ nanoparticles varies with the concentration of surface oxygen vacancy due to ferromagnetic coupling interactions between singly ionized oxygen vacancy, thus RTFM can be tailored by modulating the concentration of surface oxygen vacancy via La ions incorporating.     

Synthesis,structural and microwave absorption properties of Cr-doped zinc lanthanum nanoferrites Zn1-xCrxLa0.1Fe1.9O4 (x=0.09, 0.18, 0.27 and 0.36)

Cr-doped zinc-lanthanum nanoferrites Zn_1-xCr_xLa_0.1Fe_1.9O₄ (x = 0.09, 0.18, 0.27, and 0.36) were successfully synthesized using sonochemical reactors. Effect of powder production parameters were extensively studied and powder characterization was performed. Existence of cubic spinel structures in the prepared nanoferrites with the average crystallite size ranging from 35 to 51 nm was confirmed by X-ray diffraction studies. An electrochemical impedance analyzer was used to measure the dielectric constant (ε′), loss tangent (tan δ), and complex dielectric constant (ε") with respect to frequency and composition ratio. Maxwell–Wagner polarization and hopping mechanism were calculated to distinguish the variations in ε′, tan δ, and ε". The Nyquist impedance plots for nanoferrites revealed the pseudocapacitance as well as resistive behavior. Vibrating sample magnetometer studies reveled the ferromagnetic behavior of nanoferrites. Substantially increased saturation magnetization and decreased coercivity were noted with respect to increased Cr²⁺ ions in the prepared nanoferrites. It was found that the addition of chromium in Zn_1-xCr_xLa_0.1Fe_1.9O₄ nanoferrites enhances the optical, electrical, and magnetic properties of the nanoferrites.     

Novel S,N co-doped Ba2In2-xCrxO5+y oxides: Synthesis and optical properties

The (S, N) co-doped Ba₂In_2-xCr_xO_5+y (0 ≤ x ≤ 0.5) oxides are successfully obtained by mixing the Ba₂In_2-xCr_xO_5+y oxides and thiourea through a simple ball milling method followed by sintering at 400 °C for 3 h. The colors of the compounds change from orange-brown to yellow-green after reacting with thiourea. When Cr amount is small (x = 0.1), the crystal structure of (S, N) co-doped Ba₂In_2-xCr_xO_5+y is orthorhombic Ba₂In₂O₅ phase. When x ≥ 0.3, the crystal structure of the sample is cubic BaInO_2.5 phase. And this phase transition is the same as Ba₂In_2-xCr_xO_5+y. XPS results reveal that Cr⁶⁺ in Ba₂In_2-xCr_xO_5+y (0 ≤ x ≤ 0.5) oxides are reduced to Cr³⁺ after sintering. S exists in both cation and anion forms, and N exists in substitutional forms. UV–Vis analysis indicates that the yellow-green hue comes from the d-d transition of Cr³⁺, and the doping of S, N ions leads to a red shift of the absorption edge of the samples.     

Structural and magnetic properties of strontium substituted NiZn ferrite nanopowders

Sr-substituted NiZn ferrite nanopowders, Ni_0.5-xZn_0.5Sr_xFe_2.0O₄ (0≤x≤0.20), were synthesized by the sol-gel auto-combustion method. The effects of Sr substitution on the structural and magnetic properties have been investigated. Differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) measurements were used to characterize chemical, structural and magnetic properties. The DTA-TG results indicate that there are three steps of combustion process. XRD results indicate that the lattice parameter increases, and the average crystallite size decreases with increasing Sr substitution. The Sr₂FeO₄ and SrFe₁₂O₁₉ impurity phases formed with excess Sr substitution. The saturation magnetization monotonically decreases with the increase of Sr substitution. Meanwhile, the coercivity initially decreases with the increase of Sr substitution when x≤0.15, and increases when x>0.15.     

The magnetization reversal and high temperature dielectric response in Y1−xHoxFe0.5Cr0.5O3

The structural, magnetic, and dielectric properties of the Y_1−xHo_xFe_0.5Cr_0.5O₃ (x=0, 0.05, 0.1, 0.3, and 0.5) compounds have been investigated. Rietveld refinement of the XRD patterns shows that the compounds possess orthorhombic perovskite structure. The dual magnetization reversal is observed in the samples with x=0.05 and 0.1, and it vanishes when x≥0.3. Ferromagnetic-like behavior with large coercive fields is observed in all Ho³⁺ doped YFe_0.5Cr_0.5O₃ samples, indicating a doping induced metamagnetic behavior. This abnormal magnetization behavior can be explained by the antiparallel magnetic coupling between the Ho³⁺ and the canted Cr³⁺/Fe³⁺ moments, as well as the Ho–O–Ho magnetic interaction. The dielectric behavior in the frequency range from 100 Hz to 10 MHz is investigated. The low doped samples (x=0, 0.05, and 0.1) exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. The dual magnetization reversal under low magnetic field makes these materials attractive candidates for the magnetic dual sensor devices.     

Structural development and magnetic phenomenon in Zn–Cr–Fe multi oxide nano-crystals

The Cr³⁺ ions doped multi-oxide ZnFe_2−xCr_xO₄ ferrite nanoparticles have been synthesized by chemical co-precipitation method. Site occupancies of Zn²⁺, Cr³⁺ and Fe³⁺ ions were analyzed using X-ray diffraction data and Buerger's method. The effect of the constituent phase variation on the magnetic hysteresis behavior was examined by saturation magnetization which decreases with the increase in Cr³⁺ content in place of Fe³⁺ ions at octahedral B-site. Typical blocking temperature (T_B) around 90 K was observed by zero field cooling and field cooling magnetization study. Room temperature Mössbauer spectra show two paramagnetic doublets (tetrahedral and octahedral sites). The isomer shifts of both doublets decrease whereas quadrupole splitting and relative area of tetrahedral A-site increases with increasing Cr³⁺ substitution. The dielectric constant (measured on compositions x=0, 0.4, 0.8 and 1.0) increases when the temperature increases as in the semiconductor. This behavior is attributed to the hopping of electrons between Fe²⁺ and Fe³⁺ ions with a thermal activation.     

Synthesis and analysis of structural,compositional, morphological,magnetic, electrical and surface charge properties of Zn-doped nickel ferrite nanoparticles

Zn-doped nickel ferrite nanoparticles (Zn_xNi_(1-x)Fe₂O₄) were synthesized using the co-precipitation technique. The structural and compositional studies of the Zn_xNi_(1-x)Fe₂O₄ nanoparticles revealed their face-centred cubic spinel structure and an appropriate amount of Zn doping in nickel ferrite nanoparticles, respectively. The morphological analysis had been carried out to obtain the particle size of the synthesized nanoparticles. The magnetic studies revealed the superparamagnetic nature of the Zn_xNi_(1-x)Fe₂O₄ nanoparticles, and the maximum magnetization of 30 emu/g for the Zn_0.2N_0.8Fe₂O₄ sample. The M ? H curves were fitted with the Langevin function to obtain the magnetic particle diameter of Zn_xNi_(1-x)Fe₂O₄ nanoparticles. The electrical conduction in Zn_xNi_(1-x)Fe₂O₄ nanoparticles was explained through the Verway hopping mechanism. The Zn_0.2N_0.8Fe₂O₄ nanoparticle exhibited a higher electrical conductivity of 42 μS/cm and surface charge of ?29/7 mV due to the enhanced hopping of Fe³⁺ ions in the octahedral sites. Owing to this nature, they were identified as the suitable candidates in the applications such as thermoelectrics, hyperthermia, magnetic coating and for the preparation of conducting ferrofluids.     

Effect of Cr doping on ferroelectric and magnetic properties of Bi0.8Ba0.2FeO3

Multiferroic ceramics Bi_0.8Ba_0.2Fe_1−xCr_xO₃ (x=0, 0.05 and 0.1) were synthesized by using the conventional solid state reaction method. The pure phase with rhombohedral structure was confirmed by the X-ray diffraction measurements for all samples. It was shown that the magnetic and the ferroelectric properties were simultaneously improved, and the maximum values of the remnant magnetization (2Mr) and the remnant polarization (2Pr) at room temperature for all samples are around 1 emu/g and 1.9 μC/cm², respectively. Furthermore, the leakage current density, the low frequency dispersion in the dielectric constant and the dielectric loss decreased with increasing the Cr concentration from x=0 to 0.1. A remarkable change in the P–E loop was observed whether a bias dc magnetic field was applied or not, approving the existence of the magnetoelectric coupling indirectly therein.     

Oxide/oxide composites in the system Cr2O3-Y2O3-Al2O3

Ceramic compacts in the systems Al₂O₃–Y₂O₃, Cr₂O₃–Y₂O₃ and Y₃(Cr_yAl_1-y)₅O₁₂ (Cr-doped YAG) were prepared by solid state reaction in calcined co-precipitated powder mixtures of appropriate compositions. Various solid-solution phases were formed, e.g. Y₃(Al_1-xCr_x)₅O₁₂, YAl_yCr_1-yO₃ and Al_2-xCr_xO₃. Composite materials in the pseudo-binary or ternary systems Al₂O₃–Y₃Al₅O₁₂, Cr₂O₃–Y₂O₃ and Y₃(Al_1–xCr_x)₅O₁₂–YAl_yCr_1–yO₃–(Al_zCr_1−z)₂O₃ were obtained by hot-pressing appropriate powder precursors at 1600–1650°C for 1 h. The microstructure of the prepared materials was studied in a scanning electron microscope with element analysis facilities. X-ray diffraction was used to reveal the phases present and their lattice parameters. The chemical compatibility of these phases was investigated. The results are discussed with a special emphasis on the solubility of Cr in the YAG structure, and on the compatibility relationship between Cr-doped YAG and its neighbouring phases. A gel-coating process for preparing Al₂O₃–YAG composites with tailored microstructures is also described.     

Fabrication of Ce:YAG,Ce,Cr:YAG and Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramics for the application of white LEDs

(Ce_0.001Y_0.999)₃Al₅O₁₂ and (Ce_0.001Y_0.999)₃(Cr_xAl_1−x)₅O₁₂ (x=0.001−0.005) transparent ceramics were synthesized by the solid state reaction and vacuum sintering and their optical properties were measured. High quality white light was obtained when the Ce:YAG/Ce,Cr:YAG dual-layered composite ceramic was directly combined with commercial blue LED chip. A maximum luminous efficacy exceeding 76 lm/W at a low correlated color temperature of 4905 K was obtained. The color temperature can be controlled by variations of Cr³⁺ concentration and the ceramic thickness. Hence, the Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramic may be a promising candidate for white LEDs.