Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.     

Modifications in magnetic anisotropy of M—type strontium hexaferrite crystals by swift heavy ion irradiation.

Using vibrating sample magnetometery (VSM) 50 MeV Li³⁺ ion irradiation effects on magnetic properties of single crystals of SrGa_xIn_yFe_12−(x+y)O₁₉ (where x=0, 5, 7, 9; y=0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals decreases the value of magnetization sharply, which is attributed to shifting of collinear magnetic order to a non-collinear one. Reduction of magnetization is also explained to be as a result of the occupation of the crystallographic sites of Fe³⁺ by Ga³⁺ and In³⁺. The Li³⁺ ion irradiation decreases the value of magnetization, irrespective of whether the crystals are Ga–In substituted or unsubstituted crystals of SrFe₁₂O₁₉. The result is interpreted in terms of the occurrence of a paramagnetic doublet in crystals replacing magnetic sextuplet as a result of irradiation. Substitution of Ga–In in Strontium hexaferrite decreases the value of anisotropy constant. Irradiation with Li³⁺ ions increases the values of anisotropy field for both substituted as well as unsubstituted crystals. Substitution with Ga–In also decreases the Curie temperature (T_c) but the irradiation with Li³⁺ ions does not affect the curie temperature of either Ga–In substituted or pure SrFe₁₂O₁₉ crystals.     

Dielectric, ferroelectric and pyroelectric properties of Pb[(Ni1/3Sb2/3)xTiyZrz]O3 ceramics

In the present study, various Pb[(Ni_1/3Sb_2/3)_xTi_yZr_z]O₃ where x+y+z=1, x=0.08 and y=0.44-0.49, ceramics in the morphotropic phase boundary (MPB) range were studied by dielectric and pyroelectric methods. The results of the investigations revealed an MPB composition range of y≅0.46. The study of the dielectric properties of these compounds as a function of temperature suggests that with increase in y the permittivity maximum increases and transition temperature shifts towards higher temperature. Well-saturated polarization versus electric field (P-E) hysteresis loops were obtained and values of P_r were calculated. The samples revealed good pyroelectric properties for y=0.44 and y=0.45 at room temperature with large figures of merit F_v=0.019 m²/C and F_D=1.34×10^-5Pa^1/2.     

Electrical and electrochemical properties of molten salt-synthesized Li4Ti5−xSnxO12 (x=0.0, 0.05 and 0.1) as anodes for Li-ion batteries

Submicron-sized polyhedral Li₄Ti_5−xSn_xO₁₂ (x=0.0, 0.05, and 0.1) materials were successfully prepared by a single-step molten salt method. The structural, morphological, transport and electrochemical properties of the Li₄Ti_5−xSn_xO₁₂ were studied. X-ray diffraction patterns showed the formation of a cubic structure with a lattice constant of 8.31 Å, and the addition of dopants follows Vegard's law. Furthermore, FT-IR spectra revealed symmetric stretching vibrations of octahedral groups of MO₆ lattice in Li₄Ti₅O₁₂. The formation of polyhedral submicron Li₄Ti_5−xSn_xO₁₂ particles was inferred from FE-SEM images, and a particle size reduction was observed for Sn-doped Li₄Ti₅O₁₂. The chemical composition of Ti, O and Sn was verified by EDAX. The DC electrical conductivity was found to increase with increasing temperature, and a maximum conductivity of 8.96×10⁻⁶ S cm⁻¹ was observed at 200 °C for Li₄Ti₅O₁₂. The galvanostatic charge–discharge behavior indicates that the Sn-doped Li₄Ti₅O₁₂ could be used as an anode for Li-ion batteries due to its enhanced electrochemical properties.     

Structural and magnetic ordering in the VxNb1+yS2 system

The influence of composition on the structural ordering and magnetism in the V_xNb_1+yS₂ system has been investigated by X-ray diffraction and magnetic measurements. Stoichiometric V_1/3NbS₂ did not exhibit the structural ordering of vanadium between the NbS₂ layers. In the ordered structure, the vanadium composition deviated from the ideal value of to both higher and lower values, while the niobium composition was in the range of 0.05?y?0.18. Excess niobium, y>0, is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, a magnetic transition was observed at 20-50 K, depending on the composition. The spontaneous magnetization of 3-5×10⁻³ μ_B/V atom is thought to be intrinsic to this system. The magnetization curves consisted of a constant and a proportional parts of the magnetic field, which correspond to the spontaneous magnetization and high-field susceptibility, respectively. The magnetization curves and the temperature dependencies of the high-field susceptibility were quite similar to those of the canted antiferromagnetic NiS₂. A correlation between the structural and magnetic ordering is suggested.     

Synthesis and structure refinement of layered perovskites Ba5−xLaxNb4−xTixO15 (x=0, 1, 2, 3 and 4) solid solutions

Ba_5−xLa_xNb_4−xTi_xO₁₅ solid solutions were prepared by solid state reaction method. Structural analysis for the stoichiometric phases was performed for x=0, 1, 2 and 3 by Rietveld analysis of neutron powder diffraction data. The x=0, 1 and 2 members could be refined in the space group P-3m1 (stacking sequence chhcc, polytypoid 5 H). There is a decrease in cell volume as x increases. La³⁺ occupies preferentially the A2 site (Wyckoff site 2d) and Ti⁴⁺ the B2 site (Wyckoff site 2c). As x increases there is an increase of the global instability index (GII) (which is a measure of the extent to which the BVS rule is violated over the whole structure) indicating the presence of intrinsic strains large enough to cause instability at room temperature. This strain is responsible for a structural change for the member with x=3, which could be refined in the space group P-3c1 (stacking sequence (chhcc)₂, polytypoid 2×5H=10H). This change in space group is associated with a cooperative rotation of (Nb/Ti)O₆ octahedra around the c-axis, necessary to accommodate the smaller La³⁺ ion in the cuboctahedral cavity.     

Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3−xO4) nanoparticles

Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (Co_xFe_3−xO₄) and some indication of γ-Fe₃O₄ when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at ∼560 and ∼400 cm⁻¹, characteristic of the metal–oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was ∼3 nm with a geometric deviation of ∼0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a=8.388 Å for Co_0.61Fe_0.39O₄, which is near that of CoFe₂O₄ (a=8.394 Å). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating χ′ and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ∼10⁶ erg/cm³ were determined using the Debye model, whereas anisotropy constants in the order of ∼10⁷ erg/cm³ were calculated assuming Ωτ=1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Ωτ=1 at the temperature peak of χ′ is rigorously incorrect.     

Synthesis,structure and magnetic properties of layered manganate Sr4Mn3−xCrxO10 (x=0, 0.2)

The manganates Sr₄Mn_3−xCr_xO₁₀ (x=0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x=0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn⁴⁺) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.     

Synthesis of Gd2Ti2O7:Eu, V phosphors by sol-gel process and its luminescent properties

A red-emitting phosphor material, Gd₂Ti₂O₇:Eu³⁺, V⁴⁺, by added vanadium ions is synthesized using the sol-gel method. Phosphor characterization by high-resolution transmission electron microscopy shows that the phosphor possesses a good crystalline structure, while scanning electron microscopy reveals a uniform phosphor particle size in the range of 230-270 nm. X-ray photon electron spectrum analysis demonstrates that the V⁴⁺ ion promotes an electron dipole transition of Gd₂Ti₂O₇:Eu³⁺ phosphors, causing a new red-emitting phenomenon, and CIE value shifts to x=0.63, y=0.34 (a purer red region) from x=0.57, y=0.33 (CIE of Gd₂Ti₂O₇:Eu³⁺). The optimal composition of the novel red-emitting phosphor is about 26% of V⁴⁺ ions while the material is calcinated at 800  °C. The results of electroluminescent property of the material by field emission experiment by CNT-contained cathode agreed well with that of photoluminescent analysis.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Magnetic properties of Ho1−xMmxCo2 (x=0, 0.1, 0.2, 0.3 and 0.4) alloys and their hydrides

The influence of Mm (Mm=mischmetal) substitution and hydrogen absorption on the magnetic properties of Ho_1−xMm_xCo₂ (x=0, 0.1, 0.2, 0.3 and 0.4) alloys have been determined through the temperature dependence of ac susceptibility and thermopower measurements. The changes in magnetic-ordering temperature of Ho_1−xMm_xCo₂ alloys have been explained based on the dilution of the magnetic ions and weakening of 4f–3d exchange interactions. The gradual disappearance of the magnetic transition temperature upon increasing hydrogen concentration (y) has been interpreted by the lattice expansion and charge transfer between absorbed hydrogen and 3d-band of Ho_1−xMm_xCo₂.     

Effect of Co or Mn addition on the soft magnetic properties of amorphous Fe89−xZr11Bx (x=5, 10) alloy ribbons

The temperature and field dependent magnetic properties of melt-spun amorphous Fe_89−x−yZr₁₁B_x(Co,Mn)_y (x=5, 10 and 0≤y≤10) alloys in the temperature range 5-1200 K are reported. The Curie temperature and saturation magnetization at room temperature increase (decrease) almost linearly with Co (Mn) addition. With increasing Co concentration, the room temperature coercivity increases at the rate of 2.26 (0.28) A/m per at% for the x=5 (10) samples. The high-field magnetic susceptibility and local magnetic anisotropy decrease (increases) rapidly with increasing Co (Mn) concentration. The thermomagnetic curves show a marked increase in magnetization above 850 K corresponding to the crystallization of α-FeCo (α-Fe) phase in samples containing Co (Mn). The Curie temperature of the crystalline phase increases (remains same) with increasing Co (Mn) concentration with the formation of α-FeCo (α-Fe). Addition of Co up to 10 at% in Fe-Zr-B improves the room temperature saturation magnetization from 0.56 to 1.2 T, and Curie temperature from 315 to 476 K. Also, the coercivity increases with Co addition from 1.27 to 23.88 A/m for x=5 and from 7.64 to 10.35 A/m for x=10 alloy. The non-collinear spin structures that characterize Fe rich Fe-Zr-B amorphous alloys have been used to describe the observed results.     

Ab initio calculation of Zn0.8Mn0.2O1−yNy

Based on first-principles spin-density functional calculations, using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA), we investigated the magnetic and half-metallic properties of Mn-doped p-type ZnO and the mechanism which control these properties. Mn-doped ZnO is anti-ferromagnetic spin-glass state, but it becomes half-metallic ferromagnetic upon holes doping. The electronic structure, total magnetic moment of Zn_0.8Mn_0.2O_1−yN_y and magnetic moments of Mn and N in Zn_0.8Mn_0.2O_1−yN_y are calculated for different holes (y) concentrations. In this paper we address the origin of half-metallic and ferromagnetic properties as controlled and oriented by the nature of hybridization of the Mn (3d) state and host p(N) states. The band structure has been used to explain the strong ferromagnetism observed in Zn_0.8Mn_0.2O_0.1N_0.9.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Effect of Ti doping on the electrical transport and magnetic properties of layered compound Na0.8CoO2

Effect of Ti doping on the electrical transport and magnetic properties of layered Na_0.8Co_1−xTi_xO₂ compounds has been investigated. The lattice parameters a and c increase with x. A minor amount of Ti doping results in a metal-insulator transition at low temperatures. For samples with x>0.03, the variable-range hopping process dominates the transport behavior above a certain temperature. The temperature dependence of magnetization of all the samples is found to obey the Curie-Weiss law. The mechanism of the doping effect is discussed.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.