Investigation of structural,magnetic and Mössbauer properties of Co2+ and Cu2+ substituted Ni–Zn nanoferrites

We investigated the effects of Co²⁺ and Cu²⁺ substitution on the super-exchange interactions in Ni–Zn nanoferrites. The cation distribution technique was taken into account to explain the results. To authenticate the cation distribution, we have estimated the cation distribution in the light of X-ray diffraction method, Mössbauer spectroscopic analysis, and magnetization study. Statistical model based on the cation distribution was used to calculate the Curie temperature. The values of magneton number n_B and Curie temperature T_C calculated by using the cation distribution is found to be in agreement with the experimentally obtained values.     

Structural,morphological, magnetic hysteresis and dielectric properties of cobalt substituted barium–lead hexagonal ferrites for technological applications

M-type, Ba_0.4Pb_0.6Fe_12-xCo_xO₁₉ (x = 0.00, 0.10, 0.20, 0.30, 0.40) hexaferrites, synthesized using citrate gel auto combustion method, and heated at 950 °C, 4 h for lossless applications. XRD analysis shows the development of the M-phase, along with PbM and hematite. The microstructural analysis reveals the stacking clusters of hexagonally shaped platelets. TEM image and SAED pattern of x = 0.3 composition shows polycrystalline nature and formed particles observed to fused with neighbouring particles. M − H loops of all samples reveal hard magnetic behaviour and possess multi-domain structure. The maximum saturation magnetization of 55.427 A m²/kg is observed in x = 0.10 composition and coercivity of prepared hexaferrites was found to vary from 0.058 T to 0.390 T. The cobalt substitution has a strong influence on the dielectric properties of prepared hexaferrites. The value of ac conductivity increases with cobalt substitution from x = 0.00 to x = 0.10, and followed by a reduction from x = 0.10 to x = 0.40. The same trend is observed for the dielectric constant. The low value of loss tangent for all compositions shows apt scope for lossless application.     

Structural,magnetic and dielectric properties of Co-Zr substituted M-type calcium hexagonal ferrite nanoparticles in the presence of α-Fe2O3 phase

Polycrystalline nanoparticles of M-type Ca(ZrCo)_xFe_12?2xO₁₉ (0.0?≤?x?≤?1.0) hexaferrites were prepared using a simple heat treatment method at a low heating temperature of 650?°C. Effect of cobalt-zirconium substitution on the structural, microstructural, magnetic and dielectric properties was investigated. XRD analysis indicates that all the samples possess a hexagonal structure with anti-ferromagnetic α-Fe₂O₃ phase. The values of lattice parameters and cell volume found to be increased with increasing the cobalt-zirconium substitution along with the amount of α-Fe₂O₃ phase. Crystal symmetry has not affected by Zr–Co substitution in prepared calcium hexaferrite samples but the position of diffraction peak [108] is found to shift towards a lower angle as an increase in the substitution of Zr–Co. The crystallite size found to vary between 12 and 17?nm. SEM images show agglomerated grains and surface morphology has changed with Zr–Co substitution. EDX analysis of typical samples revealed the presence of Ca, Fe, Co, Zr. The magnetic analysis revealed the formation of multi-domain structure. Room temperature Mössbauer spectra of prepared samples show that all five sextets are merged together with a paramagnetic doublet and it confirmed that the size of particles is very small in the nano range. Single and double semicircle arcs were observed in Cole-Cole plots, due to the contributions of grain and grain boundaries resistance.     

Influence of samarium doping on magnetic and structural properties of M type Ba–Co hexaferrite

Sm doped Ba–Co hexaferrite with composition BaCo_0.8Sm_xFe_(11.2−x) O₁₉ (x=0.2, 0.4 and 0.6) were prepared via a citrate precursor method. After appropriate heat treatments, the ferrite samples were characterised by using different measurement techniques. X-ray diffraction (XRD) confirmed the formation of M phase with an average crystallite size of 35–45 nm. Observed tensile strain leads to the elongation of Ba–Co grains and was calculated using Williamson Hall plot. Surface morphology of these samples was studied by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). σ–H loops of these samples were measured at room temperature up to an applied field of 22,000 Oe using vibrating sample magnetometry (VSM). With an increase in concentration of Sm ions in Ba–Co lattice, specific saturation magnetisation (σ_s), coercivity (H_c) and retentivity (σ_r) values were found to decrease. At x=0.2, excellent values of H_c (2690.20 Oe) and squareness ratio (SQR=0.5619) were simultaneously found. These parameters make this material a promising candidate for the applications such as high density magnetic recording and enhanced memory storage.     

Effect of Mn/Co substitutions on the resistivity and dielectric properties of nickel–zinc ferrites

Nickel-zinc ferrite nanoparticles with suitable chemical modifications by Mn or Co substitutions for Zn as two systems (namely, Ni–Zn–Mn and Ni–Zn–Co) were synthesized by sol-gel autocombustion method. X-ray diffraction measurements on the synthesized nanoparticles confirm that the samples attain single phase cubic spinel structures only. The powders were then used to obtain pellets in desired dimensions by employing usual ceramic procedure for carrying out measurements of resistivity and dielectric properties. The variations of dc resistivity as a function of composition and temperature, and the corresponding variation of activation energies for both the systems are presented and discussed. Also, the results of dielectric constant as a function of substituent concentration, dielectric dispersion and dielectric loss tangent are discussed. Effect of Mn/Co substitutions in Ni–Zn ferrites and possible mechanisms responsible for variations in resistivity and dielectric properties of both the ferrite systems have been evolved independently. Also, comparison of the trends between the dielectric constant and the resistivity with substituents’ concentration and their inter-relation with conduction mechanisms has been thoroughly analyzed for both the ferrite systems.     

Influence of B-site cations ordering on magnetization and dielectric relaxations in Li–Ni spinel ferrites

Ho-substituted Li–Ni ferrites with composition L i_1.2Ni_0.4Ho_xFe_2-xO₄; 0≤ x ≤ 0.15 were synthesized by a self-ignited sol-gel process. An annealing temperature of 950 °C is estimated via thermal-gravimetric (TGA) analysis. X-ray diffraction (XRD) scans have confirmed the formation of the ferrite phase with a spinel structure in all samples. Substitution of Ho ions on the B-site significantly reduced the porosity from 38 -to 23% and the crystallite size from 23.4 -to 21.7 nm. Microstructural analysis revealed a denser structure with an increase in Ho content. Dielectric results showed that both the dielectric loss and dielectric constant depict a nonlinear variation with the addition of Ho. Complex impedance behavior with a single semicircle for all samples suggests the predominant effect of the grain boundary mechanism. The substitution of Ho ions in place of Fe ions significantly decreased the electrical conductivity. The anisotropic Ho³⁺ ions reinforce the L-S coupling which consequently enhanced the coercive force from 145 -to 389 Oe, and thus the anisotropy constant.     

Investigations on the structural,electrical, magnetic and 57Fe Mössbauer studies of YFeO3

Yttrium iron oxide (YFeO₃) sample is prepared by sol-gel method. X-ray diffraction (XRD) graph shows highly defined peaks showing that the compound possess crystalline nature. The Rietveld refined XRD data revealed that the sample has orthorhombic structure with Pbnm space group. The experimental data of structural parameters is in accordance with DFT calculated theoretical structural parameters.Fourier Transform Infrared Spectroscopy (FTIR) shows that a pure phase of YFeO₃ isbeen formed. Scanning electron micrograph of YFeO₃ reveals that the sample has non uniform grain structure with slight porosity in it. The grains have an average grain size of about 200 nm. Raman spectroscopic studyconfirms the structure and phase purity of the sample. The M-H curve of YFeO₃shows a hysteresis loop, with a coercive field of 61 Oe and a residual magnetization of 0.516 emu/gm. Weak ferromagnetic nature of YFeO₃ is attributed to the remnant magnetization value, which suggests that Fe⁺³ spins are slightly canted but not perfectly anti parallel. Room temperature P-E measurement of YFeO₃ shows lossy ferroelectric nature of the sample.Positive Up Negative Down (PUND) measurementsdone on YFeO₃ sample gives the trueswitchable polarization to be0.002 C/cm².From the ln J versus ln E plot, the slope of the line is found to be 1.25, indicating Ohmic conduction in the sample. At 300 K, ⁵⁷Fe Mössbauerspectra of YFeO₃ exhibit a single magnetic sextet, confirming the presence of a single Fe site and the absence of any magnetic impurities. The value of Isomer shift is0.37 mm/s, quadrupole splitting is found to be 0.01 mm/s, Line width (Г) = 0.47 mm/s and hyper fine magnetic field (B_hf) of 49.8 T are being calculated from ⁵⁷Fe Mössbauer measurements.     

Influence of intercalated organoclay on the phase structure and physical properties of PTT–PTMO block copolymers

Poly(trimethylene terephthalate-block-tetramethylene oxide) (PTT–PTMO) copolymer/organoclay nanocomposites were prepared by in situ polymerization. They showed an intercalated silicate structure, as determined by X-ray diffraction and transmission electron microscopy. The influence of intercalated organoclay on the two-phase structure and mechanical properties of PTT–PTMO block copolymer was examined by using DSC and tensile tests. The DSC results imply that the silicate layers (Nanofil 32) in PTT–PTMO act as nucleation agents and accelerate the crystallization of PTT hard phase during the cooling down process from the melt. The introduction of silicate layers does not have great effect on the glass transition temperature of PTMO-rich soft phase, melting temperature of PTT hard phase, and degree of crystallinity of the nanocomposites. As the organoclay loading in the nanocomposites increase, the enhanced tensile modulus and yield stress was observed. The cyclic tensile tests showed that obtained nanocomposites have values of permanent set comparable to the neat PTT–PMO copolymer.     

Study of structural,dielectric, electric,magnetic and magnetoelectric properties of K0.5Na0.5NbO3 − Ni0.2Co0.8Fe2O4 composites

Magnetoelectric composites with general formula (x) Ni_0.2Co_0.8Fe₂O₄- (1- x) (K_0.5Na_0.5)NbO₃ (x = 0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) have been synthesized by solid state reaction method. X-ray diffraction pattern asserts the existence of both the constituent phases in the synthesized composites. FESEM micrographs are used to investigate the microstructure and for calculation of average grain size of the composites. Temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with addition of ferrite in the composites. P-E hysteresis loops obtained for individual (K_0.5Na_0.5)NbO₃ (KNN) phase and composites indicate the ferroelectric ordering in the composites. Saturation and remnant magnetization show increasing trend with increase in NCFO concentration. FC-ZFC magnetization curves indicate charge ordering, metal-insulator transition (Verwey transition) in NCFO and composites. Impedance spectroscopy shows that bulk resistance reduces with increase in temperature, thereby indicating negative temperature coefficient of resistance (NTCR) behaviour of the composites. The magnetoelectric effect is confirmed by measuring magnetoelectric voltage coefficient, α_ME and the maximum value of α_ME is 5.389 mV/cm-Oe for 20% NCFO-80% KNN composite.     

Influence of annealing treatment on magnetic properties of Fe2O3/SiO2 and formation of ε-Fe2O3 phase

Magnetic properties of Fe₂O₃/SiO₂ samples were studied after being produced by sol-gel synthesis and formation of ε-Fe₂O₃ polymorph. Samples were thermally treated, using different annealing temperatures and annealing times. The size and morphological characteristics of the iron oxide nanoparticles were examined using a TEM microscope. We used the “ellipticity of shapes”, which is a measure of how much the shape of a nanoparticle differs from a perfect ellipse, in order to quantitatively describe morphological properties of nanoparticles. Coercivity measurements were used to identify and monitor the formation of the epsilon-iron oxide phase during the thermal treatments (annealing). Coercivity values were in the range from 1.2 to 15.4 kOe, which is in accordance with previous experience regarding the existence of ε-Fe₂O₃. We have determined the optimal formation conditions for the ε-Fe₂O₃ polymorph (t=1050 °C for 7 h, H_C=15.4 kOe), as well as the narrow temperature interval (1050–1060 °C) in which the polymorph abruptly vanished (H_C=2300 Oe), on the basis of results of the magnetic properties. The threshold temperature for the ε-Fe₂O₃ phase transformation was measured as 1060 °C. We found that different annealing temperatures and annealing times significantly affected magnetic properties of the examined samples.     

First-principles study on electronic and magnetic properties of twisted graphene nanoribbon and Möbius strips

The geometric, electronic, and magnetic properties of twisted zigzag-edged graphene nanoribbons (ZGNRs) and novel graphene Möbius strips (GMS) are systematically investigated with the first-principles calculations based on the density functional theory. All the structures of ZGNRs and GMS are optimized, and their structural stabilities are examined. The molecular energy levels and the spin polarized density of states of ZGNRs are also calculated. It is found that the atomic bonding energies of the twisted ZGNRs decrease quadratically with the increase of the twisted angle, and the gaps between the lowest unoccupied molecular orbital and the highest occupied molecular orbital are varied with the twisted angle. The spin densities of ZGNRs and GMS reveal that the ground states with antiferromagnetic edges persist during the twisting, and the spin flip at some positions of the zigzag edges of GMS can be observed.     

Rietveld refinement,Raman, optical,dielectric, Mössbauer and magnetic characterization of superparamagnetic fcc-CaFe2O4 nanoparticles

This article describes synthesis of superparamagnetic fcc-CaFe₂O₄ nanoparticles by a metal nitrate-citrate monohydrate sol–gel route and characterization using X-ray diffractometry, Raman spectroscopic, UV–Vis–NIR optical absorption spectroscopic, Mössbauer spectroscopy, dielectric and SQUID magnetometry measurements. Rietveld refinement of the X-ray diffraction pattern and observation of active A_1?g, T_2?g & E_g modes in the Raman spectrum confirmed formation of single phase CaFe₂O₄ nanoparticles in the spinel ferrite type fcc structure without impurity. Mössbauer and Rietveld data analysis revealed that nanocrystalline CaFe₂O₄ is dominantly an inverse ferrite in which 85% Ca atoms preferentially occupy the octahedral site in the fcc symmetry and the Fe ions are in high spin Fe⁺³ state. Nanocrystalline CaFe₂O₄ significantly absorbs optical light below 500?nm and the direct band gap energy is estimated to ~1.83?eV, which is higher than 1.26?eV reported for orthorhombic CaFe₂O₄. Temperature and field dependent magnetic studies showed that fcc-CaFe₂O₄ nanoparticles exhibit superparamagnetism at room temperature with high saturation magnetization of 1.07μ_B. The blocking temperature is ~53?K at 1000?Oe and ~72?K at 500?Oe clearly shows lowering of blocking temperature at higher magnetic field. Interestingly below the blocking temperature at 20?K, CaFe₂O₄ nanoparticles behave as non collinear soft-ferrimagnetic material with a saturation magnetization of 1.16 μ_B, coercivity of 150?Oe and remanence of 4.45?emu/g. The magnetic momenta of Fe⁺³ ions residing at the octahedral sub-lattice are canted at ~25?°.     

Structural,optical, electrical,dielectric, molecular vibrational and magnetic properties of La3+ doped Mg–Cd–Cu ferrites prepared by Co-precipitation technique

Ferrites are among the most frequently investigated materials mainly due to interesting and practically different properties. Therefore, easily and cost-effective lanthanum doped Mg_0.5Cd_0.25Cu_0.25Fe_2-xLa_xO₄ (x = 0.0, 0.0125, 0.025, 0.0375 and 0.05) ferrites were synthesized by a co-precipitation route, a comprehensive characterisation of their structural, optical, electric, dielectric, molecular vibrational, and magnetic properties were carried out. X-ray diffraction analysis confirmed the formation of a cubic spinel structure. Variations in frequency bands were also observed with amplification in optical band gap energy (2.95 – 3.38 eV) due to La³⁺ ions insertion. The electric resistivity had opposite trends at low and high temperatures with increasing La³⁺ content. The Curie temperature, activation energy, and drift mobility were also determined to have values consistent with the semiconducting behavior of the soft ferrites. The saturation magnetization (M_S) has a maximum value 49.385 emu/g with remanent magnetization (M_r) was 34.928 emu/g and coercivity 661.4 Oe for La³⁺ concentration x = 0.05. The minimum dielectric loss was observed for La³⁺ concentration x = 0.025. Moreover, the resistivity (ρ) has a maximum value of 7.95 × 10⁴ Ω cm for La³⁺ concentration x = 0.025. The calculated frequency range of La³⁺ doped Mg–Cd–Cu ferrites was detected in the microwave range (3.36 – 10.80 GHz), suggesting the potential application of the materials in longitudinal recording media and microwave absorbance.     

Rietveld refined crystal structure,magnetic, dielectric,and electric properties of Li- substituted Ni–Cu–Zn ferrite and Sm,Dy co-doped BaTiO3 multiferroic composites

The conventional solid-state reaction technique is used to fabricate the multiferroic xLi_0.1Ni_0.3Cu_0.1Zn_0.4Fe_2.1O₄(LNCZFO)+(1-x)Ba_0.95Sm_0.05Ti_0.95Dy_0.05O₃(BSTDO) composites. To determine the ferrite and ferroelectric phases, the Rietveld refinement analysis is used. The excellent fit of experimental diffraction data is confirmed by the low values of reliability factors and the goodness of fit index, and so the crystal structure is perfect. Increasing the LNCZFO phase in the composites causes the formation of more ferrite grains and enhancement of magnetization values. The anisotropy field varies due to compressive stress created by a lattice mismatch between the BSTDO and LNCZFO phases. The dielectric peak shifts to higher temperatures as the ferrite phase increases, indicating that magnetoelectric interaction between the constituent phases exists in composites. At 100 kHz, the diffuseness exponent ranged from 1.01 to 1.79, indicating that a diffuse phase transition (DPT) occurred for some composites. As the ferrite content increases, the DPT effect decreases, resulting a narrower dielectric peak. The small polaron hopping mechanism is responsible for electrical conduction, which followed Jonscher's power law. The magnitude of the angular frequency exponent factor increases with frequency, indicating an increase in charge carrier mobility from long to short range.     

Effects of Ca–Gd co-substitution on the structural,magnetic, and dielectric properties of M-type strontium hexaferrite

Sr_1−xCa_xFe_12−xGd_xO₁₉ (x = 0, 0.04, 0.08, 0.12, 0.16, 0.20) hexaferrites were characterized by several techniques, such as X-ray diffraction, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and vibrating sample magnetometer. Structural results indicate the formation of a pure-phase Sr_1−xCa_xFe_12−xGd_xO₁₉ hexaferrite with space group P6₃/mmc. SEM photography confirms that there are a smaller number of defects due to the reduced porosity and surface area (increased particle size). Magnetic investigations showed a rise of the coercive force from 5069.8 to 5757.4 Oe and saturation magnetization from 79.25 to 80.68 emu/g. The maximum values appear to be for sample x = 0.16, which may be useful in such as permanent magnets, and high-density media for magnetic storage devices. Dielectric parameters, such as conductivity, the real part of permittivity, dielectric loss, dielectric tangent loss, and complex modulus, were studied. Impedance analysis shows that the conduction process is mainly governed by the long-range movement of the charge carriers based on the Debye model for x = 0.12.     

Effect of heat treatment on structural,morphological, dielectric and magnetic properties of Mg–Zn ferrite nanoparticles

Green combustion was used to prepare a ferrite composition of Mg_0.4Zn_0.6Fe₂O₄ using a blend of fresh lemon juice as a natural fuel-reductant. Effect of heat treatment on phase, morphological, dielectric, and humidity sensor properties is discussed. The formation of a cubic spinel ferrite has been established by XRD-diffraction and vibrational spectroscopic studies. The experimental lattice parameter ranges from 8.3721 to 8.3631 Å. The broadening of octahedral band (υ₂) in the vibrational spectra is an identification for the existence of ferrite nanoparticles in various sizes. The typical crystallite size ranges from 10.2 to 36.9 nm. Using micrographs obtained from field-effect scanning electron microscopy (FESEM), researchers observed a spherical-shaped microstructure with agglomerated nanoparticles. Dielectric investigations have shown that the current ferrite composition has typical dielectric dispersion. The highest reported value for saturation magnetization (M_s) in the present study is 33 emu/g. Magnetic behaviour is primarily influenced by magnetocrystalline anisotropy, cation distribution, and crystallite size. The existence of void spaces in the sintered samples, as well as their porous nature, rendered them suitable for humidity sensor applications. Sintered samples have good sensing capability at 900 °C. The current findings are integrated in terms of cation distribution and magnetocrystalline anisotropy, assuming fine size effects of ferrite nanoparticles.     

Spinel ferrites NiFe2O4 NCs enhanced Mössbauer spectra,magnetization and Faraday rotation of diamagnetic tellurite glasses

Glass containing magnetic nanocrystals are attractive for obtaining high magnetic and magneto-optical properties. In this study, for the first time, 10 nm cubic NiFe₂O₄ (NFO) nanocrystals doped heavy metal oxide glasses were fabricated and the influence of NFO to glass structure, chemical bonds, Mössbauer spectra, magnetization and Faraday rotation was studied. X-ray diffraction, Raman and X-ray photoelectron spectra revealed the existence of multi-valence states of Fe/Ni ions, oxygen vacancies in cubic lattice and modifications on glass structure, which in turn adjusted the polarizability, oxygen packing density, Mössbauer spectra, magnetic property and magneto-optical behaviors. Through Mössbauer study, the increase of NFO amount enhanced the hyperfine field intensity of tetrahedral Fe³⁺ ions at the expense of octahedral ones. NFO doped glasses exhibited ferromagnetic behavior whose M_s and H_c increased with the NFO amount due to the NCs size effect. Verdet constant and Faraday rotation angle were studied as function of wavelength, magnetic field, polarizer angle and NFO amount. 3%NFO glass exhibited significant enhancement in magnetization and Verdet constant of 91 rad/T.m at 633 nm which is 5-fold of host glass and superior than literatures. In addition, due to the nanoscale NCs, the thermal stability of obtained NFO glasses remained higher than 100 °C which is promising for photonics devices fabrication.     

Effect of mechanical milling on structural,dielectric and magnetic properties of BaTiO3–Ni0.5Co0.5Fe2O4 multiferroic nanocomposites

The coexistence of ferroelectricity and ferromagnetism has triggered great interest in multiferroic materials. Multiferroic with strong room temperature magnetoelectric (ME) coupling can provide a platform for future technologies. In this paper, we have investigated the effect of mechanical milling on the properties of multiferroic nanocomposites synthesized by mixing barium titanate (BaTiO₃) (BT) and nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) (NCF). This process has resulted into reliable disposal of a given quantity of NCF nanoparticles in BT grid and composite samples of different particle sizes (<500 nm) have been obtained by varying the duration of ball-milling for 12, 24, and 48 h. The presence of NCF within BT powder has been confirmed by X-ray Diffraction (XRD) and magnetization measurements (MH). Structural analysis was performed by using Reitveld refinement method that shows that the tetragonality of BaTiO₃ structure get reduced in submicron range. Variations in ferroelectric and dielectric properties with reduction in particle size/milling duration have been studied by P-E loop tracer and Impedance analyzer. The dielectric constant value of 400 has been observed for BT-NCF0 that increases to 9.7 K for composite sample ball mill at 48 h whereas remnant polarization increases to 4.2 μC/cm². These composites with high dielectric constant that changes with temperature and particles size find application in energy storage devices, sensor and memory devices.