Magnetic properties and hyperfine interactions of Co1-2xNixMnxFe2O4 nanoparticles

Co1-2xNixMnxFe2O4 (0.0≤x≤0.5) nanoparticles (NPs) were prepared via citrate assisted microwave combustion route. XRD analysis confirmed the cubic structure (spinel) of all samples. Average crystallite size of products (obtained from (311) diffraction line) was in the range of 32.9–43.4 nm. The intense peak appearing at around 531 cm⁻¹ in FT-IR was attributed to the formation of a spinel ferrite. Magnetic properties of the products were investigated by room temperature vibrating sample magnetometer and Mössbauer spectroscopy. The magnetic parameters have been found to strongly depend on the Ni and Mn concentrations. The saturation magnetization continuously decreases with the increasing of the concentration (x). We found that Ni_0.5Mn_0.5Fe₂O₄ NP has superparamagnetic character at room temperature. This result was also verified by Mössbauer analysis. Scanning electron microscopic analysis revealed the cubic morphology of all products, EDX and elemental mapping analyses confirmed the expected composition of each product.     

Effect of bimetallic (Ni and Co) substitution on magnetic properties of MnFe2O4 nanoparticles

Nickel and cobalt substituted manganese ferrite nanoparticles (NPs) with the chemical composition Ni_xCo_xMn_1–2xFe₂O₄ (0.0≤x≤0.5) NPs were synthesized by one-pot microwave combustion route. The effect of co-substitution (Ni, Co) on structural, morphological and magnetic properties of MnFe₂O₄ NPs was investigated using XRD, FT-IR, SEM, VSM and Mössbauer spectroscopic techniques. The cation distribution of all products were also calculated. Both XRD and FT-IR analyses confirmed the synthesis of single phase spinel cubic product for all the substitutions. Lattice constant decreases with the increase in concentration of both Co and Ni in the products. From ⁵⁷Fe Mössbauer spectroscopy data, the variations in line width, isomer shift, quadrupole splitting and hyperfine magnetic field values with Mn²⁺, Ni²⁺ and Co²⁺ substitution have been determined. While the Mössbauer spectra collected at room temperature for the all samples are composed of magnetic sextets, the superparamagnetic doublet is also formed for MnFe₂O₄ and Ni_0.2Co_0.2Mn_0.6Fe₂O₄ NPs. The magnetization and Mössbauer measurements verify that MnFe₂O₄ and Ni_0.2Co_0.2Mn_0.6Fe₂O₄ NPs have superparamagnetic character. The saturation and remanence magnetizations, magnetic moment and coercive field were determined for all the samples. Room temperature VSM measurements reveals saturation magnetization value close to the bulk one. It has been observed that the saturation magnetization and coercive field increase with respect to the Ni and Co concentrations.     

Rapid microwave-assisted synthesis and magnetic properties of high-entropy spinel (Cr0.2Mn0.2Fe0.2Co0.2-xNi0.2Znx)3O4 nanoparticles

High-entropy oxide (HEO) has recently become popular because of its unique multifunctional performance. In this study, we developed a novel microwave-assisted method for the production of HEO nanoparticles with the composition (Cr_0.2Fe_0.2Mn_0.2Co_0.2-xNi_0.2Zn_x)₃O₄ (x = 0, 0.05, 0.1, and 0.2). The results revealed that all metallic elements were uniformly distributed throughout the single-phase cubic spinel structure of the HEO nanoparticles. The particle size distributions of four fabricated samples ranged from 10 to 50 nm. Because of its numerous advantages such as the ultrafast and low-temperature fabrication of nanoscale and high-purity products at a relatively low cost, the suggested methodology is an excellent synthesis method. The original HEO spinel (x = 0) achieved saturated magnetization (M_s) and coercivity (H_c) values of 24.3 emu/g and 160 Oe, respectively, at room temperature. Zinc substitution in the HEO composition indicated that M_s and H_c decreased with increasing zinc concentration owing to its non-magnetic nature.     

Role of Mn doping on magnetic properties of multiferroic NiCr2O4 nanoparticles

The structural and magnetic properties of Mn doped Nickel Chromite (Ni_1-xMn_xCr₂O_4, x = 0, 0.2, 0.3, 0.4, 0.6, 0.8) nanoparticles (NPs) were studied in detail. The X-ray diffraction analysis affirms normal spinel structure for all the samples and average crystallite size was found in the range 31–58 nm. The spinel structure of these nanoparticles was also confirmed by Fourier transform infrared spectroscopy which revealed the formation of tetrahedral and octahedral vibrational bands in the range 607 -628 cm^?1 and 486 - 491 cm^?1, respectively. Transmission electron microscopy images depicts less agglomerated and non-spherical shaped NPs. The temperature dependent zero field cooled and field cooled magnetic measurements revealed a paramagnetic to ferrimagnetic transition T_c at 87 K for NiCr₂O₄ NPs, which is shifted to low temperatures by Mn doping. This effect was attributed to cationic distributions between adjacent sites produced by Mn doping. M ? H loops of Ni_1-xMn_xCr₂O₄ NPs revealed enhanced saturation magnetization with increase in Mn doping which is attributed to a large magnetic moment of Mn ions. Ni_1-xMn_xCr₂O₄ (x = 0.6 and 0.8) NPs show steps in their M ? H loops because of exchange interactions between two sites of these NPs.     

Exchange-coupling effect in hard/soft SrTb0.01Tm0.01Fe11.98O19/AFe2O4 (where A = Co,Ni, Zn,Cu and Mn) composites

In this study, series of hard/soft SrTb_0.01Tm_0.01Fe_11.98O₁₉/AFe₂O₄ (where A = Co, Ni, Zn, Cu and Mn) composites were fabricated via a single-pot citrate sol-gel approach. The structure, morphology and magnetic properties of prepared composite samples were investigated via X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM - TEM) and vibrating sample magnetometer (VSM). The XRD analysis of all composite samples showed the co-existence of both hard (Sr hexaferrite) and soft (spinel ferrites) ferrite phases with minor impurity. TEM micrographs displayed well-distinguished particles of SrM and AFe₂O₄ with different symmetry. The magnetic M − H hysteresis loops were performed at room temperature (RT; T = 300 K) and low temperature (T = 10 K) using VSM instrument. The magnitudes of various magnetic parameters including saturation magnetization (M_s), squareness ratio (SQR = M_r/M_s), remanence (M_r) and coercivity (H_c) were determined. M − H loops revealed smoothed curves and the dM/dH versus H curves exposed only a single peak, indicating that the exchange-coupling effect was accomplished in one-step. Moreover, the various composites showed relatively high M_s, M_r, and H_c values. The obtained results revealed the occurrence of exchange-coupling effect among soft and hard magnetic phases. The magnetic properties of various hard/soft SrTb_0.01Tm_0.01Fe_11.98O₁₉/AFe₂O₄ composites (where A = Co, Ni, Zn, Cu and Mn) were evaluated also by ZFC-FC magnetization measurements with respect to different soft phases. A peak temperature in ZFC curves occurred for various prepared composites. This peak is attributed to competition of the movement of magnetic domain walls and thermal activation. The present study offers a simple but efficient route for the fabrication of exchange-coupled nanocomposites with the chemical formula SrFe_11.98Tb_0.01Tm_0.01O₁₉/AFe₂O₄ (where A = Co, Ni, Zn, Cu and Mn) having controllable magnetic properties. It was found that the SrTb_0.01Tm_0.01Fe_11.98O₁₉/CoFe₂O₄ composite sample displayed the strongest exchange-coupling behavior among the different prepared composite products.     

A comparative study of spinel ZnFe2O4 ferrites obtained via a hydrothermal and a ceramic route: Structural and magnetic properties

The spinel ZnFe₂O₄ specimens were obtained via a hydrothermal and a ceramic method, respectively, and their structural and magnetic properties were comparatively studied. It was found that all the specimens exhibited a single-phase and mixed spinel structure. The magnetism of specimens synthesized via the hydrothermal method is obviously better than that of specimen prepared via the ceramic method. This can be ascribed to the different occupancy of Fe ions resulted from the loss of Zn during the hydrothermal process.     

Thermodynamics and electronic structure characteristics of MFe2O4 with different spinel structures: A first-principles study

In recent years, spinel ferrites with chemical formula MFe₂O₄, have attracted much attention due to their impressive photocatalytic and electrocatalytic performances, which are significantly influenced by their spinel structures. However, it is still a big challenge to distinguish or predict spinel structures for spinel ferrites. As an attempt to address this issue, this paper presents a first-principles study of the thermodynamics and electronic structures for six spinel ferrites with different spinel structures. The configurational free energy of these spinel ferrites at different inversion degrees is calculated to determine the equilibrium inversion degree for each spinel, which successfully predicts the spinel structure type of these spinel ferrites. The partial density of states is obtained for six spinel ferrites assuming they are normal or inverse spinels. The electronic states close to the Fermi energy of each spinel ferrite are carefully examined, showing that normal spinels have weak interactions between M and Fe states, while strong interactions exist in mixed or inverse spinels. Our results offer an insightful understanding of different spinel structures, and provide a reliable approach to determine the spinel structure of spinel ferrites.     

Structural,dielectric and ferromagnetic behavior of (Zn,Co) co-doped SnO2 nanoparticles

Nanoparticles of basic composition Sn_0.94Zn_0.05Co_0.01O₂, Sn_0.92Zn_0.05Co_0.03O₂ and Sn_0.90Zn_0.05Co_0.05O₂ were synthesized by chemical precipitation method. The incorporation of Co and Zn in SnO₂ lattice introduced significant changes in the physical properties of all the three nanocrystals. The average particle size estimated from TEM data decreased from 15.71 to 6.41  nm with enhancement in concentration of oxygen vacancies as Co content is increased from 1 to 5 wt%. Increasing Co content enhanced the Sn:O atomic ratio as a result concentration of oxygen vacancies increased. The dielectric study revealed strong doping dependence. The dielectric parameters (ε′, tanδ and σ_ac) increased with increasing Co content and attained maximum values for 5% (Zn, Co) co-doped SnO₂ nanoparticles. The dielectric loss (ε′′) exhibited dispersion behavior and the Debye’s relaxation peaks observed in dielectric loss factor (tanδ), whose intensities increased with increasing Co content. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell-Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺³ and Co⁺²/Co⁺³. The large dielectric constant of all samples made them interesting materials for device application. Magnetization measurements (M (H) loops) revealed enhancement in saturation magnetization with doping which is due to the formation of large amount of induced defects and oxygen vacancies in the samples. The present study clearly reveals doping dependent properties and the oxygen vacancies induced ferromagnetism in Zn, Co co-doped SnO₂ nanoparticles having applications in ultra-high dielectric materials, high frequency devices and spintronics.     

Impact of Zn doping on the dielectric and magnetic properties of CoFe2O4 nanoparticles

Owing to its unique magnetic, dielectric, electrical and catalytic properties, ferrite nanostructure materials gain vital importance in high frequency, memory, imaging, sensor, energy and biomedical applications. Doping is one of the strategies to manipulate the spinel ferrite structure, which could alter the physico-chemical properties. In the present work, Co_1-xZn_xFe₂O₄ ($x$ = 0, 0.1, 0.2, 0.3, and 0.4 wt%) nanoparticles were prepared by sol-gel auto-combustion method and its structural, morphological, vibrational, optical, electrical and magnetic properties were studied. The structural analysis affirms the single-phase cubic spinel structure of CoFe₂O₄. The crystallite size, lattice constant, unit cell, X-ray density, dislocation density and hopping length were significantly varied with Zn doping. The Fe–O stretching vibration was estimated by FTIR and Raman spectra. TEM micrographs show the agglomerated particles and it size varies between 10 and 56 nm. The Hall effect measurement shows the switching of charge carriers from n to p type. The dielectric constant (ε′) varies from 0.2 × 10³ to 1.2 × 10³ for different Zn doping. The VSM analysis shows relatively high saturation magnetization of 57 and 69 emu/g for ZC 0.1 and ZC 0.2 samples, respectively than that of undoped sample. All the prepared samples exhibit soft magnetic behaviour. Hence, it can be realized that the lower concentration of Zn ion doping significantly alters the magnetic properties of CoFe₂O₄ through variation in the cationic distribution and exchange interaction between the Co and Fe sites of the inverse spinel structure of CoFe₂O_4.     

Magneto-optical investigation and hyperfine interactions of copper substituted Fe3O4 nanoparticles

Copper substituted Fe₃O₄ nanoparticles (NPs) (Cu_xFe_1−xFe₂O₄ (0.0≤x≤1.0)) were synthesized by polyol method and the effect of Cu²⁺ substitution on structural, magnetic and optical properties of Fe₃O₄ was investigated. X-ray diffraction (XRD), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), UV–Visible spectroscopy and Vibrating sample magnetometer (VSM) were used to study the physical properties of the products. The room temperature (RT) magnetization (σ–H) curves revealed the superparamagnetic nature of the products. The extrapolated specific saturation magnetization (σ_s) decreases from 42.69 emu/g to 14.14 emu/g with increasing Cu content (x). The particle size dependent Langevin fit studies were applied to determine the magnetic particle dimensions (D_mag). The average magnetic particle diameter is about 9.89 nm. The observed magnetic moments of NPs are in range of (0.61–1.77) µ_B and rather less than 4 µ_B of bulk Fe₃O₄ and 1 µ_B of bulk CuFe₂O₄. Magnetic anisotropy was assigned as uniaxial and calculated effective anisotropy constants (K_eff) are between 10.89×10⁴ Erg/g and 26.95×10⁴ Erg/g. The average value of magnetically inactive layer for Cu_xFe_1−xFe₂O₄ NPs was calculated as 1.23 nm. The percent diffuse reflectance spectroscopy (DR%) and Kubelka–Munk theory were applied to determine the energy band gap (E_g) of NPs. The extrapolated optical E_g values from Tauc plots are between minimum 1.98 eV to 2.31 eV. From 57Fe Mössbauer spectroscopy data, the variation in line width, isomer splitting, quadrupole splitting and hyperfine magnetic field values on Cu⁺² ion substitution have been determined. Although, the Mössbauer spectra for the sample x=0.2 and 0.8 are composed of paramagnetic doublets, ferromagnetic sextets were also formed for other products.     

Effects of calcination temperature on structure and magnetic properties of pure FeCo2O4 powders

A series of FeCo₂O₄ powders was initially synthesized using a hydrothermal method and subsequently calcined at various temperatures to produce the final product. Pure phase FeCo₂O₄ powders can only be formed in the temperature range of 950–1050 °C. In this work, we study the cation occupation, cation valence, bond length and bond angle changes of the pure phase FeCo₂O₄ powders formed in such a narrow temperature range. Octahedral lattice distortion in the pure phase FeCo₂O₄ samples has been observed. More tetrahedral Fe³⁺ and octahedral Co²⁺ are excited and exchanged their sites as the calcination temperature increases from 950 °C to 1000 °C, and part of Co³⁺ ions are reduced to Co²⁺ in the sample calcined at 1050 °C. The structure of the sample calcined at 1000 °C is close to that of the ideal FeCo₂O₄ spinel. Magnetic measurements show that ferrimagnetism and anti-ferromagnetism coexist in the pure phase FeCo₂O₄ samples. Interaction changes between ferrimagnetism and antiferromagnetism caused by the structural changes of the samples have been studied. Due to the pinning of the local anti-ferromagnetism to ferrimagnetism in the sample, the sample shows a Barkhausen jump below 150 K. As the measurement temperature increases further, the system enters into a reentrant spin glass state.     

Synthesis,structure and magnetic properties of spinel ferrite (Ni,Cu, Co)Fe2O4 from low nickel matte

Spinel ferrite (Ni, Cu, Co)Fe₂O₄ was synthesized from the low nickel matte by using a co-precipitation-calcination method for the first time. The influences of the added amount of NiCl₂·6H₂O, calcination temperature and time on the structure and magnetic properties of the as-prepared ferrites were studied in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy, and Vibrating sample magnetometer (VSM). It is indicated that pure (Ni, Cu, Co)Fe₂O₄ with cubic phase could be obtained under the experimental conditions (NiCl₂·6H₂O added amount of 3.0: 100 g mL⁻¹, calcination temperature from 800 to 1000 °C and calcination time from 1 to 3 h). With increasing calcination temperature and time, saturation magnetization (M_S) of the synthesized (Ni, Cu, Co)Fe₂O₄ increased and the coercivity (H_C) decreased. Under the optimum conditions (i.e. NiCl₂·6H₂O added amount of 3.0: 100 g mL⁻¹, 1000 °C, 3 h), the M_S and H_C values of the product were approximately 46.1 emu g⁻¹ and 51.0 Oe, respectively, which were competitive to those of other nickel ferrites synthesized from pure chemical reagents. This method explores a novel pathway for efficient and comprehensive utilization of the low nickel matte.     

MCr2O4 (M=Co,Cu, and Zn) nanospinels for 2-propanol combustion: Correlation of structural properties with catalytic performance and stability

The correlation between structure and activity of MCr₂O₄ nanospinels (M=Co, Cu, and Zn) synthesized by a sol–gel combustion method was investigated for the oxidation of 2-propanol. The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), N₂ adsorption/desorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Wide-angle XRD patterns show that the samples are pure spinel phases with cubic structure for CoCr₂O₄ and ZnCr₂O₄, and tetragonal structure for CuCr₂O₄. FTIR spectra confirmed the spinel structure of samples. The spinels were tested for total oxidation of 2-propanol as a model reaction for the catalytic combustion of oxygenated organic pollutants. ZnCr₂O₄ exhibited the highest activity and stability than the others toward the combustion of 2-propanol. The higher activity of ZnCr₂O₄ was ascribed to existence of excess surface oxygen on catalyst, active Cr³⁺–Cr⁶⁺ pair sites, and synergistic effect between ZnO and ZnCr₂O₄ confirmed by TPR and XPS techniques. The high stability of ZnCr₂O₄ and CuCr₂O₄ was explained by the existence of stable Cr⁶⁺ species on the surface of catalysts. The study showed that ZnCr₂O₄ could be used as a promising catalyst in the catalytic conversion of organic compounds.     

Magnetic enrichment behavior of monodispersed MFe2O4 nanoferrites (M= Mg,Ca, Ni,Co, and Cu)

The magnetic enrichment behavior of monodispersed MFe₂O₄ (M = Mg, Ca, Ni, Co, and Cu) ferrite nanoparticles with different size (10–130 nm) on the surface of a 15 mm o.d. NdFeB-N40 magnetic rod has been investigated. The materials were synthesized by a modified sol-gel method. They were characterized by XRD, TEM, and VSM. The magnetic field of the rod was modelled numerically using a finite element analysis software to obtain the input data for the magnitude of magnetic force. Three adsorption models can be used to describe the enrichment mechanism of ferrite nanoparticle depending on the magnetic permeability: (i) Freundlich adsorption model at low magnetic permeability (<10 μemu/Oe) which leads to the enrichment percentage below 50%, (ii) mixed (multilayer) adsorption model at intermediate permeability (10–50 μemu/Oe), and (iii) a monolayer adsorption at high permeability (>100 μemu/Oe) leading to the enrichment percentage above 90%.     

Structural changes of cerium doped copper ferrites during sintering process and magneto-electrical properties assessment

Crystalline CuCe_xFe_2-xO₄ nanoparticles with different rare earth metal content (x = 0.00, 0.03, 0.05, 0.08 and 0.10) were synthesized using a co-precipitation technique that involves a 600 °C calcination treatment. The newly obtained powders were pressed into pellets and heated up to 950 °C in order to form a compact and less porous material, ideal for dielectric measurements. X-ray investigations revealed a structural modification from typical cubic system to a tetrahedral spinel form, accompanied by Ce³⁺ cations exclusion and formation of secondary phases. The amount of phases and cations distribution were determined by Rietveld refinement method. A degradation process of cerium doped copper ferrite was proposed based on X-ray analysis and Rietveld refinement. Furthermore, the magnetic behavior of both 600 °C and 950 °C samples was investigated from the hysteresis loops recorded from a vibrating sample magnetometer (VSM) in a ±10 kOe range. Room temperature dielectric values for the 950 °C samples were determined using the parallel-plate capacitor configuration in a frequency range of 10 Hz – 1 MHz.     

Synthesis,characterization and influence of fuel to oxidizer ratio on the properties of spinel ferrite (MFe2O4, M = Co and Ni) prepared by solution combustion method

The effect of fuel to oxidizer ratio on the crystallite size and the physical properties of cobalt and nickel ferrite samples synthesized by solution combustion method using glycine as fuel are reported. Powder X-ray diffraction studies indicate that pure nanocrystalline ferrite phase is formed when the fuel to oxidizer ratio is less than one. The crystallite size can also be controlled by controlling the fuel to oxidizer ratio. The Mössbauer and Raman studies on the sample prepared at F/O 0.6 indicate that high quality samples having inverse or mixed spinel structure can be prepared by solution combustion method. Room temperature magnetization curves of the samples prepared under fuel lean condition have a systematic trend of coercivity with increasing crystallite size.     

Enhancement of magnetic properties of Ni0.5Zn0.5Fe2O4 nanoparticles prepared by the co-precipitation method

Nano crystalline Ni–Zn ferrites of composition Ni_0.5Zn_0.5Fe₂O₄have been prepared by a chemical co-precipitation method. The powdered samples were sintered at a temperature of 800 °C and 900 °C for three hours. X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Fourier Transform Infrared (FTIR) Spectroscopy were used to study their structural and morphological changes. The enhanced magnetic properties were investigated by using a Vibrating Sample Magnetometer (VSM). The saturation magnetization was found to increase from 73.88 to 89.50 emu/g as a function of sintering temperature making this material useful for high frequency applications. Electromagnetic studies showed sustained values of permittivity up to 1 GHz. These results have been explained on the basis of various models and theories.