Liquid phase epitaxy of hexagonal ferrites and spinel ferrites on non-magnetic spinel substrates

Using a PbOBaOB₂O₃ fluxed melt which had been employed previously to grow hexagonal ferrite Zn₂Y films on hexagonal ferrite M substrates, isothermal dipping liquid phase epitaxy produced epitaxial films on non-magnetic spinel substrates. The substrates studied were single crystals of three different compositions: MgAl₂O₄, MgGa₂O₄ and Mg(In,Ga)₂O₄. Two different film phases were identified: a lightly Zn-substituted M-type hexagonal ferrite and a heavily Zn-substituted magnetite.     

The growth of BaTiO3 films on (001) MgAl2O4 substrates by pulsed laser deposition technique

The microstructural evolution of the BaTiO₃ films grown on (001) MgAl₂O₄ spinel substrates at different temperatures by means of pulsed laser deposition technique is studied via transmission electron microscopy (TEM). The BaTiO₃ film grown at 850 °C consists of columnar grains of random orientations. Once the substrate temperature is over 900 °C, the BaTiO₃ films grow on (001) MgAl₂O₄ substrates epitaxially. The cross-sectional TEM study reveals that the boundaries and interfaces act as the sources to emit stacking faults and twins which are detrimental to the film quality. The quality of epitaxial films increases with the growth temperature, and is optimized at the growth temperature of 1050 °C. The evolution of film microstructures with the growth temperature is discussed in view of the growth temperature, the surface structure of MgAl₂O₄ substrates, and the phase transition of BaTiO₃.     

Liquid phase epitaxy of hexagonal ferrites

Single crystal films of the hexagonal ferrite Zn₂Y, having the chemical formula Ba₂Zn₂Fe₁₂O₂₂, were grown by the isothermal dipping method of liquid phase epitaxy using a PbOBaOB₂O₃ flux. The substrates were flux-grown M-type hexagonal ferrite crystals having the chemical formula BaFe₁₂O₁₉. The films and substrates were distinguishable by the differences in their crystallographic and magnetic properties.     

An electron microscope study of thin ferrite films prepared by the oxidation of high purity vacuum-evaporated metal thin films

The oxidation of thin films of nickel, cobalt, iron, NiFe₂ and CoFe₂ has been investigated between 200 and 1200° C. The oxidation products for the elemental metals differ from the oxidation products observed in previous work upon bulk material. The oxidation mechanism proposed for bulk material is in general still valid in the thin film situation. Above oxidation temperatures of approximately 850° C both NiFe₂ and CoFe₂ form the respective ferrites, although in the case of nickel ferrite, traces of the -Fe₂O₃ tetragonal superstructure can still be detected at oxidation temperatures of 1200° C. Films of nickel ferrite and cobalt ferrite upon single-crystal magnesium oxide substrates, produced by oxidation of vacuum-deposited NiFe₂ and CoFe₂ thin films, have been investigated by transmission and scanning electron microscopy. It has been found that (100) nickel ferrite prepared by this technique grows epitaxially upon (100) magnesium oxide.     

Effect of annealing on the microstructure of NiFe1.925Dy0.075O4 thin films

Dysprosium-doped nickel-ferrite (NiFe_1.925Dy_0.075O₄) thin films were fabricated using RF sputter-deposition. Structural studies indicate that the effect of post-deposition annealing is significant on structural evolution in NiFe_1.925Dy_0.075O₄ films. As-grown NiFe_1.925Dy_0.075O₄ films were amorphous. Annealing (T_a) in air at 450-1000 °C results in the formation of nanocrystalline NiFe_1.925Dy_0.075O₄ films, which crystallize in the inverse spinel structure. The average grain size (L) increases from 5 to 40 nm with increasing T_a from 450 to 1000 °C. Lattice constant of NiFe_1.925Dy_0.075O₄ films is higher compared to that of NiFe₂O₄ due to partial substitution of Dy³⁺ ions for Fe³⁺ ions. The lattice parameter increases from 8.353 to 8.362 Å with increasing T_a from 450 to 1000 °C which is attributed to the lattice-strain developed in the NiFe_1.925Dy_0.075O₄ films with increasing T_a. The corresponding density of NiFe_1.925Dy_0.075O₄ films increases from 3.2 to 3.9 g/cm³ with increasing annealing temperature. Magnetization measurements indicate the ferromagnetic behavior of all the films while the coercive field values at 300 K are found to be 0.0134 T and 0.0162 T for as grown and T_a = 1000 °C films, respectively.     

Stress-induced anisotropy in LPE grown Ni(Fe,Al)2O4 films

Preliminary results are reported about the growth of single crystal Ni(Fe,Al)₂O₄ films, grown by means of liquid phase epitaxy on (111)MgO and on (111)ZnGa₂O₄ substrates using a PbO-B₂O₃-Fe₂O₃ solvent. While films grown upon MgO show stress relief at the growth temperature, films grown upon ZnGa₂O₄ possess a tensile strain due to elastic deformation. Since λ₁₁₁ for NiFe₂O₄ is strongly negative a stress-induced uniaxial anisotropy is present in the films. Stripe domains can be observed with the Bitter technique and when a magnetic field is applied perpendicular to the plane of the film, magnetic bubbles with a diameter of ～2 μm appear. A bubble stability factor q exceeding unity is obtained. For the first time magnetic bubbles are found in LPE grown spinel ferrites.     

Dielectric and magnetic properties of ferrite/poly(vinylidene fluoride) nanocomposites

Particulate composite films of poly(vinylidene fluoride) and CoFe₂O₄ and NiFe₂O₄ were prepared by solvent casting and melt processing. The well-dispersed ferrite nanoparticles nucleate the piezoelectric β-phase of the polymer, but the different ferrites nucleate the whole polymer crystalline phase at different filler concentrations. The macroscopic magnetic and dielectric response of the composites demonstrates a strong dependence on the volume fraction of ferrite nanoparticles, with both magnetization and dielectric constant increasing for increasing filler content. The β-relaxation in the composite samples is similar to the one observed for β-PVDF obtained by stretching. A superparamagnetic behavior was observed for NiFe₂O₄/PVDF composites, whereas CoFe₂O₄/PVDF samples developed a hysteresis cycle with coercivity of 0.3 T.     

Epitaxial growth of CoFe2O4 on SrTiO3 (100) and MgO (100) substrates without buffer layer by laser molecular beam epitaxy technique

Among the family of ferrite materials, cobalt ferrite (CoFe₂O₄) is unique in that it has the highest values of magneto-crystalline anisotropy and magnetostriction. Recently, much of the efforts have been focused on the fabrication of single crystalline cobalt ferrite films. For the epitaxial growth of cobalt ferrite, the issues of lattice parameter and crystal symmetry mismatch with the substrate are of considerable importance. The growth of thin films of CoFe₂O₄ on MgO and SrTiO₃ single crystal substrates is reported in this paper. The key parameters on the growth modes were investigated by changing oxygen pressure and substrate temperature. Results show that the two-dimensional layer-by-layer growth mode only occurs under high oxygen pressure for epitaxy of Co ferrite. The significant observation presents the controllable lattice constant of a highly strained thin film by modulation of the substrate temperature. In this light, to grow high quality Co ferrite thin films on SrTiO₃ is of a considerable importance to modulate intrinsic magnetic properties.     

Growth and characterization of Fe3O4 films

Iron oxide films were grown on sapphire substrates by pulsed laser deposition at substrate temperatures between 100 and 700 °C. X-ray diffraction, Raman spectroscopy, and vibrational sample magnetometer analysis revealed that structural and magnetic properties of the iron oxide films strongly depend on the substrate temperature during growth. Single phase Fe₃O₄ film was successfully grown on sapphire substrate at a substrate temperature of 500 °C. The saturation magnetic moment of the single phase Fe₃O₄ film is 499 emu/cm³, which is in good agreement with the value reported for bulk magnetite, suggesting the Fe₃O₄ film is of high crystal quality without antiphase boundaries.     

Photocatalytic degradation of methyl orange dye by NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles under visible irradiation: effect of varying the synthesis temperature

Nanoparticles of nickel ferrites (NiFe₂O₄) were synthesized at different temperature of synthesis (25, 50 and 80 °C) through the chemical co-precipitation method. The synthesized powders were characterized using X-ray diffraction for crystallite size and lattice parameter calculation. It reveals the presence of cubic spinel structure of ferrites with crystallite size between 29 and 41 nm. Transmission electron microscopy and scanning electron microscopy showed uniform distribution of ferrite particles with some agglomeration. The Fourier-transform infrared spectroscopy showed absorption bonds, which were assigned to the vibration of tetrahedral and octahedral complexes. Raman spectroscopy is used to verify that we have synthesized ferrite spinels and determines their phonon modes. The thermal decomposition of the NiFe₂O₄ was investigated by TGA/DTA. The optical study UV–visible is used to calculate the band gap energy. Magnetic measurements of the samples were carried out by means of vibrating sample magnetometer and these studies reveal that the formed nickel ferrite exhibits ferromagnetic behavior. Photoluminescence showed three bands of luminescence located at 420, 440 and 535 nm. The photocatalytic properties of nickel ferrite (NiFe₂O₄) nanoparticles were evaluated by studying the photodecomposition of methyl orange as organic pollutant models and showed a good photocatalytic activity.     

Interface characterization and thermal degradation of ferrite/poly(vinylidene fluoride) multiferroic nanocomposites

Flexible multiferroic 0–3 composite films, with CoFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄ or NiFe₂O₄ ferrite nanoparticles as filler and polyvinylidene fluoride (PVDF) as the polymer matrix, have been prepared by solvent casting and melt crystallization. The inclusion of ferrite nanoparticles in the polymer allows to obtain magnetoelectric nanocomposites through the nucleation of the piezoelectric β-phase of the polymer by the ferrite fillers. Since the interface between PVDF and the nanoparticles has an important role in the nucleation of the polymer phase, thermogravimetric analysis was used in order to identify and quantify the interface region and to correlate it with the β-phase content. It is found that an intimate relation exists between the size of the interface region and the piezoelectric β-phase formation that depends on the content and type of ferrite nanoparticles. The interface value and the β-phase content increase with increasing ferrite loading and they are higher for CoFe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles. The composites shows lower thermal stability than the pure polymer due to the existence of mass loss processes at lower temperature than the main degradation of the polymer. The main degradation of the polymer matrix, nevertheless, shows increased degradation temperature with increasing ferrite content.     

Ultralight Flexible Electrodes of Nitrogen-Doped Carbon Macrotube Sponges for High-Performance Supercapacitors

Light-weight and flexible supercapacitors with outstanding electrochemical performances are strongly desired in portable and wearable electronics. Here, ultralight nitrogen-doped carbon macrotube (N-CMT) sponges with 3D interconnected macroporous structures are fabricated and used as substrate to grow nickel ferrite (NiFe₂O₄) nanoparticles by vapor diffusion–precipitation and in situ growth. This process effectively suppresses the agglomeration of NiFe₂O₄, enabling good interfacial contact between N-CMT sponges and NiFe₂O₄. More remarkably, the as-synthesized NiFe₂O₄/N-CMT composite sponges can be directly used as electrodes without additional processing that could cause agglomeration and reduction of active sites. Benefiting from the tubular structure and the synergetic effect of NiFe₂O₄ and N-CMT, the NiFe₂O₄/N-CMT-2 exhibits a high specific capacitance of 715.4 F g⁻¹ at a current density of 1 A g⁻¹, and 508.3 F g⁻¹ at 10 A g⁻¹, with 90.9% of capacitance retention after 50 000 cycles at 1 A g⁻¹ in an alkaline electrolyte. Furthermore, flexible supercapacitors are fabricated, yielding areal specific capacitances of 1397.4 and 1041.2 mF cm⁻² at 0.5 and 8 mA cm⁻², respectively. They also exhibit exceptional cycling performance with capacitance retention of 92.9% at 1 mA cm⁻² after 10 000 cycles under bending. This work paves a new way to develop flexible, light-weight, and high-performance energy storage devices.     

Microstructure and gas-sensing properties of nanocrystalline NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared by spray pyrolysis

Nanocrystalline nickel ferrite with a crystallite size from 3 to 40 nm has been prepared by spray pyrolysis. The ⁵⁷Fe Mössbauer spectrum of NiFe₂O₄ samples has been found to vary systematically with crystallite size. The sensing response of the nanocrystalline nickel ferrite to 50 ppm NH₃ has been studied using in situ conductance measurements. NiFe₂O₄ offers a strong sensing response to ammonia at the level of its maximum concentration limit. The optimum nickel ferrite crystallite size and temperature for ammonia detection are determined.     

Epitaxial growth of SrRuO<Subscript>3</Subscript> thin films by RF sputtering and study of surface morphology

We report on the epitaxial growth of SrRuO₃ (SRO) thin films on Pt (111)/γ-Al₂O₃ (111) nSi (111) substrates. The grown thin films are crystalline and epitaxial as suggested by RHEED and XRD experiments. With the use of γ-Al₂O₃ (001)/nSi (001) and γ-Al₂O₃ (111)/nSi (111) substrates, crystalline but not epitaxial films have grown successfully. This result implies that lattice mismatch between nSi and SRO prevents the epitaxial growth of SRO film directly on nSi. However, the buffer Pt (111) layer mitigates lattice mismatch that provides to grow epitaxial film on nSi of quality. Morphological study shows a good surface with moderate roughness. Film grown at 700°C is smoother than the film grown at 750°C, but the variation of temperature does not affect significantly on the epitaxial nature of the films.     

Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

The crystal structure and superconductivity of MgB₂ thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900^○C. The X-ray diffraction shows that the MgB₂ films grown on C–AL₂O₃, R–AL₂O₃, and MgO (001) are c-axis oriented while the films grown on SrTiO₃ (001), LaAlO₃ (001), and ZrO₂ (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB₂ films using the two-step method.     

Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

The crystal structure and superconductivity of MgB₂ thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900C. The X-ray diffraction shows that the MgB₂ films grown on C–AL₂O₃, R–AL₂O₃, and MgO (001) are c-axis oriented while the films grown on SrTiO₃ (001), LaAlO₃ (001), and ZrO₂ (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB₂ films using the two-step method.     

Paederia foetida Linn. promoted synthesis of CoFe2 O4 and NiFe2 O4 nanostructures and their photocatalytic efficiency

A facile method of synthesis of cobalt ferrite (CoFe₂ O₄) and nickel ferrite (NiFe₂ O₄) nanoparticles (NPs) was developed using urea as a hydroxylating agent and Paederia foetida Linn. (family: Rubiaceae) leaf extract as a bio‐template. The synthesised ferrite NPs were characterised in a detailed manner by powder X‐ray diffraction (XRD), transmission electron microscopy, Fourier transform‐infrared spectroscopy and vibrating sample magnetometer analysis. The XRD patterns revealed the formation of cubic face‐centred phase for both CoFe₂ O₄ and NiFe₂ O₄ NPs. These quasi‐spherical particles of CoFe₂ O₄ and NiFe₂ O₄ were shown to have sizes in the range of 10–80 and 5–50 nm, respectively. The photocatalytic activity of metal ferrites was evaluated in H₂ O₂ assisted oxidative degradation of methylene blue (MB) and rhodamine B (RhB) under irradiation of solar light. Both metal ferrite photocatalysts exhibited pronounced activity in degradation of MB and RhB, respectively, but relatively higher activity was observed for NiFe₂ O₄. After completion, the catalysts were recovered using an external magnet. Recycling of these recovered catalysts up to five times showed no noticeable change in the efficiency.Inspec keywords: nanoparticles, nanofabrication, photochemistry, catalysts, cobalt compounds, nickel compounds, ferrites, X‐ray diffraction, transmission electron microscopy, Fourier transform infrared spectra, crystal structure, dyesOther keywords: Paederia foetida Linn, nanostructures, photocatalytic efficiency, cobalt ferrite nanoparticles, nickel ferrite nanoparticles, hydroxylating agent, leaf extract, bio‐template, powder X‐ray diffraction, transmission electron microscopy, Fourier transform‐infrared spectroscopy, vibrating sample magnetometer analysis, cubic face‐centred phase, quasi‐spherical particles, photocatalytic activity, methylene blue, rhodamine B, size 5 nm to 80 nm, CoFe₂ O₄ , NiFe₂ O₄      

Structure and Nonlinear Electrical Properties of Ni–Fe–Mg–O Epilayers

NiFe₂O₄-based epilayers grown on MgO substrates and exhibitingS-type negative-resistance behavior were characterized by x-ray diffraction, Mössbauer spectroscopy, and electron probe x-ray microanalysis. The current–voltage characteristics of the epilayers were measured at different temperatures in air, water, and liquid nitrogen, and the activation energy for conduction was evaluated. The near-surface and interfacial regions of the films were shown to differ in the nature of disordering as a result of the through-thickness variation in composition.     

An investigation on synthesis and magnetic properties of ferromagnetic nanoparticles of nickel ferrite coated with TiO2

TiO₂-coated nickel ferrite (NiFe₂O₄) nanoparticles were obtained by the hydrolysis of titanium tetrabutyloxide in the presence of NiFe₂O₄ nanoparticles in water/oil (w/o) microemulsion. The effects of TiO₂ coating on the magnetic properties of NiFe₂O₄ nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, transmission electron microscopy (TEM) and magnetic measurements. The morphology analysis confirmed that the obtained composite nanoparticles consisted of NiFe₂O₄ coated by TiO₂. It was shown that the saturation magnetization and coercivity of NiFe₂O₄ decreased after TiO₂ coating, which can be interpreted by the interparticle dipole–dipole interactions related to the magnetic particle volume fraction in the composite nanoparticles.     

Epitaxial growth of SrRuO3 thin films by RF sputtering and study of surface morphology

We report on the epitaxial growth of SrRuO₃ (SRO) thin films on Pt (111)/γ-Al₂O₃ (111) nSi (111) substrates. The grown thin films are crystalline and epitaxial as suggested by RHEED and XRD experiments. With the use of γ-Al₂O₃ (001)/nSi (001) and γ-Al₂O₃ (111)/nSi (111) substrates, crystalline but not epitaxial films have grown successfully. This result implies that lattice mismatch between nSi and SRO prevents the epitaxial growth of SRO film directly on nSi. However, the buffer Pt (111) layer mitigates lattice mismatch that provides to grow epitaxial film on nSi of quality. Morphological study shows a good surface with moderate roughness. Film grown at 700°C is smoother than the film grown at 750°C, but the variation of temperature does not affect significantly on the epitaxial nature of the films.