Hydrothermal Synthesis and Characterization of CoFe2O4 Nanoparticles and Nanorods

This paper presents a low temperature (130 and 160 °C) synthesis route to prepare the spinel phase CoFe₂O₄ nanoparticles and nanorods. A one-dimensional (1-D) structure of Co-ferrite was successfully synthesized using Cetyl Trimethyl Ammonium Bromide (CTAB) as a surfactant at temperature 160 °C. Structural, electrical, and magnetic measurements have been performed using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and the Vibrating Sample Magnetometer (VSM). XRD patterns show a pure spinel (fcc) structure, showing a complete phase formation at a low temperature of 160 °C, without any subsequent sintering. Average crystallite sizes have been calculated by Sherrer’s and Williamson-Hall methods. As prepared CoFe₂O₄ nanorods exhibited a uniform shape of diameter 60–80 nm and 600–900 nm in length. The FTIR spectrum for Co-ferrite nanorods shows two intrinsic lattice absorption bands for tetrahedral and octahedral sublattices. DC electrical resistivity of CoFe₂O₄ nanorods is high up to ～10⁸ (Ω-cm), as compared to CoFe₂O₄ nanoparticles (～10⁷ Ω-cm) at 373 K. Dielectric parameters were measured using a LCR meter, in the frequency range of 1 kHz to 5 MHz. The real and imaginary part of the dielectric constant (ε′ and ε″) and dielectric loss tangent (tanδ) reduces for CoFe₂O₄ nanorods in comparison to nanoparticles, and has a value of 13.6 and 0.0416, respectively. Magnetic properties were characterized by VSM under a field of 10 kOe and showed that the 1-D structure reduces the magnetization of nanocrystalline CoFe₂O₄ from 65 emu/gm to 54 emu/gm.     

High temperature growth, charge distribution and magnetism in Co and Mn co-doped ZnO

Zn_1?2xCo_xMn_xO (x = 0.03 and 0.05) bulk samples were synthesised by a standard high temperature solid state reaction technique at 1,400 °C. Characterization techniques X-ray diffraction, Vibrating Sample Magnetometry and Scanning Electron Microscopy with Energy Dispersive Analysis of X-rays were used to investigate effect of transition metal element Co and Mn incorporated in the system using high temperature technique. The magnetic, structural, electron density distribution and bonding properties have been investigated for the samples prepared in this work. X-ray diffraction analysis of the synthesized samples showed a single phase ZnO wurtzite structure for x = 0.03 and very slight segregation of a spinel phase for x = 0.05. Magnetization measurements indicate no evidence of ferromagnetism in co-doped (Co and Mn) ZnO. The structural and electron density analysis presented in this work is based on the actual concentrations of Co and Mn ions introduced in the matrix which nearly coincide with the nominal concentrations.     

Magnetic dynamic behavior of nanomagnets studied by Magnetic Force Microscopy with external field

The micro/nanomagnetic behavior of magnetic systems is a key issue as the size of magnetic devices is reduced to or under the micrometer range. We study the magnetic behavior of nanomagnets under different applied magnetic field conditions by Magnetic Force Microscopy (MFM). MFM is sensitive mainly to magnetization distributions that generate magnetic fields. CoCr Magnets were deposited by electropulsed SPM onto a Si substrate with sizes ranging from 400×100 to 800×400 nm and thickness between 2 and 3 nm. MFM measurements were performed using a Digital Instruments (DI) Dimension 3100 SPM upgraded for measurements with an external magnetic field applied to the sample. The home-designed modification consists in an electromagnet with field guides towards the scanning region while measuring. Different magnetic fields up to 400 Oe were applied to the samples in-plane during the MFM measurements. The magnetic configuration for the different applied fields was then imaged by MFM.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

Low-temperature preparation of transparent conductive Al-doped ZnO thin films by a novel sol–gel method

We developed a novel sol–gel method to prepare transparent conductive Al-doped ZnO (AZO) thin film at low temperature. The AZO nanocrystals were prepared by a solvothermal method and then they were dispersed in the monoethanolamine and methanol to form AZO colloids. A (002)-oriented ZnO thin film was used as a nucleation layer to induce the (002)-oriented growth of AZO thin films. The AZO thin films were prepared on Si(100) and fused quartz glass substrates with the (002)-oriented ZnO nucleation layer and annealed at 400 °C for 60 min. All AZO thin films showed (002) orientation. For electrical and optical measurements, the films deposited on glass substrates were post-annealed at 400 °C for 30 min in forming gas (100 % H₂) to improve their conductivity. These samples had high transparency in the visible wavelength range, and also showed good conductivity. A 0.2 mol L^?1 AZO solution with 3 at.% Al content was heated in a Teflon autoclave at 160 °C for 30 min to form AZO nanocrystals, and then the AZO nanocrystals were suspended in the MEA and methanol to obtain the stable AZO colloid. The Al content in the AZO nanocrystals was 2.7 at.%, and the high Al doping coefficient was mainly attributed to the formation of AZO nanocrystals in the autoclave. The AZO thin film using this colloid had the lowest resistivity of 3.89 × 10^?3 Ω cm due to its high carrier concentration of 3.29 × 10²⁰ cm^?3.     

Ferromagnetic-to-Paramagnetic Phase Transition of MnAs Studied by Calorimetry and Magnetic Measurements

Manganese monoarsenide samples have been prepared by the sealed-ampule technique and characterized by X-ray diffraction, differential thermal analysis, and scanning electron microscopy. The hexagonal- to-orthorhombic phase transition of MnAs has been studied using differential scanning calorimetry (DSC) and magnetic measurements. The enthalpy and temperature range of the transition have been determined to be ΔH =–5.6 J/g and 312.5–319 K, respectively. The enthalpy and temperature range of the transition are influenced by the quality of the samples. The samples containing inclusions of the metastable, orthorhombic phase have a lower enthalpy and broader temperature range of the magnetostructural transformation of manganese monoarsenide. It has been demonstrated that DSC is an effective tool for assessing the quality of MnAs samples. Temperature dependences of specific magnetization and magnetic permeability for MnAs lend support to the DSC results. From these data, the Curie temperature of MnAs has been determined to be 40°C, in good agreement with previously reported data.     

Dependence of Magnetic Hysteresis on Evolving Single-Sample Sintering in Fine-Grained Yttrium Iron Garnet

Mono-dispersed yttrium iron garnet nanoparticles have been synthesized via mechanical alloying technique, and some attendant qualitative relationships between evolving microstructural parameters and magnetic properties have been clearly revealed. A rarely employed single-sample sintering scheme was adopted where only one sample was sintered repeatedly from 600 °C to 1400 °C prior to an analysis of evolution of microstructure-dependent magnetic properties after each sintering. A brief, yet detailed characterization of the sample was carried out using Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and B–H Hysteresis graph. A scrutiny of the B–H Hysteresis graph results showed a transition from disordered to ordered magnetism which belongs to three different magnetically dominant groups, namely weak ferro-, moderately strong ferro-, and strongly ferromagnetic groups. Three factors were found to strongly influence the ordered magnetism of the sample, namely the crystallinity degree of the crystalline phase, the number of grains with size larger than a critical diameter, and the number of large enough grains for magnetic order accommodation.     

Dielectric and magnetic properties of DyMnO3 ceramics

This paper reports the dielectric and magnetic properties of DyMnO₃ (DMO) ceramics sintered at 1,100 and 1,350 °C temperature for 10 h. Sample sintered at 1,100 °C showed presence of secondary phases of Dy₂O₃ whereas the monophasic orthorhombic structure of DyMnO₃ sample is synthesized only at sintering temperature of 1,350 °C. The dielectric properties of the sintered sample of DMO were investigated as a function of temperature (T ≥ 300 K) and frequency (10 kHz–1 MHz). Both the sintered samples showed frequency independent dielectric anomaly at 313 K. These samples exhibited ferroelectric behavior at room temperature which was evidenced from the polarization hysteresis loop measurement. No magnetic transition has been observed at room temperature. However, magnetic field dependent magnetization and temperature dependent magnetization showed the paramagnetic behavior of the DyMnO₃ samples.     

Large Increase of the Critical Field in a Magnet–Superconductor Nanowire Hybrid

We have fabricated two-dimensional periodic arrays of parallel magnetic and superconducting nanowires on a silicon substrate. Parallel magnetic (nickel) nanowires of cross section 90 nm by 300 nm form a periodic array with Pb₈₂Bi₁₈ superconducting nanowires of cross section 200 nm by 100 nm. These nanostructures were characterized with Scanning Electron Microscopy (SEM) and magnetic properties were studied with Magnetic Force Microscopy (MFM). The phase diagram was determined by electrical transport measurements. Depending on the temperature, the second critical field was 2 to 3 times larger than that of a homogeneous Pb₈₂Bi₁₈ superconducting control film. The superconducting phase diagram and transport properties exhibit strong hysteresis in a magnetic field. Results are explained on the basis of the theory of magnet–superconductor hybrids.     

Synthesis and characterization of nanocrystalline TiO<Subscript>2</Subscript> thin films

Nanocrystalline thin films of TiO₂ have been synthesized by sol gel spin coating technique Thin films of TiO₂ annealed at 700 °C were characterized by X-ray diffraction(XRD), Atomic Force Microscopy, High resolution TEM and Scanning Electron Microscopy (SEM), The XRD shows formation of tetragonal anatase and rutile phases with lattice parameters a = 3.7837 Å and c = 9.5087 Å. The surface morphology of the TiO₂ films showed that the nanoparticles are fine with an average grain size of about 60 nm. Optical studies revealed a high absorption coefficient (10⁴ cm^?1) with a direct band gap of 3.24 eV. The films are of the n type conduction with room temperature electrical conductivity of 10^?6 (Ω cm)^?1.     

Effect of annealing temperature on the energy transfer in Eu-doped ZnO nanoparticles by chemical precipitation method

Eu-doped ZnO nanoparticles were synthesized by the chemical precipitation method and the annealing temperature effect on the structures and photoluminescence (PL) properties of the nanoparticles were briefly investigated. The X-ray diffraction and energy dispersive spectroscopy results indicated that the Eu³⁺ was successfully incorporated into the crystal lattice of ZnO host when the annealing temperature was fixed at 400 °C, but the Eu³⁺ ions were partly precipitated from the host with the annealing temperature increasing. The as-obtained ZnO: Eu nanocrystals composed of nanoparticles had an average size of 10 nm, and the valence states of europium ions in the nanocrystals were determined as tervalent. PL spectroscopy indicated that the characteristic red emissions of Eu³⁺ ions were attributed to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, respectively. Moreover, the annealing temperature was found to have effect on the red emission of Eu³⁺ ions. That is to say, the energy transfer in the doped nanocrystals could be adjusted by different annealing temperatures.     

Measurement of an Iso‐Curie Temperature Line of a Co?Cr?Mo Solid Solution by Magnetic Force Microscopy Imaging on a Diffusion Multiple

The 24 °C iso‐Curie temperature line of a Co? Cr? Mo fcc solid solution is obtained by performing magnetic force microscopy (MFM) imaging on solid solution compositions created in a diffusion multiple. The MFM imaging clearly reveals the boundary that separates the paramagnetic region without magnetic domains from the ferromagnetic region with domains. Compositional analysis along the boundary yields a constant Curie temperature (24 °C) composition line. Such a measurement is more efficient than one‐alloy‐at‐a‐time tests and can be used to screen new ferromagnetic materials.     

Investigation of chalcopyrite film growth at various temperatures: analyses from top to the bottom of the thin films

We reported a new method to investigate the phases and structures of thin film bottom parts. The films were polished by flapping papers to reach the bottoms. The surfaces and cross sections of thin films were observed by Scanning Electron Microscopy. Grazing Incidence X-ray Diffraction, Raman spectra and X-ray Photoelectron Spectroscopy (XPS) were used to investigate the phases, structures and chemical components of the surfaces and bottoms of thin films. By this method, we studied the growth processes of chalcopyrite films after the selenization at various temperatures from 270 to 600 °C. At 270 °C, a great amount of Cu–Se nodules formed at the surface, while (In,Ga)–Se stayed in the bottom. At 380 °C, a double layer structure was observed in the film. The top part was typical CuInSe₂ polycrystalline, while the bottom part contained complicated components, like CuInSe₂, Cu(In,Ga)₃Se₅, (In,Ga)Se. At 600 °C, a single layer was formed, which was composed of Cu(In,Ga)Se₂ phase. However, a higher Ga/(In+Ga) ratio was obtained towards the back contact. In addition, XPS indicated that the Mo/Cu(In,Ga)Se₂ interface was rich in Ga and Se.     

Synthesis and characterization of zinc and calcium nanoferrites

In this work, Zn_(1?x)Ca_xFe₂O₄ nanoparticles (x = 0, 0.5 and 1) have been synthesized by sol–gel method followed by heat treatment at a temperature within the range of 300–700 °C. The samples with appropriate saturation magnetization (Ms), low coercivity and remanence were Zn₀Ca₁Fe₂O₄ treated at 300 °C (Ms ~ 25 emu/g), Zn₀Ca₁Fe₂O₄ treated at 400 °C (Ms ~ 40 emu/g) and Zn_0.50Ca_0.50Fe₂O₄ treated at 400 °C (Ms ~ 31 emu/g). These samples were analyzed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and transmission electron microscopy. The heating ability of selected nanoparticles was evaluated under a magnetic field using a solid state induction heating equipment. The obtained nanoferrites showed a particle size within the range of 13–14 nm. The Zn₀Ca₁Fe₂O₄ treated at 400 °C was able to heat the nanoferrite particles/water suspension (10 mg/2 ml) at a temperature of 44 °C under the selected magnetic field (10.2 kA/m and frequency 362 kHz). Additionally, in vitro bioactivity assessment was performed by immersing samples in a simulated body fluid for different periods of time at physiological conditions of pH and temperature. The samples showed an appropriate bioactivity. These nanoferrites are highly potential materials for hyperthermia treatment.     

Phase formation and magnetic property evolution processes of the hexaferrite with composition BaO⋅0.9Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>⋅5.1Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

The phase formation and magnetic property evolution processes of the hexaferrite with composition BaO?0.9Sc₂O₃?5.1Fe₂O₃ have been investigated. Results show that when the calcination temperature is lower than 1000 °C, the spinel phase BaFe₂O₄ and M-type hexaferrite phase BaFe₁₂O₁₉ dominate. The M-type hexaferrite BaFe_12?xSc_xO₁₉ (0?<?x?≤?1.8) appears above 1050 °C and becomes a single phase BaFe_10.2Sc_1.8O₁₉ above 1200 °C. A two-step decrease of both the coercivity and remanence ratio is observed above 1050 °C, which agrees well with the appearance of soft magnetic phase BaFe_12?xSc_xO₁₉ (0?<?x?≤?1.8). The saturation magnetization of the sample increases with calcination temperature until 1100 °C and then decreases. Raman spectra results show that the above magnetic property evolutions can be explained by a temperature dependent incorporation of Sc³⁺ into the lattice sites nearby the magnetic blocks’ interfaces. This weakens the local magnetic exchange interactions between Fe³⁺ and thus leads to a change in the magnetic structure.     

Strong correlation between the cation ordering and magnetic properties of anodically electrodeposited Mn–Co–O nanocrystals

The rock salt-to-spinel structural transformation that occurs in anodically electrodeposited Mn–Co–O nanocrystals involves a rearrangement of Mn/Co cations from octahedral interstices to tetrahedral interstices. The cation ordering process leads to distinct magnetic properties. Curie temperature (T _C) and blocking temperature (T _B) increase dramatically with annealing temperature (200–400 °C), while the corresponding change in particle size for the oxide nanocrystals is rather small. A strong correlation between the magnetic properties and the cation ordering degree in annealed Mn–Co–O nanocrystals was established. These unique magnetic properties can be attributed to the magnetic moment changes induced by Mn/Co cation ordering from octahedral interstices to tetrahedral interstices in the annealed Mn–Co oxide nanocrystals.     

Intrinsic Switching Field Distribution of Arrays of Ni80Fe20 Nanowires Probed by In Situ Magnetic Force Microscopy

The progress of magnetization reversal of weakly packed ferromagnetic Ni₈₀Fe₂₀ nanowire arrays of different diameters (40, 50, 70, and 100 nm) electrodeposited in polycarbonate membranes was studied by magnetic force microscopy (MFM). For such a low packing density of nanomagnets, the dipolar interactions between neighboring wires can be neglected. The intrinsic switching field distribution has been extracted from in situ MFM images and its width was found to be considerably smaller than for densely packed nanowire arrays.     

Structural and properties of hydrothermally synthesised M-type hexaferrites

Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized by hydrothermal method as the materials. Then, the precursors were calcinated at 1000 °C for two different time (200 and 300 min). The phase composition, micro-morphology and magnetic properties of the particles were investigated by X-ray powder diffraction analysis, scanning electron microscopy, and vibrating sample magnetometer. At hydrothermal temperature ranging from 220 to 230 °C, the phase of M-type hexaferrites includes small amounts of impurities phases, such as α-Fe₂O₃. The result shows that the single M-type phase was obtained when the hydrothermal temperature is 240 °C. The particles appear in hexagonal plate-like shape and the size of grain is about 400 nm. The effect of sintering time on magnetization and coercivity was studied in this paper.     

Synthesis and Study of Structural,Morphological and Magnetic Properties of ZnFe2O4 Nanoparticles

Superparamagnetic zinc ferrite (ZnFe₂O₄) nanoparticles were prepared by a surfactant assisted hydrothermal method and subjected to the heat treatment. The structure, vibrational, morphology, and magnetic properties of synthesized product were characterized by XRD, FT-IR, HR-SEM, and VSM measurements. XRD result confirms the formation of regular spinel structured ZnFe₂O₄ with space group of Fd3m and an average crystalline size was calculated as 21 nm and 28 nm for the samples annealed in air atmosphere at 300 °C and 600 °C. The HR-SEM image shows that the particles are in spherical shape with small aggregation. A room temperature superparamagnetic behavior was observed for both samples. The saturation magnetization (M _s) of 12.0 emu/g and 9.10 emu/g were observed for the samples annealed in air atmosphere at 300 °C and 600 °C, respectively.     

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy

Abstract

Thin (4–20 nm) yttrium iron garnet (Y₃Fe₅O₁₂, YIG) layers have been grown on gadolinium gallium garnet (Gd₃Ga₅O₁₂, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe³⁺ sublattices were studied by XMCD.