Dependence of developing magnetic hysteresis characteristics on stages of evolving microstructure in polycrystalline yttrium iron garnet

The microstructure evolution in several polycrystalline yttrium iron garnet samples as a result of a sintering scheme was studied in detail, in parallel with the changes in their magnetic properties. Samples with nanometer sized starting powder were synthesized by employing the High-Energy Ball Milling technique and then sintering toroidal compacts of the milled powder. Nine sintered samples were obtained, each corresponding to a particular sintering from 600 °C to 1400 °C. The samples were characterized for their evolution in crystalline phases, microstructure and magnetic hysteresis-loops parameters. The results showed an increasing tendency of the saturation magnetization and saturation induction with grain size, which is attributed to crystallinity increase and to reduction of demagnetizing fields in the grains. The variation in coercivity could be related to anisotropy field changes within the samples due to grain size changes. In particular, the starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped B-H loops seems to be dependent on a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. Viewed simultaneously, the hysteresis loops appear to belong to three groups with different magnetism-type dominance, respectively dependent on phase purity and three different groups of grain size distributions.     

Synthesis,magnetic properties and microstructure of Ni–Zn ferrite by sol–gel technique

In the study, the Ni–Zn ferrite powder of a Ni_0.3Zn_0.7Fe₂O₄ composition was synthesized by sol–gel route using metal acetates at low temperatures. Both the scanning electron microscope and X-ray diffraction analyses of various gel samples heated at different temperatures were used to identify the reaction stages where the amorphous-gel-to-crystalline phase transition occurred. The electrical, magnetic and microstructural properties of the toroidal cores were studied. It was found that the initial permeability increased with a large frequency band (0.1–31.39 MHz) and the magnetic loss was small. The electrical resistivity was higher as compared to the ones which were obtained by the conventional process. Therefore, well–defined polycrystalline microstructure nickel–zinc ferrite and a short processing time of gel preparation have become the major achievements of this study.     

The effect of multi-step milling and annealing treatments on microstructure and magnetic properties of nanostructured Fe-Si powders

Nanostructured Fe_1−xSi_x (x=0.05, 0.1, 0.15 and 0.2) powders are prepared by different multi-step milling and annealing treatments. The microstructure and magnetic properties are investigated for all alloys. The minimum crystallite size of as-annealed powders (∼40 nm) is found to be larger than in as-milled ones (∼15 nm). It is found that microstrains of 2- and 4-step processes are close to those of the as-received powders. The lattice parameter decreased ∼0.5% and 0.9% for the powders that experienced milling and annealing at the last step, respectively. The Fe₈₀Si₂₀ powders prepared by 1- and 4-step treatments show the maximum (40-125 Oe) and minimum (20-26 Oe) coercivity, respectively. With increase in milling time, mass magnetization increased for all processes. This can be ascribed to diminution in magneto-crystalline anisotropy due to grain refinement. The maximum mass magnetization (160-199 Am²/kg) is achieved for the 4-step process.     

Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

Annealing effect on microstructure and mechanical properties of amorphous Al-C-N films

Al-C-N thin films with different Al contents were deposited on Si (1 0 0) substrates by closed-field unbalanced reactive magnetron sputtering in the mixture of argon and nitrogen gases. These films were subsequently vacuum-annealed at 700 °C and 1000 °C, respectively. The microstructures of as-deposited and annealed films were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM); while the hardness and elastic modulus values were measured by nano-indention method. The results indicated that the microstructure of both as-deposited and annealed Al-C-N films strongly depended on Al content. For thin films at low Al content, film delamination rather than crystallization occurred after the sample was annealed at 1000 °C. For thin films at high Al content, annealing led to the formation of AlN nanocrystallites, which produced nanocomposites of AlN embedded into amorphous matrices. Both the density and size of AlN nanocrystallites were found to decrease with increasing depth from the film surface. With increasing of annealing temperature, both hardness and elastic modulus values were decreased; this trend was decreased at high Al content. Annealing did not change elastic recovery property of Al-C-N thin films.     

Changes of microstructure and magnetic properties of Nd-Fe-B sintered magnets by doping Al-Cu

The microstructural and magnetic properties of Al_100−xCu_x (15at%≤x≤45 at%) doped Nd-Fe-B magnets were studied. The distribution and alloying effects of Cu or Al on the intergranular microstructure were investigated by thermodynamic analysis, differential scanning calorimetery and microscopy techniques. It was observed that when the Cu content of Al_100xCu_x exceeds to 25 at%, the (Pr, Nd)Cu and CuAl₂ phases form in these magnets. The formation of (Pr, Nd)Cu phase depends on the negative formation enthalpy of (Pr, Nd)Cu and the exclusive distribution of Cu in the intergranular regions. The eutectic reaction between (Pr, Nd)Cu phase and (Pr, Nd) occurs at 480 °C, which forms the liquid phase that dissolves the (Pr, Nd)₂Fe₁₄B surface irregularities and thus increases the quantities of (Pr, Nd)-rich phase at the grain boundaries. These changes benefit the grain boundary microstructure, especially the distribution of (Pr, Nd)-rich phase, which effectively improves the intrinsic coercivity _iH_c due to the decreases of exchange coupling between the (Pr, Nd)₂Fe₁₄B grains.     

Influence of annealing temperature on the formation, microstructure and magnetic properties of spinel nanocrystalline cobalt ferrites

Nanocrystalline cobalt ferrites were synthesized by a simple, general, one-step sol–gel auto-combustion method. An interpretation based on the measurement of the adiabatic flame temperature and the amounts of gas evolved during reaction had been proposed for the nature of combustion. The influence of annealing temperatures on the magnetic properties was investigated. The microstructure was characterized by means of X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the particle size and magnetic properties of the as-prepared ferrite samples showed strong dependence on the annealing temperature. The coercivity initially increased and then decreased with increasing annealing temperature whereas the particle size and saturation magnetization continuously increased.     

Effects of In-substitution on the microstructure and magnetic properties of Bi-CVG ferrite with low temperature sintering

[Y_1.05Bi_0.75Ca_1.2](Fe_4.4−xIn_xV_0.6)O₁₂(In_x:Bi-CVG) ferrite material has been prepared successfully by a solid-state reaction method. The effects of In³⁺ substitution and sintering temperatures on the bulk density, microstructure and magnetic properties are performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), materials automatic test system (MATS) and microwave ferrite parameters meter. The results show that In³⁺ can lower the sintering temperatures and enhance the magnetic properties of Bi-CVG ferrite. Besides, all sintered specimens with different In³⁺ contents show a single garnet crystal structure. The specimen of [Y_1.05Bi_0.75Ca_1.2](Fe₄In_0.4V_0.6)O₁₂ sintered at 1075 °C shows homogenous distribution of grain size and densified microstructures. The ferromagnetic resonance linewidth (ΔH) has an increase with In³⁺ contents. Additionally, the sample has the optimum magnetic properties: ρ=5.23 g/cm³, B_r=31.3 mT, H_c=378.8 A/m, 4πM_s=506.2×10⁻⁴ T.     

The role of Mg substitution on the microstructure and magnetic properties of Ba Co Zn W-type hexagonal ferrites

A series of W-type hexagonal ferrites with the composition BaCoZn_1−xMg_xFe₁₆O₂₇ (0?x?0.6) were prepared by the conventional ceramic method to study their structural and magnetic properties as a function of temperature and composition. The characterization using X-ray diffraction indicated that a hexagonal W-type single-phase structure and the effect of composition on the unit cell parameters, density and porosity was studied. The variation of the magnetic susceptibility (χ_M) with temperature for all the investigated samples in the temperature range (300–800 K) shows three regions of behavior that was explained on the basis of the distribution of Zn²⁺ and Mg²⁺ ions in the lattice and leads to the anomalous behavior of the effective magnetic moment μ_eff. The Curie temperature indicated that the critical concentration is at x=0.5. Paramagnetic nature of the samples above the Curie temperature is observed. The Curie Weiss constant θ calculated from the plot of 1/χ_M vs. T (K) is in agreement with the expected value. The effective magnetic moment μ_eff decreases with increasing the intensity of magnetic field. The possible mechanisms contributing to these properties are discussed in the text.     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Study of heating effect and acoustic properties of dextran stabilized magnetic fluid

This paper presents acoustic properties of water-based biocompatible fluids in which magnetite particles (Fe₃O₄) were coated with two layers of surfactants: sodium oleate and dextran. The attenuation coefficient of ultrasonic wave measurements shows good structural stability of the fluid under the influence of a magnetic field. Hyperthermic tests proved that the magnetic fluid is suitable for therapeutic use as an agent which can release thermal energy (hyperthermia).     

The effect of different chemical compositions caused by the variation of deposition potential on properties of Ni-Co films

The magnetic and microstructural properties of Ni-Co films electrodeposited at different cathode potentials were investigated. The compositional analysis revealed that the Ni content increases from 13 at.% to 44 at.% in the films with increasing deposition potential. Magnetic measurements showed that the saturation magnetization, M_s of the films decreased with increase of Ni content as the deposition potential increased. M_s values changed between 1160 emu/cm³ and 841 emu/cm³. The X-ray diffraction revealed that the crystalline structure of the films is a mixture of the predominant face-centered cubic (fcc) and hexagonal closed packed. However, the mixture phase turns to the fcc because of increasing Ni content up to 44 at.% at the highest (−1.9 V) potential by enhancing the intensity of reflections from the fcc phase. The changes observed in the magnetic and microstructural properties were ascribed to the changes observed in the chemical composition caused by the applied different deposition potentials.     

Structural and magnetic properties of Cu1−xMnxO nanocrystal prepared by combustion synthesis

Nanoscale Cu_1−xMn_xO powder is prepared by using the combustion synthesis technique with two different fuels. The structural properties of the powder are determined using Rietveld refinement of X-ray diffraction data, high-resolution transmission electron microscopy, and Fourier transform infrared spectroscopy, while its magnetic properties are analyzed by means of hysteresis loop and temperature dependence of magnetization. The results show that (1) the Cu_1−xMn_xO nanocrystal is of monoclinic CuO structure, with grain size of 10-30 nm varying with the type of fuel, the nitrate/fuel ratio (N/F), and the Mn concentration, the doping of Mn has a little influence on the lattice parameters; (2) when the Mn concentration is higher than 7%, a small amount of impurity phase of CuMn₂O₄ appears and annihilates the potential cation vacancies; (3) all of the samples with x≥5% exhibit low-temperature ferromagnetism with the Curie temperature of ∼90 K, which increases slightly by raising the Mn concentration; (4) the paramagnetic moment per Mn ion is around 2-4 bohr magneton above the Curie temperature, which decreases with increasing Mn concentration, implying that the nearest Mn ions are antiferromagnetically coupled and the ferromagnetic order could originate from the super-exchange of next nearest Mn ions along the [1 0 1?] direction.     

Sputtering gases and pressure effects on the microstructure,magnetic properties and recording performance of TbFeCo films

The MsHc value is considered to be a key factor in high-density recording, and controlling the microstructure on the magnetic underlayer was found to be an effective way of increasing the MsHc of the amorphous TbFeCo magneto-optical (MO) medium. In this paper, we investigate the TbFeCo film's magnetic properties and the effects on the microcolumnar structure, which depends on the sputtering conditions of using various sputtering gases including Ar, Kr, and Xe, and the recording characteristics of TbFeCo memory layers. With heavy sputtering gases such as Kr or Xe, the columnar structure can be prepared in a TbFeCo film at a pressure lower than 1.0 Pa. The columnar structure of a recording layer can be effectively formed thanks to the effects of the magnetic underlayer, which has a fine surface even in the sputtering process in which Xe gas is used. The above applies to the sputtering process in which Ar gas is used. Also, when Xe gas is used in the sputtering process, coercivity Hc is increased through the formation of a well-segregated microcolumnar structure built on domain wall pinning sites, and we obtain a large MsHc and a high squareness ratio of the Kerr-hysteresis loop. Our results indicate that processing a TbFeCo film with heavy sputtering gases is suitable for tiny mark stability because the temperature gradient of Hc is increased. The objective of the low-pressure sputtering process using Xe gas to produce the columnar structure is to achieve ultra-high-density recording with tiny mark stability in the TbFeCo medium. This has been confirmed with magnetic force microscope (MFM) images of stable tiny marks recorded on TbFeCo film.     

Effects of SnO2 addition on the microstructure and magnetic properties of NiZn ferrites

The microstructure and magnetic properties of SnO₂-doped NiZn ferrites prepared by a solid-state reaction method have been investigated. Due to its low melting point (∼1127 °C), moderate SnO₂ enhanced mass transfer and sintering by forming liquid phase, which accelerated the grain growth. However, excessive SnO₂ producing much of liquid phase retarded mass transfer and sintering, leading to a decrease in grain size. The diffraction intensity of the samples doped with SnO₂ addition was stronger than that of the sample without addition. The lattice constant initially decreased up to a content of 0.10 wt% and showed an increase at higher content up to 0.50 wt%. The initial permeability (μ_i) initially increased up to a content of 0.15 wt% and showed a decrease at higher content up to 0.50 wt%; however, losses (P_L) measured at 50 kHz and 150 mT changed contrarily. Both saturation induction (B_S) and Curie temperature (T_C) decreased gradually with increasing SnO₂. Finally, the sample doped with 0.10–0.15 wt% SnO₂ showed the higher permeability and lower losses.     

Simulation study on microstructure formations in magnetic fluids

We propose the Langevin-type microscopic equations of motion for magnetic fluids. Magnetic fluids are modeled as an ensemble of interacting ferromagnetic nanoparticles suspended in a viscous fluid. The present model is described in terms of position vectors of nanoparticles and orientation vectors of their magnetic dipole moments. In this model, forces and torques arising from the magnetic origin and the surrounding fluid flow are included. Effects of non-spherical particle shape are also taken into account. From the Brownian dynamics simulations of the model, it is found that the present model exhibits various microstructure formation processes in magnetic fluids.     

Effects of doping site and pre-sintering time on microstructure and magnetic properties of Fe-doped BaTiO3 ceramics

The A-site substituted BaTiO₃ ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba²⁺. By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba_0.93Fe_0.07)TiO₃ sample, the Fe ions are present as Fe²⁺ and Fe³⁺, respectively, replacing A-site Ba²⁺ and octahedral B-site Ti⁴⁺ in hexagonal perovskite lattice. The double-exchange Fe²⁺-O²⁻-Fe³⁺ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti_0.93Fe_0.07)O₃, the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti⁴⁺ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO₃ system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti_0.93Fe_0.07)O₃ is enhanced ∼6 times.     

Texture, microstructure and magnetic properties of Fe-28Cr-15Co-3.5Mo permanent magnet

Crystallographic texture, microstructure and magnetic properties of Fe-28Cr-15Co-3.5Mo alloy were studied as a function of processing parameters. Texture studies revealed that thermo-magnetic aging leads to the development of ideal Goss type, {1 1 0} 〈0 0 1〉 and cube type {0 0 1} 〈0 1 0〉 textures. The orientation densities of these texture components become stronger after the step-aging treatments. Microstructural features show that improvement in magnetic properties were due to aligning and elongation of ferromagnetic Fe, Co-rich (α₁) particles in the preferred 〈1 0 0〉 directions. Magnetic analysis reveals that magnetic properties of the alloys are directionally dependent and influenced by the choice of thermo-magnetic treatment temperature or time. The maximum values of intrinsic coercive force, remanence and energy product, obtained in the textured magnetic alloy were 68.99 kA/m (867 Oe), 1.12 T (11.2 kG) and 43.2 kJ m³ (5.4 MG Oe), respectively.