Influence of temperature on structural and magnetic properties of Co0·5Mn0·5Fe2O4 ferrites

Co_0·5Mn_0·5Fe₂O₄ ferrites have been synthesized using a single-step sol-gel auto-combustion method in which the metal nitrate (MN)-to-citric acid (CA) ratio was adjusted to 0.5: 1 and pH to 7, respectively. The structural and magnetic properties of as-burnt and annealed samples were studied as a function of temperature. The inverse spinel structure was confirmed by X-ray diffraction (XRD) and crystallite size was estimated by the most intense peak (311) using Scherrer’s formula. Contrary to earlier studies reported in the literature, both as-burnt and annealed samples exhibit crystalline behaviour. Room temperature magnetic properties were studied using vibrating sample magnetometer (VSM) with field strengths up to ±10 kOe. Lattice constant and crystallite size increased as the annealing temperature was increased. However, the coercivity (H _c) initially increased and then decreased with the increase of crystallite size. The variation in coercivity is ascribed to the transition from a multi-domain to a single-domain configuration.     

Study of dependence of magnetic properties on seeding temperature in fine particles of Cu0·25Co0·25Zn0·5Fe2O4

Using a combination of X-ray diffraction, room temperature/low temperature Mössbauer studies and d.c. magnetization, the structural and magnetic properties of nano-sized Cu_0·25Co_0·25Zn_0·5Fe₂O₄, prepared using the coprecipitation method at different seeding temperatures, have been studied. Formation of α-Fe₂O₃ in these samples is observed to be very sensitive to the seeding temperature and is totally suppressed at a seeding temperature of 333 K. These results are explained in terms of the rate of coprecipitation of the different components as a function of temperature. Since α-Fe₂O₃ is antiferromagnetic, the presence and quantity of this impurity is also observed to deteriorate the overall magnetic properties of the spinel phase.     

High temperature magnetic properties of Sm–Co and Sm–Co/polyamide-12 materials: effects of temperature, particle size, and silanization

There is an increasing demand for polymer-bonded magnets (PBM) in high temperature applications. While most research deals with high temperature properties of NdFeB–PBM, only a few studies consider Sm–Co PBM. Therefore, this study, on the thermal and magnetic properties of Sm–Co alloy powders and blends of these with polyamide-12 (PA12), was undertaken. Since the Sm–Co powders were the product of ball milling, they contained a variety of shapes and sizes. Studies on size fractions of these showed that the thermal stability and magnetic properties were improved as the particle size increased. It was suggested that higher residual strains and smaller crystallite sizes in the small particles were responsible for a decrease in the thermal stability and magnetic properties. In addition, energy dispersive X-ray spectroscopy revealed that the oxygen content increased with decreasing particle size (larger specific surface area) and higher oxygen content was possibly also responsible for a decrease in the magnetic properties. It was shown that, in general, the surface modification by silanization, using (3-aminopropyl)trimethoxsilane, increased the saturation magnetization and remanence of both the particles and the Sm–Co/PA12 composite. The silanization also improved the thermal stability of the particles.     

Impact of annealing temperature on structural,optical, and Mössbauer properties of nanocrystalline NiFe2O4

In this paper, we have investigated the effect of annealing temperature on the structural, optical, and Mössbauer properties of nanocrystalline (NC) nickel ferrites (NFOs) synthesized by the sol–gel auto-combustion method. The NFOs were characterized by X-Ray diffraction (XRD), Raman spectroscopy, Diffusion reflectance spectroscopy (DRS), and Mössbauer spectroscopy techniques. The XRD results show that the average crystallite size increases from 27.5 to 54.3 nm when increasing the annealing temperature from 200 to 1000 °C. The Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV-DRS) measurement is used to find the optical band gap observed between 1.92 and 1.75 eV for NFOs annealed at 200 and 1000 °C, respectively. The Mössbauer study confirmed that the structure transforms from mixed spinel to inverse spinel structure when moving to higher annealing temperature.

     

Crystal structures and magnetic performance of nanocrystalline Sm–Co compounds

A variety of intermetallic compounds in the binary Sm–Co system were reviewed, and the contents were focused on the crystal structures, magnetic properties and the nanoscale effects. The representative nanocrystalline Sm–Co compounds were introduced in details, the diagrams for their lattice structures and the atomic sites and occupancies were provided. Moreover, the magnetic properties of the nanocrystalline Sm–Co compounds were compared with those of the conventional polycrystalline counterparts. It showed that the nanocrystalline Sm–Co compounds exhibit special phase stability and remarkably enhanced magnetic performance, which are promising candidates for the matrix phases to develop permanent magnets, particularly the advanced high-temperature magnetic materials.     

Magnetodielectric effect in Ni0∙5Zn0∙5Fe2O4–BaTiO3 nanocomposites

Composites comprising of nanoparticles of Ni_0?5Zn_0?5Fe₂O₄ (NZF) and BaTiO₃ (BT), respectively were synthesized by a chemical method. The particles had diameters in the range of 15–31 nm. NZF was prepared by a coprecipitation technique. This was soaked in a sol containing BT. Compositions synthesized were xNZF-(1 – x) BT, where x = 0?7, 0?5 and 0?3, respectively. The composites showed ferromagnetic hysteresis loops due to NZF phase. The analysis of coercivity variation as a function of temperature gave blocking temperatures in the range of 306–384 K depending on the diameter of the ferrite nanoparticles. This implied that superparamagnetic interactions are above these temperatures. The nanocomposites also exhibited ferroelectric behaviour arising due to the presence of BT. The remanent polarization of the samples was small. This was adduced to the nanosize of BT. The specimens showed magneto-dielectric (MD) effect in the magnetic field range 0–0?7 Tesla. The MD parameter measured at the maximum magnetic field was around 2%. This was one order of magnitude higher than that reported so far in similar composite systems. This was explained on the basis of a two-phase inhomogeneous medium model with an interface between them, the phases possessing drastically different electrical conductivities.     

High piezoelectric and dielectric properties of 0–3 PZT/cement composites by temperature treatment

Temperature treatment of 0–3 type PZT/cement composites before polarization yielded high dielectric and piezoelectric properties in materials with 50% PZT inclusions by volume and 50% cement matrix. Specimens were treated at seven temperatures from 23 °C to 150 °C and then applied by a 1.5 kV/mm poling field. The dielectric loss of the composites reduces at higher pretreatment temperatures, shorting the trigger time. Temperature treatment increased the piezoelectric strain factor d₃₃, the relative dielectric constant ε_r and the piezoelectric voltage factor g₃₃ of PZT/cement composites, but did not affect significantly the electromechanical coupling coefficient K_t. Piezoelectric factors reach stable values after 70 days of aging, and samples that were not temperature pretreated reached stable values earlier. Specimens pretreated at 150 °C exhibit d₃₃ = 106.3 pC/N and ε_r = 477 on the 70^th aging day, almost two times greater than the composites without temperature treatment. The resonance frequency of the composites on the 70th day decreases with increasing temperature, with the exception of 150 °C. Temperature pretreatment can also improve the phase angle of the composites. In addition, the effect of curing time for PZT/cement composites is an important factor to dominate the feasibility of polarization.     

Synthesis, non-isothermal crystallization and magnetic properties of Co0·75Zn0·25Fe2O4/poly(ethylene-co-vinyl alcohol) nanocomposite

The synthesis of Co_0·75Zn_0·25Fe₂O₄/poly(vinyl alcohol-co-ethylene) (ferrite/PEVA) nanocomposite was carried out through two steps: impregnation of the ferrite particules by PEVA and then mixing the ferrite/PEVA impregnated with PEVA solution. A non-isothermal study of the crystallization kinetic of ferrite/PEVA nanocomposite was carried out by differential scanning calorimetry (DSC), scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques. It was observed that the Ozawa equation describes perfectly the primary process of non-isothermal crystallization of ferrite/PEVA system. There is a strong dependence of the ferrite/PEVA composition on the crystallization parameters. The crystallization activation energy (E _a ) calculated from the Xu and Uhlmann model increased by increasing the ferrite content in ferrite/PEVA nanocomposites between 3 and 7 wt% and decreased dramatically beyond these values. The results revealed that the ferrite nanoparticles were uniformly distributed throughout the PEVA matrix. The percentage of magnetization of the composite decreases as the concentration of the ferrite increases.     

High temperature mechanical properties of La1−x Sr x Cr1−y Co y O3 for SOFC interconnect

Solid oxide fuel cells (SOFC) require an interconnect for fabrication into stacked cells. This is typically La(Sr, Ca)CrO₃, of which much data on the electrical and physical properties already exists. However, very little information exists on the high temperature mechanical properties of the material, which is a necessity for future design improvements. La_1–x Sr _x Cr_1–y Co _y O₃ samples were fabricated into green dry-pressed bars and pellets, and sintered under various heating and cooling regimes. The sinterability and high temperature mechanical properties of the material was then investigated as a function of the dopant concentration. It was observed, for example, that the modulus of rupture of the dry pressed La_0.7Sr_0.3Cr_1–yCo_yO₃ (y 0.3) gave a value of over 110 MPa at 1000 °C. This paper will provide data on the high temperature mechanical properties of the material and its application to the SOFC system.     

Soft magnetic properties and microstructure of Fe84−xNb2B14Cux nanocrystalline alloys

Magnetic properties and microstructure of new Fe_84−xNb₂B₁₄Cu_x nanocrystalline alloys were investigated. We found that the microstructure was refined and soft magnetic properties of this alloy system were enhanced with proper Cu addition and annealing conditions. It was also discovered that the mean grain size firstly increases, then decreases to a minimum, and finally increases again with increasing annealing temperature for Fe₈₃Nb₂B₁₄Cu₁ nanocrystalline alloy, and this phenomenon was interpreted by the grain growth mechanism. Moreover, after annealing at 813 K for 180 s, Fe₈₃Nb₂B₁₄Cu₁ nanocrystalline alloy shows a fine microstructure with mean grain size of 16 nm, and exhibits excellent soft magnetic properties, such as high saturation magnetic flux density (1.7 T), low coercivity (7 A/m) and high permeability (2.8 × 10⁴). The result indicates that this alloy should have a promising application in the soft magnetic industry.     

Co ion-substituted dielectric–magnetic properties of BaZn2−xCoxFe16O27 for high-frequency and miniature antennas

Journal of Materials Science: Materials in Electronics - Based on the solid-state reaction method, the effects of Co ion substitution (x?=?0.8, 1.2, 1.6, 2.0) on the crystal structure,...     

Nanostructure and magnetic properties of the MnZnO system, a room temperature magnetic semiconductor?

The magnetic properties of the system MnZnO prepared by conventional ceramic procedures using ZnO and MnO(2) starting powders are studied and related to the nanostructure. Thermal treatment at 500?°C produces a ferromagnetic phase, although this temperature is not high enough to promote proper sintering; thus the thermally treated compact shows brittle characteristics of unreacted and poorly densified ceramic samples. Scanning electron microscopy and x-ray analysis reveal the appearance of a new phase, most probably related to the diffusion of Zn into MnO(2) oxide nanocrystals. The magnetic properties deviate considerably from what would be expected of an unreacted mixture of ZnO (diamagnetic) and MnO(2) particles (paramagnetic above 100?K and anti-ferromagnetic below that temperature), exhibiting a ferromagnetic like behaviour from 5 to 300?K and beyond mixed with a paramagnetic component. The ferromagnetic phase seems to be originated by diffusion at the nanoscale of Zn into MnO(2) grains. The Curie temperature of the ferromagnetic phase, once the paramagnetic component has been subtracted from the hysteresis loops, is measured to be 450?K. EPR resonance experiments from 100 to 600?K confirm a ferromagnetic to paramagnetic like transition above room temperature for these materials.     

Structure and magnetic properties of nano-structured heterogeneous Au—Co alloys

High-resolution and analytical transmission electron microscopy, X-ray diffraction, magnetic and magnetoresistance measurements were used to investigate nanostructures of melt-spun Au₈₆Co₁₄ and Au₇₈Co₂₂ alloys. The microstructure of the Au₈₆Co₁₄ alloy was composed of very small Co precipitates inside the Au grains with some larger Co precipitates (20–35 nm) dispersed at the grain boundaries, while the microstructure of the Au₇₈Co₂₂ alloy consisted of Au/Co lamellar eutectic grains with Co precipitates (50–70 nm) dispersed at the grain boundaries. A few grains had very small (4 nm) Co precipitates. Annealing at 773 K for 10 min caused Co depletion in the Au matrix from 5.4–10 at.% to 0.9–2.0 at.%. Annealing also caused transitions from superparamagnetic to ferromagnetic and from single to multiple domain magnetic structure of some of the small and some of the larger Co precipitates, respectively. The MR ratios (/, in magnetic field of 14.5 kOe) of the as-cast Au₈₆Co₁₄ and Au₇₈Co₂₂ alloys were 2.5% and 2.6%, respectively. Annealing of the alloys at 673 K for 1 hr reduced / to 0.9–1%.     

Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite

Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300–900 K is investigated. ln ρ versus 1/T plot shows four break resulting into five regions in 300–900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (?′) with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in ?′ variation with temperature show frequency dependence. Electrical and dielectric properties of Ni_0.4Co_0.6Fe₂O₄ sample show unusual behavior in the temperature range 723–833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723–833 K for Ni_0.4Co_0.6Fe₂O₄. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed.     

The microstructure and magnetic properties of FeCo alloys with different OH ¯ /(Co 2+ , Fe 2+ ) ratio and annealing temperature

Journal of Materials Science: Materials in Electronics - The OH¯/(Co2+, Fe2+) ratio and annealing temperature (Ta) have a great influence on the microstructure and magnetic properties of FeCo...     

Microwave sintering of nanocrystalline γ-Al2O3

The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1₂O₃ sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.     

Synthesis and magnetic properties of polycrystalline films of Co x Fe y Cr3 − x − y O4 and Cr2O3/CoFe2O4 multiferroics

Magnetic properties of first obtained polycrystalline films of FeCr₂O₄, CoCr₂O₄, and CoFe_0.5Cr_1.5O₄ multiferroics and films of a Cr₂O₃/CoFe₂O₄ composite multiferroic have been studied. In particular, magnetization curves and temperature dependences of the magnetic moment of the samples were measured in the temperature range 4.2–300 K in fields of up to 10 kOe. It was shown that the Curie point of a multiferroic depends on its cation composition. It was found that an exchange bias of the hysteresis loop exists in films of the Cr₂O₃/CoFe₂O₄ composite multiferroic at temperatures below the Néel point of Cr₂O₃ (330 K).     

Some magnetic properties of γ-Fe2O3 synthesized from ferrous fumarate half-hydrate

-Fe₂O₃ synthesized from ferrous fumarate half-hydrate was studied by measurements of D.c. electrical conductivity, Seebeck coefficient, initial magnetization and magnetic hysteresis, and by Mössbauer spectroscopy and scanning electron microscopy. The phase transformation observed by electrical conductivity measurements matched well with the phase transformation observed by the variation with temperature of initial magnetization measurements of -Fe₂O₃; this magnetic study also established the single-domain character of -Fe₂O₃. The magnetic hysteresis values of the -Fe₂O₃ synthesized indicated improved values over that of a -Fe₂O₃ sample synthesized by established procedures. The scanning electron micrographs showed that the -Fe₂O₃ particles were acicular in shape and the Mössbauer spectrum showed a well-resolved six-band spectrum. The presence of a hydrogen ferrite phase was also confirmed by the electrical and magnetic measurements.Deceased, 10 October, 1985.