Synthesis and characterization of nanostructured strontium hexaferrite thin films by the sol–gel method

Nanostructured single phase strontium hexaferrite, SrFe₁₂O₁₉, thin films have been synthesized on the (100) silicon substrate using a spin coating sol–gel process. The thin films with various Fe/Sr molar ratios of 8–12 were calcined at different temperatures from 500 to 900 °C. The composition, microstructure and magnetic properties of the SrFe₁₂O₁₉ thin films were characterized using Fourier transform infrared spectroscopy, differential thermal analysis, thermogravimetry, X-ray diffraction, electron microscopy and vibrating sample magnetometer. The results showed that the optimum molar ratio for Fe/Sr was 10 at which the lowest calcination temperature to obtain the single phase strontium hexaferrite thin film was 800 °C. The magnetic measurements revealed that the sample with Fe/Sr molar ratio of 10, exhibited higher saturation magnetization (267.5 emu/cm³) and coercivity (4290 Oe) in comparison with those synthesized under other Fe/Sr molar ratios.     

Composition and magnetic studies of ultrafine Al-substituted Sr hexaferrite particles prepared by citrate sol-gel method

Ultrafine aluminum-substituted strontium hexaferrite particles have been prepared via citrate sol-gel route. Gels were synthesized with molar ratios [Al³⁺]:[Fe³⁺] of 0.4:11.6, 1:11, 1.5:10.5 and 2:10 and the ferrite particles were obtained by annealing the gels at 950 °C for 2 and 24 h. Electron energy dispersive X-ray spectroscopy (EDX) verified the presence of Al in the substituted samples. X-ray diffraction (XRD) confirmed the formation of the M-type hexaferrite phase in the samples with some indication of α-Fe₂O₃. Scanning electron microscope showed that the hexaferrite powder consists hexagonal crystals with average diameter D_av (80-186 nm) that decreases with increasing Al content and increases with increasing annealing time. Magnetic properties were determined using a pulsed-field magnetometer and a vibrating sample magnetometer (VSM). The saturation magnetization at room temperature and the Curie temperature were found to decrease while the coercivity increases with increasing the Al content. The highest coercivity of 10.1 kOe was achieved for the sample with the molar ratio [Al³⁺]:[Fe³⁺]=2:10 annealed for 24 h. The influences of the particle size, composition and impurity on the magnetic properties were discussed.     

Effect of pH value on the structural and magnetic properties of nanocrystalline strontium hexaferrite thin films

Strontium hexaferrite SrFe₁₂O₁₉ thin films have been synthesized at different pH, adjusted by NH₄OH, on the Si (1 0 0) substrate using a spin coating sol-gel process. Fourier transform infrared spectroscopy analysis and theoretical calculations were conducted for determination and controlling metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe₁₂O₁₉ thin films. Using the solution with pH 7, the approximately single phase strontium hexaferrite thin films with optimum physical properties can be obtained at calcination temperature of 800 °C. The SrFe₁₂O₁₉ thin films derived from the solution with pH 7 after calcination at 800 °C exhibited crystallite size of 42 nm and magnetic properties of M_s=267 emu/cm³ (at 10 kOe), M_r=134 emu/cm³ and H_c=4290 Oe.     

Evidence of existence of metastable SrFe12O19 nanoparticles

The existence of metastable hexaferrite is reported. Synthesis of strontium hexaferrite, SrFe₁₂O₁₉, at 400 °C was realized under controlled oxygen atmosphere. Such technique allows obtaining of SrFe₁₂O₁₉ at lower temperatures than those by traditional methods (above 800 °C). Phase transformation occurred during a measurement of magnetization vs. temperature (heating up to 625 °C). The heat treatment induces a change from SrFe₁₂O₁₉ to γ-Fe₂O₃ (as the main phase), and SrFeO_2.74 to Sr₂Fe₂O₅. Together with these phase transformations, an increment in the amount of SrCO₃ is detected. Magnetic study of the samples, before and after the heating, supports the structural analysis conclusions.     

Magnetic force microscopy characterization of heat and current treated Fe40Ni38Mo4B18 amorphous ribbons

The domain structure of a magnetostrictive Fe₄₀Ni₃₈Mo₄B₁₈ amorphous ribbon has been studied using magnetic force microscopy (MFM) at room temperature. First, the evolution of the magnetic domain patterns as a function of the annealing temperature has been investigated. In samples heat treated at 250 and 450 °C for 1 h, a transformation from 90° to 180° domain wall has been clearly observed, while the sample heat treated at 700 °C for 1 h showed a magnetic phase fixed by the crystalline anisotropy. Additionally, the evolution of the magnetic domain structure by applying a DC current was recorded by the MFM technique. For current annealed samples at 1 A for 1, 30 and 60 min, a transformation between different domain patterns has been observed. Finally, in samples treated by the current annealing method under simultaneous stress, an increase of the annealing time gives rise to a different magnetic structure arising from the development of transverse magnetic anisotropy.     

Preparation of barium hexaferrite nanopowders by the sol-gel method, using goethite

A series of barium hexaferrite nanoparticles (BaO·nFe₂O₃) with different n values were prepared by the sol-gel method, using goethite and Ba carbonate as raw materials. Phase identification of the samples was investigated by X-ray diffraction (XRD). XRD investigations show that the samples with n=5 and calcined at temperatures higher than 875 °C are single-phase Ba ferrite. An average crystallite size of 22 nm was obtained for the single-phase sample with minimum calcining temperature of 875 °C, using the Scherrer's formula. The morphology of the samples was checked by transmission electron microscope (TEM) and magnetic properties were measured by a sensitive permeameter. The results show that the samples have nonzero coercivities, which shows the particle size are not less than the critical size of Ba ferrite and then are not superparamagnet.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

Structural and magnetic properties of RF sputtered FePt/Fe multilayers

Series of [FePt(4min)/Fe(t_Fe)]₁₀ multilayers have been prepared by RF magnetron sputtering and post-annealing in order to optimize their magnetic properties by structural designs. The structure, surface morphology, composition and magnetic properties of the deposited films have been characterized by X-ray diffractometer (XRD), Rutherford backscattering (RBS), scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDX) and vibrating sample magnetometer (VSM). It is found that after annealing at temperatures above 500 °C, FePt phase undergoes a phase transition from disordered FCC to ordered FCT structure, and becomes a hard magnetic phase. X-ray diffraction studies on the series of [FePt/Fe]_n multilayer with varying Fe layer thickness annealed at 500 and 600 °C show that lattice constants change with Fe layer thickness and annealing temperature. Both lattice constants a and c are smaller than those of standard ones, and lattice constant a decreases as Fe layer deposition time increases. Only a slight increase in grain size was observed as Fe layer decreased in samples annealed at 500 °C. However, the increase in grain size is large in samples annealed at 600 °C. The coercivities of [FePt/Fe]_n multilayers decrease with Fe layer deposition time, and the energy product (BH)_max reaches a maximum in the samples with Fe layer deposition time of 3 min. Comparison of magnetic properties with structure showed an almost linear relationship between the lattice constant a and the coercivities of the FePt phase.     

Effect of annealing on phase composition,structural and magnetic properties of Sm-Co based nanomagnetic material synthesized by sol-gel process

Sm-Co based nanomagnetic material was synthesized by means of a Pechini-type sol-gel process. In this method, a suitable gel-precursor was prepared using respective metal salts and complexing agent such as citric acid. The gel-precursor was dried at 300 °C and then subjected to various reductive annealing temperatures: 350, 500 and 600 °C. The nanopowders so obtained were characterized for their structure, phase composition and magnetic properties. FT-IR studies on the gel-precursor showed the binding of metal cations with the citrate molecules in the form of metal-citrate complex. The gel-precursor, which was annealed at 350 °C showed the presence of both meta-stable cobalt carbide (Co₂C, Co₃C) and Co₃O₄ phases; while the sample annealed at 500 °C indicated the sign of SmCo₅ phase. Upon increasing the reductive annealing temperature to 600 °C, crystalline phase such as fcc-Co and Sm₂C₃ were formed prominently. FE-SEM analysis revealed the change in sample morphology from spherical to oblate spheres upon increasing the annealing temperature. VSM measurements demonstrated ferromagnetic nature at room temperature for all the nanopowders obtained irrespective of their after reductive annealing temperature.     

Magnetic properties in Fe-doped NiO synthesized by co-precipitation

The magnetic properties of 1.5 at% Fe-doped NiO bulk samples were investigated. The samples were prepared by sintering the corresponding precursor in air at temperatures between 400 and 800 °C for 6 h. The synthesis was by a chemical co-precipitation and post-thermal decomposition method. In order to allow a comparison, a NiO/0.76 at% NiFe₂O₄ mixture was also prepared. The X-ray diffraction pattern shows that the samples that were sintered at 400 and 600 °C remain single phase. As the sintering temperature increased to 800 °C, however, the sample becomes a mixture of NiO and NiFe₂O₄ ferrite phases. The samples were investigated by measuring their magnetization as a function of magnetic field. The samples sintered between 400 and 800 °C and the one mixed directly with NiFe₂O₄ nanoparticles show a coercivity value of H_c≈200, 325, 350 and 110 Oe, respectively. The magnetic properties of the samples depend strongly on the sintering temperature. Simultaneously, the field-cooling hysteresis loop shift also observed after cooling the sample sintered at 600 °C to low temperature suggests the possibility of the existence of a ferromagnetic/antiferromagnetic exchange coupling.     

Improvement of the magnetic properties of barium hexaferrite nanopowders using modified co-precipitation method

Barium hexaferrite BaFe₁₂O₁₉ powders have been synthesized using the modified co-precipitation method. Modification was performed via the ultrasonication of the precipitated precursors at room temperature for 1 h and the additions of the 2% KNO₃, surface active agents and oxalic acid. The results revealed that single phase magnetic barium hexaferrite was formed at a low annealing temperature of 800 °C for 2 h with the Fe³⁺/Ba²⁺ molar ratio 8. The microstructure of the powders appeared as a homogeneous hexagonal platelet-like structure using 2% KNO₃ as the crystal modifier. A saturation magnetization (60.4 emu/g) was achieved for the BaFe₁₂O₁₉ phase formed at 1000 °C for 2 h with Fe³⁺/Ba²⁺ molar ratio 8 using 5 M NaOH solution at pH 10 in the presence of 2% KNO₃. Moreover, the saturation magnetization was 52.2 emu/g for the precipitated precursor at Fe³⁺/Ba²⁺ molar ratio 12 in was achieved for the precipitated precursor ultrasonicated for 1 h and then annealed at 1200 °C for 2 h. Coercivities from 956.9 to 4558 Oe were obtained at different synthesis conditions.     

Improvement in magnetic properties of La substituted BaFe12O19 particles prepared with an unusually low Fe/Ba molar ratio

In this study, effect of lanthanum substitution on the phase composition, lattice parameters and magnetic properties of barium hexaferrite has been studied in samples synthesized in ammonium nitrate melt. Samples, prepared with different lanthanum amount and having various initial Fe/(Ba+La) ratios in between 12 and 2 {(Ba_1−xLa_x)·n Fe₂O₃, where 0≤x≤1 and 1≤n≤6)}, are sintered at temperatures from 800 to 1200 °C. The lattice parameters, both a and c, decreases with increasing La amount which results in a decrease of the unit cell volume. The scanning electron microscope micrographs show that the pure and La-substituted sample with x=0.3, both calcinated at 1000 °C, have grain sizes smaller than 1 μm. The coercivities of the La-substituted samples increase with increasing La amount and reaches to a maximum value of 5.73 kOe, when x=0.3. Sintering at higher temperatures (above 1000 °C) decreases the coercivity, resembling a transition from single to multi-domain behavior of the particles, while saturation magnetization of the samples continues to increase due to the increasing grain size. Magnetization measurements of the samples prepared with different Fe/(Ba+La) molar ratios, n's, revealed that the specific saturation magnetization slightly increases with decreasing n, while coercivities fluctuates around 5.5 kOe. However, a sharp increase in the saturation magnetization has been observed in the sample having n=1 and washed in HCl. It was measured as 59.2 emu/g at 15 kOe, which is higher than that of the pure sample (57.5 emu/g). Thus, the magnetic parameters are optimized in the sample Ba_0.7La_0.3Fe₁₂O₁₉ so as to maximize both coercivity and specific saturation magnetization in the HCl-washed sample synthesized by starting with an unusually low Fe/(Ba+La) molar ratio of 2 (or n=1).     

Investigating the formation and magnetic properties of the Dy0.75Fe1.25O3 orthoferrite prepared by sol-gel method

In this paper, the Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles were synthesized successfully by sol-gel method. Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles are obtained by calcining the flakes at 600 and 700 °C. The magnetic properties of the different samples are investigated using Quantum Design MPMS SQUID magnetometer and MS-500 Mössbauer spectrometer. Magnetic phase γ-Fe₂O₃ coexists in the samples calcined at 600 °C and orthoferrite phase is completely recovered in the samples calcined at 700 °C. Although excessive Fe³⁺ ions were introduced, none of these iron spins couple magnetically with Dy³⁺ ions.     

Anomalous phase transformation in magnetostrictive Fe81Ga19 alloy

In this work, we have investigated the room-temperature phase constitution of heat-treated Fe₈₁Ga₁₉ alloys cooled from 800 °C at different rates. Results show that at cooling rates in the range from 0.43 to 0.26 °C/min, in addition to the A2 matrix, an fcc phase also can be observed in Fe₈₁Ga₁₉ samples at room temperature. To investigate the precipitation of the fcc phase out of A2 matrix, a systematic study of phase constitution was carried out on the samples quenched from different temperatures during cooling from 800 °C at 0.32 °C/min, which reveals an anomalous phase transformation between A2 and fcc. Precipitation of the fcc phase from A2 matrix occurs at 500 °C and its volume fraction exhibits a sharp increase at 400 °C. However, it begins to dissolve when further decreasing the temperature and only a minor fcc phase can be retained at room temperature, which suggests that the fcc phase is metastable below 400 °C. Magnetic measurements indicate that the precipitation of fcc phase deteriorates the saturation magnetization of Fe₈₁Ga₁₉.     

Synthesis of coprecipitated strontium hexaferrite nanoparticles in the presence of polyvinyl alcohol

Strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles were synthesized by the chemical coprecipitation method and using polyvinyl alcohol (PVA) as a protective agent. The synthesized samples were characterized by differential thermal analysis, X-ray diffraction, scanning and transmission electron microscopy, particle size analyzer, sedimentation test and vibrating sample magnetometer. In the presence of PVA, the single-phase SrFe₁₂O₁₉ nanoparticles were obtained at low temperature of 650 °C. The average particle size of SrFe₁₂O₁₉ precursor was 15 nm, which increased to 61 nm after calcination at 650 °C. The magnetic measurements indicated that PVA decreased coercivity from 4711 to 3216 Oe with particle size reduction. The results showed that PVA as a protective agent could be effective in decreasing the particle size, calcination temperature and coercivity of SrFe₁₂O₁₉ nanoparticles.     

AC magnetic susceptibility at medium frequencies suggests a paramagnetic behavior of pure water

The AC magnetic susceptibility of water is studied in the frequency range of 1 kHz-1 MHz at a low magnetic field. The results show a paramagnetic behavior between 500 kHz and 1 MHz for samples at 37 °C and 25 °C, which is surprising considering that water is believed to be diamagnetic. AC magnetic susceptibilities for ice and different salt solutions were investigated as well: in such cases the susceptibility is always negative and therefore, such samples remain diamagnetic.     

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.     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

Effect of nanocrystalline particles on the magnetic properties of Z-type hexaferrites

In order to improve the magnetic properties of Z-type hexaferrites such as high initial permeability and high-quality factor, the nanocrystalline hexaferrite particles (NHPs) with the same chemical composition were introduced. The influence of NHPs on the densification, microstructures and magnetic properties of the ceramics prepared by a combined method was investigated. The results show that these NHPs, which spread around the micron-sized hexaferrite particles (MHPs), enhance the densification by increasing the inter-diffusion of the particles due to the increase of contact area; simultaneously, the grain growth in the direction of c-plane is dominant. Due to the special microstructure, high sintering density, and no addition sintering aids with different chemical composition, relatively high initial permeability and Q-factor than those of the samples with 1.0 wt% Bi₂O₃ were obtained in the samples with proper nanocrystalline particles sintered at 900 °C.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.