Hexaferrite Platelets Research Articles

Size controlled growth of barium hexaferrite platelets using salt assisted sol-gel technique

BaFe12O19 (BaM) platelets of size ranging from 0.15 to 0.90 μm were synthesized using a salt assisted sol-gel technique. The structural and morphological characteristics of the hexaferrite particles were analyzed using the x-ray diffraction technique and scanning electron microscopy. During the high-temperature calcination process, salts like KCl, KBr, KI, and K2CO3 serve as a matrix for the formation of barium hexaferrite platelets from its precursor mixture, BaCO3, and ϒ-Fe2O3. The salt matrix prevents possible particle coarsening during the high-temperature treatment and promotes the growth of plate-like shaped particles. The width and thickness of hexaferrite platelets increase with a decrease in the melting temperature of potassium halide salts. The coercivity at 300 K of the barium hexaferrite particles was analyzed as a function of the lateral dimension of platelets. Barium hexaferrite platelets of width ~0.20 μm yielded a coercivity as high as 4.6 kOe and magnetization of 71 emu/g. The outlined synthesis route is suitable for realizing isolated hard ferrite platelets with improved intrinsic coercivity.

Morphology control of hexagonal strontium ferrite micro/nano-crystals

In this study, controllable morphology evolution of hexagonal strontium ferrite (SrFe12O19) micro/nano-crystals has been demonstrated. Single phase strontium ferrite platelets with hexagonal morphology were successfully prepared by conventional ceramic process. In the hexagonal crystals, it is revealed that the anisotropic growth rate is changed, with the increasing of ball milling time, from relatively high rate along the <00l> direction (c-axis) to <hk0> direction, leading to the morphology evolution. Moreover, the optimal saturation magnetization (MS) is 69.5 emu/g, which is intensely close to the theoretical value (72 emu/g). This study provides the direct evidence of the enhanced reaction activity induced by high energy ball milling in strontium hexaferrite platelets and the obtained SrFe12O19 particles are promising for the hard magnet application and the magnetoelectric electronics.

Current-induced nonlinear magnetoelectric effects in strontium hexaferrite

We report on the observation of nonlinear magnetoelectric effects at room temperature due to a dc current in the ferrimagnetic $M$-type strontium hexaferrite platelets. Utilizing microwave measurement techniques and data on the shift in magnetic mode frequencies, it was found that a dc current along the hexagonal $c$ axis resulted in a significant decrease in the saturation magnetization and an increase in the uniaxial magnetocrystalline anisotropy field. These changes in the magnetic order parameters were directly proportional to the square of applied electric field and were found to be much higher than variations due to Joule heating. A phenomenological theory that takes into account the current-induced magnetobielectric (MBE) effects is proposed. Expressions for coupling coefficients for MBE effects have been obtained and have been calculated from the variations in magnetic order parameters. The electric field $E$ (or current) tuning of the magnetic modes in $\mathrm{Sr}M$ reported here is orders of magnitude stronger than strain mediated $E$ tuning of magnetic resonance in hexaferrite-ferroelectric composites. The nonlinear magnetoelectric effects in hexaferrite, therefore, open up an avenue for the realization of $E$-tunable broadband microwave and millimeter wave ferrite signal processing devices such as resonators and filters.

Monolithic Magneto-Optical Nanocomposites of Barium Hexaferrite Platelets in PMMA

The incorporation of magnetic barium hexaferrite nanoparticles in a transparent polymer matrix of poly(methyl methacrylate) (PMMA) is reported for the first time. The barium hexaferrite nanoplatelets doped with Sc3+, i.e., BaSc0.5Fe11.5O12 (BaHF), having diameters in the range 20 to 130 nm and thicknesses of approximately 5 nm, are synthesized hydrothermally and stabilized in 1-butanol with dodecylbenzenesulfonic acid. This method enables the preparation of monolithic nanocomposites by admixing the BaHF suspension into a liquid monomer, followed by in-situ, bulk free-radical polymerization. The PMMA retains its transparency for loadings of BaHF nanoparticles up to 0.27 wt.%, meaning that magnetically and optically anisotropic, monolithic nanocomposites can be synthesized when the polymerization is carried out in a magnetic field. The excellent dispersion of the magnetic nanoparticles, coupled with a reasonable control over the magnetic properties achieved in this investigation, is encouraging for the magneto-optical applications of these materials.

Morphology and magnetic properties of barium hexaferrite ceramics synthesized in xwt% NaCl-(100−x) wt% KCL molten salts

Micron size barium hexaferrite (BaFe12O19) platelets were prepared by the molten-salt synthesis method in various weight proportions of NaCl–KCl salt mixtures as a liquid medium. The effect of molten salt composition—x wt% NaCl and (100−x) wt% KCl—on the amount of barium hexaferrite phase formation, as well as, on the morphology and magnetic properties of the final products are discussed. Inductively coupled plasma-mass spectroscopy (ICP-MS) was used to determine the solubility of the starting materials in the salts in order to understand the formation mechanism of barium hexaferrite. X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) were used to identify the characteristics of the synthesized barium hexaferrite platelets. ICP-MS analysis showed that the Fe2O3 solubility is negligible, however, BaCO3 has very high solubility in both molten KCl and NaCl. Quantitative XRD and SEM results showed that molten salt containing 100 wt% NaCl at 900 °C (for 2 h) resulted in the highest production of barium hexaferrite. Further, SEM results showed that KCl-rich molten salt led to the formation of sharper faceted platelet morphology, whereas NaCl-rich ones resulted in more round platelets. Data from magnetic measurements showed that as the content of NaCl in the molten salt increases, hysteresis losses became higher. This is a characteristic of the achievement of a harder magnetite behavior in the synthesized barium hexaferrite ceramics. The composition of the KCl/NaCl molten salts was shown to play an important role on the extent of barium hexaferrite formation, resulting platelet morphology and the magnetic properties.

Finite-size scaling relation of the Curie temperature in barium hexaferrite platelets

High-temperature magnetic measurements were carried out on barium ferrite BaFe12 O19 nanoparticles coated with amorphous silica. We find that the Curie temperature of this material decreases with decreasing particle size, in agreement with the finite-size scaling theory. In contrast to what one expects, the observed particle-size dependence of the Curie temperature does not follow a finite-size scaling relation for a zero-dimensional magnetic system. Instead, the data follow a finite-size scaling relation for a two-dimensional magnetic system with the scaling exponent ν = 0.78±0.06. The validity of the two-dimensional scaling relation in this material is due to the fact that the nanoparticles have a platelet-like shape.

M-type barium hexaferrite synthesis and characterization for phase shifter applications

M-type barium hexaferrite films have been grown by liquid phase epitaxy and examined by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and conventional and Lorentz-mode transmission electron microscopy (TEM). These films exhibit a diamond chevron shaped “brick wall” microstructure with c-axis oriented hexaferrite platelets. The films are oriented with their c axes in-plane, and parallel to the M-plane sapphire substrate, and exhibit a 30° rotation about the c axis with respect to the substrate. Rocking curves showed (20−20) and (22−40) FWHM values of 1.09° and 1.56°, respectively, for the thinner of two samples, and 0.31° and 0.50° for the thicker sample. The magnetic domain structures have been characterized by Lorentz-mode TEM and the domain walls were found to be pinned to small angle tilt boundaries. Using the measured rocking curve values, the effect of the overall crystalline misorientation on the dispersion of the magnetocrystalline anisotropy of the samples is estimated to be less than half a percent.

Magnetic and microwave absorption properties of W-type Ba(ZnxCo1−x)2Fe16O27 hexaferrite platelets

W-type barium hexaferrite platelets Ba(ZnxCo1−x)2Fe16O27 (0.1 ≤ x ≤ 0.9) were prepared by a chemical coprecipitation route. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 0.5 to 1.5 μm. With increasing x, the coercivity decreased, whereas the saturation magnetization increased. Microwave absorption properties were investigated in the frequency range of 8–12 GHz. An optimal reflection loss (−24.8 dB) reached at 10 GHz for x = 0.5 with a thickness of 2 mm. These results indicated that our samples might be a potential candidate for permanent magnets and microwave absorption materials.

Barium Hexaferrite Prepared by Hydrothermal Synthesis

Nanoparticles of barium hexaferrite were prepared by controlled hydrothermal synthesis at 280oC. During the synthesis, the composition of the precursor and the suspension concentration were controlled. The as-synthesized crystalline barium hexaferrite platelets of about 50 nm in length and 5 nm in thickness exhibited a saturation magnetization of 40 emu/g.

MMIC circulators using hexaferrites

MMIC circulators operating at K/sub A/-band have been fabricated from single crystal and polycrystalline hexaferrite platelets integrated into glass-microwave integrated circuit wafers. Measurements on circulators utilizing single crystal BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ showed two-band circulation, below-FMR (at 36 GHz) and above-FMR (at 24 GHz), while those using single crystal BaFe/sub 12/O/sub 19/ showed above-FMR circulation at 32-35 GHz. Self-biased circulators using SrFe/sub 12/O/sub 19/ ceramics showed above-FMR circulation at 32 GHz. All circulators showed reasonable insertion losses and isolation, with the best insertion loss of 1.2 dB at an isolation of 18 dB being found for a BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ circulator operating below-FMR.

Magnetism and magneto-optics of hexaferrite layers

Recent contributions to research in magnetism and magneto-optics of hexaferrite layers, resulting from the collaboration between the above-mentioned institutions, are comprehensively reviewed. The pulsed laser deposition (PLD) technique is described and its main features, relying on the plume diagnostics and correct oxygen pressure, both being important for the deposition of hexaferrites of complex stoichiometry, are highlighted. The fabricated layers were investigated structurally and it was found that they are highly textured with the c-axis perpendicular to the film plane. Their magnetization was measured over a wide temperature range, 4.2–300 K, and in fields up to 12 T. Its dependence upon the cobalt content x in BaFe 12− x − yCo x Ti y O 19 was also determined in the interval 0 ⩽ x ⩽ 0.8. The results were interpreted in terms of Néel theory and this, when combined with our results of Mössbauer spectra measurements, led to the formulation of a consistent model for the cation distribution in CoTi-substituted barium hexaferrites. The hysteresis-loop measurements provided data for obtaining values of anisotropy, which are in agreement with those of the bulk materials. The domain structure of thin hexaferrite layers was also studied, particularly the domain period dependence upon the sample thickness and cobalt content. The domain period dependence was found to be in very good agreement with theoretical micromagnetic calculations. Ellipsometry, reflectance photometry and Kerr/Faraday polarimetry were used to determine the optical and magneto-optical properties of hexaferrite platelets and thin layers. The complex refractive index and magneto-optic parameter were determined over the spectral range 350–850 nm and the reliability of the data was tested by comparison with photometric measurements of reflectance. The Faraday rotation and absorption spectra of substituted hexaferrite thin layers were measured in the 500–2000 nm wavelength range at room temperature and 80 K. The results obtained are interpreted in terms of single-ion optical electron transitions belonging to either cobalt or iron cations occupying the tetrahedral and/or octahedral positions in the spinel block of hexagonal ferrite crystal lattice.

Domain structure of thin single crystal platelets of BaFe12-x-yCoxTiyO19

The domain structure in hexaferrite platelets was studied for various cobalt concentrations, 0≤x≤0.78, using the colloid-SEM method, with a view towards the possible applications of thin films of such materials in high density magnetic recording. Typical surface undulation of Bloch walls was observed in thicker platelets. The domain period decreases with decreasing crystal thickness and also with increasing cobalt concentration. The critical sample thickness for the transition from undulated to simple stripe structure was measured on a wedge-form sample and compared with the theoretical estimates.

Pair correlations within dispersions of strongly interacting BaFe particles

Barium hexaferrite has widely been reported as a particulate medium eminently suitable for high-density perpendicular recording. In this paper we report the results of Monte Carlo simulations of strongly interacting barium hexaferrite platelets, in a precoated, dynamic dispersion. Magnetization, light transmission, and particle correlation characteristics are investigated as a function of external applied fields up to 1000 Oe. As a result of magnetostatic particle interactions, these platelets are revealed to stack readily in zero external applied field, and show well-defined anisotropic stacking in the presence of a field.

Interaction effects in longitudinally oriented and non-oriented barium hexaferrite tapes

The observed magnetic interactions in barium hexaferrite tapes as a function of dispersion milling time and orientation are reported. Measurement of the interaction state is obtained by comparing the magnetizing and demagnetizing remanence curves. Variation in the interactions as a function of dispersion milling time in longitudinally oriented and nonoriented tapes indicates substantial changes in the interaction state of the constituent magnetic particles. In particular, the oriented tape samples exhibit an increasing net magnetizing interaction effect, whereas the nonoriented samples show a decreasing net magnetizing effect. These two markedly different effects are explained in terms of the improved dispersion and separation of barium hexaferrite platelets, which reduces the interaction parameter in the case of nonoriented tapes. In the longitudinally oriented form, however, the increased freedom of movement (associated with improved dispersion) results in the enhanced formation of stacking and particle columns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theory of domain-wall motion in magnetic films and platelets

We calculate dynamical properties of plane and cylindrical magnetic domain walls in a uniaxial film or platelet whose plane is perpendicular to the easy axis. First, we calculate the internal structure of a freely moving wall to determine the nonlinear velocity-momentum relation. Using a Bloch-line approximation to the kinetic wall energy, we find that stray magnetic fields emanating from surface poles destabilize the structure at a critical velocity Vp, above which uniform motion is not possible. For a plane wall, we have Vp=24γA/hK1/2, where γ is the gyromagnetic ratio, A is the exchange stiffness, h is the film thickness, and K is the anisotropy. This velocity is usually much less than the critical velocity for bulk wall motion derived by Walker. Combination of the velocity-momentum relation with the conservation of momentum provides a simple method of calculating the time-dependent velocity in the presence of a small drive field and small viscous damping. For very small drive, the terminal velocity equals that given by the conventional linear-mobility theory. However, for small drives exceeding a critical value corresponding to Vp, a periodic cycle of Bloch-line generation, motion, and annihilation takes place at a frequency approaching the Larmor frequency corresponding to the driving field. This is accompanied by a synchronous nonsinusoidal oscillation of the wall, with a mean forward velocity tending to an asymptote V0. Our relation V0=7.1γA/hK1/2 agrees within a factor of 3 with the onset of nonlinear behavior observed by means of bubble collapse in certain low-loss garnet specimens. Partial support is also given by data for hexaferrite platelets. For radial motion of cylinders with finite diameter D, numerically computed values of V0 are larger but never exceed this expression by more than 17%. The critical velocity Vp also varies little for D/h&amp;gt;2, but varies rapidly and attains large values for small D/h.