This study is aimed at the manufacture and the magnetic properties of polycrystalline M-type hexaferrites BaFe12O19 (barium ferrite, or BaM) materials of different magnetic texturing grades, going ...from a random distribution of the BaM crystallites to their almost complete stacking. Our target is to optimize the value of reduced-remanence magnetization MR/MS, which is among the most significant features of the self-polarized materials. In this study, we focus on the role played by the precursors hematite (isotropic spherical shape) and goethite (anisotropic lath shape). Therefore, 11 samples with a flat cylinder shape are fabricated, with an increasing hematite to goethite ratio. We demonstrate that this ratio drives the texturization of the samples by producing self-polarized materials with different MR/MS from the simple green compaction of the precursors, followed by a heat treatment. Most importantly, our study reveals the orientation of BaM particles after compaction; therefore, MR/MS, is strongly influenced by the aspect ratio of the lath-shaped goethite crystallites. Additionally, we show that finer goethite crystallites yield higher-value MR/MS. We optimize the aspect ratio of the goethite crystallites for an improved BaM texture. The optimization of the morphology of the goethite crystallites leads to an increase in the BaM particles’ orientation and stacking. The salient outcome of this work, which distinguishes it significantly from recent works, is that the particles stacking increases with the value of the shape factor η (defined as the ratio of the diameter of the laths to their length) of the goethite, evidenced by XRD results. The Rietveld refinements of powder diffractograms and the measured magnetic properties reveal a particle-stacking enhancement caused by not only the ratio of hematite: goethite but mainly by an optimal aspect ratio of the goethite crystallites. Based on this study, the BaM materials are further manufactured with a controlled magnetic texture; thus, they are partly self-polarized. They show reduced-remanence magnetization MR/MS varying from 0.5 and 0.81, while the angular dispersion of the BaM particles’ easy axis of magnetization varies from 60° to 10°. The magnetic properties of the samples are further studied in microwave experiments, from which the value of the magnetocrystalline anisotropy field HK = 16.6 kOe is deduced. The first magnetization curves of each sample are obtained using a VSM. A law of approach to the saturation suitable for the case of the uniaxial polycrystalline materials, and for which the particle stacking is only partial, is proposed for the fitting of the magnetization process. It is suggested that by using the proposed law with a known magnetocrystalline anisotropy constant K1, the angular grain-dispersion can be found.
M-type hexagonal barium ferrites Ba(M x Ti x )Fe 12-2 x O 19 (M = Co or Zn, x = 0-0.5) were synthesized by chemical topotactical reaction. Both randomly oriented and self-polarized samples were ...prepared. Experimental values of the anisotropy fields, first and second anisotropy constants, and remanent magnetizations were extracted by combining hysteresis loops and microwave measurements. A salient result of this study is that, compared with K 1 , K 2 cannot be disregarded for these co-doped BaM hexaferrites, even at low doping levels. Results show that the BaM ferrites of composition Ba(Co x Ti x )Fe 12-2 x O 19 are more suitable for microwave applications that operate at frequencies between 30 GHz and 40 GHz.
This study presents the performance of a magneto-dielectric material (MDM) specially developed to enable the tunability of antenna devices operating in the VHF band. A current flowing through the ...sample, which creates a DC magnetic field inside the sample with the same symmetry as the magnetic domains, controls the magnetic susceptibility variation of the MDM. By changing the magnetization state of the MDM with a low DC magnetic field of 740 A.m-1, a tunability of 40% can be observed. The MDM tunability at VHF frequencies is also highlighted by microwave measurements for different magnetization states.
Self-biased circulators and isolators require pre-oriented hexaferrites with a high degree of orientation in order to ensure a magnetization perpendicular to the plane of the hexaferrite sample. In ...this paper, a strontium hexaferrite is selected for this application. Angular Remanence Measurements and XRD are performed in order to quantify the distribution of magnetization directions in this polycrystalline hexaferrite. The extracted distribution law was used to develop a model to predict the evolution of remanent magnetization as a function of shape factor. This model is then used to design a self-biased isolator in Ka-band. Insertion loss of 0.85 dB at 38 GHz was demonstrated with a quite good agreement with simulations.
M-type hexagonal barium ferrites Ba(MxTix)Fe 12−2x O 19 (M=Co or Ti, x=0-0.5) are synthesized by chemical route. Experimental values of the anisotropy fields HK are extracted from microwave ...measurements. It is shown that HK values are very close to the higher switching field. Results show that the shift through lower values both for HK and the switching field distributions are the largest for CoTi substitutions. A qualitative analysis based on the cations occupation sites is proposed.
M-type hexagonal barium ferrites Ba(M x Ti x )Fe 12−2x O 19 (M=Co or Ti, x=0-0.5) are synthesized by chemical topotactical reaction. Both randomly oriented and self-polarized samples are prepared. ...Experimental values of the anisotropy fields, first and second anisotropy constants, and remanent magnetizations are extracted by combining hysteresis loops and microwave measurements. A salient result of this study is that, comparing with K1, K2 cannot be disregarded for these co-doped BaM hexaferrites, even at low doping levels. Results show that the compositions Ba(CoxTix)Fe12−2xO19 are more suitable for microwave applications that operate at frequencies between 30GHz and 40GHz.
3D printing of microwave absorbers gave rise to new shape optimization possibilities. However, new printable materials dedicated to this application have to be developed. In this study, an iron flake ...polymer composites was elaborated and characterized. Scanning Electron Microscopy observations demonstrated that iron flakes particles are naturally aligned along the extrusion flow direction without applied magnetic field during the 3D printing process. Magnetic anisotropy was demonstrated with both Vibrational Sample Magnetometer and microwave measurements. Indeed, at 5.4 GHz, a ratio of 1.71 between parallel and perpendicular susceptibility was demonstrated. These measurements demonstrated a self-induced magnetic anisotropy that can be used to control the angular robustness of free-space microwave absorbers.
This study presents the performance of a magneto-dielectric material (MDM) specially developed to enable the tunability of antenna devices operating in the VHF band. The magnetic susceptibility ...variation of the MDM material is controlled by a current flowing through the sample which creates a DC magnetic field inside the sample with the same symmetry as the magnetic domains. By changing the magnetization state of the MDM with a low DC magnetic field of 740 A/m, a tunability of 40% can be observed. The MDM tunability at VHF frequencies is also verified by microwave measurements for different magnetization states.
