Pablo Hernández-Gómez

Thermal expansion of Ti-substituted barium hexaferrite

Thermal expansion measurements in the range of 20–500 °C were carried out on both poly- and single crystalline samples of the hexagonal magnetoplumbite ferrite with composition BaTiFe11O19. The continuous scanning of the thermal expansion reveals the existence of a λ-type anomaly near the Curie temperature. In contrast to the usual observed anisotropic behavior for magnetoplumbite hexaferrites, the averaged thermal expansion coefficient for the a direction is larger than for the c direction, i.e., 12.8×10−6 and 9.5×10−6 °C−1, respectively.

An approach to the magnetic disaccommodation in Ti-substituted Ba–W hexaferrites

The relaxation of the initial permeability has been measured in polycrystalline Ti-doped Ba–W hexaferrites with nominal composition BaTixFe18−xO27, with x ranging among x=0 and 0.3. The samples have been sintered at different temperatures in the 1250 °C<T<1350 °C range in a reducing CO2 atmosphere and rapidly quenched to promote the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as main phase. In the temperature range between 80 and 500 K, the time decay of the initial permeability after sample demagnetization has been plotted by means of isochronal curves. The isochronal disaccommodation spectra show an increase in the amplitude of the different disaccommodation peaks in substituted samples regarding pure Ba–W hexaferrites. These peaks are centered at 180, 300, and 380 K and they are related to the presence of both ferrous cations and lattice vacancies. The increase of the disaccommodation maxima in Ti-substituted hexaferrites is ascribed to the additional amo...

Effect of annealing temperature and substitution of Zr-Cu on magnetic properties of strontium hexaferrite nanoparticles

ZnTe and ZnTe:Cr films were prepared onto glass substrates using the thermal evaporation method. Structural properties of the prepared samples were analyzed using X-ray diffractometry, and the presence of a ZnCrTe phase was identified along with poor crystallinity. Composition analysis was done using XPS and the Cr content in the film was found to be 0.05 atomic percent. Transmittance spectra were recorded using UV-Vis spectrophotometry. The valence state of Cr in ZnTe:Cr film is determined to be +2 using electron spin resonance (ESR) spectroscopy. Magnetic moment data as a function of magnetic field were recorded using a Superconducting Quantum Interference Device (SQUID) magnetometer at temperatures of 5, 77 and 300 K. The results showed minority ferromagnetic behavior even at room temperature. Magnetic domains were observed using Magnetic Force Microscopy and the average domain size is 3.7 nm.

Magnetic Disaccommodation in Ferrites with Nonmagnetic Divalent Substitutions

The after-effect processes in magnetite with nonmagnetic divalent substitutions are studied by means of disaccommodation techniques. We have recorded at room and high temperatures the time evolution of the initial magnetic susceptibility after sample demagnetization for polycrystalline specimens with nominal composition MxFe3-xO4+?, M being a divalent cation such as Zn2+, Mg2+, or Ca2+. These ions replace Fe2+ inside the spinel structure and, consequently, affect the original process III in magnetite. We have found that the presence of these ions involves the splitting of process III into similar processes, according to the position of the substituting ions. In addition, high-temperature processes are discussed on the basis of vacancy-mediated diffusion of nonmagnetic metallic ions.

Analysis of magnetic aftereffects in strontium hexagonal ferrites with W-type stoichiometry

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of W phase (SrO⋅9Fe2O3). The samples have been prepared by means of standard ceramic techniques at different temperatures in the 1250 °C<T<1420 °C range in air as well as CO2 sintering atmospheres and then rapidly quenched to provide the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as the main phase. In the temperature range between 80 and 500 K, the magnetic disaccommodation is represented by means of isochronal curves. The isochronal spectra show the presence of different after-effect processes whose maxima lie at 365 K, 300 K, and 165 K (respectively, A, B, and D peaks). They correspond to thermally activated relaxation processes with activation energies of 1 eV, 0.8 eV, and 0.5 eV, respectively. Processes A and B are connected to ionic reorientations of ferrous cations and lattice vacancies within the octahedral sites in spinel-like...

Measurement at microwave frequencies of the magnetic properties of small quantities of powdered or diluted samples

Transmission line techniques are a convenient way to determine the electromagnetic properties of a variety of materials in the ranges of radio and microwave frequencies. Traditional methods based on the measurement of the four scattering parameters can be successfully replaced for the method presented here, in which no change in the geometry is needed, since two independent measurements are carried out, with and without an applied magnetic field. In addition, given the small size of the sample holder, the required amount of material can be drastically reduced, and allow the use of a lumped circuit model, then reducing the inherent difficulties associated with the use of distributed parameters. Even though this kind of model requires the involved wavelengths to be much larger than the size of the system, this requirement can be overcome as long as the tested materials have relative ϵ or μ lower than 100. Furthermore, the use of short-circuited transmission lines simplifies sample holding and systematizes t...