Microscopic study of grain-boundary region in polycrystalline ferrites

Investigation of structural, dielectric, and magnetic properties of hard and soft mixed ferrite composites J. Appl. Phys. 112, 054323 (2012) Growth and ferromagnetic resonance of yttrium iron garnet thin films on metals Appl. Phys. Lett. 101, 082405 (2012) Magnetoelectric effects at microwave frequencies on Z-type hexaferrite Appl. Phys. Lett. 101, 062406 (2012) Temperature and frequency dependent giant magnetodielectric coupling in DyMn0.33Fe0.67O3 J. Appl. Phys. 112, 013920 (2012) Structure refinement and dielectric relaxation of M-type Ba, Sr, Ba-Sr, and Ba-Pb hexaferritesThe power loss of MnZn ferrites and its relation to average grain size and grain boundary properties was investigated. The power loss was found to be dependent on the average grain size and on the highly electrically resistive grain boundaries which are formed by introduction of aliovalent ions in the outer grain region of a MnZn ferrite. Analysis of ac impedance data, assisted by Auger electron spectroscopy, elucidated the chemical and electrically diversity of doped and undoped samples and its influence on eddy current loss.

Section: Resultsmentioning

confidence: 99%

“…Extensive grain boundary analyses of MnZn ferrites were performed in the past [14][15][16][17][18] where the grain boundary composition in the outer layer of MnZn ferrite grains was considered. Our results for grain boundary analyses are consistent with those previously reported.…”

Section: Resultsmentioning

confidence: 99%

Highly resistive grain boundaries in doped MnZn ferrites for high frequency power supplies

Drofenik

Žnidaršic̆

Zajc

1997

“…So, it should also be noted that the static dielectric constant of boundary layers are quite different from the bulk grains. In fact, Ca oxides contained in the powder are know to segregate under impurities forms in the boundary regions, producing at these places magnetic and dielectric properties far from that of ferrite crystallites 17,18,19 . Since the ferrite impedance is modelled by series resistance-capacitance elements, the anomalously high apparent dielectric constant of ferrites is a consequence of the high capacitance of the thin boundary regions connected in series by highly conductive bulk grains.…”

Section: Resultsmentioning

confidence: 99%

An analysis of Mn-Zn ferrite microstructure by impedance spectroscopy, scanning transmission electron microscopy and energy dispersion spectrometry characterizations

Loyau

Wang

Lobue

et al. 2012

AC resistivity measurement results on Mn-Zn sintered ferrite were analyzed in the 0.1-500 MHz range. From electrical point of view, the material could be represented by an equivalent circuit of parallel resistance-capacitance cells connected in series corresponding to the contributions from bulk grains in one hand, and grain boundary layers in the other hand. The experimental resistivity curves were fitted with the model. The as obtained parameters give information on dielectric properties and conductivity of both bulk grains and boundary layers. For the studied material, it appears that the resistivity at low frequencies is increased 27 times due to the boundary layers effects. STEM (Scanning Transmission Electron Microscopy) and EDS (Energy Dispersion Spectrometry) characterization where performed in order to detect impurities at a grain boundary layer which can explain those wide differences between bulk grains and boundary layers electrical properties. It appears that the two components have close chemical compositions, but some calcium impurities segregate at the boundary which increases dramatically the resistivity of these layers. Furthermore, the bulk grains show relative permittivity around 350 at low frequency which is much smaller than the one measured for the whole material which is in the 50,000-100,000 range. This giant-dielectric behaviour can be explained by an internal barrier layer (IBLC) at the grain boundaries. At last, the components of classical eddy current losses including losses due to ohmic effects and (true) dielectric losses on both bulk grain and boundary layers are distinguished.

“…͑2͒ is used to describe a region starting from the edge of the grain and with a width of a few nanometers, where the regular crystallographic and chemical periodicity of the crystal are disturbed. The origin of the existence of such a NMGB is basically attributed to ͑a͒ the misfit of the lattices of adjacent grains which is an inevitable by-product of the sintering process 26 and ͑b͒ the concentration of chemical secondary phases due to impurities or doping at the grain boundaries. 23,27 In the NMGB model, the grain boundary is described only by it's thickness ␦ and the specific resistivity of the grain boundary is not taken into account.…”

Section: Discussionmentioning

confidence: 99%

Materials processing issues influencing the frequency stability of the initial magnetic permeability of MnZn ferrites

Hanuszkiewicz¹,

Holz²,

Eleftheriou

et al. 2008

The frequency behavior of the initial permeability of MnZn ferrites is generally described by Snoek’s law related to gyromagnetic resonance. However, in polycrystalline systems, for the same initial permeability, the frequency stability can be tailored by modifying the specific resistivity of the grain boundaries. This is achieved through the accumulation of high resistivity doping phases along the grain boundaries. The grain boundary specific resistivity may decrease or increase with increasing the average grain size depending on the ferrite composition and the extent of dopant segregation. In order to explain the experimental results and to account for the permeability dependency on the specific resistivity, a modification of the nonmagnetic grain boundary model is proposed to a less magnetic grain boundary model, where not the thickness but the initial permeability of the grain boundary layer is the main variable that is influenced by the process parameters.