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

Abstract

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.

Frequency dispersion model of the complex permeability of soft ferrites in the microwave frequency range

“…Particularly, average grain size and relative density play an important role in the magnetization process of polycrystalline ferrites: complex magnetic permeability increases with the average grain size and relative density. 2,3,8,9,11,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]39 However, very few models relating magnetic permeability to microstructure have been developed in the past, and all of them have been uniquely tested at a specific constant angular frequency. In this regard, it is worth mentioning the models proposed by Globus, 36 Rikukawa, 38 Johnson and Visser, 25 and Pankert.…”

A comprehensive approach to broadband characterization of soft ferrites

“…But the physical interpretation of the ferrite behaviour, namely magnetic loss and permeability, across the broad frequency domain useful for the various applications in electrical, electronic, and telecommunication devices, lags behind today's technological developments. This derives in part from lack of comprehensive magnetic characterization, typically restricted in the literature to relatively narrow ranges of frequency and polarization values [3][4][5]. These materials do actually display a complex magnetic phenomenology, where domain wall (dw) processes and magnetization rotations, combining in comparable proportions under DC excitation, evolve with the magnetizing frequency through different dissipation channels, descending from eddy current and spin damping phenomena.…”

“…The typically measured resistivity values are in fact compatible, contrary to the case of the nearly insulating Ni-Zn ferrites, with the existence of eddy current generated losses [1,5,6,7]. Efforts are therefore spent towards loss reduction by increasing the resistivity through addition of segregating oxides, like CaO, SiO2, Nb2O5, TiO2, and fine control of the partial pressure of oxygen during sintering [4,8,9]. It can be shown, however, that the eddy current loss contribution can be suppressed, at least below a few MHz, if the sample is sufficiently mall [10].…”

“…These quantities are shown for the N87-type ferrite, where <s> = 16 m, in Table I. The estimated  value, of the order of the elementary cell size, appears somewhat lower than the actual grain boundary thickness, as revealed by electron transmission microscopy [4,5]. One might actually consider  as an electrically equivalent thickness, which is expected to attain the nanometer value in ferrites with high relative permittivity (say around 10 5 ) [12].…”

MnZn-ferrites: Targeted Material Design for New Emerging Application Products

“…Along the main guidelines described in paragraphs 2.1 and 2.2 a new polycrystalline MnZn-ferrite material has been synthesized by the conventional ceramic method (solid state oxide method [6]), the properties of which are summarized in Table 1 [5]. The material is suitable for wireless power charging applications and fulfils all set specifications.…”