Magnetic Permeability and Relaxation Frequency in High Frequency Magnetic Materials.
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Magnetic Permeability and Relaxation Frequency in High Frequency Magnetic Materials. M.I. Rosales, H. Montiel and R. Valenzuela Institute for Materials Research, National University of Mexico P.O. Box 70-360, Mexico D.F., 04510 Mexico.
ABSTRACT An investigation of the frequency behavior of polycrystalline ferrites is presented. It is shown that the low frequency dispersion (f < 10 MHz) of permeability is associated with the bulging of pinned domain walls, and has a mixed resonance-relaxation character, closer to the latter. It is also shown that there is a linear relationship between the magnetocrystalline anisotropy constant, K1, and the relaxation frequency. The slope of this correlation depends on the grain size. Such a relationship could allow the determination of this basic parameter from polycrystalline samples.
INTRODUCTION Many electronic devices such as switched-mode power supplies involve [1] the use of magnetic materials with high magnetic permeability (µrel > 1000) at high frequencies (f > 10 MHz). The active magnetization processes in the 1 - 50 MHz frequency range have been found to be domain wall movements and spin rotation [2,3]. At frequencies higher than ∼50 MHz, domain walls become unable to follow the excitation field, and only spin rotation remains. Recent studies [3,4] associate a resonance character to the permeability dispersion of domain walls. We think, however, that a more detailed investigation is needed. In this paper we present a study of the frequency behavior of polycrystalline Ni-Zn ferrites, with the aim of progressing also on the understanding of the influence of grain size on the relaxation frequency.
EXPERIMENTAL TECHNIQUES Polycrystalline ferrite samples in the formula NixZn1-xFe2O4 (with x = 0.30, 0.35 and 0.40) were prepared by the ceramic method, from the reactive grade oxide reagents NiO, ZnO and Fe2O3. The initial wet milling of raw materials was followed by press in the shape of toroids and sintering for various combinations of time and temperature (6-96 h at 1150 °C). The furnace atmosphere was oxidizing (100% O2 at 1 atm) in order to reduce the possibility of reduction of ferric to ferrous ions, which would reduce the electric resistivity and therefore would increase the frequency losses. Their grain size was determined by counting on scanning electron microscopy (SEM) micrographs of selected surfaces. Real and imaginary impedances were measured in a system [2] including an HP 4192 A Impedance Analyzer controlled by a PC computer. Measurements were carried out in the 5 Hz-13 MHz frequency range at temperatures from 110 to 450 K. Real and imaginary permeability U1.8.1
values were calculated from impedances by using the relationship µ* = KL* = (j/ω)Z*, where µ* is the complex permeability, K is a geometrical constant (for toroids), L* is the complex inductance, ω is the angular frequency (ω = 2πf), j is the basis of imaginary numbers (√-1) and Z* is the complex impedance. Note that the presence of j leads to a cross-over of values, since the real part of inductance (a
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