HIGH-FREQUENCY EMI NOISE SUPPRESSION BY POLYMER-BASED COMPOSITE MAGNETIC MATERIALS

The complex permeability and EM-wave absorption properties of hybrid polymer-based composite magnetic materials (with MnZn and LiZn ferrite fillers and PVC matrix) prepared with constant total filler content (65 vol%) and particle size (0-250 mu m) have been investigated in the 1-1000 MHz frequency range. Within this filler concentration the permeability of composites changed continuously with the change of ferrite filler content ratio between two types of ferrite fillers. The observed relaxation type of permeability dispersion was due to the domain wall and natural ferromagnetic resonance phenomena and was also attributed to the high damping of spin motion. Measured values of permeability were used to determine the EM-wave absorption properties (return loss RL, matching frequency f(m), matching thickness d(m) and bandwidth Delta f for RL <= -20 dB). The calculation of these properties was based on a model of single-layered absorber backed by a perfect conductor using transmission-line and EM-field theory. The composite with the volume fraction ratio of hybrid MnZn: LiZn ferrite filler set to 0.5:0.5 has shown a return loss of -57 dB (> 99 % power absorption) at f(m) = 714 MHz with the -20 dB bandwidth of Delta f= 232 MHz for an absorber thickness of 7.79 mm. The prepared composites can fruitfully be utilized for suppression of conducted and/or radiated EMI noise especially in wireless and portable electronic equipments.