1. Introduction

Applications utilizing magnetic functional fluids having both micrometer- and nanometer-sized magnetic particles have been developed [1]. The size of clusters, the meso-scale structure formation of particles and the cohesion between the magnetic particles are affected by the mixture ratio of the magnetic particles of different sizes in the magnetic functional fluids. Therefore, resistance force properties of the shock absorber using the magnetic functional fluids are influenced by the mixture ratio of the micrometer- and nanometer-sized magnetic particles. In this paper, resistance force of a shock absorber is investigated experimentally. Two magnetic functional fluids whose mixture ratio of micrometer- and nanometer-sized magnetic particles is different are prepared for the experiment. The resistance forces are measured when load is applied to the shock absorber under magnetic field.