Size effect of iron oxide nanorods with controlled aspect ratio on magneto-responsive behavior

Iron oxide (Fe₃O₄) has been widely used as a magnetic material for magnetorheological (MR) fluids due to its facile geometric control, magnetic properties, and surface functionalization. However, studies on controlling the aspect ratio of iron oxide nanoparticles applied to MR fluids have rarely been reported due to difficulties in preparing anisotropic magnetic particles. To gain insight into the application of anisotropic nanoparticle in MR fluids, in this work iron oxide nanorods are prepared by using β-FeOOH nanorods as a template. The size of the β-FeOOH nanorods is controlled by adding polyethyleneimine (PEI) as the size controlling agent, and further reduction results in iron oxide nanorods. The resulting iron oxide nanorods are uniform in size and shape and exhibit superparamagnetic properties and high magnetic saturation, which is suitable for MR applications. The influence of particle size of the iron oxide nanorods on MR properties is evaluated, and the result shows the MR fluids prepared with larger size nanorods exhibit higher shear stress due to the higher magnetic saturation ascribed to larger crystalline domain. Consequently, gaining control over the size and aspect ratio of the magnetic particles can lead to high performance MR fluids.