Submicron Magnetic Particles of ${\rm Mn}_{0.25}{\rm Fe}_{2.75}{\rm O}_{4}$ and Their Magnetorheological Characteristics

The single crystalline ferrite magnetic Mn_0.25Fe_2.75O₄ particles were synthesized via a solvothermal reaction method and applied as a magnetorheological (MR) material when being dispersed in a nonmagnetic oil. Field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HR-TEM) measurements showed that the average size of the monodispersed particles was 344 nm with single crystalline spots in the selected-area electron diffraction patterns. The crystal structure was determined to be cubic spinel with a lattice constant α₀=8.398 Å. According to the magnetization curves at 4.2 and 295 K, the saturation magnetization and coercivity of Mn_0.25Fe_2.75O₄ microspheres are determined to be 92.5, 72.8 emu/g, and 18.86, 4.53 kA/m, respectively. MR performance of the Mn_0.25Fe_2.75O₄-based MR fluid was measured by using a rotational rheometer with a parallel-plate geometry. Both controlled shear rate and controlled shear stress modes were applied to the loaded MR fluid sample at various magnetic field strengths. Dynamic and static yield stresses obtained from the two modes were compared and analyzed using a power law.