Magnetic Field-Driven Dynamics of Microdisk Aqueous Suspension
V. Novosad, S.-T. Chui, V. Yefremenko, J. Pearson, S.D. Bader
Argonne National Laboratory, US
Keywords: ferrofluid, light modulation, fluid dynamics
Abstract:We investigated magnetic-field induced optical transmittance in suspension of magnetically soft Permalloy microdisks with diameters 1-3 um, 20-100 nm thick. The samples were fabricated using magnetron sputtering, optical lithography and lift off techniques. As expected, the hysteresis loop is typical for magnetization reversal due to nucleation, displacement and annihilation of magnetic vortices. With decreasing field from the saturated state, the magnetization gradually decreases, showing an abrupt jump at the nucleation field. At this field a single magnetic vortex is formed inside of each dot. When a small external magnetic field is applied, the vortex center displaces (reversibly), and therefore induces magnetic moment proportional to the field strength. As a result, the microdisks suspended in aqueous solution will physically rotate till the disk plane is aligned along the field direction. The transmitted light intensity is therefore rapidly increased when the magnetic field is ON, and decreases when field is OFF. We find that even 0.001 vol % FeNi microdisks subjected to 5 Oe ac field provides 10 % light intensity modulation with characteristic response time of 10-100 ms (rise, and fall, respectively). This is far superior to the earlier reported tens of minutes of the response time of superparamagnetic nanoparticles (Fe3O4) with 0.1 vol % concentration and 50 Oe magnetic filed.