Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1804626 | Journal of Magnetism and Magnetic Materials | 2006 | 8 Pages |
Abstract
A very stably dispersed magnetic fluid (mother MF) and its 1000-times diluted solution were independently zero-field-cooled from room temperature to 5Â K followed by application of a magnetic field of 2.86Â MA/m for 300Â s. After the field was removed (t=0), its residual magnetization M was measured as a function of time t for 80Â 000Â s. After measurement, the MF sample was heated to room temperature, and the experiment was repeated after cooling to 5Â K and again applying and removing the 2.86Â MA/m field. We performed the same experiment several times, and obtained a different M vs t curve each time. With each cycle, the average M increased and the M vs t curve converged to a universal curve. In the initial few cycles, the value of M is very small, fluctuates and surprisingly increases with t in some time region. These characteristics are common in both the mother MF and diluted MF. We consequently propose the following physical model. When the MF is cooled, the isolated surfactant molecules in the solvent trigger the generation of magnetic colloid micelles. In other words, there occurs a phase transition from the magnetic colloids' monodispersed phase to a micelle phase. The magnetic dipoles of the micelle's colloids make a closed magnetic flux loop. That is the origin of the anomalously small value of the residual magnetization in the early cycles. After a certain time elapses the micelles spontaneously break due to their residual stress, and a finite magnetic moment of the individual micelle develops. Consequently, M increases with t during this period.
Keywords
Related Topics
Physical Sciences and Engineering
Physics and Astronomy
Condensed Matter Physics
Authors
Susamu Taketomi, Rosetta V. Drew, Robert D. Shull,