Scale and Stress Effects on the Behavior of Ferromagnetic Materials

This dissertation, "Scale and Stress Effects on the Behavior of Ferromagnetic Materials" by Ruilong, Hu, 胡瑞龍, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled SCALE AND STRESS EFFECTS ON THE BEHAVIOR OF FERROMAGNETIC MATERIALS Submitted by Hu Ruilong for the Degree of Doctor of Philosophy at The University of Hong Kong in August 2007 Magnetic materials are being more and more widely used in industry, agriculture and medical and military utilities, for example, transformer cores, magnetically levitated vehicles, transducers, actuators, nuclear reactors, magnetic recording media, biosensors and magneto-optical devices. In future, the most promising applications of magnetic materials may be found in microelectronics and nanoscaled medicine. Magnetic-bead-based high sensitivity biosensors have already been successfully used to probe and detect DNA, proteins and cells. Generally speaking, sensors made of magnetic materials work under very complicated conditions, e.g., electrically, magnetically and/or mechanically loaded environment. For a magnetic device working under external mechanical stresses, the magnetoelastic coupling effect is important especially for materials with large magnetostriction. It is a well-known fact that the distortion of crystal lattice appears as macroscopic strains, which is related to elastic energy. Besides, this kind of strain is coupled with magnetic moment. Hence, a change of the material shape leads to a change of magnetization and vice versa. It is also well-known that behaviors of magnetic materials at diminishing length scale are quite different from those at macroscopic length scale. Therefore, a better understanding of stress and scale effects on the behavior of magnetic materials appears important. The investigation of stress effects on the behaviors of ferromagnetic materials was carried out based on micromagnetism and microelasticity theories in reciprocal space. The evolution of magnetization governed by Landau-Lifshitz-Gilbert equation was solved using the fast Fourier transform technique in reciprocal space. The simulation results showed that the stress could change the distribution of easy axis, and, hence, the distribution of magnetization. The cyclic stress was found to destroy the symmetry of hysteresis loop. The simulation results showed the occurrence of stress-induced magnetization reversal. Moreover, extremely large stress could produce abnormal hysteresis loop. However, the stress had little effect on the scaling exponent. The scale effects were studied using a carefully devised model, in which the Landau-Lifshitz-Gilbert equation was solved in Fourier space with the consideration of exchange energy, magnetocrystalline anisotropy energy, magnetostatic energy, magnetoelastic energy and Zeeman energy. The simulation results showed that the coercivity reached maximum when the characteristic length was equal to the width of domain wall. Moreover, when the length scale was sufficiently small, (e.g., when it was comparable to or smaller than the exchange length), the phenomenon of coercivity almost vanished and the material was in the so-called superparamagnetic state. Fabrication of nanocrystalline magnetic materials was carried out using the electrodeposition method to study the distinct properties of such materials at nano length scale. For example, the nanocrystalline cobalt-nickel alloy of grain sizes around 30 nm was successfully fabricated by this method, which showed improved magnetic properties. The microstructures of these nanocrystalline material