Dynamics of Bloch Domain Walls in One-Dimensional Ferromagnetic Media
This thesis deals with two independent although related topics on the dynamics of Bloch domain walls in ferromagnetic media. In the first topic, which is covered in chapter two, we present a theory for the linear response of a ferromagnetic domain wall to an external d.c magnetic drive. A collective mode method is utilized to study the motion of the domain wall as it interacts with its spinwave excitations. It is found that for small magnetic fields the velocity of the domain wall is linearly proportional to the external magnetic field as seen in some materials. When the external magnetic field reaches a critical value, corresponding to the value that makes the apparent frequency of certain spinwaves in the moving frame of the domain wall vanish, the domain wall radiate spinwaves without increasing its velocity. This provides an explanation for the saturation velocity that is observed in the measurements. In the second topic we deal with the nonlinear effects associated with the dynamics of the domain wall for fields well above the threshold field of the saturation point. The center manifold technique provides us with a powerful tool to study the stability of this point. In phase space, the saturation point represents a fixed point. When the drive field is increased above the threshold, this fixed point executes small "excursions" around the saturation point but returns back to it, until the magnetic field reaches a new threshold where it undergoes a Hopf bifurcation toward a limit cycle. Therefore, the motion of the domain wall becomes the superimposition of small oscillations on the steady state regime with the saturation velocity. This second threshold, and the frequency and amplitude of these oscillations together with their stability, are given by the application of the center manifold theorem. A numerical model for the interaction of the critical single spinwave with the domain wall, is presented to support this analysis. At least one further bifurcation occurs, based on this model, into a bistable state in which the wall velocity successively oscillates with small amplitude about two widely separated values; one corresponds to the saturation value and the other to an order of magnitude larger.
- Pub Date:
- FERROMAGNETIC MEDIA;
- Physics: Condensed Matter; Mathematics