Time-resolved imaging of pulse-induced magnetization reversal with a microwave assist field

The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal

Magnetic domain configuration of La0.7Sr0.3MnO3 patterned elements

PosterInternational audienceThe magnetization configuration in small La0.7Sr0.3MnO3 elements is investigated as a function of geometry, film thickness, magnetic field, and temperature using x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM). The patterned elements were defined by focused ion beam (FIB) lithography, and consist of elements varying in shape (from circular, triangular and quadrangular) and size, from 200 nm up to 10 μm. A strong magnetic contrast is observed for all thicknesses (10-50 nm). The magnetic state in the larger elements tends to be multidomain, with complex configurations that are determined by the presence of local pinning sites. These pinning sites are overcome with increasing temperature, and the magnetic configuration evolves into lower energy states. In contrast, the magnetic configuration of the smaller elements are largely determined by the magnetostatic energy contribution, which gives rise to highly symmetric states as found in 3d ferromagnetic structures. Our results show that the magnetism of small LSMO elements is robust nearly up to the critical temperature, with magnetic configurations that can be controlled by suitable geometrical design

Electron Transport in Ferromagnetic Nanostructures

The proposal of logic- and memory devices based on magnetic domain-wall motion in nanostructures created a great demand on the understanding of the dynamics of domain walls. We describe the controlled creation and annihilation of domain walls by Oersted-field pulses as well as their internal dynamics during motion. Electric measurements of the magnetoresistance are utilized to identify permanent- or temporal creation and continuous motion of domain walls initiated by nanosecond short field pulses in external magnetic fields. The injection of domain walls into nanowires with control of their magnetic pattern (transverse or vortex), their type (head-to-head or tail-to-tail magnetization orientation) and their sense of magnetization rotation (clockwise or counter clockwise chirality) is reliably achieved. Influencing the creation process of consecutively created domain walls to obtain multiple walls inside one wire or to mutually annihilate the walls is found to be possible by changes of magnetic field parameters. The time structure of the creation process is analysed by time-resolved transmission X-ray microscopy. After complete formation wall transformations are observed above a critical driving field known as the Walker breakdown. Internal excitations of vortex domain walls are also found in low field motion. A strong interplay between internal dynamics and the macroscopic motion is identified