UC Santa Cruz

The areal density of current magnetic storage technologies is approaching the superparamagnetic limit. In order to reach densities of 1 Tb/in² and beyond, new recording techniques are needed, such as the use of patterned media or energy-assisted recording. Studying the small angle ultrafast dynamics sheds light on the intrinsic magnetic properties that determine the device speed. In this Thesis, I will discuss several material systems related to the next generation technologies, and how their dynamics can be tuned through ion irradiation, changes in composition, and three-dimensional shaping. The ability to not only characterize a material's dynamics, but to tune its resonance frequencies, adds an extra dimension of design optimization and flexibility.

First, we measured how the magnetization dynamics of CoCrPt:SiO₂ granular media is affected by irradiation with Co⁺ ions. We observe a steep decrease in the resonance frequency as the ion fluence is increased. Moreover, we quantified how the intergranular exchange can affect the dynamics, causing an increase in frequency beyond what is predicted through macrospin calculations.

Next, we used the composition of the FePt alloy to tune the dynamic response. We showed that the magnetic oscillation frequency of disordered Fe_xPt_100-x alloys can be tuned by up to 50 % by varying the iron content from 42 at. % to 100 at. %. The increasing amount of Pt causes a decrease in the saturation magnetization, and this causes a change in resonance frequency. Furthermore, the damping is enhanced as the Pt is increased due to the additional mosaicity and spin scattering in the alloy.

The main focus of this work was the first investigation of the switching behavior and magnetization dynamics of curved nanomagnets ("caps") and their comparison to flat dots of the same diameter and thickness. We find that the spherical caps reverse via coherent rotation at a larger diameter than the flat dots, and that the caps become saturated at lower applied field strengths. The spin wave spectra of the spheres also proved to be dramatically different than the flat dots, exhibiting a complex mode spectrum with atypical field dependence. The additional modes were due to the interplay of the field direction and curvature of the sphere, which caused multiple distinct regions of demagnetizing field value that were able to support localized spin waves.