Spin Dynamics in Ferromagnetic Microstructures Paul Crowell, University of Minnesota: DMR We are investigating the excitations of ferromagnetic films patterned into nanostructures. These excitations determine the ultimate “speed limit” which must be obeyed by the bits used in magnetic storage media such as a computer hard drive. In the past year, we have studied excitations in individual nanoparticles down to 50 x 150 x 20 nm in size (graduate student Mun Chan). These are among the smallest systems that have been probed using picosecond time-resolved Kerr microscopy. Graduate student Robert Compton is exploring how microstructure such as grains in a film can pin magnetic vortices. Real-time response and spectral images of a nanoparticle. Note the “beating” of the two different fundamental modes. The inset shows a magnetic force micrograph. Note the radical change in the low- frequency response of a vortex in a 500 nm radius particle when the field changes from 0 to 5 Oe. The actual displacement of the vortex is on the order of 10 nm.

Ferromagnetic films made from materials such as iron and cobalt are the backbone of magnetic storage technologies such as the hard drive in your computer. In order to be effective as a storage medium, the magnetic bit (smallest storage unit) must be stable against perturbations such as those created by heat. In future storage media, the bits might be arranged in a simple pattern such as an array of disks. We want to understand the wave-like excitations that occur when energy is dumped into these small magnetic particles. We have developed a type of microscope that allows us to look at these systems with a spatial resolution of few hundred nanometers and, a time resolution of much less than one billionth of a second. The panel on the left above shows the dynamics of nanometer-scale ellipses prepared by electron beam lithography. These are like miniature bar magnets. When the magnetization is perturbed, it rings like a bell, but with two very closely spaced frequencies. One of the frequencies is associated with energy spread out near the ends of the particle, while the other is more focused near the center (as shown in the images). On the right we show the response of a single magnetic vortex, and how it changes radically when the vortex is displaced by approximately 10 – 20 nm inside a disk by a small magnetic field (red curve). We believe that this is due to the fact that the core of the vortex, which is only 10 nm in diameter can get “stuck” on defects in the magnetic thin film. Spin Dynamics in Ferromagnetic Microstructures Paul Crowell, University of Minnesota: DMR

Spin Dynamics in Ferromagnetic Microstructures Paul Crowell: University of Minnesota: DMR Solitons in Ferrofluids Kate Raach is an undergraduate at the University of Minnesota who has worked in the laboratory as an REU student exploring the possible relationship between the solitons that form in ferrofluids and the vortices that we study in nanoparticles. Ferrofluids are the closest analog to a “liquid ferromagnet,” and they form stable (and sometimes very beautiful) structures that are determined by the competition between gravity, surface tension, and magnetic forces. A single peak, shaped like a “sombrero,” can form over a narrow range of magnetic field. Like the core of a magnetic vortex, this is a soliton in the surrounding uniform fluid background. Kate explored the interactions of these defects and whether they could be moved rapidly in the fluid. She found that they have a very small critical velocity, above which rapid movement creates additional solitons. Kate is now a senior at the University of Minnesota. A single soliton in the middle of a pool of ferrofluid. Three solitons. They are attracted by a local magnetic field.

Ordinary liquids from a simple meniscus: a smooth surface which bends up slightly to meet the walls of its container. The shape of the surface is determined by the forces of gravity and the surface tension of the fluid. Surface tension is what allows a bug to walk across water. Ferrofluids contain small magnetic nanoparticles in a colloid (or suspension). In the real world, they are used in bearings and also in high-end audio applications. In the absence of magnetic field, a ferrofluid forms a meniscus just like an ordinary liquid. When a magnetic field is turned on, however, the surface can break up into an array of peaks, an instability that was first observed in the 1960’s. This effect is responsible for the ring peaks around the edges of the container in the above images. It was recently found that isolated peaks can form when the density of particles in the fluid is high enough. These peaks are “solitons” and can be stable for days. This REU project focused on how these solitons interact and what happens when they are displaced rapidly. Spin Dynamics in Ferromagnetic Microstructures Paul Crowell: University of Minnesota: DMR