Matthias Buess

High-resolution imaging of fast magnetization dynamics in magnetic nanostructures

By combining magnetic transmission x-ray microscopy with a stroboscopic pump and probe technique using synchrotron radiation we are able to image the magnetization dynamics in micron sized magnetic particles on a sub-100 ps time scale with a lateral spatial resolution down to 21 nm. We report first observations in squared elements indicating locally varying precessional frequencies which are in agreement with micromagnetic simulations. The experiment opens a route towards a high spatiotemporal resolution of spin patterns which is needed to understand the microscopic origin of magnetization reversal of micron sized and nano-sized magnetic particles.

Spatially resolved ferromagnetic resonance: Imaging of ferromagnetic eigenmodes

Fast magnetization dynamics of ferromagnetic elements on sub-micron length scales is currently attracting substantial scientific interest. Studying the ferromagnetic eigenmodes in such systems provides valuable information in order to trace back the dynamical response to the underlying micromagnetic properties. The inherent time structure of third generation synchrotron sources allows for time-resolved imaging (time resolution: 70–100 ps) of magnetization dynamics at soft x-ray microscopes (lateral resolution down to 20 nm). Stroboscopic pump-and-probe experiments were performed on micron-sized Permalloy samples at a full-field magnetic transmission x-ray microscope (XM-1, beamline 6.1.2) at the ALS at Berkeley, CA. Complementary to these time-domain experiments a frequency-domain “spatially resolved ferromagnetic resonance” (SR-FMR) technique was applied to magnetic x-ray microscopy. In contrast to time-domain measurements which reflect a broadband excitation of the magnetization, the frequency-domain SR...

Spin dynamics of the antiferromagnetic-to-ferromagnetic phase transition in FeRh on a sub-picosecond time scale

The antiferromagnetic-to-ferromagnetic phase transition in FeRh films induced by heating with a femtosecond laser pulse was investigated using the time-resolved magneto-optical Kerr effect. An initial rise time of the magneto-optical signal of about 500fs is found as the FeRh is heated through the transition. The data offer a complementary view to previous pump–probe experiments on “simple” ferromagnetic materials and allow a glimpse at the complex interplay between lattice, electron and spin dynamics governing the first-order antiferromagnetic-to-ferromagnetic phase transition of FeRh.

Ultrafast generation of magnetic fields in a Schottky diode.

For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching current pulses into micro-coils and micro-striplines and by using high-energy electron beams. Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements, and experiments seem to confirm this. Spin injection has also been observed in hybrid ferromagnetic–semiconductor structures. Here we introduce a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of our approach is to optically pump a Schottky diode with a focused, ∼150-fs laser pulse. The laser pulse generates a current across the semiconductor–metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques with the speed of current generation at a Schottky barrier. Specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements and the ability to tune the magnitude of the field with the diode bias voltage.

Bifurcation in precessional switching

We explore the precessional motion of the magnetization vector in a model magnetic element. We find that the Landau–Lifshitz equation governing this motion allows trajectories of the magnetization vector to bifurcate. This yet unknown phenomenon is accompanied by a slowing down of the precessional motion and an abrupt shrinking of the size of the trajectory of the precessing magnetization. We discuss the implication of bifurcation for future devices using precessional switching and suggest how magnetic elements showing the classical phenomenon of bifurcation can be tuned to act as quantum bits.

Magnetic spatial non-uniformities on the picosecond timescale

The magnetization of a Co disk with an in-plane flux-closure domain structure was subject to a picosecond magnetic field pulse perpendicular to the plane. Images with sub-micron spatial resolution have been recorded every 10 ps which reveal magnetic non-uniformities of the ferromagnetic resonance (FMR) response. A detailed analysis of the images shows a difference in the FMR frequencies of about 10% as a function of radius. In addition, the magnetization at the edge responds earlier to the applied field pulse. We discuss possible origins of these phenomena.

Magnetization dynamics of Landau structures: tuning the response of mesoscopic magnetic objects using defects

Magnetic vortex cores are interacting with and can even be annihilated by artificial defects, such as holes. These defects have been fabricated by focused ion beam milling (FIB) into the magnetic domains, domain walls and the center of square-shaped vortices, known as Landau structures. We report the imaging of the magnetization dynamics of Landau structures containing holes by means of x-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM). Due to the high lateral and temporal resolution of this method, the magnetic excitation spectrum, which is characteristic for the vortex-hole interaction, is investigated in detail. We find that the vortex core as well as domain walls can be trapped by small holes. With the help of micromagnetic simulations we show that the vortex gyrotropic motion frequency is enhanced, whereas the amplitude is significantly reduced in the case of non-centric holes in domain walls.

Dynamic Aspects of Magnetism

This chapter touches some of the current interests in magnetization dynamics in the precessional regime. In particular, experiments with high spatiotemporal resolution are described. Here. we focus on experiments aimed at identifying the modal structure of inhomogeneously magnetized ferromagnetic elements. Recent developments at synchrotron sources are discussed.

Ultrafast generation of magnetic fields in a Schottky diode

Summary form only given. For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching current pulses into micro-coils and micro-striplines and by using the high-energy electron beams. Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements, and experiments seem to confirm this. Here we introduce a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of our approach is to optically pump a Schottky diode with a focused, /spl sim/150-fs laser pulse. The laser generates a current across the semiconductor-metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques with the speed of current generation at a Schottky barrier. Specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements and the ability to tune the magnitude of the field with the diode bias voltage.