Information technology, with its ever growing demand for static and dynamic data storage with both high density and high speed, is one of the driving forces behind the advancement of nanotechnology. This is illustrated by the famous Moore’s law, which describes the temporal development of transistor densities in related integrated circuits and extrapolates to a 10 nm node fabrication by 2017. Similarly, the less popular Kryder’s law projects the areal storage density for magnetic disks at 40 terabytes per square inch by 2020, which translates into approximately 20 nm per byte. Magnetic recording is the technology of choice for large data storage applications as well as for the next generation of nonvolatile, random access memory technologies. Therefore, worldwide activity in this field (including the choice of magnetic materials as well as their resulting properties when fabricated as nanoscale units) is a natural consequence. Thus, it also seems natural that the Beilstein Nanotechnology Symposia series would focus on the topic “Magnetization dynamics on the nanometer scale” for the 2015 event.
Although the Symposium was dedicated to basic physics with a focus on fundamental problems related to magnetization dynamics, these applications were the driving force that triggered the idea to arrange the oral and poster contributions around two extended talks reviewing the state-of-the-art and envisioning possible future trends for magnetic data storage. The first, also serving as the opening talk, was given by Manfred Albrecht (University Augsburg) on Sunday evening. This talk concentrated on bit patterned media and suitable materials optimized for that purpose. In this context, Fe/Pt/Cu was mentioned as a promising candidate material where patterning could be performed by a combination of nanoimprinting and etching. The second talk, delivered by Stuart Parkin (MPI Halle) on Tuesday evening, reported on a recent improvement in racetrack memory that was previously introduced by his group. The new design, comprised of a pair of antiparallel-magnetized nanowires separated by a heavy-metal layer, leads to a much higher domain wall velocity due to current-induced spin-orbit torque in the heavy metal and compensated dipolar fields for the synthetic antiferromagnetic structure.
The main topic of the Symposium was immediately introduced in the Monday morning session by a description of how theory tackles the problem of ultrafast, light-induced magnetization/demagnetization. Emphasis was placed on the essential mechanisms involved with a distinction between local and nonlocal spin-flip processes. All the points addressed in the theoretical introduction were picked up again in the subsequent experimental contributions based on pump–probe experiments performed on multilayer systems. Such systems allow for the analysis of light-induced spin transfer torque (STT) and even the distinction between the transfer of “hot” electrons and the spin-Seebeck effect due to the high temperature gradients generated. Additional insights were reported, delivering element-specific information on fast and ultrafast magnetization dynamics in complex, multi-sublattice magnets and multilayer magnetic structures by employing a transverse magneto-optical Kerr geometry and using extreme ultraviolet pulses from high-harmonic generation. Similarly, time-resolved photoemission (employing femtosecond, higher-order harmonic, vacuum ultraviolet pulses) and parallel recording the 4f magnetic linear dichroism (MLD) and the exchange splitting of the 5d minority and majority spin valence bands in gadolinium revealed a surprising breakdown of the intra-atomic exchange upon femtosecond laser excitation. Spin-transfer torque played an important role quite generally and was addressed quite often, including the idea to exploit STT to compensate for magnetic damping, and thus, to possibly establish spin-torque oscillators. The most interesting point in this context was that from an experimental point-of-view, the confirmation of local or nonlocal spin flips as the cause of magnetization dynamics appears to be by no means clear cut. Rather, both mechanisms appear to contribute to ultrafast demagnetization. Yet another recent example employing nanolithography to fabricate nanopillars of Co/Cu layers with a megahertz lock-in X-ray spectromicroscopy technique demonstrated STT. The application of a nanosecond, electrical pulse produced a strong spin-polarized current flow between thin Co and Cu layers. This transferred an average transient magnetic moment of 1 × 10−5·μB per Cu atom, which was found to be parallel to the magnetization in Co.