Novel exchange-coupled composite Nanomagnets

The fundamental behaviour of tailored magnetic islands with dimensions below 20 nm is an unchartered area deserving focus because of the possibility for new physics at the nanoscale and the potential for a wide range of applications including magnetic recording, sensors, magnetic RAM and magnetic oscillators. To this aim, we focus on novel nanoscale exchange-coupled composite (ECC) magnets incorporating a hard and a soft magnetic component. Our intention is to obtain a detailed understanding of both the static and dynamic behaviour, so allowing us to tailor the magnetic properties specifically for magnetic data storage, which requires high thermal stability, low switching fields and a narrow switching field distribution (SFD). Arrays of ECC nanoscale magnetic islands at areal densities of 1 Tbit/in2 and beyond will be fabricated with extreme ultraviolet interference lithography (EUV-IL) at the Swiss Light Source and state-of-the-art electron beam lithography. In the first instance, ECC thin films and nanoislands combining high anisotropy films of L10 FePt alloys with softer FePt layers will be investigated. We will then also explore the potential of the more exotic rare earth-transition metal (RE-TM) materials, namely Fe1-xGdx. These materials have the advantage of minimal structural imperfections due to their amorphous nature, a property particularly important for reproducible magnetization reversal in device applications. In addition, in the Fe1-xGdx materials, the net saturation magnetization is dependent on the Gd content, being zero at the compensation temperature, which leads to an infinite increase in coercivity. The combination of FePt layers with the Fe1-xGdx layers will lead to novel magnetic behaviour. In particular, such a system can be used to study systematically the influence of the saturation magnetization and magnetic anisotropy on the reversal characteristics (switching field, SFD) of the hard magnetic FePt layer in the ECC layer stack. In order to understand the magnetic behaviour, in particular to minimize the SFD, a de-tailed understanding of the underlying mechanisms and their relationship to the material micro-structure needs to be gained. Therefore, transmission electron microscopy (TEM) will be deployed to determine the grain structure and crystallography of individual magnetic islands, as well as more advanced x-ray and neutron scattering methods, and this information will be correlated with the magnetic properties. Our intention is then to be able to control the island grain structure by modification of the seed layers and processing conditions. Both the magnetic and microstructural information will be fed into micromagnetic simulations, which will give a vital understanding of the detailed spin configurations, strengthening our understanding of the static and dynamic re-versal behaviour, and elucidating the microstructural contribution to the SFD. In terms of the dynamics, we intend to undertake measurements and micromagnetic simulations to explore new possibilities of energy assisted reversal using microwave excitations to reverse the magnetization in materials with a high magnetic anisotropy. If this is realisable ex-perimentally, then this innovative approach of addressing small, but still thermally stable, mag-netic nanoislands could become a reality. Experimental work in this area is very new and we in-tend to magnetically excite the ECC islands by passing pulsed and continuous wave currents through a copper stripline. We will employ a variety of techniques to detect the magnetic response including scanning transmission x-ray microscopy, MFM and Hall measurements. We apply here for funding for three years to work on this highly topical area of arrays of ECC magnetic nanoislands. This is a collaborative project between L. Heyderman (Paul Scherrer Institute, Switzerland), R. Schäublin (Swiss Federal Institute of Technology Lausanne - Paul Scherrer Institute, Switzerland), M. Albrecht (Chemnitz University of Technology, Germany), and T. Schrefl (St. Pölten University of Applied Sciences, Austria), making use of the Lead Agency Process available between the Swiss National Science Foundation (SNF), the German Research Foundation (DFG) and Austrian Science Fund (FWF). This complementary approach offers a sig-nificant opportunity for a cross-border team, combining expertise from Switzerland, Germany and Austria to make real progress on this important and scientifically fascinating area. It will enhance collaboration between leading research groups in the field and providing an ideal environment to train young people in nanofabrication, measurements at large facilities and computer simulations.

The project entitled Novel exchange coupled nanomagnets addressed the fundamental physics of bilayer magnetic media for data storage. Nowadays, hand held devices like tablets, laptops and cell phones are linked with cloud storage. The data in data centres is stored on hard disks despite the advance of flash memory for mobile computing. Today storage densities of 1 Tbit/in can be achieved in hard disk recording. An increase of the storage density goes hand in hand with substantial energy saving for cloud storage. Increasing the storage density requires new concepts for magnetic data storage. The collaborative project between St. Pölten University of Applied Sciences, Austria, the Paul Scherrer Institute (PSI), Switzerland, and the University of Chemnitz, Germany, targeted the development of novel storage layers for the improvement of hard disk recording. The projects focus was on nanostructured multilayer media. With this concept bits are stored on single magnetic islands, in contrast to conventional hard disk media where a bit is stored on several grains of a continuous media. Additionally the bilayer structure helps to write the data to the disk despite the small magnetic field from the write head in high density magnetic recording. PSI manufactured the nanostructured bilayers, the University of Chemnitz made magnetic measurements, and St. Pölten University provided computer simulation, in order to explore the potential of the new magnetic media concept and to guide the experimental work. One major success was the development of a computer model for optimizing the recording parameters for the new media. With an optimized design the write errors can be reduced to ten errors in one billion written bits. Further the simulations show the details of the magnetization reversal process and how the structure of the material influences the evolution of magnetic features as a function of time. Particular magnetic structures investigated include: (1) magnetic vortices and their movement in amorphous magnetic layers, (2) domain wall the interface between differently oriented magnetic regions in the magnet and how they deform and pin at material inhomogeneities. Future magnetic recording systems will use write heads that provide energy for magnetization reversal in addition to the magnetic write field. One particular energy assisted method is microwave assisted magnetic recording. Again using simulations the bit error rate for microwave assisted recording on bilayer media was calculated.