Novel exchange-coupled composite nanomagnets

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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 com-posite (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 stabil-ity, 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 nanois-lands combining high anisotropy films of L10 FePt alloys with softer FePt layers will be investi-gated. 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 imper-fections due to their amorphous nature, a property particularly important for reproducible mag-netization 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 re-versal 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.