Novel materials and media concepts for thermally assisted magnetic recording.

Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, K$_{U}$, combined with a thermal assist to overcome thermal stability and write field limitations are widely seen as a potential extension of current magnetic recording technology. Here we present an overview of recent work on adapting the thermo-magnetic properties of FePt-based high-K$_{U}$ materials for the requirements of such a recording system. In [1] we recently proposed a novel media structure consisting of two exchange coupled films, a high anisotropy film like, e.g., FePt, and a FeRh film. At close to equiatomic compositions FeRh is an antiferromagnet at low temperatures. Interestingly, upon heating beyond a critical temperature, T$_{AF-FM}$, FeRh becomes ferromagnetic for temperatures T$_{AF-FM}<$T$<$T$_{C}$. This opens interesting possibilities for media applications for thermally assisted recording: at a storage temperature, T$_{S}<$T$_{AF-FM}$, the magnetic information is stored in the high-K$_{U}$ FePt layer. For writing at increased temperature, T$_{AF-FM}<$T$_{W}<$T$_{C-FeRh}$, the FeRh becomes ferromagnetic, effectively lowering K$_{U}$ and increasing the total magnetic moment of the bilayer, thus lowering its coercivity via an exchange spring mechanism and helping magnetization reversal at temperatures well below T$_{C}$ of the FePt layer. A related area of great interest is the magnetization dynamics upon rapid heating and cooling of FeRh films using \textit{fs}-laser pump-probe techniques. First results indicate that the AF-FM transition can be driven on a timescale below 1 \textit{ps} [2], yielding interesting insight into the interaction of the spin, electron and lattice subsystems. \newline \newline [1] J.-U. Thiele, S. Maat, E. E. Fullerton, Appl. Phys. Lett. \textbf{82} (2003) p2859-2861 \newline [2] J.-U. Thiele, M. Buess, C. H. Back, Appl. Phys. Lett. \textbf{85} (2004) p2857-2859 and G. Ju \textit{et al.}, Phys. Rev. Lett. \textbf{93} (2004) 197403.