oa Dynamic Non-Local Coherent Phase Approximation (Dnlcpa) Model For Spin Wave Dynamics In Ultrathin Magnetic Fe-Gd Films Presenting Interfacial Structural Disorder

It is widely believed in the semiconductor community that the progress of the electronics based information technology is coming to an end (ITRS 2007), owing to fundamental electronic limitations. A promising alternative to electrons is the use of spin wave excitations. This has ushered a potentially powerful solution towards fabricating devices that use these excitations to transmit and process information (Khitun and Wang 2006). This new approach to information-processing technology, known as magnonics, is rapidly growing (Kruglyak and Hicken 2006), and key magnonic components such as spin wave guides, emitters, and filters are currently explored (Choi et al. 2007). The first working spin wave based logic device has been experimentally demonstrated by Kostylev et al (2005). In the present paper we develop and apply a model to analyze the spin dynamics for iron-gadolinium films of a few Gd(0001) atomic planes between two Fe(110) atomic planes. These ultrathin systems may be deposited layer by layer on a nonmagnetic substrate using techniques like dc-sputtering or pulsed laser deposition. They constitute prototypes for iron-gadolinium nanojunctions between iron leads in magnonics. In this system the Fe/Gd interfaces present structural disorder due to the mismatch between the Fe_bcc and Gd_hcp lattices. This engenders a quasi-infinite ensemble of Fe-Gd cluster configurations at the interface. In the absence of DFT or ab initio results for the magnetic Fe-Gd exchange, we have developed an integration based analytic approach to determine the spatial dependence of this exchange using available experimental data from the corresponding multilayer systems. A dynamic non-local CPA method is also developed to analyze the spin dynamics for the disordered systems. This DNLCPA introduces the idea of a scattering potential built up from the phase matching of the spin dynamics on structurally disordered Fe-Gd interface clusters with the spin dynamics of the virtual crystal. This method accounts properly for the quasi-infinite ensemble of interfacial structural configurations, and yields the configurationally averaged Green's function for the disordered system. The computations yield the spin wave eigenmodes, their energies, life-times, and local densities of states, for any given thickness of the ultrathin magnetic Fe-Gd film. Fig.1 DNLCPA calculated dispersion branches for the spin waves propagating in the ultrathin magnetic 1Fe-5Gd-1Fe film (7 atomic planes) presenting structural interfacial disorder. The normalized energies are in units of iron exchange and spin J(Fe-Fe)S(Fe). The curves are plotted as a function of the y-component of the wave-vector (inverse angstroms), the z-component = 0, in both figures. Fig.2 DNLCPA calculated life-times in picoseconds of the spin waves propagating in the ultrathin magnetic 1Fe-5Gd-1Fe film. Acknowledgements: The authors acknowledge QNRF financial support for the NPRP 4-184-1-035 project. References - S. Choi, K.S. Lee, K.Y. Guslienko, S.K. Kim, Phys. Rev. Lett. 98, 087205 (2007) - A. Khitun and K. L. Wang, Proceedings of the Third International Conference on Information Technology, New Generations ITNG, 747 (2006) - M.P. Kostylev, A.A. Serga, T. Schneider, B. Leven, B. Hillebrands, Appl. Phys. Lett. 87 153501 (2005) - V.V. Kruglyak and R.J. Hicken, J. Magn. Magn. Mater. 306, 191 (2006)